Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
High Performance Powertrain Materials Lab. Okanagan Campus
Banerjee, Siddharth
Stroh, Joshua
Sediako, Dimitry
Tjong, Jimi
This study evaluates the tribological characteristics of quasi-monolithic engine cylinder coatings and piston rings using a custom-built linear reciprocating tribometer. The coatings were deposited on an Al-Si alloy cylinder bore using the Plasma Transfer Wire Arc (PTWA) and Electrolytic Jet Plasma Oxidation (EJPO) processes. The coatings’ tribological performances were investigated in the boundary lubrication regime. The performance of conventional chrome-coated cast iron piston rings was tested and compared to that of EJPO- and PTWA-coated engine cylinder samples that were extracted from a cast Al-Si engine block. Scanning electron microscopy and profilometry were used to compare the evolution of wear and the prevalent wear mechanism. This paper also presents the verification and repeatability analysis of a custom-built tribometer against a standard industry-calibrated tribometer. The wear test results showed that the EJPO coating had 0.05% to 10.35% lower wear rates than its PTWA counterpart throughout a wide range of loading conditions and sliding distances. The variation in the counter-face behavior is likely due to the different surface topographic parameters such as skewness, kurtosis, and porosity.
Academic Editors: Jose Outeiro andJoao NobreReceived: 1 March 2025Revised: 22 March 2025Accepted: 23 March 2025Published: 27 March 2025Citation: Banerjee, S.; Stroh, J.;Sediako, D.; Tjong, J. TribologicalInvestigation of Plasma-BasedCoatings for Use in Quasi-MonolithicEngine Cylinder Bores. Metals 2025,15, 370. https://doi.org/10.3390/met15040370Copyright: © 2025 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license(https://creativecommons.org/licenses/by/4.0/).ArticleTribological Investigation of Plasma-Based Coatings for Use inQuasi-Monolithic Engine Cylinder BoresSiddharth Banerjee , Joshua Stroh *, Dimitry Sediako and Jimi TjongHigh Performance Powertrain Materials Laboratory, The University of British Columbia, 1540 Innovation Drive,Kelowna, BC V1V 1V7, Canada; besidd11@gmail.com (S.B.); dimitry.sediako@ubc.ca (D.S.);jtjongdr@gmail.com (J.T.)* Correspondence: joshua.leo.stroh@gmail.comAbstract: This study evaluates the tribological characteristics of quasi-monolithic enginecylinder coatings and piston rings using a custom-built linear reciprocating tribometer. Thecoatings were deposited on an Al-Si alloy cylinder bore using the Plasma Transfer Wire Arc(PTWA) and Electrolytic Jet Plasma Oxidation (EJPO) processes. The coatings’ tribologicalperformances were investigated in the boundary lubrication regime. The performance ofconventional chrome-coated cast iron piston rings was tested and compared to that of EJPO-and PTWA-coated engine cylinder samples that were extracted from a cast Al-Si engineblock. Scanning electron microscopy and profilometry were used to compare the evolutionof wear and the prevalent wear mechanism. This paper also presents the verification andrepeatability analysis of a custom-built tribometer against a standard industry-calibratedtribometer. The wear test results showed that the EJPO coating had 0.05% to 10.35% lowerwear rates than its PTWA counterpart throughout a wide range of loading conditions andsliding distances. The variation in the counter-face behavior is likely due to the differentsurface topographic parameters such as skewness, kurtosis, and porosity.Keywords: plasma coating; porosity; repeatability; roughness; tribometer; wear rate1. IntroductionIn the current era of accelerated urbanization, maintaining appropriate/high airquality is a significant challenge that confronts society. One of the major contributors topoor air quality is the emission of greenhouse gases (GHGs), which are produced fromsectors such as manufacturing, transportation, and agriculture. In 2021, the GHGs producedfrom the transportation sector alone accounted for about 25% of global GHGs [1]. A reportpublished by The International Environment Agency’s (IEA) indicates that global fueleconomy initiatives have the potential to reduce net emissions by 1 Gt-CO2 equivalentper year, which is 3% of the total global annual GHG emissions [2]. The primary methodthat demonstrates a significant potential for increasing the fuel efficiency of vehicles is toincrease the specific thermal load management of the engine. This can be achieved throughapplication of lighter and more conductive materials, as well as reduce the frictional lossesduring engine operation. For the latter, the primary contributor for frictional losses liesin the interaction of the piston, piston ring, and cylinder liner. It has been reported thatapproximately 50–68% of the engine’s frictional losses are caused by this piston–cylinderassembly [3]. These frictional losses have a dramatic effect on the engine’s overall efficiency.The enhancement of fuel efficiency through vehicle weight reduction witnessed initialefforts in the 1970s when grey cast iron engine blocks were substituted with lightweight Al-Si engine blocks. However, the use of aluminum is impeded by various limitations, such asMetals 2025, 15, 370 https://doi.org/10.3390/met15040370Metals 2025, 15, 370 2 of 24insufficient wear resistance and hardness, inadequate Young’s modulus, and substandardtensile strength [4]. To counter the poor wear properties of aluminum alloys, cast iron linersare commonly press-fit or cast-in as the cylinder bore surface. Although the wear resistanceis improved, these liners lead to an overall increase in the weight of the engine. Potentiallymore problematic than the increased mass, is the effects that the differential in thermalexpansion coefficient between the Fe-based liner and Al-based cylinder block materials,which often leads to unwanted residual stress, distortion, and expensive postprocessingheat treatments and machining [5].The development of hyper-eutectic Al-Si alloys (e.g., AlSi17Cu4Mg), fortified withuniformly distributed hard Si crystals on the surface of the cylinder bore, contributed tosolving the issues related to the weight and wear resistance of cylinder liners. The softeraluminum matrix surrounding the Si crystals gets eroded during the honing process andengine run-in phase, exposing the Si crystals on the surface of the cylinder bore. Thiscreates crevices between the Si particles, which retains engine oil for lubrication of thesliding piston and piston ring on the cylinder bore surface [6]. The disadvantage of usinghyper-eutectic Al-Si liners is the high cost of the manufacturing process due to complexitiesin the inhomogeneous distribution of primary Si particles and poor machinability [7].In the last two decades, thermal spray coatings have emerged as a cost-efficient andreliable method of improving engine efficiency and tribological properties [8,9]. Oneindustry-accepted thermal spray coating process, Plasma Transferred Wire Arc (PTWA),has gained traction in the production of quasi-monolithic engines due to its faster coatingdeposition rates [9] This method employs low-carbon steel wire as its feedstock to deposita thick (about 200 µm) coating using a supersonic plasma jet that melts and propelsthe atomized feedstock onto the Al-Si substrate [10]. As a result of the high-velocitymaterial deposition process, some of the molten particles oxidize and solidify before hittingthe substrate. This leads to the formation of interfacial oxide regions with low fracturetoughness that could be susceptible to delamination under frictional contact [11]. However,a 300 h endurance test was conducted with PTWA-coated engines in a laboratory at FordMotor Company, which provided clear evidence that oil and fuel consumption decreasedbecause of the diminished friction. Specifically, the fuel consumption was reduced by 6.8%in comparison to engine bores with traditional cast iron liners [12]. This PTWA technologycan potentially be utilized in the restoration of older cast iron or aluminum engine blocksand in the refurbishment of worn engines that would otherwise have been discarded [13].Recently, a new coating process that is a viable option for protecting and enhancingthe surface of aluminum cylinders is Electrolytic Jet Plasma Oxidation (EJPO) [14]. It hasemerged as an environmentally safe option that utilizes chemically benign, electrolyticbaths that are typically low concentration, and the generated waste can directly be drainedpost-pH adjustment. It is an electrochemical technique to generate an oxide coating on alightweight metal substrate. The EJPO coating exhibits bi-directional growth, expandingboth towards the interior and exterior of the underlying substrate, resulting in strongerbonding strength [14]. The outer layer of the coating is porous due to escaping gases, whichresults in lower hardness at the surface. The coating process is done at room temperature,so the thermal stresses are low. The microcracks and porosity networks in its microstructurerelieve residual stresses as oxygen gas is liberated. Although the EJPO coating processhas benefits, compared to other plasma-based coatings, the wear performance of the EJPOcoating, for cylinder liner application under realistic engine load and lubrication scenarios,remains unexplored. Thus, to enable the use of this advanced coating, it is imperative toevaluate its wear performance using conditions that replicate piston–cylinder interactionsduring engine operation.Metals 2025, 15, 370 3 of 24During the power stroke of a spark-ignition engine, air–fuel combustion generatesa rapid increase in gas pressure, which causes the piston to apply a radial force on thecylinder liner in the Top Compression Ring (TCR) reversal zone near the Top Dead Center(TDC). The elevated radial force leads to frictional contact between the piston ring andthe cylinder liner. The severity of this phenomenon is worse during cold start conditionsdue to the delay in the arrival of the more viscous lubricating oil [15,16]. In situ studiesdemonstrate that wear on the engine liner surface is greatest around the TDC during thepower stroke where the lubrication oil thickness is minimum [17]. Experimental findingsindicate that frictional energy dissipation during cold start is 72% greater than duringwarmed up start across six engine cycles [15]. In a warmed up engine, the overall energyloss per engine cycle due to friction is 16 J, whereas, during a cold start, the energy lossvalue ranges from 74 to 96 J [18]. The wear caused by the increased frictional contact of thepiston ring and cylinder liner has a direct influence on the energy loss due to the increase inblow-by gases [19]. In summary, the worst-case tribological interaction of a cylinder lineroccurs when the engine is starved of fresh lubrication and maximum radial pressure exertedby the piston near TDC. Therefore, it is imperative for automakers who are developingadvanced cylinder liner coatings to analyze their tribological behavior in test conditions thatsimulate the worst-case scenario. It should be noted that simulating real-world tribologicalconditions in a controlled environment is considerably difficult, as there are many variables(e.g., temperature, humidity, load variations, and material inhomogeneity).The present research examines the evolution of surface topographic features such asroughness, skewness, kurtosis, and porosity of the EJPO and PTWA coatings throughoutwear test experiments. The sliding counter-face wear characteristics of the coated samplesagainst a CrN-coated piston ring were investigated using simplified component wear testsutilizing test parameters that simulate the pressure and lubrication conditions near theTDC position during the power stroke in cold start conditions. The analysis excludes theinfluence of combustion byproducts or thermal oil degradation on the lubricant and theeffect of piston ring radial pressure distribution. This