DLC-Coated Thermoplastics: Tribological Analyses under Lubricated Rolling-Sliding Conditions

Vol.:(0123456789) 1 3 Tribology Letters (2022) 70:121 https://doi.org/10.1007/s11249-022-01664-6 ORIGINAL PAPER DLC‑Coated Thermoplastics: Tribological Analyses underLubricated Rolling‑Sliding Conditions S.Reitschuster1 · E.Maier1 · T.Lohner1 · K.Stahl1 · K.Bobzin2 · C.Kalscheuer2· M.Thiex2 · P.Sperka3 · M.Hartl3 Received: 20 July 2022 / Accepted: 2 October 2022 / Published online: 31 October 2022 © The Author(s) 2022 Abstract The goal of this work is to evaluate the potential of diamond-like carbon (DLC) coatings on thermoplastic polymers for friction and wear reduction in highly stressed rolling–sliding contacts. Therefore, hydrogen-containing DLC coatings were deposited on the polymer surface by a low-temperature high power pulsed magnetron sputtering (HPPMS) physical vapor deposition (PVD) process. The rolling-sliding contact between coated polyamide 66 (PA66) or coated polyether ether ketone (PEEK) against case-hardened steel 16MnCr5 is investigated in a twin-disk tribometer at normal loads up to FN = 1,000N, sum velocities between 1m/s ≤ vΣ ≤ 16m/s and slip ratios up to s = 50%. Results show a friction reduction with the application of DLC on the considered polymers compared to uncoated polymers under specific lubrication conditions. High solid losses caused by the polymer’s internal damping properties dominate the temperature behavior of the polymer, even when coated with DLC. Regarding the wear behavior, DLC coatings show potential especially under severe mixed lubrication conditions with high-solid load portion and sliding. The knowledge gained about coated polymers can be used to improve the overall tribological performance in terms of friction and wear of thermoplastic machine elements like gears. Keywords DLC coating· Polymers· Lubrication· Friction· Temperature· Elastohydrodynamics 1 Introduction The well-known advantages of thermoplastic polymers such as cost-efficient production and low density, compete with moderate strength and strong temperature-dependent material behavior [1, 2]. Particularly under dry conditions, severe wear can limit the lifetime of thermoplastic machine elements like gears. Therefore, in applications with high power transmission, oil lubrication is used to reduce wear and bulk temperature [3, 4]. The combination of high loads and associated very low material stiffness (high elastic deformations resp. mechanical compliance) of polymers forms a (thermo-) elastohydrodynamically lubricated (TEHL) contact [5, 6]. As a result, convex contact pairings between polymer and steel like in a gearing show a pronounced contact conformity [7]. The large deformation of the contact region results in low hydrodynamic pressure. Significant pressure-induced increase in the lubricant viscosity is therefore not present, which is also reflected in the frictional behavior [7, 8]. As shown in [8], interfacial friction in polymer/steel contacts is low and depends largely on the surface characteristics. Furthermore, experimental investigations on a twin-disk tribometer and numerical calculations demonstrate that very low coefficients of friction in the range of superlubricity are possible, even in the mixed lubrication regime [8, 9]. Friction leads to increased temperatures in TEHL contacts. Besides heat sources from the shearing and compression of the lubricant as interfacial friction, some thermoplastic polymers also show solid losses. Even at pure rolling conditions, a bulk temperature increase is observed, indicating that heat generated by interfacial friction is subordinate [8]. Solid losses can originate from the viscoelastic material behavior and show a strong load and frequency dependency * S. Reitschuster stefan.reitsc[email protected] 1 School ofEngineering & Design, Department ofMechanical Engineering, Technical University Munich, Gear Research Center (FZG), Boltzmannstraße 15, 85748GarchingnearMunich, Germany 2 Surface Engineering Institute (IOT), RWTH Aachen University, Kackertstraße 15, 52072Aachen, Germany 3 Brno University ofTechnology (BUT), Technicka 2896/2, 61669Brno, CzechRepublic Tribology Letters (2022) 70:121 1 3 121 Page 2 of 16 that varies for each material. For example, high performance polyether ether ketone (PEEK) shows much lower solid losses under same operating conditions compared to polyamide (PA) [8]. Nevertheless, it should be noted that bulk temperatures and contact temperatures differ due to the low thermal diffusivity of polymers. This can lead to an accumulation of detrimental contact heat on the polymer surface [7]. In general, owing to the strong temperature dependency and the limited operating temperature range, the temperatures occurring in the polymer contact are important. If temperatures are too high, premature failure can occur far below the expected load limit. The application of diamond-like carbon coatings (DLC) can improve the tribological performance of polymer components in terms of friction and wear. These coatings composed of graphitc sp2 bindings