Tribochemical Interactions Between Graphene and ZDDP in Friction Tests for Uncoated and W-DLC-Coated HS6-5-2C Steel

materials

Article Tribochemical Interactions between Graphene and ZDDP in Friction Tests for Uncoated and W-DLC-Coated HS6-5-2C Steel

Joanna Kowalczyk 1,*, Monika Madej 1 , Wojciech Dzi˛egielewski 2, Andrzej Kulczycki 2 , Magdalena Z˙ ółty 3 and Dariusz Ozimina 1

1 Faculty of Mechatronics and Mechanical Engineering, Kielce University of Technology, 25-614 Kielce, Poland; [email protected] (M.M.); [email protected] (D.O.) 2 Air Force Institute of Technology, 01-494 Warsaw, Poland; [email protected] (W.D.); [email protected] (A.K.) 3 Oil and Gas Institute-National Research Institute, 31-503 Cracow, Poland; [email protected] * Correspondence: [email protected]

Abstract: If a lubricant contains structures capable of conducting energy, reactions involving zinc dialkyldithiophosphate (ZDDP) may take place both very close to and away from the solid surfaces, with this indicating that ZDDP can be a highly effective anti-wear (AW) additive. The central thesis of this article is that the tribocatalytic effect is observed only when the energy emitted by the solids is transmitted by ordered molecular structures present in the lubricant, e.g., graphene. The friction tests were carried out for 100Cr6 steel balls in a sliding contact with uncoated or W-DLC-coated HS6-5-2C steel discs in the presence of polyalphaoleﬁn 8 (PAO 8) as the lubricant, which was enhanced with graphene and/or ZDDP. There is sufﬁcient evidence of the interactions occurring between ZDDP

and graphene and their effects on the tribological performance of the system. It was also found that the higher the concentration of zinc in the wear area, the lower the wear. This was probably due

Citation: Kowalczyk, J.; Madej, M.; to the energy transfer resulting from the catalytic decomposition of ZDDP molecules. Graphene, Dzi˛egielewski,W.; Kulczycki, A.; playing the role of the catalyst, contributed to that energy transfer. Zółty,˙ M.; Ozimina, D. Tribochemical Interactions between Graphene and Keywords: graphene; diamond-like carbon; zinc dialkyldithiophosphate; lubricant additive; surface ZDDP in Friction Tests for Uncoated layers of solid elements and W-DLC-Coated HS6-5-2C Steel. Materials 2021, 14, 3529. https:// doi.org/10.3390/ma14133529 1. Introduction Academic Editor: Sergei Yu Tarasov Friction is a phenomenon with either negative or positive effects. When undesirable, it needs to be reduced. Researchers all over the world are looking for ways to achieve Received: 1 June 2021 this. One of the basic methods to handle this problem, especially in automotive and Accepted: 18 June 2021 Published: 24 June 2021 mechanical systems, is by applying lubricants [1]. Lubricants help reduce friction between two moving metal surfaces, but their effectiveness can be improved by properly selecting

Publisher’s Note: MDPI stays neutral the additives [1,2]. with regard to jurisdictional claims in One of the most common lubricant additives is zinc dialkyldithiophosphate, ZDDP, published maps and institutional affil- added, for instance, to engine oils. Originally, in 1940 [3], it was developed as an antioxidant iations. and detergent [4]; its other potential applications were suggested later [3]. In the literature, ZDDP is classified as an antiwear and anticorrosion additive [5]. The presence of ZDDP contributes to the formation of protective lubricating films [4]. In the case of ZDDP, however, the lubrication mechanism is quite complicated because there are three [6,7] interactions with the active elements [6–8], i.e., zinc, phosphorus and sulfur. Water and oxygen are Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. also active, but their presence increases the complexity of the lubrication mechanism. The This article is an open access article composition of the film forming on a solid surface is dependent on the operating conditions, distributed under the terms and type of contact between the moving parts, as well as the length of its aliphatic chains [6,7]. conditions of the Creative Commons Lubricant additives containing phosphorus and sulfur are reported to be effective as Attribution (CC BY) license (https:// extreme pressure and antiwear (EP–AW) additives. The proposed mechanism of opera- creativecommons.org/licenses/by/ tion involves tribofragmentation, during which an additive is split into highly reactive 4.0/). fragments, reacting with an uncoated (unprotected) metal surface [9].