research utilized a custom-designedreciprocating tribometer to investigate the tribological properties of the two coatings inboundary-lubricated conditions. For real-world applications, the tribological evaluationof actual engine cylinder specimens in simplified component bench tests serves as a linkbetween expensive full-scale durability tests and frequently inaccurate simulations todetermine component durability. The tribological data obtained from these tests willserve as a benchmark for refining the coating process parameters, thereby enhancing thewear resistance of coatings, and potentially facilitating the application of EJPO coatings innext-generation engines.2. Materials and Methods2.1. Calibration TestsFor the validation tests, Al 6061 with a known hardness value of 90–100 HV and ayield strength of 240 MPa was used as the flat sample specimen. The 100Cr6 bearing steelspherical ball, with a hardness of 650 HV and a higher yield strength of 835 MPa, wasselected for the custom device validation. The operating parameters for the validationtests are shown in Table 1. It should be noted that the ASTM G133-22 standard [20] forlinear oscillating tests mentions that ambient conditions like relative humidity and ambienttemperature affect the accuracy of the wear and friction devices up to a limit of ±3%. In thepresent study, the tests were conducted in a controlled ambient environment in a researchlaboratory, where both apparatuses were operated under the same conditions.Metals 2025, 15, 370 4 of 24Table 1. Operating parameters for the comparative analysis of wear testing devices.Parameters ValueNormal Load (N) 3, 5, 10, and 20Initial Contact Pressure (MPa) 10.42, 13.23, 17.09, and 32.68Final Contact Pressure (MPa) 2.22, 2.40, 2.94, and 3.76Frequency (Hz) 4Sliding Distance (m) 50Stroke Length (m) 0.01Oscillation Speed (m/s) 0.08Material Alloy 61S (flat sample specimen) and100Cr6 (6 mm diameter)Temperature (◦C) 21–23Composite Roughness (nm) 241–383Relative Humidity (%) 16.1–17.4Lubrication Regime DryThe commercial tribometer is equipped with weighed disks provided by the man-ufacturer, with a maximum load capacity of 20 N. Table 2 describes the profilometermeasurement settings. The calibration of the Bruker Dektat XT profilometer platform (Bil-lerica, Middlesex, MA, USA) was performed using a reference glass sheet with a preciselymachined specific roughness profile. The calibration is necessary for the vertical scale ofthe profilometer to ensure accurate measurements of surface height or depth variations.Table 2. Commercial profilometry specifications.Profilometry Labels ValueMeasurement Type Hills and ValleysScan Duration 30 sScan Length 4000 µmStylus Orientation 90◦Stylus Force 10 mgStylus Type Radius: 12 µm2.1.1. Sample PreparationTo prepare the samples for the validation tests, 40 mm long, 22 mm wide, and 8 mmthick specimen were cut using a water jet. To ensure consistency in surface roughnessparameters across all samples, a manual polisher was utilized. The rotational velocity andpolishing time were kept identical. The first step involved using coarse grit grinding paper,starting from abrasive sizes 120 to FEPA 1200 grit SiC paper. The next step involved roughpolishing using 9 to 6 µm diamond, followed by fine polishing using a 3 µm diamondpolishing pad. It is worth noting that the surface of the sample was inspected at each stepusing an optical microscope to ensure a consistent pattern of abrasive machining. Theobjective of this sequence of steps was to achieve uniformity in surface characteristics. Theaverage surface roughness of the 24 samples for these tests was observed using the BrukerDektat 2-D profilometer, at 30.26 nm.2.1.2. Procedure for the Validation TestsThe general steps for the validation tests are as follows:(i). The polished flat specimen and the test ball were held using solvent-proof gloves.Both were cleaned in IP alcohol for 5 min to get rid of oil and dirt. They were taken outfrom the solvent and allowed to dry. It is recommended to clean the sample surfacenot more than 10 min before the test.Metals 2025, 15, 370 5 of 24(ii). The weight of the sample on the digital scale with a resolution of 0.0001 g and diameterof the ball using a micrometer with a resolution of 0.001 mm were recorded, and thesample is mounted on the device.(iii). The ball holder is disassembled and cleaned using pressurized air. The test ball isplaced in the ball holder with tweezers and then tightened.(iv). The normal load is applied on the system by placing weighted disks on the AP ballholder, and in the case of the HPPM tribometer, the load was applied by tighteningthe top bolt, and the lock nut was tightened. It is recommended to allow the load cellreading to stabilize for 2–5 min and then start the test.(v). Upon completion of the test, pressurized air is used to clean the sample surface andcarefully unmounted and the weight of the flat specimen was noted, and diameter ofthe ball was recorded.(vi). The specimen is cleaned using IP alcohol for 1 min and allowed to dry and thenlabelled with the test number and operating condition details. The sample waswrapped in soft tissue and then placed in a plastic cover to avoid contamination.2.1.3. Tribometer Setup and ValidationTribometers are devices that are used to evaluate tribological characteristics of materialpairs (also referred to as tribo-pairs) such as friction or resistance to wear in controlled oper-ating conditions. A reciprocating tribometer was designed to precisely control two criticaloperational parameters, namely, the normal load and the sliding velocity (see Figure 1a).The custom-designed apparatus (HPPM tribometer) was fabricated for oscillating weartests with piston ring segments and coated engine cylinder samples. Two separate detach-able sliding styluses are designed to mount a piston ring segment and a 100Cr6 steel ballas the sliding counter-face material. A DC motor-driven cam-follower linkage propels thesliding component on the specimen surface. The average normal force acting on the ringholder is controlled using a force transducer with a load fluctuation deviation between±2% of the test load (see Figure 1b) per ASTM G181 standard [21].Metals 2025, 15, x FOR PEER REVIEW 5 of 26 2.1.2. Procedure for the Validation Tests The general steps for the validation tests are as follows: (i). The polished flat specimen and the test ball were held using solvent-proof gloves. Both were cleaned in IP alcohol for 5 min to get rid of oil and dirt. They were taken out from the solvent and allowed to dry. It is recommended to clean the sample sur-face not more than 10 min before the test. (ii). The weight of the sample on the digital scale with a resolution of 0.0001 g and diam-eter of the ball using a micrometer with a resolution of 0.001 mm were recorded, and the sample is mounted on the device. (iii). The ball holder is disassembled and cleaned using pressurized air. The test ball is placed in the ball holder with tweezers and then tightened. (iv). The normal load is applied on the system by placing weighted disks on the AP ball holder, and in the case of the HPPM tribometer, the load was applied by tightening the top bolt, and the lock nut was tightened. It is recommended to allow the load cell reading to stabilize for 2–5 min and then start the test. (v). Upon completion of the test, pressurized air is used to clean the sample surface and carefully unmounted and the weight of the flat specimen was noted, and diameter of the ball was recorded. (vi). The specimen is leaned using IP alcohol for 1 min and allowed to dry and then la-belled with the test number and operating condition details. The sample was wrapped in soft issu and then placed in plastic cover to avoid contamination. 2.1.3. Tribometer Setup and Validation Tribometers are devices that are used to evaluate tribological characteristics of mate-rial pairs (also referred to as tribo-pairs) such as friction or resistance to wear in controlled operating conditions. A reciprocating tribometer was designed to precisely control two critical operational parameters, namely, the normal load and the sliding velocity (see Fig-ure 1a). The custom-designed apparatus (HPPM tribometer) was fabricated for oscillating wear tests with piston ring segments and coated engine cylinder samples. Two separate detachable sliding styluses are designed to mount a piston ring segment and a 100Cr6 steel ball as the sliding counter-face material. A DC motor-driven cam-follower linkage propels the sliding component on the specimen surface. The average normal force acting on the ring h lder is controlled using a force transducer with a load fluctu ti deviation between ±2% of he test load (se Figure 1b) per ASTM G181 standard [21]. (a) (b) Figure 1. (a) Schematic arrangement of the novel experimental apparatus. (b) Normal load fluctua-tions during unlubricated friction tests. Figure 1. (a) Schematic arrangement of the novel experi t s. ( ) ormal load fluctua-tions during unlubricated friction tests.The repeatability and precision of the wear results produced by the custom HPPMtribometer were compared with that of a standard industry-certified Anton Paar (AP)sliding ball contact type tribometer. The variation of wear volume results within thetribometers was used to compare the devices. The wear profile readings were taken atthe center of the wear track, perpendicular to the stroke length on each sample at the endof the test at different load values (Figure 2). The loads were selected based on contactpressures relevant to the piston ring-cylinder liner (PRCL) interaction in a passenger vehicleMetals 2025, 15, 370 6 of 24where combustion gas pressures range from 2.5 to 10 MPa [22]. The volume of materiallost during the wear test is calculated by the gravimetric method which, is determinedby the weight differential of homogenous specimen material. Compliance with a certifieddevice can be claimed when using tribo-pairs with identical surface profiles and materialproperties under standardized operating conditions [20]. Origin Pro was used to assessmachine-to-machine variability using the analysis of variance approach.Metals 2025, 15, x FOR PEER REVIEW 6 of 26 The repeatability and precision of the wear results produced by the custom HPPM tribometer were compared with that of a standard industry-certified Anton Paar (AP) slid-ing ball contact type tribometer. The variation of wear volume results within the tribo-meters was used to compare the devices. The wear profile readings were taken at the cen-ter of the wear track, perpendicular to the stroke length on each sample at the end of the test at different load values (Figure 2). The loads were selected based on contact pressures relevant to the piston ring-cylinder liner (PRCL) interaction in a passenger vehicle where combustion gas pressures range from 2.5 to 10 MPa [22]. The volume of material lost dur-ing the wear test is calculated by the gravimetric method which, is determined by the weight differential of homogenous specimen material. Compliance with a certified device can be claimed when using tribo-pairs with identical surface profiles and material prop-erties under standardized operating conditions [20]. Origin Pro was used to assess ma-chine-to-machine variability using the analysis of variance approach. (a) (b) (c) (d) Figure 2. Validation test profile readings at (a) 3 N, (b) 5 N, (c) 10 N, and (d) 20 N loads for both of the wear testing apparatuses. An intriguing observation arises from the mean wear width values at lower loads, as presented in Table 3. The tribometers exhibit a disparity in the mean width of wear, with values of 0.69 mm and 0.41 mm. Although, the difference has little effect on the wear vol-ume as wear width is measured on the surface. It can be linked to the two devices’ differ-ing normal load application mechanisms. The HPPM tribometer is designed for loading conditions ranging from 0 N to 490 N. It limits