<<

applied sciences

Review Molecular Science of Lubricant Additives †

Ichiro Minami

Division of Machine Elements, Luleå University of Technology, 97187 Luleå, Sweden; [email protected] † Dedicated to the late Professor Keiji Yamamoto in memory of his contributions to organo- chemistry.

Academic Editor: Jun Kubota Received: 8 March 2017; Accepted: 21 April 2017; Published: 28 April 2017

Abstract: This review aims at introducing an engineering field of to researchers who are not familiar with , thereby emphasizing the importance of lubricant chemistry in applied science. It provides initial guidance regarding additive chemistry in lubrication systems for researchers with different backgrounds. The readers will be introduced to molecular sciences underlying lubrication engineering. Currently, lubricant chemistry, especially “additive technology”, looks like a very complicated field. It seems that scientific information is not always shared by researchers. The cause of this is that lubrication engineering is based on empirical methods and focuses on market requirements. In this regard, engineering knowhow is held by individuals and is not being disclosed to scientific communities. Under these circumstances, a bird’s-eye view of lubricant chemistry in scientific words is necessary. The novelty of this review is to concisely explain the whole picture of additive technology in chemical terms. The roles and functions of additives as the leading actors in lubrication systems are highlighted within the scope of molecular science. First, I give an overview of the fundamental lubrication model and the role of lubricants in machine operations. The existing additives are categorized by the role and work mechanism in lubrication system. Examples of additives are shown with representative molecular structure. The second half of this review explains the scientific background of the lubrication engineering. It includes interactions of different components in lubrication systems. Finally, this review predicts the technical trends in lubricant chemistry and requirements in molecular science. This review does not aim to be a comprehensive chart or present manufacturing knowhow in lubrication engineering. References were carefully selected and cited to extract “the most common opinion” in lubricant chemistry and therefore many engineering articles were omitted for conciseness.

Keywords: tribology; tribo-chemistry; lubrication mechanism; lubricant formulation; molecular design

1. Introduction

1.1. Tribology and Lubrication Engineering Tribology, defined as “the science of interacting surfaces in relative motion” [1], was coined in 1966 [2] by combination of “tribos” (“rubbing” in Greek) [1] + “logy”. This term is becoming popular as we can find the term in dictionaries, not only in science and engineering fields but also in general culture. Although this name is relatively young compared to other engineering fields, empirical knowhow had been known from ancient days back to 2000 BC [3]. Now we can find various tribological phenomena in our daily life; dental care, shaving, wiping, cooking, etc. Needless to say, we enjoy modern civilized life using many machines; house equipment, vehicles, IT devices, industrial manufacturing, transportations, etc. We need tribology every time in everywhere. Thanks to lubrication engineering, we usually use these devices without being aware of tribology. However, we experience occasionally that the breakdown of a machine and this mostly happens due to failures of moving elements. This indicates the importance of lubrication engineering in prolonging the lifetime

Appl. Sci. 2017, 7, 445; doi:10.3390/app7050445 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 445 2 of 33 of machines. Lubrication also plays a significant role in improving energy efficiency. Moreover, lubrication engineering can contribute to comfortable machine operation by reducing unpleasant vibrations and noises that were developed from the moving contacts. Therefore, lubrication engineering does work hard in the background to support our civilized activities. Here, most readers of Applied Sciences would agree with the importance of lubrication engineering but might have little interest in it as research subject, probably because lubrication is a mature technology and hence is not a subject in applied sciences. Actually, lubrication engineering, especially lubricants, are still being developed by mostly trial and error procedures. So far, this has worked well for industrial applications. From now on, industrial R&D has to pay attention to more energy efficiency with sustainability. Since conventional procedures consume time and energy, a smart method should be considered. In this regard, the author believes that the application of fundamental sciences in the right methods brings successful R&D. Although many lubricants are manufactured by a combination of different substances, the understanding of the chemistry in lubricants seems rather underestimated. Actually, chemical information of lubricants is provided without detail and precise description, mainly due to the complexity of contents and the business matter. Although many tribologists state the importance of chemistry in tribology, they sometimes disregard “chemistry” because it looks like “a black box.” This article surveys the chemistry of lubricant additives from viewpoint of fundamental chemistry, with the hope that it to a better understanding of tribological phenomena with molecular science. The author wishes the readers to be good supporters of tribology, especially those who are not familiar with machine elements. It would be an unexpected pleasure of the author if this article motivates a reader to undertake a tribology project with scientific aims.

1.2. The Functions of Lubricants Lubricants are those substances that intervene between rubbing surfaces, thereby preventing any negative influences upon moving. The three major functions of lubricants are below.

• Controlling —This is the primary role of lubricants. Reducing friction and preventing and seizure (or failure) are necessary in most applications. Well-controlled, high friction is required for clutches, breaks, etc. Since this is the central objective of the lubricant, we discuss the phenomena with a lubrication model (Stribeck curve, see Figure1). Optimized performances with minimized side effects could be achieved by proper selection of lubricants for specific machine elements. As modern machines are required to work in a more energy efficient way, the roles of lubricants are increasing. The importance of additives is emphasized (see Section 2.1 tribo-improvers). • Cooling the contact—Heat generated by rubbing motion can have many negative influences on surface materials, such as transformation of microstructure or thermal failure. Ageing of lubricants is accelerated at higher temperatures. Heat accumulation could be prevented by circulating a lubricant. The heat capacity of a lubricant is the controlling factor for this function. This function is mainly supported by the properties of base fluids. Some additives contribute to the heat conductivity of primary (see Section 2.5.2) or secondary (see Section 2.3.5) base fluids. • Cleaning the contact—Wear particles, external dusts, or deposits by aged lubricants could appear during machine operation. These contaminants negatively influence lubrication performances. Circulating a lubricant can wash out these nuisances physically. Advanced lubricants contain some substances to help the cleaning process (see Sections 2.3.2 and 2.3.3).

Lubricants can appear as a liquid, as a semi-solid (greases), or as a solid (including particles and coatings) form. Since cleaning and cooling functions need fluidity, greases and solid lubricants could be applied to machine elements where heat and contamination are not serious factors. Because of the three functions and the benefit of handling, liquid lubricants are most ubiquitous in many practical applications. Therefore, various additives have been developed and are being applied in Appl. Sci. 2017, 7, 445 3 of 33

Appl. Sci. 2017, 7, 445 3 of 33 liquid lubricants. In this regard, this review mainly focuses on additives in liquid lubricants. Of course, lubricants. In this regard, this review mainly focuses on additives in liquid lubricants. Of course, the the demands of greases and solid lubricants are also substantial mainly for task-specific purposes. demands of greases and solid lubricants are also substantial mainly for task‐specific purposes.

Fluid between sphere/flat tribo-contact

L R2 V

λ < 0.7 mixed  1

m R

boundary lubrication λ > 2 H lubrication hydrodynamic lubrication Nomenclature V Velocity L Load R1,R2 Surface roughness 

Friction coefficient Hm Minimum film thickness

-1 1 2 Hersey number (η・V・L ) λ = 2·Hm/(R +R )

Note: Hm (Minimum thickness of liquid film developed from base fluid between the surfaces, clearance of the surfaces) could be calculated from the operating parameters (load and velocity) and material parameters (elastic modulus and pressure-viscosity relations). For detail, see [5].

FigureFigure 1. 1.The The Stribeck Stribeck curve and and lubrication lubrication regime. regime.

1.3. The1.3. StribeckThe Stribeck Lubrication Lubrication Model Model ThereThere are many are many types types of machine of machine elements elements with with different different configurations configurations that that work work under under different different operating parameters. Lubrication systems are individually designed in most cases. Despite operating parameters. Lubrication systems are individually designed in most cases. Despite this, this, we have a versatile model for simulation and explanation of the lubrication performances, the we haveStribeck a versatile curve [4]. model The for model simulation concept and is the explanation dependence of of the friction lubrication coefficient performances, on the operating the Stribeck curveparameters [4]. The model (load and concept velocity), is the the dependencematerial properties of friction (surface coefficient roughness on and the elastic operating modulus), parameters and (loadthe and lubricant velocity), properties the material (viscosity properties and pressure (surface‐viscosity roughness relations). and elasticAs Figure modulus), 1 depicts, and the the friction lubricant propertiescoefficient (viscosity of a lubrication and pressure-viscosity system varies with relations). the Hersey As Figure number1 depicts, calculated the from friction viscosity, coefficient load, of a lubricationand velocity. system When varies a withliquid the (lubricating Hersey number ) is applied calculated to tribo from‐contact, viscosity, it develops load, and a liquid velocity. film When a liquidbetween (lubricating the surfaces. oil) The is applied thickness to of tribo-contact, liquid film Hm it (or develops clearance a between liquid film the surfaces between supported the surfaces. by the liquid) depends both on the properties of the solid and the liquid materials [5]. A viscous The thickness of liquid film Hm (or clearance between the surfaces supported by the liquid) depends liquid tends to provide a thicker liquid film at the contact, while a low viscous liquid tends to both on the properties of the solid and the liquid materials [5]. A viscous liquid tends to provide provide a thinner film between surfaces. If the thickness of the liquid film is greater than the surface a thicker liquid film at the contact, while a low viscous liquid tends to provide a thinner film between roughness R1 and R1 (more precisely λ > 2), the surfaces could be completely separated by the liquid. 1 1 surfaces.Therefore, If the the thickness surfaces can of themove liquid without film contact is greater between than solids the and surface this results roughness in lowR friction.and RThis(more preciselylubricationλ > 2), regime the surfaces is defined could as be “hydrodynamic completely separated lubrication”. by theIdeally, liquid. machine Therefore, elements the surfacesare can movedesigned without to operate contact in this between regime solids since low and friction this results as well in as low negligible friction. wear This could lubrication be achieved. regime is definedWhen as the “hydrodynamic Hersey number lubrication”. becomes smaller, Ideally, the lubrication machine elements regime shifts are to designed the boundary to operate regime. in this regimeAsperities since low on the friction surfaces as come well to as interfere negligible with wear each couldother. As be a achieved. result, friction When becomes the Hersey higher and number becomessubstantial smaller, wear the occurs. lubrication This regime regime is defined shifts to as theboundary boundary lubrication. regime. The Asperities properties on of surfaces the surfaces comeare to interfereimportant withto achieve each other.lubrication As a performances result, friction in becomesthis lubrication higher regime. and substantial Mixed lubrication wear occurs. regime is an intermediate regime between hydrodynamic lubrication and boundary lubrication. This regime is defined as boundary lubrication. The properties of surfaces are important to achieve Although the phenomena in the real world are more complicated, this simplified model helps us to lubrication performances in this lubrication regime. Mixed lubrication regime is an intermediate understand the role of each substance in lubrication systems. Today, the Stribeck model is a regimeconsensus between model hydrodynamic to predict the lubricationlubrication performance. and boundary lubrication. Although the phenomena in the real world are more complicated, this simplified model helps us to understand the role of each substance in lubrication systems. Today, the Stribeck model is a consensus model to predict the lubrication performance. Relative importance of three functions varies with individual application. For example, metalworking fluids have to wash out cutting dusts from the contact and have to cool the tool surface simultaneously. High water based fluids are frequently used in metalworking applications. Although the lubricity is inferior to that of -based fluids, water-based fluids have strong advantages in cleaning and cooling functions together with cost effectiveness. Of course, the lubricity Appl. Sci. 2017, 7, 445 4 of 33 of metalworking fluid is necessary to prevent tool wear, for example, but cooling and cleaning are a technical priority in most cases. On the other hand, friction reduction is the major concern for applications. This is why greases are mostly used in bearing lubrication where cooling and cleaning functions are of secondary importance. The differences in requirements of lubricants for different applications could be understood more clearly if we look at the Stribeck curve [4]. Bearings should be operated under hydrodynamic (precisely under elasto-hydrodynamic) conditions where wear of material and heat generation are trivial. On the other hand, metalworking processes are “heavily” influenced by boundary conditions. When we look at the Stribeck curve carefully, friction increases as Hersey number increases under a hydrodynamic lubrication regime. This means that if the operating conditions are constant, friction increases as viscosity of the fluid increases. Since viscosity is defined as “a measure of the resistance to flow” [6], increased friction by viscosity could be understood as “internal friction in fluids”. Anyhow, viscosity of fluids has to be controlled t a proper level; that is as low as possible for achieving hydrodynamic lubrication. Additive technology for controlling viscosity is discussed in Section 2.2.1. Although machine elements are designed to work under a hydrodynamic regime at steady state, they experience boundary regimes at least during start-stop operations where the velocity is 0. Therefore, lubricants should support all lubrication regimes, from hydrodynamic to boundary lubrications, and hence different functions are required. This is the main reason for the complexity of the lubricant in which different additives are required in a lubricating fluid. Thanks to successful research by both experiments and simulations, the Stribeck curve well represents the phenomena under (elasto-)hydrodynamic regimes. In general, we can use lower viscosity lubricants for fast-running machines in comparison with slow-running machines. Similarly, heavy-load machines need higher viscosity lubricants, compared to light-load machines. Wear of tribo-material happens under boundary and mixed lubrication regimes where direct surface-surface contact occurs. This leads to changes in surface roughness and a possible shift in lubrication regimes as a consequence. However, the Stribeck curve is based on the assumption that the material parameter (surface roughness in this case) is constant. It should be noted that the Stribeck curve does not fully support for boundary and mixed lubrication regimes. Nonetheless, it indicates inferior lubrication performances under boundary and mixed lubrication regimes compared to those under hydrodynamic lubrication.

1.4. Tribo-Chemical Reactions Mechano-chemical reactions have a long history from the alchemist ages [7–10]. Since then, various chemical reactions have been empirically examined under mechanical stresses such as agitation in pestle-mortar or collision of solids. In addition to scientific approaches [11,12], industrial applications of mechano-chemical reaction could be found in preparation of materials [13,14] and waste treatments [15,16]. The term, “tribolysis” was first published in 1928 [17]. It seems to indicate “tribo-chemical reaction” but this article is describing alterations of petroleum products by mechanical agitation (traditional tool for mechano-chemical reactions) compared with photolysis and thermolysis of these substances. Tribo-chemical reactions could be categorized as a part of mechano-chemical reactions. t should be defined precisely as “conversion of molecules to others under the influence of rubbing”. Additionally, this review focuses on this phenomenon “especially under lubricated conditions”. In this regard, chemical reactions of lubricants, both base fluids and additives, are of interest. It is generally agreed that tribo-chemical reactions are complicated phenomena. There are two major causes of the complexity. One is the problem with the analytes. Lubricants are mixture of various compounds, including base fluids and different additives. In most cases, their principle structure is hydrocarbons and hence clear identification of each substance is difficult. When we focus on the reaction mechanism of an additive of interest, the problem of the detection limit with instrumental analysis usually arises. Concentration of additive could be as low as in the range of mmol·kg−1. Specific analytical tools, such as surface Appl. Sci. 2017, 7, 445 5 of 33 sensitive analysis, combined with a model lubricant (e.g., a solution of single additive in a solvent) could be a solution for the analyte problem. Actually, many articles are being published on the mechanism of tribo-improvers (see Section 2.1) with this procedure. On the other hand, much progress in understanding of the energy for reaction is necessary. Chemical reaction is an arrangement of atoms within or between molecule(s), and is accompanied with breaking and making chemical bonds. This process needs energy to occur; tribo-chemical reaction is a conversion of mechanical energy into chemical energy in one sense. From this point, possible driving forces (sources of energy for chemical reactions) for tribo-chemical reactions are summarized in Figure2. Six possible driving forces are considered for the initiation process of tribo-chemical reactions [18].

• Heat induces various chemical reactions. It was estimated that rubbing surface could reach as high as 250–450 ◦C[19]. Friction generates heat and this is considered as the major cause of tribo-chemical reactions. • Nascent surfaces could be exposed by wear of solid surfaces under mixed and boundary conditions. Similarly, a mechanical stress to crystalline materials can sometimes cause lattice defects. These produce a chemically active site on the rubbing surfaces [20]. The chemical activity of transition metals is induced by vacant d-atomic orbital. • Exo-electrons could be emitted from rubbing surfaces. Those electrons have low energy to promote chemical reactions but can produce radical intermediates for further chemical reactions [21]. • Elevated pressure up to Giga-Pascal (>104 bar) could be generated at tribo-contact [22]. Since chemical reactions are initiated by a collision of molecules, compression of reactant raises the probability of the collision. Chemical reactions at high pressure under static conditions are well known [23]. • Orientation of molecules may occur if they were flown through a narrow area [24]. This means the reacting functional groups are close to each other and increases the probability of reaction. This, together with elevated pressure, contributes to the entropy factor of the reaction. • Shearing can dissociate chemical bonds in a molecule directly. It is a well-known phenomenon that molecular mass of can decrease under shearing conditions. The formation of radical species by the dissociation of a –carbon bond had been reported [25]. Typical shear rate at tribological contact is in the range of 105–107 s−1 [19].

Appl. Sci. 2017, 7, 445 6 of 33

exo-electrons Orientation Elevated (electrochemical of molecules pressure reactions) (entropy factor) (entropy factor)

+e→ • → →

L V

Friction heat Nascent surface Shearing ( factor) ( of metal) (mechanical bond dissociation)

M • → → • → •

: hydrocarbon CxHy, : carbon radical • Revised from figure 13 in ref. 18 : functional group M : metal with d-orbital Figure 2. Possible driving forces for chemical reactions at tribo‐contact. Figure 2. Possible driving forces for chemical reactions at tribo-contact. 1.5. Components in Liquid Lubricants So‐called lubricant formulation is a process of mixing substances to prepare a lubricant. The substances should be properly selected to meet the requirements of machine operation. The components in liquid lubricants are classified in two groups—base fluids and additives. Base are the major contents of lubricating fluids. It has been empirically known from ancient times that viscous fluids provide lubrication performances. It was found, about a hundred years ago, that certain substances can improve lubrication performances if dissolved in base fluids [26]. Then many engineering knowledge was accumulated and additive technology for lubricant became a common technique later on. Today, lubricating fluids are manufactured from base oils and 5–20 mass% of different additives depending on the practical requirement (Graphic abstract). Base fluids are viscous having proper viscosity for the application. API defines the category of base oil, as summarized in Table 1. API Groups I–III are made from crude oil through distillation and refinery processes [27]. They are so‐called mineral oils and they constitute the majority of base fluids today because of their availability at a reasonable cost. In brief, mineral oils are composed of various hydrocarbons (normal and branched , cycloalkanes, aromatics, heterocyclic compounds, etc.). It seems difficult to show the chemical contents in mineral oils, because they vary with production area and refinery processes. Therefore, viscosity represents the properties of base oil. Conventionally, kinematic viscosity at 40 °C and at 100 °C is considered for lubricants. The refinery processes have two major purposes; they are purification and modification of molecular structure. Typically, organic sulfides and unsaturated hydrocarbons were extracted by solvents to produce Group I, or decomposed by a catalytic to produce Group II. The importance of these processes for additive technology is that these impurities often show antagonism to many types of lubricant additives. On the other hand, a catalytic isomerization of hydrocarbon molecules produces Group III oils. The differences between Groups I‐II and Group III oils are their rheological properties. Appl. Sci. 2017, 7, 445 6 of 33

A tribo-chemical reaction can beneficially or detrimentally influence to the lubrication performance. There is a consensus in the tribology community that tribo-chemical reactions concern mainly boundary lubrication conditions. However, from the viewpoint of a reaction mechanism, they can occur under (elasto-)hydrodynamic conditions as well; elevated pressure, heat by compression, and shearing are three examples.

