Effect of Chemical Structure on Tribological Behavior of Base Oils

ABSTRACT

EFFECT OF CHEMICAL STRUCTURE ON TRIBOLOGICAL BEHAVIOR OF BASE OILS

by Kun Qian

Friction and wear cost significant energy and reduce the machine life in engineering applications. Lubricant is widely used to reduce friction and wear with a tribofilm generated on the surfaces during sliding. The tribofilm formation is closely related to the additives and base oils in the lubricants. A commercial lubricant in automobile industry usually contains about 85% base oil and 15% additives. The effect of additives on tribofilm formation has been investigated widely. However, the effect of base oil still needs more investigation. This thesis investigates the effect of different base oils and their chemical composition and structure on tribofilm formation.

iii

THE EFFECT OF CHEMICAL STRUCTURE OF BASE OILS ON TRIBOFILM FORMATION

Thesis

Submitted to the

Faculty of Miami University

in partial fulfillment of

the requirements for the degree of

Master’s degree

Kun Qian

Miami University

Oxford, Ohio

2021

Advisor: Dr. Zhijiang Ye

Advisor: Dr. Timothy Cameron

Committee member: Dr. Mark Sidebottom

This Thesis titled

THE EFFECT OF CHEMICAL STRUCTURE IN BASE OIL ON ZDDP TRIBOFILM FORMATION

Kun Qian

has been approved for publication by

Department of Mechanical Engineering

______Zhijiang Ye

______Timothy Cameron

______Mark Sidebottom

Table of Contents

1. Introduction ……………………………………………………………………………….….1 1.1 Motivation…………………………………………………………………………1 1.2 Problem definition……………………………………………………………...... 2 1.3 Objective....…………………………………...…………………………………...3

2. Research background…………………………………………………………….….5 2.1 Background……………………...………………………………………………...6 2.2 Base oil……………………...…….….………………………………………...….6 2.3 ZDDP additive…………...…….….……………………………………….……...7 2.4 ZDDP tribofilm………...…….…………………………………………….……...8 2.4.1 Anti-wear function….…………………………………..………...……...10 2.4.2 Structure and element distribution……………………………………….11 2.4.3 Observation and experiment method …………….…………….….…….12

3. Methods……………………………………………………………….………….….13 3.1 Sample preparation.…….….……………………………………………....…….13 3.2 Friction experiment.…….….……………………………………………....…….14 3.3 Analysis method.…….….……………………………………….………. ….….17

4. Result & Discussion ……………………………………….……………………….19 4.1 Types of ZDDP additive ………………………………………….……….…….19 4.2 Secondary alkyl group ZDDP ……………………………….…………….…….20 4.2.1 General load effect…………………………...…………………….…….20 4.2.2 Various base oil group………………………………….………….……..22 4.2.3 Various on different load…………………….…………………….….….23 4.2.4 Temperature effect ………………………………………………...…….26 4.2.5 Wear track under Electron microscopy ………………………………….27 4.3 Primary alkyl group ZDDP …………………………………………….….…….27 4.3.1 General load effect ………………………………………………. ….….29 4.3.2 Temperature effect…………………………………………...…….…….34 4.3.3 Wear track under Electron microscopy……………………….………….35 4.3.4 Surface morphology………………………………….……………….….37

5. Conclusions………………………………………………………………………….41

6. Appendix………………………………….……………………………………...….46 6.1 MATLAB code 7. Reference…………………………………………………………………..….…….43

iii

List of Table 1. Phosphor and sulfur concentration in engine oil …………………………………………9

List of Equation 1. Hertzian contact model…………………………………………………………..21

List of Figures

1. Rtec Multifunction tribometer rotation stage ………………………………..……4 2. Two type of Zinc Dialkyldithiophosphates (ZDDP)……………………….….…..7 3. Cross-section of wear track under TEM…………………………………….……..8 4. ZDDP tribofilm formation under AFM at different period…………………....…..9 5. Wear track under various temperature in ZDDP tribofilm formation……....……10 6. Different base oil mix with two type of ZDDP lubricant sample …………..…....13 7. Demonstration experiment and equipment ...…………………………….….…...15 8. Temperature versus time under different power output……………………..……16 9. COF plot for two type of ZDDP……………………………………………....….19 10. COF under 10N load for long period….………………………………….…...….20 11. Steady state COF for secondary ZDDP…………………………………….….…21 12. Weight present of chemical structure in base oil samples... ……………….…….22 13. Secondary ZDDP with different base oil under different load …………….…….23 14. Secondary ZDDP with different base oil under 10N and 20N …………….…….24 15. Secondary ZDDP with different base oil under 20N and 40N …………….…….25 16. Thermal film formation after 120 °C test …………………………………….….…26 17. Group 5 with secondary ZDDP under various temperature……………….….….27 18. SEM and EDS image for group 5 with secondary ZDDP under 10N……….…...28 19. SEM and EDS image for group 5 with secondary ZDDP under 20N……….…...29 20. Steady state COF for primary ZDDP………………………………………..……30 21. Primary ZDDP with different base oil under different load ……………….…….31 22. Comparison between secondary and primary ZDDP under different load….……32 23. Primary ZDDP with different base oil under 10N and 20N ……………….…….33 24. Primary ZDDP with different base oil under 20N and 40N ….…………….……34 25. Group 2 with primary ZDDP under various temperature…………………………35 26. SEM and EDS image for group 5 with primary ZDDP under 10N……………….36 27. SEM and EDS image for group 4 with primary ZDDP under 20N……………….37 28. Wear track for primary ZDDP with group 5 under 10N load……………….……38 29. Wear track for primary ZDDP with group 5 under 20N load……………….……38 30. Wear track for primary ZDDP with group 5 under 40N load……………….……39

Acknowledgements

I would like to express my gratitude to my advisors Dr. Zhijiang Ye and Dr. Timothy Cameron for supporting my masters study and research. Their patient guidance helped me in all the research time and thesis writing.

Besides my advisors, I would like to thank the Dr. Mark Sidebottom for encouragement, insightful comments and help on research.

Thanks to my friends in the Tribology Research Group, Xiaoyun Fan and Holden Rittenhouse-Starbuck, for helping with the experiments and equipment adjustment.

Thank you, also, to Dr. Mark Devlin, of Afton Chemical Corporation, for providing the base oils and ZDDP additives, the great support and the useful discussions of research.

vii

Chapter 1: Introduction

1.1 Motivation

Friction and wear play a significant role in the modern world, often dominating the energy transport behavior of many engineered systems with sliding contact. Contacting interfaces are typically regarded negatively, often resisting the flow of thermal and electrical energy or irreversibly converting useful mechanical work into heat, plastic deformation and wear. Indeed, such concern is financially justified, as some estimates attribute ~$200 billion/year in losses to friction and wear in the US alone [1,2,3]. Friction always resists relative motion and contributes to wear, which directly affects the lifetime of parts. Ways to achieve friction reduction include, reducing the normal load, improving surface finish, and applying lubricant. The former two ways are less cost effective, so lubrication is the most common way to reduce friction.

Three general forms of lubrication include solid (dry), liquid, and gas (vapor) phase. Dry lubricants are often used under high temperature or low pressure with low humidity working conditions and vapor lubricants can only be applied under extremely low load [4,28]. Liquids have fewer restrictions under typical conditions, which makes liquid lubrication the most widely used form. The common usage of liquid lubricant is often seen in machines, such as automobiles. Liquid lubricants also provide other functions, such as heat removal and scavenging dirt or wear debris, which the other forms of lubrication cannot achieve.

The use of liquid lubricants to reduce friction can be traced back to early civilization with water or plant oil used in machines and mechanisms [5]. In the 1900s, petroleum-based lubricants were developed, which had great performance achieving friction reduction. However, achieving friction reduction is not always enough. Wear and oxidation are other parameters that have great impact on the lifespan of machines [6]. Thus, additives to improve anti-friction, anti-wear, and anti- corrosion performance, as well as other functions, are typically added to base oil.

Zinc dialkyldithiophosphates (ZDDP), which have extraordinary anti-wear and anti-oxidation functions, are one of the remarkable additives used in the automobile industry. After the 1940s, most commercial lubricants in US had ZDDP additives [7]. The mechanism of its excellent anti- wear performance was found to be due to chemical reactions that occur during stressed contact between surfaces with lubricant. These chemical reactions form a solid film up to a few hundred nanometers thick, referred to as a tribofilm, on the sliding surfaces [7,8]. This tribofilm is constantly being generated and worn off, which prevents direct contact between the two surfaces. This gives ZDDP excellent anti-wear performance [7]. However, drawbacks of ZDDP were discovered later. The phosphorus in ZDDP harms the environment and the ZDDP formed tribofilm is microscopically rough, which increases the coefficient of friction. It is important to understand the characteristic of ZDDP tribofilm and to optimize its properties to reduce friction while maintaining strong anti-wear performance.

1.2 Problem definition

The objective of this thesis is to investigate the effects of different base oils on ZDDP tribofilm formation and analyze the tribolfilm properties on 52100 steel. The behavior and condition of ZDDP tribofilm formation show consistent results that provide good anti-wear performance as well as an increase in the coefficient of friction. Even though the main elements contributing to the tribofilm are the phosphorous and sulfur in the ZDDP, the base oil constitutes more than 85% of the lubricant and its effect on tribofilm formation cannot be neglected. Base oils are grouped into five categories. Groups 1-3 are mineral oils, which are directly refined from crude oil, and groups 4 and 5 are synthetic oils formulated from carbon monoxide and hydrogen [6]. Synthetic oil is specifically formulated for its designed purpose. Group 4 is polyalphaolefin (PAO), a common base for gear oil, which can be applied under both cold and hot conditions. All other formulated synthetic oils are classified as Group 5 base oil. Groups 1, 2, and 3 base oil from petroleum are distinguished by their compounds, each with specific properties, viscosity index, and saturates. Compared to mineral oils, synthetic oils have a higher viscosity index, superior oxidation stability, and can adjust their properties by formulating different chemical structures and compounds [6]. Tribofilm formation is a chemical reaction in which the different chemical structures in base oil, which constitutes about 85% of a lubricant, has non-negligible effects. A fundamental understanding of tribofilm formation is still lacking. The ZDDP tribofilm forming process has different stages and the different chemical structures in the base oil may affect the chemical reactions at any stage in the process.