and diamond-like sp3 bindings are established on steel-based automotive components like tappets, piston, piston rings and camshafts since the early 2000s [10]. Nevertheless, transferring the deposition of the DLC coating systems from steel to polymer substrates is a major challenge. This is due to the chemical structure of the polymer substrate material and its incompatibility with metallic interlayers, since DLC coatings are primarily developed for deposition on metals. In addition, the effort for pre-treatment by means of grinding, polishing and etching of polymers is significantly increased compared to metallic materials but mandatory to ensure a sufficient adhesion between coating and substrate material. Lugscheider etal. [11] showed, that the adhesion between PVD coating and polymer substrate is improved by using combined pulsed magnetron sputtering compared to direct current (dc) or pulsed sputter process solely. Moreover, the coating properties have to be adjusted in accordance with soft polymer substrates, whereas hardened steels possess a sufficient load carrying capacity for the DLC coatings. There is a large variety of studies concerning the application of DLC on elastomers improving the friction and wear behavior, which is shown in the review of Martinez-Martinez and De Hosson [12]. In the field of endoprosthetics for artificial knee and hip joints, a multitude of studies describe the positive influence of DLC on the wear behavior of thermoplastics such as polyethylene in direct comparison with uncoated references, e. g. Rothammer etal. [13], Puertolas etal. [14] and Onate etal.[15]. In contrast thereto, less studies are conducted with regard to the application of DLC on high performance thermoplastic materials such PEEK and PA66. Kaczorowski etal. [16] analyzed DLC-coated PEEK against zirconium oxide under fluid-free conditions, which led to reduced friction and wear compared to uncoated surface modified PEEK at an initial Hertzian pressure pH = 117MPa. Furthermore, the adhesion between DLC and PEEK can be improved by N2 etching instead of O2 etching before deposition of the DLC coating. A friction and wear reduction was also measured by Kapinski etal. [14] by testing DLC coated PA66 under dry lubrication against uncoated 100Cr6 at an initial Hertzian pressure pH = 160MPa. Analyses of DLC coated polymers under boundary and mixed friction at an initial Hertzian pressure pH = 345MPa prove, that besides a friction and wear reduction a chemical interaction with lubricant and additive is prevented [17]. Analytical and numerical calculations of Elsharkawy etal. [18] and Ziegltrum etal. [7] on the TEHL contact with DLC coated polymers against uncoated steel specimens demonstrate that the influence of the coating on the mechanical properties such as polymer stiffness and thus on hydrodynamic pressure and lubricant film thickness in the contact is low. In contrast to this, the thermophysical properties of DLC affect the contact temperature distribution also for very thin coatings. Due to a higher thermal effusivity of DLC compared to polymers, the heat removal in the contact is more pronounced, resulting in a lower surface and lubricant temperature [7]. Besides that, the DLC microstructure is affected by the tribological contact conditions, which can be analyzed my means of Raman spectroscopy. Thereby, the transformation of sp2 chain bindings into sp2 ring bindings, reduction of density and rearrangement of sp2 bindings and the effusion of hydrogen, only valid for hydrogen-containing DLC, can be described as shown by Kalish etal. [19]. These three steps can be summarized by the term relaxation. The Raman spectrum of amorphous carbon is characterized by the D and G peak, which arise from different vibrational states of the molecules generated by visible light (VIS) laser or ultraviolet light (UV) excitation as described by Robertson and Ferrari [20]. Changes in accordance with the relaxation can be quantified by the intensity ratios of the D and G peak, I(D)/I(G), under VIS and UV excitation. The full width half maximum FWHM(G) allows the analysis of changes of bond angle and bond length distortions, so called structural disorder, in accordance with the findings of Casiraghi etal. [21]. Therein, the quotient dispersion Disp(G) of the difference of the G peak positions under VIS and UV excitation divided through the difference of the excitation wavelengths is also described. It enables an analysis of the topological disorder which combines the information of size and shape distribution of sp2 clusters of chains. Additional information regarding the Raman spectroscopy of DLC in this study are provided in [22]. Scientific studies on coated thermoplastics are currently limited mainly to applications exposed to moderate tribological stresses and standard materials such as PE. However, for the highly stressed EHL contact, occurring in machine elements as for example polymer gears, there is still comparatively limited insight into the influence of DLC coatings, especially for the relevant high-performance polymers like PA66 and PEEK. Therefore, this study investigates the Tribology Letters (2022) 70:121 1 3 Page 3 of 16 121 tribological behavior of DLC coated PA66 and PEEK. A related paper of the authors [22] focuses on the development and analyses of hydrogenated DLC coatings (a-C:H) for the application on thermoplastic polymers and evaluates their friction and wear behavior under dry and lubricated sliding conditions on a pin-on-plate tribometer. The development and analyses of the a-C:H coatings are explained in detail. This paper is concerned with the friction and temperature behavior of coated thermoplastic polymers under lubricated rolling-sliding conditions in a twin-disk tribometer. 