lubricants

Article The Influence of a Ring Coating on the Wear and Friction Generated during Linear Oscillation

Arthur Rozario 1,*, Christoph Baumann 2 and Raj Shah 3,*

1 Electrical and Computer Engineering Department, Stony Brook University, Stony Brook, NY 11794, USA 2 Optimol Instruments Prüftechnik GmbH, Westendstraße 125, 80339 München, Germany; [email protected] 3 Koehler Instrument Company, Inc., 85 Corporate Drive, Holtsville, NY 11742, USA * Correspondence: [email protected] (A.R.); [email protected] (R.S.); Tel.: +1-631-589-3800 (R.S.)  Received: 24 June 2018; Accepted: 10 January 2019; Published: 14 January 2019 

Abstract: The piston group is responsible for contributing to ~50% of the frictional losses of an , which ultimately leads to the waste of fuel. This coupled with the fact that is a finite resource linked to CO2-emissions, there is an increased demand of higher performance , which coincidently further loads the . As of yet, there are plenty of studies that already study the piston ring’s contact with the liner. However, this study focuses on a cost-effective Schwing, Reib, Verschleiss (SRV) instrumentation that allows to pre-screen lubricants, additives, materials and coatings for their friction, wear and load carrying capacity including scuffing resistance. As a result of the pre-screening conducted outside of engine by using the SRV instrument, it brings us to the following conclusion: the PVD CrN-TiN 1o Group coating on the piston ring produces the least wear, as well as the lowest coefficient of friction. Moreover, it is concluded that a coating that is based from CrN and TiN allows the piston ring to perform better in engine settings. A continued understanding of the piston-cylinder-contact assembly only helps engineers, scientists and any other stakeholder to improve on the piston ring and cylinder liner interaction.

Keywords: internal combustion engine; compression rings; viscosity

1. Introduction The purpose of piston rings (PR) is to provide a sealing between the gap of the piston and the cylinder wall, whilst distributing and controlling the oil and transferring the heat along with stabilizing the piston. This is a departure from earlier days, where the sole purpose of the PR was just to seal off the . The PR in the internal combustion engine (ICE) is accountable for a significant amount of friction loss and the optimization of this tribosystem is very important to increase the overall fuel economy of the engine [1]. For example, frictional losses of the piston-cylinder-contact (PCC, where the PR is in contact with the cylinder liner) in heavy duty diesel of trucks can contribute to the overall 4–15% of fuel consumption. As a result, billions of liters of fuel are lost to just overcoming the friction in these vehicles [2]. Significant studies have been conducted in this area, such as R. Rahmani et al. explaining that the internal combustion engine will continue as the main ‘source of vehicular propulsion for the foreseeable future.’ The authors present a study that takes an integrated approach to the frictional losses, as well as discuss the emission losses of the piston-cylinder system. The conclusions of this experiment state that the cylinder liner temperature is critical in mitigating power loss, in addition to reducing Hydrocarbon and Nitrogen oxide emissions from the compression ring [3]. It is important to note that the compression ring is located nearest to the piston head, which is the top of the piston [4].

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ThisThis paperpaper offersoffers aa novelnovel wayway toto investigateinvestigate thethe wearwear andand frictionfriction generatedgenerated byby thethe PRPR oscillatingoscillating againstagainst thethe cylinder cylinder liner liner using using the the SRV SRV tester. tester. This This is otherwise is otherwise known known as SRV, as SRV, which which translated translated from Germanfrom German means means oscillation, oscillation, friction friction and wear. andThe wear. SRV The instrument SRV instrument can be used can tobeclosely used to simulate closely specificsimulate operating specific operating conditions conditions relevant for relevant the functioning for the functioning of the PR inof thethe ICE.PR in the ICE. TheThe primaryprimary investigationinvestigation ofof thisthis paperpaper is:is: HowHow differentdifferent pistonpiston ringring coatingscoatings influenceinfluence thethe wearwear andand thethe frictionfriction generatedgenerated fromfrom thethe slidingsliding motionmotion ofof thethe PCC.PCC. ToTo thethe authors’authors’ knowledge,knowledge, therethere areare studiesstudies thatthat dodo considerconsider PRPR coatingscoatings andand thethe effecteffect ofof itit onon thethe PCCPCC assemblyassembly butbut itit isis thethe noveltynovelty ofof howhow thisthis investigationinvestigation waswas conductedconducted thatthat presentspresents aa realistic,realistic, cost-efficientcost-efficient wayway ofof studyingstudying thethe PCCPCC movement.movement. Ultimately, itit isis thethe goalgoal ofof suchsuch screeningscreening toto suggestsuggest possiblepossible coatingscoatings ofof thethe PRPR inin thethe PCCPCC assembly,assembly, so so thatthat thethe PRPR cancan bebe possiblypossibly furtherfurther optimized.optimized. By choosing this parameter of study, itit offersoffers aa viableviable wayway thethe PRPR cancan bebe advancedadvanced withoutwithout aa needneed forfor aa largelarge changechange inin thethe currentcurrent settingssettings ofof thethe PCCPCC assemblyassembly inin thethe engine,engine, makingmaking itit aa feasiblefeasible optionoption toto bebe consideredconsidered forfor moremore studies.studies. ThisThis approachapproach isis alsoalso suitedsuited forfor developmentdevelopment ofof alternativealternative andand lowlow viscosity viscosity engine engine oils. oils.

