Influence of ta‑C, a‑C, and a‑C:H properties and low-viscosity lubricants on the mild wear behaviour of DLC

Influence of ta‑C, a‑C, and a‑C:H properties and low-viscosity lubricants on the mild wear behaviour of DLC | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Influence of ta‑C, a‑C, and a‑C:H properties and low-viscosity lubricants on the mild wear behaviour of DLC Manuel Zellhofer, Martin Jech, Josef Brenner, Bernhard Fickl, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6717519/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Diamond-like carbon (DLC) coatings are widely employed to extend the service life of components exposed to severe tribological conditions, particularly where uncoated surfaces would fail under boundary and mixed lubrication. For reliable application, a mechanistic understanding of mild wear processes in such regimes is essential. In this study, in-situ wear rate measurements were performed using a reciprocating tribometer to evaluate DLC-coated specimens against steel counter bodies under low-viscosity lubricants with distinct chemical compositions, namely water and diesel fuels (fresh and aged). A series of hydrogenated and hydrogen-free DLC coatings with varying sp 3 /sp 2 hybridisation ratios and hardness were investigated. The results indicate that under mild wear conditions, lubricant properties — most notably dynamic viscosity (~ 1–3 mPa·s) and oxidation state — dominate the wear response, while the DLC coating’s sp 3 /sp 2 ratio and mechanical properties exert a secondary influence. Hydrogenated, sp 2 -rich DLCs exhibited superior wear resistance due to their enhanced ability to form passivating hydroxyl layers, which suppress graphitisation-induced degradation. In contrast, water-based lubrication, while promoting hydroxylation, led to increased wear due to steel counter body corrosion and third-body abrasion. Aged diesel lubricants resulted in lower DLC wear rates, attributable to increased oxidation promoting surface passivation. These findings highlight the necessity of jointly considering lubricant chemistry and DLC structural characteristics when designing tribological systems for low-wear applications. Diamond-Like Carbon (DLC) Radio-Isotope Concentration (RIC) method sp3/sp2 ratio lubricant influence boundary regime continuous wear measurement Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 1. Introduction Diamond-like carbon (DLC) coatings are commonly regarded as an adequate approach for extending the lifetime of components that are opposed to critical wear conditions [ 1 , 2 ]. Additionally, DLC achieves low friction [ 3 , 4 ], resulting in energy savings in both dry [ 5 ] and lubricated boundary conditions [ 6 , 7 ]. Due to these beneficial properties, DLC plays an important role in the increasing emphasis on sustainability [ 8 – 10 ] and reducing fuel consumption [ 11 , 12 ]. In such applications, DLC is used against a steel counter body. The manufacturing processes for DLC determine its main properties, including the sp 3 /sp 2 ratio and its hardness [ 13 – 15 ], which strongly affect its wear behaviour [ 16 ]. The classification of DLCs can be determined by the presence of hydrogen (hydrogenated, a-C:H) or absence of hydrogen (hydrogen-free or H-free, a-C or ta-C) [ 16 , 17 ], where a-C stands for amorphous and ta-C (with sp 3 content > 80 %) for tetrahedralDLC. The application of a-C:H as a protective overcoat is typically applied with the objective of preventing corrosion and abrasive wear to components [ 18 ]. Likewise, ta-C is used to increase the performance, durability, and lifetime [ 18 ]. Furthermore, particular doped DLCs are regarded for specific applications [ 18 – 25 ]. For example, doping of DLC coatings has been demonstrated to enhance their capacity to interact with lubricants [ 18 ]. Moreover, doping of DLC coatings can enhance their adhesion with the substrate, increase wear resistance, and reduce internal compressive stresses [ 18 ]. The combination of DLC and lubricant variants, especially with different chemistry or additives [ 26 – 28 ], outlines a wide range of applications that require experimental exploration and/or simulation [ 21 , 26 ]. From the authors’ point of view, this is particularly true for the lubricated boundary regime. DLC coatings are commonly employed for systems in the boundary regime, for which high-load and/or low-viscosity represent critical situations. Although DLC coatings possess excellent properties, they may wear out over time due to mild-polishing wear progression [ 29 ]. In particular, DLC will wear out or fail when used with low-viscosity lubricants such as diesel fuel [ 23 , 29 – 34 ]. Consequently, the automotive industry is confronted with the challenge of optimising DLC coatings and investigating the influences on wear [ 35 – 38 ], which aims towards low wear and increased lifetime. In the initial Hertzian pressure range of 1.10 to 1.45 GPa, premature failures (e.g. due to delamination) and mild wear rate occur alternately in such applications [ 29 ]. Therefore, the present study focuses on mild wear regimes with pressure ranges below 1.10 GPa, where mild wear rate of DLC is supposed and premature failures are mainly negligible. In the case of mild DLC wear, the questions arise, what are the major influencing factors limiting the lifetime of the DLC coatings and how these factors can be distinguished? The Radio-Isotope Concentration (RIC) method is suitable for continuous wear measurement of DLC and well suited to this study, where the detection of nanoscopic wear is essential due to mild wear rates and thin coatings [ 29 , 39 ]. Zahid et al. [ 26 ] and Nuruzzaman et al. [ 40 ] provide an overview of the factors that influence DLC wear. The factors include coating hardness, hydrogen content, hybridisation (sp 3 /sp 2 ), coating thickness, and interlayers, coating roughness, applied load, viscosity, and lubrication. Previous work by the authors has already investigated the role of the remaining coating thickness on failure [ 29 ] and the role of the applied load [ 29 , 39 ]. Thus, the present study focuses on the influence of coating hardness, hydrogenated/H-free DLC, sp 3 /sp 2 ratio, coating roughness, viscosity, and lubrication in the mild wear regime. As a side note in the DLC mild wear rate regime of ~ 25 nm/h (0.05 ± 0.02 × 10 6 µm³/h with a wear area of 2 mm² [ 29 ]), the influence of the different interlayers can be neglected. With respect to the lubricants, the focus of this paper is also on the influence of lubricant viscosity, type and oxidation number, particularly for water and diesel. Water was chosen for use in combination with DLC-coated components [ 41 – 46 ] due to its potential future applications and sustainability [ 9 ], while diesel is commonly used in the transportation industry [ 2 , 23 , 29 – 37 , 39 , 47 – 52 ]. Another factor, that has been particularly considered here with regard to DLC mild wear, is aged lubricants, especially aged diesel. In summary, the aim of this study is to investigate DLC wear behaviour and influence factors at mild wear rates in the boundary regime. Two groups, comprising variations of DLC coatings on one side and variations of lubricants on the other side, are investigated and compared in this study. 2. Materials and methods 2.1. DLC samples and surface analysis The DLC coatings ta-C, a-C, a-C:H(1), a-C:H(2), and a-C:H(3), Table 1 , were selected for the present study to represent different properties in terms of hydrogen-free and hydrogenated DLC. In addition, the a-C:H coatings have different sp 3 contents. The five different DLC coated samples were provided by two company partners and have a DLC coating thickness according to the company partners information of approximately 2.0 to 2.3 µm, with the exception of a-C:H(2) which has a DLC coating thickness of ~ 1.0 µm, Table 1 . The substrate of all DLCs was hot work tool steel W300. A layer of ~ 0.5 µm chromium, or ~ 0.5 µm titanium, or ~ 3.0 µm chromium nitride served as an interlayer between the DLC and the steel substrate, Table 1 . These coatings were characterised by applying light microscopy for visual appearance, topographical measurements for roughness analysis, nanoindentation for hardness and Young´s modulus evaluation, Raman spectroscopy for sp 3 content evaluation, and X-ray photoelectron spectroscopy (XPS) for elemental wear track analysis, respectively. Table 1 DLC coatings, their composition and thickness, and their application to the experiments (tribometer test and lubricants) DLC coating label DLC coating thickness / µm Interlayer sp 3 content according to manufacturer / % Test Info ta-C ~ 2.0 Ti ~ 80 Activated DLC coating; offset tribometer tests; water , diesel , and aged diesel OX10 lubricant; DLC-supplier 1; 1 to 2 repetitions per lubricant; a-C ~ 2.0 Cr ~ 60 a-C:H(1) ~ 2.0 Cr ~ 50 a-C:H(2) ~ 1.0 CrN (3.0–4.0 µm) not specified a-C:H(3) ~ 2.3 Cr (~ 0.5 µm) not specified Activated steel counter body; water , diesel , aged diesel OX2 , and aged diesel OX10 lubricant; DLC-supplier 2; 2 to 3 repetitions per lubricant; Roughness analysis was performed on the basis of 3D-topographical measurements using a chromatic-confocal profiler (Jr25, Nanovea) with a lateral resolution of ~ 1.5 µm and a vertical resolution of ~ 5.5 nm (PS2 sensor with 300 µm max height range) with subsequent data evaluation (Leica Map, Leica Microsystems). Hardness and Young´s modulus evaluation was performed by nanoindentation (Bruker, Hysitron Triboindenter TI 950, PerforMech 2 Transducer, Berkovich indenter) with a load of 10 mN, resulting in penetration depths of 141 ± 58 nm. For such loads, Tischler et al. [ 53 ] achieved substrate independent results for distinguishing the DLC properties. For statistical reasons, a total of ten indentations were performed at evenly distributed locations across the entire sample´s surface for each DLC coating. The sp 3 contents of the various DLC coatings were assessed by Raman spectroscopy, following the methodology of Cui et al. [ 54 ]. This method combines and compares previous Raman studies on DLC coatings and constructs models for hydrogenated DLC via G-peak dispersion and for H-free DLC coatings via G-peak full width at half maximum (FWHM) analysis, respectively [ 55 ]. To this end, a Raman system (WITec alpha 300 RSA+) with three excitation wavelengths 633 nm, 532 nm, and 488 nm was employed. All Raman spectra were fitted with two Gaussian peaks representing D- and G-peaks, respectively, after linear background subtraction using the dedicated software (CasaXPS©). The two peak Gaussian peak fit model was selected, as it is the most widely used and most consistent [ 56 ]. Because the peak position and FWHM of both D- and G-peak may vary depending on sp 3 content [ 55 ], the fit parameters were not constrained in any way. However, the ranges for the position (pos(D): 1300–1400 cm − 1 ; pos(G): 1500–1680 cm − 1 ) and FWHM (< 400 cm − 1 ) were not exceeded. Dispersions were calculated between all three used wavelengths and thus obtained sp 3 contents via Cui et al. [ 54 ] were averaged from all three dispersions for each hydrogenated DLC. For the FWHM analysis of H-free samples, the FWHMs for 488 nm were calculated to obtain sp 3 contents via Cui et al. [ 54 ]. Choices in peak shape are not expected to significantly change the calculated sp 3 content for the dispersion model used for hydrogenated samples. Cui et al. [ 54 ] also derived a general equation to calculate the sp 3 content in H-free DLC coatings for arbitrary excitation wavelengths using the G-peak FWHM. The details of XPS analysis are discussed in Chap. 2.5. 2.2. RIC wear measurement / Irradiation of samples Continuous monitoring of DLC wear with appropriate sensitivity is necessary to address the issues of wear rate quantification in the mild wear regime. For the wear measurement, radioactive tracer technology is applied by producing radioactive isotopes in the DLC and substrate by Thin Layer Activation (TLA) [ 57 ]. The wear particles carrying the isotopes are transported to the gamma-ray detector through the lubricant circuit, Fig. 1 . Detailed information about the used Radioactive Isotope Concentration (RIC) wear measurement can be found in the literature [ 29 , 39 , 58 – 62 ]. During each experiment, 100 ml of lubricant is circulated through a squeeze pump at a flow rate of 30 ml/min at 25°C temperature. The components of the RIC circuit are analysed after the experiments for any sedimentation or deposition of the wear particles, proving that the essential majority of the particles are detectable in the lubricant. Consequently, the RIC measurement is significant, and no filters are used in the circuit. DLC activation was achieved by irradiating the carbon with helium ( 3 He) [ 63 ] along a 2 mm wide strip across the sample surface, Fig. 1 . To investigate the wear behaviour of the steel counter body in separate tests, the X90CrMoV18 piston rings with a diameter of 83 mm, radius of 5 mm, and width of 1.2 mm, were activated with deuterons [ 64 ]. To avoid wear superposition between the steel counter body and the steel substrate, the DLC was not activated in the examinations of the steel counter body. All specimens were irradiated using low-intensity beam parameters to prevent any alteration of their material properties. This wear measurement method is designed to investigate wear progression under different loading conditions without affecting the tribological performance of DLC [ 39 ]. 