The formation of tribofilm typically requires the application of shear stress and normal load in the presence of a lubricant. Temperature is another key parameter related to the activation energy needed to initiate the chemical reactions that form tribofilm. Other factors such as roughness also affect the tribofilm formation. The tribofilm formed by ZDDP on steel can be divided into two types: a monolayer film formed due to adsorption behavior below 40 °C and a multi-layer film formed by chemical reaction above 60 °C [1]. The latter multi-layer film prevents contact between the two surfaces, and it is the properties of this multi-layer film that affect the anti-wear and friction properties. However, according to previous research, ZDDP additives have poor thermal stability that causes a thermal film to form around the wear track whenever the flash temperature is above 150°C, even without the rubbing process. This thermal film is relatively thin and easily decomposes at high temperature. In order to observe the tribofilm without disturbing the thermal film, removal steps are necessary before analyzing the wear track. For this work, a sonication bath was used to clean the samples.

Analyzing the trend of the coefficient of friction (COF) versus time can be an estimation of whether there is a tribofilm on the wear track. In the case where no tribofilm forms on the wear track, the COF trend should be similar to abrasive wear which increases dramatically at the initial stage then keeps increasing until the experiment ends. However, if tribofilm is formed, after an initial increase, the COF value will gradually decrease until it reaches steady state. Under extreme load conditions, however, the tribofilm formation rate is lower than the wear rate and the COF will continue to increase. This technique does not prove the existence of ZDDP tribofilm.

There are two methods to confirm the existence of ZDDP tribofilm. The first method is to analyze the wear track using Energy Dispersive x-ray Spectroscopy (EDS) with electron microscopy. The

EDS can analyze the energy of electrons from the wear track to determine if the chemical elements that are characteristic of ZDDP tribofilm components (phosphorous and zinc) are involved. After cleaning the substrate surface, the remaining elements can tell whether the reaction occurred or not. Clear tracks of zinc and phosphorus in the wear track confirm that a ZDDP tribofilm formed. The second method is to measure the height of the wear track using Transmitted Electron Microscope (TEM) profilometry or Atomic Force Microscopy (AFM). AFM can directly measure the height of the wear track and TEM can only measure the thickness of tribofilm by cutting the wear track into a cross-section after the experiment. Measuring the height of the wear track often cannot determine the existence of ZDDP tribofilm under the extreme load condition because a tribofilm may have formed and been worn away. At lower load, the tribofilm can grow to a few hundred nanometers thick in the wear track during stressed sliding. The growing solid surface is evidence of ZDDP tribofilm formation.

Some preliminary experiments show the different conditions resulting in the various formation of tribofilm and friction coefficient and there are some special cases like unstressed metal surfaces can also form a ZDDP tribofilm. These different operation conditions will change the tribofilm formation time, friction coefficient and steady state behavior. Temperature is directly relating to the energy boundary of reaction which is a major factor to controlling chemical reactions. Shear and normal stresses are also necessary to form tribofilm. These can lower the barrier of activation energy to initiate the reaction and higher stress can make the reaction start faster, however, stresses also have significant impact on wear rate which will remove the tribofilm on the surface which high stress will remove more tribofilm and may result in significant wear. The other parameters such as sliding to roll ratio and shear stress will also impact the formation process. The sliding-to- roll ratio can pull the fluids into contact area results in more fluid between contacting surfaces that are different contact conditions to pure sliding contact [9]. However, these cases will not be further discussed in this thesis. This thesis will focus on investigating the effect of chemical structure in base oil on forming ZDDP tribofilm under different temperatures and normal load.

1.3 Objective

This thesis investigates temperature and normal stress effects on ZDDP tribofilm formation with two types of ZDDP additive and five different base oil groups. The effect of base oil chemical structure on ZDDP tribofilm formation is also considered. The test conditions were designed by considering previous research and recalibrating the parameters to form a stable tribofilm in pure sliding conditions. The stable tribofilm was defined as the removal of surface material and formation tribofilm reaches a dynamic equilibrium, the COF under this condition will fluctuate in a small range due to the machine vibration and wear debris.

The experiments are operated on Pin-on-Disc using Rtec multifunction tribometer. Friction, real time load and COF are measured and recorded by this tribometer. The disc sample will be installed in a liquid tank and fully submerged into the lubricant sample. The disc will be locked on the bottom of the liquid tank with a pin lock beneath the disc to prevent it sliding against the bottom of the tank. A hood will cover the top of the sample allowing lubricant flow back to the center in figure 1. The quarter inch ball will be fixed on a probe with a load sensor on it.

Figure 1 Rtec Multifunction tribometer liquid rotational stage with hood to get back flow of liquid

The tribometer will apply load on the probe as well as measuring the friction force. The test conditions were designed at various temperatures and normal load. For time-saving purposes when testing under different load conditions, the lubricant will not be changed to a new one due to the amount of oil considering the time. The effect of wear debris is neglected. Each time when testing under a different load, a new surface of the ball will be used. The ball will not be changed unless testing a new type of lubricant sample as well as the steel disc. The different tracks on the disc will be used for each different load. When conduction experiments under different temperatures, the test starts at a low temperature. The temperature measurement was taken after the experiment, then adjusted the heating module to achieve the next temperature level. After reaching the desired temperature, move to a new track, and switch a new surface of the ball, then start the next experiment.

After the experiments were done on the disc, the disc was labeled and cleaned using acetone with a chem wipe. The observation of ZDDP tribofilm formation on wear track on disc surface was done using electron microscopy (EM). Before using EM, a further clean process was done to prevent the contamination from oil residual on the surface to the vacuum chamber. The oil residual will vaporize under vacuum pressure resulting in damaging lenses in the EM. The element distribution will be analyzed as well as the surface morphology.

Chapter 2: Literature Review

2.1 Background

In the automobile industry, liquid lubricant usually consists of two parts, around 85% base oil and the rest are additives. Base oil is one type of product refined from petroleum or crude oil using a fractional distillation process to separate different sizes of hydrocarbon chains [5]. Base oils are mainly used in lubricant and metal processing fluids depending on their properties. However, base oil itself cannot provide enough anti-wear performance that additives are necessary for anti-wear purposes. In this circumstance, additives are added to the base oil to provide extra functionality. In the 1930s, phosphorus and sulfur were found to have a huge improvement on anti-wear performance under metal-metal contact boundary lubrication [7,10]. At the atomic scale level, surfaces are never perfectly smooth. The high part is asperity, and the low part is valley. Boundary lubrication happens when the asperities are contacting each other with the lubricant trapped in the valleys [11]. Under boundary lubrication, the contact area between two surfaces is only a small part where asperities made contact. [12].

A new type of additive was developed is ZDDP which contains both phosphorus and sulfur in 1930. It has an astonishing excellent performance in antioxidant, anti-wear, and cost-effective [7] which results in more and more usage in additives, table 1 shows there are no limits for phosphorus and sulfur concentrate in engine oil before 1989. The way to provide the anti-wear function of phosphorus and sulfur-containing additives was investigated experimentally. A solid-state film referred to as tribofilm is generated on the metal contact surface through a tribo-chemical reaction. Phosphor and metal are involved in the reaction, constantly generating during sliding motion between two surfaces, thus providing the anti-wear function [7,8,13]. Formation and wear occur simultaneously that the performance of anti-wear function depends on the formation rate and tribofilm properties.

1989 NO LIMITS for P and S 1994 ≤0.12 % wt. P 1997 ≤0.10 % wt. P 2000 ≤0.10 % wt. P 2004 ≤0.06 % wt. P ≤0.50 % wt. S Table 1 Phosphor and sulfur concentration in engine oil from “The history and mechanisms of ZDDP” H. Spikes 2004

In the 1990s, phosphorus emitted into the environment will cause water eutrophication resulting in increased growth of aquatic plants and microorganisms. This makes the lack of oxygen in water and disappearance of life in this area [14]. Phosphorus is not the only source from engine oil that can damage the environment, oils in the environment may form a thin film between the atmosphere and water which disturb the oxygen-gas exchange [14]. To protect the environment, exhaust after- treatment was installed on vehicles to reduce harmful emissions. However, phosphorus and sulfur oxides were found that they can make exhaust catalyst effectiveness and block filter which significantly impact the exhaust after-treatment life span [7,15]. These drawbacks led to the

5 limitation on phosphorus-containing lubricant usage after 1994. Until recently, ZDDP is still one of the most effective additives in engine oil which the replacement product is still not developed yet.

2.2 Base oil

The first appearance of petroleum-based oil was in 1852, which replaced the animal fats lubricants [5]. Petroleum is also referred to as crude oil which is a natural resource that can be extracted by drilling or pumping underground. However, crude oil has a good property that it cannot be directly used unless transformed into products through several physical and chemical processes. In general, there are four steps to transfer crude oil to products, which are distillation, Conversion process, treatment process, and blending [6]. Distillation processes occur in distillation units that use physical processes to achieve the separation which is also known as fraction distillation. In the distillation unit, boiled crude oil is separated into different usage of products by using crude oil will condense at a different temperature to obtain different oils including gas, naptha, gasoline, kerosene, diesel, lubricant oil, heavy gas oil, and bitumen from low temperature to high temperature [5,6]. Conversion processes occur in cracking units which recompositing, unification and reforming the size or structure of hydrocarbons. In the third step, the different products have different treatment processes to obtain the composition material of the final products.