load fluctuations with a lock nut and rigid load transmission mechanism that eliminates reactionary lift when the sliding counter Figure 2. Validation test profile readings at (a) 3 N, (b) 5 , (c) 10 , and (d) 20 loads for both ofthe wear testing apparatuses.An intriguing observation arises from the mean wear width values at lower loads,as presented in Table 3. The tribometers exhibit a disparity in the mean width of wear,with values of 0.69 mm and 0.41 mm. Although, the difference has little effect on the wearvolume as wear width is measured on the surface. It can be linked to the two devices’differing normal load application mechanisms. The HPPM tribometer is designed forloading conditions ranging from 0 N to 490 N. It limits load fluctuations with a lock nutand rigid load transmission mechanism that eliminates reactionary lift when the slidingcounter face encounters a smoother asperity. In contrast, the AP device to tends to slideover smoother asperities at lower loads such as 3 N. The HPPM tribometer tends to gothrough the asperities at lower and higher loads, which is why the wear width is greater atlower loads, but the wear depth remains unaffected (discussed in next sub-section).Repeatability of the wear results is evaluated using the coefficient of variation (COV).It is a dimensionless statistical measure that indicates the relative dispersion or variabilityof a data set. The formula used to calculate % COV is:Metals 2025, 15, 370 7 of 24% COV =(SDMean)× 100 (1)As the test load increases from 3 N to 20 N, the coefficient of variation values of thewear width data decreases for the AP device (see Table 3). As shown in Figure 3, the %COVof the wear width results for the HPPM device is lower at 10 N and 20 N loads. Thisdecrease is in line with the AP device, as the maximum COV for the AP device is 8.76% forthe 3 N test and 7.48% for the 5 N test.Table 3. Comparison of wear volume data obtained for validation of custom-made device.Tribometer Loads(N)Mean(mm3)SD(mm3)±95% ConfidenceIntervalMax(mm3)Min(mm3)COV(%)Standard(AP)3 N 0.43 0.01 0.02 0.44 0.42 2.335 N 0.74 0.02 0.03 0.75 0.73 2.3410 N 3.10 0.05 0.08 3.14 3.06 1.6520 N 6.51 0.03 0.04 6.53 6.49 0.41Custom(HPPM)3 N 0.69 0.01 0.02 0.70 0.68 1.455 N 0.95 0.01 0.02 0.96 0.94 1.0510 N 3.10 0.08 0.12 3.16 3.04 2.5220 N 6.59 0.04 0.07 6.62 6.55 0.63Metals 2025, 15, x FOR PEER REVIEW 7 of 26 face encounters a smoother asperity. In contrast, the AP device to tends to slide over smoother asperities at lower loads such as 3 N. The HPPM tribometer tends to go through the asperities at lower and higher loads, which is why the wear width is greater at lower loads, but the wear depth remains unaffected (discussed in next sub-section). Repeatability of the wear results is evaluated using the coefficient of variation (COV). It is a dimensionless statistical measure that indicates the relative dispersion or variability of a data set. The formula used to calculate % COV is: % COV = � SDMean� × 100 (1) As the test load increases from 3 N to 20 N, the coefficient of variation values of the wear width data decreases for the AP device (see Table 3). As shown in Figure 3, the %COV of the wear width results for the HPPM device is lower at 10 N and 20 N loads. This decrease is in line with the AP device, as the maximum COV for the AP device is 8.76% for the 3 N test and 7.48% for the 5 N test. Another parameter to represent the repeatability of results is Confidence Interval. It indicates the degree of certainty in the measurements and establishes the precision of the resultant wear data for each load group in their respective tribometers. It means that if tests are repeated, there is a probability of 0.95 that the mean of the results lie within upper and lower bound of the Confidence Interval. The formula used to calculate 95% Confi-dence Interval is: CI = Mean × ± 1.96 × (SD√n) (2) The narrow range of the 95% confidence interval of the HPPM tribometer quantifies the within-instrument data, as depicted in the figure below. The 95% confidence interval values for test loads of 3 N, 5 N, 10 N, and 20 N were ±0.07, ±0.18, ±0.16, and ±0.09, respec-tively, for the HPPM tribometer, compared to ±0 1 , ±0.22, ±0.24, and ± 09, respectively, for the AP tribometer. These wear width values of the custom-made tribometer demon-strate a remarkable congruence based on the three conducted tests when compared to the AP confidence interval values. Both the indicators show that at higher loads, both AP and HPPM tribometer show accurate and precise wear width values. Figure 3. Comparison of wear volumes at different applied loads for the tribometers. Figure 3. Comparison of wear volumes at different applied loads for the tribometers.Another parameter to represent the repeatability of results is Confidence Interval. Itindicates the degree of certainty in the measurements and establishes the precision of theresultant wear data for each load group in their respective tribometers. It means that if testsare repeated, there is a probability of 0.95 that the mean of the results lie within upper andlower bound of the Confidence Interval. The formula used to calculate 95% ConfidenceInterval is:CI = Mean×± 1.96×(SD√n)(2)The narrow range of the 95% confidence interval of the HPPM tribometer quantifies thewithin-instrument data, as depicted in the figure below. The 95% confidence interval valuesfor test loads of 3 N, 5 N, 10 N, and 20 N were ±0.07, ±0.18, ±0.16, and ±0.09, respectively,for the HPPM tribometer, compared to ±0.13, ±0.22, ±0.24, and ±0.09, respectively, forMetals 2025, 15, 370 8 of 24the AP tribometer. These wear width values of the custom-made tribometer demonstratea remarkable congruence based on the three conducted tests when compared to the APconfidence interval values. Both the indicators show that at higher loads, both AP andHPPM tribometer show accurate and precise wear width values.A similar study was done using two commercial tribometers with a scotch yoke drivemechanism comparable to the AP device used in this investigation. The wear volumefindings published in that interlaboratory study indicated a maximum standard deviationof 0.078 mm3 with a mean value of 0.543 mm3 and an average 95% confidence intervalvalue of 0.22 [20]. The wear volume data between two readings from the same deviceshowed more repeatability in comparison to the volume loss readings between the twodevices. The reproducibility of the results between the two devices is dependent on thetest environment and precision limits of the testing apparatus. Hence, it is important todetermine the precision limits of an apparatus under standard test conditions, which allowsother tribologists to run the test with the same operating conditions listed in Table 1 ondifferent devices, thereby creating a useful wear volume loss database for a wide variety ofmaterial-pairs and establishing a more reliable reproducibility limit.2.2. Engine Cylinder Coating Sample MaterialThe material combinations used for the test were a chromium nitride-coated cast iron(CI) piston ring running against a coated cylinder bore specimen. The steel coating wasdeposited on the engine cylinder bore surface by the PTWA process on a cast engine block,and the porous oxide coating was developed using the EJPO process. Both the engineblocks were supplied by the industry partners. The coating surface produced by the PTWAprocess was finished with a crosshatch honing procedure, and the coating surface producedby the EJPO process was unidirectionally polished, as shown in Figure 4. The honinggrooves observed on the PTWA-coated cylinder bore surface have a cross-hatch patternwith grooves inclined at 30◦ to each other. Honing groove angles between 30 and 60◦ isa prevalent practice in manufacturing engine cylinder surfaces to minimize friction bycreating oil flow channels [23]. The EJPO-coated surface exhibited fine uniaxial polishat 20◦ to the cylinder bore longitudinal axis. The angular alignment of the asperities inthe sliding direction is a crucial factor in determining the wear properties of the cylinderbore surface.Metals 2025, 15, x FOR PEER REVIEW 9 of 26 (a) (b) Figure 4. (a) SEM micrograph of the surface topography of the PTWA and (b) EJPO plasma-based engine cylinder coatings. The typical dimensions of the coated cylinder sample used for the tribological tests were 40 mm × 22 mm × 8 mm, as shown in Figure 5b. ASTM G181-21 [21] states that the contact area between sliding tribo-pairs must be consistent throughout the oscillation stroke. Conformal contact between the tribo-pairs was ensured with a piston ring cut in segments of 10 mm that matched the radius of curvature of the cut cylinder samples, as shown in Figure 5a. This configuration produced a uniform wear track area for the tests, which met the criterion for comparison of wear rate between coated cylinder specimens. The shape and dimensions of the liner sample were developed for a line contact between the ring and the liner coating that is 10 mm in width. After extracting each cylinder liner sample, they were cleaned with acetone in a bath for fifteen minutes and dried with pres-surized air. (a) (b) (c) Figure 5. (a) 10 mm CrN-coated CI piston ring segment. (b) Unworn coated engine cylinder samples. (c) Samples cut from V8 Ford EJPO-coated engine block. 2.3. Lubrication Concept The test method is based on ASTM G181-21 [21] standards, but adapted to simulate starved boundary lubrication conditions during a cold start when fresh oil is yet to arrive in the engine cylinder. The sample preparation emulates the worst-case lubrication sce-nario at TDC. As combustion pressure peaks, the surface of the bore is starved of Figure 4. (a) SEM micrograph of the surface top graphy of the PTWA and (b) EJPO plasma-basedengine cylinder coatings.Metals 2025, 15, 370 9 of 24The typical dimensions of the coated cylinder sample used for the tribological testswere 40 mm × 22 mm × 8 mm, as shown in Figure 5b. ASTM G181-21 [21] states that thecontact area between sliding tribo-pairs must be consistent throughout the oscillation stroke.Conformal contact between the tribo-pairs was ensured with a piston ring cut in segmentsof 10 mm that matched the radius of curvature of the cut cylinder samples, as shown inFigure 5a. This configuration produced a uniform wear track area for the tests, which metthe criterion for comparison of wear rate between coated cylinder specimens. The shapeand dimensions of the liner sample were developed for a line contact between the ring andthe liner coating that is 10 mm in width. After extracting each cylinder liner sample, theywere cleaned with acetone in a bath for fifteen minutes and dried with pressurized air.Metals 2025, 15, x FOR PEER REVIEW 9 of 26 (a) (b) Figure 4. (a) SEM micrograph of the surface topography of the PTWA and (b) EJPO plasma-based engine cylinder coatings. The typical dimensions of the coated cylinder sample used for the tribological tests were 40 mm × 22 mm × 8 mm, as shown in Figure 5b. ASTM G181-21 [21] states that the contact area between sliding tribo-pairs must be consistent throughout the oscillation stroke. Conformal contact between the tribo-pairs was ensured with a piston ring cut in segments of 10 mm that matched the radius of curvature of the cut cylinder samples, as shown in Figure 5a. This configuration produced a uniform wear track area for the tests, which met the criterion for comparison of wear rate between coated cylinder specimens. The shape and dimensions of the liner sample were developed for a line contact between the ring and the liner coating that is 10 mm in width. After extracting each cylinder liner sample, they were cleaned with acetone in a bath for fifteen minutes and dried with pres-surized air. (a) (b) (c) Figure 5. (a) 10 mm CrN-coated CI piston ring segment. (b) Unworn coated engine cylinder samples. (c) Samples cut from V8 Ford EJPO-coated engine block. 