2.2. ModelingModeling thethe FrictionFriction ofof thethe Piston-Cylinder-ContactPiston-Cylinder-Contact duringduring EngineEngine OperationOperation ◦ DuringDuring operationoperation cyclecycle ofof 720720° inin thethe ICE,ICE, thethe PRPR passespasses allall lubricationlubrication modesmodes onon thethe cylindercylinder linerliner andand willwill generategenerate differentdifferent levelslevels ofof frictionfriction [5[5].]. PistonPiston coatingscoatings havehave thethe potentialpotential toto reducereduce frictionfriction and/orand/or wear of the piston’s sliding components [[6]6] but alsoalso toto preventprevent scuffing.scuffing. ToTo measuremeasure reductionreduction inin wearwear andand frictionfriction ofof thethe PCC,PCC, twotwo variablesvariables areare considered:considered: LossLoss ofof massmass ofof thethe PRPR andand coefficientcoefficient ofof frictionfriction (COF)(COF) of the PCC as a function of of time. time. The The following following explanations explanations provide provide a abasis basis of of why why both both these these parameters parameters are are valid valid to to observe. observe. FigureFigure1 illustrates1 illustrates the the surface surface of contact of contact between between the PR the and PR the and cylinder the cylinder liner. Over liner. the distance:Over the ∆x = x − x the PCC can be assumed to experience mixed friction, due to the combination of pure distance:2 Δx1 = x2 − x1 the PCC can be assumed to experience mixed friction, due to the combination of hydrodynamicpure hydrodynamic lubrication points points (where (where sufficient sufficient amount amount of lubricant of lubricant exists exists to separate to separate the two the surfacestwo surfaces in contact) in contact) and and pure pure boundary boundary friction friction points points (where (where there there is is an an insufficient insufficient amount amount ofof lubricantlubricant betweenbetween thethe twotwo surfacessurfaces inin contact)contact) [ [7].7].

FigureFigure 1.1. PistonPiston ringring (PR)(PR) andand CylinderCylinder LinerLiner Contact.Contact.

Furthermore,Furthermore, scuffingscuffing failure failure can can bebe categorizedcategorized asas catastrophiccatastrophic failurefailurecoupled coupled withwith severesevere formform ofof wearwear byby substantial substantial material material transfer, transfer, which which can can occur occur during during the the PCC PCC operation operation [8]. [8]. The The location location in whichin which the oilthe film oil isfilm penetrated is penetrated is where is thewhere sliding the on sliding the PR on of thethe cylinderPR of the will cylinder cause deformation will cause anddeformation therefore and wear therefore [9]. wear [9]. ToTo extrapolateextrapolate fromfrom thesethese foundationalfoundational ideas,ideas, whenwhen therethere isis aa penetrationpenetration ofof thethe oiloil film,film, thethe PRPR willwill slide slide over over the the cylinder cylinder liner liner without without sufficient sufficient lubrication lubrication between between the two the surfaces. two surfaces. This will This cause will cause deformation and wear. Additionally, the metal to metal contact and the subsequent sliding of it leads to increased friction [10]. This potentially increases the COF leads to higher fuel consumption