The lubricant oil will be distinguished into three groups referred to as base oil group I, II, and III. There are two main steps in the base oil treatment process, the first is aromatics removal and the second is dewaxing [5]. These two steps are also called solvent refining processes. Aromatic is a reactive component in base oil causing faster oxidation and shortening the base oil lifetime. Moreover, aromatic is also an important component related to temperature, which its properties do not has god response to temperature. The designed automobile lubricant can be used in various conditions including engine ignition at low temperature and operating at high temperature. Desired base oil has viscosity is low enough for operation at low temperature and becomes high viscosity at regular high temperature operating conditions. This viscosity changing property is characterized as viscosity index (V.I.) that high V.I. represents a smaller response to temperature change. The desired lubricant is having high V.I. which can meet the requirements operating under different temperature conditions. Thus, the aromatics are removed in the first step. The second dewaxing process has a similar purpose in which wax will freeze at low temperature which will damage the machine.

2.3 ZDDP additive

The first appearance of ZDDP additives in lubricant is in 1941 [7]. The discovery of the high anti- wear performance of phosphorus and sulfur leads to an increasing amount of use in weight percent of lubricant additives. The scientists developed a type of chemical product called Dialkyldithiophosphates (DDP) that contains both phosphorus and sulfur. DDP is main compound to anti-wear and friction reduction property. DDP is relatively unstable which can link to other metal elements to form metal Dialkyldithiophosphates compounds (MDDP). Comparisons were done on different metal element link with DDP to get a cost-effective and efficient product. Zinc has better performance than other metals like copper, lead, and iron which are more toxic [7].

There are two types of alkyl group ZDDP which are primary alkyl ZDDP and secondary alkyl ZDDP, the difference between them is alkyl group treatment. The primary ZDDP only has one bond to the alkyl group, secondary ZDDP has two bonds alkyl group in figure 2. On the tribological performance, primary ZDDP has less friction but a higher wear rate compared to secondary ZDDP. Secondary alkyl ZDDP has lower thermal stability than primary alkyl ZDDP which so secondary alkyl ZDDP has a faster reaction rate [16,17].

(a)

, (b) Figure 2 (a) Primary ZDDP structure (b) Secondary ZDDP structure

ZDDP was the most successful additive to the engine oil lubricant till now. It has excellent anti- wear properties which the amount of ZDDP did not limit to use in engine oil before the 1990s when the researchers found ZDDP significantly shortened the lifetime of exhaust catalysts in exhaust after-treatment. To reduce the phosphorus and sulfur in engine oil as well as improve the performance of lubrication, ZDDPs are now usually combining with other additives like molybdenum Dithiocarbamates (MoDTC) to perform an anti-wear function with less phosphorus containment [18]. Until recently, the replacement of ZDDP with such great anti-wear additives has not been developed.

2.4 ZDDP tribofilm

In the 1950s, a solid-state film with 120 nm thickness ZDDP tribofilm was discovered from radio tracing on phosphor 32P which formed under shear and normal stresses at a quite low temperature. Furthermore, a similar thickness of ZDDP tribofilm is also formed on an unstressed surface at 150 °C with extended immersion [18]. This solid film usually has about 50-200nm thickness [7,19-23] and mainly contain phosphorus, sulfur, zinc and surface metal element. The hardness, wear property and formation rate heavily depend on temperature, stresses, and chemical compounds. The ZDDP tribofilm reaches dynamic equilibrium between formation and mechanical removal results in a great performance in the anti-wear. However, ZDDP tribofilm was found to be microscopically rough so that forming the ZDDP tribofilm increases friction. The ZDDP tribofilm formed through chemical reaction by breaking carbon chains then bonding with surface metal elements. However, this tribofilm is not elementally equally distributed, it is categorized as three different layers which have different reactions at different formation stages. The element concentration and structure are also different by layers. The forming process of ZDDP tribofilm initiates soon after stressed sliding occurs. At the initial stage, metal sulfide formed on the substrate surface. Then zinc phosphate is generated and mixed with the sulfide layer. In figure 3 shows the cross-section of ZDDP tribofilm that has two layers. The top layer has many porous, closer to surface less porous in the layer and prevents direct contact between asperities of two surfaces that reduce the wear and stress between two surfaces. The third layer is a very thin layer that does not have porous and it will dissolve afterward [8]. This layer is considered as a thermal film that can also form without applying normal stress at 150°C above [7,16]. The thermal film has low thermal stability that it will disappear after a certain period, and this film does not provide great anti-wear properties.

Figure 3 The cross section of wear track with tribofilm under transmission electron microscopy (TEM) From “Understanding Tribofilm Formation Mechanisms in Ionic Liquid Lubrication” Yan Zhou, Donovan N, Lenard, Wei Guo & Jun Qu Scientific reports, Aug. 2017

The formation is the chemical reaction process that requires several conditions. At first, the condition to form tribofilm is normal and shear stress under high temperature. However, later researchers found that high temperature is not necessary for forming processes, the tribofilm formation can be achieved at room temperature. The ZDDP tribofilm formation does not form a solid film, it forms in small patches which have 2-6µm size and start to grow to large pad with the 8 rubbing process in figure 4(a). After more than 30 minutes of sliding, these patches are well developed into large pads in figure 4(b). After 1 hour, the large pads start to break down to smaller pads and form a uniform steady tribofilm in figure 4(c & d) [20,21]. These patch-like films have been observed through Atomic Force Microscopy under 100 °C until fully developed tribofilm.

Figure 4 ZDDP tribofilm formation under AFM rubbing test at different period (a)(b)from “Tribofilms generated from ZDDP and DDP on steel surfaces: Part 1, growth, wear and morphology Z.Zhangm E.S. Yamaguchim M. Kasrai and G.M. Bancroft (2005)”

The steady-state friction will decrease with the increased length of alkyl chains of ZDDP in steel-steel contact. Moreover, the chain length also affects the tribofilm formation status. The short-chain alkyl group (C3, C4, C6) forms a more uniform tribofilm compared to long-chain alkyl group (C8, C12), which forms a patchy film. [7,16] The film formation rate is faster with secondary ZDDP than primary ZDDP [7, 16,17,24]. The overall tribofilm formation rate depends on the strength of the P-O or C-O bond in ZDDP [17]. ZDDP tribofilm has the same hardness that will not change even the material was removed from abrasion. When operating temperature is over 100 °C a thermal film formation will occur on a steel surface which can reach 200nm thickness. The thermal film has a smoother structure compared to tribofilm and did not contain any surface element. The tribofilm formation rate is proportional to the temperature that the formation process only occurs when actual sliding contact.

Figure 5 shows the wear track under different temperature using ZDDP in group 4 base oil [9]. The increasing temperature will increase the reaction rate which accelerates the ZDDP tribofilm formation from the increasing dark area with the temperature increase. However, this trend does not appear in 80 °C which has less area of darkness than 100 °C . The parameters can affect the ZDDP tribofilm formations not only including temperature, applied load but also sliding conditions. Rubbing is a pure sliding condition which is not always the contact case between two surfaces. Rolling is the other parameter affecting the ZDDP tribofilm formation. The slide roll ratio between two surfaces is quantified that the more this ratio is close to pure sliding, the faster ZDDP tribofilm forms [9]. The COF of ZDDP tribofilm is important which directly impacts the energy consumption, in which the ZDDP tribofilm COF value is measure when it reaches a steady-state. The steady-state COF is determined when the tribofilm fully developed and reaches a dynamic equilibrium of formation and removal. The ZDDP tribofilm is relatively rough on a micro level, which results in increasing COF. However, this phenomenon does not show under extreme load conditions. When contact pressure around 3.5 GPa, the COF value does not change from initial value to steady-state value [22,23].

Figure 5 Wear track under various temperature using Mini Traction Machine-Space Layer Imaging (MTM-SLIM)

2.4.1 Anti-wear Function

The generally accepted view of anti-wear performance on ZDDP tribofilm is that it prevents direct contact and smoothens the surface to reduce stress concentration on asperities. The formed tribofilm will substitute surface directly contact with other surfaces and be worn out, the formation and removal occurring simultaneously which provide such good anti-wear function. The key factor for anti-wear at the initial stage is the temperature which directly relates to the ZDDP tribofilm formation rate. Only formed tribofilm covers the surface and minimizes the interaction between contacting surfaces. Moreover, the shear stress is another factor co-relate to the temperature that the high shear stress will raise temperature fast at contact points which also result in faster formation. The main element contributing to anti-wear performance is phosphor [7, 24]. The anti-

10 wear function layer has roughly 50nm thickness [7] The film thickness has no direct relation to anti-wear performance. [20]. From the X-ray photoemission microscopy that ZDDP anti-wear film contains very long chain polyphosphate, the short-chain mixed iron, and zinc orthophosphate is in small patches close to the substrate. [26]

The anti-wear performance also relates to the concentration of ZDDP additives. From 0.1% to 1% concentration, the tribofilm will perform a great anti-wear function. However, when the concentration exceeds 2%, the formed tribofilm thickness did change significantly, ticker formed tribofilm cannot suffer from load and break to a fragment which results in a higher wear rate. After the concentration exceeded 10% the wear rate did not change much. However, the wear performance under extreme load condition which above 3.35Gpa does not depend on the concentration of secondary alkyl ZDDP [22].