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 To analyze the influence of a:C–H coatings for PEEK and PA66 on the tribological behavior of lubricated rollingsliding contacts, friction and temperature measurements are carried out at a twin-disk tribometer. Furthermore, the longterm performance in terms of wear and friction behavior of coatings is investigated and chemically analyzed following the experiments. 2.1 Twin‑Disk Tribometer The tribological investigations were carried out on a twindisk tribometer, Fig.1 (left). This is an analogy test rig for rolling–sliding contacts as found in machine elements like gears and roller bearings. The following description is based on Reitschuster etal. [8]. Two independently driven cylindrical disks (Ø 80mm) are loaded by the adjustable normal force FN that a pneumatic air cylinder applies to the end of a pivot arm, where the lower disk is mounted. The upper disk is firmly fixed in a skid, which is connected to the frame via thin steel sheets. Any horizontal displacement of the skid due to frictional forces from the rolling-sliding contact is resisted and recorded via a load cell. The coefficient of friction μ relates the horizontally acting force to the contact normal force (μ = FR/FN). To prevent starved lubrication, a steady lubricant volume of  Q = 1.5l/min is injected centrally into the disk contact via an injection nozzle. Using an oil pump, the lubricant circulates in a closed circuit, containing an additional filter system and a heater to adjust the oil temperature. The control system also records the oil injection temperature ϑOil, surface velocities v1 and v2 of the lower and upper disk, as well as the bulk temperature ϑM of the upper disk determined by an Pt-100 sensor around 4mm underneath the polymer surface. The sum velocity v∑ = v1 + v2, the sliding velocity vg = v1-v2 (v1 ≥ v2) and the slip ratio s = vg/v1 characterize the kinematics of the disk contact. The test configuration consisting of a case-hardened uncoated lower steel disk (16MnCr5), and a coated polymer upper disk is shown in Fig.1 (right). Note that, the measuring principle detects only horizontally acting friction forces. Non-horizontal friction forces, which are likely to occur in highly-loaded polymer contact due to the high compliance of polymers, cannot be quantified by the load cells for FR or FN. Further losses, caused for example by internal damping inside the polymer, are seen in the bulk temperature. Therefore the measured coefficient of friction is referred to as µ* in the following in analogy to the results shown in [8]. 2.2 Specimens andMaterials Steel disks made of case-hardened steel 16MnCr5 were axially ground to an averaged mean arithmetic roughness of Ra ≈ 0.24µm (roughness measurement in circumferential direction). Polymer disks were injection-molded around a Pivot arm load cell FN Mount Skid Load cell FR Steel sheet Pivot Frame Oil injection Pneumatic cylinder FN v1 FN Steel hub Steel disk: -casehardened -cross-grinded -witdh: 12 mm Thermoplasticdisk: -injection-molded around steel hub -width: 10 mm Pt-100 sensor Coatingv2 Fig. 1 Layout of the FZG twin-disk tribometer (left) and specimen design (right) in accordance with [8] Tribology Letters (2022) 70:121 1 3 121 Page 4 of 16 steel hub for connection to shaft in the twin-disk tribometer (see Fig.1 right). Two thermoplastic polymers were considered: polyamide 66 (PTS–Creamid-A4H9) and polyether ether ketone (EvonikPEEK-VESTAKEEP® 4000 G). Basic material parameters for each polymer are given in Table1 for dry conditions and at ambient temperature. Note that material properties of thermoplastics can strongly dependent on the temperature and ambient conditions. In accordance with [22] a newly developed amorphous hydrogenated carbon coating (a:C–H-1) was applied on PEEK and a:C–H-2 on PA66 by low-temperature physical vapor deposition (PVD) process. Prior to the coating process, PEEK and PA66 specimens were pretreated to improve the adhesion properties between the coating and the substrate. For this purpose, the specimens were first ground and then polished to an average surface roughness of less than Ra < 0.1µm. The coating process itself was carried out in an industrial scale coating unit CC800/9 Custom, CemeCon AG, Würselen, Germany, with two high power pulsed magnetron sputtering (HPPMS) cathodes. The C targets used for the