1.5. Components in Liquid Lubricants So-called lubricant formulation is a process of mixing substances to prepare a lubricant. The substances should be properly selected to meet the requirements of machine operation. The components in liquid lubricants are classified in two groups—base fluids and additives. Base oils are the major contents of lubricating fluids. It has been empirically known from ancient times that viscous fluids provide lubrication performances. It was found, about a hundred years ago, that certain substances can improve lubrication performances if dissolved in base fluids [26]. Then many engineering knowledge was accumulated and additive technology for lubricant became a common technique later on. Today, lubricating fluids are manufactured from base oils and 5–20 mass% of different additives depending on the practical requirement (Graphic abstract). Base fluids are viscous liquids having proper viscosity for the application. API defines the category of base oil, as summarized in Table1. API Groups I–III are made from crude oil through distillation and refinery processes [ 27]. They are so-called mineral oils and they constitute the majority of base fluids today because of their availability at a reasonable cost. In brief, mineral oils are composed of various hydrocarbons (normal and branched alkanes, cycloalkanes, aromatics, heterocyclic compounds, etc.). It seems difficult to show the chemical contents in mineral oils, because they vary with production area and refinery processes. Therefore, viscosity represents the properties of base oil. Conventionally, kinematic viscosity at 40 ◦C and at 100 ◦C is considered for lubricants. The refinery processes have two major purposes; they are purification and modification of molecular structure. Typically, organic sulfides and unsaturated hydrocarbons were extracted by solvents to produce Group I, or decomposed by a catalytic hydrogenation to produce Group II. The importance of these processes for additive technology is that these impurities often show antagonism to many types of lubricant additives. On the other hand, a catalytic isomerization of hydrocarbon molecules produces Group III oils. The differences between Groups I-II and Group III oils are their rheological properties. Here we introduce the key properties of base fluids; viscosity index: dependency of viscosity by temperature. Viscosity of hydrocarbons drops as temperature increases. Viscosity index (VI) represents the rate of changes by temperature [28]. “High” viscosity index is defined as fewer changes in viscosity by temperature change, as illustrated in Figure3. Both fluid A and B pose similar viscosity at 40 ◦C, while those at 100 ◦C are different. Needless to say, higher VI is desirable because many lubrication systems generate friction heat during machine operations. When considerable viscosity drop happens during machine operation, it causes a shift of the lubrication regime to boundary (see Figure1). As a consequence, it raises the risk of high friction and wear. We also discuss this issue in Section 2.2.1. VI could be calculated according to ASTM D2270, for those fluids of VI < 100. It is recommended to use software on computers, since currently base fluids with VI > 100 are commonly used [29]. Besides mineral oils being the major resources of base fluids, natural (TG, also known as vegetable oils, plant oils, or animal tallow) were the only resources of lubricants before the petroleum age. Although they were used before petroleum products, TGs are currently classified in Group V because of the fewer demands compared to mineral oils. Technically, those additives for mineral oils do not always work well for TGs. One reason for this is that additives are manufactured by petroleum industries. Therefore, industries are interested in improving the performances of petroleum products. The relevant scientific discussions are given in Section 4.4. Appl. Sci. 2017, 7, 445 Appl. Sci. 2017, 7, 445 7 of 33 7 of 33 Appl. Sci. 2017, 7, 445 7 of 33 Appl. Sci. 2017, 7, 445 7 of 33 Appl. Sci. 2017, 7, 445 7 of 33 Table 1. API base oil category and examples. Table 1. API base oil category and examples. Appl. Sci. 2017, 7, 445 Table 1. API baseTable oil 1. API categoryContents base oil andcategory examples. and examples. 7 of 33 Category Classification Table 1. API base oil category and examples. Viscosity Index Remarks Saturates, Mass% Aromatics, Mass%ContentsSulfur, ppm Examples Category Classification Contents Viscosity Index Remarks Category Classification Saturates, Mass% Aromatics, TableMass% 1.Contents APISulfur base oil, ppm category and examples.Examples Viscosity Index Remarks Category Classification Saturates, Mass% Aromatics,Contents Mass% , ppm Examples Viscosity Index Remarks Category Group I ClassificationSolvent‐refined Saturates65–85, Mass% Aromatics,15–35 Mass% SulfurContents300–3000, ppm Examples Viscosity80–119 Index Extraction of impuritiesRemarks by solvents Category Saturates,Classification Mass% Aromatics, Mass% Sulfur, ppm Examples Viscosity Index Remarks Group I Solvent‐refined mineral oil 65–85 Saturates, Mass%15–35Aromatics, Mass%300–3000Sulfur , ppm Examples 80–119 Extraction of impurities by solvents Group I Solvent‐refined mineral oil 65–85 15–35 300–3000 80–119 Extraction of impurities by solvents Group I Solvent‐refined mineral oil 65–85 15–35 300–3000 80–119 Extraction of impurities by solvents Group I Solvent-refined mineralGroup oil I Solvent‐refined 65–85 mineral oil 65–85 15–35 15–35 300–3000300–3000 80–11980–119 Extraction Extractionof impurities by of solvents impurities by solvents Decomposition of organic sulfides by ≧ Group II Hydro‐processed mineral oil 93 <7 5–300 80–119 Decomposition of organic sulfides by Group II Hydro‐processed mineral oil ≧93 <7 5–300 80–119 catalytic hydrogenolysis DecompositionDecomposition ofcatalytic organic sulfides hydrogenolysis of byorganic sulfides by Group II HydroGroup‐processed II mineralHydro‐processed oil mineral oil ≧93 ≧93 <7 <7 5–3005–300 80–119 80–119 DecompositionDecomposition of of organic organic sulfides sulfides by by Group II Hydro-processedGroup II Hydro mineral‐processed oil mineral oil =93≧93 <7<7 5–3005–300 80–11980–119 catalytic hydrogenolysiscatalytic hydrogenolysis catalytic hydrogenolysis hydrogenolysis

Group III Hydro‐cracken mineral oil ≧95 <5 0–30 ≧120 Isomerization of hydrcarbons ≧ ≧ Group III Hydro‐Groupcracken III mineralHydro oil ‐cracken mineral oil 95 ≧95 <5 <5 0–30 0–30 ≧120 120 IsomerizationIsomerization of hydrcarbons of hydrcarbons Group IIIGroup Hydro-cracken III mineralHydro‐cracken oil mineral oil 95≧95 <5<5 0–300–30 ≧120120 IsomerizationIsomerization of of hydrcarbons hydrcarbons Group III Hydro‐cracken mineral oil = ≧95 <5 0–30 =≧120 Isomerization of hydrcarbons Synthetic hydrocarbons,so‐called Group IV Oligomers of 1‐ ‐ ‐ ‐ ‐ Synthetic hydrocarbons,so‐called Group IV Oligomers of 1‐alkene ‐ ‐ ‐ ‐ PAO(polySyntheticSynthetic alpha hydrocarbons,so hydrocarbons,so-called‐olefin) in the‐called market Group IVGroup Oligomers IV ofOligomers 1-alkene of 1‐alkene ‐ ‐ ‐ - - - ‐ - PAO(poly alphaSynthetic‐olefin) in thehydrocarbons,so market ‐called Group IV Oligomers of 1‐alkene ‐ ‐ ‐ ‐ PAO(polyPAO(polySynthetic alpha alpha-olefin) hydrocarbons,so‐olefin) in the in ‐market thecalled market Group IV Oligomers of 1‐alkene ‐ ‐ ‐ ‐ PAO(poly alpha‐olefin) in the market PAO(poly alpha‐olefin) in the market Non‐PAO synthetics, plant oils, and Group V All fluids not included in groups I–IV ‐ ‐ ‐ ‐ NonNon-PAO‐PAO synthetics, synthetics, plant plant oils, oils, and and Group V AllGroup fluids V not includedAll fluids innot groups included I–IV in groups I–IV ‐ ‐ ‐ --- ‐ - some low quality mineral oils Nonsomesome‐PAO low synthetics, low quality quality mineral plant mineral oils, oils and oils Group V All fluids not included in groups I–IV ‐ ‐ ‐ ‐ Non‐PAO synthetics, plant oils, and Group V All fluids not included in groups I–IV ‐ ‐ ‐ ‐ Nonsome‐PAO low synthetics, quality mineral plant oils,oils and Group V All fluids not included in groups I–IV ‐ ‐ ‐ ‐ some low quality mineral oils some low quality mineral oils

Appl. Sci. 2017, 7, 445; doi:10.3390/app7050445 www.mdpi.com/journal/applsci

Appl. Sci. 2017, 7, 445; doi:10.3390/app7050445 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 445; doi:10.3390/app7050445 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 445; doi:10.3390/app7050445 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 445; doi:10.3390/app7050445 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 445 8 of 33

Here we introduce the key properties of base fluids; viscosity index: dependency of viscosity by temperature. Viscosity of hydrocarbons drops as temperature increases. Viscosity index (VI) represents the rate of changes by temperature [28]. “High” viscosity index is defined as fewer changes in viscosity by temperature change, as illustrated in Figure 3. Both fluid A and B pose similar viscosity at 40 °C, while those at 100 °C are different. Needless to say, higher VI is desirable because many lubrication systems generate friction heat during machine operations. When considerable viscosity drop happens during machine operation, it causes a shift of the lubrication regime to boundary (see Figure 1). As a consequence, it raises the risk of high friction and wear. We also discuss this issue in Section 2.2.1. VI could be calculated according to ASTM D2270, for those Appl.fluids Sci. of2017 VI ,<7 ,100. 445 It is recommended to use software on computers, since currently base fluids 8with of 33 VI > 100 are commonly used [29]. 200

150 Fluid A exhibits higher VI than fluid B -1

s VM mitigates the viscosity drop by temperature 2

100 Viscosity, mm Viscosity,

50 Fluid B + VM

Fluid A Fluid B 0 20 40 60 80 100 120 Temperature, °C

FigureFigure 3. TemperatureTemperature-viscosity‐viscosity relation and viscosity index.

BesidesIn addition mineral to MOs oils andbeing TGs, the theremajor is resources growing of demand base fluids, for base natural fluids triglycerides prepared artificially, (TG, also knownso-called as syntheticvegetable fluidsoils, plant (SF). oils, Huge or animal types of tallow) SFs have were been the only introduced resources and of arelubricants being developed before the petroleumfor lubricants. age. AmongAlthough them, they syntheticwere used hydrocarbons before petroleum prepared products, by the TGs oligomerization are currently classified of 1-aklens in Groupform an V individual because of category, the fewer Group demands IV. They compared are called to mineral “poly-alpha-olefin oils. Technically, (PAO)” those as a additives market code. for Thismineral pseudo-chemical oils do not always naming work is anwell example for TGs. that One makes reason chemists for this confused.is that additives are manufactured by petroleumOther synthetic industries. hydrocarbons, Therefore, synthetic industries , are and interested poly-ethers in improving are also being the used performances as base fluids. of Thepetroleum importance products. of SF The is, in relevant contrast scientific to MOs discussions and TGs, the are possibility given in Section of molecular 4.4. design for various purposesIn addition [30]. Similar to MOs to the and issue TGs, with there TGs, is conventionalgrowing demand additive for forbase MOs fluids cannot prepared always artificially, be applied soto‐ SFs,called as synthetic discussed fluids in Section (SF). 4.4Huge. types of SFs have been introduced and are being developed for lubricants.Although Among the them, concept synthetic of molecular hydrocarbons design canprepared create by molecules the oligomerization with required of 1 properties‐aklens form as anlubricants, individual it is category, difficult Group to support IV. They all requirements are called “poly for a‐alpha lubrication‐olefin system (PAO)” with as a basemarket fluid code. alone This in pseudoa cost effective‐chemical manner. naming Here is an we example shall recognize that makes the chemists importance confused. of task-specific lubricant additives. TheyOther are defined synthetic as thosehydrocarbons, substances synthetic that improve esters, the and total poly performances‐ethers are also if added being toused base as fluids base properly.fluids. The The importance performances of SF include is, in contrast both tribological to MOs and and TGs, material the relatedpossibility ones. of Themolecular former design contribute for variousto reducing purposes friction [30]. and Similar preventing to the wear issue and with seizure TGs, conventional during machine additive operation. for MOs The cannot latter always mainly becontribute applied to SFs, the lifetimeas discussed of the in lubricant Section 4.4. and/or tribo-materials. The method of improvements is idealAlthough to enhance the the concept advantages of molecular of base fluids design with can a create synergistic molecules effect. with In the required real world, properties additives as lubricants,make up for it theis difficult shortages to insupport specific all properties requirements of base for fluids a lubrication in most cases.system Additives with base as fluid liquid alone forms in ahave cost advantages effective manner. of handling Here inwe liquid shall lubricants.recognize the Solid importance additives, of either task‐ asspecific solution lubricant or as suspension, additives. Theyare also are commonly defined as used. those Greases substances could that be usedimprove in some the cases.total performances if added to base fluids properly.The The application performances of lubricant include additiveboth tribological is relatively and material recent inrelated the longones. history The former of lubrication contribute toengineering. reducing friction To the best and of preventing the author’s wear knowledge, and seizure the first during publication machine of operation. the phenomena The latter in a scientific mainly contributejournal was to nearly the lifetime 100 years of the ago lubricant [31]. According and/or totribo the‐materials. publication, The lubricant method additivesof improvements have been is practiced in the market since the 1950s [26]. Since then, lubricant additives have been developed Appl. Sci. 2017, 7, 445; doi:10.3390/app7050445 www.mdpi.com/journal/applsci by engineering knowhow. This might be the main reason why the descriptions in this field are less-scientific; confusing terminology, unclear chemical structure, complexity of contents, etc.

1.6. Category of Lubricant Additives Many types of lubricant additives can be found in the market. In most cases, they are being sold according to their main roles in lubrication systems, such as anti-wear or anti-oxidation agents, and friction modifiers. It would be instructive to classify those substances according to their working functions, the means of action (how does it work), and their working site. Appl. Sci. 2017, 7, 445 9 of 33

1.6.1. Working Function Category This is an upgraded categorization of existing methods, grouping “substances by their role” into large groups. All additives belong to one of the following groups.

• Tribo-improvers (tribology-improving additives) directly contribute to improve the tribological performances of the lubricant. In short, they are responsible for the primary role of lubricants (see Section 1.2). Friction modifiers (FM), anti-wear agents (AW), extreme pressure additives (EP) are representative ones. They stand at a central position in the technology of lubricant additives. • Rheo-improvers (rheology-improving additives) concern the fluidity of the base oil. Viscosity modifiers (VM) are the main additives in this group. Pour point depressants (PPD) make the base oil applicable in a chilly environment. They indirectly contribute to the lubrication performances, mainly in hydrodynamic regime. • Maintainers help to keep the substances (both lubricant and materials of machine elements) in good condition through preventing the degradation of substances participating in the lubrication system. They mainly contribute to prolonging the lifetime of the lubrication system and partly contribute to the lubrication performances in some cases. (AO) play a decisive role in preventing the ageing process of lubricants. Detergents and dispersants can mitigate the negative influences of contaminants on lubrication. inhibitors (rust preventives) can protect the tribo-materials from corrosion. Air bubbles may be incorporated in lubricants during machine operation. They cause lubricant starvation at the contact and promote the autoxidation processes. Anti-foam agents (foam breaking agents, ) can break bubbles. Water is a ubiquitous contaminant in most applications. It drops the viscosity of the lubricant and causes ageing of both lubricants and materials. Demulsifiers (emulsion breaking agents) are beneficial for separating contaminated water in a lubricant. • Auxiliaries are sometimes incorporated with specific purpose in addition to above additives.

1.6.2. Working Site Category From another viewpoint, the working site of each additive can be characterized, as below. This category focuses on the action mechanism (the way the additive works) at a molecular scale rather than practical performances.

• Interface agents work at the interfaces between different phases. Lubricated contacts involve solid (tribological material)—liquid (lubricant) interfaces. Examples are tribo-improvers as the main actors in this category. Corrosion inhibitors deactivate the surfaces to be attacked by corrosive matters. A lubricant may intake air bubbles while circulating, and thereby develops various forms. The interaction of foam decomposing additives at the border between gas–liquid phases can destroy the bubbles. • Bulk agents concern the properties and/or stability of liquid as uni-phase matter. Rheo-improvers are representative examples of this group. Antioxidants (except metal deactivators), and other auxiliaries are in this group.

1.6.3. Working Mechanism Category This gives more insight into the action mechanism, how a molecule behaves and/or changes while working. It also connects to the level of energy to activate the functions.