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In the case of phosphorus- and sulfur-containing oils, the layer that forms on the solid surface is rich in phosphorus when the load is moderate; at higher loads it is rich in sulfur. Numerous studies confirm that ZDDPs are efficient as lubricant additives. Much progress has been made in the area of elemental analysis of the films formed. Martin et al. [10,11] and Bell et al. [12,13] describe the formation of a layer of vitreous polyphos- phorus about 500 nm in thickness in the presence of ZDDP, above which there is an approximately 100 nm thick organic layer. They also report the occurrence of iron sulfide between the phosphorus layer and the iron-based substrate, showing an adsorptive or antiwear function. The zinc compound is considered to be the best antiwear inhibitor of all metals studied [14]. Phosphorus has good EP and AW properties when added to base oils [5]. Compared with sulfur, phosphorus is a better antiwear agent [5,15]. There has been much national and international research into this additive [16]. Investigations by Luo et al. [17] concerning the tribological properties of an aqueous solution of nonylphenol polyoxyethylene ether phosphate ester (PPE) [17] show that it is possible to reduce the coefficient of friction to 0.15 and wear even up to 50%. Concentrations higher than 0.5% by weight guarantee that the lubricant is likely to satisfy both the lubricating and antiwear requirements. Ren et al. [18] consider the use of a P-N-type ionic liquid in synthetic ester (PETO) and water-based emulsion and they indicate that the tribological response of the materials in contact is better in the presence of oligomeric phosphorus. Wu et al. [19] analyze the performance of three different triazine compounds added to water-glycol fluids to act as water-soluble antiwear additives. The results show that the compounds provide higher resistance to extreme pressure and wear [16]. The tribological tests concerning the performance of polyalphaolefin 8 (PAO 8) with and without ZDDP and carbon nanotubes (CNTs) [20] show clear interactions between the two additives. The addition of ZDDP resulted in lower linear wear on uncoated steel discs but higher linear wear when these were coated with hydrogenated amorphous carbon (a-C:H). Amorphous carbon or diamond-like carbon (DLC) has properties similar to crystalline diamond, which vary depending on the bonds between carbon atoms [21]. DLC coatings can be classified according to a number of different criteria. When the presence of foreign elements in the structure is considered, DLC coatings fall into doped and undoped. The former can be subdivided into metal-doped and nonmetal-doped, with metals and non-metals including Ti, W, Mo, Cr, and Si, F, N, respectively [22]. The development of nanotechnology, which enabled the manufacture of advanced materials, led to the development of lubricants with better properties, which was achieved by introducing antiwear additives in the form of nanomaterials. Carbon nanomaterials have good lubricating properties, hence their common use as additives. Carbon-based nanomaterials include carbon nanotubes, fullerenes and graphene [1]. Nanotubes with their varied morphology can have different mechanical, electronic, chemical and magnetic properties [23]. Graphene is a two-dimensional carbon allotrope, characterized by good mechanical, electrical and thermal properties, which is why it is used in various applications, including engineering, chemistry and physics [24–26]. Additionally, carbon atoms in graphene bond easily, which makes it an effective additive, reducing friction in mechanical systems, e.g., automotive engines. Zhang et al. [27] used graphene oxides altered with oleic acid to create graphene nanosheets; they obtained a lubricant suitable for antiwear applications. Azman et al. [28], on the other hand, mixed graphene with PAO 10 synthetic oil (95 vol%) and palm oil based trimethylolpropane ester (15 vol% of TMP), which reduced the wear scar size by 15% [1]. Saurín et al. [29] experimented with 1–2- and 1–10-layer graphene dispersed in ionic liquid at a concentration of 0.1% by weight. The lubricant variants were tested in a system with polymer-steel or ceramic-steel contact surfaces. The ionic liquid containing 1–10-layer graphene was reported to reduce the coefficient of friction and the wear rate. Materials 2021, 14, 3529 3 of 16

Garcia et al. [30] compared the performance of different lubricants enhanced with graphene. The study was conducted for commercial PAO 6 and ‘green’ choline chloride (ChCl)-based deep eutectic solvents combined with urea, ethylene glycol (EG) or malic acid. A steel–steel contact was analyzed using a tribometer with a block-on-ring configu- ration. The lowest coefficient of friction was observed for ChCl-EG + graphene, which is biodegradable. The objective of this study was to determine whether graphene and ZDDP, added separately or together, would reduce the coefficient of friction and linear wear. It was a novelty to use graphene as an additive taking part in the tribocatalytic reaction with the solid surfaces and ZDDP as a friction modifier additive. The friction tests conducted at relatively low loads showed that, like other structured carbon forms, especially carbon nanotubes (CNTs), graphene enhanced the effectiveness of ZDDP as an antiwear additive. That, however, differed depending on the structure and properties of the materials in contact.

2. Experiment The tribological tests were carried out using a TRB3 tribometer. The ball-on-disc conﬁguration operated at a humidity of 37 ± 15% and an ambient temperature (T0) of 24 ± 4 ◦C. The test parameters were as follows: load 10 N, sliding speed 0.1 m/s and sliding distance 1000 m. Table1 shows the four lubricants used in the friction tests. It is important to mention that no dispersing agents were introduced to prevent the interaction between the additives.

Table 1. Lubricants subjected to tribological testing.

Lubricant Composition Symbol PAO 8 PAO 8 PAO 8 + 1.5% ZDDP PAO 8 + ZDDP PAO 8 + 0.005% graphene PAO 8 + graphene PAO 8 + 1.5% ZDDP + 0.005% graphene PAO 8 + ZDDP + graphene

The testing was performed using polyalphaolefin 8 (PAO 8) as the base oil (Table2). The hydrocarbon structure of the oil is a result of catalytic oligomerization of linear α-oleﬁns having from 8 to 12 carbon atoms in the chain. Table2 illustrates the basic properties of polyalphaoleﬁn 8.

Table 2. Properties of polyalphaoleﬁn 8 (PAO 8).

Property Value Speciﬁc gravity (at 15.6 ◦C) 833 kg/m3 Kinematic viscosity at a given temperature [◦C]: 40 46.4 mm2/s −40 19 570 mm2/s Viscosity Index 138

ZDDP was introduced to act as the friction modifier additive. The concentration of ZDDP used as the AW/EP additive in commercial applications (engine oils, hydraulic fluids or transmission fluids) remains at about 2 mmol/kg. From the authors’ earlier research as well as a review of the literature on the subject, e.g., [31] it is clear that dimers predominate at concentrations higher than approx. 15 mmol/kg, with this affecting the adsorption and chemisorption on the contact surfaces. As the investigations focused on the interaction between ZDDP and graphene in friction at a relatively low load, ZDDP was added to the different lubricants at appropriate concentrations so that dimers would be the prevalent form. Thus, ZDDP was introduced at a concentration of 1.5% (m/m), which corresponds to about 20 mmol/kg. Materials 2021, 14, 3529 4 of 16

The concentration selected for graphene was 0.005% (m/m); it was equal to that of carbon nanostructures studied by the authors earlier [20]. The lubricants were prepared by dispersing graphene and ZDDP ultrasonically. Table3 shows the properties of PAO 8 after ZDDP was added.

Table 3. The basic properties of zinc dialkyldithiophosphate (ZDDP).

Property Value Density (at a temperature of 25 ◦C) 1160 kg/m3 Kinematic viscosity (at a temperature of 40 ◦C) 150 mm2/s Content: Zn 9.0 wt% P 8.5 wt% S 16.5 wt%

Graphene ﬂakes with a very low oxidation level produced by Advanced Graphene Products were used in the tests. Table4 shows the main properties of graphene.

