DLC-Coated Thermoplastics: Tribological Analyses Under Dry and Lubricated Sliding Conditions

Vol.:(0123456789) 1 3 Tribology Letters (2023) 71:2 https://doi.org/10.1007/s11249-022-01663-7 ORIGINAL PAPER DLC‑Coated Thermoplastics: Tribological Analyses Under Dry andLubricated Sliding Conditions K.Bobzin1 · C.Kalscheuer1· M.Thiex1 · P.Sperka2 · M.Hartl2 · S.Reitschuster3 · E.Maier3 · T.Lohner3 · K.Stahl3 Received: 20 July 2022 / Accepted: 2 October 2022 © The Author(s) 2022 Abstract The purpose of this study was to increase the limited wear resistance of polymers in highly stressed tribological contacts. Therefore, diamond-like carbon (DLC) coatings were firstly developed and deposited on polymers. The development of the low temperature coating process was conducted on an industrial scale coating unit. Due to the use of polyether ether ether ketone (PEEK) and polyamide as substrate material, the coatings were tailored in accordance with different mechanical properties of the polymers. Furthermore, analyses regarding morphology, roughness as well as compound adhesion between coatings and polymers were conducted. The tribological analyses were accomplished for reciprocating sliding kinematics on pin-on-plate tribometer under dry conditions as well as boundary lubricated conditions. The results prove the ability of the coatings to undergo high plastic deformations without delamination. In addition, the possible formation of a composite layer of DLC and PEEK in the wear track due to high energy input under dry conditions was shown. Furthermore, the transformations of sp2 chain into ring bindings as well as a change of size and distribution of the sp2 clusters during dry contact conditions were observed. Keywords DLC· PEEK· PA66· Raman spectroscopy· Wear· Polymers 1 Introduction Nowadays, the mechanical engineering sector needs to meet increasing demands on products in terms of lightweight construction, cost efficiency and performance under the prerequisite of saving resources. In order to meet these requirements, the substitution of metallic materials with polymers is a target-oriented approach. A typical representative of a substitute is poly ether ether ketone (PEEK), which has been established as alternative substrate material for components in a wide range of technical applications like gears, sliding bearings or compressor plate valves under low tribological loads [1]. This is due to a high chemical and temperature resistance as well as promising mechanical properties for technical applications. However, this leads to high costs compared to other conventionally used polymers like polyamide 66 (PA66). This polymer possesses lower mechanical and chemical properties in relation to PEEK. Polyamide is oftenused for structural components and tanks in the automotive industry. Nevertheless, a disadvantage arises in the use of polymers in tribological systems due to the reduced load carrying capacity compared to metals. This is accompanied by increased wear of the polymers and thus limits the field of application under high loads. There is a long-term effort to decrease wear of PEEK mainly by addition of various fillers, fibers and solid lubricants to form better performing composites [2–12]. The most often carbon and glass fibers are used for reinforcement and PTFE particles as a solid lubricant. Wear improvement up to three orders of magnitude and friction decrease by up to 75% were reported. Laux etal. showed that wear depends * M. Thiex [email protected] S. Reitschuster stefan.reitsc[email protected] 1 Surface Engineering Institute (IOT), RWTH Aachen University, Kackertstraße 15, 52072Aachen, Germany 2 Brno University ofTechnology (BUT), Technicka 2896/2, 61669Brno, CzechRepublic 3 Gear Research Center (FZG), Department ofMechanical Engineering, School ofEngineering & Design, Technical University ofMunich, Boltzmannstraße 15, 85748Garching, Germany Tribology Letters (2023) 71:2 1 3 2 Page 2 of 15 on PEEK molecular weight and that sliding perpendicular to the roughness lay leads to thinner transfer film [13]. Zalaznik etal. pointed out that processing temperature influences hardness and tribological performance of PEEK [14]. A promising approach to increase the wear resistance of polymers is the application of physical vapor deposition (PVD) coatings, which enable a friction and wear reduction in tribological systems among