• Chemical additives are those additives that undergo chemical reaction(s) while working. A chemical reaction is defined as a rearrangement of electrons that bind atoms in a molecule. Chemical changes at interfacial phenomena in tribology are mostly irreversible, although there is a certain possibility of reversible reactions in the liquid phase. The initiation of chemical reaction needs much more energy than physical phenomena. Examples are anti-wear agents that provide Appl. Sci. 2017Appl., 7 Sci., 445 2017 , 7, 445 10 of 33 10 of 33 Appl. Sci. 2017, 7, 445 10 of 33 various forms.various The forms. interaction The interaction of foam ofdecomposing foam decomposing additives additives at the border at the between border between gas–liquid gas–liquid phases canvariousphases destroy canforms. thedestroy Thebubbles. interactionthe bubbles. of foam decomposing additives at the border between gas–liquid  Bulk agentsphasesBulk concern agents can destroy concernthe properties the the bubbles. properties and/or stability and/or stabilityof liquid of as liquid uni‐phase as uni matter.‐phase Rheo matter.‐improvers Rheo‐improvers are representative Bulkare representative agents examples concern examples the of thisproperties group. of this and/or Antioxidants group. stability Antioxidants (except of liquid metal(except as uni deactivators), ‐metalphase deactivators),matter. and Rheo other‐ improversand other auxiliariesareauxiliaries are representative in this are group. in this examples group. of this group. Antioxidants (except metal deactivators), and other auxiliaries are in this group. 1.6.3. Working1.6.3.Appl. Working Mechanism Sci. 2017, 7, Mechanism445 Category Category 10 of 33 1.6.3. Working Mechanism Category This givesThis variousmore gives insightforms. more The intoinsight interaction the intoaction of foamthe mechanism, actiondecomposing mechanism, additiveshow a molecule athow the border a molecule behaves between behavesgas–liquidand/or changes and/or changes while working.whileThis working. Itphases givesalso canconnects Itmore destroy also insightconnects tothe thebubbles. intolevel to thethe of energylevelaction of to mechanism,energy activate to activatethe how functions. a the molecule functions. behaves and/or changes  Bulk agents concern the properties and/or stability of liquid as uni‐phase matter. Rheo‐improvers while working. It also connects to the level of energy to activate the functions.  Chemical Chemical additivesare representative additives are those examples are additives those of this group.additives that Antioxidants undergo that undergo (exceptchemical metal chemical reaction(s) deactivators), reaction(s) whileand other working. while working. auxiliaries are in this group. A chemical ChemicalA chemical reaction additives reactionis defined isare definedas those a rearrangement asadditives a rearrangement that of electronsundergo of electrons thatchemical bind that atoms reaction(s) bind in atoms a molecule. while in a molecule.working. Appl. Sci. 2017Chemical, 7, 445 changes at interfacial phenomena in tribology are mostly irreversible, although there10 of 33 1.6.3.AChemical chemical Working changes Mechanismreaction at isCategoryinterfacial defined asphenomena a rearrangement in tribology of electrons are mostly that irreversible, bind atoms althoughin a molecule. there is a certainChemical possibility changes of reversibleat interfacial reactions phenomena in the in liquidtribology phase. are mostlyThe initiation irreversible, of chemical although there is aThis certain gives morepossibility insight intoof reversiblethe action mechanism, reactions how in athe molecule liquid behaves phase. and/or The changesinitiation of chemical reactionis reactionneeds a certain much needs possibility more much energy more of reversible thanenergy physical than reactions physical phenomena. in thephenomena. liquid Examples phase. Examples are The anti initiation ‐arewear anti agents‐ wearof chemical agents boundary filmwhile on working. rubbing It also surfaces connects to through the level of tribo-chemicalenergy to activate the reactions. functions. Antioxidants also belong to that providereactionthat provideboundary needs boundary filmmuch on more rubbingfilm energyon rubbingsurfaces than surfaces throughphysical throughtribo phenomena.‐chemical tribo‐chemical Examplesreactions. reactions. Antioxidantsare anti ‐Antioxidantswear agents this group. Chemical additives are those additives that undergo chemical reaction(s) while working. also belongthatalsoA to provide belongchemical this group. to boundaryreaction this group. is defined film on as arubbing rearrangement surfaces of electrons through that tribo bind‐ chemicalatoms in a molecule.reactions. Antioxidants • Physical Physical additivesalsoPhysical additivesChemical belong are additives are changesto those thisthose at group.are substances interfacialsubstances those substancesphenomena that work work in that tribology without without work are withoutany mostly any chemical chemicalirreversible, any chemical changes. although changes. changes. Examples there Examples Examples are are are nano-particlenano ‐particlePhysicalnano additivesis ‐a additivesparticle certain additives possibility suchadditives such are as as ofthose tribo-improvers tribo suchreversible substances‐ asimprovers tribo reactions‐improvers thatand andin the workviscosity viscosity liquid and without phase.viscosity modifiers. modifiers. Theany initiationmodifiers. chemical Physical Physical of chemicalchanges.Physical changes changes Examples changesneed need need are less activationreaction energy needs thanmuch chemicalmore energy changes than physical and phenomena.are usually Examples reversible are anti processes.‐wear agents They last less activationnanolessthat energyactivation‐particle provide than boundaryadditives energy chemical film thansuch on rubbing chemicalas changes tribo surfaces‐improvers changes and through are and and tribo usually areviscosity‐chemical usually reversible reactions. modifiers. reversible Antioxidants processes. Physical processes. changes They They last need last longerlonger than thanlesslonger thealso theactivation chemical belongthan chemical the to thisenergy ones. chemical ones. group. than ones. chemical changes and are usually reversible processes. They last  The abovelonger Physicalcategories than additives the arechemical areindependent those ones. substances of that each work other. without The any chemicalfunction changes. category Examples is a areuniversal Thenano above‐particle categories additives suchare asindependent tribo‐improvers ofand each viscosity other. modifiers. The Physicalfunction changes category need is a universal The above categories are independent of each other. The function category is a universal criterion criterioncriterion mainlyTheless formainlyabove activation engineering categoriesfor energyengineering applications than are chemical independent applications andchanges overviewing andofand eachare overviewing usually other. the reversibletechnology. The the function technology.processes. Phase category They and Phase last scientific is and a universal scientific mainlycategories for engineeringcriterioncategories focus longeron mainly focus investigation applications than onfor the investigation engineeringchemical of andmechanism ones. overviewing applicationsof mechanism and developing and the and overviewing technology. developing new technologies thenew Phase technology. technologies and through scientific Phase throughmolecular and categories molecular scientific focusdesign. on investigation Tablecategoriesdesign. 2 depictsTableThe focus above of 2 an depicts mechanismon categories interactive investigation an interactiveare andcharacterindependent of developing mechanism character of of these each ofandcategoriesother. new these developing The technologies categories function for individual new category for throughtechnologies individual lubricantis a universal molecular lubricant through additives. molecularadditives. design. Table2 depictsdesign. ancriterion interactive Table mainly 2 depicts for character engineering an interactive of applications these character categories and overviewing of these for individual thecategories technology. for lubricant Phase individual and scientific additives. lubricant additives. categories focus on investigation of mechanism and developing new technologies through molecular Table 2. RelationsTable 2. Relations of different of differentclassification classification of additives. of additives. design. Table 2 depicts an interactive character of these categories for individual lubricant additives. Table 2. RelationsTable 2. Relations of different of different classification classification of additives. of additives. WorkingWorking WorkingWorking Site Site Table 2. Relations of different classification of additives. MechanismMechanismWorking InterfaceInterface Working Site Bulk Bulk Working Site Working Working Site Working MechanismMechanism Interface Bulk Mechanism InterfaceInterface Bulk Bulk

chemically chemically chemically chemically chemically

physically physically physically physically

physically

2. Individual Lubricant Additives and Their Working Mechanism 2. Individual2. Individual2. Individual Lubricant LubricantThis chapter Lubricant Additives Additives introduces Additives and and individual Their Their and Working additive Their Workingcategories Mechanism Mechanism relatedMechanism to the role in lubrication. Although the description follows conventional ways to introduce lubricants [32–34], this chapter This2. Individualchapter introduces Lubricant individual Additives additiveand Their categories Working Mechanismrelated to the role in lubrication. This chapterprimarilyThis introduces chapter focuses individualonintroduces how molecules additiveindividual behave for categories lubricationadditive relatedperformances.categories to the Representativerelated role into lubrication. themolecular role in Althoughlubrication. Although the description follows conventional ways to introduce lubricants [32–34], this chapter the descriptionAlthoughstructureThis follows chapterthe and conventionaldescription the processintroduces of followsaction ways individual are conventionalgiven to introduce briefly additive to helpways lubricants categoriesthe toreaders’ introduce understanding [related32– 34lubricants], to this andthe chapter further[32–34],role in primarilythislubrication. chapter primarilyAlthough focusesscientific onthe investigations. how description molecules The follows science behave ofconventional forlubricant lubrication additives, ways performances. especially to introduce their Representativeaction lubricants mechanism, [32–34], molecular is this chapter focuses on howprimarily molecules focuses behave on how for molecules lubrication behave performances. for lubrication Representative performances. Representative molecular structure molecular structureprimarily andunder the discussionprocessfocuses onofin the actionhow tribology molecules are communitygiven behave briefly and isforto still help lubrication under the investigation readers’ performances. understandingby different Representative researchers. and further molecular structureHere the and author the processtried to showof action the consensus are given mechanism, briefly to as help the “greatestthe readers’ common understanding measure” and further and thescientific process investigations. of action are The given science briefly of lubricant to help additives, the readers’ especially understanding their action and mechanism, further scientific is structurescientificextracted andinvestigations. from the carefully process selected Theof action science articles. are of given lubricant briefly additives, to help the especially readers’ theirunderstanding action mechanism, and further is investigations. The science of lubricant additives, especially their action mechanism, is under under discussionscientificunder discussion ininvestigations. the tribology in the tribology Thecommunity science community andof lubricant is still and under isadditives, still investigation under especially investigation by different their by action different researchers. mechanism, researchers. is discussionHere the inunderHere theauthor tribologythediscussion triedauthor to community in triedshow the tribologyto the show consensus and communitythe is stillconsensus mechanism, under and mechanism,is investigation still as under the “greatest asinvestigation bythe different “greatest common by researchers. differentcommon measure” researchers. measure” Here the authorextracted triedHereextracted from to the carefully show fromauthor the carefully selected consensustried to selectedarticles. show mechanism, articles.the consensus as the mechanism, “greatest common as the “greatest measure” common extracted measure” from carefully selectedextracted articles. from carefully selected articles.

2.1. Tribo-Improvers Those additives contribute to the primary purpose of a lubricant and many R&D activities have been and are being performed intensively. Therefore, this review puts special emphasis on this category. There is a consensus among tribologists and lubrication engineers that tribo-improvers could be conveniently classified into FMs, AWs, and EPs. Generally, FMs effectively work for “relatively light-load” mixed lubrication while AWs prevent wear for “harder” mixed lubrication, depicted as a simple Stribeck curve in Figure4. It is empirically known that their effects overlap each other. That is, friction reduction by FM is usually associated with wear reduction to some extent. Similarly, AW primary prevents wear and sometimes reduces friction, but not always. In consequence, the demarcation between FM and AW is not clear. This can be understood by their working mechanism below. Appl. Sci. 2017, 7, 445 11 of 33

2.1. Tribo‐Improvers Those additives contribute to the primary purpose of a lubricant and many R&D activities have been and are being performed intensively. Therefore, this review puts special emphasis on this category. There is a consensus among tribologists and lubrication engineers that tribo‐improvers could be conveniently classified into FMs, AWs, and EPs. Generally, FMs effectively work for “relatively light‐load” mixed lubrication while AWs prevent wear for “harder” mixed lubrication, depicted as a simple Stribeck curve in Figure 4. It is empirically known that their effects overlap each other. That is, friction reduction by FM is usually associated with wear reduction to some extent. Similarly, AW primary prevents wear and sometimes reduces friction, but not always. In consequence, Appl. Sci.the2017 demarcation, 7, 445 between FM and AW is not clear. This can be understood by their working11 of 33 mechanism below.

higher Supposed to Base FM fluid Supposed to Supposed

Wear rate Wear AW

AW FM lower lower

lower Friction coefficient higher

FigureFigure 4. 4.Overlap Overlap effectseffects of of tribo tribo-improvers.‐improvers.

2.1.1.2.1.1. Friction Friction Modifiers Modifiers (FM) (FM) Historically,Historically, carboxylic carboxylic acids acids with with a a straight straight hydrocarbon chin chin (higher (higher fatty fatty acids) acids) were were the first the first recognizedrecognized “lubricity “lubricity improving improving substances” substances” when when they they were were dissolved dissolved in in mineral mineral oils oils [ 35[35].]. TheyThey have have been well‐studied from both a scientific and engineering perspective in the past 70 years. The been well-studied from both a scientific and engineering perspective in the past 70 years. The benefit benefit of carboxylic acids in scientific research is the availability of pure compounds as commercially of carboxylicavailable reagents, acids in although scientific their research practical is theapplications availability are trivial of pure [36]. compounds However, an as analogous commercially availablemolecular reagents, structure although could be their found practical in various applications practical additives, are trivial indicating [36]. the However, importance an of analogous good molecularunderstanding structure in could the working be found mechanism in various from practical a scientific additives, viewpoint. indicating the importance of good understandingFigure in5 theexplains working the effective mechanism working from mechanism a scientific of viewpoint. carboxylic acids (octadecanoic acid, Figuren‐C17H355COOH explains is the the most effective frequently working employed mechanism in fundamental of carboxylicresearch) for acids steel as (octadecanoic a tribo‐material. acid, n-C17EngineeringH35COOH issurfaces the most are frequentlyusually covered employed with contaminants in fundamental or oxide research) layers, for which steel can as be a tribo-material. removed Engineeringby rubbing. surfaces This areexposes usually active covered surfaces with that contaminants interact with or oxidethe FM layers, molecule. which Further can be rubbing removed by rubbing.provides This the exposes energy active to the surfaces molecule that for interactsurface reactions. with the It FM produces molecule. organic Further salt (salt rubbing of carboxylic provides the acid), and results in chemisorption ( with between adsorbent and energy to the molecule for surface reactions. It produces organic salt (salt of carboxylic acid), and results adsorbant). Chemisorption possesses a strong interaction with the surface compared to that of in chemisorptionphysisorption (adsorption [37], and hence with it chemicalis beneficial bond for protecting between adsorbent the film against and adsorbant). rubbing [38]. Chemisorption The alkyl possessesgroup a in strong the molecule interaction has withsignificant the surface role in compared lubrication. to Direct that of asperities, physisorption contact [ 37upon], and rubbing hence it is beneficialcould for be protectingmitigated by the the film adsorbed against FM rubbing molecules. [38]. The Van alkyl der group Waals in attraction the molecule force has could significant be role ingenerated lubrication. between Direct oriented asperities, molecules. contact It strengthens upon rubbingthe robustness could of be the mitigated film to support by the the adsorbedload FM molecules.applied to the The contact Van der [35]. Waals In fact, attraction alkyl chains force having could >12 be methylene generated (‐CH between2‐ unit) are oriented beneficial molecules. for It strengthensboundary lubrication the robustness [39]. This of themodel film has to been support proved the under load dynamic applied conditions to the contact (so‐called [35 ].“in In lubro”) fact, alkyl using optical interferometry [40] or liquid cell scanning probe microscope [41]. chains having >12 methylene (-CH2- unit) are beneficial for boundary lubrication [39]. This model has been proved under dynamic conditions (so-called “in lubro”) using optical interferometry [40] or liquid cell scanning probe microscope [41]. Although the interaction of the acid moiety with the surface is a chemical process, the hydrocarbon group (commonly straight hydrocarbons having 15–17 ), a major part of the molecule, is supposed to be unreacted. However, common spectroscopy cannot provide clear evidence of additive molecules that are mainly composed of the hydrocarbon moiety. It should be noted that the concentration of FM is usually around 10 mmol·kg−1 in hydrocarbons as base fluids and hence it is difficult to identify additive molecules as highly diluted solutions. Nonetheless, clear evidence of chemisorption was obtained by surface mass spectroscopy using deuterium-labeled molecules as model FM [42]. This is frequently misunderstood by tribologists as “those FMs physisorp on surfaces”, probably because the alkyl group is unreacted. A term “quasi-physical adsorption” might be convenient to distinguish those FMs from “tribo-chemical types” explained below. Some lubricant additives take advantage of tribo-chemical reactions. Figure6 illustrates the working mechanism of organic salts of molybdate. The first step is the interaction of additive molecules Appl. Sci. 2017, 7, 445 12 of 33 with the rubbing surface, which is an analogous mechanism in Figure5. The difference appears in the next step. When the tribo-process provides enough energy to the molecule, the molecule decomposes to inorganic salts. Since the reaction is associated with the dissociation of many chemical bonds, it needs relatively higher energy than the chemisorption process. The inorganic salts ( disulfide as the example in Figure6) are usually more robust than organic salts to heat and mechanical shearing. Contrary to the chemisorption mechanism, the organic moieties in an additive molecule were decomposed during the tribo-film formation. In this regard, the main role of the organic moiety is to dissolve the precursor of inorganic salt in base fluids (mostly hydrocarbons). Some contributions of carbon-deposit to the tribological properties have been also suggested for the tribo-chemically derived film [43]. Appl. Sci. 2017, 7, 445 12 of 33

FigureFigure 5. Working 5. Working mechanism mechanism of of adsorption adsorption type type tribo tribo-improvers.‐improvers. Appl. Sci. 2017, 7, 445 13 of 33 Although the interaction of the acid moiety with the surface is a chemical process, the hydrocarbon group (commonly straight hydrocarbons having 15–17 carbons), a major part of the molecule, is supposed to be unreacted. However, common spectroscopy cannot provide clear evidence of additive molecules that are mainly composed of the hydrocarbon moiety. It should be noted that the concentration of FM is usually around 10 mmol∙kg−1 in hydrocarbons as base fluids and hence it is difficult to identify additive molecules as highly diluted solutions. Nonetheless, clear evidence of chemisorption was obtained by surface mass spectroscopy using deuterium‐labeled molecules as model FM [42]. This is frequently misunderstood by tribologists as “those FMs physisorp on surfaces”, probably because the alkyl group is unreacted. A term “quasi‐physical adsorption” might be convenient to distinguish those FMs from “tribo‐chemical types” explained below. Some lubricant additives take advantage of tribo‐chemical reactions. Figure 6 illustrates the working mechanism of organic salts of molybdate. The first step is the interaction of additive molecules with the rubbing surface, which is an analogous mechanism in Figure 5. The difference appears in the next step. When the tribo‐process provides enough energy to the molecule, the molecule decomposes to inorganic salts. Since the reaction is associated with the dissociation of many chemical bonds, it needs relatively higher energy than the chemisorption process. The inorganic salts ( as the example in Figure 6) are usually more robust than organic salts to heat and mechanical shearing. Contrary to the chemisorption mechanism, the organic moieties in an additive molecule were decomposed during the tribo‐film formation. In this regard, the main role of the organic moiety is to dissolve the precursor of inorganic salt in base fluids (mostly hydrocarbons). Some contributions of carbon‐deposit to the tribological properties have been also suggested for the tribo‐chemically derived film [43]. Molybdenum disulfides are well‐known solid lubricants [44]. Certain oil‐soluble molybdenum FigureFigure 6. Working 6. Working mechanism mechanism of of tribo-chemical tribo‐chemical reaction type type tribo tribo-improvers.‐improvers. salts are practical FMs [45–47] as precursors of molybdenum disulfide. InorganicSome task particles,‐specific tribosuch‐ improversas soft metals have or beencarbon developed. allotropes, For can example, physically the interact importance with theof rubbing“anti‐shudder surfaces agent” and therebyis sometimes improve pointed the tribological out for driveline properties systems. [48–52]. It prevents They could stick be‐slip used due not to onlyhigher for static FMs frictionbut also than AWs dynamic or EPs dependingfriction, which on their results properties. in the vibration Their stable of material dispersion being has in to the be sameprepared direction for liquid as the lubricants. motion. This The belongs details toare FM omitted and analogous because thissubstances review topays FMs much are employed. attention to molecular structure. 2.1.2. Anti‐Wear Agents (AW) Wear is a change of shape on surfaces through eliminating a small portion of the material from the triboogical contact. It happens under relatively hard mixed lubrication conditions where adsorption type FMs cannot support the lubrication performances. As discussed in the mechanism of reaction‐type FMs, inorganic salts generated by tribo‐chemical reactions are suitable. The differences between AWs and reaction‐type FMs are not the chemical process of tribo‐film formation but the physical properties of the tribo‐film provided at the tribo‐contact. In general, hard materials are more robust to wear compared to soft material. Inorganic phosphates on steel surfaces are well known as the protecting film [53]. bis(dialkyldithiophosphates) (ZnDTPs) are the most widely used AWs in practical lubricants so far [54]. Intensive studies on working mechanism of ZnDPTs by surface analysis have revealed the formation of inorganic compounds composed of phosphorus, sulfur, and zinc [55]. The process of boundary film formation has been studied experimentally [56–59] and in‐silico [60]. Co‐existence of other tribo‐improvers such as molybdenum carbamates displays improvements in lubrication performances [61–64]. Organic esters of phosphoric and phosphonic acids also display AW properties for steel surfaces [19].

