the base pitch p et based on ISO/ TR 14179-2 [4]. To reduce the load-dependent gear power loss P LGP under dry lubrication, it is essential to reduce the coefficient of friction µ. Solid lubricants are used to achieve this. These are typically transition metal dichalcogenides (TMD), such as molybdenum disulfide (MoS 2 ) and tungsten disulfide (WS 2 ), as reviewed in [5]. The solid lubrication mechanism involves the formation of a lubricating layer when a tribological stress is applied. This lubricating tribofilm then transfers and redistributes itself across the contact surfaces by means of shearing the solid lubricant, with the objective of spreading it uniformly. The transfer mechanism of solid lubricant coatings has been investigated on gears, revealing that the wear resistance of coated gears is influenced by both the coating material and the specific gear being coated [6]. Studies on the application of WC/ C coatings on the gear wheel results in improved tribological properties, with the uncoated pinion undergoing a polishing process due to interaction with the coating. The transfer Science and Research 20 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0009 1 Introduction Lubricants in tribological contacts of machine elements, such as gears, are intended to reduce friction and wear and to remove frictional heat generated in the contact. Oil and grease lubrication cannot be used in applications with extreme ambient conditions such as in a vacuum, investigated in [1], with low weight or extreme thermal requirements, reviewed in [2], and in hygienic applications such as systems in the food or pharmaceutical industries, investigated in [3]. Dry lubrication results in a limited heat transfer by convection. Therefore, a low load-dependent gear power loss P LGP is essential to stay below critical temperature regimes for durable and safe operation. Equation (1) shows the load-dependent gear power loss P LGP , (1) with the coefficient of friction µ, the line load f N and the sliding velocity v g across the area of tooth contact, and = 1 ⋅ ( ̅ , ) ⋅ ( ̅ , ) ⋅ ( ̅ , ) ̅ ̅ Dry Lubrication of Spur Gears Coated with CrAlN+Mo: W: S Felix Farrenkopf, Thomas Lohner, Karsten Stahl, Kirsten Bobzin, Christian Kalscheuer, Max Philip Möbius, Marta Miranda Marti* submitted: 1.08.2024 Accepted: 25.04.2025 Presented at GfT-Conference 2024 Cylindrical gears are coated with CrAlN+Mo: W: S using High Power Pulsed Magnetron Sputtering and direct current Magnetron Sputtering. Their power loss, bulk temperature, and wear behavior are analyzed on the FZG gear efficiency test rig under dry lubrication. Coated and uncoated gears are examined using laser microscopy, energy-dispersive spectroscopy, and Raman spectroscopy. For operating points with low gear power loss, a tribofilm containing MoS 2 as well as Mo and W oxides is detected on the pinion and wheel tooth flank, resulting in a stable tribosystem for several thousands load cycles. Cylindrical gear pairs with CrAlN+Mo: W: S coated wheels exhibit reduced power loss and lower bulk temperatures compared to uncoated gears. Keywords PVD, CrAlN+Mo: W: S, Solid Lubricants, Gears, Efficiency, Friction, Wear Abstract * Felix Farrenkopf, M.Sc. 1 (corresponding author) Orcid-ID: https: / / orcid.org/ 0000-0002-8675-3040 Dr.-Ing. Thomas Lohner 1 Orcid-ID: https: / / orcid.org/ 0000-0002-6067-9399 Prof. Dr.-Ing. Karsten Stahl 1 Orcid-ID: https: / / orcid.org/ 0000-0001-7177-5207 Prof. Dr.-Ing. Kirsten Bobzin 2 Orcid-ID: https: / / orcid.org/ 0000-0003-3797-8347 Dr.-Ing. Christian Kalscheuer 2 Orcid-ID: https: / / orcid.org/ 0000-0002-1606-5090 Max Philip Möbius, M.Sc. 2 Orcid-ID: https: / / orcid.org/ 0000-0002-2982-8720 Marta Miranda Marti, M.Sc.(corresponding author) Orcid-ID: https: / / orcid.org/ 0000-0002-2048-7042 1 Gear Research Center (FZG), Department of Mechanical Engineering, School of Engineering and Design, Technical University of Munich, Boltzmannstraße 15, 85748 Garching near Munich, Germany 2 Surface Engineering Institute (IOT), RWTH Aachen University, Kackertstraße 15, 52072 Aachen, Germany mechanisms of solid lubricant coatings such as W-S-C [7], h-BN [8], Ti-WS 2 [9], Mo 2 N/ MoS 2 / Ag [10] have been extensively investigated in various studies but primarily on flat samples. A promising approach for low friction and high wear resistance is by applying a CrAlN coating enriched with solid lubricants components such as Mo, W and S. Previous studies explored the use of graded CrAlN+Mo: W: S deposited by means of physical vapor deposition (PVD), demonstrating a significant reduction in friction compared to uncoated substrates [11]. These studies also investigated the tribofilm and transfer mechanism on flat samples [12]. However, these coatings have not been analyzed in machine elements such as gears tested under stationary operating conditions, and the transfer mechanisms of these triboactive coatings remain not fully understood. Therefore, the subject of the present study is the investigation of the triboactive CrAlN+Mo: W: S coating in the gear contact and the analysis of the tribofilm and transfer film formation mechanism. 2 Methods and Materials This section describes the FZG efficiency test rig, the CrAlN+Mo: W: S coating and application process used, the test procedure for the coated gears in the FZG efficiency test rig, and the gear analysis procedure. 