others. Since the fields of application for polymers steadily increased during the 90s, first analyses on PVD coating of thermoplastics like polycarbonate (PC), polybutylene terephthalate(PBT), polyamide (PA6.6) were conducted by Lugscheider etal. [15]. Therein the authors showed the possibility of depositing TiN on polymers by upscaling from a laboratory scale coating unit on an industrial unit. Riester etal. analyzed the influence of Ar, O2 or N as plasma etching gases on the surface activation of PBT before coating and found the highest potential for surface activation using Ar or O2 in contrast to N2 [16]. Further analysis of Lugscheider etal. using a combined pulsed magnetron sputtering technology for the deposition of TiN on polymers like PA, PEEK amongst others showed an improved compound adhesion between coating and substrate material compared to the sole use of direct current (dc) or pulsed magnetron sputtering [17]. The deposition of a zirconium modified DLC coating on PEEK was analyzed for the targeted application on a PEEK based cage for spindle bearings [18]. Therein, the results demonstrate an improved adhesion between coating and PEEK after plasma etching with argon or alternatively an argon ethyne mixture combined with a radio frequency (RF) pulsed bias voltage. These basic research studies prove the possible application of PVD coatings on polymers by process parameter adjustments. Also, first tribological analyses were conducted by Kapinski etal., testing a–C:H coated PA66 against uncoated 100Cr6 under dry conditions at an initial Hertzian pressure of pH=160MPa [19]. The results prove that a–C:H coatings developed for steel substrates can be transferred to PA66 substrate and are able to reduce friction and wear and enhance lifetime. Kaczorowski etal. analyzed the effect of an O2/CH4 etching plasma compared to a N2/CH4 etching plasma on the surface activation and found, that remains of N2 are integrated into the coating structure [20]. The addition of N2 into the a–C:H coating also led to a friction reduction under dry conditions against zirconium oxide at a Hertzian pressure of pH=117MPa compared to the use of an unmodified a–C:H coating. Another approach by Dufils etal. is texturing of a–C:H coated PEEK tested against alumina and lubricated with distilled water at a Hertzian pressure of pH=53MPa [21]. It was found that dimples with a size between 12 and 21µm and a density on the coated surface area between 10 and 30% lead to a significant friction reduction compared to uncoated PEEK. Tribological tests at higher initial Hertzian pressure of pH=345MPa under boundary and mixed lubrication against uncoated 100Cr6 also prove a friction and wear reduction compared to uncoated PEEK [22]. The overview of tribological investigations shows that DLC has a high potential to affect the wear and friction behavior of polymers positively. Nevertheless, there are only a few studies in this field so far, so that further investigations regarding the mechanical and chemical interactions during the tribological contact are necessary for a broad knowledge base. Besides the achieved friction and wear reduction, the DLC microstructure is affected by the tribological contact conditions, which can be analyzed my means of Raman spectroscopy. The effect on the microstructure is called relaxation as described by Kalish etal. [23] and is dominated by the two main mechanisms clustering of graphitic sp2 bindings and the transformation of diamond-like sp3 bindings into sp2-bindings. Furthermore, clustering subdivides itself into the three processes: • Transformation of sp2 chain bindings into sp2 ring bindings • Reduction of density and rearrangement of sp2 bindings • Effusion of hydrogen, which is only valid for hydrogencontaining DLC The Raman spectrum of carbon is primarily determined by the presence of the G peak, which is based on bond stretching vibrations of chain and ring sp2-bindings [24]. The D peak is solely based on breathing modes of sp2 ring bindings. Ferrari and Robertson analyzed the influences of different bonding states and composition of hydrogen-free amorphous carbon coatings [25]. Thereby, the position and intensity of D and G peak are affected by clustering, bond disorder, share of sp2 chain bindings and sp3 bindings. A qualitative evaluation of the binding states of hydrogen-free DLC is possible through the 3-stage