2.1.3. Extreme Pressure Additives (EP) Formerly, these were so‐called as load carrying agents. When the tribological conditions become severe, at higher load and/or higher temperature, welding of the material may occur. EP can prevent such failure of materials upon rubbing contact. Their functions are unique compared to FMs and AWs. EPs first react with the surface, which is analogous to AWs. However, the product does not protect the surface but mildly removes the atoms on the uppermost surfaces (Figure 7). The reaction of an EP molecule (dialkyl disulfide in this case) with the atom−α on the surface is associated with migration of a bonded‐electron from the material, as depicted by a red arrow. Appl. Sci. 2017, 7, 445 13 of 33

Molybdenum disulfides are well-known solid lubricants [44]. Certain oil-soluble molybdenum salts are practical FMs [45–47] as precursors of molybdenum disulfide. Inorganic particles, such as soft metals or carbon allotropes, can physically interact with the rubbing surfaces and thereby improve the tribological properties [48–52]. They could be used not only for FMs but also AWs or EPs depending on their properties. Their stable dispersion has to be prepared for liquid lubricants. The details are omitted because this review pays much attention to molecular structure. Some task-specific tribo-improvers have been developed. For example, the importance of “anti-shudder agent” is sometimes pointed out for driveline systems. It prevents stick-slip due to higher static friction than dynamic friction, which results in the vibration of material being in the same direction as the motion. This belongs to FM and analogous substances to FMs are employed.

2.1.2. Anti-Wear Agents (AW) Wear is a change of shape on surfaces through eliminating a small portion of the material from the triboogical contact. It happens under relatively hard mixed lubrication conditions where adsorption type FMs cannot support the lubrication performances. As discussed in the mechanism of reaction-type FMs, inorganic salts generated by tribo-chemical reactions are suitable. The differences between AWs and reaction-type FMs are not the chemical process of tribo-film formation but the physical properties of the tribo-film provided at the tribo-contact. In general, hard materials are more robust to wear compared to soft material. Inorganic phosphates on steel surfaces are well known as the protecting film [53]. Zinc bis(dialkyldithiophosphates) (ZnDTPs) are the most widely used AWs in practical lubricants so far [54]. Intensive studies on working mechanism of ZnDPTs by surface analysis have revealed the formation of inorganic compounds composed of phosphorus, sulfur, and zinc [55]. The process of boundary film formation has been studied experimentally [56–59] and in-silico [60]. Co-existence of other tribo-improvers such as molybdenum carbamates displays improvements in lubrication performances [61–64]. Organic esters of phosphoric and phosphonic acids also display AW properties for steel surfaces [19].

2.1.3. Extreme Pressure Additives (EP) Formerly, these were so-called as load carrying agents. When the tribological conditions become severe, at higher load and/or higher temperature, welding of the material may occur. EP can prevent such failure of materials upon rubbing contact. Their functions are unique compared to FMs and AWs. EPs first react with the surface, which is analogous to AWs. However, the product does not protect the surface but mildly removes the atoms on the uppermost surfaces (Figure7). The reaction of an EP molecule (dialkyl disulfide in this case) with the atom−α on the surface is associated with migration of a bonded-electron from the material, as depicted by a red arrow. It weakens the interaction of the atom−α with other atoms in the material resulting facile migration of atom−α from the material. The wear process releases the mechanical energy at the contact point, otherwise the accumulated energy can induce serious damages to the tribo-materials. The function is so-called “controlled wear mechanism”. This is closely related to the mechanism of “corrosive wear” or “chemical wear” where certain chemical substances promote the wear of material. Contrary to the other wear processes such as abrasive and adhesive wear, corrosive wear produces relatively smooth surfaces (less surface roughness). Chemically aggressive substances can induce corrosive wear. Similarly, EPs are relatively reactive with surfaces are compared to substances for FMs and AWs [65]. It is empirically known that EPs can help the running process in an efficient way. This could be understood by the smooth removing of the initial asperity on the surfaces. In some cases, EPs can provide “rescue tasks” when the machine elements are under fatal conditions with severe wear. EPs are expected to lower the contact stress through the corrosive wear. This can make the lubrication regime milder. However, the effect should not last. Appl. Sci. 2017, 7, 445 14 of 33

It weakens the interaction of the atom−α with other atoms in the material resulting facile migration of atom−α from the material. The wear process releases the mechanical energy at the contact point, otherwise the accumulated energy can induce serious damages to the tribo‐materials. The function is so‐called “controlled wear mechanism”. This is closely related to the mechanism of “corrosive wear” or “chemical wear” where certain chemical substances promote the wear of material. Contrary to the other wear processes such as abrasive and adhesive wear, corrosive wear produces relatively Appl.smooth Sci. 2017 ,surfaces7, 445 (less surface roughness). Chemically aggressive substances can induce corrosive14 of 33 wear. Similarly, EPs are relatively reactive with surfaces are compared to substances for FMs and AWs [65].

Wear of weakly bound α through breaking ●− α bonds releases mechanical energy

S

CnH2n+1− S S−CnH2n+1 S S S α

α α rubbing rubbing

Transferring one electron from ●−● bond and one electron from S−S bond forms a new ●−S bond

The arrow mark shows the origin and the destination of the transferring electron Figure 7. Working mechanism of EP (controlled wear). Figure 7. Working mechanism of EP (controlled wear). It is empirically known that EPs can help the running process in an efficient way. This could be understoodContrary to by the the extensivesmooth removing academic of the researches initial asperity on FMs on the and surfaces. AWs, scientific In some cases, approaches EPs can to EP functionsprovide are “rescue still fewer, tasks” mainlywhen the due machine to the elements difficulties are under of analysis. fatal conditions Evaluation with ofsevere EP suchwear. asEPs ASTM D2783are results expected in weldedto lower surfacesthe contact with stress relatively through poor the corrosive repeatability. wear. This can make the lubrication regimeIn summary, milder. allHowever, tribo-improvers the effect should work not on thelast. surface under dynamic processes. In other words, Contrary to the extensive academic researches on FMs and AWs, scientific approaches to EP the mechanical energy continuously stresses the rubbing contact. It damages the surface more or functions are still fewer, mainly due to the difficulties of analysis. Evaluation of EP such as ASTM less, resulting in wear, for example. Here, the importance of recovering the tribo-film has arisen, D2783 results in welded surfaces with relatively poor repeatability. as pointedIn summary, out in Figure all tribo8.‐ Asimprovers discussed work supra, on the thesurface function under ofdynamic all tribo-improvers processes. In other start words, with the interactionthe mechanical to the rubbing energy continuously surface. Then stresses the intermediate the rubbing contact. material It damages (the transition the surface state) more is converted or less, to the tribo-filmresulting in through wear, for adsorption example. Here, or tribo-chemicalthe importance of reaction, recovering according the tribo‐film to the has lubrication arisen, as pointed conditions. Whenout the in machineFigure 8. As works discussed under supra, high the wear function rate conditions, of all tribo‐ theimprovers regeneration start with process the interaction should beto done quickly.the Anrubbing increase surface. in affinity Then the of the intermediate additive to material tribo-materials, (the transition or an increasestate) is inconverted reactivity to under the the tribologicaltribo‐film conditions through adsorption is the usual or tribo method‐chemical of lubricant reaction, chemistry. according to An the increase lubrication in robustness conditions. of the tribo-filmWhen isthe challenging. machine works Multi-functional under high wear tribo-improversrate conditions, the are regeneration possible by process a proper should combination be done of quickly. An increase in affinity of the additive to tribo‐materials, or an increase in reactivity under each moiety in FM, AW, and EP molecules (Figure9). theAppl. tribological Sci. 2017, 7 ,conditions 445 is the usual method of lubricant chemistry. An increase in robustness15 of of 33 the tribo‐film is challenging. Multi‐functional tribo‐improvers are possible by a proper combination of each moiety in FM, AW, and EP molecules (Figure 9).

FigureFigure 8. The 8. The importance importance of of dynamic dynamic process process for for tribo-improvers.tribo‐improvers.

EP AW FM EP AW FM

severe Tribological conditions mild

Reaction Reaction Adsorption

EPAW FM

Figure 9. Multi‐functional tribo‐improver (model).

2.2. Rheo‐Improvers (Rheological Properties Improvers) Viscosity is the primary property of base fluids for liquid lubricants, as discussed in Section 1.1. Those substances that are responsible for fluidity of base oil are rheological property improvers. They contribute to the lubrication performance by modifying the bulk properties of the liquid, mainly under hydrodynamic conditions. From the viewpoint of functions, rheo‐improvers could be categorized as “indirect” tribo‐improvers between mixed and hydrodynamic lubrication regimes.

2.2.1. Viscosity Modifiers (VM) Formerly, VM were called viscosity index improvers (VII). The retired name indicates the role of the additive more directly. VI (see Figure 2 and Section 1.5) is an engineering parameter to qualify the lubricants in practice. The bulk properties are dependent on the structure of the base fluids. In fact, the manufacturing process of the API Group III includes the isomerization of hydrocarbons. Structural features of VM are polymers [66]. The molecule can be compactly packed at lower temperatures, while it expands at higher temperatures (Figure 10). It is expected that molecules with large volume possess resistance to flow, thereby displaying high viscosity [33]. Applicable Appl. Sci. 2017, 7, 445 15 of 33

Appl. Sci. 2017, 7, 445 15 of 33 Figure 8. The importance of dynamic process for tribo‐improvers.

EP AW FM EP AW FM

severe Tribological conditions mild

Reaction Reaction Adsorption

EPAW FM

Figure 9. MultiMulti-functional‐functional tribo tribo-improver‐improver (model).

2.2. Rheo Rheo-Improvers‐Improvers (Rheological Properties Improvers) Viscosity is the primary property of base fluids fluids for liquid lubricants, as as discussed discussed in in Section Section 1.1.1.1. Those substancessubstances that that are are responsible responsible for for fluidity fluidity of baseof base oil areoil rheologicalare rheological property property improvers. improvers. They Theycontribute contribute to the lubricationto the lubrication performance performance by modifying by modifying the bulk properties the bulk of properties the liquid, of mainly the liquid, under mainlyhydrodynamic under hydrodynamic conditions. From conditions. the viewpoint From the of functions, viewpoint rheo-improvers of functions, rheo could‐improvers be categorized could be as “indirect”categorized tribo-improvers as “indirect” tribo between‐improvers mixed between and hydrodynamic mixed and lubricationhydrodynamic regimes. lubrication regimes.

2.2.1. Viscosity ModifiersModifiers (VM) Formerly, VMVM werewere called called viscosity viscosity index index improvers improvers (VII). (VII). The The retired retired name name indicates indicates the the role role of ofthe the additive additive more more directly. directly.VI (seeVI (see Figure Figure2 and 2 and Section Section 1.5) 1.5) is an is engineering an engineering parameter parameter to qualify to qualify the thelubricants lubricants in practice. in practice. The bulkThe bulk properties properties are dependent are dependent on the on structure the structure of the baseof the fluids. base Influids. fact, Inthe fact, manufacturing the manufacturing process ofprocess the API of Groupthe API III Group includes III theincludes isomerization the isomerization of hydrocarbons. of hydrocarbons. Structural Structuralfeatures of features VM are polymersof VM are [66 polymers]. The molecule [66]. The can molecule be compactly can be packed compactly at lower packed temperatures, at lower whiletemperatures,Appl. Sci. it 2017 expands, 7, 445while at higher it expands temperatures at higher (Figuretemperatures 10). It is(Figure expected 10). thatIt is expected molecules that with molecules large volume16 with of 33 possesslarge volume resistance possess to flow, resistance thereby displayingto flow, thereby high viscosity displaying [33]. Applicablehigh viscosity temperature [33]. Applicable range of lubricantstemperature was range made of wider lubricants by improving was made VMs. wider A technical by improving challenge VMs. for advancedA technical VMs challenge is stability for underadvanced shearing VMs is (see stability Figure under3 and Sectionshearing 1.4 (see)[67 Figure]. 3 and Section 1.4) [67].

Base fluid molecule Principle chain of VM molecule

low Temperature high

Figure 10. Working mechanism of viscosity modifier.modifier.

2.2.2. Pour Point Depressant (PPD) Lubricants may freeze if while machine pauses at low temperatures. This causes lubrication failure and leads to serious damages to machine elements. A pre‐heating system can prevent the problem, but it consumes considerable time and energy. PPDs are beneficial for machineries under cold climates. PPDs are typically branched hydrocarbons [68]. Each branched alkyl group can interact with oil molecules, thereby preventing the crystallization of the oil molecules together. Because of the branching structure, the PPD molecule can keep free volume around the molecule that this makes the partial structure mobile (Figure 11). Base fluid molecule Alkyl chain of PPD molecule (flexible)

Mobility around flexible PPD

Solidified, Fluid Without PPD

chilly Temperature normally Figure 11. Working mechanism of pour point depressant.

Both VM and PPD are macro‐molecules (polymers) compared to the molecular mass of base oil molecule. However, the functions of VM and PPD are different. The main function of VM is to suppress mobility at higher temperature thereby increasing the viscosity. On the other hand, branched structure is beneficial for PPDs because the molecule has to be responsible for mobility at lower temperatures. The functions of both additives involve a physical effect and the changes are reversible by temperature change unless the molecule decomposes. Appl. Sci. 2017, 7, 445 16 of 33

temperature range of lubricants was made wider by improving VMs. A technical challenge for advanced VMs is stability under shearing (see Figure 3 and Section 1.4) [67].

Base fluid molecule Principle chain of VM molecule

Appl. Sci. 2017, 7, 445 16 of 33 low Temperature high

2.2.2. Pour Point Depressant (PPD)Figure 10. Working mechanism of viscosity modifier.

Lubricants2.2.2. Pour may Point freeze Depressant if while (PPD) machine pauses at low temperatures. This causes lubrication failure and leadsLubricants to serious may freeze damages if while to machine machine pauses elements. at low temperatures. A pre-heating This system causes lubrication can prevent the problem,failure but itand consumes leads to serious considerable damages time to machine and energy. elements. PPDs A pre are‐heating beneficial system for can machineries prevent the under cold climates.problem, PPDs but it are consumes typically considerable branched time hydrocarbons and energy. [PPDs68]. Eachare beneficial branched for alkylmachineries group under can interact with oilcold molecules, climates. thereby PPDs are preventing typically branched the crystallization hydrocarbons of the[68]. oil Each molecules branched together. alkyl group Because can of the branchinginteract structure, with oil the molecules, PPD molecule thereby can preventing keep free the volume crystallization around of the the molecule oil molecules that together. this makes the Because of the branching structure, the PPD molecule can keep free volume around the molecule partial structurethat this makes mobile the (Figure partial structure 11). mobile (Figure 11). Base fluid molecule Alkyl chain of PPD molecule (flexible)

Mobility around flexible PPD

Solidified, Fluid Without PPD

chilly Temperature normally

FigureFigure 11. Working11. Working mechanism mechanism of pour pour point point depressant. depressant.

Both VM and PPD are macro‐molecules (polymers) compared to the molecular mass of base oil Bothmolecule. VM and However, PPD are the macro-molecules functions of VM and (polymers) PPD are different. compared The tomain the function molecular of VM mass is to of base oil molecule.suppress However, mobility at the higher functions temperature of VM thereby and increasing PPD are the different. viscosity. The main function of VM is to suppress mobilityOn the atother higher hand, temperature branched structure thereby is beneficial increasing for the PPDs viscosity. because the molecule has to be Onresponsible the other for hand, mobility branched at lower structure temperatures. is beneficial The functions for PPDsof both becauseadditives theinvolve molecule a physical has to be effect and the changes are reversible by temperature change unless the molecule decomposes. responsible for mobility at lower temperatures. The functions of both additives involve a physical effect and the changes are reversible by temperature change unless the molecule decomposes.

2.3. Maintainers Tribology is an entropy-increasing process that generates heat and worn particles as mechanical energies are lost. It is usually accompanied with retarding of lubricants and tribo-materials, resulting in inferior lubrication performances called “lubricant ageing”. Maintainers mitigate any negative influences on the lubrication performance. In most cases, the ageing processes are accelerated by internal and external contaminants accumulated in the system while the machine works. The function of maintainers is either pro-active or post-active.

2.3.1. Antioxidants (AO) Autoxidation is the oxidative degradation of organic compounds that occurs spontaneously under aerobic conditions without any active treatments such as reagents, catalysts, or driving forces. Most organic materials experience this reaction as ageing processes, and lubricants are no exceptions. Therefore, they have been well studied both from engineering and scientific approaches [69–73]. The consensus reaction pathway is the radical chain reaction mechanism, which is initiated by the hemolytic dissociation of a C–H bond by heat or ultraviolet irradiation. Figure 12 explains the possible mechanism of radical formation under tribo-chemical conditions. • Thermal dissociation of a carbon– bond to yield hydrogen radical and carbon radical. Appl. Sci. 2017, 7, 445 17 of 33

2.3. Maintainers Tribology is an entropy‐increasing process that generates heat and worn particles as mechanical energies are lost. It is usually accompanied with retarding of lubricants and tribo‐materials, resulting in inferior lubrication performances called “lubricant ageing”. Maintainers mitigate any negative influences on the lubrication performance. In most cases, the ageing processes are accelerated by internal and external contaminants accumulated in the system while the machine works. The function of maintainers is either pro‐active or post‐active.

2.3.1. Antioxidants (AO) Autoxidation is the oxidative degradation of organic compounds that occurs spontaneously under aerobic conditions without any active treatments such as reagents, catalysts, or driving forces. Appl. Sci. 2017Most, 7, 445 organic materials experience this reaction as ageing processes, and lubricants are no exceptions. 17 of 33 Therefore, they have been well studied both from engineering and scientific approaches [69–73]. The consensus reaction pathway is the radical chain reaction mechanism, which is initiated by the hemolytic dissociation of a C–H bond by heat or ultraviolet irradiation. Figure 12 explains the • Shear stress can dissociate a carbon–carbon bond though direct mechanical forces, yielding two possible mechanism of radical formation under tribo‐chemical conditions. carbon radical intermediates. Polymers can decompose through this mechanism.  Thermal dissociation of a carbon–hydrogen bond to yield hydrogen radical and carbon radical. • Wear ofShear tribo-materials stress can dissociate exposes a carbon–carbon nascent bond surfaces though that direct maymechanical catalyze forces, theyielding dissociation two of a carbon–hydrogencarbon radical bond. intermediates. Polymers can decompose through this mechanism.  Wear of tribo‐materials exposes nascent surfaces that may catalyze the dissociation of a carbon–hydrogen bond.

X CH2 X CH2 CH CH

CH HC H H C C H2 H2

X CH2 X CH2 X CH2 CH Shearing CH C H

H2C CH H2C C H C C H2 H2 H2

X CH2 X CH2 CH CH

M M CH HC H H C C H2 H2

H C Depicts hydrogen radical and carbon radical, respectively

L Vacant d orbital Deactivation L coordinates to the vacant d orbital, thereby blocks the site on metal atom Where M is transition metals such as Fe, Ni, Cu, L is Lewis acid M M

Figure 12.FigureInitiation 12. Initiation step step of autoxidationof autoxidation (generation (generation of radical of radical intermediates). intermediates).