Table 4. The basic properties of graphene.

Property Value Oxygen content 1–2% Chemical composition–C 99.8%

HS6-5-2C high-speed steel with and without a W-DLC coating was used in the experi- ments. The composition of HS6-5-2C steel is given in Table5.

Table 5. Chemical composition of HS6-5-2C high-speed steel.

Element Percentage Composition C 0.82–0.92 W 6–7 Mo 4.5–5.5 Cr 3.5–4.5 V 1.7–2.1 Co ≥0.5 Si ≥0.5 Mn ≥0.4 Ni ≥0.4 Cu ≥0.3 P ≥0.03 S ≥0.03

To obtain high hardness of about 65 HRC, the steel can be subjected to austenitization at a temperature of 1150 ◦C, which is followed by tempering at 560 ◦C[32]. The system consisted of a ball made of 100Cr6 steel and a disc made of HS6-5-2C steel. Uncoated as well as W-DLC coated discs were used to compare the lubricant behavior under different friction conditions. Diamond-like carbon (DLC) coatings are very popular because they are characterized by good tribological and anticorrosion properties as well as high hardness [17,18]. The W-DLC coating was 3.5 µm in thickness. As shown in Figure1, the tribological tests involved measuring: the coefﬁcient of friction, linear wear and the wear scar surface area. Materials 2021, 14, x FOR PEER REVIEW 5 of 16

Materials 2021, 14, x FOR PEER REVIEW 5 of 16

Diamond-like carbon (DLC) coatings are very popular because they are characterized by goodDiamond-like tribological and carbon anticorrosion (DLC) coatings properties are very as well popular as high because hardness they [17,18].are characterized The W- DLC coating was 3.5 μm in thickness. As shown in Figure 1, the tribological tests involved Materials 2021, 14, 3529 by good tribological and anticorrosion properties as well as high hardness [17,18]. The5 of W- 16 measuring:DLC coating the wascoefficient 3.5 μm of in friction,thickness. linear As shown wear and in Figure the wear 1, the scar tribological surface area. tests involved measuring: the coefficient of friction, linear wear and the wear scar surface area.

Figure 1. Tribological measurements. FigureFigure 1.1.Tribological Tribological measurements. measurements. The values measured with two friction sensors mounted on the TRB3 tribometer were saved Theautomatically.The valuesvalues measuredmeasured The software with with two two then friction friction calculated sensors sensors the mounted mounted coefficient on on theof the friction TRB TRB33tribometer tribometer from the fric- were were tionsavedsaved force, automatically. automatically. dividing it Theby The the software software applied then then load, calculated calculated P = 10 theN. the Each coefﬁcient coefficient tribological of frictionof friction test from was from therepeated frictionthe fric- threeforce,tion times. force, dividing Thedividing itwear by theitscar by applied areathe applied was load, studied Pload, = 10 byP N. = means Each10 N. tribologicalofEach a Leica tribological DCM8 test was testconfocal repeated was repeatedmicro- three scopetimes.three operating times. The wear The in scarwearthe interferometry area scar was area studied was mode. studied by meansThe by linear means of a wear Leica of a was DCM8Leica measured DCM8 confocal confocal as microscopethe depth micro- tooperating scopewhich operating the in ball the had interferometry in theremoved interferometry the mode. material The mode. linearon theThe wear disc linear wassurface. wear measured was measured as the depth as the to which depth theto whichThe ball hadelemental the removed ball hadconstituents theremoved material ofthe onthe material thecontact disc on surface.surfaces the disc weresurface. determined at the start of each testTheThe and elementalelemental after it constituents wasconstituents completed of of the usingthe contact contact a Phenom surfaces surfaces XL were scanningwere determined determined electron at the microscope,at start the ofstart each of fittedtesteach andwith test after anand EDS itafter was detector it completed was forcompleted analyzing using ausing Phenomthe chemicala Phenom XL scanning composition XL scanning electron of materials.electron microscope, microscope, ﬁtted withfitted an with EDS an detector EDS detector for analyzing for analyzing the chemical the chemical composition composition of materials. of materials. 3. Results 3. Results 3.1.3. a-C:HResults Coating 3.1. a-C:H Coating 3.1.Figure a-C:H 2Coating presents the cross-section of the W-DLC coating and a thickness measurement ofFigure 3.5 μ2m. presents the cross-section of the W-DLC coating and a thickness measurement of 3.5Figureµm. 2 presents the cross-section of the W-DLC coating and a thickness measurement of 3.5 μm.

Figure 2. SEM microphotograph and results of the EDS line proﬁle analysis of the W-DLC coating. Figure 2. SEM microphotograph and results of the EDS line profile analysis of the W-DLC coating. 3.2.Figure Tribological 2. SEM microphotograph Tests and results of the EDS line profile analysis of the W-DLC coating. Figure3 illustrates the tribological properties of all the systems tested. The performance of PAO 8 and PAO 8 + ZDDP with and without the addition of graphene is also depicted [20].

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3.2. Tribological Tests

Materials 2021, 14, 3529 Figure 3 illustrates the tribological properties of all the systems tested.6 of 16The performance of PAO 8 and PAO 8 + ZDDP with and without the addition of graphene is also depicted [20].

(a) disc with a W-DLC coating

disc without a coating

(b) disc with a W-DLC coating

disc without a coating

Figure 3. The tribological parameters: (a) coefﬁcient of friction and (b) linear wear.