model established by Ferrari and Robertson. Therein, the G peak position and ratio of the intensities of G and D peak are considered to describe the amount of sp3 bindings in DLC. The interpretations of the 3-stage model are extended for hydrogen-containing DLC coatings by Casiraghi etal. [26]. Furthermore, the parameters dispersion of the G peak Disp(G) and the full width half maximum FWHM(G) of the G peak can be determined. The Disp(G) predominantly describes the topological disorder, which combines the information of size and shape distribution of sp2 clusters of chains. At the same time, changes of sp2 ring bindings do not affect Disp(G) based on the results for a hydrogenated DLC coating ta–C:H of Ferrari and Robertson [27]. Furthermore, also the structural disorder of amorphous carbon can be described by determining the full width half maximum of the G peak FWHM(G)UV under UV excitation. Based on the findings Tribology Letters (2023) 71:2 1 3 Page 3 of 15 2 of Casiraghi etal., structural disorder arises mainly from bond angle and bond length distortions [26]. It must be noted, that the influence of the structural disorder on the Disp(G) and the topological disorder on the FWHM(G)UV cannot be completely excluded. In this study, durable DLC coatings applicable on PEEK and PA66 substrates to improve tribological performance are developed. The a–C:H coated specimens were analyzed after tribological testing to identify possible changes in the coating structure and further correlated with findings based on frictional and wear behavior. 2 Methods 2.1 Pretreatment ofPEEK andPA66 Specimens Squared polymer samples of PEEK, TECAPEEK, Ensinger GmbH, Nufringen, Germany, and PA66, TECAMID 66 LA natural, Ensinger GmbH, Nufringen, Germany, with a size of 30mm×30mm and a height of h = 8mm were grinded and polished by means of Phoenix 4.000×, ITW Test & Measurement GmbH, Esslingen am Neckar, Germany, to a roughness ofRa≈0.05µm. Therefore, SiC paper with a grain size of 1200, 2000 and 4000 and diamond suspension with a grain size of 3µm was used. The samples were chemically purified in a cleaning unit with the water-soluble basic detergents GALVAPREP SU 2012, NGL Cleaning Technology SA, Nyon, Switzerland, and a subsequent washing with water and isopropanol C3H8O. 2.2 Coating Process Two a–C:H coatings, called a–C:H-1 and a–C:H-2, were developed and deposited in an industrial scale coating unit CC800/9 Custom, CemeCon AG, Wuerselen, Germany. The coating unit was equipped with two high power pulsed magnetron sputtering (HPPMS) cathodes. The arrangement of the cathodes and the layout of the deposition chamber are depicted in Fig.1. For the deposition of the a–C:H coatings, four C targets with a purity of C = 99.9% were used. The process and reactive gases were argon (Ar), helium (He) and ethyne (C2H2). The coating processes were adjusted in accordance with mechanical properties, which show higher values for PEEK in comparison to PA66. A reduced helium gas flow lowers the degree of ionization of carbon in the coating process. The resulting structure shows a reduced content of diamond-like sp3 bindings in favor of more graphitic sp2 bindings. The reduction of the bias voltage UB influences the morphology as well as the mechanical properties due to the lower kinetic energy of the ionized particles during the coating growth. The helium gas flow was reduced from Q = 70sccm to Q = 50sccm and the bias voltage from UB = −250V to UB = −200V for the deposition on PA66, Table1. In addition, the bias voltage UB was pulse-synchronized with the HPPMS cathodes, which reduces the thermal transfer on the substrate material. The PVD coating process is basically divided into the four steps heating, etching, coating and cooling. In between the steps heating and etching there is a process step called off-gassing, where the starting pressure of p = 3mPa is reached again. In order to analyze the substrate temperature monitored during the coating Fig. 1 Schematic representation of the industrial coating unit CC800/9 Custom and target Tribology Letters (2023) 71:2 1 3 2 Page 4 of 15 process, temperature measurements were carried out using Insitemp, which is described in [28]. Therefore, specimen dummies made out of PEEK and PA66 were used. 2.3 Coating Analysis The coating thicknesses and morphology were analyzed by scanning electron microscopy (SEM), Oxford Link