Among the three initiation mechanisms, causes by shearing and nascent surfaces are unique in Amonglubrication the three systems initiation (see Section mechanisms, 1.4). The carbon causes radicals by shearingare ready to and react nascent with surfaces to yield are unique in lubricationperoxy systems‐radical (seespecies Section that further1.4). The react carbon with another radicals hydrocarbon are ready tomolecule react with(Figure oxygen 13). to yield The reaction results in regeneration of the carbon radical intermediate and oxidized molecule. peroxy-radical species that further react with another hydrocarbon molecule (Figure 13). The reaction This regeneration step is the characteristic to the radical chain reaction mechanism. Theoretically, results in regenerationthe autoxidation of can the proceed carbon again radical and again intermediate until the depletion and oxidized of the hydrocarbon molecule. once This the regeneration carbon step is the characteristic to the radical chain reaction mechanism. Theoretically, the autoxidation can proceed again and again until the depletion of the hydrocarbon once the carbon radical species were generated. It should be noted that the radical intermediates can react with oxygen, even while the machine stops. Therefore, the radical intermediates should be deactivated as soon as they were generated. The radical scavengers can deactivate the active intermediates, thereby breaking the chain reaction. Phenol derivatives are commonly used for this role [74–76]. Even with these reagents, a small portion of radical species can react with oxygen and yield peroxide. In other words, the function of the radical scavengers is not always perfect in preventing the autoxidation reaction. Organic peroxides possess strong oxidative attack to other organic molecules and metallic materials. Aromatic amines [77,78], alkyl sulfides [79], and ZnDTP [80] are frequently employed to decompose peroxy radicals or peroxides. Peroxide decomposers are post-active AO that deactivate the active intermediates after the reaction with oxygen. On the other hand, radical scavengers and metal deactivators are proactive AO that suppress the reaction prior to the oxidation step. It is empirically known that certain metal catalyzes the autoxidation of hydrocarbons. The wear process produces nascent surfaces that display catalytic activities. The surface reactions become significant when tiny wear particles are dispersed in the lubricant, because they have large surface area. Metal deactivators can interact with metal surfaces and block the active sites on the surfaces. From the mechanistic viewpoint, metal deactivators work at a solid-liquid interface and the others work in the liquid phase. Appl. Sci. 2017, 7, 445 18 of 33

Appl. Sci. 2017radical, 7, 445 species were generated. It should be noted that the radical intermediates can react with 18 of 33 oxygen, even while the machine stops. Therefore, the radical intermediates should be deactivated as soon as they were generated.

OH OOH O O H H H H H H H C C C C C C C C C C and/or C OH H H H H Peroxide H H H H H decomposer Peroxides Organic oxides

Post-active AO

H H H C C C H H H Regeneration step of Radical species after The oxidation O Metal 2 (inactive) Metal Metal (catalyst) R H H H H deactivator C C C C H C C H H Radical H Carbon radical scavenger H H Proactive AO

Heat or shearing

R R Red arrow depicts initiation step (Fig. 12) Blue arrow depicts anti-oxidation R R

Hydrocarbon R R Figure 13. Oxidation of lubricants and working mechanism of AO (in ellipse). Figure 13. Oxidation of lubricants and working mechanism of AO (in ellipse). The radical scavengers can deactivate the active intermediates, thereby breaking the chain Differentreaction. types Phenol of AOs derivatives are frequently are commonly used togetherused for this in arole lubricant [74–76]. [Even81]. Sincewith these tribological reagents, processes a small portion of radical species can react with oxygen and yield peroxide. In other words, the usually inducefunction different of the radical causes scavengers of autoxidation, is not always different perfect in AO preventing complement the autoxidation each other reaction. to prevent the ageing processesOrganic happeningperoxides possess simultaneously. strong oxidative attack to other organic molecules and metallic materials. Aromatic amines [77,78], alkyl sulfides [79], and ZnDTP [80] are frequently employed to decompose 2.3.2. Detergentperoxy radicals or peroxides. Peroxide decomposers are post‐active AO that deactivate the active intermediates after the reaction with oxygen. On the other hand, radical scavengers and metal Severaldeactivators tribo-improvers are proactive contain AO that , suppress phosphorus,the reaction prior or to sulfur the oxidation atoms step. in the molecule. Oxidation of these compoundsIt is empirically results known in mineral that certain acids metal and catalyzes their derivatives. the autoxidation These of hydrocarbons. acidic compounds The wear can attack tribo-materialsprocess and/or produces further nascent promote surfaces thethat ageing display ofcatalytic the lubricant. activities. DetergentsThe surface reactions neutralize become or deactivate significant when tiny wear particles are dispersed in the lubricant, because they have large surface these aggressivearea. Metal chemicals deactivators (Figure can interact 14)[ 82with]. metal Fine particlessurfaces and of block calcium the active carbonate sites on the could surfaces. be dispersed in hydrocarbonsAppl.From Sci. the2017 with mechanistic, 7, 445 the aid viewpoint, of surfactant. metal deactivators This releases work at thea solid alkali‐liquid salt interface upon and formation the19 others of 33 of acidic compounds.work Because in the liquid of the phase. structural features, they are called “over-based type detergent”. The base acidic compounds. Because of the structural features, they are called “over‐based type detergent”. number and meanDifferent particle types sizeof AOs are are the frequently measure used of the together quality in fora lubricant these additives. [81]. Since tribological Theprocesses base number usually and induce mean different particle sizecauses are ofthe autoxidation, measure of the different quality AO for complementthese additives. each other to prevent the ageing processes happening simultaneously. reservoir 2.3.2. Detergent

Lubricant Circulation FuelSeveral & air tribo‐improvers contain nitrogen, phosphorus, or sulfur atoms in the molecule. ICE Oxidation of these compounds results in mineral acids and their derivatives. These acidic compounds can attack tribo‐materials and/or further promote the ageing of the lubricant. Detergents neutralize S

O SO 3

or deactivate these aggressive chemicals (Figure 14) [82]. Fine particles of calcium3 carbonate could be

SO3 S O

dispersed in hydrocarbonsExhaust gas with the aid of surfactant. This releases the alkali3 salt upon formation of

3 O

S 3

3

O SO S

Sulfuric, phosphoric acids (oxidation of AW)

Nitric acid (oxidation of N2)

CaCO3

Over-based detergent Neutralization

Figure 14. Working mechanism of detergent (example diesel oils). Figure 14. Working mechanism of detergent (example oils). 2.3.3. Dispersant An internal combustion engine often yields soot. When it disperses in the lubricant, abrasive wear, damaged seals, or depleted tribo‐improvers may occur [83]. When the ageing proceeds further, organic oxides are converted to polymers. This yields insoluble matters or deposits. In contrast to wear particles of metallic materials, these oligomers and polymers are gel‐like and are not easily filtered off. A dispersant molecule can interact with these organic contaminants and disperse in the liquid phase (Figure 15) [84]. The primary function of dispersants is surfactant, which is analogous to detergent. However, the dispersant molecule has more polar groups in order to capture organic contaminants sufficiently and to keep it dispersed in the liquid phase.

reservoir

R

Lubricant Circulation Fuel & air O ICE O N

HN

O HN Exhaust gas H N N

R O

where R = CnH2n+1 Contaminated soot Multiple interactions

Dispersant molecules capture the contaminants

Figure 15. Working mechanism of dispersant (example diesel engine oils).

Appl. Sci. 2017, 7, 445 19 of 33 acidic compounds. Because of the structural features, they are called “over‐based type detergent”. The base number and mean particle size are the measure of the quality for these additives.

reservoir

Lubricant Circulation Fuel & air ICE

S

O SO 3

3

SO3 S O

Exhaust gas 3

3 O

S 3

3

O SO S

Sulfuric, phosphoric acids (oxidation of AW)

Nitric acid (oxidation of N2)

CaCO3

Over-based detergent Neutralization

Appl. Sci. 2017, 7, 445 19 of 33 Figure 14. Working mechanism of detergent (example diesel engine oils).

2.3.3.2.3.3. Dispersant Dispersant AnAn internal internal combustion combustion engine engine often often yields yields soot. soot. When When it it disperses disperses in in the the lubricant, lubricant, abrasive abrasive wear,wear, damaged damaged seals, or or depleted depleted tribo tribo-improvers‐improvers may occur [83]. [83]. When When the the ageing ageing proceeds proceeds further, further, organicorganic oxides areare convertedconverted to to polymers. polymers. This This yields yields insoluble insoluble matters matters or deposits. or deposits. In contrast In contrast to wear to wearparticles particles of metallic of metallic materials, materials, these oligomers these oligomers and polymers and polymers are gel-like are and gel‐ arelike not and easily are filterednot easily off. filteredA dispersant off. A moleculedispersant can molecule interact can with interact these organic with these contaminants organic contaminants and disperse and in the disperse liquid in phase the liquid(Figure phase 15)[ 84(Figure]. The 15) primary [84]. The function primary of dispersants function of isdispersants surfactant, is which surfactant, is analogous which is to analogous detergent. toHowever, detergent. the However, dispersant the molecule dispersant has molecule more polar has groups more polar in order groups to capture in order organic to capture contaminants organic contaminantssufficiently and sufficiently to keep it and dispersed to keep in it the dispersed liquid phase. in the liquid phase.

reservoir

R

Lubricant Circulation Fuel & air O ICE O N

HN

O HN Exhaust gas H N N

R O

where R = CnH2n+1 Contaminated soot Multiple interactions

Dispersant molecules capture the contaminants

FigureFigure 15. 15. WorkingWorking mechanism mechanism of of dispersant dispersant (example (example diesel diesel engine engine oils). oils).

2.3.4. , Rust Preventive Corrosion and rust are defined as an oxidative degradation of metallic materials through electrochemical processes. The Galvanic corrosion process is the representative corrosion process in lubrication systems. The mechanism in brief is as follows. Metal surfaces are usually covered with an oxide layer. The tribological process the oxide off and exposes metal surfaces. The electrical potential between metal and metal oxide is different. When a conductive liquid exists between the surfaces, Galvanic cell is formed. The metal atom liberates electrons and forms metal ion by increasing the oxidation number (this process is the oxidation of metal). The electron liberated from the metal reduces an oxygen molecule in the system (Figure 16). Base fluids (hydrocarbons) are insulators but some additives can increase the electrical conductivity of the lubricant. Another cause of corrosion is the ionization of metal by acidic compounds that occur without oxygen. Both corrosion mechanisms can take place under static conditions while the machine is pausing. Corrosion inhibitors provide a protecting layer on metal surfaces [85]. The protection layers could be either organic (adsorption type, providing organic layers) or inorganic (reaction type, providing passive layers), which are analogous to FMs (see Section 2.1.1). In contrast to FMs, those protecting films from corrosion do not need mechanical robustness. Appl. Sci. 2017, 7, 445 20 of 33

2.3.4. Corrosion Inhibitor, Rust Preventive Corrosion and rust are defined as an oxidative degradation of metallic materials through electrochemical processes. The Galvanic corrosion process is the representative corrosion process in lubrication systems. The mechanism in brief is as follows. Metal surfaces are usually covered with an oxide layer. The tribological process wears the oxide off and exposes metal surfaces. The electrical potential between metal and metal oxide is different. When a conductive liquid exists between the surfaces, Galvanic cell is formed. The metal atom liberates electrons and forms metal ion by increasing the oxidation number (this process is the oxidation of metal). The electron liberated from the metal reduces an oxygen molecule in the system (Figure 16). Base fluids (hydrocarbons) are insulators but some additives can increase the electrical conductivity of the lubricant. Another cause of corrosion is the ionization of metal by acidic compounds that occur without oxygen. Both corrosion mechanisms can take place under static conditions while the machine is pausing. Corrosion inhibitors provide a protecting layer on metal surfaces [85]. The protection layers could be either organic (adsorption type, providing organic layers) or inorganic (reaction type, providing passive Appl. Sci. 2017, 7, 445 20 of 33 layers), which are analogous to FMs (see Section 2.1.1). In contrast to FMs, those protecting films from corrosion do not need mechanical robustness.

Liquid phase Worn surface − 2+ (1/2) O2+ H2O2OHMetal Non-rubbed surface Uppermost surface

2e−

Metal + [base]−H Solid phase Metal [Metal]+[base]− + H+ Galvanic corrosion Ionization with acids

Oxidants or acids

Surface film keeps corrosion-promoting substances away from the surfaces

Organic Inorganic (adsorption of molecule) (formation of passive layer) FigureFigure 16. Corrosion 16. Corrosion of metalof metal and and the the workingworking mechanism mechanism of corrosion of corrosion inhibitor. inhibitor.

2.3.5. Anti‐Foam Agent () 2.3.5. Anti-Foam Agent (Defoamer) Lubricants can intake air during work and circulation. This develops an air bubble in the Lubricantslubricants, can and results intake in air drop during of viscosity work [86]. and In addition circulation. to the physical This develops changes, the an autoxidation air bubble in the lubricants,of base and oils results is facilitated in drop ofbecause viscosity the [probability86]. In addition of oil–oxygen to the physicalcontact is changes, increased the through autoxidation an of base oilsextended is facilitated area of because the gas‐liquid the probability interface. In of this oil–oxygen regard, non contact‐visible istiny increased bubbles are through problematic. an extended area of theAnti gas-liquid‐form agents interface. are composed In this of surfactant regard,‐ non-visiblelike molecules tiny that bubbles can interact are between problematic. gas‐liquid Anti-form interfaces. The function is to merge tiny bubbles into a large one, thereby helping to release the agents are composed of surfactant-like molecules that can interact between gas-liquid interfaces. molecule from the liquid phase (Figure 17) [87]. The function is to merge tiny bubbles into a large one, thereby helping to release the molecule from the liquidAppl. Sci. phase 2017, 7 (Figure, 445 17)[87]. 21 of 33

Gas

Gas

Liquid Liquid

Anti-foam agent molecule

The surfactant promotes the coalescing of tiny bubbles into large ones. As a result, large bubbles can rise to the surface and pop.

Figure 17. Working mechanism of anti-foamanti‐foam agentagent (defoamer).(defoamer).

2.3.6. DemulsifiersDemulsifiers (Emulsion Breaking Agents)Agents) Water isis aa ubiquitousubiquitous contaminantcontaminant forfor almostalmost allall lubricants.lubricants. It causescauses manymany problemsproblems inin lubricationlubrication systems includingincluding viscosity viscosity drop, drop, corrosion corrosion of metallic of metallic materials, materials, hydrolysis hydrolysis of additives, of additives, etc. Therefore, etc. lubricationTherefore, lubrication systems are systems better if are they better have if water they have release water systems release from systems the lubricant. from the It lubricant. is reasonable It is toreasonable drain contaminated to drain contaminated water from water the lubricant from the (density lubricant less (density than 1) less while than the 1) lubricant while the is lubricant settling in is settling in a reservoir. As discussed supra, and summarized in Table 2, many additives work at the interface. These additives have surfactant nature more or less. Therefore, water can diffuse in the lubricant in the form of water‐in‐oil emulsion with the aid of those additives. Demulsifiers break water‐in‐oil emulsion by deactivating the surfactant type additives as illustrated in Figure 18. The role of anti‐foam agents and demulsifiers is to release contamination, gas or liquid respectively, from the lubricants. However, they work in the opposite way; anti‐foam agents activate the gas‐liquid interface, while demulsifiers deactivate the water‐oil interface.

oil oil water

water

Interface-type additive Demulsifier Deformer interacts with interface-type additive thereby destabilizes the water-in-oil emulsion.

Figure 18. Working mechanism of Demulsifier.

Appl. Sci. 2017, 7, 445 21 of 33

Gas

Gas

Liquid Liquid

Anti-foam agent molecule

The surfactant promotes the coalescing of tiny bubbles into large ones. As a result, large bubbles can rise to the surface and pop.

Figure 17. Working mechanism of anti‐foam agent (defoamer).

2.3.6. Demulsifiers (Emulsion Breaking Agents) Water is a ubiquitous contaminant for almost all lubricants. It causes many problems in lubrication Appl.systems Sci. 2017 including, 7, 445 viscosity drop, corrosion of metallic materials, hydrolysis of additives,21 ofetc. 33 Therefore, lubrication systems are better if they have water release systems from the lubricant. It is reasonable to drain contaminated water from the lubricant (density less than 1) while the lubricant is a reservoir. As discussed supra, and summarized in Table2, many additives work at the interface. settling in a reservoir. As discussed supra, and summarized in Table 2, many additives work at the These additives have surfactant nature more or less. Therefore, water can diffuse in the lubricant in the interface. These additives have surfactant nature more or less. Therefore, water can diffuse in the form of water-in-oil emulsion with the aid of those additives. Demulsifiers break water-in-oil emulsion lubricant in the form of water‐in‐oil emulsion with the aid of those additives. Demulsifiers break by deactivating the surfactant type additives as illustrated in Figure 18. water‐in‐oil emulsion by deactivating the surfactant type additives as illustrated in Figure 18. The role of anti-foam agents and demulsifiers is to release contamination, gas or liquid respectively, The role of anti‐foam agents and demulsifiers is to release contamination, gas or liquid from the lubricants. However, they work in the opposite way; anti-foam agents activate the gas-liquid respectively, from the lubricants. However, they work in the opposite way; anti‐foam agents activate interface, while demulsifiers deactivate the water-oil interface. the gas‐liquid interface, while demulsifiers deactivate the water‐oil interface.

oil oil water

water

Interface-type additive Demulsifier Deformer interacts with interface-type additive thereby destabilizes the water-in-oil emulsion.

Figure 18. WorkingWorking mechanism of Demulsifier.Demulsifier.

2.4. Multi-Functional Additives Some additives have inter-category functions, i.e., display more than two roles out of three (tribo-improving, rheo-improving, and maintaining, see graphic abstract and Table2) functions. ZnDTPs are well-known and the most widely used additives in lubricants that act as FM, AW, AO, and corrosion inhibitors, depending on the structure of the alkyl group [88,89]. This means that ZnDTPs take the role as both tribo-improvers and maintainers. Other examples in practices are: over-based calcium sulfonates as detergent and AW [90], and functionalized poly(methacrylate) as VM and AW [91].

2.5. Auxiliaries Not the all but some specific lubricant may include certain substances that are not categorized as the major groups.

2.5.1. Antibiotics High water content fluids could be used for some hydraulic fluids or metal working fluids. These fluids occasionally experience biological decay during storage. Antibiotics can prevent ageing.

2.5.2. Conductivity Improver These additives are not very common but required for certain applications. The conductivity is one of the bulk liquid properties. Hydrocarbons, most commonly used as base oils, possess relatively poor thermal conductivity. Although this property is of importance for the cooling effect, other advantages with hydrocarbons are usually considered. Dispersion of inorganic particles may increase the conductivity [92]. Appl.Appl. Sci. 2017 Sci. ,2017 7, 445, 7 , 445 22 of22 33 of 33

2.4. 2.4.Multi Multi‐Functional‐Functional Additives Additives SomeSome additives additives have have inter inter‐category‐category functions, functions, i.e., i.e.,display display more more than than two two roles roles out outof threeof three (tribo(tribo‐improving,‐improving, rheo rheo‐improving,‐improving, and and maintaining, maintaining, see seegraphic graphic abstract abstract and and Table Table 2) functions.2) functions. ZnDTPsZnDTPs are arewell well‐known‐known and and the themost most widely widely used used additives additives in lubricants in lubricants that that act asact FM, as FM, AW, AW, AO, AO, andand corrosion corrosion inhibitors, inhibitors, depending depending on theon thestructure structure of theof thealkyl alkyl group group [88,89]. [88,89]. This This means means that that ZnDTPsZnDTPs take take the therole role as bothas both tribo tribo‐improvers‐improvers and and maintainers. maintainers. Other Other examples examples in practicesin practices are: are: overover‐based‐based calcium calcium sulfonates sulfonates as detergent as detergent and and AW AW [90], [90], and and functionalized functionalized poly(methacrylate) poly(methacrylate) as as VMVM and and AW AW [91]. [91].