2.3. Selection of tribological load The irradiated DLC coated-plates were tested against the piston ring using a model tribometer (Universal Mechanical Tester – UMT, Bruker) with oscillating movement. The oscillation frequency was set at 25 Hz with a 4 mm stroke. The reciprocating motion imitates start-up motion and provokes boundary conditions. To ensure that the activation area is fully utilized, the wear track was arranged perpendicular to the 2 mm wide activated strip in order to perform multiple wear tracks on one sample, Fig. 1 . Subsequently, several tribological tests were conducted across the activated zone on one sample. The length of the wear track on the flat DLC plate is ~ 4 mm in the direction of the stroke, with a width of ~ 1 mm dependent on the loading and wearing condition, giving a wear area of up to 4 mm². Of this, the activated wear area is about half of the wear track. The wear area on the piston ring is ~ 0.5 mm², 1 mm perpendicular to the stroke direction and 0.5 mm in stroke direction due to the barrel-like geometry, Fig. 1 . The test parameters result in an average velocity of ~ 0.3 m/s in the activated area. The initial Hertzian contact pressure was 875 ± 152 MPa, estimated for a 50 N load, further details in Table 2 . After the running-in phase, the estimated test pressures are ~ 100 ± 60 MPa, considering the effects of smoothing and enlarging of the wear track area (~ 0.5 mm²), obtainable at the steel counter body at the end of the test. The estimated pressures are significantly below the yield strength of DLC [ 65 ]. Therefore, the selected loading parameters were set to prevent (premature) DLC failure and to investigate the influence of the different DLCs and lubricants in the mild wear regime [ 29 ]. Although friction was also measured for a complete evaluation, the focus of this study was clearly on DLC and steel counter body wear behaviour and wear rates. The coefficient of friction (COF) was evaluated using the root mean square over 333 ms of the measured signal with 3000 Hz sampling frequency. The RIC wear data was averaged at 10-minute intervals. Table 2 Parameters for the estimation of initial Hertzian contact pressure and film thickness. Parameter DLC Piston ring steel counter body Radius of curvature in X and Y direction in mm 0 and 0 41.5 and 5 Young´s modulus in MPa 151 to 244 GPa (refer to chapter “DLC coating classification”) 210 Poisson ratio 0.22 [ 67 ] 0.3 Dynamic viscosity at 25 °C in mPas (refer to chapter “Lubricant viscosity”) Water…0.94, Diesel…2.17, Aged diesel OX2…2.27, Aged diesel OX10…2.92; Pressure-viscosity coefficient in GPa − 1 Water ~ 5, Diesel 9.5 [ 68 ] Normal load in N 50 Average velocity in m/s 0.3 2.4. Test procedure for wear measurement Four low-viscosity lubricants with different chemical compositions were selected for this study: deionised water (DEI, Electrolube, Germany), diesel GDK650, and aged diesel GDK650 with oxidation numbers of 2 (OX2), and aged diesel GDK650 with oxidation numbers of 10 (OX10). The lubricant viscosities were measured using a Stabinger viscosimeter (SVM 3000, Anton Paar, Austria), Table 2 . As these lubricants are different, e.g., regarding oxidation, the tribological experiments were set up to take oxidation reactions without relative motion into account. Additionally, two different wear measurement experiments were conducted using firstly activated DLC and secondly activated steel counter body: The experiments with activated DLC were executed with break times or idle times, defined as “offset” tribometer tests, Table 1 . In these tests the tribometer runs for an appropriate time (period 1), which allows for the discrimination of running-in and constant wear. The appropriate time for the duration of the tribometer's operation during the test was individually determined (6 ± 1 hours). Following period 1, there was an offset time of 8 ± 2 hours (period 2) during which the tribometer was stopped, but the interacting bodies remained in contact and were flushed with lubricant. After the offset time, the tribometric load was continued again (period 3). The continuous wear measurement was active throughout the whole experiment. For the determination of the wear behaviour of the steel counter body, tests against one selected DLC (a-C:H(3), Table 1 ) with water, diesel, aged diesel OX2, and aged diesel OX10 were performed. The test time for investigating the steel counter body was set to 2 hours. A total of 38 tests were carried out with the different DLC-samples and with different lubricants, Table 1 , of which 30 tests ( Appendix 2) with activated DLC and a further 8 tests with activated steel counter body. The parameters of the tests with activated DLC are listed in Appendix 1, the obtained wear rates averaged over the tests with the same parameters are listed in Appendix 2. 2.5. Analysis of the wear track Selected areas within and outside the wear tracks were analysed by X-ray photoelectron spectroscopy (XPS). XPS data was acquired using a Thermo Fisher Scientific Thetaprobe with a monochromatic Al Kα X-ray source (1486.6 eV). High-resolution spectra were obtained at 50 eV pass energy with an energy step size of 0.2 eV. The C1s peak was analysed to compare the graphitic change and oxidation of ta-C and a-C:H(1). C1s peak was divided into sp 3 peak at 285.5 eV, sp 2 peak at 284.7 eV, O = C-O at 290.2 eV, and C-O at 286.7 eV according to literature [ 69 – 71 ]. Furthermore, for a detailed examination of the oxidation, the O1s region was fitted by C-O at 531.9 eV, MeOx at 530.3 eV, OH 535.1 eV, and C-OH at 533.5 eV [ 72 ]. Peak fitting was performed with the Thermo Fisher software (Thermo Fisher Scientific, Avantage), using Gaussian/Lorentzian curve fitting for the evaluation. Peak backgrounds were subtracted using a modified Shirley algorithm [ 73 ]. To analyse the influence of oxidation during transport from the tribological test to the XPS, not-sputtered and sputtered areas were analysed by XPS spectroscopy. For comparison, each surface measurement spot was sputtered with 1 keV Ar ions (20 seconds, 1 µA) and another XPS inspection was performed. 3. Results - DLC and tribosystem characterisation 3.1. DLC coating classification The visual appearance of the DLC surfaces can be described as following: aC, aC:H(1), and taC occur smoother than a-C:H(3), where else aC:H(3) seems smoother than a-C:H(2), Fig. 2 . Pores or dotted surface texture can be recognized on each DLC sample, whereby a-C:H(2) shows the largest and roughest pore structure. The line structure originates from the substrate surface pre-preparation process before coating, as reported in the previous study [ 29 ]. A similar trend to the appearance can be seen in the roughness values Sa, Fig. 2 . The coatings are ordered from the lowest to the highest roughness values of the unloaded (initial, as obtained) DLC coated surface: a-C < a-C:H(1) < ta-C < a-C:H(3) < a-C:H(2) It seems that generally H-free DLC has lower roughness values than hydrogenated DLC. However, it cannot be said that H-free is always smoother taking Ma et al. [ 74 ] into consideration. Based on Raman spectroscopy, the sp 3 content was derived from FWHM for hydrogenated DLC and from Disp(G) for H-free DLC, Table 3 . This gives the following sequence of DLC layers in terms of Raman-evaluated sp 3 content: ta-C > a-C > a-C:H(1) > a-C:H(2) > a-C:H(3) Based on the formula of Cui et al. [ 54 ] an uncertainty of ± 8 % i estimated for the sp 3 content based on the FWHM of the Raman measurements. In the present analysis, the sp³ content of ta-C has been determined to be 69 %, hich outlines a bigger uncertainty when compared to the values documented in the literature [ 13 ] and given by the manufacturer, Table 1 , to be ≥ 80 % for s 3 content of ta-C. The derived values for the sp³ content based on Raman measurements are dependent on the fitting method and parameters. As we use the same fitting method for both hydrogenated and H-free samples, there may be a higher systematic error as given by the above-mentioned uncertainty. However, this applies in the same way to all our sp³ estimations, the ranking of our DLC samples according to the sp³ content can be seen as reliable. The ta-C sample exhibits a sp³ content of 69%, while the a-C sample displays a sp 3 content of 56%. In contrast, the a-C:H(1), a-C:H(2), and a-C:H(3) samples exhibit sp 3 contents of 54, 46, and 40%, respectively. In this study, the sp 3 content is higher for H-free DLC coatings than for hydrogenated DLC coatings, Table 3 . Table 3 List of Raman evaluated Disp(G) for hydrogenated DLC, FWHM for H-free DLC, and sp 3 content for all DLCs. DLC coating label Disp(G) FWHM @532 nm Raman evaluated sp 3 content ± 8% ta-C - 223 69 a-C - 202 56 a-C:H(1) 0.256 - 54 a-C:H(2) 0.217 - 46 a-C:H(3) 0.203 - 40 The hardness and the Young´s modulus of the samples in this study are higher for H-free DLC coatings (ta-C and a-C) than for hydrogenated DLC coatings (a-C:H(1), a-C:H(2), and aC:H(3)) Fig. 3 . The highest hardness of DLC was determined to be 29 MPa for the a-C type, with the second-highest hardness observed in ta-C at 27 MPa. In comparison, the aC:H(1), a-C:H(2), and a-C:H(3) variants exhibited lower hardness values of 19, 16, and 20 MPa, respectively, Fig. 3 . The trend of the hardness values correlates with the trend of the averaged sp 3 contents. This is in accordance with literature [ 13 , 75 ]. 3.2. Lubricant viscosity Concerning the viscosities, water and diesel lubricant viscosities show a similar behaviour over temperature, Fig. 4 , goes in line with literature [ 76 , 77 ]. Aged OX2 diesel shows a small increase in viscosity compared to diesel, where else aged OX10 diesel shows a significant increase in viscosity. An increase in viscosity due to ageing has also been reported [ 78 ]. The viscosity values at 25°C were used to estimate the film thickness, Fig. 6 , and to visualise the dependence of wear rate on viscosity in the discussion in this study, Fig. 10 and Fig. 12 , as the wear experiments were conducted at 25°C room temperature. 3.3. DLC wear tracks topography Based on the optical inspection and topographical results, Fig. 5 , no delamination areas or failure occurred on the DLC coated surfaces during the experiments due to the mild wear loading conditions, as expected [ 29 ]. Only in four experiments (ta-C lubricated with diesel, a-C lubricated with water, a-C:H(1) lubricated with diesel, and a-C:H(2) lubricated with water) a pronounced wear track can be observed, Fig. 5 . Furthermore, scratches in the direction of the relative motion of the counter body are present after testing ta-C with water lubrication, a-C with diesel and aged diesel lubrication, and a-C:H(1) lubricated with water. Although there are some roughness changes, see next paragraph, wear cannot be quantified via chromatic confocal profilometer measurements for the following combinations, Fig. 5 , (marked with red dots): ta-C, a-C and a-C:H(1) with aged diesel lubrication, a-C:H(2) lubricated with diesel and aged diesel, and all a-C:H(3). If changes in wear hight are smaller than typical roughness values, for example Sa, topographical wear measurements are subjected to serious uncertainty and consequently not suitable for distinguishing the effects investigated in the present study. The roughness values after tribological loading indicate a reduction in the wear area compared to the values of the initial unloaded DLC surfaces, Fig. 6 . Estimation of the lubricant film thickness using the equation of Chittenden et al. [ 66 ], the parameters listed in Table 2 , and the average sliding velocity gives values of 3 to 10 nm, mostly dependent on lubricant viscosity. The film thickness values are smaller than all the roughness values (initial and after the test in the wear area) by a factor of 5 to 100. It can consequently be assumed that the experiments are mainly running in the boundary lubrication regime. At the highest speeds in the centre of the stroke, partially mixed lubrication can be assumed. 3.4. DLC wear progression The experiments in the present study show no significant DLC running-in behaviour and consequently the wear rates were evaluated using linear regression from the beginning to the end for each of the periods, Fig. 7 . For each lubricant and each DLC coating the wear rates were averaged for periods 1 and 3 (and over several tests, when such repetitions were available, see list of all experiments in Appendix 1). The grey areas (Fig. 7 ) correspond to the offset periods, in which the tribometer was stationary, but the DLC-steel contact was maintained, and the system was flushed with the lubricant. The DLC wear rates for ta-C show a significant difference depending on the lubricant, water (Fig. 7 a), diesel (Fig. 7 b), and aged diesel OX10 (Fig. 7 c). The ta-C wear rates (average of period 1 and 3 of two tests) were found to be 31 ± 22 nm/h for water, 25 ± 6 nm/h for diesel, and 12 ± 12 nm/h for aged diesel OX10. Surprisingly, when flushing with water, there is a DLC wear rate observable during offset period 2 (in this example of 20 ± 8 nm/h). This behaviour was also observed for the other DLC coatings tested when lubricated with water, Appendix 2. Side note: A wear rate of 20 nm/h over an average wear area of 2 mm² (intersection of activated area and worn area) corresponds to a wear volume rate of 0.04 × 10 6 µm³/h. This is in the range of mild wear for such testing conditions, for comparison with 0.05 ± 0.02 × 10 6 µm³/h as given in [ 29 ]. At this wear rate, a 2 µm DLC layer loaded at 25 Hz would achieve a lifetime of approximately 9 × 10 6 cycles. For the tests with water lubrication, deposits next to and in the wear-tracks were observable by optical means (for example a-C and a-C:H(1)), Fig. 8 a und b. These deposits were within groves in the wear track or in elliptical shapes next to the wear track, indicating the probable contact area of the piston ring with the DLC plate during the offset flushing period. In comparison, the experiments with diesel and aged diesel OX10 showed no DLC wear rate and no deposits during the offset flushing period. 4. Wear rate results and discussion 4.1. Main influences on DLC wear rate A clear trend can be observed for the DLC wear rates with respect to the lubricant property viscosity and the DLC property sp 3 content, Fig. 9 . Specifically, the DLC wear rates decrease with viscosity from water to diesel to aged diesel, and increase with sp 3 content from a-C:H(2) over a-C:H(1) and a-C to ta-C. Even though the wear processes are in the order magnitude of of nm/h and there are unavoidable uncertainties due to tribological testing, as documented by the uncertainty bars in Fig. 9 , trends can nevertheless be ascertained on the basis of the wear rates. Considering the influence of the lubricant, the wear rates for the different DLC coatings are similar to water lubrication and do not show any trend concerning the DLC sp 3 content, Fig. 9 . Otherwise, tests with diesel lubrication show a significant tendency with sp 3 content. Considering the influence of the DLC-characteristics, hydrogenated DLC with a sp 3 content in the range of 40–55% exhibits less wear than H-free or tetrahedral DLC with sp 3 content greater than 55%, Table 3 and Fig. 9 . At first glance, this result seems to be contractionary to the standard tribological knowledge, as higher sp 3 content results in higher hardness, Fig. 3 , and should therefore lead to less wear. Instead, higher hardness content shows higher wear in the present study. In literature, this is related to several effects, including the presence of hydrogen, the appearance of abrasive particles and graphitisation, as discussed in the following. Law et al. [ 49 ] reported improved performance of hydrogenated DLC compared to H-free DLC in lubricated contact of engine components. The ability of DLC to inhibit wear processes may therefore be related to the contribution of hydrogen, as also reported by Zhang et al. [ 42 ]. For ta-C it is reported that abrasive DLC wear particles are generated during the tribological loading [ 79 ]. These spalled particles exhibit high hardness and consequently accelerate wear as abrasives [ 78 ] [ 80 ]. In summary, increased hardness of DLC coatings is related to increased sp 3 content, but also to increased wear, for example, due to the generation of abrasive particles or a higher degree of graphitisation. These understanding models focus on the wear behaviour of DLC. However, we obtained some remarkable results during the flushing period for the tests with water lubrication associated with some debris at the contact with the steel counter body. This debris are assumed to be corrosive products of the steel counter body. Nevertheless, we need to understand the role of the steel counter body, before going into further details about the DLC wear behaviour. As described in the methods, a separate test series was carried out with activated steel piston rings (counter body) for online wear measurement. The a-C:H(3) plates served as base body due to availability and are assumed to be similar to a-C:H(1) and a-C:H(2) for reasonable comparison in terms of wear behaviour. 