ISIS, Oxford Instruments plc, UK, at the Central Facility for Electron Microscopy (GfE), RWTH Aachen University, Aachen, Germany. A nanoindenter of type TI 950 TriboIndenter, Bruker Corporation, Billerica, Massachusetts, USA, with a Berkovich indenter with a nominal edge radius of approximately R = 20nm was applied for the determination of the indentation hardness HIT and modulus of indentation EIT. The samples were unprepared and the used constant indentation force was F = 2mN. Thereby, the penetration depth was kept below 10% of the top layer thickness. Thus, the influence of the substrate material can be minimized as explained by Fischer-Cripps [29]. In order to determine HIT and EIT, x = 40 individual measurements were conducted and subsequently averages out of the force–displacement curves were calculated. Calculations of the modulus of indentation are based on Oliver and Pharr’s equations [30]. Further informations on the nanoindentation method are presented in [31]. A constant Poisson's ratio of υ = 0.25 was assumed for the coatings. Nuclear reaction analysis(NRA) were carried out at Helmholtz Zentrum Dresden Rossendorf(HZDR), Dresden Rossendorf, Germany, to determine the hydrogen content of the a–C:H coatings in the as-deposited status. Two positions were analyzed on each coating for the penetration depths γ = 35nm and γ = 500nm with 20 repetitions. Thereby, the resulting hydrogen contents had to be evaluated by an exponential fitting method with approximation to a limit value in contrast to the determination by mean value. 2.4 Compound Analysis The analysis of the compound adhesion between the a–C:H coatings and PEEK respectively PA66 were conducted by means of scratch tester Gesellschaft für Fertigungstechnik und Entwicklung Schmalkalden e.V. (GFE), Schmalkalden, Germany. Scratch tests, with one repetition each, were conducted with a Rockwell C diamond at different constant forces between 10 ≤ FN ≤ 250N in steps of 10N. Every scratch was generated with a length of xs = 4mm and a feed speed vf = 10mm min−1. The load was applied during the first xs = 0.2mm of the total scratch length. In accordance to DIN EN ISO 20502, the compound adhesion between coating and substrate was evaluated by scratch tests and confocal laser scanning microscopy (CLSM), VK-X210, Keyence, Neu-Isenburg, Germany. It has to be stated that the scratch testing is not standardized for coated plastics. Nevertheless, the indention and scratching with a Rockwell C diamond through coated plastics at different loads simulates a high stress collective for the compound of coating and substrate material. 2.5 Tribological Analysis byMeans ofPin‑on‑Plate Tribometer Reciprocating pin-on-plate (PoP) tribological tests were carried out on an UMT Tribolab, Bruker Nano, California, San Jose, USA, under dry and lubricated conditions. In the device, normal load is close-loop controlled and measured simultaneously with friction by two axis sensor. The pin is stationary and the plate performs a harmonic motion. Acircular point contact was formed between 10mm in diameter 100Cr6 bearing steel ball and flat coated and uncoated PEEK and PA66 specimens with a geometry 30 × 30mm and height of h = 8mm. Dry and oillubricated conditions were measured. A mineral base oil FVA3 was used as lubricant, Table2. Operating conditions of the tests are listed in Table3. Tribological performance over time was studied in five steps of test duration. Table 1 Process parameters of a–C:H-1 on PEEK and a–C:H-2 on PA66 shown in [22] Process parameter and unit a–C:H-1on PEEK a–C:H-2on PA66 Argon gas flow, QAr in sccm 200 200 Process pressure, p in mPa 500 500 Ethyne gas flow, QC2H2 in sccm 10 10 Helium gas flow, QHe in sccm 70 50 HPPMS cathode power, P in W4000 4000 Pulse-on-time, ton in µs 100 100 Frequency, f in Hz 500 500 Bias voltage, UB in V −250 −200 Coating time, t in s 7200 7200 Table 2 Properties of the consideredmineral oil FVA3 [32] Characteristic value and unit FVA3 Kinematic viscosity ν (40°C) in mm2 s−1 95 Kinematic viscosity ν (60°C) in mm2 s−1 40 Kinematic viscosity ν (100°C) in mm2 s−1 11 Density ρ in g cm−3 0.90 (ϑOil = 15°C) Tribology Letters (2023) 71:2 1 3 Page 5 of 15 2 In initial tests, a higher frequency compared to f = 2Hz was tested. It led to wear tracks that have signs of wear markedly influenced by contact temperature rise, thus the frequency was decreased to f = 