2.5. 2.5.Auxiliaries Auxiliaries NotNot the theall butall butsome some specific specific lubricant lubricant may may include include certain certain substances substances that that are arenot notcategorized categorized as theas majorthe major groups. groups.

2.5.1.2.5.1. Antibiotics Antibiotics

Appl. Sci. 2017, 7, 445 HighHigh water water content content fluids fluids could could be usedbe used for forsome some hydraulic hydraulic fluids fluids or metalor metal working working fluids. fluids.22 of 33 TheseThese fluids fluids occasionally occasionally experience experience biological biological decay decay during during storage. storage. Antibiotics Antibiotics can canprevent prevent ageing. ageing.

2.5.2.2.5.2. Conductivity Conductivity Improver Improver 2.5.3. Colorings TheseThese additives additives are arenot notvery very common common but butrequired required for certainfor certain applications. applications. The The conductivity conductivity is is one of the bulk liquid properties. Hydrocarbons, most commonly used as base oils, possess This ingredient is notone necessary of the bulk for liquid lubrication properties. Hydrocarbons, performances most but commonly is sometimes used as base beneficial oils, possess for relativelyrelatively poor poor thermal thermal conductivity. conductivity. Although Although this thisproperty property is of is importance of importance for thefor thecooling cooling effect, effect, lubricant maintenance.other Specificother advantages advantages coloring with with hydrocarbons canhydrocarbons prevent are areusually mistakesusually considered. considered. by Dispersion mixing Dispersion of wrong inorganic of inorganic lubricant particles particles may when may replacement or replenishmentincreaseincrease isthe taken. conductivitythe conductivity In some [92]. [92]. applications, leakage could be found easily by coloring of the liquid. 2.5.3.2.5.3. Colorings Colorings ThisThis ingredient ingredient is not is notnecessary necessary for forlubrication lubrication performances performances but butis sometimes is sometimes beneficial beneficial for for 3. Evaluation of Tribo-Improverslubricantlubricant maintenance. maintenance. Specific Specific coloring coloring can canprevent prevent mistakes mistakes by mixingby mixing wrong wrong lubricant lubricant when when replacementreplacement or replenishmentor replenishment is taken. is taken. In someIn some applications, applications, leakage leakage could could be foundbe found easily easily by by Even if a researchercoloring developscoloring of the of liquid.the an liquid. excellent additive, it would be never commercialized directly. The R&D processes of lubricants has been undertaken step-by-step as briefly shown in Table3. Among 3. Evaluation3. Evaluation of Tribo of Tribo‐Improvers‐Improvers three additive categories, rheo-improvers and maintainers are analogous substances for other industrial EvenEven if a ifresearcher a researcher develops develops an excellent an excellent additive, additive, it would it would be never be never commercialized commercialized directly. directly. materials. DevelopmentThe andThe R&D R&D primary processes processes of evaluation lubricantsof lubricants has of hasbeen these been undertaken undertaken additives step step‐by could‐‐stepby‐step as follow brieflyas briefly shown the shown procedure in Tablein Table 3. 3. for those materials. On theAmong otherAmong three hand, three additive additive tribo-improvers categories, categories, rheo rheo‐improvers are‐improvers unique and and maintainers to maintainers lubricants. are areanalogous analogous This sectionsubstances substances focuses for for on tribo-improvers. otherother industrial industrial materials. materials. Development Development and and primary primary evaluation evaluation of these of these additives additives could could follow follow the the procedureprocedure for forthose those materials. materials. On Onthe theother other hand, hand, tribo tribo‐improvers‐improvers are areunique unique to lubricants.to lubricants. This This sectionsection focuses focuses on tribo on tribo‐improvers.‐improvers. Table 3. Research phase and tribo-testing. TableTable 3. Research 3. Research phase phase and andtribo tribo‐testing.‐testing.

Test SampleTest Test SampleSample TestTest Test Test CostCost Costof of of R&D Phase FocusR&DR&D Phase Phase FocusFocus Test Equipment OutcomeOutcomeOutcome Category LubricantCategoryCategory LubricantLubricant EquipmentEquipment PeriodsPeriodsPeriods EvaluationEvaluationEvaluation IncubateIncubate‐ ‐ SpecificSpecific UniversalUniversal The roleThe role Incubate-break breakbreak PossibilityPossibility Specific The role Possibility Universalcomponentscomponents tribo-testertribotribo‐tester‐tester componentcomponent through throughthrough Laboratory components component LaboratoryLaboratory TriboTribo‐tester‐tester test test testTribo-tester according to Technical Improve Applicability Prototype accordingaccording to to TechnicalTechnical ImproveImprove ApplicabilityApplicability PrototypePrototype industrial standardsindustrialindustrial benefitbenefitbenefit standardsstandards Engineering Optimize Feasibility Bench test Machine componentMachine Engineering Optimize Feasibility FullBench test Machine benefitEngineering Optimize Feasibility Bench test component benefit Full component benefit formulated Full Industrial Verity & tune ProductivityVerity&Verity& Field testformulated Realformulated machine IndustrialIndustrial ProductivityProductivity FieldField test test Real Realmachine machine benefit tune tune benefitbenefit

Because of variety of machine elements—their configurations, operating conditions, and environment—lubricants should be formulated for each machine element. The performances of these lubricants are evaluated using the practical machine under real conditions and environments. This so-called field test is a time- and cost-intense procedure and is usually employed just before the introduction of the product into the market. Prior to the field test, lubricants have to pass the laboratory tests that are classified as “bench tests”. This tests lubricants using the real machine elements (part of whole machine) under simulated operating conditions and environments in the laboratory. These tests should focus on optimizing, verifying or tuning of the machine. Therefore, fully formulated lubricants are employed for evaluation. Since these phases test long-term durability and safety of the machine, requirements for lubricants are close to practical qualities. The first two phases in R&D are carried out using laboratory equipment in a time- and cost-effective manner. Standard tests regulated by the authorized organizations such as ASTM are usually employed in these early research stages. Each standard test clearly defines the equipment and the procedure and mainly focuses on the quality control of industrial materials. Another advantage of the standard tests is that equipment and related parts including consumables are commercially available in most cases. Therefore, these tests are also employed for evaluation of prototype lubricants in an “improving” phase. It should be noted that the standard tests define the minimum requirements as industrial products for individual applications. These procedures are supposed to evaluate fully formulated lubricants. Therefore, these procedures may involve over-loaded conditions for lubricant additives under an incubate-breakthrough phase. Contrary to well-established methods and apparatuses for chemical analysis such as spectroscopies, there is no consensus as to a universal tribo-testing procedure for tribo-improvers in laboratories. Simple tribo-testers (not a part of machine element) are beneficial in an incubating phase. Factors to be considered in conducting a tribo-test of lubricants are described below. Appl. Sci. 2017, 7, 445 23 of 33

• Configuration of tribo-contact: a point contact generates high contact stress while line and square contact generate low contact stress. A careful setup of the test specimen is needed to ensure the alignment of contact. This influences the repeatability of the test considerably. Ball-on-flat type point contact provides a comparatively easy setup. • Type of relative motion: Reciprocation sliding, unidirectional sliding, rolling, and a combination of rolling and sliding are most commonly employed. • Tribo-materials: Various materials are used in different machine elements. Simple tribo-tests are beneficial for evaluating the compatibility of tribo-improvers with specific materials in machine elements. Surface roughness of the materials should be considered. • Operating conditions: Load, velocity, and temperature of lubricant should be controlled in a proper way. • Testing environment: Should be kept away from contaminants as much as possible to obtain the results with good repeatability. Humidity of room air can influence the tribological properties.

In summary, there are countless test conditions that combine the above factors. Under these circumstances, simple comparison of the results, i.e., friction coefficient or wear rate, between different groups does not always make sense. Performances of tribo-improvers should be evaluated by comparing the base fluid (without the tribo-improver, see Figure4).

4. Discussion as Multi-Component Systems

4.1. Formulation of a Lubricant Based on the understandings of each component for lubricants, this section discusses the combination of additives to achieve lubrication performances. Manufacturing a lubricant, so-called “formulation”, requires high-level engineering knowledge; what base oil(s) and additives should be chosen? What is the concentration of each component?—The contents in lubricants are in a black box in most cases. It seems an implicit agreement that everybody can produce a fake-lubricant if the detailed contents in a lubricant were disclosed. That might indeed be possible but this is not a good reason to attempt this. Here is a simple question; can you serve a high-quality dinner by merely following the recipe of a three-star Michelin restaurant? Note that lubricants cannot be formulated by simple mixing of base fluids and additives. The reason for non-disclosure of lubricant contents might be an economical matter. Everybody can calculate the total cost of the raw materials in a lubricant and then might feel that the product price is too expensive, without considering the cost for the quality. The cost discussion is not in the scope of this review and the following sections highlight the science of additive technology.

4.2. The Quality of Each Ingredient The total performances of formulated lubricant are often influenced by the quality of each component. In chemistry, quality of substance is nearly equal to the purity. In this regard, it is a tacit knowledge that even a small amount of impurities sometimes play a decisive role, especially in tribological performances. This phenomenon could be understood by a competitive interaction of impurities against the tribo-improvers with tribological surfaces. Actually, many lubricants are made of industrial grade chemicals that are usually less pure compared to reagent grade. Laboratory tribo-tests are a reasonable way to check the quality of the ingredients of lubricants.

4.3. The Interaction of Ingredients Because of the nature of multiple components in a lubricant, an interaction between each component (either as additive or as base oil) occurs inherently. The interaction can result in a positive, negative, or neutral effect on lubrication performances including lifetime of usage. A positive effect is a synergistic effect; improved performance by combination compared to one component alone. Appl. Sci. 2017, 7, 445 24 of 33

detailed contents in a lubricant were disclosed. That might indeed be possible but this is not a good reason to attempt this. Here is a simple question; can you serve a high‐quality dinner by merely following the recipe of a three‐star Michelin restaurant? Note that lubricants cannot be formulated by simple mixing of base fluids and additives. The reason for non‐disclosure of lubricant contents might be an economical matter. Everybody can calculate the total cost of the raw materials in a lubricant and then might feel that the product price is too expensive, without considering the cost for the quality. The cost discussion is not in the scope of this review and the following sections highlight the science of additive technology.

4.2. The Quality of Each Ingredient The total performances of formulated lubricant are often influenced by the quality of each component. In chemistry, quality of substance is nearly equal to the purity. In this regard, it is a tacit knowledge that even a small amount of impurities sometimes play a decisive role, especially in tribological performances. This phenomenon could be understood by a competitive interaction of impurities against the tribo‐improvers with tribological surfaces. Actually, many lubricants are made of industrial grade chemicals that are usually less pure compared to reagent grade. Laboratory tribo‐tests are a reasonable way to check the quality of the ingredients of lubricants.

4.3. The Interaction of Ingredients Appl. Sci. 2017, 7, 445 24 of 33 Because of the nature of multiple components in a lubricant, an interaction between each component (either as additive or as base oil) occurs inherently. The interaction can result in a positive, Creatingnegative, new function(s) or neutral effect that on were lubrication not provided performances by each including ingredient lifetime is of also usage. within A positive this category. effect The oppositeis casea synergistic is an antagonistic effect; improved effect. performance In the neutral by combination case, each compared ingredient to worksone component independently, alone. even thoughCreating they interact new function(s) with each that other were in not a provided molecular by level.each ingredient is also within this category. The opposite case is an antagonistic effect. In the neutral case, each ingredient works independently, Antagonismeven though of they ingredients interact with is each frequently other in a observed molecular level. in preparation and evaluation of prototype lubricants. AAntagonism simple example of ingredients is shown is frequently in Figure observed 19; each in preparation tribo-improver and evaluation A or B prevented of prototype wear by a laboratorylubricants. test, A while simple mixing example them is shown both in displayed Figure 19; higher each tribo wear‐improver than the A or base B prevented oil alone. wear Antagonism by should bea laboratory notified test, if A while + B displays mixing them much both more displayed wear higher than wear B (not than base the oil), base inoil thisalone. example. Antagonism A reaction of A withshould B was be notified suggested if A + inB displays this case much [93 more]. Depletion wear than ofB (not tribo-improvers base oil), in this example. by dispersants A reaction [94 ,95] is of A with B was suggested in this case [93]. Depletion of tribo‐improvers by dispersants [94,95] is categorized as antagonism in a broad sense. categorized as antagonism in a broad sense.

1.2 3 1.0 mm -2 0.8 10 × 0.6

0.4

0.2 Wear volume,

0.0 Base A Additive B Additives A&B Sample Figure 19. Antagonism of two additives. Figure 19. Antagonism of two additives. The term “synergism” seems to be often used in the literature, aiming to show the result is Thepositive. term Here “synergism” we introduce seems a clear to definition be often for usedlubricant in chemistry. the literature, In molecular aiming science, to show the molarity the result is positive.of a Here substance we introduce (unit as mol a∙kg clear−1) is definition important. forTherefore, lubricant the lubrication chemistry. performance In molecular should science, be the molarity of a substance (unit as mol·kg−1) is important. Therefore, the lubrication performance should Appl. Sci. 2017, 7, 445 25 of 33 be compared using samples with the same molarity of the additive. In short, “synergism” should be stated ascompared an improvement using samples by combination, with the same ifmolarity the total of the molarity additive. of In the short, combined “synergism” ingredients should be is exactly same asstated the single as an component. improvement Improvement by combination, by if simply the total mixing molarity could of the be combined the effect ingredients of the total is amount of ingredientsexactly same (better as the performances single component. by higher Improvement concentration by simply ofmixing ingredients), could be the as effect concisely of the total illustrated amount of ingredients (better performances by higher concentration of ingredients), as concisely in Figure 20. In this regard, mass concentration is the most common unit in lubrication engineering. illustrated in Figure 20. In this regard, mass concentration is the most common unit in lubrication Therefore,engineering. it is better Therefore, to reconsider it is better the to published reconsider the results. published results.

The same molarity

worse 0.3

0.2

0.2

0.1 Lubrication performances performances Lubrication

0.1Normalized wear rate better Synergism

0.0 Couldof be effect Concentration BF BF + AW-A BF + AW-B BF + AW-A&B BF + AW-A&B (keep molarity) (simply mixed) Sample

AW-B BF (base fluid) AW-A (anti-wear additive) (anti-wear additive) Figure 20. Synergism model (example of AW). Figure 20. Synergism model (example of AW). It is uncertain in terms of engineering knowledge whether a combination of tribo‐improvers, e.g., FM and AW, can provide better lubrication performances. This could be classified into 3 phenomena, as illustrated in Figure 21.  Synergy is defined as the combination improves BOTH friction and wear compared to each ingredient alone.  Enhanced performance of one while scarifying the other seems to have an improved effect but should not be defined as synergism.  Antagonism is any other phenomena except the above two. BF + FM BF higher

worse Enhanced FM by scarifying AW

BF + AW Wear rate Wear Enhanced AW by AW + FM scarifying FM Lubrication performances performances Lubrication Synergy better lower lower

lower Friction coefficient higher

better Lubrication performances worse

Figure 21. Synergy chart (example of FM and AW). Appl. Sci. 2017, 7, 445 25 of 33

compared using samples with the same molarity of the additive. In short, “synergism” should be stated as an improvement by combination, if the total molarity of the combined ingredients is exactly same as the single component. Improvement by simply mixing could be the effect of the total amount of ingredients (better performances by higher concentration of ingredients), as concisely illustrated in Figure 20. In this regard, mass concentration is the most common unit in lubrication engineering. Therefore, it is better to reconsider the published results.

The same molarity

worse 0.3

0.2

0.2

0.1 Lubrication performances performances Lubrication

0.1Normalized wear rate better Synergism

0.0 Couldof be effect Concentration Appl. Sci. 2017, 7, 445 BF BF + AW-A BF + AW-B BF + AW-A&B BF + AW-A&B 25 of 33 (keep molarity) (simply mixed) Sample

AW-B BF (base fluid) AW-A It is uncertain in terms of engineering knowledge(anti-wear whether additive) a combination(anti-wear additive) of tribo-improvers, e.g., FM and AW, can provide better lubricationFigure 20. Synergism performances. model (example This of could AW). be classified into 3 phenomena, as illustrated in Figure 21. It is uncertain in terms of engineering knowledge whether a combination of tribo‐improvers, • Synergye.g., FM is definedand AW, as can the provide combination better lubrication improves performances. BOTH friction This andcould wear be classified compared into to each ingredient3 phenomena, alone. as illustrated in Figure 21. • Enhanced Synergy performance is defined ofas onethe combination while scarifying improves the BOTH other friction seems and to have wear ancompared improved to each effect but should notingredient be defined alone. as synergism. • Antagonism Enhanced is any performance other phenomena of one while except scarifying the the above other two. seems to have an improved effect but should not be defined as synergism.  Antagonism is any other phenomena except the above two. BF + FM BF higher

worse Enhanced FM by scarifying AW

BF + AW Wear rate Wear Enhanced AW by AW + FM scarifying FM Lubrication performances performances Lubrication Synergy better lower lower

lower Friction coefficient higher

better Lubrication performances worse

Figure 21. Synergy chart (example of FM and AW). Figure 21. Synergy chart (example of FM and AW).

As is common knowledge, different AOs (radical scavengers, peroxide decomposers, and metal Appl. Sci. 2017, 7, 445 26 of 33 deactivators) are dissolved in a lubricant [83]. From a mechanistic viewpoint, these AOs are on standby forAs different is common causes knowledge, of autoxidation different AOs (Figures (radical 12 scavengers, and 13). peroxide Therefore, decomposers, this is understood and metal by the complementeddeactivators) effect. are dissolved in a lubricant [83]. From a mechanistic viewpoint, these AOs are on Thestandby interaction for different of different causes of additivesautoxidation highly (Figures concerns 12 and 13). the Therefore, formulation this is understood process of by a the lubricant. complemented effect. It occasionally happens that the lubrication performances can differ by the order of a mixing process. The interaction of different additives highly concerns the formulation process of a lubricant. AlthoughIt occasionally this involves happens high-level that the engineering lubrication performances knowledge can in differ formulating by the order a lubricant, of a mixing the process. phenomena could beAlthough explained this involves by the interactionhigh‐level engineering of additives, knowledge as illustrated in formulating in Figure a lubricant, 22. the As phenomena discussed supra (Table2),could a lubricant be explained contains by the different interaction surfactant-type of additives, as moleculesillustrated in that Figure work 22. atAs interfaces. discussed supra Because of the “interface(Table 2), activating a lubricant nature”, contains thesedifferent molecules surfactant aggregate‐type molecules themselves that work or at incorporate interfaces. Because other additives.of the “interface activating nature”, these molecules aggregate themselves or incorporate other additives.

Base fluid Additive-A Additive-B

FigureFigure 22. Possible 22. Possible variation variation of of performanceperformance by by formulation formulation process. process.