From the test results, it is clear that the lowest coefﬁcient of friction was reported for the coated discs lubricated with PAO 8 + ZDDP. The results concerning the uncoated discs indicate that the coefﬁcient of friction was lower when lubrication was provided by: Materials 2021, 14, x FOR PEER REVIEW 7 of 16

Materials 2021, 14, 3529 7 of 16

Figure 3. The tribological parameters: (a) coefficient of friction and (b) linear wear. Materials 2021, 14, x FOR PEER REVIEW 7 of 16

PAOFrom 8, PAOthe test 8 + results, ZDDP it or is PAO clear 8 that + ZDDP the lowest + graphene. coefficient The of highest friction value was reported of the coefficient for theof coated friction discs for lubricated both the with coated PAO and 8 + uncoated ZDDP. The discs results was concerning obtained inthe the uncoated presence discs of PAO Figure 3. Theindicate tribological8 + graphene. that parameters: the coefficient The linear (a) coefficient of wear, friction however, of wa frictions lower and reaches when (b) linear the lubrication lowestwear. value was provided for PAO by: 8 + PAO graphene 8, PAOlubricating 8 + ZDDP the or coated PAO discs8 + ZDDP and for + graphene. PAO 8 + ZDDPThe highest applied value on theof the uncoated coefficient discs. of The frictionFromhighest the for test valueboth results, the of linearcoated it is clear wear and thatuncoated was the observed lowest discs coefficient forwas the obtained coated of friction in discs the presence inwas the reported presence of PAO for of 8 PAO+ 8 the coatedgraphene.and discs for The the lubricated uncoatedlinear wear, with discs however,PAO lubricated 8 + ZDDP reaches by. The PAO the results lowest 8 + graphene. concerning value for PAOthe uncoated 8 + graphene discs lu- indicatebricating thatFigures the coefficientcoated4 and discs5 show of and friction the for resultsPAO was lower8 obtained+ ZDDP when applied with lubrication the on confocal the was uncoated provided microscope discs. by: The PAO operating high- in 8, PAOest thevalue 8 + interferometry ZDDP of linear or PAO wear mode. 8 was + ZDDP observed Figure + graphene.4 illustrates for the coatedThe the highest values discs invalue recorded the presenceof the prior coefficient of to PAO the tribological of8 and frictionfortests. the for uncoatedboth As canthe becoateddiscs seen, lubricated and W-DLC uncoated by coatings PAO disc 8s contributed +was graphene. obtained to in lower the presence peaks and of shallowerPAO 8 + valleys graphene.thanFigures The was linear 4 the and casewear, 5 show of however, uncoated the results reaches surfaces obtained the [20 lowest wi].th The the value isometric confocal for PAO microscope images 8 + graphene on theoperating left lu- show in the bricatingthesurface interferometry the coated topographies discs mode. and along Figurefor PAO the 4 illustrates8 height. + ZDDP The appliedthe roughness values on recordedthe profiles uncoated prior are discs. onto the the The tribological right; high- the x -axis est valuetests.depicts Asof linearcan the be wear length seen, was W-DLC of theobservedsample coatings for in thecontributed nm, coated and discs the to y-axislower in the peaks illustrates presence and ofshallower the PAO roughness 8 andvalleys height for thethanin uncoated wasµm. the discscase lubricatedof uncoated by surfaces PAO 8 + [20]. graphene. The isometric images on the left show the surfaceFigures Figuretopographies 4 and5 5 provides show along the theresults the measurement height. obtained The wiroughness valuesth the obtainedconfocal profiles microscope for are the on discsthe operatingright; and the balls xin-axis after the the depictsinterferometrytribological the length testsmode. of [ 20the Figure]. sample On the4 illustrates left,in nm, there and th aree the values the y-axis isometric recorded illustrates images prior the to and, roughness the on tribological the right,height there in are tests.μ m.Asthe can roughness be seen, profiles.W-DLC coatings contributed to lower peaks and shallower valleys than was theThe case surface of uncoated texture surfaces parameters [20]. obtained The isometric for the images discs andon the balls left before show and the after the (a) surfacetribological topographies tests, along shown the inheight. Table Th6, are:e ( broughness) Sa—arithmetic profiles mean are on height, the right; Sq—root the x-axis mean square depictsheight, the length Sp—maximum of the sample peak in nm, height, and Sv—maximumthe y-axis illustrates valley the height, roughness Sz—maximum height in height, µm µm μm. Ssk—skewness3 and Sku—kurtosis. 3 2 2

1 1 (a) 0 (b) 0 -1 -1

-2 -2 µm µm -3 -3 3 3 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 mm 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 mm 2 2 1 1

(c) 0 0 -1 µm -1 -2 20 -2 -3 10 -3 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 mm 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 mm 0 -10 (c) -20 0.0 0.2 0.4 0.6 0.8 1.0 mm µm 20 10 Figure 4. Surface topography:0 (a) the W-DLC coated HS6-5-2C steel disc, (b) the uncoated HS6-5-2C steel disc and (c) the -10 100Cr6 steel ball, before the -20tribological tests. 0.0 0.2 0.4 0.6 0.8 1.0 mm Figure 5 provides the measurement values obtained for the discs and balls after the FigureFigure 4. Surface 4. Surface topography: topography: (atribological) the W-DLC (a) the W-DLCtests coated [20]. HS6-5-2C coated On the HS6-5-2C steel left, disc, there steel (b )are disc,the theuncoated (b )isometric the uncoated HS6-5-2C images HS6-5-2C steel and, disc steelonand the ( discc) theright, and (therec) the 100Cr6100Cr6 steel ball, steel before ball, beforethe tribological theare tribologicalthe roughnesstests. tests. profiles.