2Hz. Thetest temperature for oil lubricated conditions was set to ϑOil = 90°C to decrease oil viscosity and minimize formation of fluid film. The goal was to perform tests at severe mixed and boundary lubrication (see Table3). The average coefficient of friction and volume of material removed by wear were evaluated. Thewear volume was evaluated from surface topography measurement of the complete wear track after the experiment by optical profilometer Contour GT-X8, Bruker, Arizona, Tucson, USA. To be precise, the wear volume was evaluated as a net missing volume of material from topography data with reference plane established on surface out of the wear track. In this way of evaluation, the transported material on the plate is not included in the wear volume. The volume was calculated from the area of wear track and the closest surroundings by masking out the rest of sample. This approach decreases uncertainties of the volume measurement due to surface geometry imperfections (waviness). The estimated uncertainty of wear volume is around 1 105µm3. It is mainly given by precision of reference plane establishment and local sample waviness in the area of wear testing. 2.6 Chemical Analysis After Tribological Testing byMeans ofRaman Spectroscopy The molecule structure of the hydrogen-containing amorphous carbon coatings before and after tribological testing was analyzed by means of Raman spectroscopy. Therefore, the Raman spectrometer, Renishaw InVia Reflex, Renishaw GmbH, Pliezhausen, Germany, with a λVIS = 532nm laser with a spot size of d≈1µm and a diffraction grating gVIS = 1800l mm−1 was used. In addition, Raman spectra were taken with an ultraviolet(UV) laser λ = 325nm with a spot size d≈6µm and a diffraction grating gUV = 2400l mm−1. The lasers were calibrated before the measurement using a silicon reference sample. To enable an evaluation of statistic valid data, a mapping measuring method was used. Hereby, a matrix with 2 × 3 measurement points with size of 100 × 100µm and a step width of 50µm was positioned horizontal over the middle of the wear track on a–C:H coated disk for the analysis by VIS and UV laser. The measurement parameters are shown in Table4. For visible light (VIS) laser excitation the G peak position ranges between 1560 cm−1 ≤ ῦG,VIS ≤ 1600 cm−1 and in the area between 1600 cm−1 ≤ ῦG,UV ≤ 1690 cm−1usi ng an ultraviolet (UV) laser. The D peak occurs in the area 1360 cm−1 ≤ ῦD ≤ 1400 cm−1 under VIS excitation, as far as ring bindings are present in the DLC structure. Higher wave numbers of the D peak position are measured by UV laser excitation in the area between 1400 cm−1 ≤ ῦD,UV ≤ 1500 cm−1. All spectra were taken using the software Wire® 5.4, Renishaw plc, Wotton-under-Edge, United Kingdom. Thereby, the spectra were evaluated section by section of the Raman shift by a combined Gaussian and Lorentzian fitting. Based on the measured spectra, the parameters I(D)/I(G)VIS ratio, I(D)/ I(G)UV ratio, FWHM(G)UV and Disp(G) were determined. In case of the characteristic value FWHM(G)UV, the UV excitation enables to exclude the photoluminescence of the spectra as shown by Casiraghi etal. [26]. Furthermore, the dispersion of the G peak Disp(G) can be calculated by determining the position at visible light ῦG and UV ῦG,UV excitation divided by the excitation wavelengths. The interpretations of the a–C:H coatings were performed in analogy to the findings of Casiraghi etal. [26]. Correspondingly, the coatings were classified as graphite-like a–C:H (GLCH) with a hydrogen content Ψ > 20% and sp2 clustered ring bindings. (G1. 1) Disp (G)= 𝜐 G,UV −𝜐 G 𝜆 UV −𝜆 [ cm−1 nm ] Table 3 Operating conditions of PoP tribological tests Characteristic value and unit Dry Lubricated Lubricant – FVA3 Test temperature T in°C 60 90 Relative humidity RH in % 40 - Frequency f in Hz 2 2 Stroke δ in mm 2.2 2.2 Mean speed v in m s−1 0.01 0.01 Load F in N25 25 Test duration N≙number of passes {1,600; 3,200; 8,000; 16,000; 32,000} Table 4 Raman measurement parameters for chemical analysis of a–C:H coatings before and after tribological testing Characteristic value and unit Vis UV Wavelength λ in nm 532 325 Laser power PL in mW 2.6 3 Accumulations N1 3 Exposure time tE in s 10 10 Tribology Letters (2023) 71:2 1 3 2 Page 6 of 15 3 Results andDiscussion 3.1 Coating andCompound Analysis 3.1.1 Substrate Temperature Measurement During Coating