4.2. Steel counter body wear Steel counter body wear rates show a dependency on viscosity, Fig. 10 a. Compared to DLC wear behaviour, Fig. 7 , which showed no pronounced running-in wear, the wear of the steel counter bodies show a significant running-in behaviour, Fig. 10 b. The running-in behaviour of the steel piston rings is rather independent of the applied lubricant, Fig. 10 b. This lubricant independent behaviour can be attributed to the barrel-shape of the steel counter body piston ring compared to the flat DLC plate samples as well as the difference in contact areas during the reciprocal movement. Steel counter body running-in wear is ~ 23 times higher in comparison to steel counter body wear rate when lubricated with diesel or aged diesel OX2 or OX10, Fig. 11 , compare left and right y-axis. However, after running-in the wear rates of the steel counter bodies and the wear rates of the DLC coatings, Fig. 9 , are in a comparable range for diesel and aged diesel lubrication. However, aged diesel (OX2 and OX10) resulted to a sharp equal drop in wear rate and running-in wear compared to diesel, indicated by the arrow in Fig. 11 . This suggests that viscosity is not the only influencing factor when considering lubricant properties, as discussed in the following chapter. 4.3. Influence of DLC and lubricant properties In order to better understand the influence of certain parameters, a correlation analysis was carried out. To this end, the various DLC and lubricant properties were evaluated and compared with the DLC wear rates, Fig. 12 a and c, and the COFs, Fig. 12 b. Positive values (in the bars) indicate a direct correlation and negative values an indirect correlation between the outlined parameter and the DLC wear rate or COF. The viscosity inhibits the highest influence on DLC wear, Fig. 12 a, although we are in the boundary and mixed regime in this study. The difference in wear rates with water and diesel lubricants can be seen as the main driving force, indicating the high correlation with viscosity. As the factors viscosity and type of lubricant cannot be considered independently, water is excluded for the correlation details, outlined in Fig. 12 c. The factors sp 3 content, hardness, and hydrogenated or H-free cannot be regarded as independent of each other and show similar correlations with DLC wear rates, Fig. 12 c at least for the applied loading conditions. The roughness has the lowest correlation with DLC wear rates. The roughness in the area measured in this study is therefore negligible for the mild wear behaviour. These indications suggest that in boundary regimes lubrication properties are ultimately the driving force regarding the DLC wear rate. Choosing the right DLC coating is of secondary importance. However, when considering only the C-based lubricants (diesel and aged diesel), the DLC properties reach a higher correlation with the DLC wear rates than the lubricant properties, Fig. 12 c. Nevertheless, roughness has the lowest correlation with wear rate in both correlation approaches. Summarising the findings regarding DLC wear rate: Due to the lower viscosity, water leads to higher DLC wear rates than the diesel lubricants, as the tests are run in boundary and mixed lubrication regimes. When excluding water, the impact of the sp³ content (or hardness) of the DLC coatings is recognizable within the tests with diesel lubricants as well as the impact of the oxidation number (aging) of the diesel lubricants. The correlation between DLC wear rate and COF is only 22%, Fig. 12 a, which is also observable in the direct comparison, Fig. 12 d. COF correlates indirectly with oxidation number, viscosity, sp 3 content, and roughness, Fig. 12 b. Generally, diesel lubrication results in a higher COF (0.21 ± 0.05) compared to water (0.19 ± 0.05) or aged diesel (0.13 ± 0.04) lubrication. This behaviour is somehow surprising, as the main factor for the COF trend is the oxidation number followed by the minor factor viscosity. In order to clarify the impact of oxidation and if there is a chemical interaction between the aged lubricant and the DLC surface, wear track analyses are discussed in the following chapter. 4.4. XPS analyses of DLC wear surface The XPS analyses were carried out on the samples, as received after the tribological test and named “not sputtered”. As such, they showed a relatively high content of C-O with additional O = C-O bindings within the C1s peak. Details of all measured peaks can be found in Appendix 3. These peaks (O = C-O and C-O) are regarded to be some sort of tribofilm (or deposition) formed from or formed together with the lubricant. For gaining the unimpaired sp 3 /sp 2 ratio, the peaks O = C-O and C-O were subtracted, resulting in sp 3 together with sp 2 being 100%, Fig. 13 a and b. The percentage of sp 3 is indicated in the results. The sp 3 ratios of the reference areas (outside of wear tracks) of ta-C and a-C:H(1) from the non-sputtered XPS data are within their respective uncertainties consistent with the Raman derived sp 3 content estimates above. Regarding Fig. 13 a, the sp 3 contents of the ta-C reference area (82%) and the a-C:H reference area (61%) are higher compared to the sp³ content inside the wear tracks (ta-C: from 66 to 72% and a-C:H: from 20 to 31%). This indicates that the loading conditions lead to friction and wear but also lead to a decrease in sp³ content, which is in first instance referred to as graphitisation. As the C-O tribofilm or deposit may limit the significance of the above observations, the surfaces were sputtered in order to clean the surface from the tribofilm. The C1s peak analysis of the sputtered wear tracks, Fig. 13 b, show a lower sp³ content compared to the not sputtered tracks, Fig. 13 a. Here some kind of radiation damage of the sputtering must be assumed, that leads to an additional transition from sp³ to sp². Consequently, all sp 3 /sp 2 ratios of the sputtered surfaces are lower than the ratios for the not sputtered surfaces. Nevertheless, all ta-C wear tracks still show a higher sp 3 /sp 2 ratio compared to a-C:H for the sputtered surfaces, and the trend of higher sp 3 /sp 2 ratio for the reference area compared to the wear tracks is as well recognizable, Fig. 13 b. Thus, graphitisation from sp 3 to sp 2 can be supposed due to tribological loading and in accordance with literature [ 47 , 81 ]. Regarding the analyses of the O1s peaks, the content of C-OH is higher in the wear tracks of a-C:H (between 89 and 94%) compared to ta-C (between 69 and 84%) for not sputtered surfaces, Fig. 13 c. The analyses of the sputtered wear tracks also show a higher content of C-OH in the a-C-H wear tracks (between 32 and 40%) compared to the ta-C wear tracks (between 13 and 20%), Fig. 13 d. C-OH is a hydroxy group. Zahid et al. [ 18 ] describe in their review that hydrogen passivates the dangling carbon bonds. The passivation of hydroxyl groups can dominate the wear behaviour of DLC coatings, according to [ 82 ]. Dorner-Reisel et al. [ 47 ] found passivation due to diesel lubrication and low wear of DLC is closely attributed to the formation of hydrophilic group C-OH [ 45 ]. The analysis shows that with hydrogenation and less sp 3 (so higher sp 2 ) results in higher C-OH content and a lower DLC wear rate. As a side comment and in addition to Chap. 2.5, a higher content of MeOx was found in the O1s peaks for the deposits (in the range of 3 to 7%) compared to the reference or the wear tracks (from 1 to 3%), Appendix 3. Iron oxides (likely a mixture of Fe 2 O 3 and Fe 3 O 4 ) are therefore clearly present. The deposits are formed during water lubrication; mainly during the offset period (flushing period with no relative movement) while the tribometer is stationary. When the relative movement is turned on again after the offset flushing period, the iron oxides of the counter body can cause higher DLC wear with water lubrication compared to the diesel lubricants, for which no deposits were observed on or next to the DLC wear tracks. This additional 3rd particle wear mechanism is assumed to be also the cause for the outstanding wear rates of DLC with water lubrication compared to the general trend of wear rates versus COF based on the diesel lubricants, Fig. 12 d. 5. Summary and Conclusions This study demonstrates the successful application of the RIC method for continuously monitoring the mild wear behaviour of DLC-coated plates and steel piston ring counter bodies during tribological testing. Using this approach, wear rates in the nm-per-h range were obtained and differentiated based on influencing factors, such as coating composition and lubricant type. In boundary and mixed lubrication regimes, lubricant properties - particularly viscosity - proved to be more influential on wear performance than the DLC coating properties. However, when only carbon-based lubricants were used, the characteristics of the DLC coatings became more decisive for wear behaviour. Interestingly, higher sp 3 content (or hardness) in DLC coatings led to increased wear rates when lubricated with diesel, whereas water-based lubrication rendered the sp 3 content largely irrelevant. Additionally, both DLC and steel counter bodies exhibited improved wear resistance when lubricated with aged diesel rather than fresh diesel, likely due to initial running-in processes and chemical changes in the lubricant. The formation of hydroxyl groups on the DLC surface was found to correlate with a transformation from sp 3 to sp 2 hybridisation, commonly referred to as graphitisation. Under the mild wear and boundary lubrication conditions applied here, hydrogenated DLCs with higher sp 2 content showed superior wear performance compared to harder, hydrogen-free coatings with higher sp 3 content. This suggests that sp 2 structures may facilitate hydroxyl group formation, which acts as a passivation layer and contributes to anti-wear behaviour. In contrast, while higher hardness is traditionally seen as beneficial, it plays a lesser role under the specific conditions of this study. Water-based lubrication, despite promoting hydroxyl group formation, also introduced corrosive effects on the steel counter body, leading to increased wear due to third-body abrasion. In contrast, aged diesel lubricants reduced DLC wear rates, with oxidation appearing more influential than viscosity changes. This aligns with the proposed model in which oxidation products assist in forming protective surface layers. In conclusion, the study highlights the importance of understanding the interplay between lubricant chemistry and DLC coating properties. The findings show that under realistic loading and lubrication conditions, a tailored combination of DLC type and lubricant can significantly reduce wear, thereby extending the service life of coated components. Selecting the appropriate lubricant and DLC is therefore essential to maintaining low wear rates, particularly under boundary lubrication conditions. Declarations Author Contribution The authors have no relevant financial or nonfinancial interests to disclose. All authors contributed to the study conception and design. Material preparation was performed by M. Zellhofer, T. Wopelka, F. Ditrói (Institute for Nuclear Research, Debrecen, Hungary), A. Kuebler (Robert Bosch GmbH, Feuerbach, Germany), and F. Rovere (Oerlikon Balzers Coating AG, Balzers, Switzerland). Nanoindentation was conducted by C. Tomastik and H. Rojacz using the facilities at AC2T research GmbH. Diesel ageing and oxidation number measurements were carried out by A. Agocs at AC2T research GmbH. Raman measurements, analysis, and proportionate contribution to this paper were carried out by B. C. Bayer-Skoff and B. Fickl using the facilities of the Institute of Materials Chemistry at the Vienna University of Technology. XPS measurements and analysis were carried out by J. Brenner and E. Charfi. Data collection and preparation were performed by M. Zellhofer. Data analysis and interpretation were performed by M. Zellhofer and M. Jech. The first draft of the manuscript was written by M. Zellhofer and M. Jech, and detailed revision work was performed by P.H. Mayrhofer, E. Badisch, M. Zellhofer and M. Jech. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Acknowledgement This work was carried out as part of the COMET Centre InTribology (FFG no. 906860 and 872176), a project of the “Excellence Centre for Tribology” (AC2T research GmbH). InTribology is funded within the COMET – Competence Centres for Excellent Technologies Programme by the federal ministries BMK and BMAW as well as the federal states of Niederösterreich and Vorarlberg based on financial support from the project partners involved. 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Sci. 22 , 243–254 (2018) Additional Declarations No competing interests reported. Supplementary Files Appendix.