2.4.2 Structure and Element Distribution

The ZDDP tribofilm only contains oxygen, phosphorus, sulfur zinc, and iron from the substrate. The structure and element distribution of tribofilm was discovered by X-RAY Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) in the 1970s that has high sulfur- containing layer to the substrate, phosphorus, and zinc-rich layer lies on top of it containing less sulfur, [7] which is also stated in to [21] by using XPS sputtering that 15% phosphor, 25% zinc and only 8% sulfur atomic percent on the surface. When close to the substrate, the sulfur has a slightly higher atomic percent than Zinc and phosphorus, as well as the iron element. The phosphorus and sulfur compositions in tribofilm are also affected by contacting conditions, that mild contact will consist of thick phosphate film, and high load or high sliding speed will build up a thinner sulfur film. The carbon element only exists on top of the film which can be assumed as the thermal film [7]. From Auger electron spectroscopy results indicates the tribofilm has no native iron oxide between tribofilm and substrate that the iron in the tribofilm gradually increases until substrate. [20] Zinc has relatively more concentrated on the out layer around 30 to 40% base on X-ray photoelectron spectroscopy. All oxygens are bonded with zinc that no iron oxide was found in tribofilm that the formation of ZDDP tribofilm reduces the oxidation of the steel. [26] Other chemicals including phosphate glasses and zinc phosphate are also found in tribofilm. According to [26], “tribofilm is made of mixed short-chain iron and zinc orthophosphate layer covered by a thin long zinc pyrophosphate one.” However, the element distribution is not the same for different types of ZDDP. Short-chain secondary alkyl group ZDDP tribofilm contains more phosphorus, oxygen in the outer layer, compared to primary alkyl group ZDDP has more sulfur on the outer layer. The friction coefficient is dominated by sulfur-rich out layer which indicates the primary ZDDP has a lower coefficient of friction [24].

There are no direct findings on ZDDP tribofilm formation under mechanical contact, thermal, or rubbing process. [7] The thickness of ZDDP tribofilm is up to 200 nm which formation is conducted by chemical reaction, however, the key parameter driven to the reaction has various

11 ideas including cation exchange at low temperature, electron or particle emission, or stressed sliding and contacting. The detail tribofilm formation is still unknown.

2.4.3 Observation & experiment method

In current tribofilm research, tribofilm formation experiments are always designed to simulate the process that occurs in the machine operation condition which is either reciprocating sliding or single direction sliding. Both experiments conduct under lubrication conditions using flood lubrication or minimum quantity lubrication. The experiments are usually designed as spherical to flat contact like a ball on disc or flat on flat contact. The operation of these experiments using mini traction machine (MTM) [9,13,16,25], atom force microscopy (AFM) [25] and multifunction tribometer. MTM and multifunction tribometer can operation contact experiments in both reciprocating and rotating conditions. Compared to AFM which can only operate linear reciprocating under relatively low load. The difference between AFM and other machines is that AFM can only operate on an extremely small scale (nm to um) and at low speed (nm/s to um/s).

The observation of experiment results for sample surface is focusing on the wear track which includes the element distribution, wear track width, and depth. Transmission electron microscopy (TEM) and Scanning electron microscopy (SEM) and profilometer are used to analyze surface morphology. Chemical composition on wear track is using energy dispersive X-ray Spectroscopy (EDS) and X-ray Absorption Near-Edge Structure (XANES). The hardness, modulus, and other mechanical properties can be examined by Hysitron Triboscope (Nano indenter)

Chapter 3: Experiment Setup

3.1 Sample preparation

The lubricant samples contain base oil and additives. To analyze the influence of chemical structure in base oil on specific ZDDP additives, the samples used in this experiment are only applied to one type of ZDDP with one base oil group. There are five base oil from Group 1 to 5 used in the experiment and two types of ZDDP additives. There are two types of ZDDP additive, primary and secondary alkyl ZDDP. However, these two types of sample ZDDP additives are not told which is primary and secondary. The label on the sample is 1 and 2 to represent the sample type. The actual type of ZDDP additive can be analyzed after comparing the experiment results from secondary ZDDP having a faster tribofilm formation rate than the primary ZDDP.

Base oil Group I to V mixed with 1 weight percent single type ZDDP additive in figure 6. The mixed solution was filled in different bottles with label and then put into an ultra-sonication bath that pre-heated up to 60-degree C. Then the solution was sonicated for 60 minutes to ensure the additive well mixed with base oil and was kept for at least 24 hours before experiments. The sample can only be used when there is no separation of layer between base oil and additives after 24-hour standstill.

Figure 6 Base oil mixed with ZDDP additive. Left five bottles labeled as ZDDP type 1 mixed with base oil Group 1 to 5(From left to right) Right five bottles labeled as ZDDP type 2 mixed with base oil Group 1 to 5

The ball and disc material are 52100 steel alloy. The ball is a quarter inch in diameter and is ordered for McMaster-Carr. The disc is ordered from Rtec instruments having around 15nm surface roughness measured from the profilometer. The thickness of the disc is 0.2 inch and the radius is 2 inches. Each disc will run the test using a single type of lubricant sample. To avoid the wear track overlap with the previous results, a 1.5 mm gap was taken between each test run. The thickness of the disc is limited by the liquid tank hold and its hood. The limited thickness is 1/4

13 inch which currently is unavailable for our machine to produce such disc, the repeated tests were held until a cost-effective way to produce the same disc sample is found.

3.2 Friction Experiments

To formulate ZDDP tribofilm, a pin on disk tribo-experiment is performed in this study uses 52100-bearing steel ball and substrate disk illustrated in figure 7a. Tribo-tests are done using the Rtec Multifunction tribometer which applies the load on the ball, rotating disk to the experiment. Unique software is provided for this tribometer to conduct the experiments. In figure 7b, this tribometer has a rotational stage that can contain liquid and a probe attached to a load cell which can apply load ranging from 0.5 N to 50 N as well as measuring the force on the tip of the probe. The disk is mounted in a liquid tank and fully submerged in lubricant. A hood is also attached on top of the tank to prevent the liquid spill out at high rotational speed. The precision of force measurement is 0.001 N. The probe can hold ¼ inch ball and the stage can mount a disc size up to 0.25-inch thickness.

To operate the test under different temperatures, a heating element was mounted on the tribometer. A custom-built heating system with temperature control was installed. as shown in figure 7c and 4d. There are two type system control compared between duty cycle control and proportional integral derivative (PID) controller. PID controller reads the temperature from the sensor probe in the liquid tank then compares it with the desired value to determine the power is on or off. Duty cycle control is set up as a period that power on for a certain time then off for a certain time which creates a loop to achieve temperature. However, the heating source thermal pad is not directly in contact with the lubricant so the power output adjustment at a certain temperature needs to be tested. Several demo tests using the only Group 4 base oil without additives were designed to adjust and compare both temperature control methods. By considering the amount of additives in the sample, the thermal properties of additives can be neglected. Both methods were tested and compared the temperature for 30 mins under 60 rpm for liquid tank rotational speed to simulate the experiment condition. The results show that PID controller has about plus-minus 10 °C which is a larger temperature variation than duty cycle control only plus-minus 5°C. The less temperature variation duty cycle control is selected and mounted to the tribometer. The setup point temperature relates to the power output that was adjusted in figure 8. However, during the experiment, the sliding of two surfaces will generate heat which creates more temperature variation in the experiment. The final result for duty cycle control heating during the experiment is about plus- minus 7 °C.

Figure 7 (a) Demonstration of pin-on-disc experiment (b) Rotational stage with liquid holder of Rtec Tribometer (c) Heating pad mounted on Rtec Tribometer (d) Duty cycle temperature control box

Duty cycle power control to temperatrue 140

120

100

80 40 60 50 60

temperature C temperature 40 100 20

0 0 20 40 60 80 100 120 time min

Figure 8 Temperature under different power output in duty cycle control

The tribometer will record the rotation speed, normal load, and friction force simultaneously during the experiment with 1000 Hz sampling rate. The coefficient of friction will be automatically calculated by software operating the tribometer by dividing friction force by normal load. The tribo-test was set up in applying load 1.2 Gpa, 1.56 Gpa, and 2 Gpa using Hertzian contact model to calculate the applied force. However, the tribometer can only set up contact load in newton which the actual contact pressure relates to the contact area. Therefore, it is necessary to use a contact model to calculate the actual contact area and find corresponding load and contact pressure to make a comparison to another research. The Hertzian contact model is chosen due to its modeling of the spherical contact in cases without adhesive force. [27]. The contact pressure 3퐹 convert to applied load is though 푃 = , where P is contact pressure and F is applied load. The 2휋푎2 1−푣2 1−푣2 3 3퐹∗[ 1+ 2] 퐸1 퐸2 true contact area 푎 is calculated though 푎 = √ 1 1 Where, 푅1 , 푅2 are Radius of contact 4( + ) 푅1 푅2 objects , 퐸1, 퐸2 are Elastic modulus and 푣1, 푣2 are Poisson’s ratio of contacting objects. The flat disc is considered to have an infinite radius. The results for applied load are 10N, 20N, and 40N. The sliding speed is 0.1 m/s linear speed with full slide, under various temperatures from 25 °C to 120 °C, which is 25°C is considered as room temperature. The tests operate on the disc which the linear speed is convicted to the rotation speed in revelation per minute (RPM). The experiment duration is from 30 minutes to 120 minutes. The surface roughness of the disk is Ra 10nm. The first batch of experiments is done at 25 °C for 30 minutes for 5 groups various with three loads. The second batch of experiments is done for 120 minutes for base oil group 2 and 5 various with different temperatures.

When doing the different loads using the same lubricant sample, the lubricant will not change each time with different loads. But new track and ball surfaces will be used with different load experiments. After finishing the one lubricant sample, the remaining lubricant will be discarded into oil disposal, then the liquid tank and prob will be cleaned using acetone. The disc will also be cleaned using a chem wipe and acetone and labeled on the backside.