Process The results of the substrate temperature measurement during the coating processes of PEEK and PA66 are shown in Fig.2. Thereby, the coating process is divided into the five phases heating, off-gasing, etching, coating and cooling. The highest substrate temperature for a–C:H-1 is reached during the etching phase with Tmax,1 = 174°C, whereby the maximum substrate temperature for a–C:H-2 was measured at Tmax,2 = 186°C. It is believed, that the deviation between the maximum substrate temperatures results predominantly from differences of the PEEK and PA66 specimen properties and to a minor extend due to differences in the coating processes. In general, the measured substrate temperatures are far below the melting temperatures of PEEK with T = 340°C and PA66 with T = 255°C, given by Domininghaus etal. [1]. In contrast thereto, the glass transition temperatures of PEEK with T = 143°C and PA66 with T = 70°C are surpassed. On this point, it must be stated that the PEEK and PA66 specimens are mounted using holders which allow movement such that no significant thermal stresses occur during coating process. 3.1.2 Morphology ofa–C:H Coatings bySEM Analysis The cross-sectional images and top layer images of a–C:H-1 and a–C:H-2 are shown in Fig.3. Thereby a columnar structure with the transition into an amorphous character of both coatings a–C:H-1 and a–C:H-2 can be observed. The cross-sectional image for a–C:H-1 shows a wavy structure and it can be assumed that this is caused by the preparation of the specimens for the microscopy. Hereby, it is necessary to apply high mechanical forces to break the specimens. Even though there are high plastic deformations of the substrate material visible, no cracking or delamination at the interface between coating and substrate can be observed. At the top view of a–C:H-1 interspaces can be seen in between the cauliflower structure, which can be a result of the sample preparation during grinding and polishing. A coarse columnar structure can be observed for a–C:H-2 on PA66. Furthermore, a very rough structure can be seen at the top layer of a–C:H2, where the columns show locally heightened areas. It can be assumed, that among others the structural differences between a–C:H-1 and a–C:H-2 result from the growth process on the ring binding structure of PEEK and chain binding structure of PA66. The coatings should be predominantly composed of sp2 ring bindings due to the HPPMS sputtering of graphite in contrast to the share of sp2 chain bindings resulting out of the low ethyne gas flow with  Q C2H2 = 10sccm. Thus, the “epitaxial” coating growth of a–C:H-2 on PA66 is affected in comparison to the coating growth of a–C:H-1 on PEEK and leads to the observed uneven structure. Due to the small differences of bias voltage with ∆UB = 50V and helium gas flow with ∆QHe = 20sccm compared to a–C:H-1, it is not believed, that the process parameters lead to the observed changes of the coating structure. Catena etal. came to a similar conclusion by analyzing the structure of a–C:H on different thermoplastic substrate materials like polyoxymethylene (POM), high density polyethylene (HDPE) and polyethylene terephthalate (PET) [33]. 3.1.3 Roughness ofUncoated andCoated Samples The results of the roughness measurements of the arithmetical mean roughness Ra and root mean square roughness Rq Fig. 2 Substrate temperature during coating process of a–C:H-1 and a–C:H-2 Tribology Letters (2023) 71:2 1 3 Page 7 of 15 2 before and after coating of PEEK and PA66 are shown in Fig.4. The increase in roughness for the as-deposited state of a–C:H-1 compared to uncoated PEEK is moderate. An explanation can be given by small defects on the a–C:H-1 coating, which result out of the coating growth process. In contrast, the roughness of the PA66 specimen increases significantly after coating deposition. The measurements of Rq show a similar trend to the changes of Ra. Overall, the roughness measurements confirm quantitatively the observations of the SEM images. 3.1.4 Chemical Analysis ofHydrogen Content ofa–C:H Coatings The hydrogen content Ψ of a–C:H-1 and a–C:H-2 at γ = 35nm and γ = 500nm depths from the top layer are Fig. 3 Cross sectional images and topography images of a–C:H-1 on PEEK, shown in [22], and a–C:H-2 on PA66 Fig. 4 Roughness measurement of uncoated and coated PEEK and PA66 Tribology Letters (2023) 71:2 1 3 2 Page 8 of 15 shown in Fig.5. It can be observed, that the hydrogen content at the top layer in γ = 35nm depth varies between Ψ≈5.0at.