2.3. Lubrication Concept The test method is based on ASTM G181-21 [21] standards, but adapted to simulate starved boundary lubrication conditions during a cold start when fresh oil is yet to arrive in the engine cylinder. The sample preparation emulates the worst-case lubrication sce-nario at TDC. As combustion pressure peaks, the surface of the bore is starved of Figure 5. (a) 10 m CrN-coate I iston ring segment. (b) Unworn coated engine cylinder samples.(c) Samples cut from V8 Ford EJPO-coated engine block.2.3. Lubrication ConceptThe test method is based on ASTM G181-21 [21] standards, but adapted to simulatestarved boundary lubrication conditions during a cold start when fresh oil is yet to arrive inthe engine cylinder. The sample preparation emulates the worst-case lubrication scenarioat TDC. As combustion pressure peaks, the surface of the bore is starved of l bricatingoil, and the engine is yet to reach operating temperatures. Standard SAE 0W30 syntheticengine oil for gasoline engines was utilized as the lubricant in all of the wear tests. Themanufacturer rating of oil density at 20 ◦C is 841 kg/m3. Scuffing wear may also manifest atthe bottom dead center (BDC), albeit to a comparatively lesser degree, or in the mid-strokearea, in the presence of abrasive particles. The primary factors contributing to wear in themid-stroke region are the oscillatory movement of the rings and the existence of abrasiveparticles. Nevertheless, the present study does not prioritize the investigation of the wearcharacteristics exhibited by said zones.To simulate the boundary lubrication regime during a cold start, the samples werefirst suspended in an acetone bath for 10 min to remove foreign particles and oil content.Then, 1 mL of engine oil was added to the surface of the coated cylinder bore samples andleft for 30 min. Subsequently, the oil was drained from the surface of the sample underthe influence of gravity for 30 min. This procedure was designed to simulate the start–stop system mode of the engine or the process of restarting the engine following a periodof inactivity. The coating’s surface profiles served as oil reservoirs. Additionally, the honingMetals 2025, 15, 370 10 of 24or polishing patterns help to retain oil and sustain a layer of lubrication on the coatings, evenwhen the engine is inactive. It is essential to prevent the oil from draining due to gravity,which could lead to metal-to-metal contact between the tribo-pair during engine startup.The thickness of the oil film was measured using the OLS51000 3D Laser Microscope(Tokyo, Japan), as shown in Figure 6. Table 3 shows readings of film thickness measurementon the coated samples based on the lubrication concept. Ting et al. [4] reported that thelower limit of the film thickness below which boundary lubrication contact occurs betweena piston ring and cylinder liner is 50 × 10−6 in (1.27 µm). As the values reported in Table 4for both the coatings fall below the critical lower limit value of 1.27 µm, it is assumed thatcontact of ring-bore occurs, and wear takes place.Metals 2025, 15, x FOR PEER REVIEW 10 of 26 lubricating oil, and the engine is yet to reach operating temperatures. Standard SAE 0W30 synthetic engine oil for gasoline engines was utilized as the lubricant in all of the wear tests. The manufacturer rating of oil density at 20 °C is 841 kg/m3. Scuffing wear may also manifest at the bottom dead center (BDC), albeit to a comparatively lesser degree, or in the mid-stroke area, in the presence of abrasive particles. The primary factors contributing to wear in the mid-stroke region are the oscillatory movement of the rings and the exist-ence of abrasive particles. Nevertheless, the present study does not prioritize the investi-gation of the wear characteristics exhibited by said zones. To simulate the boundary lubrication regime during a cold start, the samples were first suspended in an acetone bath for 10 min to remove foreign particles and oil content. Then, 1 mL of engine oil was added to the surface of the coated cylinder bore samples and left for 30 min. Subsequently, the oil was drained from the surface of the sample under the influence of gravity for 30 min. This procedure was designed to simulate the start–stop system mode of the engine or the process of restarting the engine following a period of inactivity. The coating’s surface profiles served as oil reservoirs. Additionally, the hon-ing or polishing patterns help to retain oil and sustain a layer of lubrication on the coat-ings, eve when the engine is i active. It i essential to prevent the il from draining due to gravity, which could lead to metal-to-metal co tact between the tribo-pair du ing en-gine startup. he thickne s of the oil film was measured using the OLS51000 3D Laser Microscope (Tokyo, Japan), a show in Figure 6. Table 3 shows readings of film thickn s measure-ment on th co ted sampl s based on the lubrication concept. Ting et al. [4] reported that the lower limit of the film thickness below which boundary lubri ation c ntact occurs be-tween a pisto ring and cylinder liner is 50 × 0−6 in (1.27 µm). As the valu s report d in Table 4 for both the coatings fall below the critical lower limit value of 1.27 µ , it is as-sumed that contact of ring-bore occurs, and wear takes place. Figure 6. Laser interferometry to determine the oil film thickness. Each colored line denotes the measurement path for oil film thickness. The measurements were recorded at an identical distance from the centerline on both cylindrical cross-section samples. Table 4. Line readings taken on the laser interferometer. Coated Samples Oil Film Thickness Measurement Readings (µm) 1 2 3 4 5 6 7 Mean SD PTWA 0.977 0.968 0.977 0.977 0.981 0.968 0.863 0.959 0.0393 EJPO 0.968 0.977 1.164 1.055 1.023 0.997 0.968 1.022 0.0652 The Hutchings equation [24] (see Equation (3)) was used to calculate the minimum oil film thickness between the surfaces of the piston ring segment and the coated cylinder bore samples at different load values at the start of the wear test. Figure 6. Laser interferometry to determine the oil film thickness. Each colored line denotes themeasurement path for oil film thickness. Th m asure ents were recorded at an identical di tancefrom the centerline on b th cylindrical cross-section samples.Table 4. Line readings taken on the laser interferometer.CoatedSamplesOil Film Thickness Measurement Readings (µm)1 2 3 4 5 6 7 Mean SDPTWA 0.977 0.968 0.977 0.977 0.981 0.968 0.863 0.959 0.0393EJPO 0.968 0.977 1.164 1.055 1.023 0.997 0.968 1.022 0.0652The Hutchings equation [24] (see Equation (3)) was used to calculate the mini um oilfilm thickness between the surfaces of the piston ring segment and the coated cylinder boresamples at different load values at the start of the wear test.hmin = 1.79 R0.47α0.49 · η0.680 · (E∗)−0.12 ·V0.68 · p−0.07 (3)where R is the radius of the ring (m), V is the sliding speed (m/s), p is the applied normalload (N), η0 represents viscosity properties of the lubricating oil (Pa·s), E* is the combinedelastic modulus of sliding surfaces, and α represents the viscosity pressure coefficient(Pa·s) [24]. Figure 7 helps infer that increasing the load decreases minimum film thickness ata constant relative velocity. As load is applied on two surfaces under lubricated conditions,the minimum lubricating film thickness resembles the minimum amount of lubrication oilthat will always be present between the sliding surfaces. The calculations confirmed thatthe oil film thickness on the test sample’s surface at the start of the test is greater than theminimum amount of film thickness when the load is applied.Metals 2025, 15, 370 11 of 24Metals 2025, 15, x FOR PEER REVIEW 11 of 26 hmin = 1.79 R0.47α0.49 ⋅ η00.68 ⋅ (E∗)−0.12 ⋅ V0.68 ⋅ p−0.07 (3) where R is the radius of the ring (m), V is the sliding speed (m/s), p is the applied normal load (N), η0 represents viscosity properties of the lubricating oil (Pa⋅s), E* is the combined elastic modulus of sliding surfaces, and α represents the viscosity pressure coefficient (Pa⋅s) [24]. Figure 7 helps infer that increasing the load decreases minimum film thickness at a constant relative velocity. As load is applied on two surfaces under lubricated condi-tions, the minimum lubricating film thickness resembles the minimum amount of lubri-cation oil that will always be present between the sliding surfaces. The calculations con-firmed that the oil film thickness on the test sample’s surface at the start of the test is greater than the minimum amount of film thickness when the load is applied. Figure 7. Minimum oil film thickness between tribo-pairs at different applied loads. 2.4. Wear Test Configuration Fresh cylinder specimens, piston ring segments, and engine oil were used for each wear test. After the lubricated sample was mounted on the sample holder, the contact point of the piston ring was set at the center of the cylinder specimen. There was no ap-plied force during this step, so the piston ring and cylinder sample did not touch. The only lubrication provided during the test is by the oil trapped in the valleys and micropores within the surface of the coatings. During the fitment of samples on the tribometer, a light 2 N load was applied to check for conformal contact between the liner’s surface and the piston ring segment. The alignment of the tribo-pair curvature is monitored using the light gap method. The line contact between the piston ring segment and the coated cylinder bore section was checked using a blue dye. Thereafter, the test load was applied and al-lowed to stabilize before initiating the reciprocating tests. The operating parameters are mentioned in Table 4. All factors, such as load, temperature, and relative humidity, were constant for the test duration. It is not straightforward to compare the reproducibility of wear volume results for the piston ring-cylinder bore tribo-pair due to its dependence on environmental condi-tions and operating parameters. The use of wear rate data enables comparison between interlaboratory results unaffected by the contact variations and test duration. For wear rate calculation, the average of three readings of the surface profiles was used to deter-mine the planimetric cross-sectional wear area, as shown in Figure 8. The ASTM G99-17 [25] standard defines the formula of specific wear rate (mm3/N-m) as: Figure 7. Minimum oil film thickness between tribo-pairs at different applied loads.2.4. Wear Test ConfigurationFresh cylinder specimens, piston ring segments, and engine oil were used for eachwear test. After the lubricated sample was mounted on the sample holder, the contact pointof the piston ring was set at the center of the cylinder specimen. T ere was no appliedforce during this step, so th piston ri g and cylinder sam le did not touch. The onlylubrication provided during the test is by the oil trapped in the valleys and microporeswithin the surfac of the coatings. During t fitment of sampl s on the tribo eter, a light2 N load was applied to check for co formal contact between the line ’s surface nd thepist n ring egment. The alignment of the trib -p ir curvature is monitored using the lightgap method. The line contact between the piston ring segment and the coated cylinder boresection was checked usi g a blue dye. T reaf er, the te t load w s applied nd allowed tostabilize before initiating the reciprocating tests. Th opera ing p rameters are me tionedin Table 4. All factors, such as load, t mperature, and relative humidity, were constant forthe test uration.It is not straightforward to compare the reproducibility of wear volume results for thepiston ring-cylinder bore trib -pair du o its dependence on envi onmental conditi nsand operating parameters. The use of wear rate data enables comparison between inter-laboratory results unaffected by the contact variations and test duration. For wear ratecalculation, the average of three readings of the surface profiles was used to determinethe p animetric cross-sectional