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6717519","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":463258103,"identity":"2cd34943-ec82-4108-aa28-1490a2a16174","order_by":0,"name":"Manuel Zellhofer","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABA0lEQVRIiWNgGAWjYNCCAiBmZ25jYLBhYOAnTosBEDMzArWkMTBINsCFiNVicICAFt32s8ckPhgw2PMDtTz4kGCTb3x+dZoEQ80fnFrMzuSlSc4wYGCWbGZsN5yRkGa57cbbzQYMx3DbYnYgx0yax4CBzeAwY5s074/DBmY3zm58AOTj1nL+jZn0HwMGHnuQFp6E/wbGM85uOMDwD4+WG0BbgH6VMGAGazlgYMDfu/EBYxs+LW+MLXsMJAwkgLZIzkhINpC4wbvZILHPGI/Dcgxv/KiwsedvbwYGXYKdAX//2W0SH77J4dQCBRLI7AQGhgRCGlAB/wHS1I+CUTAKRsGwBwC0nE03N1U6agAAAABJRU5ErkJggg==","orcid":"","institution":"TU Wien","correspondingAuthor":true,"prefix":"","firstName":"Manuel","middleName":"","lastName":"Zellhofer","suffix":""},{"id":463258104,"identity":"06487978-b2f5-44ad-8f77-44822f476e9d","order_by":1,"name":"Martin Jech","email":"","orcid":"","institution":"AC2T Research (Austria)","correspondingAuthor":false,"prefix":"","firstName":"Martin","middleName":"","lastName":"Jech","suffix":""},{"id":463258105,"identity":"7e8c35bd-9dd3-4d7b-835c-727b4030db44","order_by":2,"name":"Josef Brenner","email":"","orcid":"","institution":"AC2T Research (Austria)","correspondingAuthor":false,"prefix":"","firstName":"Josef","middleName":"","lastName":"Brenner","suffix":""},{"id":463258106,"identity":"64d82c8b-eab8-41f9-8320-f9250e501c95","order_by":3,"name":"Bernhard Fickl","email":"","orcid":"","institution":"TU Wien","correspondingAuthor":false,"prefix":"","firstName":"Bernhard","middleName":"","lastName":"Fickl","suffix":""},{"id":463258107,"identity":"1a92e24e-6dbf-437d-b58f-77f3bc211944","order_by":4,"name":"Bernhard Bayer","email":"","orcid":"","institution":"TU Wien","correspondingAuthor":false,"prefix":"","firstName":"Bernhard","middleName":"","lastName":"Bayer","suffix":""},{"id":463258108,"identity":"d562e3af-ce4a-4c84-a617-72c1f8e128ff","order_by":5,"name":"Ewald Badisch","email":"","orcid":"","institution":"AC2T Research (Austria)","correspondingAuthor":false,"prefix":"","firstName":"Ewald","middleName":"","lastName":"Badisch","suffix":""},{"id":463258109,"identity":"f4d6375a-9b1a-4d9d-b3d8-bfeb8de0ad12","order_by":6,"name":"Paul Heinz Mayrhofer","email":"","orcid":"","institution":"TU Wien","correspondingAuthor":false,"prefix":"","firstName":"Paul","middleName":"Heinz","lastName":"Mayrhofer","suffix":""}],"badges":[],"createdAt":"2025-05-21 14:38:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6717519/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6717519/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":83662008,"identity":"02e6802c-0d58-4a17-871d-9023c79e6c01","added_by":"auto","created_at":"2025-05-30 10:23:06","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":190259,"visible":true,"origin":"","legend":"\u003cp\u003eLeft: schematic display of intersection of wear track and activated area; Right: schematic representation of the lubricant circuit, including the DLC-sample piston-ring contact in the Universal Mechanical Tester (UMT) and the RIC detector.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/6c6d334adc1fbaace192aa9f.png"},{"id":83662009,"identity":"ae3641f5-a9ae-4151-9827-66a80fe8caeb","added_by":"auto","created_at":"2025-05-30 10:23:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":327907,"visible":true,"origin":"","legend":"\u003cp\u003eThe visual appearance of the initial DLC coatings and their corresponding roughness value Sa in µm (Image size: 1 × 1 mm).\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/a6a7ed239bc00199ba07bd2c.png"},{"id":83661917,"identity":"0985fbc5-9770-4a01-8e52-a86d24e5bf33","added_by":"auto","created_at":"2025-05-30 10:15:06","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":184957,"visible":true,"origin":"","legend":"\u003cp\u003eHardness (±11\u0026nbsp;%), Young´s modulus (±10\u0026nbsp;%) and sp\u003csup\u003e3\u003c/sup\u003e content (±8\u0026nbsp;%) values of the different DLC coatings. ta-C and a-C belong to the H-free DLC and a-C:H are the representatives of hydrogenated DLC. DLCs are ordered beginning from the highest to the lowest sp\u003csup\u003e3\u003c/sup\u003e content.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/58b182ad7006e684a2c45ae2.png"},{"id":83661913,"identity":"ae8eefd5-0871-438c-b6db-c0475599cda7","added_by":"auto","created_at":"2025-05-30 10:15:06","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":48000,"visible":true,"origin":"","legend":"\u003cp\u003eMeasured viscosity values over temperature for the different lubricants: water, diesel, aged diesel OX2, and aged diesel OX10.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/d68ddaa34e80cbf67dfde3f2.png"},{"id":83661919,"identity":"ed89cbef-8c8e-4db5-89ef-03673a55986d","added_by":"auto","created_at":"2025-05-30 10:15:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":699696,"visible":true,"origin":"","legend":"\u003cp\u003e3D-Topography of middle wear track segments of the different DLC coatings in false colour illustration: from top to bottom ta-C, a-C, a-C:H(1), a‑C:H(2), and a-C:H(3); from left to right: wear tracks lubricated with water, diesel, and aged diesel OX10. False colour scaling from -1.0 µm to +0.5 µm on top right. Red dots mark those samples, on which wear cannot be quantified by optical means.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/0ef83bca73327d0d5a45da23.png"},{"id":83661924,"identity":"9c72c228-2732-4212-a25c-c8ee65a8d64e","added_by":"auto","created_at":"2025-05-30 10:15:06","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":113881,"visible":true,"origin":"","legend":"\u003cp\u003eThe estimate of the maximum lubricant film thickness (red dashed line at 10 nm) is significantly lower than the roughness of the DLC coatings before and after the experiments, indicating that the experiments of this study are mainly running in boundary or mixed lubrication condition.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/569985f3f55c1fe933b7511d.png"},{"id":83662012,"identity":"2d282083-f901-4339-8443-64915c42b290","added_by":"auto","created_at":"2025-05-30 10:23:06","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":121038,"visible":true,"origin":"","legend":"\u003cp\u003eExamples of ta-C wear progression lubricated with a) water, b) diesel, and c) aged diesel OX10. During the offset periods (period 2) the tribometer was stationary and the lubricant was circulated through the circuit, whereby the steel piston ring counter body was in contact with the DLC in the centre of the activated zone.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/d71b8f50cb8187c399e7ac21.png"},{"id":83661921,"identity":"ac677646-0a3e-4fb4-ac99-4da5142e020f","added_by":"auto","created_at":"2025-05-30 10:15:06","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":147201,"visible":true,"origin":"","legend":"\u003cp\u003eDeposits appeared next to and in the wear track, when lubricated with water on a-C a) and a-C:H(1) b).\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/4e3ec6ac3b3d7ca452eab552.png"},{"id":83662013,"identity":"ea3f44fc-1fda-4ac6-b24c-2ffb4308fc5c","added_by":"auto","created_at":"2025-05-30 10:23:06","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":171864,"visible":true,"origin":"","legend":"\u003cp\u003eDLC wear rates with respect to the sp\u003csup\u003e3\u003c/sup\u003e content of the DLC coatings and with respect to the viscosity, representing water, diesel, and OX10. The wear rates are averaged for the periods of tribological loading (periods 1 and 3). The individual wear rates (also for flushing during period 2) can be found in Appendix 2.\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/3906bbdf852e3a23b1c72d5b.png"},{"id":83661916,"identity":"f670d340-f7e2-476d-8516-4deb3b039d96","added_by":"auto","created_at":"2025-05-30 10:15:06","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":134727,"visible":true,"origin":"","legend":"\u003cp\u003ea) Wear rate of steel counter body (against a-C:H(3)) over viscosity in constant regime and b) during run-in.\u003c/p\u003e","description":"","filename":"image10.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/2d1cfef0ca08f336966ad9a5.png"},{"id":83661928,"identity":"a35e2fd7-f935-4da5-b2f9-d1ccf2f97a4e","added_by":"auto","created_at":"2025-05-30 10:15:06","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":136248,"visible":true,"origin":"","legend":"\u003cp\u003eDetailed steel counter body wear rate and running-in height depending on the viscosity of diesel, OX2 and OX10 (counter body of a-C:H(3)). Aged diesel (OX2 and OX10) resulted to a sharp drop in both constant and running-in wear compared to diesel.\u003c/p\u003e","description":"","filename":"image11.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/9c5e463d1fc2c61367d5ab9d.png"},{"id":83661929,"identity":"f93c9d2e-87c6-4601-9582-ab60b895f35b","added_by":"auto","created_at":"2025-05-30 10:15:06","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":341797,"visible":true,"origin":"","legend":"\u003cp\u003ea) Correlation of influencing factors (viscosity, sp\u003csup\u003e3\u003c/sup\u003e content, etc.) with DLC wear rate. b) Correlation of influencing factors with COF. c) Correlation of influencing factors with DLC wear rate without experiments with water. d) DLC wear rate over COF. The circle shows 99\u0026nbsp;% of the data and the box in the middle shows 25\u0026nbsp;% of the data.\u003c/p\u003e","description":"","filename":"image12.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/c0b99598059b1e26bcd58229.png"},{"id":83661927,"identity":"274e476c-e62f-497d-a39f-caa7d2c1375d","added_by":"auto","created_at":"2025-05-30 10:15:06","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":182409,"visible":true,"origin":"","legend":"\u003cp\u003eXPS analysis of not sputtered and sputtered DLC surfaces regarding sp³ content, a) and b), and C-OH content, c) and d), of ta-C and a-C:H(1). The reference (outside the wear track) and the content within the wear tracks lubricated with water, diesel and aged diesel OX10 are shown. (Uncertainty of ±12 % is determined with repeated measurements at samples with water lubricant and assumed in the same for the other samples; all contents of C1s and O1s peak are listed in Appendix 3)\u003c/p\u003e","description":"","filename":"image13.png","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/371aab175fc597ef828f10d1.png"},{"id":86603490,"identity":"bbeeba8d-52d0-43eb-8ce9-d6fdad1a97a5","added_by":"auto","created_at":"2025-07-13 14:24:09","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3911392,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/e5309f5c-1e9a-4712-bf2d-64a18d9f6756.pdf"},{"id":83661912,"identity":"2a83f8a2-c4af-4761-b82f-6c9c8fcb317e","added_by":"auto","created_at":"2025-05-30 10:15:06","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":41856,"visible":true,"origin":"","legend":"","description":"","filename":"Appendix.docx","url":"https://assets-eu.researchsquare.com/files/rs-6717519/v1/dd01b8a1ed235268b08c53c2.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Influence of ta‑C, a‑C, and a‑C:H properties and low-viscosity lubricants on the mild wear behaviour of DLC","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eDiamond-like carbon (DLC) coatings are commonly regarded as an adequate approach for extending the lifetime of components that are opposed to critical wear conditions [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Additionally, DLC achieves low friction [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], resulting in energy savings in both dry [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] and lubricated boundary conditions [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Due to these beneficial properties, DLC plays an important role in the increasing emphasis on sustainability [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] and reducing fuel consumption [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. In such applications, DLC is used against a steel counter body.\u003c/p\u003e \u003cp\u003eThe manufacturing processes for DLC determine its main properties, including the sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e ratio and its hardness [\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], which strongly affect its wear behaviour [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. The classification of DLCs can be determined by the presence of hydrogen (hydrogenated, a-C:H) or absence of hydrogen (hydrogen-free or H-free, a-C or ta-C) [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], where a-C stands for amorphous and ta-C (with sp\u003csup\u003e3\u003c/sup\u003e content\u0026thinsp;\u0026gt;\u0026thinsp;80 %) for tetrahedralDLC. The application of a-C:H as a protective overcoat is typically applied with the objective of preventing corrosion and abrasive wear to components [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Likewise, ta-C is used to increase the performance, durability, and lifetime [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Furthermore, particular doped DLCs are regarded for specific applications [\u003cspan additionalcitationids=\"CR19 CR20 CR21 CR22 CR23 CR24\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. For example, doping of DLC coatings has been demonstrated to enhance their capacity to interact with lubricants [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Moreover, doping of DLC coatings can enhance their adhesion with the substrate, increase wear resistance, and reduce internal compressive stresses [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe combination of DLC and lubricant variants, especially with different chemistry or additives [\u003cspan additionalcitationids=\"CR27\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], outlines a wide range of applications that require experimental exploration and/or simulation [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. From the authors\u0026rsquo; point of view, this is particularly true for the lubricated boundary regime. DLC coatings are commonly employed for systems in the boundary regime, for which high-load and/or low-viscosity represent critical situations. Although DLC coatings possess excellent properties, they may wear out over time due to mild-polishing wear progression [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. In particular, DLC will wear out or fail when used with low-viscosity lubricants such as diesel fuel [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan additionalcitationids=\"CR30 CR31 CR32 CR33\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Consequently, the automotive industry is confronted with the challenge of optimising DLC coatings and investigating the influences on wear [\u003cspan additionalcitationids=\"CR36 CR37\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], which aims towards low wear and increased lifetime. In the initial Hertzian pressure range of 1.10 to 1.45 GPa, premature failures (e.g. due to delamination) and mild wear rate occur alternately in such applications [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Therefore, the present study focuses on mild wear regimes with pressure ranges below 1.10 GPa, where mild wear rate of DLC is supposed and premature failures are mainly negligible.