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Lubricantsdeformation 2019, and7, x FOR wear. PEER Additionally, REVIEW the metal to metal contact and the subsequent sliding of it leads3 of to9 increased friction [10]. This potentially increases the COF leads to higher fuel consumption and lower andfuel efficiency.lower fuel It efficiency. is particularly It is difficult particularly to model difficult scuffing to model failure ofscuffing normal failure engine of operations normal engine due to operationsvarying piston due slidingto varying speeds piston during sliding its strokespeeds operations, during its where operatio the lubricationns, where also the varies lubrication during alsoengine varies operation during asengine a function operation of strokeas a function operation of stroke [8]. This operation speed [8]. of slidingThis speed motion, of sliding temperature motion, temperatureand load are and simplified load are insimplified this study. in this This study. material This transfermaterial thattransfer occurs thatduring occurs during the sliding the sliding of the ofPCC the isPCC studied is studied by the by loss the ofloss mass of mass of the of the PR PR and and COF COF as as a functiona function of of time. time. Therefore,Therefore, a low overall COF, combined with a small loss of mass, is is an an indicator indicator that that it it is is a a well-adjusted well-adjusted tribosystem. tribosystem. The Stribeck-type diagram can can be be used to model th thee rubbing of the PR with the cylinder liner in presence of a lubrication, as shown below [[11].11]. From FigureFigure2 ,2, the theη V/W𝜂𝑉/𝑊factor factor is a is lubrication a lubrication parameter parameter thatcan that be can related be related to the COF to the through COF throughthe Stribeck the Stribeck Curve. ForCurve. small For values small ofvaluesηV/W of, in𝜂𝑉/𝑊 the boundary, in the boundary lubrication lubrication region, region, the COF the is COF high isdue high to due the decreasedto the decreased oil viscosity oil viscosity causing causing metal-to-metal metal-to-metal to occur. to occur. These These two metalstwo metals then then can can rub rubtogether together causing causing the highthe high COF. COF. In the In mixed-film the mixed-film lubrication lubrication region, region, as the as load the appliedload applied decreases decreases or oil orviscosity oil viscosity increases, increases, and/or and/or sliding sliding velocity velocity (or linear (or oscillation) linear oscillation) increases, increases, the ηV/W thefactor 𝜂𝑉/𝑊 increases. factor increases.Moreover, Moreover, the COF decreases the COF untildecreases it reaches until aitminimum, reaches a minimum, as shown inas the shown mixed-film in the mixed-film lubrication lubricationregion. For region. higher For values higher of ηvaluesV/W, inof the𝜂𝑉/𝑊 fluid-film, in the fluid-film lubrication lubrication region there region is negligible there is negligible asperity asperitycontact and contact COF and increases COF increases linearly withlinearly increasing with increasing values of valuesηV/W of[11 ].𝜂𝑉/𝑊 [11].

FigureFigure 2. 2. DiagramDiagram of of Stribeck Stribeck Curve, Curve, 𝜂:η: oil viscosity, VV:: slidingsliding velocity,velocity, WW:: normalnormal loadload applied.applied.

The frictionfriction generated generated by by the the PR isPR significantly is significantly affected affected by the pressureby the inpressure the cylinder in the throughout cylinder throughoutthe four-stroke the four-stroke operation [ 7operation]; cylinder [7]; temperatures cylinder temperatures and pressures and pressures contribute contribute to corrosive to corrosive wear [9]. wearThe cylinder [9]. The is cylinder the volume is the in volume which the in pistonwhich movesthe piston up and moves down up causing and down the crankshaftcausing the to rotate [12]. toThese rotate factors [12]. areThese considered factors are during considered the experiment, during the so experiment, that when the so loss that of when mass the and loss COF ofof mass each and PR COFcoating of each are collected, PR coating it reflectsare collected, the PCC it reflects as accurately the PCC as as possible accurately during as possible the operation during cycle. the operation cycle. 3. Materials and Methods 3. Materials and Methods 3.1. Design of Instrument 3.1. DesignOriginal of Instrument engine parts for the experiment were used, as shown in Figure3, to replicate the PCC interaction during normal engine operation. All tests are run using a SRV®III by Optimol Instruments Original engine parts for the experiment were used, as shown in Figure 3, to replicate the PCC Prueftechnik GmbH, Westendstraße 125, 80339 München, Germany. The SRV consists of a fixed interaction during normal engine operation. All tests are run using a SRV®III by Optimol Instruments bottom specimen support and a mobile replaceable top specimen holder. The holder is responsible for Prueftechnik GmbH, Westendstraße 125, 80339 München, Germany. The SRV consists of a fixed clamping the different ring and liner geometries as shown in Figure4a and is dipped in an oil bath as bottom specimen support and a mobile replaceable top specimen holder. The holder is responsible shown above in Figure4b. for clamping the different ring and liner geometries as shown in Figure 4a and is dipped in an oil bath as shown above in Figure 4b.