4.4. Additive Technology for Synthetic Fluids Historically, additive technology in lubrication engineering arose in the early period of the petroleum age. Mass production of mineral oils, together with naphtha and fuels from crude oils, provided considerable benefits in the market. Liquids with proper viscosity for lubrication had become available at reasonable costs. However, neat mineral oils (without additives) usually display poor lubrication performances, especially in reducing friction and wear, in comparison with natural triglycerides (plant oils and animal tallow that had been the resources of lubricants since the ancient days until the petroleum age). After the advantage of additives had been recognized, the demands of additive technology sprang up. It was proved in many applications that additive technology enables machine operation with enhanced reliability, improved energy efficiency, and prolonged lifetime, etc. It should be emphasized that the improvements in lubricant performance motivated an advanced design of machine elements. This further promoted the improvement of lubricant performances. Today and in the future, additive technology is and will be indispensable in lubrication systems. The majority of base oils in the lubricant industry had been and is still Group I oils. Therefore, additive technology has been focused on improvement of Group I oils. Demands of quality lubricants from users are promoting the shift of base fluids to Group II or III from Group I. Then the problem with the solubility of additives in these base fluids arose. The same problem happens with Group IV. Although Groups I−IV are composed of hydrocarbons, Group I oils contain a considerable amount of cycloalkanes (naphthenic) and aromatics that are responsible for good solvency. Solvency‐improving fluids are usually blended in these poor solvents (Group II–IV oils). Solvency could be understood by the affinity of a solute to the solvent. If a compound does not dissolve in the base oil, formulation of a liquid lubricant is impossible. On the other hand, high solubility of a solute in the solvent is not desirable. A proper solubility, especially for tribo‐improvers, is required in formulating a lubricant. Figure 23 depicts a model of solubility−adsorptivity relation on lubrication performance, in consideration with an adsorption isotherm. From the working mechanism of tribo‐improvers (Figures 5–7), it is possible to ascertain that they start by adsorption Appl. Sci. 2017, 7, 445 26 of 33

4.4. Additive Technology for Synthetic Fluids Historically, additive technology in lubrication engineering arose in the early period of the petroleum age. Mass production of mineral oils, together with naphtha and fuels from crude oils, provided considerable benefits in the market. Liquids with proper viscosity for lubrication had become available at reasonable costs. However, neat mineral oils (without additives) usually display poor lubrication performances, especially in reducing friction and wear, in comparison with natural triglycerides (plant oils and animal tallow that had been the resources of lubricants since the ancient days until the petroleum age). After the advantage of additives had been recognized, the demands of additive technology sprang up. It was proved in many applications that additive technology enables machine operation with enhanced reliability, improved energy efficiency, and prolonged lifetime, etc. It should be emphasized that the improvements in lubricant performance motivated an advanced design of machine elements. This further promoted the improvement of lubricant performances. Today and in the future, additive technology is and will be indispensable in lubrication systems. The majority of base oils in the lubricant industry had been and is still Group I oils. Therefore, additive technology has been focused on improvement of Group I oils. Demands of quality lubricants from users are promoting the shift of base fluids to Group II or III from Group I. Then the problem with the solubility of additives in these base fluids arose. The same problem happens with Group IV. Although Groups I−IV are composed of hydrocarbons, Group I oils contain a considerable amount of cycloalkanes (naphthenic) and aromatics that are responsible for good solvency. Solvency-improving fluids are usually blended in these poor solvents (Group II–IV oils). Solvency could be understood by the affinity of a solute to the solvent. If a compound does not dissolve in the base oil, formulation of a liquid lubricant is impossible. On the other hand, high solubility of a solute in the solvent is not desirable. A proper solubility, especially for tribo-improvers, is required in formulating a lubricant. Figure 23 depicts a model of solubility−adsorptivity relation on lubrication performance, in consideration with an adsorption isotherm. From the working mechanism of tribo-improvers (Figures5–7), it is possible to ascertain that they start by adsorption on rubbing surfaces. Assume that there is a threshold of surface coverage at C by which adsorption enables the tribo-improver to work. When additive-A and additive-B were compared at the same concentration of XA, the former displays expected performances while the later displays none, due to insufficient amount of molecules on the surface. A higher concentration of XB is needed for additive-B to perform. However, using an additive at high concentration rises the risk of disadvantages by the interaction with other additives. Therefore, additives should have proper solvency to the base fluids. The solubility in the base fluid is the second key factor in choosing additives after the primary lubrication function. Esters and polyethers belong to Group V oils and are receiving growing demands as fast biodegradable fluids [96]. Contrary to Groups I−IV oils, they contain oxygen atoms in their molecule. Here we discuss the difference between hydrocarbon and oxygen-containing molecules (esters and polyethers) in terms of polarity. Polarity is a result of unequaled sharing of the bonding electron pair, due to the differences in electron negativity of the atoms that form the covalent bond. The electron negativity of hydrogen, carbon, and oxygen are 2.2, 2.5, and 3.5, respectively. Hydrocarbons are being constructed with the combination of a carbon–carbon bond and carbon–hydrogen bonds. Since the differences in the electron negativity of the atoms of these bonds are low (0 for C–C bond, 0.3 for C–H bond), hydrocarbons display “non-polar” properties. On the other hand, esters and polyethers have carbon–oxygen bonds (the difference in the electron negativity is 1.0), together with C–C and C–H bonds. Some substances involve hydrogen–oxygen bond(s) in which the difference in the electron negativity is 1.3. These oxygen involving bonds make the molecule polar. Furthermore, the chemical structure should be considered for the polarity of the molecule, which is omitted here for simplicity. Generally, polar liquids are good solvents and hence esters and polyethers can dissolve common additives for Groups I oils up to high concentrations. This is an analogous problem to that of additive-B in Figure 23. By and large, polar molecules display an affinity to steel surfaces. This induces adsorption of polar fluids on tribo-surfaces. Here, competitive adsorption of a base fluid against a tribo-improver Appl. Sci. 2017, 7, 445 27 of 33

on rubbing surfaces. Assume that there is a threshold of surface coverage at C by which adsorption enables the tribo‐improver to work. When additive‐A and additive‐B were compared at the same concentration of XA, the former displays expected performances while the later displays none, due Appl. Sci. 2017, 7, 445 27 of 33 to insufficient amount of molecules on the surface. A higher concentration of XB is needed for additive‐B to perform. However, using an additive at high concentration rises the risk of disadvantages occurs.by the When interaction the base with fluid other moleculesadditives. Therefore, adsorb on additives the surface should and have block proper the solvency surface to the from base being interactedfluids. with The solubility the additive, in the additive base fluid effects is the second will not key result. factor in choosing additives after the primary lubrication function. Solubility low proper high Base fluid Liquid phase Tribo-improver Tribo-contact Interface Lubrication poor optimum fair

1.00

0.80 C additive-B additive-A

0.60

0.40

0.20

Surface coverage (normalized) coverage Surface 0.00 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 XA Xb Appl. Sci. 2017, 7, 445 Concentration (normalized) 28 of 33 Figure 23. Solubility of a tribo‐improver and lubrication performance. at reasonable concentrations,Figure 23. Solubility while conventional of a tribo-improver AW needs and lubrication a much higher performance. concentration to work in polyether.Esters Conventional and polyethers AWs belongusually to work Group in Vhydrocarbon oils and are oilsreceiving at a concentration growing demands of 10 as mmol fast ∙kg−1 (310 Itppmbiodegradable was of anticipated phosphorus fluids that [96]. content) increasedContrary or below. to polarity Groups I in−IV an oils, additive they contain molecule oxygen can atoms enhance in their the molecule. adsorption activityTheHere from relation we solutiondiscuss between the in difference a polar chemical solvent. between structure Examples hydrocarbon and havephysical and enhanced oxygen properties‐containing polarity for bysynthetic molecules chemical esters(esters modification hasand been ofreviewed phosphatespolyethers) in [98]. such in termsNon as AW ‐ofpolarity polarity. for polar index Polarity synthetic was is a introducedresult fluids, of as unequaled shown therein in sharing Figureas an of engineering24 the[ 97bonding]. This electron preventedparameter. pair, wear The atperformance reasonabledue to the concentrations,of differences model AW in electron whilefor polar negativity conventional synthetic of the AW fluidsatoms needs thatwere a form much evaluated the higher covalent and concentration bond. analyzed The electron toregarding work in negativity of hydrogen, carbon, and oxygen are 2.2, 2.5, and 3.5, respectively. Hydrocarbons are polyether.polarity index Conventional as minor revision AWs usually of non work‐polarity in hydrocarbon index. As shown oils at ain concentration Figure 25, performance of 10 mmol of·kg the−1 being constructed with the combination of a carbon–carbon bond and carbon–hydrogen bonds. (310 ppm of phosphorus content) or below. anti‐wearSince propertiesthe differences decreases in the electron as the polaritynegativity of of the the base atoms fluid of these increases bonds [99].are low (0 for C–C bond, 0.3 for C–H bond), hydrocarbons display “non‐polar” properties. On the other hand, esters and polyethers have carbon–oxygen bonds (the difference in the electron negativity is 1.0), together with C–C and C–H bonds. Some substances involve hydrogen–oxygen bond(s) in which the difference in the electron negativity is 1.3. These oxygen involving bonds make the molecule polar. Furthermore, the chemical structure should be considered for the polarity of the molecule, which is omitted here for simplicity. Generally, polar liquids are good solvents and hence esters and polyethers can dissolve common additives for Groups I oils up to high concentrations. This is an analogous problem to that of additive‐B in Figure 23. By and large, polar molecules display an affinity to steel surfaces. This induces adsorption of polar fluids on tribo‐surfaces. Here, competitive adsorption of a base fluid against a tribo‐improver occurs. When the base fluid molecules adsorb on the surface and block the surface from being interacted with the additive, additive effects will not result. It was anticipated that increased polarity in an additive molecule can enhance the adsorption activity from solution in a polar solvent. Examples have enhanced polarity by chemical modification of phosphates such as AW for polar synthetic fluids, as shown in Figure 24 [97]. This prevented wear

Figure 24. Performances of different AW in synthetic fluidfluid (polyether).(polyether).

Figure 25. AW performances for different synthetic fluids ().

4.5. Tribo‐Improvers for Non‐Ferrous Materials Most tribo‐improvers are developed for lubrication of steels. When it comes to other materials, affinity of each additive to the material is different from that of steel. Therefore, the kinetics in the adsorption step (Figure 6) would be influenced. The processes of tribo‐chemical reaction and the property of the tribo‐film could be different from those with steel. Various materials including Appl. Sci. 2017, 7, 445 28 of 33

at reasonable concentrations, while conventional AW needs a much higher concentration to work in polyether. Conventional AWs usually work in hydrocarbon oils at a concentration of 10 mmol∙kg−1 (310 ppm of phosphorus content) or below. The relation between chemical structure and physical properties for synthetic esters has been reviewed in [98]. Non‐polarity index was introduced therein as an engineering parameter. The performance of model AW for polar synthetic fluids were evaluated and analyzed regarding polarity index as minor revision of non‐polarity index. As shown in Figure 25, performance of the anti‐wear properties decreases as the polarity of the base fluid increases [99].

Appl. Sci. 2017, 7, 445 28 of 33

The relation between chemical structure and physical properties for synthetic esters has been reviewed in [98]. Non-polarity index was introduced therein as an engineering parameter. The performance of model AW for polar synthetic fluids were evaluated and analyzed regarding polarity index as minor revision of non-polarity index. As shown in Figure 25, performance of the anti-wear propertiesFigure decreases 24. Performances as the polarity of different of the AW base in fluid synthetic increases fluid (polyether). [99].

FigureFigure 25.25. AWAW performancesperformances forfor differentdifferent synthetic synthetic fluids fluids (ester). (ester).

4.5.4.5. Tribo-ImproversTribo‐Improvers for for Non-Ferrous Non‐Ferrous Materials Materials MostMost tribo-improverstribo‐improvers areare developeddeveloped forfor lubricationlubrication ofof steels.steels. WhenWhen itit comescomes toto otherother materials,materials, affinityaffinity ofof eacheach additiveadditive toto thethe materialmaterial isis differentdifferent from from that that of of steel. steel. Therefore,Therefore, thethe kineticskinetics inin thethe adsorptionadsorption stepstep (Figure(Figure6 )6) would would be be influenced. influenced. The The processes processes of of tribo-chemical tribo‐chemical reaction reaction and and the the propertyproperty of of the the tribo-film tribo‐film could could be differentbe different from from those those with steel. with Various steel. Various materials materials including including coatings are being developed for tribological purposes. Molecular structure of tribo-improvers should be tuned for the materials.

4.6. Additive Technology for Environmentally Acceptable Lubricants Because of the concern for reducing the impacts on the global environment, demands of EAL (environmentally acceptable/adapted lubricants) are increasing. The biodegradability is one of the key factors in choosing base fluids for EAL [96]. Synthetic esters, natural triglycerides, polyethers, and certain synthetic hydrocarbons are candidates of the base fluids in EALs. Significant investigation in additive technology is needed, especially in the solubility of additives, as discussed in the previous section. Besides the problem of solubility and adsorption for synthetic fluids, toxicity and bioaccumulation tendency of additives are also a concern. Although the contents are minor portions, it has been pointed out that certain ingredients in industrial products should be or might be better if replaced by safer substances [100]. “Greener” candidates have been proposed for 10 specific cases therein. Among them, antimicrobials, small amines, boron alternatives, corrosion inhibitors, alkanolamides, and surfactants relate to lubricant chemistry.

5. Summary Even though they constitute the minor contents in a lubricant, additives are responsible for various functions in lubricating a machine element. The functions include not only tribological properties but also the material properties and maintenance. Many machines are in use under various lubrication conditions. Therefore, the required functions of each lubricant are different. This leads to a vast variety of commercial lubricants. Publications in lubrication engineering and lubricant chemistries Appl. Sci. 2017, 7, 445 29 of 33 are mostly focused on individual engineering phenomena. Unfortunately, scientific viewpoints have thus far underestimated such lubricant technology. This review helps the readers to survey the additive technology for lubricants. The main role and functions of individual additive category were introduced with an emphasis on molecular science. Similarity and dissimilarity in functions and molecular structure were discussed by comparing different additive categories. The continuous developments in machine elements require advanced lubricants more and more. The improvement in efficiency, reliability, and lifetime together with environmental acceptability are core concerns. These technological requirements should be solved by additive technology. The availability of synthetic fluids and new tribo-materials open the opportunities for designing advanced machine elements. This requires a much bigger lineup of task-specific lubricants. This would constitute a paradigm shift from empirical knowhow to a smart technology in research and development of advanced lubricants. Here we wish to propose the algorithm of molecular science in the R&D of lubricant chemistry. Lubricant chemistry is an ever-improving science.

Acknowledgments: A part of this work is supported by Maritime Technologies II Research Programme in the framework of the ERA-NET MARTEC II project CA 266111 ArTEco ‘Arctic Thruster Ecosystem’ and “Austrian COMET-Program” in the frame of K2 XTribology (project No. 849109) within the “Excellence Centre of Tribology” (AC2T research GmbH). Conflicts of Interest: The author declares no conflict of interest.

Nomenclature

Abbreviation Description AO API The American petroleum Institute, http://www.api.org/ ASTM The American Society for Testing and Materials, http://www.astm.org/ AW Anti-wear, anti-wear additive BF Base fluid (base oil) EP Experme pressure, extreme pressure additive FM Friction modifying, fricition modifier ICE Internal conbustion engine MO Mineral oil PPD Pour pont depressant SF Synthetic fluids STLE The Society of Tribologists and lubricationEngineers, http://www.stle.org/ TG VI Viscosity index VM Viscosity modifying, Viscosity modifier ZnDTP Zinc bis(dialkyldhitiophosphate)

References

1. Stevenson, A. “Tribology” in Oxford Dictionary of English, 3rd ed.; Oxford University Press: Oxford, UK, 2015. 2. Jost, H.P. “Tribology or Lubrication” in Lubrication (Tribology) Education and Research; Department of Education and Science, Her Majesty’s Stationery Office London: London, UK, 1966. 3. Dowson, D. “The Early Civilizations” (Chapter 4) in History of Tribology, 2nd ed.; Professional Engineering Publishing: London, UK, 1998. 4. Wang, Y.; Wang, Q.J. “Stribeck Curves” in Encyclopedia of Tribology; Wang, Q.J., Chung, Y.W., Eds.; Springer: New York, NY, USA, 2013; Volume 5. 5. Zhu, D.; Wang, Q.J. “EHL History (Elastohydrodynamic Lubrication)” in Encyclopedia of Tribology; Wang, Q.J., Chung, Y.W., Eds.; Springer: New York, NY, USA, 2013; Volume 2. 6. Rennie, R. “Viscosity” in Oxford Dictionary of Chemistry, 7th ed.; Oxford University Press: Oxford, UK, 2016. 7. Kaupp, G. Mechanochemistry: The varied applications of mechanical bond-breaking. CrystEngComm 2009, 11, 388–403. [CrossRef] Appl. Sci. 2017, 7, 445 30 of 33