w ar area, a shown in Figure 8. The ASTM G99-17 [25]standard defines the formula of specific wear r te (mm3/N-m) as:Wsp = V/LD (4)where V represents the total wear volume (mm3), L represents the normal load (N), andD represents the total sliding distance (m). A Bruker Dektat stylus profilometer was usedto generate a 2D-profilogram (see Figure 8). The generated 2D profile was then used tocalculate the planimetric wear volumes of the worn engine cylinder samples. Wear volumeis the product of the wear width and the average planimetric cross-sectional area of thewear track shown in the schematic below. The surface profile measurements were takenat the center of the wear width and along the stroke length. The Origin Pro graphinganalysis software (Origin Pro 2023, https://www.originlab.com/2023, accessed 1 June2023) calculated the planimetric wear area enclosed within the stroke length and datumline. The stroke length is bounded by vertical bars on the left and right borders. A datumline represents the mean surface roughness line. The surface area that has undergone wearin the planimetric wear plane is denoted by the grey region in Figure 8.Metals 2025, 15, 370 12 of 24Metals 2025, 15, x FOR PEER REVIEW 12 of 26 W𝑠𝑠𝑠𝑠 = V/LD (4) where V represents the total wear volume (mm3), L represents the normal load (N), and D represents the total sliding distance (m). A Bruker Dektat stylus profilometer was used to generate a 2D-profilogram (see Figure 8). The generated 2D profile was then used to calculate the planimetric wear volumes of the worn engine cylinder samples. Wear vol-ume is the product of the wear width and the average planimetric cross-sectional area of the wear track shown in the schematic below. The surface profile measurements were taken at the center of the wear width and along the stroke length. The Origin Pro graphing analysis software (Origin Pro 2023, https://www.originlab.com/2023, accessed 1 June 2023) calculated the planimetric wear area enclosed within the stroke length and datum line. The stroke length is bounded by vertical bars on the left and right borders. A datum line represents the mean surface roughness line. The surface area that has undergone wear in the planimetric wear plane is denoted by the grey region in Figure 8. Figure 8. Surface profile measured along the stroke at the center of the wear track. The net radial force experienced by the PRCL system originates from the pressure of the gases in the cylinder acting on the surface area of the piston. The primary contributors to this radial load experienced by the cylinder liner can be attributed to two main sources: (i) the radial force transmitted from the connecting rod to the piston as a function of com-bustion pressure, piston axial velocity, and displacement, and (ii) the elastic tension of the piston rings [26]. The test loads were selected based on the contact area of the piston ring segment, as mentioned in Table 5. A spectrum of contact pressures can be observed in the contact scenario between the PRCL tribo-pair. Determining contact pressure values is con-tingent upon the combustion pressure and engine design specifications [17]. The normal loads used to evaluate the coated engine cylinder samples were derived from the nominal contact pressure values that correlate with the combustion pressure observed in passenger gasoline vehicles. Figure 8. Surface profile easured along the stroke at the center of the ear track.The net radial force experienced by the PRCL system originates from the pressure ofthe gases in the cylinder acting on the surface area of the piston. The primary contributorsto this radial load experienced by the cylinder liner can be attributed to two main sources:(i) the radial force transmitted from the connecting rod to the piston as a function ofcombustion pressure, piston axial velocity, and displacement, and (ii) the elastic tension ofthe piston rings [26]. The test loads were selected based on the contact area of the pistonring segment, as mentioned in Table 5. A spectrum of contact pressures can be observedin the contact scenario between the PRCL tribo-pair. Determining contact pressure valuesis contingent upon the combustion pressure and engine design specifications [17]. Thenormal loads used to evaluate the coated engine cylinder samples were derived from thenominal contact pressure values that correlate with the combustion pressure observed inpassenger gasoline vehicles.Table 5. Operating parameters for the PRCL wear test.Parameters ValueNormal Loads (N) 50, 100, 165Nominal Contact Pressure (MPa) 3.72, 8.44, 13.8Contact Area (mm2) 13.4, 11.9, 12.1Frequency (Hz) 5Sliding Distances (m) 500, 100, 3000Composite Surface Roughness of CylinderSample and Piston Ring (µm) 1.72 (PTWA), 1.97 (EJPO)Stroke Length (m) 0.01Material Coated engine bore sample (stationary)and piston ring segment (oscillating)Initial Temperature (◦C) 22 ± 1Lubrication Regime Boundary lubricationRelative Humidity (%) 14.5–16Oscillation Speed (m/s) 0.10Test Duration (min) 100, 200, 600Metals 2025, 15, 370 13 of 243. Results and Discussion3.1. RMS Roughness, Skewness, and KurtosisThe wear behavior of tribo-pairs largely depends on the surface roughness param-eters [27]. Similarly, the frictional behavior of tribo-pairs is also influenced by surfaceroughness and distribution of the peaks and valleys within the surface profile. Before thetribological tests, surface topography profiles of the coated cylinder samples were recorded,as shown in Figure 9. Both unworn-coated cylinder liner surfaces exhibit flattened peaksas seen in the 2D-profilogram. This indicates that all three samples of both coatings showminimal probability of wear due to the peak asperity contact. The roughness averages,kurtosis, and skewness were calculated using the Bruker XT postprocessing software.Metals 2025, 15, x FOR PEER REVIEW 13 of 26 Table 5. Operating parameters for the PRCL wear test. Parameters Value Normal Loads (N) 50, 100, 165 Nominal Contact Pressure (MPa) 3.72, 8.44, 13.8 Contact Area (mm2) 13.4, 11.9, 12.1 Frequency (Hz) 5 Sliding Distances (m) 500, 100, 3000 Composite Surface Roughness of Cylinder Sample and Piston Ring (µm) 1.72 (PTWA), 1.97 (EJPO) Stroke Length (m) 0.01 Material Coated engine bore sample (stationary) and piston ring segment (oscillating) Initial Temperature (°C) 22 ± 1 Lubrication Regime Boundary lubrication Relative Humidity (%) 14.5–16 Oscillation Speed (m/s) 0.10 Test Duration (min) 100, 200, 600 3. Results and Discussion 3.1. RMS Roughness, Skewness, and Kurtosis The wear behavior of tribo-pairs largely depends on the surface roughness parame-ters [27]. Similarly, the frictional behavior of tribo-pairs is also influenced by surface roughness and distribution of the peaks and valleys within the surface profile. Before the tribological tests, surface topography profiles of the coated cylinder samples were rec-orded, as shown in Figure 9. Both unworn-coated cylinder liner surfaces exhibit flattened peaks as seen in the 2D-profilogram. This indicates that all three samples of both coatings show minimal probability of wear due to the peak asperity contact. The roughness aver-ages, kurtosis, and skewness were calculated using the Bruker XT postprocessing soft-ware. Figure 9. Profile data of unworn EJPO And PTWA-coated specimen samples. The unworn PTWA coating surface roughness (Ra) and RMS roughness (Rq) values are lower than measured for the unworn EJPO coating, as shown in Table 6. Roughness averages give information about the surface profile deviation of the mean datum line. Typically, smoother surfaces experience less wear. However, with less lubrication, more severe frictional wear is possible due to abrasive asperity contact. The distinct difference between the unworn surface profile of both coatings is represented by the deepest valley depths (Rv) value. Skewness (Rsk) denotes the distribution of peaks and valleys about the surface roughness mean line (see schematic in Figure 10). A slightly negative Rsk value for both coatings explains the choice of automakers to have fine asperities that encounter Figure 9. Profile data of unworn EJPO And PTWA-coated specimen samples.The unworn PTWA coating surface roughness (Ra) and RMS roughness (Rq) valuesare lower than measured for the unworn EJPO coating, as shown in Table 6. Roughnessaverages give information about the surface profile deviation of the mean datum line.Typically, smoother surfaces experience less wear. However, with less lubrication, moresevere frictional wear is possible due to abrasive asperity contact. The distinct differencebetween the unworn surface profile of both coatings is represented by the deepest valleydepths (Rv) value. Skewness (Rsk) denotes the distribution of peaks and valleys about thesurface roughness mean line (see schematic in Figure 10). A slightly negative Rsk value forboth coatings explains the choice of automakers to have fine asperities that encounter thepiston ring and enable the initial running-in wear. Kurtosis is defined as the density of thesharpness of peaks in the surface profile. The negative skewness and high kurtosis valuesin Table 6 represent low friction between sliding contacts due to the relatively flat surfacewith deep valleys, as depicted in Figure 10.Table 6. Initial surface roughness parameters of the coated-cylinder samples (average of five pro-file readings).Coating Rq (µm) Rv (µm) Ra (µm) Rku RskPTWA 0.75 ± 0.26 7.60 ± 2.3 0.53 ± 1.04 24.34 ± 0.65 −3.59 ± 0.45EJPO 2.87 ± 0.35 19.60 ± 1.4 1.92 ± 0.42 16.74 ± 0.51 −4.10 ± 0.35Metals 2025, 15, x FOR PEER REVIEW 14 of 26 the piston ring and enable the initial running-in wear. Kurtosis is defined as the density of the sharpness of peaks in the surface profile. The negative skewness and high kurtosis values in Table 6 represent low friction between sliding contacts due to the relatively flat surface with deep valleys, as depicted in Figure 10. Figure 10. Schematic diagram depicting the effect of skewness and kurtosis on friction. Table 6. Initial surface roughness parameters of the coated-cylinder samples (average of five profile readings). Coating Rq (µm) Rv (µm) Ra (µm) Rku Rsk PTWA 0.75 ± 0.26 7.60 ± 2.3 0.53 ± 1.04 24.34 ± 0.65 −3.59 ± 0.45EJPO 2.87 ± 0.35 19.60 ± 1.4 1.92 ± 0.42 16.74 ± 0.51 −4.10 ± 0.35The impact of surface roughness parameters on friction and wear can be attributed to two factors. Firstly, it regulates the thickness of the oil film, which leads to scuffing initiation. Secondly, it influences the contact area between the sliding surfaces. Pawlus et al. [28] found that lower Rq values lead to reduced friction between sliding surfaces, re-sulting in smoother contact surfaces and better lubricant retention. The study suggests that the resulting surface exhibits a distinct texture derived from the amalgamation of the two phenomena. The recommended surface texture for honed cylinder liner surfaces of-fered by engine manufacturers includes minimal surface roughness values and the bear-ing area parameters comprising smaller peaks that provide larger initial contact area for the counter face surface aimed to reduce contact stress during sliding. The variation in roughness parameters post-tribological tests of the PTWA and EJPO coatings are shown in Figure 11. Initially, prior to the wear tests (i.e., at 0 m, 0 min bar plots of Figure 11a), the EJPO coating has a higher average Rq value, which is to be ex-pected due to the nature of the manufacturing procedure and lack of postprocessing that the EJPO coating process receives. However, as the load and sliding distance increase, the Rq values for both coatings merge closer together. In general, it appears that the EJPO coating continues to have a slightly higher Rq than the PTWA coating. After the initial run-in phase, the 2D bar plots show a reducing trend in the RMS surface roughness value, until the load and sliding distance is increased to 165 N and 3000 m, respectively. The