\u003c/p\u003e \u003cp\u003eIn the case of mild DLC wear, the questions arise, what are the major influencing factors limiting the lifetime of the DLC coatings and how these factors can be distinguished? The Radio-Isotope Concentration (RIC) method is suitable for continuous wear measurement of DLC and well suited to this study, where the detection of nanoscopic wear is essential due to mild wear rates and thin coatings [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eZahid et al. [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] and Nuruzzaman et al. [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e] provide an overview of the factors that influence DLC wear. The factors include coating hardness, hydrogen content, hybridisation (sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e), coating thickness, and interlayers, coating roughness, applied load, viscosity, and lubrication. Previous work by the authors has already investigated the role of the remaining coating thickness on failure [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e] and the role of the applied load [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Thus, the present study focuses on the influence of coating hardness, hydrogenated/H-free DLC, sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e ratio, coating roughness, viscosity, and lubrication in the mild wear regime. As a side note in the DLC mild wear rate regime of ~\u0026thinsp;25 nm/h (0.05\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e \u0026micro;m\u0026sup3;/h with a wear area of 2 mm\u0026sup2; [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]), the influence of the different interlayers can be neglected.\u003c/p\u003e \u003cp\u003eWith respect to the lubricants, the focus of this paper is also on the influence of lubricant viscosity, type and oxidation number, particularly for water and diesel. Water was chosen for use in combination with DLC-coated components [\u003cspan additionalcitationids=\"CR42 CR43 CR44 CR45\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e] due to its potential future applications and sustainability [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], while diesel is commonly used in the transportation industry [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan additionalcitationids=\"CR30 CR31 CR32 CR33 CR34 CR35 CR36\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan additionalcitationids=\"CR48 CR49 CR50 CR51\" citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. Another factor, that has been particularly considered here with regard to DLC mild wear, is aged lubricants, especially aged diesel.\u003c/p\u003e \u003cp\u003eIn summary, the aim of this study is to investigate DLC wear behaviour and influence factors at mild wear rates in the boundary regime. Two groups, comprising variations of DLC coatings on one side and variations of lubricants on the other side, are investigated and compared in this study.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. DLC samples and surface analysis\u003c/h2\u003e \u003cp\u003eThe DLC coatings ta-C, a-C, a-C:H(1), a-C:H(2), and a-C:H(3), Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, were selected for the present study to represent different properties in terms of hydrogen-free and hydrogenated DLC. In addition, the a-C:H coatings have different sp\u003csup\u003e3\u003c/sup\u003e contents. The five different DLC coated samples were provided by two company partners and have a DLC coating thickness according to the company partners information of approximately 2.0 to 2.3 \u0026micro;m, with the exception of a-C:H(2) which has a DLC coating thickness of ~\u0026thinsp;1.0 \u0026micro;m, Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The substrate of all DLCs was hot work tool steel W300. A layer of ~\u0026thinsp;0.5 \u0026micro;m chromium, or ~\u0026thinsp;0.5 \u0026micro;m titanium, or ~\u0026thinsp;3.0 \u0026micro;m chromium nitride served as an interlayer between the DLC and the steel substrate, Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. These coatings were characterised by applying light microscopy for visual appearance, topographical measurements for roughness analysis, nanoindentation for hardness and Young\u0026acute;s modulus evaluation, Raman spectroscopy for sp\u003csup\u003e3\u003c/sup\u003e content evaluation, and X-ray photoelectron spectroscopy (XPS) for elemental wear track analysis, respectively.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDLC coatings, their composition and thickness, and their application to the experiments (tribometer test and lubricants)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDLC coating label\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDLC coating thickness / \u0026micro;m\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eInterlayer\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003esp\u003csup\u003e3\u003c/sup\u003e content\u003c/p\u003e \u003cp\u003eaccording to manufacturer / %\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTest Info\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eta-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e~\u0026thinsp;80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eActivated \u003cb\u003eDLC\u003c/b\u003e coating;\u003c/p\u003e \u003cp\u003eoffset tribometer tests;\u003c/p\u003e \u003cp\u003e\u003cb\u003ewater\u003c/b\u003e, \u003cb\u003ediesel\u003c/b\u003e, and aged diesel \u003cb\u003eOX10\u003c/b\u003e lubricant; DLC-supplier 1;\u003c/p\u003e \u003cp\u003e1 to 2 repetitions per lubricant;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ea-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e~\u0026thinsp;60\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ea-C:H(1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e~\u0026thinsp;50\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ea-C:H(2)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCrN\u003c/p\u003e \u003cp\u003e(3.0\u0026ndash;4.0 \u0026micro;m)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003enot specified\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ea-C:H(3)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;2.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003cp\u003e(~\u0026thinsp;0.5 \u0026micro;m)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003enot specified\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eActivated \u003cb\u003esteel\u003c/b\u003e counter body;\u003c/p\u003e \u003cp\u003e\u003cb\u003ewater\u003c/b\u003e, \u003cb\u003ediesel\u003c/b\u003e, aged diesel \u003cb\u003eOX2\u003c/b\u003e, and\u003c/p\u003e \u003cp\u003eaged diesel \u003cb\u003eOX10\u003c/b\u003e lubricant;\u003c/p\u003e \u003cp\u003eDLC-supplier 2;\u003c/p\u003e \u003cp\u003e2 to 3 repetitions per lubricant;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eRoughness analysis was performed on the basis of 3D-topographical measurements using a chromatic-confocal profiler (Jr25, Nanovea) with a lateral resolution of ~\u0026thinsp;1.5 \u0026micro;m and a vertical resolution of ~\u0026thinsp;5.5 nm (PS2 sensor with 300 \u0026micro;m max height range) with subsequent data evaluation (Leica Map, Leica Microsystems).\u003c/p\u003e \u003cp\u003eHardness and Young\u0026acute;s modulus evaluation was performed by nanoindentation (Bruker, Hysitron Triboindenter TI 950, PerforMech 2 Transducer, Berkovich indenter) with a load of 10 mN, resulting in penetration depths of 141\u0026thinsp;\u0026plusmn;\u0026thinsp;58 nm. For such loads, Tischler et al. [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e] achieved substrate independent results for distinguishing the DLC properties. For statistical reasons, a total of ten indentations were performed at evenly distributed locations across the entire sample\u0026acute;s surface for each DLC coating.\u003c/p\u003e \u003cp\u003eThe sp\u003csup\u003e3\u003c/sup\u003e contents of the various DLC coatings were assessed by Raman spectroscopy, following the methodology of Cui et al. [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. This method combines and compares previous Raman studies on DLC coatings and constructs models for hydrogenated DLC via G-peak dispersion and for H-free DLC coatings via G-peak full width at half maximum (FWHM) analysis, respectively [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. To this end, a Raman system (WITec alpha 300 RSA+) with three excitation wavelengths 633 nm, 532 nm, and 488 nm was employed. All Raman spectra were fitted with two Gaussian peaks representing D- and G-peaks, respectively, after linear background subtraction using the dedicated software (CasaXPS\u0026copy;). The two peak Gaussian peak fit model was selected, as it is the most widely used and most consistent [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. Because the peak position and FWHM of both D- and G-peak may vary depending on sp\u003csup\u003e3\u003c/sup\u003e content [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e], the fit parameters were not constrained in any way. However, the ranges for the position (pos(D):\u003c/p\u003e \u003cp\u003e1300\u0026ndash;1400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; pos(G): 1500\u0026ndash;1680 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and FWHM (\u0026lt;\u0026thinsp;400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) were not exceeded. Dispersions were calculated between all three used wavelengths and thus obtained sp\u003csup\u003e3\u003c/sup\u003e contents via Cui et al. [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e] were averaged from all three dispersions for each hydrogenated DLC. For the FWHM analysis of H-free samples, the FWHMs for 488 nm were calculated to obtain sp\u003csup\u003e3\u003c/sup\u003e contents via Cui et al. [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. Choices in peak shape are not expected to significantly change the calculated sp\u003csup\u003e3\u003c/sup\u003e content for the dispersion model used for hydrogenated samples. Cui et al. [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e] also derived a general equation to calculate the sp\u003csup\u003e3\u003c/sup\u003e content in H-free DLC coatings for arbitrary excitation wavelengths using the G-peak FWHM. The details of XPS analysis are discussed in Chap.\u0026nbsp;2.5.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. RIC wear measurement / Irradiation of samples\u003c/h2\u003e \u003cp\u003eContinuous monitoring of DLC wear with appropriate sensitivity is necessary to address the issues of wear rate quantification in the mild wear regime. For the wear measurement, radioactive tracer technology is applied by producing radioactive isotopes in the DLC and substrate by Thin Layer Activation (TLA) [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]. The wear particles carrying the isotopes are transported to the gamma-ray detector through the lubricant circuit, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Detailed information about the used Radioactive Isotope Concentration (RIC) wear measurement can be found in the literature [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan additionalcitationids=\"CR59 CR60 CR61\" citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e]. During each experiment, 100 ml of lubricant is circulated through a squeeze pump at a flow rate of 30 ml/min at 25\u0026deg;C temperature. The components of the RIC circuit are analysed after the experiments for any sedimentation or deposition of the wear particles, proving that the essential majority of the particles are detectable in the lubricant. Consequently, the RIC measurement is significant, and no filters are used in the circuit. DLC activation was achieved by irradiating the carbon with helium (\u003csup\u003e3\u003c/sup\u003eHe) [\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e] along a 2 mm wide strip across the sample surface, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo investigate the wear behaviour of the steel counter body in separate tests, the X90CrMoV18 piston rings with a diameter of 83 mm, radius of 5 mm, and width of 1.2 mm, were activated with deuterons [\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e]. To avoid wear superposition between the steel counter body and the steel substrate, the DLC was not activated in the examinations of the steel counter body.\u003c/p\u003e \u003cp\u003eAll specimens were irradiated using low-intensity beam parameters to prevent any alteration of their material properties. This wear measurement method is designed to investigate wear progression under different loading conditions without affecting the tribological performance of DLC [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Selection of tribological load\u003c/h2\u003e \u003cp\u003eThe irradiated DLC coated-plates were tested against the piston ring using a model tribometer (Universal Mechanical Tester \u0026ndash; UMT, Bruker) with oscillating movement. The oscillation frequency was set at 25 Hz with a 4 mm stroke. The reciprocating motion imitates start-up motion and provokes boundary conditions. To ensure that the activation area is fully utilized, the wear track was arranged perpendicular to the 2 mm wide activated strip in order to perform multiple wear tracks on one sample, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Subsequently, several tribological tests were conducted across the activated zone on one sample. The length of the wear track on the flat DLC plate is ~\u0026thinsp;4 mm in the direction of the stroke, with a width of ~\u0026thinsp;1 mm dependent on the loading and wearing condition, giving a wear area of up to 4 mm\u0026sup2;. Of this, the activated wear area is about half of the wear track. The wear area on the piston ring is ~\u0026thinsp;0.5 mm\u0026sup2;, 1 mm perpendicular to the stroke direction and 0.5 mm in stroke direction due to the barrel-like geometry, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The test parameters result in an average velocity of ~\u0026thinsp;0.3 m/s in the activated area.