The experiment duration was set up at 30 mins. The demo tests were done to analyze how long it will take to reach steady-state COF. Several experiments using different types of lubricant samples were done under 2 hour, 1 hour, and 30 minutes test run. The results show that if the sample cannot reach a steady-state COF value, it will not reach it even running the test for 2 hours. It seems if the sample can reach steady-state COF, mostly it can achieve in 30 mins. The different temperature experiments were done in a set that started at low room temperature. After the first experiment was finished, the heating element will be adjusted to a higher temperature for the second experiment. Repeat this process until reaches the highest temperature designed for the experiment. After finishing all experiments, the liquid tank and probe are above 100 °C that cannot be disassembled for cleaning. The sample will stay in the lubricant until it cooled down which is about 6 to 8 hours. Then the sample will be taken out for cleaning and labeling.

3.3 Analysis method

We characterized the COF, element mapping, and wear to evaluate the tribological performance of tribofilm. COF analysis was done by plotting COF value versus time on a graph. COF can not be directly measured but the friction measurements can be recorded from the tribometer and it automatically calculates the COF value by dividing friction over normal load by its operating software. The friction measuring data sampling rate is 1000 Hz which brings a massive amount of data points in the COF versus time plot and the vibrations from pure sliding between two surfaces create a lot of noise. A MATLAB CODE for smoothing data in appendix 1 was developed to operate moving average and plotting. Moving average algorithm is sum up the 50 points before the object time point and 49 points after it, then divide 100 to get a mean value at the specific time point. After plotting time versus COF, the results will be analyzed and compared to the other testing conditions using different lubricant samples.

The element mapping is a reliable way to prove the existence of the ZDDP tribofilm formation. The unique element concentration on the wear track shows the reaction that occurs during the experiment. Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDS) are used to analyze the element distribution on the disc surface. When using SEM and EDS, the sample must put into a vacuum chamber to get a magnified image on the wear track. However, the vacuum chamber will make the oil on the sample vaporized and condense on the lens inside the vacuum chamber which will barely obtain clear imaging and affect the accuracy of element distribution analysis. A two-step cleaning process was done on the sample before put into the vacuum chamber. First, use a sonication bath filled with water, for a short time sonication to remove the most oil covered on the surface. The second step is to use a chem wipe with acetone to carefully wipe out the surface. Acetone can easily and fast vaporize in air and it can also dissolve oils which these characteristics will not bring additional impact on the sample. The wipe process

17 requires lots of care that the chem wipe can make scratches on the surface which will disturb the wear track analysis under SEM. Because the experiments were done in a rotation direction the wear track is a circle on the steel disc. To distinguish the wipe starches to wear track, the wipe direction is following the single direction and take the image using SEM on the location where the wipe scratch is relatively normal to the wear track. After finding the wear track corresponds to the experiment number. Then use EDS to analyze the element distribution on the wear track and surrounding surface area. Only four elements were analyzed including sulfur, phosphor, zinc, and oxygen are inspected that these elements are majority contained in base oil and ZDDP additive and the previous research shows that these elements do involve in the chemical reactions on forming ZDDP tribofilm. The concentration of certain elements like Zinc, sulfur, and phosphorus in wear tracks from ZDDP is evidence of the existence of ZDDP tribofilm even carbon and sulfur also contained from base oils. Oxygen cannot be the evidence of the existence of tribofilm due to oxidation can also occur during the sliding process.

Wear analysis is using a profilometer to investigate the wear track morphology. The profilometer is using the reflection of lights to analyze the surface morphology. Before examining the wear track on the sample surface, a chem wipe with acetone and air spray was used to clean the dirt, and the wipe process is also following the direction when cleaning before putting it into EM. Thus, this will not make a further disturbance to the surface when investigating the disc under EM another time. These steps are aiming at removing dirt to get a more accurate measurement from the profilometer. The scratches from chem wipe do not affect the measurement a lot due to its scales being relatively large than the SEM. During the sample surface morphology measurement, the surface is always not laid perfectly normal to the sensor results in the additional flatten fit process was taken to get the results.

Chapter 4: Results & Discussion

4.1 Verify type of ZDDP

The time to reach a steady-state COF trend was compared between two types of ZDDP with the base oil group 3 and 5 base oil under room temperature in figure 9. From previous research, the secondary alkyl group has a faster reaction speed which indicates the time required to fully develop a dynamic steady state COF value is less than the primary alkyl group ZDDP. In figure 9 left plot, at Group 5 40N load test, ZDDP type 1 reaches steady-state value around 450 seconds where ZDDP type 2 reaches a steady state after 600 seconds. In the ZDDP type 2 COF plot, there is fluctuation after 600 which un-fully developed larger size pads like ZDDP tribofilm was worn out and interpreted the COF value. In figure 9 right plot, Group 3 20N with two types of ZDDP additives were also compared. The plot results also show that the ZDDP type 1 reaches a steady- state around 600 seconds which is faster than the ZDDP type 2 hitting steady-state COF value. According to these two plots, The ZDDP type 1 can be indicated as secondary alkyl group ZDDP though this less time to develop the tribofilm, and the ZDDP type 2 is primary alkyl group ZDDP that requires a longer time.

Figure 9 Comparison between two types of ZDDP under different load (ZDDP 2 is secondary ZDDP, ZDDP 1is primary ZDDP)

The duration of the experiment is an important factor to consider which impacts the chemical reaction and costs. The original experiment was set up at 30mins duration that the necessity of extending the experiment time was also verified. To investigate COF trend under 10N load, a 2- hour experiment was designed to verify the impact on extending sliding time in figure 10. Figure 10 shows COF values fluctuated after 1000 seconds, which indicates multiple wear debris coming out of the wear track results in the small amount of increase and decrease of COF in a small range. However, the overall COF does not appear an increasing or decreasing trend. The longer the test operates, the more wear debris coming out of the wear track results in the COF trend. Thus, there is no necessary to extend the experiment duration for those who did not reach a flat COF trend.

Figure 10 Group 5 with secondary ZDDP under 20N load Experiment duration 1.6 hours

4.2 Secondary alkyl group ZDDP

4.2.1 General load effect

Load dependence of friction for different base oils has been tested and results are shown in Figure 11. For all base oil groups, we observed a decreased friction with increasing load. Group 4 shows the lowest friction coefficient at all loads we tested compared to the other based groups. The friction performance of different base oils can be explained through the different chemical compositions and chemical structures of based oils, as shown in Figure 11. For all base oils, we characterized the main chemical composition, including paraffin, cycloalkanes, benzothiophene, and carbazoles dibenzothiophene. The group 4 based oils showed the lowest weight percentage of cycloalkanes which is a ring structure molecule. Our hypothesis is that the ring structure molecules can provide the lowest activation energy for bond-breaking process compared to the other types of molecules, therefore, it could assist the tribofilm formation and thus help reduce the friction.

The steady-state COF is defined as when the formation of ZDDP and material removal reaches a dynamic equilibrium. In this case, the patch like ZDDP tribofilm has grown to a large size and breaks down to the smaller size patches to get a fully developed tribofilm which has much fine

20 surface. The steady-state value in the experiments was taken from the plot where a flat trend period lasts at least 400 seconds. For those COF trends are fluctuating within a range, and an average value was calculated. In figure 11. the decreases COF with increasing load trend is shown among all base oil groups. This phenomenon was explained that increase load will increase the contact area by the deformation of material resulting in less contact pressure and wear. Thus, it will reduce the COF [29]. The amount of COF reduction decreases with the increasing load from 10-20N and 20-40N, which is shown in Group 1,3,4,5. However, Group 2 base oil does not experience the same reduction.

COF in Different Load 0.125

0.12

0.115

0.11

COF 0.105

0.1

0.095

0.09 Group 1 Group 2 Group 3 Group 4 Group 5

10N 20N 40N . Figure 11 At 25 °C for 5 different base oil with secondary ZDDP Value was taken at steady state by from average at COF flat trend last at least 400 seconds

Figure 12 shows the weight percent of different chemical structures in base oil groups. cycloalkanes are hydrocarbon ring structures shown in light blue, cycloalkanes are monoaromatics which is closed ring structure with at least three hydrocarbons in light blue, Benzothiophenes are diaromatics and Carbazoles Dibenzothiophenes are tri-aromatics. These data are provided by its supplier (Afton chemical). No further analyses were done on these samples. The base oil group 4 has a lower steady-state COF than on other base oil group under every load. The steady-state COF value correlates to the concentration of the cycloalkanes which group 4,5,1,3,2 contains from low to high. The lower concentration of cycloalkanes, the lower the steady-state COF will get. Group 4 has the lowest COF which contains the least cycloalkanes. The assumption that cycloalkane concentration will correlate to the COF. The more cycloalkanes in base oil the lower the steady- state COF will reach in secondary alkyl group ZDDP additive.

Figure 12 Weight present of Chemical structure in Base oil Samples

4.2.2 Various on different load

The COF versus time plot in figure 13 is tested using base oil group 1 2 3 4 and 5 mixed with 1 weight percent secondary alkyl group ZDDP. The COF trend in group 1 3 and 5 experiments is relatively similar under different loads. The COF trend for base oil group 1,3 and 5 decreases immediately after experiment initiates then increases for a short period then start decrease again. This increase and decreases COF trend show the reaction of ZDDP tribofilm. The tribofilm forms on asperities of the surface causing increasing friction. With the sliding continuously happens, the tribofilm wear out to become a relatively smoother surface which appears in decreasing COF value. When the COF reaches a flat trend, a fully developed ZDDP tribofilm is considered to be formed on the wear track. However, not all experiments reach a flat COF towards the end of experiments. The Group 2 and 4 COF trend has obvious changes between low load 10 and high load 20N, 40 N. Under low load conditions group 4 and group 2, 10N load kept increasing trend and did not reach a steady-state until the experiment ended. The other two loads reach a steady- state value after some period. The fully developed ZDDP tribofilm does not be considered formed on the wear track under low loads for base oil groups 2 and 4. The reason is that these loads at room temperature did not prove enough energy to allow the chemical reactions to fully react. Further analysis will be done on element mapping to verify whether the tribofilm reactions occurred or the increasing COF trend is simply from abrasive wear.