large increase in Rq value at 165 N-3000 m is potentially due to the presence of wear burrs and ploughing tracks formed by a highly abrasive three-body wear phenomenon. The observed Rq trends shown in Figure 11a suggest that wear induces the formation of pointed crests on the contact surface during the tests, followed by a gradual flattening of asperities. Figure 10. Schematic diagram depicting the effect of skewness and kurtosis on friction.Metals 2025, 15, 370 14 of 24The impact of surface roughness parameters on friction and wear can be attributed totwo factors. Firstly, it regulates the thickness of the oil film, which leads to scuffing initiation.Secondly, it influences the contact area between the sliding surfaces. Pawlus et al. [28]found that lower Rq values lead to reduced friction between sliding surfaces, resultingin smoother contact surfaces and better lubricant retention. The study suggests that theresulting surface exhibits a distinct texture derived from the amalgamation of the twophenomena. The recommended surface texture for honed cylinder liner surfaces offeredby engine manufacturers includes minimal surface roughness values and the bearing areaparameters comprising smaller peaks that provide larger initial contact area for the counterface surface aimed to reduce contact stress during sliding.The variation in roughness parameters post-tribological tests of the PTWA and EJPOcoatings are shown in Figure 11. Initially, prior to the wear tests (i.e., at 0 m, 0 min bar plotsof Figure 11a), the EJPO coating has a higher average Rq value, which is to be expecteddue to the nature of the manufacturing procedure and lack of postprocessing that theEJPO coating process receives. However, as the load and sliding distance increase, theRq values for both coatings merge closer together. In general, it appears that the EJPOcoating continues to have a slightly higher Rq than the PTWA coating. After the initialrun-in phase, the 2D bar plots show a reducing trend in the RMS surface roughness value,until the load and sliding distance is increased to 165 N and 3000 m, respectively. The largeincrease in Rq value at 165 N-3000 m is potentially due to the presence of wear burrs andploughing tracks formed by a highly abrasive three-body wear phenomenon. The observedRq trends shown in Figure 11a suggest that wear induces the formation of pointed crestson the contact surface during the tests, followed by a gradual flattening of asperities.The skewness and kurtosis values for the coated samples differ significantly froma standard Gaussian surface (Rsk = 0 and Rku = 3) [28]. The kurtosis values for bothcoated specimens clearly exhibit a decreasing trend over time as shown in Figure 11b.The observed trend indicates a reduction in surface asperity peak heights, and increasingcontact pressure that signifies deteriorating wear resistance. A declining Rku value impliesthat the contact surface will eventually attain uniformity upon completion of the running-in phase as the fine peaks get eroded. The rate at which the skewness increases for thePTWA-coated specimen is 20% higher than the EJPO-coated samples for the increment insliding distances. This means that at the asperity contact level, it can be estimated that thewear rate of the PTWA coating could be higher than that of the EJPO-coated bore surface.This is because the surface fatigue resistance decreases with an increase in skewness [29].Surfaces with fewer peaks above the mean line may better retain impurities or debrisfrom oxide film breakdown that otherwise might function as a third abrasive body at thecontact. The EJPO-coated sample consisted of a high density of micropores when comparedto the PTWA-coated sample, as seen in the initial Rv values shown in Figure 9. The oilheld by porous valleys ensures a better wear resistance of the EJPO-coated sample. Inthe boundary lubrication regime, Rsk is the dominant surface roughness parameter thatnegatively correlates to friction experienced by the tribo-pairs [27]. The initial unwornprofiles showed that the PTWA coating has lower skewness than the EJPO coated specimenowing to its more extensive surface preparation of the cylinder bore, as evident in the lowerroughness values. The EJPO surface profile showed a lower value of Rsk when comparedto its PTWA counterpart after the wear tests as shown in Figure 11c, suggesting that theEJPO coating may result in lower friction and thus wear rates during engine operation.Metals 2025, 15, 370 15 of 24Metals 2025, 15, x FOR PEER REVIEW 15 of 26 (a) (b) (c) Figure 11. Evolution of surface roughness parameters during the wear tests. (a) RMS roughness average, (b) kurtosis, and (c) skewness. The skewness and kurtosis values for the coated samples differ significantly from a standard Gaussian surface (Rsk = 0 and Rku = 3) [28]. The kurtosis values for both coated specimens clearly exhibit a decreasing trend over time as shown in Figure 11b. The ob-served trend indicates a reduction in surface asperity peak heights, and increasing contact pressure that signifies deteriorating wear resistance. A declining Rku value implies that the contact surface will eventually attain uniformity upon completion of the running-in phase as the fine peaks get eroded. The rate at which the skewness increases for the PTWA-coated specimen is 20% higher than the EJPO-coated samples for the increment in sliding distances. This means that at the asperity contact level, it can be estimated that the wear rate of the PTWA coating could be higher than that of the EJPO-coated bore surface. This is because the surface fatigue resistance decreases with an increase in skewness [29]. Surfaces with fewer peaks above the mean line may better retain impurities or debris from oxide film breakdown that otherwise might function as a third abrasive body at the con-tact. The EJPO-coated sample consisted of a high density of micropores when compared to the PTWA-coated sample, as seen in the initial Rv values shown in Figure 9. The oil held by porous valleys ensures a better wear resistance of the EJPO-coated sample. In the . tests. r, ( ) t i , ( ) s .3.2. Dependence of Wear Rate on Sliding DistanceThe wear rate during an engine operation is characterized by a high initial abrasivewear run-in phase of the plateau-honed cylinder surface, which then transitions to a lowersteady rate of wear and is followed by marginally severe corrosive wear. In the earlystages of engine operation, the engine liner experiences a smoothing process of the highestsurface irregularities. Wear in the PRCL tribo-system initiates in the form of deformedasperities when the load surpasses a critical value, and the lubricant tribo-film reachesminimal thickness. Subsequently, the liner surface demonstrates a condition of steady-statewear during the useful service life period characterized by a low steady wear rate.Figure 12 illustrates the results of three wear tests on both coated engine cylinderspecimens in the boundary lubrication regime with a 100 N normal load applied on thetribo-pair. The wear rate value for the three tests was statistically close to the average ofthe sample set of readings, as shown in Table 6. ASTM 133-22 [20] states that taking threeprofile readings for a difference between planimetric wear areas within 25% is adequate fortribological tests.Metals 2025, 15, 370 16 of 24Metals 2025, 15, x FOR PEER REVIEW 17 of 26 (a) (b) Figure 12. (a) 2D-Profilogram depicting wear for 100 N load at varying sliding distances. (b) Calcu-lated wear rate based on the planimetric wear area acquired from the wear profile at different slidingdistances at 100 N load.Metals 2025, 15, 370 17 of 24The wear performance improved with increased sliding distances for the coatedsamples, and the running-in and steady-state wear trend was observed. The average wearrates for a 500 m sliding distance and 100 N normal load are 1.72 × 10−8 mm3/N-m (EJPOcoating) and 1.79 × 10−8 mm3/N-m (PTWA coating), which represent the abrasive run-inphase. Increasing the sliding distance to 1000 m reduces the wear rate of the EJPO andPTWA coating by ~38 and 29%, respectively. Further increasing the sliding distance to3000 m results in a further decrease in the wear rate for the EJPO (54% reduction comparedto 500 m distance) and PTWA (51%) coatings. Overall, it was observed that the EJPO-coatedspecimen exhibited better wear resistance when compared to its PTWA counterpart duringthe running in and steady-state wear stages of the test. The uncertainty in the experimentalwear rate values was evaluated by the 95% confidence interval shown in Table 7.Table 7. Statistical significance of wear rate data.CoatedBore SampleSlidingDistance (m) S.D.The Margin ofError (95% CI)EJPO500 3.78 × 10−10 ±2.49%1000 3.51 × 10−10 ±3.69%3000 2.19 × 10−10 ±3.20%PTWA500 6.11 × 10−10 ±3.70%1000 2.51 × 10−10 ±2.23%3000 2.41 × 10−10 ±3.08%3.3. Dependence of Wear Rate on Normal LoadThe applied load is proportional to the friction force experienced by the contactsurfaces and is dependent on the contact area. The planimetric wear area augmentedas expected with the increase in applied load. The surface profiles of the worn cylindersamples are shown in Figure 13. The wear rate values for varying load and sliding distancedepicted in the illustrations shown in Figure 14 were derived by computing the mean ofeach wear test outcomes for both coated cylinder samples.The findings show the superior wear resistance of the EJPO-coated sample, as com-pared to the PTWA coating, except in the 50 N-500 m tests, where the wear rates varyinsignificantly. The similar wear rate values of both coatings during the 50 N-500 m testscan be correlated to the complex microstructure of the EJPO coating. The EJPO coatingconsists of three layers due to the complex nature of the electrochemical reaction that leadsto the formation of the oxide coating on the substrate. A top porous layer (6–10 m thickness)is made of an amorphous structure due to the rapid quenching effect from the electrolyte,an inner dense layer, and a thin diffusion layer (100 nm thick) with Si-rich Al substrate.The lower hardness of the highly porous top surface of the EJPO coating allows for a fasterrun-in wear of the cylinder bore. Archard’s wear law states that the wear volume of thedebris volume is inversely proportional to the hardness of the softer tribo-pair [30], whichin this case is the coated cylinder bore specimen. However, as load and sliding durationwere incremented, the wear rate of the EJPO-coated sample showed better resistance towear, as compared to the PTWA sample.The specific wear rate of the coating increased with load, as seen in Figure 14. Maxi-mum wear was observed at 165 N loads due to higher friction and contact pressure. Underlower load test conditions, the EJPO coating exhibited lower wear rates as the slidingdistance increased.Metals 2025, 15, 370 18 of 24Metals 2025, 15, x FOR PEER REVIEW 19 of 26 Figure 13. 2D-Profilogram depicting wear caused by variation of applied load at constant sliding distances. Figure 14. Wear rates plot for EJPO- and PTWA-coated cylinder bore specimens. Figure 13. 2D-Profilogram depicting wear caused by variation of applied load at constant slid-ing distances.Metals 2025, 15, x FOR PEER REVIEW 19 of 26 Figure 13. 