(a) PAO 8 W-DLC Figure 5 provides the measurementPAO values8 obtained for the discs and balls after the tribologicalµm tests [20]. On the left, there are the isometric imagesµm and, on the right, there 3 3 are the2 roughness profiles. 2 1 1 0 0 (a) PAO 8 W-DLC -1 PAO 8 -1 -2 -2 µm µm -3 -3 3 0.0 0.5 1.0 1.5 mm 3 0.0 0.5 1.0 1.5 mm 2 2 1 Max. depth 1.46 μm 1 Max. wear depth 0.60 μm 0 Wear scar surface area 98.94 μm2 0 Wear scar surface area 186.9 μm2

-1 -1 -2 -2 -3 Figure 5. Cont. -3 0.0 0.5 1.0 1.5 mm 0.0 0.5 1.0 1.5 mm Max. depth 1.46 μm Max. wear depth 0.60 μm Wear scar surface area 98.94 μm2 Wear scar surface area 186.9 μm2

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µm µm 20 20 10 10 0 0 -10 -10 -20 -20 0.0 0.2 0.4 0.6 0.8 1.0 mm 0.0 0.2 0.4 0.6 0.8 1.0 mm (b) PAO 8 + ZDDP W-DLC PAO 8 + ZDDP [15] µm µm 3 3 2 2 1 1 0 0 -1 -1 -2 -2 -3 -3 0.0 0.5 1.0 1.5 mm 0.0 0.5 1.0 1.5 mm Max. depth 2.54 μm Max. depth 0.18 μm Wear scar surface area 232.4 μm2 Wear scar surface area 13.65 μm2

µm µm 20 20 10 10 0 0 -10 -10 -20 -20 0.0 0.2 0.4 0.6 0.8 1.0 mm 0.0 0.2 0.4 0.6 0.8 1.0 mm

(c) PAO 8 + graphene W-DLC PAO 8 + graphene µm µm 3 2 2 1 0 0 -1 -2 -2 -3 0.0 0.5 1.0 1.5 mm 0.0 0.5 1.0 1.5 mm Max. depth 1.04 μm Max. depth 1.01 μm Wear scar surface area 513.7 μm2 Wear scar surface area 649.3 μm2

µm µm 20 20 10 10 0 0 -10 -20 -10 0.0 0.2 0.4 0.6 0.8 1.0 mm -20 0.0 0.2 0.4 0.6 0.8 1.0 mm (d) PAO 8 + ZDDP + graphene W-DLC PAO 8 + ZDDP + graphene µm nm 3 3000 2 2000 1 1000 0 0 -1 -1000 -2 -2000 -3 -3000 0.0 0.5 1.0 1.5 mm 0.0 0.5 1.0 1.5 mm Max. depth 0.55 μm Max. depth 0.24 μm Wear scar surface area 166.8 μm2 Wear scar surface area 59.46 μm2

µm µm 20 20 10 10 0 0 -10 -10 -20 -20 0.0 0.2 0.4 0.6 0.8 1.0 mm 0.0 0.2 0.4 0.6 0.8 1.0 mm

FigureFigure 5. 5. IsometricIsometric views views and and primary primary profiles proﬁles of ofthe the discs discs and and balls balls after after the thetribological tribological tests: tests: (a) PAO (a) PAO 8, (b 8,) PAO (b) PAO 8 + 8 + ZDDP,ZDDP, ( (cc)) PAO PAO 8 8 + + graphene graphene and and (d ()d PAO) PAO 8 8+ +ZDDP ZDDP + graphene. + graphene.

The surface texture parameters obtained for the discs and balls before and after the tribological tests, shown in Table 6, are: Sa—arithmetic mean height, Sq—root mean

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Table 6. Surface texture parameters for (a) the uncoated steel discs and (b) the W-DLC -coated steel discs in contact with the steel balls before and after the tribological tests. square height, Sp—maximum peak height, Sv—maximum valley height, Sz—maximum height, Ssk—skewness and(a) Sku—kurtosis. Before Test; PAO 8 + PAO 8 + ZDDP + Table 6. Surface texture parameters for (a) the uncoatedPAO 8 steel discs PAO and 8 + (b ZDDP) the W-DLC -coated steel discs in contact with Surface Texture Uncoated Disc Graphene Graphene Parametersthe steel balls before and after the tribological tests. Disc Ball Disc Ball Disc Ball Disc Ball Disc Ball (a) Sa (µm) 0.40 0.14 0.44 0.64 0.39 2.09 0.28 6.56 0.32 7.93 Before Test; PAO 8 + Gra- PAO 8 + ZDDP + SurfaceSq (µm) Texture Pa- 0.49 0.40 0.56PAO 0.90 8 0.49 PAO 2.46 8 + ZDDP 0.36 7.58 0.41 9.17 µ Uncoated Disc phene Graphene Sp ( rametersm) 1.52 3.43 1.90 1.52 1.78 4.89 1.19 13.12 1.40 15.84 Sv (µm) 1.33Disc 23.62Ball 5.03 Disc 4.08 Ball 2.02 Disc 7.71 Ball 1.27 Disc 13.75 Ball 1.70Disc 17.25Ball Sz (µSam) (μm) 2.850.40 27.050.14 6.930.44 5.600.64 3.80 0.39 12.61 2.09 2.46 0.28 26.88 6.56 3.080.32 33.087.93 − − − − − − − − − − SskSq (-) (μm) 0.170.49 47.210.40 0.32 0.56 1.72 0.90 0.24 0.49 0.14 2.46 0.36 0.360.00 7.58 0.510.41 0.019.17 Sku (-) 2.39 2723 3.83 7.05 2.85 2.05 3.13 1.80 3.57 1.82 Sp (μm) 1.52 3.43 1.90 1.52 1.78 4.89 1.19 13.12 1.40 15.84 Sv (μm) 1.33 23.62 5.03 (b) 4.08 2.02 7.71 1.27 13.75 1.70 17.25 Sa (µSzm) (μm) 0.392.85 0.1427.05 0.646.93 0.365.60 0.56 3.80 0.48 12.61 0.47 2.46 0.26 26.88 0.383.08 0.1033.08 Sq (µm)Ssk (-) 0.50−0.17 0.40−47.21 0.82−0.32 0.47 −1.72 0.74 −0.24 0.59 −0.14 0.61 −0.36 0.33 −0.00 0.51−0.51 0.12−0.01 Sp (µm)Sku (-) 1.60 2.39 3.43 2723 2.87 3.83 3.95 7.05 6.47 2.85 1.60 2.05 1.68 3.13 3.65 1.80 2.08 3.57 0.16 1.82 (b) Sv (µm)Sa (μm) 2.11 0.39 23.62 0.14 3.70 0.64 5.320.36 4.090.56 1.780.48 2.990.47 1.10 0.26 2.650.38 0.390.10 Sq (μm) 0.50 0.40 0.82 0.47 0.74 0.59 0.61 0.33 0.51 0.12 µ Sz ( m)Sp (μm) 3.71 1.60 27.05 3.43 6.56 2.87 9.273.95 10.576.47 3.381.60 4.671.68 1.49 3.65 4.732.08 0.550.16 Ssk (-)Sv (μm) −0.39 2.11 −47.21 23.62 −0.70 3.70 0.87 5.32− 0.914.09 0.54 1.78− 0.29 2.99− 1.13 1.10 −0.712.65 −0.810.39 Sz (μm) 3.71 27.05 6.56 9.27 10.57 3.38 4.67 1.49 4.73 0.55 Sku (-)Ssk (-) 3.66−0.39 2723−47.21 3.54−0.70 5.500.87 5.07−0.91 2.61 0.54 3.85−0.29 3.65 −1.13 5.02−0.71 3.04−0.81 Sku (-) 3.66 2723 3.54 5.50 5.07 2.61 3.85 3.65 5.02 3.04