\u003c/p\u003e \u003cp\u003eThe initial Hertzian contact pressure was 875\u0026thinsp;\u0026plusmn;\u0026thinsp;152 MPa, estimated for a 50 N load, further details in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. After the running-in phase, the estimated test pressures are ~\u0026thinsp;100\u0026thinsp;\u0026plusmn;\u0026thinsp;60 MPa, considering the effects of smoothing and enlarging of the wear track area (~\u0026thinsp;0.5 mm\u0026sup2;), obtainable at the steel counter body at the end of the test. The estimated pressures are significantly below the yield strength of DLC [\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e]. Therefore, the selected loading parameters were set to prevent (premature) DLC failure and to investigate the influence of the different DLCs and lubricants in the mild wear regime [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAlthough friction was also measured for a complete evaluation, the focus of this study was clearly on DLC and steel counter body wear behaviour and wear rates. The coefficient of friction (COF) was evaluated using the root mean square over 333 ms of the measured signal with 3000 Hz sampling frequency. The RIC wear data was averaged at 10-minute intervals.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eParameters for the estimation of initial Hertzian contact pressure and film thickness.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eParameter\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDLC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePiston ring steel\u003c/p\u003e \u003cp\u003ecounter body\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRadius of curvature in X and\u003c/p\u003e \u003cp\u003eY direction in mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0 and 0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e41.5 and 5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eYoung\u0026acute;s modulus in MPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e151 to 244\u0026nbsp;GPa (refer to chapter \u0026ldquo;DLC coating classification\u0026rdquo;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e210\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePoisson ratio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.22 [\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDynamic viscosity at 25\u0026nbsp;\u0026deg;C in mPas\u003c/p\u003e \u003cp\u003e(refer to chapter \u0026ldquo;Lubricant viscosity\u0026rdquo;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eWater\u0026hellip;0.94,\u003c/p\u003e \u003cp\u003eDiesel\u0026hellip;2.17,\u003c/p\u003e \u003cp\u003eAged diesel OX2\u0026hellip;2.27,\u003c/p\u003e \u003cp\u003eAged diesel OX10\u0026hellip;2.92;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePressure-viscosity coefficient in GPa\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eWater\u0026thinsp;~\u0026thinsp;5,\u003c/p\u003e \u003cp\u003eDiesel 9.5\u0026nbsp;[\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNormal load in N\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAverage velocity in\u0026nbsp;m/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Test procedure for wear measurement\u003c/h2\u003e \u003cp\u003eFour low-viscosity lubricants with different chemical compositions were selected for this study:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003edeionised water (DEI, Electrolube, Germany),\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003ediesel GDK650, and\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eaged diesel GDK650 with oxidation numbers of 2 (OX2), and\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eaged diesel GDK650 with oxidation numbers of 10 (OX10).\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eThe lubricant viscosities were measured using a Stabinger viscosimeter (SVM 3000, Anton Paar, Austria), Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eAs these lubricants are different, e.g., regarding oxidation, the tribological experiments were set up to take oxidation reactions without relative motion into account. Additionally, two different wear measurement experiments were conducted using firstly activated DLC and secondly activated steel counter body:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe experiments with activated DLC were executed with break times or idle times, defined as \u0026ldquo;offset\u0026rdquo; tribometer tests, Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. In these tests the tribometer runs for an appropriate time (period 1), which allows for the discrimination of running-in and constant wear. The appropriate time for the duration of the tribometer's operation during the test was individually determined (6\u0026thinsp;\u0026plusmn;\u0026thinsp;1 hours). Following period 1, there was an offset time of 8\u0026thinsp;\u0026plusmn;\u0026thinsp;2 hours (period 2) during which the tribometer was stopped, but the interacting bodies remained in contact and were flushed with lubricant. After the offset time, the tribometric load was continued again (period 3). The continuous wear measurement was active throughout the whole experiment.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eFor the determination of the wear behaviour of the steel counter body, tests against one selected DLC (a-C:H(3), Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) with water, diesel, aged diesel OX2, and aged diesel OX10 were performed. The test time for investigating the steel counter body was set to 2 hours.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eA total of 38 tests were carried out with the different DLC-samples and with different lubricants, Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, of which 30 tests (\u003cspan refid=\"Sec19\" class=\"InternalRef\"\u003eAppendix\u003c/span\u003e 2) with activated DLC and a further 8 tests with activated steel counter body. The parameters of the tests with activated DLC are listed in \u003cspan refid=\"Sec19\" class=\"InternalRef\"\u003eAppendix\u003c/span\u003e 1, the obtained wear rates averaged over the tests with the same parameters are listed in \u003cspan refid=\"Sec19\" class=\"InternalRef\"\u003eAppendix\u003c/span\u003e 2.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5. Analysis of the wear track\u003c/h2\u003e \u003cp\u003eSelected areas within and outside the wear tracks were analysed by X-ray photoelectron spectroscopy (XPS). XPS data was acquired using a Thermo Fisher Scientific Thetaprobe with a monochromatic Al Kα X-ray source (1486.6 eV). High-resolution spectra were obtained at 50 eV pass energy with an energy step size of 0.2 eV. The C1s peak was analysed to compare the graphitic change and oxidation of ta-C and a-C:H(1). C1s peak was divided into sp\u003csup\u003e3\u003c/sup\u003e peak at 285.5 eV, sp\u003csup\u003e2\u003c/sup\u003e peak at 284.7 eV, O\u0026thinsp;=\u0026thinsp;C-O at 290.2 eV, and C-O at 286.7 eV according to literature [\u003cspan additionalcitationids=\"CR70\" citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e]. Furthermore, for a detailed examination of the oxidation, the O1s region was fitted by C-O at 531.9 eV, MeOx at 530.3 eV, OH 535.1 eV, and C-OH at 533.5 eV [\u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e]. Peak fitting was performed with the Thermo Fisher software (Thermo Fisher Scientific, Avantage), using Gaussian/Lorentzian curve fitting for the evaluation. Peak backgrounds were subtracted using a modified Shirley algorithm [\u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e]. To analyse the influence of oxidation during transport from the tribological test to the XPS, not-sputtered and sputtered areas were analysed by XPS spectroscopy. For comparison, each surface measurement spot was sputtered with 1 keV Ar ions (20 seconds, 1 \u0026micro;A) and another XPS inspection was performed.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results - DLC and tribosystem characterisation","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1. DLC coating classification\u003c/h2\u003e \u003cp\u003eThe visual appearance of the DLC surfaces can be described as following: aC, aC:H(1), and taC occur smoother than a-C:H(3), where else aC:H(3) seems smoother than a-C:H(2), Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Pores or dotted surface texture can be recognized on each DLC sample, whereby a-C:H(2) shows the largest and roughest pore structure. The line structure originates from the substrate surface pre-preparation process before coating, as reported in the previous study [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA similar trend to the appearance can be seen in the roughness values Sa, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The coatings are ordered from the lowest to the highest roughness values of the unloaded (initial, as obtained) DLC coated surface:\u003c/p\u003e \u003cp\u003ea-C\u0026thinsp;\u0026lt;\u0026thinsp;a-C:H(1)\u0026thinsp;\u0026lt;\u0026thinsp;ta-C\u0026thinsp;\u0026lt;\u0026thinsp;a-C:H(3)\u0026thinsp;\u0026lt;\u0026thinsp;a-C:H(2)\u003c/p\u003e \u003cp\u003eIt seems that generally H-free DLC has lower roughness values than hydrogenated DLC. However, it cannot be said that H-free is always smoother taking Ma et al. [\u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e] into consideration.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eBased on Raman spectroscopy, the sp\u003csup\u003e3\u003c/sup\u003e content was derived from FWHM for hydrogenated DLC and from Disp(G) for H-free DLC, Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. This gives the following sequence of DLC layers in terms of Raman-evaluated sp\u003csup\u003e3\u003c/sup\u003e content:\u003c/p\u003e \u003cp\u003eta-C\u0026thinsp;\u0026gt;\u0026thinsp;a-C\u0026thinsp;\u0026gt;\u0026thinsp;a-C:H(1)\u0026thinsp;\u0026gt;\u0026thinsp;a-C:H(2)\u0026thinsp;\u0026gt;\u0026thinsp;a-C:H(3)\u003c/p\u003e \u003cp\u003eBased on the formula of Cui et al. [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e] an uncertainty of \u0026plusmn;\u0026thinsp;8 % i estimated for the sp\u003csup\u003e3\u003c/sup\u003e content based on the FWHM of the Raman measurements. In the present analysis, the sp\u0026sup3; content of ta-C has been determined to be 69 %, hich outlines a bigger uncertainty when compared to the values documented in the literature [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] and given by the manufacturer, Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, to be \u0026ge;\u0026thinsp;80 % for s\u003csup\u003e3\u003c/sup\u003e content of ta-C. The derived values for the sp\u0026sup3; content based on Raman measurements are dependent on the fitting method and parameters. As we use the same fitting method for both hydrogenated and H-free samples, there may be a higher systematic error as given by the above-mentioned uncertainty. However, this applies in the same way to all our sp\u0026sup3; estimations, the ranking of our DLC samples according to the sp\u0026sup3; content can be seen as reliable.\u003c/p\u003e \u003cp\u003eThe ta-C sample exhibits a sp\u0026sup3; content of 69%, while the a-C sample displays a sp\u003csup\u003e3\u003c/sup\u003e content of 56%. In contrast, the a-C:H(1), a-C:H(2), and a-C:H(3) samples exhibit sp\u003csup\u003e3\u003c/sup\u003e contents of 54, 46, and 40%, respectively. In this study, the sp\u003csup\u003e3\u003c/sup\u003e content is higher for H-free DLC coatings than for hydrogenated DLC coatings, Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eList of Raman evaluated Disp(G) for hydrogenated DLC, FWHM for H-free DLC, and sp\u003csup\u003e3\u003c/sup\u003e content for all DLCs.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDLC coating label\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDisp(G)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eFWHM @532 nm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRaman evaluated\u003c/p\u003e \u003cp\u003esp\u003csup\u003e3\u003c/sup\u003e content\u003c/p\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;8%\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eta-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e223\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e69\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ea-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e202\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e56\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ea-C:H(1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.256\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e54\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ea-C:H(2)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.217\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e46\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ea-C:H(3)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.203\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe hardness and the Young\u0026acute;s modulus of the samples in this study are higher for H-free DLC coatings (ta-C and a-C) than for hydrogenated DLC coatings (a-C:H(1), a-C:H(2), and aC:H(3)) Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The highest hardness of DLC was determined to be 29 MPa for the a-C type, with the second-highest hardness observed in ta-C at 27 MPa. In comparison, the aC:H(1), a-C:H(2), and a-C:H(3) variants exhibited lower hardness values of 19, 16, and 20 MPa, respectively, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The trend of the hardness values correlates with the trend of the averaged sp\u003csup\u003e3\u003c/sup\u003e contents. This is in accordance with literature [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e75\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Lubricant viscosity\u003c/h2\u003e \u003cp\u003eConcerning the viscosities, water and diesel lubricant viscosities show a similar behaviour over temperature, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, goes in line with literature [\u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e76\u003c/span\u003e, \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e77\u003c/span\u003e]. Aged OX2 diesel shows a small increase in viscosity compared to diesel, where else aged OX10 diesel shows a significant increase in viscosity. An increase in viscosity due to ageing has also been reported [\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e78\u003c/span\u003e]. The viscosity values at 25\u0026deg;C were used to estimate the film thickness, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, and to visualise the dependence of wear rate on viscosity in the discussion in this study, Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e, as the wear experiments were conducted at 25\u0026deg;C room temperature.