Figure 13 Combined chat with secondary alkyl group ZDDP mixed in five type of base oil at different load

4.2.3 Various on different base oil group

The different load results in the different trend of COF which can be inferred from the ZDDP tribofilm formation. The low load may not provide sufficient energy to reach the energy barrier to activate the reaction, that between 10N and 20N the base oil group 1 2 3 4 has apparent changes on COF trend. The COF in figure 14 shows the time versus COF under 10N and 20N load use five different base oils mixed with secondary ZDDP. The left plot in figure 14 shows that the COF trends of group 1 2 3 4 at 10N load are continuously increasing throughout the experiment. The COF trend for a tribofilm formation needs to experience an increase then decrease trend, which only group 2 has this trend. However, the COF trend for group 2 is continuously increasing which makes it hard to tell whether a fully developed ZDDP tribofilm is formed or not. However, group 5 base oil does not have such a typical ZDDP tribofilm formation COF trend. It reaches a flat trend very soon around 500 seconds and is kept until the experiment ends which such a long period flat trend can deduce a fully developed tribofilm form on the wear track. This assumption will be verified through element distribution mapping to ensure the reaction occurs on the wear track. Other base oil groups mostly have an increasing trend of COF which can not be strong evidence to tribofilm formation that the increasing trend can also be the abrasive wear COF trend.

The right-side plot of figure 14 shows the 20N load using secondary ZDDP. The COF lines are differing from the low load 10N on group 1, 3, and 4. Base oil group 5 and 2 remain a similar trend for these two different loads. The COF trend for base oil Group 1,3 and 4 has a significant change between 10N and 20N load. They have only experienced an increasing trend at 10N load but when under 20N load. These base oil groups have an increasing then decrease COF trend. These base oil groups reach a flat COF trend after 1000 seconds which can be inferred as fully developed ZDDP tribofilm which has a steady-state COF value. This phenomenon can be assumed as normal stress is stimulating the chemical reaction of tribofilm formation. The activation energy for the chemical reaction is lowered at the condition of 20N sliding speed and 0.1m/s under 25 °C. The ZDDP tribofilm can be assumed as formed by this trend of COF. However, until the EDX analysis performs the element analysis on wear track, this cannot be the evidence for ZDDP tribofilm formed. From previous research, some cases of abrasive wear COF on steel-steel contact also have a similar COF trend.

Figure 14 Secondary ZDDP with five different base oil under 10N & 20N under room temperature 25 °C

In figure 15, the left plot shows secondary ZDDP at 20N and the right plot shows 40N load. These COF trends are not much changed between the two plots, except group 2 base oil. Under 10N and 20N load, COF trend for base oil group 2 appears to decrease then increase COF trend. Under 40N load, the base oil group 1 to 4 reaches a steady-state value which is the lowest value after the initial increase then decrease trend. However, all of them did not experience long term steady state COF trend. This can be explained by the high load makes more wear debris which affects the friction. Moreover, comparing with the lower load the steady-state values decrease with the increased load from 20N to 40N. However, in the right plot for Group 4 base oil experiment, the COF certainly increases at 1000 seconds. This increase may be caused by large wear of debris affecting the contact properties damaging the surface. Other base oil groups have a much flat trend and lower value in 40N load than 20N load. The formation of tribofilm COF trends was different in each base oil. Base oil group 1, 3, and 4 have an increasing trend from initial to 200 seconds, then gradually drop off until they reach a steady-state tribofilm. The base oil group 2 and 5 have different trends than others which group 5 always have decreasing trend until it reaches steady- state value and group 2 decreases soon after sliding occurs then increases to its steady-state value.

Figure 15 Secondary ZDDP with five different base oil under 10N & 40N under room temperature 25 °C

Base oil group 1,3,4 has similar trend which they also contain more cycloalkanes in the base oil. Steady-state values of friction for different base oil are determined by the amount of cycloalkanes which group 2 has most and it has the highest steady-state COF under all loads. Group 4 has the least amount of cycloalkanes which shows the lowest COF among all load conditions. Group 5 reaches the steady-state faster than all other groups which contain the carbazoles and dibenzothiophenes. Group 1 has the second amount of carbazoles dibenzothiophenes which on the second fast to reach the steady-state value. There are no direct relations on the benzothiophene by just analyzing the COF trend.

4.2.4 Temperature effect

After the high-temperature experiments, a thermal film was observed on the wear track. When the test was finished at 120 °C, a visible darker color film had formed beside the wear track in figure 16 and after several hours of cooling time, most of this film had disappeared. This film can be characterized as a thermal film that has very similar properties. Most wear tracks turn to dark color at the end of experiments under 20N and 40N. The 10N experiments wear track does not have visible color changes between the new surface and wear track no matter which type of lubricant sample was used.

Figure 16 Steel Disc observed immediately after 120 °C test finished (Red arrow is pointing wear track, Blue arrow is pointing a film like thermal film)

In figure 17, temperature various experiments were conducted to test the ZDDP tribofilm formation. The higher temperature will reduce the viscosity of the fluids which will affect the COF measured from the tribometer. However, the probe did not emerge into the fluid, only the ball held by the probe emerged in the lubricant that the high-temperature effect on viscosity was neglected in this research. On the other hand, temperature is one of the most important factors in lowering the energy barrier of the chemical reaction. The high-temperature experiments are expected to have a faster reaction speed and less time to form a fully developed. The decreasing COF values at a steady state with the increasing temperature are also expected. These two experiments are done in the same load with different temperatures. The two types of samples were chosen to verify the temperature effects. The reason is that Group 5 reaches the steady-state faster than other base oil groups and it did not appear to significantly fluctuate in the experiment with a decreasing COF

26 trend. On the contrary, group 2 base oil has a different COF trend but also has less fluctuation during the experiment. 20N load was chosen due to this load can activate the chemical reaction and has less wear than 40N load.

Fig 17 shows the primary alcohol ZDDP mixed with Group 5 base oil under 20N load. The COF trend under various trends follows the expectation that the 120 °C has the highest steady-state COF value and 25 °C has the lowest steady-state COF value. However, this expectation did not see under 50 °C and 80°C, which steady-state COF value under 50 °C is higher than 80 °C. In the 50 °C COF trend line at 400 to 600 seconds, three bumps with the sudden COF increases occurred which the wear debris affect the COF trend multiple times and causing significant damage to steel surfaces results in an increasing trend toward the end of the experiment. By considering the overall COF trend which bases oil group 5 under various temperatures, the time required to reach the lowest COF value decreases with the increased temperature in figure 17 pointed by arrows. 120°C experiment reaches the lowest COF at 200 seconds, 80°C experiments at 235 seconds, 50°C experiments at 300 seconds, and room temperature 25°C experiment at around 1000 seconds.

Figure 17 Group 2 mixed with secondary ZDDP under different temperature. The arrows are pointing the time when COF reaches lowest value

4.2.5 Wear track under EM

In Figures 18 and 19, evidence of the existence of ZDDP tribofilm formation was found. The dots on the right side of the four pictures represent the element concentrated at that point. The dot

27 intensity represents the amount of element in that dot relatively compared to the surroundings which the intensity brighter of the dot, the more element concentration at that point. From the element mapping of concentration in Zinc, Phosphor has a clear trace on the wear track. Oxygen This represents the chemical reactions that occurred during the pure sliding process in which a ZDDP tribofilm formation is confirmed under these two cases. Oxygen is also having a clear trace on element mapping. However, the oxygen is not only contributing to tribofilm formation reaction but also contributes to the oxidization on the material and it will become intense at a high temperature which temperature will increase dramatically under sliding contact conditions. Thus, oxygen can not be the evidence for ZDDP tribofilm formation. In figure 18, the concentration of sulfur does not have a clear trace on the wear track. This result is expected that the sulfur-rich tribofilm only forms at the initial stage and been worn out through the sliding contact

Comparing the dot intensity between fig 18 and 19, the relative intensity between track wear and surrounding surfaces represents chemical reaction intensity. The zinc element mapping on 40N wear track in figure 19 is much brighter than it on 10N load, which the reactions occur more severely under higher load.

Figure 18 SEM and EDS image of Group 5 with secondary ZDDP under 10 N (1) SEM image (2) phosphor (3) oxygen (4) Sulfur (5) Zinc

Figure 19 SEM and EDS image of Group 5 with secondary ZDDP under 20 N (1) SEM image (2) phosphor (3) oxygen (4) Sulfur (5) Zinc

4.3 Primary alkyl Group ZDDP

4.3.1 General load effect

The load effect does not meet the increasing load with the decrease steady-state COF value in Fig 20. Overall, the group 2 based oils showed the best performance and lowest friction COF at all loads we tested. For group 1 and 3, we observed a decreased COF with increasing load. For Group 2, an opposite friction-load trend was observed. For group 4 and 5, a non-monotonic trend of friction with increasing load was observed. Comparing primary alkyl group and secondary alkyl group ZDDP, COF values are mostly lower on the primary alkyl group ZDDP than on secondary alkyl group, except for base oil group 4 in fig 21. Base oil group 4 was experiencing a high COF among other groups. The COF relates to the concentration of cycloalkanes that the more cycloalkanes the less COF will get under primary ZDDP additive.