% by comparing a–C:H-1 and a–C:H-2. By increasing the penetration depth to γ = 500nm the hydrogen contents of both coatings tend to equalize. It can be assumed, that the deviations of the hydrogen content at the top layer result from the differences of the coating process parameters as well as the substrate material properties of PEEK and PA66 on the coating growth process. Information about the binding structure of hydrogen in the a–C:H coatings could be obtained out of the evaluation of the hydrogen content. Based on extrapolation method against zero, the absolute hydrogen contents could be calculated. The reason for the exponential fitting method is due to an effusion of hydrogen during the measurement, which indicates that hydrogen is primarily interstitial embedded in the a–C:H coating structure. It can be assumed, that this enables an easier effusion to the surface and affects the tribological behavior. 3.1.5 Indentation Hardness andModulus ofIndentation ofa–C:H Coatings The indentation hardness HIT and the modulus of indentation EIT of the DLC coatings are shown in Fig.6. In order to minimize an influence of the substrate material on the hardness measurement, the penetration depth of the Berkovich indeter was kept at 10% of the individual coating thickness. The difference between the indentation hardness and the modulus of indentation of a–C:H-2 compared to a–C:H-1 can be explained based on the reduced helium gas flow and changed bias voltageUB during the coating process. The changes lead to a lower ionization energy of the C–C and C–H clusters in the a–C:H-2 coating process, which negatively affects the formation of sp3 diamond bindings and reduces the share in relation to the sp2 bindings. In addition, the hydrogen content at the top layer is higher for a–C:H-2 in contrast to a–C:H-1, which also reduces the hardness of the coating. 3.1.6 Compound Adhesion ofa–C:H onPolymers The compound adhesion between a–C:H-1 and PEEK as well as a–C:H-2 and PA66 was tested by means of scratch testing, Fig.7. Even though the scratch test is not standardized for the analysis of coated polymers, important insights can be gained. The results show a high deformation of ∆h = −64.3µm of the a–C:H-1 on PEEK compound at a critical load Lc = 150N in the center of the scratch without a coating delamination. In the edge regions of the scratch, Fig. 5 Hydrogen content of a–C:H-1 and a–C:H-2 at different depths Fig. 6 Indentation hardness HIT and modulus of indentation EIT of a–C:H-1 on PEEK and a–C:H-2 on PA66 shown in [22] Tribology Letters (2023) 71:2 1 3 Page 9 of 15 2 high plastic deformation up to a height of h = 21.4µm only show partial cracks of a–C:H-1. With regard to the a–C:H-2 on PA66 compound, higher critical loads up to Lc = 250N could be applied without coating delamination. Higher critical loads also result in increased plastic deformation up to ∆h = −132.7µm. At Lc = 250N, the maximum load of the analyzing unit was achieved. Nevertheless, the elevations at the edge of the scratch remain at a comparable level around ∆h≈20µm compared to the a–C:H-1 on PEEK compound. In the case of a–C:H-2 on PA66 compound, cracks form at the top layer within the left side of the scratch and not in the edge region. Both a–C:H coatings are able to undergo high deformation without delaminating from the polymers and significant differences in the hardness and modulus of indentation. This offers a high potential for a subsequent tribological analysis under high loads, to investigate the effect on friction and wear in highly stressed sliding and rolling-sliding contacts. Fig. 7 Compound adhesion between a–C:H-1 on PEEK and a–C:H-2 on PA66 a-C:H-1onPEEK at Lc =150 N height hinµm 50 0 distance dinµm 100 height hinµm 100 0 distance dinµm 200 0300 600 1,2001,800 100 µm 100µm a-C:H-2onPA66atLc=250 N ∆h =21.4µm∆h =16.5µm ∆h =-64.3µm ∆h =-132.7µm ∆h =18.0µm ∆h =27.6µm 9001,500 0300 600 1,200 1,8009001,500 cracks cracks profile profile Fig. 8 a Wear volume and b coefficient of friction of PEEK/100Cr6 and a–C:H1/100Cr6 under dry conditions 020,00040,000 number of passes N 0.1 0.3 0.5 020,00 04 0,00 0 coefficient of friction µ number of passes N wear volume Wv in µm3 10 10 106 104 10 8 PEEK, dry at T = 60 °C a-C:H-1PEEK a-C:H-1PEEK (a) (b)