deposition of the a-C:H coating have a purity of C=99.9%. As process as well as reactive gas argon (Ar), helium (He) and ethyne (C2H2) were used. The coatings show a columnar structure with the transition into an amorphous character of both coatings. Furthermore, measurements by means of scratchtest showed a high compound adhesion between coating and polymer. A more detailed description of the coating process its characterization is described in [22]. An impression of the polymer surface before and after the coating is applied is given in Fig.2. Measurements of the layer thickness tc result in tc ≈ 1.0 -2.0µm for both coatings. The detailed characterization of the compound and the coating process are described in [22]. The measured mean arithmetic roughness Ra for both polymermaterials at the process steps injection-molding, polishing, and coating is shown in Fig.3. The measurements on the polymer surface were determined using a tactile profile method, carried out across the disk width direction with a measuring length of Lt = 4.80mm and a cut-off wavelength of λc = 0.80mm according to DIN EN ISO 4288 [23]. Compared to the polished state, the coating process shows an increase in the surface roughness due to the coating deposition for both materials. Table 1 Material parameters at dry state at ambient temperature for PA66 and PEEK [35, 36] PA66 (CreamidA4H9) PEEK (VESTAKEEP® 4000 G) Young’s modulus E in N/mm23,100 3,600 Poisson number ν 0.33 0.37 Thermal conductivity 𝜆 in W/(mK) 0.21 0.29 Glass transition temperature 𝜗G in°C 66 143 a-C:H-1 on PEEK (b) Disk width d ≈ 10 mm (a) PEEK 1 mm Rolling-sliding direction (c) PA66 a-C:H-2 on PA66 (d) Fig. 2 Surface impression of the thermoplastic surfaces for uncoated PEEK (a); a-C:H-1 on PEEK (b); uncoated PA66 (c) and a-C:H-2 on PA66 (d) PEEK PA66 0,10 0,20 0,30 0,40 0,50 Ra in µm 0.60 0.50 0.30 0 0.40 0.20 0.10 Coated Injection-molded Polished min. max. Fig. 3 Arithmetic mean roughness Ra after each process step for PEEK and PA66 disks Tribology Letters (2022) 70:121 1 3 Page 5 of 16 121 2.3 Lubricants The considered lubricants are mineral oil (FVA3 [24]) and a water-containing polyglycol (WAT) in analogy to [8], both are ISO VG 100. No further surface-active additives were added. An overview of the key properties of the lubricants is given in Table2. 2.4 Chemical Analysis byMulti Wavelength Raman Spectroscopy The molecule structure of the hydrogen-containing amorphous carbon coatings before and after tribological tests were analyzed by means of Raman spectroscopy. Therefore, the Raman spectroscope, 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 of gVIS = 1,800l/mm was used. In addition, Raman spectra were taken with an ultraviolet (UV) laser λ = 325nm with a spot size of d ≈ 6µm and a diffraction grating of gUV = 2,400l/ mm. The lasers were calibrated before the measurement by using a silicon reference sample. To enable an evaluation of statistically valid data, a mapping measuring method was used. Hereby, a matrix with 1 × 8 measurement points with size of 1µm × 1,750µm and a step width of 250µm is positioned horizontally over the middle of the running surface of the a-C:H coated disk for the analysis by UV laser. Furthermore, a matrix with 2 × 3 measurement points with a size of 100 × 160µm and a step width of 80µm is positioned horizontally over the middle of the running surface of the a-C:H coated disk using the λvis = 532nm laser. The measurement parameters are shown in Table3. All spectra are taken using the software Wire® 5.4, Renishaw plc, Wotton-under-Edge, United Kingdom. Thereby, the spectra are 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, FWHMUV(G) and Disp(G) are 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. [21]. Furthermore, the dispersion of the G peak Disp(G) can be calculated by determining the difference of the position at visible ῦG and UV ῦG,UV excitation divided through the difference of the excitation wavelengths. Based on the coating analysis in [22] the interpretations of the a-C:H coatings are orientated on the findings of Casiraghi etal. [21]. Correspondingly, the coatings were classified as graphite-like a-C:H (GLCH) with a hydrogen content Ψ > 20% and sp2-clustered ring bindings. 2.5 Operating Conditions andExperimental Procedure The operating conditions arechosen in analogy with previous investigations on uncoated polymers in [8] to ensure comparability are shown in Table4. Using Hertzian theory, the Hertzian pressure for a load of FN = 1000N corresponds to pH = 81MPa for PEEK and pH = 74MPa for PA66. Numerical studies indicate that the influence of a coating on the contact stiffness and thus on the contact pressure is negligible for practical PVD coating thicknesses [7, 18]. Friction is investigated by stepwise increase of the slip ratio at a constant sum velocity and normal load (friction curve). Each time a quasi-stationary state is reached for a slip ratio, defined by a bulk temperature change of less than 0.5K per minute (∆ϑM ≤ 0.5K/min), the slip ratio is increased as shown in Table4. The coefficients of