COF in Different Load 0.125

0.12

0.115

0.11

COF 0.105

0.1

0.095

0.09 Group 1 Group 2 Group 3 Group 4 Group 5

10N 20N 40N

Figure 20 At 25 °C for 5 different base oil. Values are taken from average at COF flat trend last at least 400 seconds

In Figure 21, base oil group 1, 2, 3 and 4 show a clear COF trend changes between different loads. COF trend under 10N load in base oil group 1 shows an increasing trend and under 20N and 40N the COF shows flat and reduced trend. This phenomenon also appears in group 2 which COF trend increases under 10N and 20N, then appears a decreasing trend under 40N. In base oil group 3, the COF plot shows a relative flat trend under 10N and 20N, when reaches 40N load, the COF appears a decreasing trend. The normal stress brings down the activation energy barrier to initiate the reaction results in the COF decreasing changes. However, the base oil group 4 and 5 have different trend from other three groups. Base oil group 4 and 5 have all increasing COF trend under different loads.

Figure 21 Primary alkyl group ZDDP mixed with five base oil group under different load

Comparing the primary alkyl group and secondary alkyl group ZDDP, the overall COF trend appears more unstable trend on the primary alkyl group than secondary. Figure 22 shows the comparison with COF trend using base oil group 4 and 2 mixed with primary or secondary ZDDP additives. The experiments use base oil group 4 and 2 using primary ZDDP additive does not reaches a flat trend that a fully developed ZDDP tribofilm does not form. However, under the same

31 conditions using secondary ZDDP additive, COF trend appears to reach steady-state which a fully developed tribofilm is build up on the wear track. This case explains that the primary alkyl group ZDDP has a slower reaction than the secondary group which requires long periods to build up a fully developed tribofilm. The shape of COF trend is also different for group 2 and 4 for two type of alkyl group ZDDP in figure 22. In secondary alkyl group ZDDP, COF trend for group 4 decreases first, then increases to a peak, and finally decreases to the steady-state. However, when applied primary ZDDP additive, the COF trend only increases until reaches a steady state. The COF trend changes from applied normal load from 20N to 40N when applying base oil group 2. COF under 20N load decreases first then increases, and under 40N load COF trend increases first then decreases.

Figure 22 Comparison between secondary alkyl group and primary alkyl group ZDDP under different

The COF trend in figure 23 between 10N and 20N are same, that COF of group 2, 3 and 4 increases after initiating the sliding. Group 5 kept a similar COF trend under different and even using

32 different alkyl group ZDDP additive, which the COF trend kept decreasing until reach steady-state value. The rising pattern can be inferred as wear debris embedded into the wear track affect the friction or the abrasive wear changes the surface morphology increasing the friction. This rising pattern occurs in most base oil groups under the 10N and 20N loads except group 3. However, this trend did not continue in 40N group.

Figure 23 Primary alkyl group ZDDP under room temperature at 10N & 20N

In figure 24 shows all base oil group under 40N load, base oil group 1, 2, and 3 have different COF trends compare to the low load condition. They are appearing an increasing then decrease COF trend which represents the tribofilm formation. The situation is explained in high normal load lower the activation energy enough to trigger the chemical reaction to develop ZDDP tribofilm which 20N load did not provide enough energy to let tribofilm formation reacts completely. The base oil group 5 remains a similar COF trend which decreases to the lowest point then increases towards the end of the experiment. The COF trend changes from 20N to 40N under group 4 base oil. The COF reaches a relatively flat under 20N load but it kept increase when the load increased to 40N. The high load will exceed the ability which ZDDP tribofilm can surfer results in the increasing trend of COF.

Figure 24 Primary alkyl group ZDDP under room temperature at 20N & 40N

The primary alkyl group ZDDP mix with group 4 base oil creates a high COF value, unlike other lubricant samples. Base oil group 1 also has it pattern but the increasing amount is not significant as group 4. Group 2 and 3 have decreased COF values from secondary to primary alkyl group ZDDP. Base oil group 5 has a flat and smooth COF trend in secondary ZDDP, and it is different in primary ZDDP that after reach the lowest point then increases towards the end of the experiment. There is no obvious trend for paraffin structure to affect the primary alkyl group ZDDP. However, the amount of ring structures like cycloalkanes, benzothiophene, and carbazoles dibenzothiophenes lower the COF value.

4.3.2 Temperature variation

The primary alkyl group ZDDP meets the expectation that increases temperature with the decrease COF value in figure 25. None heated experiment was done under room temperature which only operated 30 min duration. The exception of COF trend in the temperature test is that the higher temperature the lower COD will be which high the temperature will decrease the viscosity of the lubricant sample results in the COF decreases. The results in figure 25 show that primary ZDDP meets the expectation. The overall COF trend for 50°C, 80°C, and 120°C also have many fluctuations which are similar under room temperature conditions which increases with the experiment goes. There is a phenomenon shown that the increasing temperature will delay the time to reach the lowest COF point. Under 50°C experiment, COF takes about 400 seconds to reach the lowest value where under 80°C and 120°C take 900 seconds and 1500 seconds. The increasing temperature should accelerate the tribofilm formations and the extended time to reach the lowest value can be assumed that the primary alkyl group ZDDP has poor friction reduction performance that the faster reaction rate will delay the time for COF to reach the lowest value.

Comparing results for primary alkyl group ZDDP to secondary alkyl group ZDDP, the COF trend in primary ZDDP increases with the sliding proceeds where the secondary ZDDP has a relatively flat COF trend. The increasing COF after reaching the lowest COF value can be explanted by the

34 wear debris from substate material and tribofilm patches increase the COF value. Primary ZDDP has different COF values with various temperatures to secondary ZDDP. COF using primary ZDDP decreases with the increasing temperature, however, the COF using secondary ZDDP increases with increasing temperature. The both two types of ZDDP show that COF behavior differently at 80 °C , which matches the results that ZDDP tribofilm formation are altered 80 °C in Figure 5.

Figure 25 Group 2 mixed with primary ZDDP under different temperature

4.3.3 SEM & EDS image

The EM image for group 5 and 4 under 10N load were taken in Figures 26 and 27. Element distribution on element mapping expects that a clear trace of zinc, oxygen, and phosphors appears on the wear track. The element mapping in both figures shows the clear trace of oxygen and zinc concentrated on the wear track. The oxygen trace on element mapping can not be the confirmation of ZDDP tribofilm formation. The oxygen-involved chemical reaction is not only tribofilm formation but also oxidization which also appears in the sliding motion. The zinc element concentrated on the wear track proves there is a tribofilm formation that occurs on the wear track. Primary alkyl group ZDDP mixed with group 4 and 5 in Figures 26 and 27 has no clear trace of sulfur on the wear track. The sulfur element is only involved in the early stage of the tribofilm formation process. It is not expected to have a clear track of sulfur on the wear track in element

35 mapping if a fully tribofilm is formed. The phosphors distribution unlike the secondary ZDDP element mapping it does not appear a clear trace on the wear track. These results show that the ZDDP tribofilm reaction will initiate for all the experiment cases.

The intensity of concentrated dots in element mapping represents the amount of target element around that area than the other element. Figure 26 shows more tense dots in oxygen and zinc elements than in figure 27. This represents the different reaction rates on tribofilm formation. The experiment duration is 30 mins for all tests which the more intense zinc element mapping shows more zinc involved in the reaction for base oil group 5 than group 4.

Figure 26 Image of Group 5 with primary ZDDP under 10 N (1) SEM image (2) phosphor (3) oxygen (4) Sulfur (5) Zinc

Figure 27 Image of Group 4 with primary ZDDP under 10 N (1) SEM image (2) phosphor (3) oxygen (4) Sulfur (5) Zinc

4.3.4 Surface morphology

The wear track investigated in Figures 28, 29, and 30 were using primary alkyl group ZDDP with base oil group 5 under different loads. The results show the higher load will create a higher wear track width. The wear depth can be found in each figure left bottom plot which the higher load has deep average wear depth. In figure 28, 29, and 30 the material exists above the disc surfaces were observed surface morphology on the wear track. This phenomenon can be explained by abrasive wear with the defects in the ball causing the material been squeezed above disc surfaces or the formation of ZDDP tribofilm which often occurs under low load in AFM experiments. The evidence of tribofilm formation is not confirmed through analysis of surface morphology. In Figure 29, the wear track does not appear a relatively flat bottom which two conditions were assumed to cause this phenomenon. The first assumption is that there exists a machinery defect on the steel ball which creates a shaped edge on the contacting location. The second assumption is that large wear debris adhered to the contact area creates a deep wear scar.

By comparing the wear depth of different loads in Figures 28, 29, and 30, the wear depth does not show increasing load with increasing wear depth. The wear depth is calculated by averaging the relatively flat bottom of the wear track which is expected under abrasive wear. The wear depth under10 N load was measure from x=0.42 to 0.52mm in figure 28 which is roughly 250 nm wear depth, 20N load was measure from x=0.58 to 0.62mm in figure 29 which gives about 125 nm and 40N load was measure from x=0.44 to 0.52 mm in figure 30 less than 250 nm. These results show that there are anti-wear tribofilm was formed under 20N load experiment. Wear track width is another parameter to measure the wear which is increases with the load increase. Under 10N load, the wear track width is 21µm, and the width increase to 38 µm under 20N and 41µm under 40N.

The wear track width increases with the load increase. This data shows that ZDDP tribofilm anti- wear function is on reduction of the wear depth but not width.