FigureFigure6 depicts 6 depicts wear wear surface surface areas areas on on the the discs discs and and balls balls after after friction friction under under lubri- lubri- catedcated conditions conditions observed observed with with a scanninga scanning electron electron microscope microscope as as well well as as EDS EDS elemental elemental analysisanalysis data data obtained obtained for for selected selected microareas. microareas The. The elemental elemental analysis analysis took tookinto intoaccount account thethe concentration concentration of of elements, elements, depending depending on on the the type type of of disc disc and and lubricant lubricant used. used. ForFor thethe DLC-coatedDLC-coated discs, discs, the the contents contents of suchof such elements elements as C,as W,C, CrW, andCr and Fewere Fe were studied, studied, while while for thefor uncoated the uncoated steel discs, steel thediscs, contents the contents of Fe, C,of CrFe, andC, Cr Si wereand Si measured. were measured. However, However, when lubricantwhen lubricant was enhanced was enhanced with ZDDP, with the ZDDP, elements the elements of interest of interest were Zn, were P and Zn, S. P and S.

PAO 8 W-DLC

Element C W Fe Element Fe C Cr Si wt.% 98.09 1.75 0.16 wt.% 92.31 5.13 1.50 1.06

Figure 6. Cont.

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PAO 8

Element Fe C Cr Si Element Fe C Cr Si wt. % 76.96 21.32 1.02 0.70 wt. % 90.37 7.78 1.01 0.84

PAO 8 + ZDDP W-DLC [20]

Element C W Zn Fe Cr Element Fe C S Si Cr Zn P wt.% 98.00 1.64 0.25 0.05 0.05 wt.% 54.41 42.07 1.01 0.96 0.69 0.51 0.34

PAO 8 + ZDDP

Element Fe C Cr Zn Si Element Fe C Cr Zn S Si P wt.% 87.95 9.68 1.59 0.40 0.39 wt.% 91.83 4.67 1.27 1.14 0.49 0.46 0.14

PAO 8 + graphene W-DLC

Element C Si Fe Element Fe C Cr Si wt.% 98.81 0.93 0.25 wt.% 92.72 5.56 1.28 0.44

Figure 6. Cont.

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PAO 8 + graphene

Element Fe C Cr Si Mn Ni Element Fe C Cr Si Cu Mn wt.%. 89.41 8.50 1.12 0.54 0.24 0.20 wt.% 91.98 5.92 1.17 0.44 0.36 0.13

PAO 8 + ZDDP + graphene W-DLC

Element C W Fe Zn Cr Element Fe C Cr Si Zn P S Mo wt.%. 97.03 2.04 0.52 0.35 0.05 wt.% 91.51 5.23 1.51 0.66 0.66 0.21 0.15 0.08

PAO 8 + ZDDP + graphene

Element Fe C Cr Si Zn P Element Fe C Cr Si Zn S P wt.% 94.01 3.58 1.23 0.74 0.41 0.02 wt.% 91.94 4.83 1.57 0.67 0.50 0.33 0.16

FigureFigure 6. 6.SEM SEM images: images: surfacesurface morphologiesmorphologies and characteristic spectra spectra (EDS) (EDS) in in the the selected selected microareas microareas along along the the wear wear tracks on the discs and balls after fiction contact. tracks on the discs and balls after ﬁction contact.

4.4. Discussion TheThe experimental data data shows shows that that the the tribol tribologicalogical parameters parameters were were affected affected by by the the lubricantlubricant additives. The The presence presence or or absence absence of of a acoating coating on on the the disc disc surface surface proved proved to tobe be significant, too. signiﬁcant, too. From the chemical analysis of the W-DLC -coated disc, it is clear that the outer layer From the chemical analysis of the W-DLC -coated disc, it is clear that the outer layer consists of carbon (Figure 2). The interlayer, however, is made up of two sublayers: tung- consists of carbon (Figure2). The interlayer, however, is made up of two sublayers: tungsten sten and chromium. The interlayer improves the adhesion of the coating to the substrate; and chromium. The interlayer improves the adhesion of the coating to the substrate; it it modifies the transfer of energy from the solid material to the lubricant layer. There was modiﬁes the transfer of energy from the solid material to the lubricant layer. There was interaction between W and carbon from W-DLC with formation of WC carbides. Through interaction between W and carbon from W-DLC with formation of WC carbides. Through this tungsten diffuse to the surface [33]. this tungsten diffuse to the surface [33]. The data in Figure 3a,b suggest that: The data in Figure3a,b suggest that: (a) uncoated HS6-5-2C steel