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Figure 3. Ready prepared samples of the PR directly from the . Figure 3. Ready prepared samples of the PR directly from the engine block. Figure 3. Ready prepared samples of the PR directly from the engine block.

(a) (b) (a) (b) FigureFigure 4. 4.( a(a)) Piston Piston Ring Ring Holder, Holder, ( b(b)) Schematic Schematic of of the the testing testing chamber chamber inside inside the the SRV SRV with with the the upper upper andFigureand lower lower 4. ( specimen.a specimen.) Piston Ring Holder, (b) Schematic of the testing chamber inside the SRV with the upper and lower specimen. TheThe upperupper andand lowerlower specimensspecimens areare pressedpressed togethertogether byby anan adjustableadjustable normalnormal forceforce andand areare oscillatedoscillatedThe upper tangentially,tangentially, and lower employing employing specimens anan adequate adequate are pressedfrequency. frequency. together This This by oscillation anoscilla adjustabletionmimics mimics normalthe the motion forcemotion andof of the arethe strokeoscillatedstroke ofof the thetangentially, PR. PR. The The contact contact employing area area depends andepends adequate strongly strong frequency.ly on on the the This ring’s ring’s oscilla geometry—such geomettion ry—suchmimics the as as diameter motiondiameter of and andthe cross-section—andstrokecross-section—and of the PR. The the the wearcontact wear of theofarea the system. depends system. We strong considerWe consly onider the the PCC the ring’s asPCC the geomet as system the ry—suchsystem in this study.in as this diameter The study. friction andThe forcecross-section—andfriction is transferred force is transferred to the the wear sensor to of the located the sensor system. in located the We block consinbeneath theider block the the beneathPCC test piece,as thethe from testsystem piece, an applied in fromthis loadstudy.an applied range The thatfrictionload is range varied force that tois transferredis mimic varied the to external tomimic the sensor the pressure external located in pres a in PCC. thesure block The in a head PCC.beneath plate The the head is connectedtest plate piece, is connectedfrom to the an applied to the vialoadmachine a singlerange via orthat a (two) issingle varied piezo or to crystal(s).(two) mimic piezo the This/these externalcrystal(s). pres crystal(s) This/thesesure in measure a PCC.crystal(s) The the transverseheadmeasure plate the force is connectedtransverse created. to Withforce the themachinecreated. knowledge Withvia a the ofsingle knowledge both or normal (two) of andpiezoboth transversenormal crystal(s). and force, This/thesetransverse the COF crystal(s)force, can the be COFmeasure calculated can bethe calculated as transverse their quotient. as forcetheir Ancreated.quotient. oil With An providesoil the pump knowledge lubricationprovides of bothlubrication with normal a controlled with and atr controlledansverse flow rate, force, flow with rate,the the COF oilwith being can the be oil pre-heated. calculated being pre-heated. Aas hightheir temperaturequotient.A high temperature An setting oil pump can setting provides be selected, can lubrication be as selected, the walls with as of the a the controlled walls test chamber of theflow test canrate, chamber be with heated the can tooil high bebeing heated temperatures. pre-heated. to high ThisAtemperatures. high along temperature with This a preheated along setting with lubricant can a preheatedbe selected, oil, allows lubricant as thethe PCCwalls oil, systemallows of the the totest operate PCC chamber system in realistic can to operatebe engineheated in settings. realisticto high Frictiontemperatures.engine coefficientssettings. This Friction along are recorded withcoefficients a preheated as a functionare lubricantrecorded of time oiasl, by allowsa function data the collection PCCof time system in by acomputer. todata operate collection in This realistic datain a collectionenginecomputer. settings. is This continuous. data Friction collection coefficients is continuous. are recorded as a function of time by data collection in a computer. This data collection is continuous. 3.2.3.2. ApplicationApplication ofof DifferentDifferent Piston Piston Rings Rings Coatings Coatings 3.2. Application of Different Piston Rings Coatings TheThe experiment experiment consistedconsisted ofof aa centrifugallycentrifugally casted,casted, pearliticpearlitic castcast ironiron cylindercylinder linerliner mademade ofof ATAT 182182 Tarabusi TarabusiThe experiment Cie. Cie. with with consisted medium medium phosphorusof a phosphoruscentrifugally content, content,casted, with anpearliticwith inner an diametercast inner iron diameter cylinder of 100 mm. linerof The100 made followingmm. of TheAT three182following Tarabusi PR coatings three Cie. PR were withcoatings selected medium were for selected thephosphorus PR. Thefor the PRcontent, PR. had The a diameterwith PR had an ofainner diameter 90 mm: diameter of 90 mm:of 100 mm. The following three PR coatings were selected for the PR. The PR had a diameter of 90 mm: • Chromated 126 = Chromium Plated • Chromated 126 = Chromium Plated • CrN-TiN 1o Group 127 = PVD multi-layer TiN-Ti-CrN-CrN coating • ChromatedCrN-TiN 1º 126Group = Chromium 127 = PVD Plated multi-layer TiN-Ti-CrN-CrN coating • NIPCO + Si3N4 128 = Electroless nickel-phosphorus coating with dispersed silicon nitride • CrN-TiNNIPCO + 1ºSi 3GroupN4 128 127= Electroless = PVD multi-layer nickel-phosphorus TiN-Ti-CrN-CrN coating withcoating dispersed silicon nitride (Si3N4) (Si N ) particles • NIPCOparticles3 4 + Si3N4 128 = Electroless nickel-phosphorus coating with dispersed silicon nitride (Si3N4) particles