friction µ* and the bulk temperatures ϑM are averaged values over one minute in quasi-stationary state. The lubrication regime for different operating conditions is approximated by the relative film thickness λrel = 2·hm/ (Ra1 + Ra2). Therein, the minimum film thickness hm is calculated according to Myers etal. [25]. For the associated Ra values, the disk surface roughnesses in the initial state are used (see Fig.3). The determined relative film thicknesses λrel are shown in Fig.4. A low sum velocity of vΣ = 1m/s results in a relative lubricant film thickness λrel < 2 for all materials, which can be associated with the mixed lubrication [26]. Higher sum velocity leads to an increase in the relative lubricant film thickness λrel. Fluid film lubrication is assumed for a relative lubricant film thickness λrel > 2. (1) Disp (G)= 𝜐 G,UV −𝜐 G 𝜆 UV −𝜆 [ cm−1 nm ] Table 2 Properties of the mineral oil (FVA3) and water-containing polyglycol (WAT) FVA3 WAT Kinematic viscosity ν (40°C) in mm2/s 95 90 Kinematic viscosity ν (60°C) in mm2/s 40 40 Kinematic viscosity ν (100°C) in mm2/s 11 15 Density ρ in g/cm30.90 (15°C) 1.09 (20°C) Table 3 Raman measurement parameters for chemical analysis of a-C:H coatings before and after tribological testing Vis UV Wavelength λ in nm 532 325 Laser power PL in mW 2.6 3 Accumulations N 1 3 Exposure time tE in s 10 10 Tribology Letters (2022) 70:121 1 3 121 Page 6 of 16 In the preparation of each friction test, the disks were thermally conditioned through oil injection at vΣ = 1m/s and no-load conditions to achieve quasi-stationary bulk temperatures ϑM,0 (ϑOil = 60°C) ≈ 34–35°C. In total, a test cycle consists of the three different sum velocities (vΣ = 1, 4, 16m/s), whereby each sum velocity was in total repeated three times. The first sum velocity is vΣ = 4m/s, followed by vΣ = 1m/s and vΣ = 16m/s. The first of three runs at vΣ = 4m/s were not used for the evaluation of the results, as it is considered as a conditioning run. A further running-in process was not performed. After each of the described test cycle, new pairings of test specimens were used. For the investigation of the long-term performance under critical lubrication conditions, tests were conducted at mixed lubrication at v∑ = 1m/s and a slip ratio s = 50% over a period of t = 8h. Hereby the focus was on the development of the coefficient of friction over time as well as changes of the surface condition and possible wear marks. The surface condition were examined each time before and after the long-term tests by means of high-resolution images and roughness measurements. 3 Results andDiscussion The following results shows the tribological performance of coated thermoplastic polymers as measured at the twindisk tribometer. For the sake of comparability, results for uncoated polymers under tribological conditions from [8] are included. 3.1 Friction andTemperature Behavior inFriction Curves This section shows results of the friction curve measurements by illustrating the coefficient of friction µ* and the bulk temperature ϑM for variation of the slip ratio s. Hereby, the coefficient of friction µ* is mainly related to the interfacial friction from the disk contact (see Sect.2.1). 3.1.1 Influence ofOperating Conditions andMaterial First, the influence of the a-C:H coatings on the friction and temperature behavior at different sum velocities is Table 4 Operating conditions for investigations on the FZG twin-disk tribometer Friction curves Long-term tests Normal force FN in N 1,000 Sum velocity v∑ in m/s 4/1/16 1 Slip ratio s in % 0/5/10/20/30/40/50/60/70 50 Lubricant FVA3; WAT (ϑOil = 60°C) FVA3 (ϑOil = 60°C) Material PEEK; PA66 Surface finish Uncoated (injection-molded); a-C:H-1 coating (on PEEK); a-C:H-2 coating (on PA66) Setup of disks Polymer-steel contact Fig. 4 Relative lubricant film thickness λrel for uncoated and coated PEEK/PA66 with FVA3 and WAT 0 2 4 6 8 10 12 ssenkcihtmliftnacirbulevitaleRλ re l v ∑ = 1 m/sv ∑ = 4 m/ sv ∑ = 16 m/s FVA3 WAT PEEK a-C:H-2 on PA66 a-C:H-1 on PEEKPA66 Tribology Letters (2022) 70:121 1 3 Page 7 of 16 121 considered for mineral oil FVA3 in Fig.5. Under conditions with fluid film lubrication (v∑ = 16m/s, λrel > 2, see Fig.4), friction is characterized by very low coefficients of friction µ* for both materials. Coated and uncoated variants of PEEK and PA66 show both a linearly rising friction curve with increasing slip rates. This indicates that the effective viscosity in the contact region remains low which agrees with [7, 18] where no relevant influence of the coating thickness on the contact stiffness was detected. However, in mixed lubrication regime (v∑ = 1m/s, λrel < 2, see Fig.4), differences can be seen between the coated and the uncoated variants regarding the frictional behavior. The uncoated PEEK as well as the uncoated PA66 show overall lower coefficients of friction and comparable friction curves with a continuous gradient for increasing slip ratios. Although the surface roughness of the coated variants is lower (see Fig.3), higher interfacial friction for coated PA66 (a-C:H-2 on PA66) and especially for coated PEEK (a-C:H-1 on PEEK) is observed. For both coated variants, the friction curve is characterized by a rapid rise at the transition from pure rolling to rolling-sliding conditions. Afterwards, both friction curves become almost steady, which indicates nearly constant shear stress and interfacial friction despite increasing sliding velocity. Similar results were found in a pin-on-plate tribometer configuration [22]. To exclude delamination of the coating and inspect the coating for possible changes as a result of the high loading stress, the condition of the surfaces were examined in each case using Raman spectroscopy and SEM microscopy (see Sect.4.2). 