Figure 28 Primary alkyl Group ZDDP with base oil group 5 under 10N load

Figure 29 Primary alkyl Group ZDDP with base oil group 5 under 20N load

Figure 30 Primary alkyl Group ZDDP with base oil group 5 under 40N load

Chapter 5: Conclusions

The effect of base oils on tribofilm formation was investigated using pin on disk experiments. The ZDDP tribofilm was characterized using SEM and EDX. The COF trend has an increase then decree trend to the lowest value then keep it for a dynamic equilibrium state. . The increase then decrease COF trend represents the tribofilm formation to prevent the direct contact between two surface results in wear protection and friction reduction. The tribofilm formation process constantly occurs as well as the wear, thus create dynamic equilibrium results in a steady-state COF value. The time to reach the lowest COF represents the reaction speed for ZDDP tribofilm develop which has also been confirmed in these experiments that secondary alky group ZDDP additive has faster reactions than the primary alkyl group ZDDP additive.

In conclusion, the ZDDP tribofilm was found on both primary and secondary alkyl group ZDDP additives with all base oil group under 40N load. Even the base oil group 4 mixed with primary ZDDP has an increasing COF trend in the entire experiment that is more likely to be COF trend under abrasive wear, the EDS element mapping shows the zinc involved in the chemical reaction which a tribofilm formation process was initiated. From the EDS element mapping image, there is no obverse trace of sulfur concentration on the wear track which indicates the fully developed ZDDP tribofilm formed in 30 mins experiments, even at room temperature. However, the fully developed ZDDP tribofilm under base oil group 4 mixed with primary ZDDP additives at 10N and 20N does not have a decreeing trend of COF, which makes the experiment does not strongly prove the existence of a fully developed ZDDP tribofilm. The other lubricant samples show a decreeing COF trend to the lowest value and kept for a period under certain load conditions and the element mapping shows the clear trace on the wear track has strongly proved the existence of a fully developed ZDDP tribofilm. The lowest COF correlate to the chemical structure in different base oil are for both ZDDP additives is cycloalkanes. The primary ZDDP behaves with the high COF value with the increasing concentration of cycloalkanes where the secondary ZDDP behaves low COF value with the increasing concentration of cycloalkanes. The effect of chemical structure has opposite behavior under two types of ZDDP additive.

The COF under most experiment increase after reaching a steady-state value. One reason for this is that the small amount of lubricant sample on the surface without filtering process leads to the wear debris back into contacting area causing the additional wear and increase the friction. A flood circulating lubricant apply method may help to reduce this situation in future experiments. The direct analyzing the properties of ZDDP tribofilm is using Focus iron beam SEM to obtain a cross- section of wear track and observe the structural layout from ZDDP additives under different chemical structures.

Appendix

MATLAB CODE function smoothing

%%A = xlsread('EXP7 E3 20N 15MM ccw.xlsx', 'A4:H180012'); %%% 721812 2hour %%% 180012 0.5hr

A = csvread('smoothREP C3 20N 7MM ccw.csv',4 ,0,[4,0,180000,7]); %%%300000 60c %%% for csv file %%% 2161806 6hour %%% 2161210 for -0.1min step %%% for csv file %%% 1441810 6hour %%% 2161210 for -0.1min step time = A(:,2); cof = A(:,6); %%cof = A(:,4); %% for 2020 winter data, coloumn of data set has been changed Cofs = cof; %%cofs is the smoothed cof b=size(time); np=b(1);

pta = 100; %%point to average ptp= 100; %% point to plot for i =1:np n=1; %% count for forward m=0; %% count for backward sum = 0; if i<=pta/2 %%for i from first 50 for n=1:i sum=sum+cof(n); end for m=1:(pta/2 -1) sum=sum+cof(i+m); end Cofs(i)=sum/(n+m); elseif i<=(np-pta/2) && i>pta/2 %%% for i from middle except 50 at initial and end for n=1:pta/2 sum=sum+cof(i-n); end for m=1:(pta/2 -1) sum=sum+cof(i+m); end Cofs(i)=sum/pta; elseif i>(np-pta/2) && i

end end plot(time,Cofs,'r') %%%% 5 'b' refer A G5 120 %%%% 4 'k' B G4 80 %%%% 3 'r' C G3 50 %%%% 2 'g' E G2 14 %%%% 1 'y' D G1

%%% 10N 'b' %%% 20N 'k' %%% 40N 'r' hold on axis([0 1800 0.08 0.12]) %{ xlabel('Time') ylabel('COF') title('ZDDP 1 10N') legend('Group 5','Group 3','Group 2','Group 1') axis([0 1800 0.08 0.12]) exp2 axis([0 1800 0.08 0.125]) %} end

Reference

[1] Jost, H. P. (2005). “Tribology micro & macro economics: A road to economic savings.” Tribology & Lubrication Technology, 61(10), 18.

[2] Neville, A., Haque, T., & Morina, A. (2008). “Friction and its importance in fuel economy— probing the nanoscale characteristics of surfaces in order to understand lubricant/surface interactions.” Journal of Physics: Condensed Matter, 20(35), 354019.

[3] Holmberg, K., Andersson, P., Nylund, N. O., Mäkelä, K., & Erdemir, A. (2014). “Global energy consumption due to friction in trucks and buses.” Tribology International, 78, 94-114.

[4] Andrew J. Gellman “Vapor lubricant transport in MEMS devices” Tribology Letters, Vol. 17 No. 3, 2004

[5] David C. Karmer, Brent K. Lok, Russ R. Krug “The Evolution of Base Oil Technology” Turbine Lubrication in the 21st Century ASTM STP #1407 Page 3,1 2001.

[6] James H. Gary, Glenn E. Handwerk, Mark J. Kaiser “Petroleum Refining: Technology and Economics, Fifth Edition”, Mar5, 2007.

[7] H. Spikes 2004 “The history and mechanisms of ZDDP”

[8] Yan Zhou, Donovan N.Lenard (2017) “Understanding Tribofilm Formation Mechanisms in Ionic Liquid Lubrication.” National

[9] Yasunori Shimizu, Hugh A. Spikes (2016) “The Influence of Slide-Roll Ratio on ZDDP Tribofilm Formation.” Springer

[10] Allyson M. Barnes *, Keith D. Bartle, Vincent R.A. Thibon “A Review of Zinc Dialkydithiophosphates (ZDDPS): Characterization and Role in the Lubricating Oil” Tribology International Vol.34,2001

[11] Mathias Woydt, Rolf Wasche“The history of the stribeck curve and ball bearing steels the role of adlof martens” Wear, vol. 268, 2010

[12] Bharat Bhushan “Contact mechanics of rough surface in tribology: multiple asperity contact” Tribology letter4 1998,1-35

[13] Mark Devlin (2017) “Effect of Metallurgy on the Formation of Tribofilm”

[14] Paaulina Nowak, Karolina Kucharska Marian Kaminski “Ecological and Health Effects of lubricant Oils Emitted into the Environment” International Journal of Environmental Research and Public Health, Vol 16, Aug 16, 2019

[15] Lili Yan, Wen Yue, Chengbiao Wang, Danping Wei, Bo Xu,”Comparing Tribological Behaviors of Sulfur- and Phosphorus-free Organomolybdenum additive with ZDDP and MoDTC” Tribology International Vol.53, 2012

[16] Joanna Dawczyk, Neal Morgan, Koe Russo, Hugh Spikes “Film Thickness and Friction of ZDDP Tribofilms” Tribology letter 2019

[17] Ksenija Topolovec-Miklozic, T. Reg Forbus and Hugh A. Spikes “Film Thickness and Roughness of ZDDP antiwear films” tribology Letter Vol.26 2007

[18] A. Morina, A. Neville (2005) “ZDDP and MoDTC interaction in boundary lubrication-The effect of temperature and ZDDP/MoDTC ratio.” Tribology International 39

[19] Kosuke Ito, JeanMichel Martin (2006) “Low-Friction tribofilm formed by reacting of ZDDP on iron oxide.” Tribology International 39

[20] Z.Zhang, E.S. Yamaguchi, M. Kasrai, G.M. Bancroft (2004) “Tribofilm generated from ZDDP and DDP on steel surface Part I : Growth wear and morphology.” Tribology Letters

[21] Z.Zhang, E.S. Yamaguchi, M. Kasrai, G.M. Bancroft (2004) Tribofilm generated from ZDDP and DDP on steel surface Part II : Chemistry. Tribology Letters

[22] Ramoun Mourhatch, Pranesh B. Aswath (2011) Tribological behavior and nature of tribofilms generated from fluorinated ZDDP in comparison to ZDDP under extreme pressure conditions Part I: Structure and Chemistry of tribofilms. Tribology International 44

[23] Ramoun Mourhatch, Pranesh B. Aswath (2011) Tribological behavior and nature of tribofilms generated from fluorinated ZDDP in comparison to ZDDP under extreme pressure conditions Part II: Morphology and nanoscale properties of tribofilms. Tribology International44

[24] Hikaru Okubo, Seiya Watanabe, Chiharu Tadokoro, Shinya Sasaki, “Effects of structure of zinc dialkyldithiophosphates on tribological properties of tetrahedral amorphous carbon film under boundary lubrication” Tribology International vol. 98 2016

[25] Yasunori Shimizu, Hugh A. Spikes (2016) “The Tribofilm formation of ZDDP under reciprocating pure sliding conditions.” Tribology Letter 2016

[26] C. Minfary J.M. Martin, C. Esnouf, T.Lemogne, R. Kersting, B. Hagenhoff “A multi- technique approach of tribofilm characterization” Thin Solod Films Vol. 447-448, Jan. 2004 Page 272-277

[27] J-F. Antoine C.Visa C.Sauvey G.Abba”Approximate analytical model for hertzian elliptical contact problems” Journal of Tribology

[28] A. Erdemis “Review of Engineered Tribological Interfaces for Improved Boundary Lubrication” Tribology International, Vol.28 2005

[29] K.H.Zum Gahr R.Wahl, K.Wauthier “Experimental study of the effect of microtexturing on oil lubrication ceramic/ steel friction pairs” Wear Vol. 267,Jun 2009