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The following test conditions were selected, and are summarized in Table1 below:

Table 1. The testing conditions that were selected for each of the applied PR coatings.

Parameter Condition Setting Load 300 N Contact Pressure 10 MPa Test Time 5400 s Frequency 50 Hz Stroke 3 mm Linear Speed 0.3 m/s Temperature 200 ◦C SAE 15W-40 ((η40C = 107 mm2/s, η100C = 14.6 mm2/s, Lubricant VI = 140, API SL/CF, ACEA A3/B3, A3/B4-08)

The oil temperature of 200 ◦C reflects operating conditions of engines at the piston ring or specifically at fired top dead center (FTDC), where adhesive failure (scuffing) frequently occurs [13].

4. Results From Figure5, this graph shows the COF of the different PR coatings over a duration of 5400 s.

Figure 5. COF for different piston-ring coatings as a function of time.

The following graph (Figure6) visually shows the PR mass loss: Lubricants 2019, 7, 8 6 of 9 Lubricants 2019, 7, x FOR PEER REVIEW 6 of 9

AT 182 - PR Mass Loss

0.18 0.16 0.14 0.12 0.1 0.08 0.06

Mass Lost (mg) 0.04 0.02 0 Chromated CrN-TiN 1º Group NIPCO + Si3N4 PR Coating

Figure 6. Loss of mass for each piston-ring coating. Figure 6. Loss of mass for each piston-ring coating. The table (Table2) below summarizes Figures5 and6: The table (Table 2) below summarizes Figures 5 and 6: Table 2. Summary of the Average COF and Ring Mass Lost for PR Coating. Table 2. Summary of the Average COF and Ring Mass Lost for PR Coating. Piston Ring Average Coefficient of Friction Ring Mass Lost (mg) Piston Ring Average Coefficient of Friction Ring Mass Lost (mg) Chromium plated 0.158 0.16 ChromiumCrN-TiN 1o platedGroup 0.158 0.156 0.01 0.16 CrN-TiNNIPCO 1º + SiGroup3N4 0.1560.151 0.05 0.01 NIPCO + Si3N4 0.151 0.05 The chromium plated PR has the highest COF amongst the tested PR coatings at 0.158. Moreover, it alsoThe has chromium the greatest plated loss PR of has mass, the highest 0.16 mg, COF of allamongs threet PRsthe tested tested. PR Incoatings this screening, at 0.158. Moreover, the PVD multi-layerit also has the TiN-Ti-CrN-CrN greatest loss of andmass, the 0.16 electroless mg, of al nickel-phosphorusl three PRs tested. In coating this screening, with dispersed the PVD silicon multi- layer TiN-Ti-CrN-CrN and the electroless nickel-phosphorus coating with dispersed silicon nitride nitride (Si3N4) particles coatings displayed a strong potential for wear reduction under scuffing conditions(Si3N4) particles [14]. coatings displayed a strong potential for wear reduction under scuffing conditions [14]. 