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• Coefficient of friction: For PAO + graphene, the coefficient was much higher than that obtained for the other lubricant variants. • Wear scar size: When ZDDP was added to PAO, the wear scar area and wear depth on the disc were greater. Similar observations were made for PAO + graphene. Lubricant containing both additives, i.e., ZDDP and graphene, caused wear much smaller in size, yet still its value was higher than that reported for PAO with no additives. • Linear wear: The addition of ZDDP to PAO resulted in lower values of linear wear. For PAO + graphene, they were even lower. The wear measured for PAO + ZDDP + graphene was similar to that obtained for PAO + ZDDP. (b) W-DLC -coated HS6-5-2C steel • Coefficient of friction: There were hardly any differences in the values between the mixtures tested. • Wear scar size: The addition of ZDDP to PAO was responsible for a considerable decrease in the wear scar area and the wear depth on the disc. The values obtained for PAO + graphene were higher than those reported for PAO. Adding both ZDDP and graphene caused a significant decrease in the wear scar size; it was much smaller than that measured when PAO with no additives was used. • Linear wear: The tests carried out for PAO + ZDDP show that there was a substantial decline in linear wear. For the other lubricant variants, the values of this parameter were approximately the same. From the above observations, it is evident that the W-DLC coating alters the behavior of the tribological system operating at low loads. Obviously, there is a strong relationship between the composition of the lubricant tested and the wear of the steel elements in contact, as the addition of ZDDP reduces wear. However, for a disc coated with DLC, the experimental results were different: ZDDP appeared to promote wear. A new finding is that the wear was very high on a disc coated with W-DLC as well as on an uncoated disc when the lubricant was PAO 8 + graphene. Similar observations were made for CNTs [20], but the effect was much smaller than for graphene. It can be concluded that both graphene and ZDDP contribute to the worsening of the surface properties of the W-DLC-coated steel disc and the uncoated steel ball, and consequently to their more intensive wear. This is particularly surprising for PAO 8 + graphene. Graphene can be seen as abrasive responsible for significant wear of the elements in contact. However, unlike ZDDP, it is not a chemically active substance reacting with the solid surface. Thus, the mechanism of intensive wear must be different. It is known that at low loads, friction modifier additives, particularly those becoming active when in contact with such materials as 100Cr6 steel (e.g., ZDDP), may react with the solid surface to weaken it, which will lead to higher wear. This mechanism does not apply when the disc is coated with W-DLC or when PAO 8 + graphene is used. As indicated in [20], ordered carbon structures (CNTs, graphene) may be effective to remove the energy from the solid surface. This energy transfer may affect the bonding in the surface layer, which will cause the weakening of the surface layer, making it more susceptible to wear. This mechanism seems likely for the test conditions described here. Additionally, there is a clear influence of graphene on the kinetics of the tribochemical reactions involving ZDDP. The differences in the concentrations of Zn, P and S accumulated on the contact surfaces (ball and disc) indicate that graphene modifies the reaction process. This effect is dependent on the structure of the surface layer of the disc being in sliding contact with the ball. For a disc coated with W-DLC, graphene catalyzes the molecular changes in ZDDP (higher concentration of Zn on both contact surfaces). In the case of a steel disc, graphene inhibits the reaction process (lower concentration of Zn on the disc surfaces and unchanged concentration of Zn on the ball surface). The analysis of the surface topographies after the friction tests reveals that for the different lubricant variants, the surface texture of the disc is less varied (lower values of Sp, Sv and Sz) when the surface texture of the ball is more varied (higher lower values of Sp, Materials 2021, 14, x FOR PEER REVIEW 13 of 16 Materials 2021, 14, x FOR PEER REVIEW 13 of 16

The analysis of the surface topographies after the friction tests reveals that for the Materials 2021, 14, 3529 The analysis of the surface topographies after the friction tests reveals that 13for of the 16 different lubricant variants, the surface texture of the disc is less varied (lower values of different lubricant variants, the surface texture of the disc is less varied (lower values of Sp, Sv and Sz) when the surface texture of the ball is more varied (higher lower values of Sp, Sv and Sz) when the surface texture of the ball is more varied (higher lower values of Sp, Sv and Sz). The variability of results was greater for a system with an uncoated disc, Sp, Sv and Sz). The variability of results was greater for a system with an uncoated disc, whichSv and may Sz). Thesuggest variability that the of resultstransfer was of greaterenergy for from a system the solids with anin uncoatedcontact and, disc, conse- which which may suggest that the transfer of energy from the solids in contact and, consequently,may suggest the degradation that the transfer of their of energysurfaces from cannot the be solids fully in prevented. contact and, consequently, the quently, the degradation of their surfaces cannot be fully prevented. degradationFigure 7 ofillustrates their surfaces the wear cannot rate be of fully discs prevented. after tribological tests. The experimental Figure 7 illustrates the wear rate of discs after tribological tests. The experimental data showFigure that7 illustrates the lower the wear wear rate rate was of discs observed after tribological for uncoated tests. disc The when experimental the lubricant data data show that the lower wear rate was observed for uncoated disc when the lubricant wasshow PAO that 8 the + ZDDP. lower wear The ratehigher was wear observed rate forwas uncoated noticed discfor coated when thedisc lubricant after lubrication was PAO was PAO 8 + ZDDP. The higher wear rate was noticed for coated disc after lubrication with8 + ZDDP. the PAO The 8 higher + graphene. wear rate For was all cases, noticed th fore lower coated wear disc rate after was lubrication observed with for thethe PAOun- with the PAO 8 + graphene. For all cases, the lower wear rate was observed for the un- coated8 + graphene. discs. After For all lubrication cases, the lowerPAO 8 wear + graphene rate was the observed wear rate for thewas uncoated the higher discs. for After uncoated discs. After lubrication PAO 8 + graphene the wear rate was the higher for un- coatedlubrication and coated PAO 8 +disc. graphene After adding the wear the rate ZDDP was to the PAO higher 8 + for graphene uncoated the and wear coated rate disc.has coated and coated disc. After adding the ZDDP to PAO 8 + graphene the wear rate has decreasedAfter adding drastically. the ZDDP For to the PAO disc 8 + with graphene the a-C:H the wear coating rate hasthe decreasedlower wear drastically. rate was ob- For decreased drastically. For the disc with the a-C:H coating the lower wear rate was ob- servedthe disc after with lubrication the a-C:H coatingPAO 8 + the ZDDP lower + weargraphene rate wasthan observed after lubrication after lubrication PAO 8 + PAOZDDP. 8 + served after lubrication PAO 8 + ZDDP + graphene than after lubrication PAO 8 + ZDDP. InZDDP this case, + graphene graphene than reduced after lubrication the wear. PAO 8 + ZDDP. In this case, graphene reduced theIn this wear. case, graphene reduced the wear.