2D-Profilogram depicting wear caused by variation of applied load at constant sliding distances. Figure 14. Wear rates plot for EJPO- and PTWA-coated cylinder bore specimens. Figure 14. Wear rates plot for EJPO- and PTWA-coated cylinder bore specimens.Metals 2025, 15, 370 19 of 24Following a visual and compositional inspection using scanning electron microscopycoupled with energy dispersive X-ray spectroscopy, no signs of mass transfer from theCI piston ring to the cylinder bore specimen, and vice versa, were observed. Hence,adhesive wear mechanisms are ruled out in the PRCL interaction. The prevalent wearmechanism is solely abrasive in nature. A detailed description of various wear mechanisms,in similar applications, can be found in Refs. [31,32]. At the end of 3000 m of pistonring reciprocation with a 165 N normal load, the CI piston ring weight was reducedby 4.892 × 10−3 g when run against the PTWA samples and 4.710 × 10−3 with EJPOsamples. Considering the density of CI is 7.12 g/cm3, the average wear rate of the CIpiston ring was calculated as 1.38 × 10−6 mm3/Nm for the piston ring run with PTWAsamples and 1.33 × 10−6 mm3/Nm for the piston ring run with EJPO samples. The CIpiston ring wear rate values agree closely with the boundary lubrication wear rate reportedby Kragelskii et al. [32]. The wear rates of different piston ring-cylinder liner materialpairings significantly affect the service life prediction and durability of the components.The increased wear affects the seepage of blow-by into the crankcase, resulting in inefficientfuel combustion.At high loads, the elevation of surface roughness is due to the formation of surfaceprotrusions, as shown in Figure 11a. The presence of lubricating films, oxide films, andsimilar substances between surfaces results in a decrease in adhesion propensity. However,in the boundary lubrication regime, the lubricating film saturates with the wear debris,and it starts to aggravate the wear rate of both the coated specimens caused by three-bodywear mechanisms. Three-body wear is brought on by a hard particle caught betweenthe rubbing surfaces, which in this case is wear debris, which increases the specific wearrate [33] This phenomenon was observed in the 165 N-3000 m wear test, where the volumeof material lost was greater (see Figure 15). During engine operation, the continuous flowof fresh lubricating oil effectively prevents three-body abrasion within the PRCL tribo-system. However, this protective mechanism can be compromised if foreign contaminantsor hard soot particles are introduced into the system. Given this, the EJPO coating’s higherwear resistance to foreign particles is a valuable feature. The greater redced valley depth(i.e., Rvk) of EJPO coating surface profiles enhances the probability of wear debris to getinto these crevices, thereby reducing its contribution to wear [34]. signs of ploughingidentified by the dotted lines in Figure 15 show an accumulation of material on the side ofthe three-body wear region.Metals 2025, 15, x FOR PEER REVIEW 21 of 26 (a) (b) Figure 15. SEM micrographs showing three-body abrasive wear mechanism at 165 N load, 3000 m duration tests on the (a) PTWA sample and (b) EJPO sample. The PTWA coating morphology consists of delaminated solidified splats, pores, and thin oxide layers. After wear tests, the surface is devoid of honing grooves caused by the reciprocating contact of the piston ring. The delamination pits have signs of crack initia-tion, as seen by the red arrows in Figure 16. During the PTWA coating process, low-carbon steel interacts with the oxygen in the air and forms iron oxide phases. Some of these mol-ten plasma particles solidify before hitting the surface, leading to the formation of regions susceptible to delamination due to high shear stresses on shallow pits [34]. The microcrack propagation initiates at these non-melted splat sites. The weak bonds at splat interfaces separate under shear stress induced by the sliding piston ring. The high susceptibility to three-body abrasive wear shows that the PTWA coated engine cylinder is susceptible to an increase in the clearance between piston ring and the cylinder liner due to external particles during engine operation. This could result in a greater amount of blow-by, thereby reducing engine efficiency. For the EJPO coated specimens under boundary lu-brication conditions, the wear branches out from the microcracks, which opens into a net-work of porosity pits, thereby further acting as reservoirs for the wear debris-rich lubri-cating oil. (a) (b) Figure 16. SEM micrographs to demonstrate wear propagation on the (a) PTWA sample and (b) EJPO sample. i iti t t t ( ) l ( ) J s l .Metals 2025, 15, 370 20 of 24The PTWA coating morphology consists of delaminated solidified splats, pores, andthin oxide layers. After wear tests, the surface is devoid of honing grooves caused by thereciprocating contact of the piston ring. The delamination pits have signs of crack initiation,as seen by the red arrows in Figure 16. During the PTWA coating process, low-carbon steelinteracts with the oxygen in the air and forms iron oxide phases. Some of these moltenplasma particles solidify before hitting the surface, leading to the formation of regionssusceptible to delamination due to high shear stresses on shallow pits [34]. The microcrackpropagation initiates at these non-melted splat sites. The weak bonds at splat interfacesseparate under shear stress induced by the sliding piston ring. The high susceptibilityto three-body abrasive wear shows that the PTWA coated engine cylinder is susceptibleto an increase in the clearance between piston ring and the cylinder liner due to externalparticles during engine operation. This could result in a greater amount of blow-by, therebyreducing engine efficiency. For the EJPO coated specimens under boundary lubricationconditions, the wear branches out from the microcracks, which opens into a network ofporosity pits, thereby further acting as reservoirs for the wear debris-rich lubricating oil.Metals 2025, 15, x FOR PEER REVIEW 21 of 26 (a) (b) Figure 15. SEM micrographs showing three-body abrasive wear mechanism at 165 N load, 3000 m duration tests on the (a) PTWA sample and (b) EJPO sample. The PTWA coating morphology consists of delaminated solidified splats, pores, and thin oxide layers. After wear tests, the surface is devoid of honing grooves caused by the reciprocating contact of the piston ring. The delamination pits have signs of crack initia-tion, as seen by the red arrows in Figure 16. During the PTWA coating process, low-carbon steel interacts with the oxygen in the air and forms iron oxide phases. Some of these mol-ten plasma particles s lidify before hitting the surface, le ding t the f rmation of r gions susceptible to delamination due to high shear stresses on s llow pits [34]. The microcrack propagation initiates at these non-melted splat sites. The weak bonds at splat interfaces separate under shear stress induced by the sliding piston ring. The high susceptibility to three-body abrasive wear shows that the PTWA coated engine cylinder is susceptible to an increase in the clearance between piston ring and the cylinder liner due to external particles during engine operation. This could result in a greater amount of blow-by, thereby reducing engine efficiency. For the EJPO coated specimens under boundary lu-brication conditions, the wear branches out from the icrocracks, which opens into a net-work of porosity pits, thereby further acting as reservoirs for th wear debris-rich lubri-cating o l. (a) (b) Figure 16. SEM micrographs to demonstrate wear propagation on the (a) PTWA sample and (b) EJPO sample. Figure 16. SEM micrographs to demonstrate wear pro ti n the (a) PTWA sample and(b) EJPO sample.3.4. Evolution of PorosityCompared to a hypereutectic Al-Si cylinder bore liner that employs Si particles with ahigh hardness that forms oil retention channels after initial running-in wear, the hypoeutec-tic Al-Si alloy coated with EJPO consists of micropores on its surface that function similarlyto the protruded Si particles in hypereutectic Al-Si liners. The significant quantity of mi-cropores on the EJPO-coated surface provides an additional ability to retain lubricatingengine oil. Previous porosity studies done on EJPO coatings applied on an aluminum alloysubstrate exhibited a porosity of around 20% [35]. Similar 2D porosity studies conducted onPTWA coatings by Ford Motor Company reported an average porosity of around 2–8% [36].The 2D surface SEM micropore analysis is the most straightforward approach forporosity analysis of the plasma-based electrolytic oxidation layer on diverse substrates.This method does not consider enclosed voids, and the morphology of subsurface voidsremains imperceptible. However, it provides quantifiable data for average top surfaceporosity distribution. EJPO coatings have a higher volume fraction of pores within thecoating structure. After the initial duration of run-in, the 1000 m sliding distance wear testMetals 2025, 15, 370 21 of 24specimen forms fracture networks, as shown in Figure 16b. The process of wear entailsthe elimination of asperities while simultaneously filling existing pores with wear debris,resulting in a surface that possesses diminished porosity.To quantify the percentage porosity, image stack processing software (Image-J, version1.8.0_345) was utilized. The porosity can be determined by calculating the ratio of thetotal area occupied by the detected pores to the overall area of the analyzed region ofinterest (see Equation (5)). SEM micrographs offer the required accuracy in establishingthe measurement scale per pixel. The lower limit on the width of the micropores is set at10 µm. The blue region denotes the areas where porosities are observed, and the greenregion denotes regions where surface burrs or protrusions are observed.Porosity (%) = (Total area of micro− pores/Total area of analyzed region) (5)High-porosity EJPO coatings have a higher volume fraction of interconnected poreswithin the coating structure. During the short duration of the initial run-in wear testof 500 m of sliding distance, the pores show signs of branching out. Thereafter, as thewear depth increases, the network of pores is exposed. The initial porosity level of PTWAcoating is 3.7%. As the sliding distance is increased, the signs of scuffing are seen clearly(see Figure 17). The initial porosity levels of the EJPO coating are 16%, which is signifi-cantly higher compared to the PTWA coating. No signs of scuffing are seen even up tothe maximum 3000 m sliding distance, which is an extremely desirable trait for enginecylinder applications.Metals 2025, 15, x FOR PEER REVIEW 23 of 26 Figure 17. Porosity evolution during wear tests with 101 N normal load. 4. Conclusions The wear behavior of two plasma-based engine cylinder coatings was examined with chrome-coated CI piston rings in the boundary lubrication cold start engine conditions. To simulate the contact conditions within the piston ring cylinder liner system, a new bench-scale setup was developed as an alternative to full-scale wear test equipment. The following conclusions can be drawn from the results: • As the normal load and sliding distances were incremented during the boundary lu-brication wear tests, the EJPO-coated engine cylinder samples show better wear re-sistance than its PTWA counterpart due to its ability to hold oil in its porous surface morphology. • The boundary-lubricated wear test on EJPO-coated engine cylinder samples shows better wear resistance than its PTWA counterpart across all loading conditions and test duration, except during the lowest load and sliding distance (i.e., 50 N-500 m), where the difference in wear rates of the two coatings is minimal. The maximum wear rate is observed at the initial 500 m of the tribo-test for the PTWA- (2.16 × 10−8 mm3/Nm) and EJPO-coated (2.13 × 10−8 mm3/Nm) specimens. • In the boundary-lubricated wear tests, 24% of the total wear volume was lost in the first 500 m sliding distance for the EJPO-coated sample. For PTWA-coated samples, around 26% of the coating volume was removed in the initial 500 m of the test. This comparable value of run-in wear of tribo-pairs shows that both coatings are suitable for cylinder bore applications, which employ CI piston rings. However, the EJPO coating showed lower wear rates across all loading scenarios. • A 20% slower rate of increase in skewness value of the EJPO samples compared to the PTWA samples during the wear tests indicates that the surface is filled with fine peaks and deep valleys. This aspect contributes to the lower wear rate due to the reduced counter-face friction. • The research findings highlight notable trends in surface roughness, skewness, and kurtosis of the EJPO coating comparable to the industry accepted PTWA coating Figure 17. Porosity evolution during wear tests with 101 N normal load.4. ConclusionsThe wear behavior of two plasma-based engine cylinder coatings was examined withchrome-coated CI piston rings in the boundary lubrication cold start engine conditions.To simulate the contact conditions within the piston ring cylinder liner system, a newbench-scale setup was developed as an alternative to full-scale wear test equipment. Thefollowing conclusions can be drawn from the results:Metals 2025, 15, 370 22 of 24• As the normal load and sliding distances were incremented during the boundarylubrication wear tests, the EJPO-coated engine cylinder samples show better wearresistance than its PTWA counterpart due to its ability to hold oil in its porous sur-face morphology.