5. Discussion 4. Discussion From Figure6, the results are as follows: the PR coating of CrN-TiN 1 o Group lost the least amount of massFrom and Figure the Chromium 6, the results plated are PR as coating follows: lost the 16 timesPR coating more in of mass CrN-TiN compared 1º Group to the lost PR CrN-TiNthe least amounto of mass and the Chromium plated PR coating lost 16 times more in mass compared to the PR 1 Group coating; and the PR NIPCO + Si3N4 coating lost 5 times more in mass compared to the PR CrN-TiNCrN-TiN 11ºo GroupGroup coating.coating; and The the PR PR coating NIPCO of CrN-TiN+ Si3N4 coating 1o Group lost 5 also times has more the lowestin mass average compared COF. to Thethe PR oil bathCrN-TiN of SAE 1º Group 15W-40 coating. provides The the PR lubrication coating of betweenCrN-TiN the 1º Group PR and also the has cylinder the lowest liner. average This is doneCOF. to The help oil prevent bath of SAE the wear 15W-40 and provides yet each the PR haslubrication experienced between a loss the of PR mass. and Asthe explained, cylinder liner. over This the ∆isx donedistance to help of the prevent surface the on wear the cylinder and yet liner, each there PR has is aexperienced mixture of asperity a loss of points mass. and As well-lubricatedexplained, over points.the Δx Therefore,distance of forthe each surface of the on threethe cylinder PR tested, liner, the th oilere film is a was mixture penetrated of asperity allowing points for and wear well- to developlubricated by points. the rubbing Therefore, of the for PR each on theof the cylinder three PR liner. tested, However, the oil itfilm appears was penetrated that the PR allowing coating for of CrN-TiNwear to develop 1o Group by had the the rubbing least oil of film the penetration. PR on the Thiscylinder reasoning liner. isHowever, valid, because it appears of it havingthat the both PR thecoating least of loss CrN-TiN of mass 1º and Group the lowest had the COF least among oil film the penetration. tested PR coatings. This reasoning is valid, because of it havingA very both important the least loss physical of mass property and the of alowest lubricant COF is among its viscosity. the tested Lubricant PR coatings. viscosity is dependent on theA shearvery rate,important temperature physical and property contact pressureof a lubricant in the is tribocontact its viscosity. [15 ].Lubricant The SAE viscosity 15W-40 isis dependent on the shear rate, temperature and contact pressure in the tribocontact [15]. The SAE 15W- 40 is a multigrade oil that experiences an elevated temperature of 200 °C and its viscosity can be calculated at any temperature through the Vogel Equation [15]: (1) 𝜂=𝜅∙𝑒

Lubricants 2019, 7, 8 7 of 9 a multigrade oil that experiences an elevated temperature of 200 ◦C and its viscosity can be calculated at any temperature through the Vogel Equation [15]:

( θ1 ) η = κ·e θ2+T (1)