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.3. DLC wear tracks topography\u003c/h2\u003e \u003cp\u003eBased on the optical inspection and topographical results, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, no delamination areas or failure occurred on the DLC coated surfaces during the experiments due to the mild wear loading conditions, as expected [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Only in four experiments (ta-C lubricated with diesel, a-C lubricated with water, a-C:H(1) lubricated with diesel, and a-C:H(2) lubricated with water) a pronounced wear track can be observed, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Furthermore, scratches in the direction of the relative motion of the counter body are present after testing ta-C with water lubrication, a-C with diesel and aged diesel lubrication, and a-C:H(1) lubricated with water.\u003c/p\u003e \u003cp\u003eAlthough there are some roughness changes, see next paragraph, wear cannot be quantified via chromatic confocal profilometer measurements for the following combinations, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, (marked with red dots):\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eta-C, a-C and a-C:H(1) with aged diesel lubrication,\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003ea-C:H(2) lubricated with diesel and aged diesel, and\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eall a-C:H(3).\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eIf changes in wear hight are smaller than typical roughness values, for example Sa, topographical wear measurements are subjected to serious uncertainty and consequently not suitable for distinguishing the effects investigated in the present study.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe roughness values after tribological loading indicate a reduction in the wear area compared to the values of the initial unloaded DLC surfaces, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Estimation of the lubricant film thickness using the equation of Chittenden et al. [\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e], the parameters listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, and the average sliding velocity gives values of 3 to 10 nm, mostly dependent on lubricant viscosity. The film thickness values are smaller than all the roughness values (initial and after the test in the wear area) by a factor of 5 to 100. It can consequently be assumed that the experiments are mainly running in the boundary lubrication regime. At the highest speeds in the centre of the stroke, partially mixed lubrication can be assumed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.4. DLC wear progression\u003c/h2\u003e \u003cp\u003eThe experiments in the present study show no significant DLC running-in behaviour and consequently the wear rates were evaluated using linear regression from the beginning to the end for each of the periods, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. For each lubricant and each DLC coating the wear rates were averaged for periods 1 and 3 (and over several tests, when such repetitions were available, see list of all experiments in \u003cspan refid=\"Sec19\" class=\"InternalRef\"\u003eAppendix\u003c/span\u003e 1). The grey areas (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) correspond to the offset periods, in which the tribometer was stationary, but the DLC-steel contact was maintained, and the system was flushed with the lubricant.\u003c/p\u003e \u003cp\u003eThe DLC wear rates for ta-C show a significant difference depending on the lubricant, water (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea), diesel (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb), and aged diesel OX10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). The ta-C wear rates (average of period 1 and 3 of two tests) were found to be 31\u0026thinsp;\u0026plusmn;\u0026thinsp;22 nm/h for water, 25\u0026thinsp;\u0026plusmn;\u0026thinsp;6 nm/h for diesel, and 12\u0026thinsp;\u0026plusmn;\u0026thinsp;12 nm/h for aged diesel OX10.\u003c/p\u003e \u003cp\u003eSurprisingly, when flushing with water, there is a DLC wear rate observable during offset period 2 (in this example of 20\u0026thinsp;\u0026plusmn;\u0026thinsp;8 nm/h). This behaviour was also observed for the other DLC coatings tested when lubricated with water, \u003cspan refid=\"Sec19\" class=\"InternalRef\"\u003eAppendix\u003c/span\u003e 2.\u003c/p\u003e \u003cp\u003eSide note: A wear rate of 20 nm/h over an average wear area of 2 mm\u0026sup2; (intersection of activated area and worn area) corresponds to a wear volume rate of 0.04 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e \u0026micro;m\u0026sup3;/h. This is in the range of mild wear for such testing conditions, for comparison with 0.05\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e \u0026micro;m\u0026sup3;/h as given in [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. At this wear rate, a 2 \u0026micro;m DLC layer loaded at 25 Hz would achieve a lifetime of approximately 9 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e cycles.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor the tests with water lubrication, deposits next to and in the wear-tracks were observable by optical means (for example a-C and a-C:H(1)), Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea und b. These deposits were within groves in the wear track or in elliptical shapes next to the wear track, indicating the probable contact area of the piston ring with the DLC plate during the offset flushing period. In comparison, the experiments with diesel and aged diesel OX10 showed no DLC wear rate and no deposits during the offset flushing period.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Wear rate results and discussion","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e4.1. Main influences on DLC wear rate\u003c/h2\u003e \u003cp\u003eA clear trend can be observed for the DLC wear rates with respect to the lubricant property viscosity and the DLC property sp\u003csup\u003e3\u003c/sup\u003e content, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. Specifically, the DLC wear rates decrease with viscosity from water to diesel to aged diesel, and increase with sp\u003csup\u003e3\u003c/sup\u003e content from a-C:H(2) over a-C:H(1) and a-C to ta-C. Even though the wear processes are in the order magnitude of of nm/h and there are unavoidable uncertainties due to tribological testing, as documented by the uncertainty bars in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, trends can nevertheless be ascertained on the basis of the wear rates.\u003c/p\u003e \u003cp\u003eConsidering the influence of the lubricant, the wear rates for the different DLC coatings are similar to water lubrication and do not show any trend concerning the DLC sp\u003csup\u003e3\u003c/sup\u003e content, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. Otherwise, tests with diesel lubrication show a significant tendency with sp\u003csup\u003e3\u003c/sup\u003e content.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eConsidering the influence of the DLC-characteristics, hydrogenated DLC with a sp\u003csup\u003e3\u003c/sup\u003e content in the range of 40\u0026ndash;55% exhibits less wear than H-free or tetrahedral DLC with sp\u003csup\u003e3\u003c/sup\u003e content greater than 55%, Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. At first glance, this result seems to be contractionary to the standard tribological knowledge, as higher sp\u003csup\u003e3\u003c/sup\u003e content results in higher hardness, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, and should therefore lead to less wear. Instead, higher hardness content shows higher wear in the present study. In literature, this is related to several effects, including the presence of hydrogen, the appearance of abrasive particles and graphitisation, as discussed in the following.\u003c/p\u003e \u003cp\u003eLaw et al. [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e] reported improved performance of hydrogenated DLC compared to H-free DLC in lubricated contact of engine components. The ability of DLC to inhibit wear processes may therefore be related to the contribution of hydrogen, as also reported by Zhang et al. [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. For ta-C it is reported that abrasive DLC wear particles are generated during the tribological loading [\u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e79\u003c/span\u003e]. These spalled particles exhibit high hardness and consequently accelerate wear as abrasives [\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e78\u003c/span\u003e] [\u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e80\u003c/span\u003e]. In summary, increased hardness of DLC coatings is related to increased sp\u003csup\u003e3\u003c/sup\u003e content, but also to increased wear, for example, due to the generation of abrasive particles or a higher degree of graphitisation.\u003c/p\u003e \u003cp\u003eThese understanding models focus on the wear behaviour of DLC. However, we obtained some remarkable results during the flushing period for the tests with water lubrication associated with some debris at the contact with the steel counter body. This debris are assumed to be corrosive products of the steel counter body. Nevertheless, we need to understand the role of the steel counter body, before going into further details about the DLC wear behaviour. As described in the methods, a separate test series was carried out with activated steel piston rings (counter body) for online wear measurement. The a-C:H(3) plates served as base body due to availability and are assumed to be similar to a-C:H(1) and a-C:H(2) for reasonable comparison in terms of wear behaviour.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Steel counter body wear\u003c/h2\u003e \u003cp\u003eSteel counter body wear rates show a dependency on viscosity, Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea. Compared to DLC wear behaviour, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, which showed no pronounced running-in wear, the wear of the steel counter bodies show a significant running-in behaviour, Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eb. The running-in behaviour of the steel piston rings is rather independent of the applied lubricant, Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eb. This lubricant independent behaviour can be attributed to the barrel-shape of the steel counter body piston ring compared to the flat DLC plate samples as well as the difference in contact areas during the reciprocal movement.\u003c/p\u003e \u003cp\u003eSteel counter body running-in wear is ~\u0026thinsp;23 times higher in comparison to steel counter body wear rate when lubricated with diesel or aged diesel OX2 or OX10, Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e, compare left and right y-axis. However, after running-in the wear rates of the steel counter bodies and the wear rates of the DLC coatings, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, are in a comparable range for diesel and aged diesel lubrication.\u003c/p\u003e \u003cp\u003eHowever, aged diesel (OX2 and OX10) resulted to a sharp equal drop in wear rate and running-in wear compared to diesel, indicated by the arrow in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e. This suggests that viscosity is not the only influencing factor when considering lubricant properties, as discussed in the following chapter.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e4.3. Influence of DLC and lubricant properties\u003c/h2\u003e \u003cp\u003eIn order to better understand the influence of certain parameters, a correlation analysis was carried out. To this end, the various DLC and lubricant properties were evaluated and compared with the DLC wear rates, Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ea and c, and the COFs, Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003eb. Positive values (in the bars) indicate a direct correlation and negative values an indirect correlation between the outlined parameter and the DLC wear rate or COF. The viscosity inhibits the highest influence on DLC wear, Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ea, although we are in the boundary and mixed regime in this study. The difference in wear rates with water and diesel lubricants can be seen as the main driving force, indicating the high correlation with viscosity. As the factors viscosity and type of lubricant cannot be considered independently, water is excluded for the correlation details, outlined in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ec.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe factors sp\u003csup\u003e3\u003c/sup\u003e content, hardness, and hydrogenated or H-free cannot be regarded as independent of each other and show similar correlations with DLC wear rates, Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ec at least for the applied loading conditions. The roughness has the lowest correlation with DLC wear rates. The roughness in the area measured in this study is therefore negligible for the mild wear behaviour. These indications suggest that in boundary regimes lubrication properties are ultimately the driving force regarding the DLC wear rate. Choosing the right DLC coating is of secondary importance. However, when considering only the C-based lubricants (diesel and aged diesel), the DLC properties reach a higher correlation with the DLC wear rates than the lubricant properties, Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ec. Nevertheless, roughness has the lowest correlation with wear rate in both correlation approaches.\u003c/p\u003e \u003cp\u003eSummarising the findings regarding DLC wear rate:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eDue to the lower viscosity, water leads to higher DLC wear rates than the diesel lubricants, as the tests are run in boundary and mixed lubrication regimes.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eWhen excluding water, the impact of the sp\u0026sup3; content (or hardness) of the DLC coatings is recognizable within the tests with diesel lubricants as well as the impact of the oxidation number (aging) of the diesel lubricants.