• The boundary-lubricated wear test on EJPO-coated engine cylinder samples showsbetter wear resistance than its PTWA counterpart across all loading conditions and testduration, except during the lowest load and sliding distance (i.e., 50 N-500 m), wherethe difference in wear rates of the two coatings is minimal. The maximum wear rate isobserved at the initial 500 m of the tribo-test for the PTWA- (2.16 × 10−8 mm3/Nm)and EJPO-coated (2.13 × 10−8 mm3/Nm) specimens.• In the boundary-lubricated wear tests, 24% of the total wear volume was lost in thefirst 500 m sliding distance for the EJPO-coated sample. For PTWA-coated samples,around 26% of the coating volume was removed in the initial 500 m of the test. Thiscomparable value of run-in wear of tribo-pairs shows that both coatings are suitablefor cylinder bore applications, which employ CI piston rings. However, the EJPOcoating showed lower wear rates across all loading scenarios.• A 20% slower rate of increase in skewness value of the EJPO samples compared to thePTWA samples during the wear tests indicates that the surface is filled with fine peaksand deep valleys. This aspect contributes to the lower wear rate due to the reducedcounter-face friction.• The research findings highlight notable trends in surface roughness, skewness, andkurtosis of the EJPO coating comparable to the industry accepted PTWA coatingduring the different stages of the wear test. After the initial run-in stage of the EJPOcoating, kurtosis and RMS surface roughness decreased, suggesting a reduction insurface peak heights and improved surface uniformity, proving its fitness for servicedespite less comprehensive surface preparation than the PTWA coating.• The wear process is initiated by abrasive wear. During the initial period of use, surfacepeaks, weak edges of cracks, and pores undergo removal, resulting in the generation ofwear debris and the formation of wear scars on the surface. In the case of PTWA-coatedsamples, the wear is subsequently intensified by the separation of splats, which occursdue to the propagation of cracks at the interfaces between the oxides. The coatinglayers undergo removal due to surface fatigue and serve as the third-body particleduring subsequent abrasive wear processes. The EJPO coating shows a much lesserpropensity for scuffing and three-body abrasive wear as the greater amount of surfaceporosities can retain impurities or debris in the lubricating oil.Author Contributions: Conceptualization, S.B., J.S. and D.S.; methodology, S.B.; validation S.B., J.S.and D.S.; formal analysis, S.B., J.S. and D.S.; investigation, S.B., J.S. and D.S.; resources, D.S. andJ.T.; data curation, S.B.; writing—original draft preparation, S.B.; writing—review and editing, S.B.,J.S., D.S. and J.T.; visualization, S.B.; supervision, J.S. and D.S.; project administration, D.S.; fundingacquisition, D.S. and J.T. All authors have read and agreed to the published version of the manuscript.Funding: This research was funded by Ford and a NSERC Alliance grant. The NSERC Alliance grantnumber is ALLRP 566184-21.Data Availability Statement: The original contributions presented in this study are included in thearticle. Further inquiries can be directed to the corresponding author.Acknowledgments: The authors would like to thank Nasim Bahramian and Sina Kianfar for theircontribution to prototyping the original version of the custom tribometry system.Conflicts of Interest: The authors declare no conflicts of interest.Metals 2025, 15, 370 23 of 24AbbreviationsThe following abbreviations are used in this manuscript:PTWA Plasma transfer wire arcEJPO Electrolytic jet plasma oxidationGHG Greenhouse gasSI Spark ignitionTCR Top compression ringTDC Top dead centerPRCL Piston ring-cylinder linerBDC Bottom dead centerCI Cast ironSEM Scanning electron microscopeReferences1. Kathryn, P. (Ed.) Second Global Sustainable Transport Conference Interagency Report; United Nations: New York, NY, USA, 2021.2. IPCC. Climate Change Synthesis Report; Core Writing Team, Lee, H., Romero, J., Eds.; Intergovernmental Panel on Climate Change(IPCC): Geneva, Switzerland, 2023.3. Wong, V.W.; Tung, S.C. Overview of automotive engine friction and reduction trends–Effects of surface, material, and lubricant-additive technologies. Friction 2016, 4, 1–28.4. Ting, L.; Mayer, J. Piston ring lubrication and cylinder bore analysis, part ii—Theory verification. ASME J. Lubr. Technol. 1974, 96,258–266.5. Sediako, D.; D’Elia, F.; Machin, C.; Lombardi, A.; Ravindran, C. Analysis of residual stress profiles in the cylinder web region ofan As-Cast V6 Al engine block with cast-in Fe liners using neutron diffraction. SAE Int. J. Mater. Manuf. 2011, 4, 138–151.6. KS Aluminum Technology. Aluminum Cylinder Blocks—Designs, Materials, Casting Methods and Cylinder Bore Surface Technologies forLight Weight Passenger-Car Engines; Springer: New York, NY, USA, 2009.7. Su, J.F.; Nie, X.; Hu, H.; Tjong, J. Friction and counterface wear influenced by surface profiles of plasma electrolytic oxidationcoatings on an aluminum A356 alloy. J. Vac. Sci. Technol. 2012, 30, 061402.8. Lenny, J. Replacing the Cast Iron Liners for Aluminum Engine Cylinder Blocks: A Comparative Assessment of Potential Candidates;Graduate Faculty of Rensselaer Polytechnic Institute: Hartford, CT, USA, 2011.9. Wuest, G.; Barbezat, G.; Keller, S. The key advantages of the plasma-powder spray process for the thermal spray coating ofcylinder bores in automotive industry. SAE Int. Congr. Expo. 1997, 106, 33–43. [CrossRef]10. Fauchais, P.L.; Heberlein, J.V.R.; Boulos, M.I. Thermal Spray Fundamentals: From Powder to Part; Springer Science & Business Media:Boston, MA, USA, 2014.11. Darut, G.; Liao, H.; Coddet, C.; Bordes, J.; Diaby, M. Steel coating application for engine block bores by Plasma Transferred WireArc spraying process. Surf. Coat. Technol. 2015, 268, 115–122.12. Bobzin, K.; Ernst, F.; Richardt, K.; Schlaefer, T.; Verpoort, C.; Flores, G. Thermal spraying of cylinder bores with the PlasmaTransferred Wire Arc process. Surf. Coat. Technol. 2008, 202, 4438–4443.13. Krishna, L.R.; Somaraju, K.R.C.; Sundararajan, G. The tribological performance of ultra-hard ceramic composite coatings obtainedthrough microarc oxidation. Surf. Coat. Technol. 2003, 163–164, 484–490.14. Bahramian, N.; Kianfar, S.; Stroh, J.; Sediako, D.; Tjong, J. An investigation of the microstructure and oil retention of electrolyticjet plasma oxidation (EJPO) coating. Miner. Met. Mater. Soc. 2002, 121, 629–638.15. Fanga, C.; Meng, X.; Kong, X.; Zhao, B.; Huang, H. Transient tribo-dynamics analysis and friction loss evaluation of piston duringcold- and warm-start of a SI engine. Int. J. Mech. Sci. 2017, 133, 767–787.16. Forero, J.D.; Ochoa, G.V.; Alvarado, W.P. Study of the piston secondary movement on the tribological performance of a singlecylinder low-displacement diesel engine. Lubricants 2020, 8, 80–97. [CrossRef]17. Morris, N.; Rahmani, R.; Rahnejat, H.; King, P.D.; Fitzsimons, B. Tribology of piston compression ring conjunction under transientthermal mixed regime of lubrication. Tribol. Int. 2013, 59, 248–258.18. Usman, A.; Park, C.W. Numerical investigation of frictional behavior and energy loss in mixed-hydrodynamic contact of pistonring pack with deformed cylinder liner during warm-up period of SI-engine. Energy Convers. Manag. 2016, 117, 115–131.19. Abril, S.O.; Rojas, J.P.; Flórez, E.N. Numerical methodology for determining the energy losses in auxiliary systems and frictionprocesses applied to low displacement diesel engines. Lubricants 2020, 8, 103. [CrossRef]20. ASTM G133-22; Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear. ASTM International: West Con-shohocken, PA, USA, 2022.Metals 2025, 15, 370 24 of 2421. ASTM G181-21; Standard Test Method for Conducting Friction Tests of Piston Ring and Cylinder Liner Materials Under LubricatedConditions. ASTM International: West Conshohocken, PA, USA, 2021.22. Duronio, F.; Vita, A.D.; Montanaro, A.; Villante, C. Gasoline direct injection engines—A review of latest technologies and trends.Fuel 2020, 265, 116947.23. Hartfield-Wunsch, S.E.; Tung, S.; Rivard, C. Development of a bench wear test for the evaluation of engine cylinder components.SAE Trans. 1993, 1131–1138. [CrossRef]24. Hutchings, I.; Shipway, P. Tribology: Friction and Wear of Engineering Materials; Butterworth-Heinemann: Oxford, UK, 1992.25. ASTM G99-17; Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus. ASTM International: West Conshohocken,PA, USA, 2023.26. Totaro, P.P.; Westerfield, Z.; Tian, T. Introducing a new piston skirt profile to reduce engine friction. In Proceedings of the SAE2016 World Congress and Exhibition, Detroit, MI, USA, 12–14 April 2016; SAE International Technical Paper.27. Sedlacˇek, M.; Podgornik, B.; Vižintin, J. Correlation between standard roughness parameters skewness and kurtosis andtribological behaviour of contact surfaces. Tribol. Int. 2012, 48, 102–112.28. Pawlus, P.; Reizer, R.; Wieczorowsk, M. Functional importance of surface texture parameters. Materials 2021, 14, 5326. [CrossRef]29. Yan, X.-L.; Wang, X.-L.; Zhang, Y.-Y. Influence of roughness parameters skewness and kurtosis on fatigue life under mixedhydrodynamic lubrication point contact. J. Tribol. 2014, 136, 50–62.30. Archard, J.F. Contact and rubbing of flat surfaces. J. Appl. Phys. 1953, 24, 981–988.31. Aslantas, K.; Hasçelik, A.; Erçetin, A.; Danish, M.; Alatrushi, L.K.H.; Rubaiee, S.; Bin Mahfouz, A. Effect of cutting conditionson tool wear and wear mechanism in micro-milling of additively manufactured titanium alloy. Tribol. Int. 2024, 193, 109340.[CrossRef]32. Kragelskii, I.; Mikhin, N.; Myshkin, N.; Bronovets, M.A.; Dobychin, M.N.; Dubnyakov, V.N.; Litvinov, V.N. The mechanism of theinitial stage of selective transfer during frictional contact. Wear 1978, 47, 133–138. [CrossRef]33. Kumar, T.; Raghu, R.; Thankachan, T.; Cˇep, R.; Kalita, K. Mechanical property analysis and dry sand three-body abrasive wearbehaviour of AZ31/ZrO2 composites produced by stir casting. Sci. Rep. 2024, 14, 1543. [CrossRef]34. Wopelka, T.; Cihak-Bayr, U.; Lenauer, C.; Ditrói, F.; Takács, S.; Sequard-Base, J.; Jech, M. Wear of different material pairings for thecylinder liner—Piston ring contact. Ind. Lubr. Tribol. 2018, 70, 687–699. [CrossRef]35. Curran, J.; Kalkancı, H.; Magurova, Y.; Clyne, T. Mullite-rich plasma electrolytic oxide coatings for thermal barrier applications.Surf. Coat. Technol. 2007, 201, 8633–8687. [CrossRef]36. Ford Motor Company. Power Cylinder Friction Reduction and Through Coatings, Surface Finish and Design; Ford Motor Company:Dearborn, MI, USA, 2021.Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individualauthor(s) and contributor(s) and not of MDPI and/or the editor(s). 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Tribological Investigation of Plasma-Based Coatings for Use in Quasi-Monolithic Engine Cylinder Bores
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