0.00005 0.00005 0.000045 0.000045 0.00004 /N·m

3 0.00004 /N·m

3 0.000035 0.000035 0.00003 0.00003 0.000025 0.000025 0.00002 0.00002 0.000015 0.000015 0.00001

Wear rate, mm Wear rate, 0.00001

Wear rate, mm Wear rate, 0.000005 0.000005 0 0 PAO 8 PAO 8 PAO 8 + PAO 8 + PAO 8 + PAO 8 + PAO 8 + PAO 8 + PAO 8 PAO 8 PAO 8 + PAO 8 + PAO 8 + PAO 8 + PAO 8 + PAO 8 + W-DLC ZDDP ZDDP graphene graphene graphene + graphene + W-DLC ZDDP ZDDP graphene graphene graphene + graphene + W-DLC W-DLC ZDDP ZDDP W-DLC W-DLC ZDDP ZDDP W-DLC W-DLC

Figure 7. Wear rate of discs after tribological tests. FigureFigure 7. 7.Wear Wear rate rate of of discs discs after after tribological tribological tests. tests. From the data depicted in Figure 8, which correspond to those presented in Figure FromFrom the the data data depicted depicted in in Figure Figure8, which8, which correspond correspond to those to those presented presented in Figure in Figure3b, 3b, it is apparent that, for W-DLC -coated discs, the higher the concentration of Zn accu- it3b, is apparentit is apparent that, forthat, W-DLC for W-DLC -coated -coated discs, thedisc highers, the thehigher concentration the concentration of Zn accumulated of Zn accumulated on the ball surface, the higher the linear wear on the disc surface. Additionally, onmulated the ball on surface, the ball the surface, higher the linearhigher wear the lin onear the wear disc surface.on the disc Additionally, surface. Additionally, conversely, conversely, if the concentration of Zn accumulated on the ball surface increases, there is a ifconversely, the concentration if the concentration of Zn accumulated of Zn accumulat on the balled surface on the increases,ball surface there increases, is a decrease there is in a decreasethe linear in wear the linear on the wear disc. on the disc. decrease in the linear wear on the disc.

Figure 8. Concentrations of elements in the wear scar surface area, depending on the type of lubri- FigureFigure 8. 8.Concentrations Concentrations of of elements elements in in the the wear wear scar scar surface surface area, area, depending depending on theon typethe type of lubricant of lubricant and the type of disc used. andcant the and type the of type disc of used. disc used.

It is difﬁcult, however, to relate the concentration of accumulated Zn to the size of wear scars. Generally, when the disc was coated with W-DLC, the concentration of accumulated

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Zn was lower than that observed on an uncoated disc. Similar results were obtained for CNTs, as described in [20]. There is a clear difference in the concentrations of Zn, P and S accumulated on both contact surfaces between the systems studied, which resulted from the composition of the lubricant; the addition of graphene to PAO, separately or together with ZDDP, affected the system performance. It was also found that the wear scar size was smaller at higher concentrations of elements occurring after friction, i.e., Zn, S and P, when the lubricant contained ZDDP. This was probably due to the transfer of energy. The analysis of the interactions of ZDDP and graphene shows that graphene modiﬁes the tribological reactions of ZDDP. ZDDP used as the antiwear additive undergoes physical adsorption on the lubricated surface as well as tribochemical reactions leading to the separation of Zn from the molecules containing P and S. According to the literature, the products of the ZDDP decomposition modify the surface structure of the materials in contact, preventing their wear. When tested tribologically, the additive (ZDDP) contributed to smaller wear scar sizes in a system with uncoated discs. For DLC-coated discs, the linear wear was almost always higher (Figure4b) than that observed for uncoated discs. A considerable increase in linear wear was reported in the presence of PAO 8 + ZDDP + graphene when W-DLC -coated discs were employed. Different results were obtained for steel discs without a W-DLC coating, when the use of ZDDP led to a reduction in linear wear.

5. Conclusions The results of this study show that: • The addition of ZDDP to the lubricant tested resulted in lower linear wear when the steel discs were uncoated. • The use of ZDDP resulted in smaller wear scars when the disc had a W-DLC coating. • Graphene added to the lubricant causes more intensive transport of energy from the solid to the system. This could be the reason why for PAO + graphene the wear was very high. In the presence of both additives, part of this energy reaches ZDDP molecules, which undergo triboreactions faster than when ZDDP is the only additive (higher concentrations of Zn, P and S). The tribological phenomena are greatly dependent on the surface properties of the solid elements in contact. The results obtained for W-DLC-coated steel discs differed from those reported for uncoated discs. It is thus evident that graphene and ZDDP interact, and the mechanism of this interaction is likely to be as follows. Graphene increases the transfer of energy from the solid surface to the molecules inside the lubricating ﬁlm. Part of this energy is supplied to ZDDP molecules, initiating reactions leading to their decomposition. The rate of triboreactions involving ZDDP decreases, and so does the friction and wear of the contact surfaces (ball and disc). The above hypothesis is consistent with the hypothesis presented in [20] but both should still be veriﬁed through further research.

Author Contributions: Conceptualization, D.O. and A.K.; methodology, A.K., M.M.; formal analysis, A.K., W.D.; writing—original draft preparation, A.K, J.K.; writing—review and editing, A.K., D.O. and M.Z.;˙ supervision, A.K.; funding acquisition, D.O. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Materials 2021, 14, 3529 15 of 16

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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