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eThe correlation between DLC wear rate and COF is only 22%, Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ea, which is also observable in the direct comparison, Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ed. COF correlates indirectly with oxidation number, viscosity, sp\u003csup\u003e3\u003c/sup\u003e content, and roughness, Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003eb. Generally, diesel lubrication results in a higher COF (0.21\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05) compared to water (0.19\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05) or aged diesel (0.13\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04) lubrication. This behaviour is somehow surprising, as the main factor for the COF trend is the oxidation number followed by the minor factor viscosity.\u003c/p\u003e \u003cp\u003eIn order to clarify the impact of oxidation and if there is a chemical interaction between the aged lubricant and the DLC surface, wear track analyses are discussed in the following chapter.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e4.4. XPS analyses of DLC wear surface\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe XPS analyses were carried out on the samples, as received after the tribological test and named \u0026ldquo;not sputtered\u0026rdquo;. As such, they showed a relatively high content of C-O with additional O\u0026thinsp;=\u0026thinsp;C-O bindings within the C1s peak. Details of all measured peaks can be found in \u003cspan refid=\"Sec19\" class=\"InternalRef\"\u003eAppendix\u003c/span\u003e 3. These peaks (O\u0026thinsp;=\u0026thinsp;C-O and C-O) are regarded to be some sort of tribofilm (or deposition) formed from or formed together with the lubricant. For gaining the unimpaired sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e ratio, the peaks O\u0026thinsp;=\u0026thinsp;C-O and C-O were subtracted, resulting in sp\u003csup\u003e3\u003c/sup\u003e together with sp\u003csup\u003e2\u003c/sup\u003e being 100%, Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ea and b. The percentage of sp\u003csup\u003e3\u003c/sup\u003e is indicated in the results. The sp\u003csup\u003e3\u003c/sup\u003e ratios of the reference areas (outside of wear tracks) of ta-C and a-C:H(1) from the non-sputtered XPS data are within their respective uncertainties consistent with the Raman derived sp\u003csup\u003e3\u003c/sup\u003e content estimates above.\u003c/p\u003e \u003cp\u003eRegarding Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ea, the sp\u003csup\u003e3\u003c/sup\u003e contents of the ta-C reference area (82%) and the a-C:H reference area (61%) are higher compared to the sp\u0026sup3; content inside the wear tracks (ta-C: from 66 to 72% and a-C:H: from 20 to 31%). This indicates that the loading conditions lead to friction and wear but also lead to a decrease in sp\u0026sup3; content, which is in first instance referred to as graphitisation.\u003c/p\u003e \u003cp\u003eAs the C-O tribofilm or deposit may limit the significance of the above observations, the surfaces were sputtered in order to clean the surface from the tribofilm. The C1s peak analysis of the sputtered wear tracks, Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003eb, show a lower sp\u0026sup3; content compared to the not sputtered tracks, Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ea. Here some kind of radiation damage of the sputtering must be assumed, that leads to an additional transition from sp\u0026sup3; to sp\u0026sup2;. Consequently, all sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e ratios of the sputtered surfaces are lower than the ratios for the not sputtered surfaces. Nevertheless, all ta-C wear tracks still show a higher sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e ratio compared to a-C:H for the sputtered surfaces, and the trend of higher sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e ratio for the reference area compared to the wear tracks is as well recognizable, Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003eb. Thus, graphitisation from sp\u003csup\u003e3\u003c/sup\u003e to sp\u003csup\u003e2\u003c/sup\u003e can be supposed due to tribological loading and in accordance with literature [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e, \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e81\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eRegarding the analyses of the O1s peaks, the content of C-OH is higher in the wear tracks of a-C:H (between 89 and 94%) compared to ta-C (between 69 and 84%) for not sputtered surfaces, Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ec. The analyses of the sputtered wear tracks also show a higher content of C-OH in the a-C-H wear tracks (between 32 and 40%) compared to the ta-C wear tracks (between 13 and 20%), Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ed. C-OH is a hydroxy group. Zahid et al. [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] describe in their review that hydrogen passivates the dangling carbon bonds. The passivation of hydroxyl groups can dominate the wear behaviour of DLC coatings, according to [\u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e82\u003c/span\u003e]. Dorner-Reisel et al. [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e] found passivation due to diesel lubrication and low wear of DLC is closely attributed to the formation of hydrophilic group C-OH [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. The analysis shows that with hydrogenation and less sp\u003csup\u003e3\u003c/sup\u003e (so higher sp\u003csup\u003e2\u003c/sup\u003e) results in higher C-OH content and a lower DLC wear rate.\u003c/p\u003e \u003cp\u003eAs a side comment and in addition to Chap.\u0026nbsp;2.5, a higher content of MeOx was found in the O1s peaks for the deposits (in the range of 3 to 7%) compared to the reference or the wear tracks (from 1 to 3%), \u003cspan refid=\"Sec19\" class=\"InternalRef\"\u003eAppendix\u003c/span\u003e 3. Iron oxides (likely a mixture of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) are therefore clearly present. The deposits are formed during water lubrication; mainly during the offset period (flushing period with no relative movement) while the tribometer is stationary. When the relative movement is turned on again after the offset flushing period, the iron oxides of the counter body can cause higher DLC wear with water lubrication compared to the diesel lubricants, for which no deposits were observed on or next to the DLC wear tracks. This additional 3rd particle wear mechanism is assumed to be also the cause for the outstanding wear rates of DLC with water lubrication compared to the general trend of wear rates versus COF based on the diesel lubricants, Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ed.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Summary and Conclusions","content":"\u003cp\u003eThis study demonstrates the successful application of the RIC method for continuously monitoring the mild wear behaviour of DLC-coated plates and steel piston ring counter bodies during tribological testing. Using this approach, wear rates in the nm-per-h range were obtained and differentiated based on influencing factors, such as coating composition and lubricant type.\u003c/p\u003e \u003cp\u003eIn boundary and mixed lubrication regimes, lubricant properties - particularly viscosity - proved to be more influential on wear performance than the DLC coating properties. However, when only carbon-based lubricants were used, the characteristics of the DLC coatings became more decisive for wear behaviour. Interestingly, higher sp\u003csup\u003e3\u003c/sup\u003e content (or hardness) in DLC coatings led to increased wear rates when lubricated with diesel, whereas water-based lubrication rendered the sp\u003csup\u003e3\u003c/sup\u003e content largely irrelevant. Additionally, both DLC and steel counter bodies exhibited improved wear resistance when lubricated with aged diesel rather than fresh diesel, likely due to initial running-in processes and chemical changes in the lubricant.\u003c/p\u003e \u003cp\u003eThe formation of hydroxyl groups on the DLC surface was found to correlate with a transformation from sp\u003csup\u003e3\u003c/sup\u003e to sp\u003csup\u003e2\u003c/sup\u003e hybridisation, commonly referred to as graphitisation. Under the mild wear and boundary lubrication conditions applied here, hydrogenated DLCs with higher sp\u003csup\u003e2\u003c/sup\u003e content showed superior wear performance compared to harder, hydrogen-free coatings with higher sp\u003csup\u003e3\u003c/sup\u003e content. This suggests that sp\u003csup\u003e2\u003c/sup\u003e structures may facilitate hydroxyl group formation, which acts as a passivation layer and contributes to anti-wear behaviour. In contrast, while higher hardness is traditionally seen as beneficial, it plays a lesser role under the specific conditions of this study.\u003c/p\u003e \u003cp\u003eWater-based lubrication, despite promoting hydroxyl group formation, also introduced corrosive effects on the steel counter body, leading to increased wear due to third-body abrasion. In contrast, aged diesel lubricants reduced DLC wear rates, with oxidation appearing more influential than viscosity changes. This aligns with the proposed model in which oxidation products assist in forming protective surface layers.\u003c/p\u003e \u003cp\u003eIn conclusion, the study highlights the importance of understanding the interplay between lubricant chemistry and DLC coating properties. The findings show that under realistic loading and lubrication conditions, a tailored combination of DLC type and lubricant can significantly reduce wear, thereby extending the service life of coated components. Selecting the appropriate lubricant and DLC is therefore essential to maintaining low wear rates, particularly under boundary lubrication conditions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eThe authors have no relevant financial or nonfinancial interests to disclose. All authors contributed to the study conception and design. Material preparation was performed by M. Zellhofer, T. Wopelka, F. Ditr\u0026oacute;i (Institute for Nuclear Research, Debrecen, Hungary), A. Kuebler (Robert Bosch GmbH, Feuerbach, Germany), and F. Rovere (Oerlikon Balzers Coating AG, Balzers, Switzerland). Nanoindentation was conducted by C. Tomastik and H. Rojacz using the facilities at AC2T research GmbH. Diesel ageing and oxidation number measurements were carried out by A. Agocs at AC2T research GmbH. Raman measurements, analysis, and proportionate contribution to this paper were carried out by B. C. Bayer-Skoff and B. Fickl using the facilities of the Institute of Materials Chemistry at the Vienna University of Technology. XPS measurements and analysis were carried out by J. Brenner and E. Charfi. Data collection and preparation were performed by M. Zellhofer. Data analysis and interpretation were performed by M. Zellhofer and M. Jech. The first draft of the manuscript was written by M. Zellhofer and M. Jech, and detailed revision work was performed by P.H. Mayrhofer, E. Badisch, M. Zellhofer and M. Jech. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was carried out as part of the COMET Centre InTribology (FFG no. 906860 and 872176), a project of the \u0026ldquo;Excellence Centre for Tribology\u0026rdquo; (AC2T research GmbH). 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Epub ahead of print 1 March 2020. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s11249-019-1248-6\u003c/span\u003e\u003cspan address=\"10.1007/s11249-019-1248-6\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eErdemir, A., Martin, J.M.: Superior wear resistance of diamond and DLC coatings. Curr. Opin. Solid State Mater. Sci. \u003cb\u003e22\u003c/b\u003e, 243\u0026ndash;254 (2018)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Diamond-Like Carbon (DLC), Radio-Isotope Concentration (RIC) method, sp3/sp2 ratio, lubricant influence, boundary regime, continuous wear measurement","lastPublishedDoi":"10.21203/rs.3.rs-6717519/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6717519/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDiamond-like carbon (DLC) coatings are widely employed to extend the service life of components exposed to severe tribological conditions, particularly where uncoated surfaces would fail under boundary and mixed lubrication. For reliable application, a mechanistic understanding of mild wear processes in such regimes is essential. In this study, \u003cem\u003ein-situ\u003c/em\u003e wear rate measurements were performed using a reciprocating tribometer to evaluate DLC-coated specimens against steel counter bodies under low-viscosity lubricants with distinct chemical compositions, namely water and diesel fuels (fresh and aged). A series of hydrogenated and hydrogen-free DLC coatings with varying sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e hybridisation ratios and hardness were investigated. The results indicate that under mild wear conditions, lubricant properties \u0026mdash; most notably dynamic viscosity (~\u0026thinsp;1\u0026ndash;3 mPa\u0026middot;s) and oxidation state \u0026mdash; dominate the wear response, while the DLC coating\u0026rsquo;s sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e ratio and mechanical properties exert a secondary influence. Hydrogenated, sp\u003csup\u003e2\u003c/sup\u003e-rich DLCs exhibited superior wear resistance due to their enhanced ability to form passivating hydroxyl layers, which suppress graphitisation-induced degradation. In contrast, water-based lubrication, while promoting hydroxylation, led to increased wear due to steel counter body corrosion and third-body abrasion. Aged diesel lubricants resulted in lower DLC wear rates, attributable to increased oxidation promoting surface passivation. These findings highlight the necessity of jointly considering lubricant chemistry and DLC structural characteristics when designing tribological systems for low-wear applications.\u003c/p\u003e","manuscriptTitle":"Influence of ta‑C, a‑C, and a‑C:H properties and low-viscosity lubricants on the mild wear behaviour of DLC","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-30 10:15:01","doi":"10.21203/rs.3.rs-6717519/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"bc3c9e73-039d-4dd9-ae98-08aa6b9146b8","owner":[],"postedDate":"May 30th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-07-13T14:23:53+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-30 10:15:01","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6717519","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6717519","identity":"rs-6717519","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}