0,00 0,01 0,02 0,03 0,04 0,05 010203040506070 20 30 40 50 60 70 80 90 010203040506070 Bulk temperature ϑ M,s in °C µnoitcirffotneiciffeo C* PEEK F N= 1,000 N ϑ Oil = 60 °C (FVA3) a-C:H-1 on PEEK | v ∑ = 1 m/s a-C:H-1 on PEEK | v ∑ = 16 m/s ϑ M,0 0.05 0.04 0.03 0.02 0.01 0.06 0 Slip rate s in % Slip rate s in % PEEK | v ∑ = 1 m/s PEEK | v ∑ = 16 m/s v Σ =1 m/s v Σ =16 m/s v Σ =16 m/s v Σ =1 m/s min. max. (a) 20 30 40 50 60 70 80 90 010203040506070 0,00 0,01 0,02 0,03 0,04 0,05 010203040506070 a-C:H-2 on PA66 | v∑= 1 m/s a-C:H-2 on PA66 | v∑= 16 m/s Bulk temperature ϑ M,s in °C µnoitcirffotneiciffeoC* 0.05 0.04 0.03 0.02 0.01 0.06 0 Slip rate s in % PA66 | v∑= 1 m/s PA66 | v∑= 16 m/s vΣ=1 m/s vΣ=16 m/s ϑ M,0 vΣ=16 m/s v Σ =1 m/s PA66 FN= 1,000 N ϑ Oil = 60 °C (FVA3) Slip rate s in % min. max. (b) Fig. 5 Friction and temperature behavior of uncoated [8]/coated PEEK (a) and PA66 (b) for different sum velocities v∑ with mineral oil FVA3 Tribology Letters (2022) 70:121 1 3 121 Page 8 of 16 The corresponding bulk temperatures ϑM shows for uncoated and coated variants an increase of the bulk temperature depending mainly on the loading frequency and hardly on the slip ratio. PA66 has a significantly higher increase in bulk temperature in contrast to PEEK. Despite different coefficients of friction between uncoated and coated polymer, the impact of the coating on the bulk temperature is small. Indications of an thermal insulation effect like in DLCcoated steel contacts under elastohydrodynamic lubrication as shown in [27] cannot be observed. This is related with similar thermal effusivities of the a-C:H coatings and the considered polymers. However, as Ziegltrum etal. [7] have shown, a marginal reduction in the polymer/steel contact temperature directly at or just below the surface is possible. A direct correlation between interfacial friction and bulk temperature cannot be determined. Instead, the internal solid losses appear to be the dominant influencing factor on the bulk temperature for both coated and uncoated polymers. This is in agreement with the results in [7], which have shown that the mechanical characteristics, like the overall contact stiffness and the associated material elongation of the polymers are hardly affected by the coating. Therefore, in accordance with Sect.1, PA66 reaches significantly higher bulk temperatures than PEEK, due to its more pronounced damping behavior compared to PEEK. Marginal differences in the measured bulk temperatures between uncoated and coated variants are attributed mainly to the ambient influences during the test procedure. 3.1.2 Influence oftheLubricant In addition to the mineral oil FVA3, a water-containing fluid (WAT) is considered at an oil temperature of ϑOil = 60°C. The coefficients of friction µ* for uncoated and a-C:H-1 on PEEK in the mixed lubrication regime (v∑ = 1m/s) is shown in Fig.6 for both lubricants. The results with the mineral oil FVA3 follow the linear increase of the coefficient of friction with slip ratio and a strong increase as soon as a sliding velocity is applied. In direct comparison, the water-containing fluid WAT shows significantly lower interfacial friction under the same operating conditions. According to Chen etal. [28–30], a possible reason for this is the friction-reducing formation of a microscopic layer of FeOOH on the steel counter-part in ambient conditions, accumulating a hydrogen-bonded film of glycol and freewater molecules. Besides, also the almost 20% higher density of WAT compared to FVA3 can enhance the formation of the lubricant film by increasing the film thickness up to 10% according to Myers etal. [25]. Due to low pressure conditions, the influence of the pressure viscosity coefficient on the lubricant film thickness is comparably small. As a result, the relative lubricant film thickness for a-C:H-1 on PEEK/steel increases from λrel,FVA3 = 1.69 to λrel,WAT = 1.77 (v∑ = 1m/s), which indicates a decreasing interfacial friction. Presumably, due to the lower surface roughness compared to the uncoated PEEK, a-C:H-1 on PEEK additionally leads to reduced interfacial friction in mixed lubrication. In combination with the water-containing lubricant, this results in very low coefficients of friction in the range of superlubricity despite the present mixed lubrication regime. 