8. Takacs, L. The historical development of mechanochemistry. Chem. Soc. Rev. 2013, 42, 7649–7659. [CrossRef] [PubMed] 9. Cravotto, G.; Gaudino, E.C.; Cintas, P. On the mechanochemical activation by ultrasound. Chem. Soc. Rev. 2013, 42, 7521–7534. [CrossRef][PubMed] 10. Šepelák, V.; Düvel, A.; Wilkening, M.; Becker, K.-D.; Heitjans, P. Mechanochemical reactions and syntheses of oxides. Chem. Soc. Rev. 2013, 42, 7507–7520. [CrossRef][PubMed] 11. Ribas-Arino, J.; Marx, D. Covalent Mechanochemistry: Theoretical Concepts and Computational Tools with Applications to Molecular Nanomechanics. Chem. Rev. 2012, 112, 5412–5487. [CrossRef][PubMed] 12. Li, J.; Nagamani, C.; Moore, J.S. Mechanochemistry: From Destructive to Productive. Acc. Chem. Res. 2015, 48, 2181–2190. [CrossRef][PubMed] 13. Todres, Z.V. Organic Mechanochemistry and Its Practical Applications; CRC Press Inc.: Boca Raton, FL, USA, 2006. 14. James, S.L.; Adams, C.J.; Bolm, C.; Braga, D.; Collier, P.; Frišˇci´c,T.; Grepioni, F.; Harris, K.D.M.; Hyett, G.; Jones, W.; et al. Mechanochemistry: Opportunities for new and cleaner synthesis. Chem. Soc. Rev. 2012, 41, 413–447. [CrossRef][PubMed] 15. Birke, V.; Mattik, J.; Runne, D. Mechanochemical reductive dehalogenation of hazardous polyhalogenated contaminants. J. Mater. Sci. 2004, 39, 5111–5116. [CrossRef] 16. Nah, W.; Hwang, K.-Y.; Shul, Y.-G. Effect of metal and glycol on mechanochemical dechlorination of polychlorinated biphenyls (PCBs). Chemosphere 2008, 73, 138–141. [CrossRef][PubMed] 17. Bruere, P. Alterations by light, heat and agitation of the heavy petroleum oils refined for therapeutic use. Bull. Travaux Societe de Pharmacie Bordeaux 1928, 9, 142–144. 18. Minami, I. Ionic liquids in tribology. Molecules 2009, 14, 2286–2305. [CrossRef][PubMed] 19. Chao, K.K.; Saba, C.S. Tribo-Evaluation of High Temperature Candidate Fluids in a Sliding “TBOD” Bench Tester. Tribol. Trans. 1995, 38, 63–68. [CrossRef] 20. Philippon, D.; De Barros-Bouchet, M.-I.; Le Mognea, T.; Lerasle, O.; Bouffet, A.; Martin, J.-M. Role of nascent metallic surfaces on the tribochemistry of phosphite lubricant additives. Tribol. Int. 2011, 44, 684–691. [CrossRef] 21. Warsaw, C.K.; Furey, M.J.; Ritter, A.L.; Molina, G.J. Triboemission as a basic part of the boundary friction regime: A review. Lubr. Sci. 2002, 14, 223–254. [CrossRef] 22. Wang, Q.J.; Zhu, D. “Hertz Theory: Contact of Spherical Surfaces” in Encyclopedia of Tribology; Wang, Q.J., Chung, Y.W., Eds.; Springer: New York, NY, USA, 2013; Volume 5. 23. Schettino, V.; Bini, R. Molecules under extreme conditions: Chemical reactions at high pressure. Phys. Chem. Chem. Phys. 2003, 5, 1951–1965. [CrossRef] 24. Cann, P.M.; Spikes, H.A. In Lubro Studies of Lubricants in EHD Contact Using FTIR Absorption Spectroscopy. Tribol. Trans. 1991, 34, 248–256. [CrossRef] 25. Kauzmann, W.; Eyring, H. The Viscous Flow of Large Molecules. J. Am. Chem. Soc. 1940, 62, 3113–3125. [CrossRef] 26. Carnes, C. The ten greatest events in tribology history. Tribol. Lubr. Technol. 2005, 61, 38–47. 27. Papke, B.L. “Mineral Oil Base Fluids” in Encyclopedia of Tribology; Wang, Q.J., Chung, Y.W., Eds.; Springer: New York, NY, USA, 2013; Volume 4. 28. Michael, J. Covitch Viscosity Index Additives in Encyclopedia of Tribology; Wang, Q.J., Chung, Y.W., Eds.; Springer: New York, NY, USA, 2013; Volume 2. 29. Gresham, R.M. Viscosity index’s new importance. Tribol. Lubr. Technol. 2017, 73, 18–19. 30. Nyberg, E.; Respatiningsih, C.Y.; Minami, I. Molecular design of advanced lubricant base fluids: Hydrocarbon-mimicking ionic liquids. RSC Adv. 2017, 7, 6364–6373. [CrossRef] 31. Hardy, W.B.; Doubleday, I. Boundary Lubrication. The Paraffin Series. Proc. R. Soc. Lond. Ser. A 1922, 100, 550–574. [CrossRef] 32. Papay, A.G. Antiwear and Extreme-Pressure Additives in Lubricants. Lubr. Sci. 1998, 10, 209–224. [CrossRef] 33. Rizvi, S.Q.A. Additives for Automotive Fuels and Lubricants. Lubr. Eng. 1999, 55, 33–39. 34. Taylor, R.I. Tribology and energy efficiency: From molecules to lubricated contacts to complete machines. Faraday Discuss. 2012, 156, 361–382. [CrossRef][PubMed] 35. Bowden, F.P.; Gregory, J.N.; Tabor, D. Lubrication of metal Surfaces by Fatty Acids. Nature 1945, 156, 97–101. [CrossRef] 36. Spikes, H.A. Friction Modifier Additives. Tribol. Lett. 2015, 60, 1–26. [CrossRef] Appl. Sci. 2017, 7, 445 31 of 33

37. Rowe, G.W. The chemistry of tribology, friction, lubrication and wear. R. Inst. Chem. Rev. 1968, 1, 135–204. [CrossRef] 38. Loehlé, S.; Matta, C.; Minfray, C.; le Mogne, T.; Iovine, R.; Obara, Y.; Miyamoto, A.; Martin, J.M. Mixed lubrication of steel by C18 fatty acids revisited. Part I: Toward the formation of carboxylate. Tribol. Int. 2015, 82, 218–227. [CrossRef] 39. Jahanmir, S. Chain Length Effects in Boundary Lubrication. Wear 1985, 102, 331–349. [CrossRef] 40. Spikes, H.A. Film-forming additives—Direct and indirect ways to reduce friction. Lubr. Sci. 2002, 14, 147–167. [CrossRef] 41. Campen, S.; Green, J.H.; Lamb, G.D.; Spikes, H.A. In Situ Study of Model Organic Friction Modifiers Using Liquid Cell AFM; Saturated and Mono-unsaturated Carboxylic Acids. Tribol. Lett. 2015, 57, 18. [CrossRef] 42. Minami, I.; Kubo, T.; Nanao, H.; Mori, S.; Okuda, S.; Sagawa, T. Investigation of Tribo-Chemistry by Means of Stable Isotopic Tracers, Part 2: Lubrication Mechanism of Friction Modifiers on Diamond-Like Carbon. Tribol. Trans. 2007, 50, 477–487. [CrossRef] 43. Gellman, A.J.; Spencer, N.D. Surface chemistry in tribology, Proceedings of the Institution of Mechanical Engineers, Part J. J. Eng. Tribol. 2002, 216, 443–461. 44. Haider, J. “MoSx Coatings by Closed-Field Magnetron Sputtering” in Encyclopedia of Tribology; Wang, Q.J., Chung, Y.W., Eds.; Springer: New York, NY, USA, 2013; Volume 2. 45. Mitchell, P.C.H. Oil-Soluble Mo-S Compounds as Lubricant Additives. Wear 1984, 100, 281–300. [CrossRef] 46. Graham, J.; Spikes, H.; Korcek, S. The Friction Reducing Properties of Molybdenum Dialkyldithiocarbamate Additives: Part I—Factors Influencing Friction Reduction. Tribol. Trans. 2001, 44, 626–636. [CrossRef] 47. Graham, J.; Spikes, H.; Korcek, S. The Friction Reducing Properties of Molybdenum Dialkyldithiocarbamate Additives: Part II—Durability of Friction Reducing Capability. Tribol. Trans. 2001, 44, 637–647. [CrossRef] 48. Sunqing, Q.; Junxiu, D.; Guoxu, C. A Review of Ultrafine Particle as Antiwear Additives and Friction Modifiers in Lubricating Oils. Lubr. Sci. 1999, 11, 217–226. [CrossRef] 49. Tevet, O.; Von-Huth, P.; Popovitz-Biro, R.; Rosentsveig, R.; Wagner, H.D.; Tenne, R. Friction mechanism of individual multilayered nanoparticles. Proc. Natl. Acad. Sci. USA 2011, 108, 19901–19906. [CrossRef] [PubMed] 50. Joly-Pottuz, L. “Nanolubricants” in Encyclopedia of Tribology; Wang, Q.J., Chung, Y.W., Eds.; Springer: New York, NY, USA, 2013; Volume 4. 51. Wang, X.-B.; Liu, W.-M. “Nanoparticle-Based Lubricant Additives” in Encyclopedia of Tribology; Wang, Q.J., Chung, Y.W., Eds.; Springer: New York, NY, USA, 2013; Volume 4. 52. Daia, W.; Kheireddinb, B.; Gaob, H.; Lianga, H. Roles of nanoparticles in oil lubrication. Tribol. Int. 2016, 102, 88–98. [CrossRef] 53. Guan, B.; Pochopien, B.A.; Wright, D.S. The chemistry, mechanism and function of tricresyl phosphate (TCP) as an anti-wear lubricant additive. Lubr. Sci. 2016, 28, 257–265. [CrossRef] 54. Spikes, H.A. The history and mechanisms of ZDDP. Tribol. Lett. 2004, 17, 469–489. [CrossRef] 55. Nicholls, M.A.; Do, T.; Norton, P.R.; Kasrai, M.; Bancroft, G.M. Review of the lubrication of metallic surfaces by zinc dialkyl-dithiophosphates. Tribol. Int. 2005, 38, 15–39. [CrossRef] 56. Cen, H.; Morina, A.; Neville, A.; Pasaribu, R.; Nedelcu, I. Effect of water on ZDDP anti-wear performance and related tribochemistry in lubricated steel/steel pure sliding contacts. Tribol. Int. 2012, 56, 47–57. [CrossRef] 57. Parsaeian, P.; Ghanbarzadeh, A.; Eijk, M.C.P.V.; Nedelcu, I.; Neville, A.; Morina, A. A new insight into the interfacial mechanisms of the tribofilm formed by zinc dialkyl dithiophosphate. Appl. Surf. Sci. 2017, 403, 472–486. [CrossRef] 58. Parsaeian, P.; Ghanbarzadeh, A.; Wilson, M.; Eijk, M.C.P.V.; Nedelcu, I.; Dowson, D.; Neville, A.; Morina, A. An experimental and analytical study of the effect of water and its tribochemistry on the tribocorrosive wear of boundary lubricated systems with ZDDP-containing oil. Wear 2016, 358–359, 23–31. [CrossRef] 59. Parsaeian, P.; Eijk, M.C.P.V.; Nedelcu, I.; Neville, A.; Morina, A. Study of the interfacial mechanism of ZDDP tribofilm in humid environment and its effect on tribochemical wear; Part I: Experimental. Tribol. Int. 2017, 107, 135–143. [CrossRef] 60. Onodera, T.; Martin, J.-M.; Minfray, C.; Dassenoy, F.; Miyamoto, A. Antiwear Chemistry of ZDDP: Coupling Classical MD and Tight-Binding Quantum Chemical MD Methods (TB-QCMD). Tribol. Lett. 2013, 50, 31–39. [CrossRef] Appl. Sci. 2017, 7, 445 32 of 33

61. Grrossiord, C.; Martin, J.-M.; Varlot, K.; Vacher, B.; Mogne, T.L.; Yamada, Y. Tribological interactions between Zndtp, Modtc and calcium borate. Tribol. Lett. 2000, 8, 203–212. [CrossRef] 62. De Barros, M.I.; Bouchet, J.; Raoult, I.; Le Mogne, T.; Martin, J.M.; Kasrai, M.; Yamada, Y. Friction reduction by metal sulfides in boundary lubrication studied by XPS and XANES analyses. Wear 2003, 254, 863–870. [CrossRef] 63. Khaemba, D.N.; Neville, A.; Morina, A. New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): A Raman spectroscopic study. RSC Adv. 2016, 6, 38637–38646. [CrossRef] 64. Khaemba, D.N.; Jarnias, F.; Thiebaut, B.; Neville, A.; Morina, A. The role of surface roughness and slide-roll ratio on the decomposition of MoDTC in tribological contacts. J. Phys. D Appl. Phys. 2017, 50, 085302. [CrossRef] 65. Allum, K.G.; Forbes, E.S. The Load-carrying Properties of Organic Sulphur Compounds. Part II. The Influence of Chemical Structure on the Anti-wear Properies of Organic Disulphides. J. Inst. Petrol. 1967, 53, 173–185. 66. Covitch, M.J.; Weiss, J.; Kreutzer, I.M. Low-temperature rheology of engine lubricants subjected to mechanical shear: Viscosity modifier effects. Lubr. Sci. 1999, 11, 337–364. [CrossRef] 67. Snyder, C.E., Jr.; Gschwender, L.J.; Paciorek, K.; Kratzer, R.; Nakahra, J. Development of a Shear Stable Viscosity-Index Improver for Use in Hydrogenated Polyalphaolefin-Based Fluids. Lubr. Eng. 1986, 42, 547–557. 68. Souchik, J. “Pour Point Depressants” in Lubricant Additives, 2nd ed.; Rudnick, L.R., Ed.; Marcel Dekker, Inc.: New York, NY, USA, 2009; pp. 339–353. 69. Ingold, K.U. Inhibition of the Autoxidation of Organic Substrates in the Liquid Phase. Chem. Rev. 1961, 6, 563–589. [CrossRef] 70. Jensen, R.K.; Korcek, S.; Mahoney, L.R.; Zinbo, M. Liquid-Phase Autoxidation of Organic Compounds at Elevated Temperaturess. 1. The Stirred Flow Reactor Technique and Analysis of Primary Products from n-Hexadecane Autoxidation at 120–180 ◦C. J. Am. Chem. Soc. 1979, 101, 7574–7584. [CrossRef] 71. Jensen, R.K.; Korcek, S.; Mahoney, L.R.; Zinbo, M. Liquid-Phase Autoxidation of Organic Compounds at ElevatedTemperaturess. 2. Kinetics and Mechanisms of the Formation of Cleavage Products n-Hexadecane. J. Am. Chem. Soc. 1981, 103, 1742–1749. [CrossRef] 72. Jensen, R.K.; Korcek, S.; Zinbo, M.; Jensen, M.D. Initiation in Hydrocarbon Autoxidation at Elevated Temperatures. Int. J. Chem. Kinet. 1990, 22, 1095–1107. [CrossRef] 73. Maleville, X.; Faure, D.; Legros, A.; Hipeaux, J.C. Oxidation of mineral base oils of petroleum origin: The relationship between chemical composition, thickening, and composition of degradation products. Lubr. Sci. 1996, 9, 3–60. [CrossRef] 74. Horswill, E.C.; Ingold, K.U. The Oxidation of Phenols I. The Oxidation of 2,6-Di-t-butyl-4-methylphenol, 2,6-Di-t-butylphenol, and2,6-Dimethylphenol with Peroxy Radicals. Can. J. Chem. 1966, 44, 263–268. [CrossRef] 75. Horswill, E.C.; Ingold, K.U. The Oxidation of Phenols II. The Oxidation of 2,4-Di-t-butylphenol with Peroxy Radicals. Can. J. Chem. 1966, 44, 269–277. [CrossRef] 76. Dong, J.; Migdal, C.A. “Antioxidants” in Lubricant Additives, 2nd ed.; Rudnick, L.R., Ed.; Marcel Dekker, Inc.: New York, NY, USA, 2009; pp. 3–50. 77. Zeman, A.; Romer, R.; Roenne, V. Fate of Amine Antioxidants During Thermal Oxidative Aging of Neopentylpolyol Ester Oils: Part 1. J. Synth. Lubr. 1987, 3, 309–326. [CrossRef] 78. Zeman, A.; Roenne, V.; Trebert, Y. Fate of Amine Antioxidants During Thermal Oxidation Ageing of Neopentylpolyl Ester Oils. J. Synth. Lubr. 1987, 4, 179–201. [CrossRef] 79. Holdsworth, J.D.; Scott, G.; Williams, D. Mechanism of Antioxidant Action: Sulpur-containing Antioxidants. J. Chem. Soc. 1964, 1964, 4692–4699. [CrossRef] 80. Burn, A.J. The Mechanism of the Antioxidant Action of Zinc Dialkyl Dithiophosphates. Tetrahedron 1966, 22, 2153–2161. [CrossRef] 81. Gatto, V.J.; Moehle, W.E.; Cobb, T.W.; Schneller, E.R. The relationship between oxidation stability and antioxidant depletion in turbine oils formulated with Groups II, III and IV base stocks. J. Synth. Lubr. 2007, 24, 111–124. [CrossRef] 82. O’Connor, S.P.; Crawford, J.; Cane, C. Overbased Lubricant Detergents—A Comparative Study. Lubr. Sci. 1994, 6, 297–325. [CrossRef] 83. Olomolehin, Y.; Kapadia, R.; Spikes, H. Antagonistic Interaction of Antiwear Additives and Carbon Black. Tribol. Lett. 2010, 37, 49–58. [CrossRef] Appl. Sci. 2017, 7, 445 33 of 33

84. Bartha, L.; deak, G.; Baladincz, J.; Auer, J.; Kocsis, Z. Polyfunctional PIB Succinimide Type Engine Oil Additives. Lubr. Sci. 2001, 13, 313–328. [CrossRef] 85. Costello, M.T. “Corrosion Inhibitors and Rust Preventatives” in Lubricant Additives, 2nd ed.; Rudnick, L.R., Ed.; Marcel Dekker, Inc.: New York, NY, USA, 2009; pp. 421–444. 86. Duncanson, M. Effects of Physical and Chemical Properties on Foam in Lubricating Oil. Lubr. Eng. 2003, 59, 9–13. 87. Centers, P.W. Behavior of Antifoam Additives in Synthetic Ester Lubricants. Tribol. Trans. 1993, 36, 381–386. [CrossRef] 88. Rowe, C.N.; Dickert, J.J. The Relation of Antiwear Function to Thermal Stability and Structure for Metal O,O-Dialkylphosphorodithioates. ASLE Trans. 1967, 10, 85–90. [CrossRef] 89. McDonald, R.A. “Zinc Dithiophosphates” in Lubricant Additives, 2nd ed.; Rudnick, L.R., Ed.; Marcel Dekker, Inc.: New York, NY, USA, 2009; pp. 51–62. 90. Hong, H.; Riga, A.T.; Cahoon, J.M.; Vinci, J.N. Evaluation of Overbased Sulfonates as Extreme-Pressure Additives in Metalworking Fluids. Lubr. Eng. 1993, 49, 19–24. 91. Keromest, C.; Durand, J.-P.; Born, M.; Gateau, P.; Tessier, M.; Marechal, E. Phosphosulphurised Antiwear and Extreme Pressure VI Improver Polymer Additives: Synthesis, Properties, and Lubricant Applications. Lubr. Sci. 1988, 10, 179–197. [CrossRef] 92. Ota, J.; Hait, S.K.; Sastry, M.I.S.; Ramakumar, S.S.V. Graphene dispersion in hydrocarbon medium and its application in lubricant technology. RSC Adv. 2015, 5, 53326–53332. [CrossRef] 93. Minami, I.; Hong, H.S.; Mathur, N.C. Lubrication Performance of Model Organic Compounds in High Oleic Sunflower oil. J. Synth. Lubr. 1999, 16, 3–12. [CrossRef] 94. Palacios, J.M. The Performance of some Antiwear Additives and Interference with Other Additives. Lubr. Sci. 1992, 4, 201–209. [CrossRef] 95. Guerret-Piecourt, C.; Grossiord, C.; Mogne, T.L.; Martin, J.M.; Palermo, T. Role of Complexation in the Interaction between Antiwear and Dispersant Additives in Lubricants. Lubr. Sci. 2001, 13, 201–218. [CrossRef] 96. Kuhlman, R.E. “Environmentally Friendly Lubrication Issues” in Encyclopedia of Tribology; Wang, Q.J., Chung, Y.W., Eds.; Springer: New York, NY, USA, 2013; Volume 2, pp. 985–991. 97. Minami, I.; Kikuta, S.; Okabe, H. Anti-wear and friction reducing additives composed of ortho-phenylene phosphate-amine salts for polyether type base stocks. Tribol. Int. 1998, 31, 305–312. [CrossRef] 98. Van der Waal, G. The Relationship between the Chemical Structure of Ester Base Fluids and their Influence on Elastomer Seals, and Wear Characteristics. J. Synth. Lubr. 1985, 1, 280–301. [CrossRef] 99. Minami, I.; Mori, S. Concept of molecular design towards additive technology for advanced lubricants. Lubr. Sci. 2007, 19, 127–149. [CrossRef] 100. Jessop, P.G.; Ahmadpour, F.; Buczynski, M.A.; Burns, T.J.; Green, N.B., II; Korwin, R.; Long, D.; Massad, S.K.; Manley, J.B.; Omidbakhsh, N.; et al. Opportunities for greener alternatives in chemical formulations. Green Chem. 2015, 17, 2664–2678. [CrossRef]

© 2017 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).