◦ ◦ where η is the viscosity (mPa·s, where 1 mPa·s = 1 cP) at temperature T ( C) and κ (mPa·s), θ1 ( C) and ◦ θ2 ( C) are constants. Given that κ, θ1 and θ2 are constants, for a positive range of T values η is inversely proportional to T. The kinematic viscosity at 200 ◦C estimates to 3.18 mm2/s using this equation to model the viscometrics of the SAE 15W-40 lubricant during the experiment. In addition, multigrade oils are sheer thinning fluids [16]. As such, the viscosity of the oil is affected by the sheer rate, which in turn is dependent on the liner oscillation speed [7]. As the liner oscillation speed increases from static to a speed of 0.3 m/s, this may also contribute to the decrease in oil viscosity. Furthermore, the oil film thickness between the PR and cylinder liner increases with oil viscosity [17]; then conversely given the suspected decrease in viscosity, it hints to a decrease in the oil film layer, leading to a higher chance of wear on the PR. The Stribeck curve described earlier is a model that can be applied to further explain the results. Due to the suspected decrease in oil viscosity, along with the normal load applied and linear oscillation speed remaining constant after its initial increase, the ηV/W factor probably falls. As this factor continues to fall, due to the eroding of the oil lubrication, the PR and cylinder liner rub together which causes wear on the PR, which also explains the loss of mass of the PRs but this model can simultaneously be used to explain Figure5. In Figure5, for both the PR coating: CrN-TiN 1 o Group and Chromium plated, there is an initial increase in COF, with the chromium plated having a larger initial peak, then an immediate decrease in COF and finally increasing slightly in COF as time goes on. The Stribeck model explains that an initial increase in COF is followed by a decrease in COF till a minimum value. The cause for an immediate increase in COF can be attributed to the PCC contact going from no friction, as there is no linear velocity at time zero, to then a sudden increase in linear velocity. This in turn can cause the sudden increase in friction but is followed by an immediate decrease in COF. The reason for this being the high contact pressure in which the COF is generated in is also related to the oil viscosity [11]. The oil viscosity causes a reduction in friction in the initial stages, as there is enough lubrication to cause a decrease in COF. But as aforementioned, the linear oscillation is suspected to cause a decrease in oil lubrication making the COF rise eventually and cause wear to the PR. The anomaly to this trend is the NIPCO + Si3N4 coating, which has an initial decrease in COF and then a rise in COF as time moves on. A possible explanation is that over the contact distance, there were more pure hydrodynamic lubrication points, causing a reduction in COF. This did not last long as the COF rose quickly after, of which can be attributed to how the temperature and the linear oscillation speed change the lubrication pattern between the PR and cylinder liner. Ultimately, it did follow the trend of decreasing oil viscosity leading to PR mass loss and increased COF between the metals. The difference in the yellow and blue curve is due to running-in phenomena, which are mainly ruled by statistical events and can therefore produce significantly different COF-curves. These COF-curves depend on the many asperities that form through the actual contact between the piston ring and cylinder liner contact and how the lubricant reacts in these situations. Therefore, in this case the difference is over-pronounced by the scale of the graph. This is necessary to make the differences in COF between the coatings clearly visible and this all takes place within the range of the COF which is smaller than 0.02. However, the PR coating of CrN-TiN 1o Group still lost the least amount of mass. This can be attributed to the fact that CrN (Chromium nitrate) and TiN (Titanium Nitrate) are known for their good wear resistance [18]. More specifically, CrN and TiN are applied in racing engines, where the PCC experiences greater piston speeds [19] due to the car travelling at high speeds during a race. For example, a Formula 1 engine V10-type experiences 37–48 m/s, whilst a regular European 2.0-L engine type Inline 4 has a maximum piston speed of up to 32 m/s [15]. The greater piston speeds that Lubricants 2019, 7, 8 8 of 9 a piston experiences in race indicate of the greater frictional force. Given that CrN and TiN are used for race car applications, it can be expected for them to exhibit good wear resistance, especially at a low speed of 0.3 m/s, as used during this experiment.

6. Conclusions It was the hope of this study to help contribute more information to the field of optimizing the PR of the ICE. More specifically, the primary aim of this was to see the influence of the PR coating on the wear and friction on the PCC assembly. There is no proper correlation between wear and friction but if scuffling failure occurs, it can lead to material transfer and therefore wear. The coatings can show resistance to wear. Based on the experimental findings, it shows that a PR coating based from CrN and TiN has greater resistance to wear and friction. The wear and the COF experienced by the PR is a result of the complete tribosystem settings selected. This consists of the PR sliding against the cylinder liner, the elevated temperature, the surrounding atmosphere, load applied and the contact pressure. As such, the SRV offers these settings to help mimic the reality of what the PCC assembly faces during normal engine operations. Continued contributions to the understanding of the PCC is a critical area that will help improve efficiency and hopefully in a broader sense help automotive transports to be more effective with their fuel consumption.

Author Contributions: Conceptualization, A.R.; Formal Analysis, A.R.; Writing-Original Draft Preparation, A.R.; Writing-Review & Editing, C.B., A.R. and R.S.; Supervision, C.B. and R.S. Funding: This research received no external funding. Acknowledgments: The authors acknowledge the support, editing and reviewing of this paper by Mathias Woydt. Conflicts of Interest: The authors declare no conflict of interest.

References

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