3.1.3 Influence ofConditioning Run In contrast to one conditioning run at v∑ = 4m/s and subsequent measurements at v∑ = 4m/s, v∑ = 1m/s and v∑ = 16m/s (see Sect.3.1.1), the conditioning procedure was changed to one conditioning run at v∑ = 1m/s followed by measurements at v∑ = 1m/s, v∑ = 4m/s and v∑ = 16m/s. All other operating conditions remained the same (see Table4). For “slow” conditioning at v∑ = 1m/s, mixed lubrication is present and the sliding velocities are low. “Fast” conditioning at v∑ = 4m/s shows fluid film lubrication and high sliding velocity. Hence, “fast” conditioning comes along with zero specific boundary friction power [31]. The terms “fast” and “slow” are kept in the following. Figure7 presents the coefficients of friction for both conditioning procedures. In mixed lubrication at v∑ = 1m/s, the “slow” conditioning shows lower coefficients of friction than the “fast” conditioning for both coating variants. The observed effect is more pronounced for a-C:H-1 on PEEK (see Fig.7 (a)). However, in the area of fluid film lubrication (λrel > 2) at v∑ = 4, no effect of the different conditioning processes can be seen. While the conditioning of the surfaces does 0,00 0,01 0,02 0,03 0,04 0,05 010203040506070 a-C:H-1 on PEEK | FVA3 FN= 1,000 N ϑOil= 60 °C v ∑ = 1 m/s min. max. µnoitcirffotneiciffeo C* 0.05 0.04 0.03 0.02 0.01 0.06 0 Slip rate s in % a-C:H-1 on PEEK | WAT PEEK | FVA3 PEEK | WAT coated uncoated Fig. 6 Friction behavior of uncoated [8]/coated PEEK in comparison of different lubricants Tribology Letters (2022) 70:121 1 3 Page 9 of 16 121 not seem to affect friction in fluid film lubrication, the coefficients of frictions differ in mixed lubrication. It appears that a “fast” conditioning causes an alternation of the surfaces that prohibits low friction in the subsequent measurement at v∑ = 1m/s. The cause of the alternation could be found in the higher sliding velocity and the associated frictional power. The coating structure and chemistry was analyzed in detail for further insight (see Sect.4.2). In this case, it appears that the “slow” conditioning improves the tribological performance of the coating. 3.2 Long‑term Tests underCritical Lubrication Conditions It has been shown that surface wear increasingly becomes a relevant damage type under severe lubrication condition for polymer/steel contacts [3]. The long-term performance of the two developed a-C:H coatings was tested with regard to friction reduction and wear protection of the polymers PEEK and PA66 under severe mixed lubrication. Therefore, tests were carried out for 8h under constant normal force of FN = 1000N (no previous running-in procedure), an oil injection temperature of ϑOil = 60°C, a sum velocity of 0,00 0,01 0,02 0,03 0,04 0,05 010203040506070 0,00 0,01 0,02 0,03 0,04 0,05 010203040506070 FN= 1,000 N ϑOil= 60 °C (FVA3) Slip rate s in %S lip rate s in % µ noitcirf fo tneiciffeo C* 0.05 0.04 0.03 0.02 0.01 0.06 0 0.05 0.04 0.03 0.02 0.01 0.06 0 Coefficient of friction µ* „fast“ conditioning„slow“ conditioning vΣ=1 m/s vΣ=4m/s vΣ=1 m/s vΣ=4m/s min. max. a-C:H-1 on PEEK a-C:H-2 on PA66 (a) (b) Fig. 7 Friction behavior of coated PEEK (a) and coated PA66 (b) for different conditioning procedures (ϑOil = 60°C; FN = 1,000N; FVA3) 0 0,01 0,02 0,03 0,04 07001400210028003500420049005600 a-C:H-2 on PA66PA66 a-C:H-1 on PEEKPEEK FN= 1,000 N ϑOil= 60 °C (FVA3) v∑= 1 m/s s = 50 % 20 25 30 35 40 45 50 55 60 07001400210028003500420049005600 Bulk temperature ϑ M,s in °C 60 120 240180 300 360 480 420 Ti me t in min 60 120 240180 300 360 480420 Ti me t in min µnoitcirffotneiciffeo C* 0.05 0.04 0.03 0.02 0.01 0 (a)(b) Fig. 8 Friction (a) and temperature behavior (b) over time in comparison of uncoated and coated PEEK/PA66 Tribology Letters (2022) 70:121 1 3 121 Page 16 of 16 References 1. Erhard, G.: Konstruieren mit Kunststoffen. Carl Hanser Verlag, München, Wien (2004) 2. Ehrenstein, G.W.: Mit Kunststoffen konstruieren. Carl Hanser Verlag, Eine Einführung. München (2007) 3. Fürstenberger, M.: Betriebsverhalten verlustoptimierter Kunststoffzahnräder, Technische Universität München Dissertation, München (2013) 4. Hasl, C., Illenberger, C., Stahl, K., Tobie, T.U., Oster, P.: Potential of oil-lubricated cylindrical plastic gears. J. Adv. Mech. Design Syst. Manuf. 12(1), S. 1-9 (2018) 5. Bartel, D.: Simulation von Tribosystemen: Grundlagen und Anwendungen. Vieweg+Teubner, Wiesbaden (2010) 6. Vincente, J.D., Stokes, J.R.U., Spikes, H.A.: The frictional properties of Newtonian fluids in rolling–sliding soft-EHL contact. Tribol. Lett. 20(3–4), S. 273-286 (2005) 7. Ziegltrum, A., Maier, E., Lohner, T.U., Stahl, K.: A numerical study on thermal elastohydrodynamic lubrication of coated polymers. Tribol. Lett. 68, 71 (2020) 8. Reitschuster, S., Maier, E., Lohner, T.U., Stahl, K.: Friction and temperature behavior of lubricated thermoplastic polymer contacts. Lubricants 8, 67 (2020) 9. Maier, E., Ziegltrum, A., Lohner, T.U., Stahl, K.: Characterization of TEHL contacts of thermoplastic gears. Forsch. Ingenieurwes. 81, S. 317-324 (2017) 10. Kano, M.: Overview of DLC-coated engine components. Springer International Publishing (2015) 11. Lugscheider, E., Bobzin, K., Maes, M.U., Krämer, A.: On the coating of polymers—basic investigations. Thin Solid Films 459, 313–317 (2004) 12. Martinez-Martinez, D.U., de Hosson, J.T.M.: On the deposition and properties of DLC protective coatings on elastomers: a critical review. Surface Coat. Technol. 258, 677–690 (2014) 13. Rothammer, B., Marian, M., Neusser, K., Bartz, M., Böhm, T., Krauß, S., Schroeder, S., Uhler, M., Thiele, S., Merle, B., Kretzter, J.P.U., Wartzack, S.: Amorphous carbon coatings for total knee replacements—Part II: tribological behavior. Polymers 13, 1880 (2021) 14. Puertolas, J.A., Martinez-Nogues, V., Martinez-Morlanes, M.J., Mariscal, M.D., Medel, F.J., Lopez-Santos, C.U., Yubero, F.: Improved wear performance of ultra high molecular weight polyethylene coated with hydrogenated diamond like carbon. Wear 269(5–6), 458–465 (2010) 15. Onate, J.I., Comin, M., Braceras, I., Garcia, A., Viviente, J.L., Brizuela, M., Garagorri, N., Peris, J.L.U., Alava, J.I.: Wear reduction effect on ultra-high-molecular-weight polyethylene by application of hard coatings and ion implantation on cobalt chromium alloy, as measured in a knee wear simulation machine. Surface Coat. Technol. 142–144, 1056–1062 (2001) 16. Kaczorowski, W., Szymanski, W., Batory, D.U., Niedzielski, P.: Tribological properties and characterization ofdiamond like carbon coatings deposited byMW/RF and RF plasma-enhanced CVD method on poly(ether-ether-ketone). Plasma Process Polym. 11, 878–887 (2014) 17. Bobzin, K., Brögelmann, T., Kalscheuer, C. u. Thiex, M.: Steigerung der Leistungsfähigkeit technischer Kunststoffe durch DLC-Beschichtungen. Tribologie und Schmierungstechnik 67 (2020) 18. Elsharkaxy, A.A., Holmes, M.J.A., Evans, H.P., Snidle, R.W.: Micro-elastohydrodynamic lubrication of coated cylinders using coupled differential deflection method. Proc. Inst. Mech. Eng. 220, 29–41 (2006) 19. Kalish, R., Lifshitz, Y., Nugent, K., Prawer, S.: Thermal stability and relaxation in diamond-like-carbon: a Raman study of films with different sp3 fractions (ta-C to a-C). Appl. Phys. Lett. 74, 2936–2938 (1998) 20. Ferrari, A.C., Robertson, J.: Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B 61, 14095–14107 (2000) 21. Casiraghi, C., Ferrari, A.C., Robertson, J.: Raman spectroscopy of hydrogenated amorphous carbons. Phys. Rev. B (2005). https:// doi. org/ 10. 1103/ PhysR evB. 72. 085401 22. Bobzin, K., Kalscheuer, C., Thiex, M., Sperka, P., Hartl, M., Reitschuster, S., Maier, E., Lohner, T., Stahl, K.: DLC Coated thermoplastics: tribological analyses under dry and lubricated sliding conditions. Status: Submitted Simultaneously to Tribology Letters (2022) 23. Norm DIN EN ISO 4288:1998. Geometrical Product Specifications (GPS)–surface texture: profile method–rules and procedures for the assessment of surface texture 24. Laukotka, E.M.: Referenzöle Datensammlung. FVA-Heft Nr. 660 Forschungsvereinigung Antriebstechnik e.V. (2007) 25. Myers, T.G., Hall, R.W., Savage, M.D., Gaskell, P.H.: The transition region of elastohydrodynamic lubrication. Proc. Math. Phys. Sci. 432(1886), S469-479 (1991) 26. Niemann, G., Winter H.: Maschinenelemente Band 2: Getriebe allgemein, Zahnradgetriebe—Grundlagen, Stirnradgetriebe. Berlin, Heidelberg: Springer-Verlag (2003) 27. Bobzin, K., Brögelmann, T., Kalscheuer, C., Thiex, M., Ebner, M., Lohner, T., Stahl, K.: A contribution to the thermal effects of DLC coatings on fluid friction in EHL contacts. Lubrication Sci. 30, S285-299 (2018) 28. Chen, Z., Liu, Y., Zhang, S., Luo, J.: Superlubricity achieved with mixtures of polyhydroxy alcohols and acids. Langmuir 29, 5239–5245 (2013) 29. Wang, H., Liu, Y., Li, J., Luo, J.: Investigation of superlubricity achieved by polyalkylene glycol aqueous solutions. Adv Mater Interfaces 3, 1600531 (2016) 30. Yilmaz, M., Lohner, T., Michaelis, K., Stahl, K.: Minimizing gear friction with water-containing gear fluids. Forsch Ingenieurwes 83, S327-337 (2019) 31. Lohner, T., Mayer, J., Michaelis, K., Höhn, B.-R., Stahl, K.: On the running-in behavior of lubricated line contacts. Proc. Inst. Mech. Eng. 231, 441–452 (2017) 32. Bobzin, K., Brögelmann, T., Kalscheuer, C., Thiex, M., Stahl, K., Lohner, T., Maier, E., Reitschuster, S.: Steigerung der Leistungsfähigkeit technischer Kunststoffe durch DLC-Beschichtungen. Proceeding of the 60th German Tribology Conference (GfT) Göttingen (2019) 33. Ferrari, A. C., Robertson, J.: Resonant Raman spectroscopy of disordered, amorphous, and diamondlike carbon. Phys. Rev. B 64 (2001) 34. Erdemir, A.: Design Criteria for Superlubricity in Carbon Films and Related Microstructures. Tribiology International 37, 577– 583 (2004) 35. Computer Aided Material Preselection by Uniform Standards CAMPUS: CAMPUS—a material information system for the plastics industry Computer Aided Material Preselection by Uniform Standards. https:// www. campu splas tics. com/ 36. Martienssen, W., Warlimont, H.: Handbook of condensed matter and materials data. Springer-Verlag, Berlin, Heidelberg (2005) Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.