2.1 Test Rig and Gears The considered FZG efficiency test rig is a modified FZG back-to-back test rig with a center distance of a = 91.5 mm according to DIN ISO 14635-1 [13]. It is based on the principle of power circulation in which the load torque is applied by the load clutch in-between the test and transmission gearbox. The test rig is driven by an electric engine that provides the loss torque of the power circle. The load torque is measured in the power circle, the power loss is measured between the power circle and the electric engine. Further information can be found in [14,15]. The gear contact in the test gearbox is dry-lubricated and bearings of the type 6406 are used, which are both sided sealed. In the transmission gearbox, bearings of the type NU406 are used and injection lubrication with 2 l/ min of an ISO VG 100 mineral oil at ϑ oil = 60 °C is applied. The test gears used in the test and transmission gearbox are of FZG type C mod (cf. Figure 1) made of case-hardened 16MnCr5 steel with ground and superfinished gear flanks. A tip relief C a of 35 µm is applied. According to [16] and using the loaded tooth contact analysis (LTCA) software RIKOR [17], a local geometric tooth power loss factor H VL of 0.166 is derived for T 1 = 94.1 Nm. The bulk temperature ϑ M is measured by a Pt100 temperature sensor in the middle of one tooth. The main gear geometry data is given in Table 1. Science and Research 21 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0009 (a) (b) Figure 1: FZG test gear of type C mod (a) and transverse section of the gear mesh (b) Parameter Value Center distance, a in mm 91.5 Common tooth width, b in mm Normal module, m n in mm 4.5 Pressure angle, α n in ° 20 Number of teeth, z 1 | z 2 2 1 b | 16 | 24 14 | 14 Tip relief, C a in μm 35 Table 1: Main geometry data of the FZG test gear of type C mod Pinion (1) Wheel (2) Pinion (1) Wheel (2) d 1 d 2 1.2 0.9 0.6 0.3 0A B C D E 10 8 6 4 2 0 p H in GPa R red in mm p H R red 4.5 3 1.5 0A B C D E v in m/ s v Σ |v g | (a) (b) To achieve a homogeneous coating of the gears, a substrate holder with a spit geometry is employed for a twofold rotation of the gear. Prior to coating, a chemical cleaning process is conducted. 2.3 Test Procedure The operating point considered is selected from the method FVA 345 [21]. A moderate load of T 1 = 94.1 Nm and medium circumferential speed of v t,C = 5 m/ s at ambient temperature and atmosphere are applied. Figure 2 shows the Hertzian pressure p H , the radius of curvature R red , the sliding velocity v g and the sum velocity v ∑ along the path of contact for the FZG test gear of type C mod at T 1 = 94.1 Nm and v t,C = 5 m/ s. Due to the test gears’ tip relief C a of 35 µm (cf. Table 1) designed for T 1 = 302 Nm, there is no tooth contact close to A and E for the considered operating condition. While there is pure rolling |v g | = 0 m/ s at the pitch point C, the sliding portion increases with increasing distance to the pitch point C. Science and Research 22 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0009 The superfinished gear flanks have a roughness of Ra = 0.07…0.12 µm. Only the wheels are coated, resulting in a roughness of Ra = 0.06…0.11 µm. The surface roughness was measured according to DIN EN ISO 4287 [18] and DIN EN ISO 4288 [19] with a limiting wavelength of λ c = 0.8 mm on a length of L t = 4.8 mm in tooth profile direction. Due to the manufacturing and coating process (cf. section 2.2), the coated wheels have an average lead crowning of up to C β = 2.4 µm. 2.2 Coating The CrAlN+Mo: W: S coating is deposited using an industrial CC800/ 9 coating unit, CemeCon AG, Würselen, Germany. This coating unit is equipped with four direct current Magnetron Sputtering (dcMS) cathodes and two High-Power Pulsed Magnetron Sputtering (HPPMS) cathodes. The cathodes operate simultaneously to achieve a hybrid dcMS/ HPPMS process, resulting in a coating thickness of d = 3 µm. Table 2 displays the deposition process parameters. Further coating parameters can be found in [20]. Process phase Process parameters Unit Value Interlayer and Pressure, p mPa 490 - 560 ramps Bias voltage, U B V -150 to (-75) Argon flow, j Ar sccm 200 Nitrogen flow, j N sccm pressure controlled Pulse frequency f Hz 500 Pulse duration, t on μs 40 Speed of table rotation, v rot rpm 3 Toplayer Pressure, p mPa 590 Bias voltage, U B V -75 Argon flow, j Ar sccm pressure controlled Nitrogen flow, j N sccm 100 - 140 Table 2: Process parameters of coating deposition Figure 2: Hertzian pressure p H and radius of curvature R red (a), and sliding velocity v g and sum velocity v ∑ (b) along the line of action of the FZG test gear of type C mod (T 1 = 94.1 Nm, v t,C = 5 m/ s) For the coated gear tests, only the wheel is coated while the pinion is uncoated. For reference tests, an uncoated pinion and wheel are used. Upon reaching the maximum annealing temperature of ϑ M = 200 °C, as defined by DIN EN ISO 683-3 [22], the tests are aborted. A calibrated transmission gearbox is used for all tests. For calibration, bearings of type NU406 and uncoated gears are mounted in the test gearbox, to ensure the same set-up as in the transmission gearbox. Calibration tests are carried out under no-load and load conditions with both gearboxes are injection-lubricated. Since the test and transmission gearbox are equipped with FZG test gears of type C mod , featuring a symmetrical gear mesh behavior, the measured power loss can be approximately halved to derive the no-load and load-dependent power loss of the transmission gearbox, P L0,cal and P LP,cal . With the calibrated transmission gearbox, the bearings of type 6406 and test gears are mounted in the test gearbox. Before each test under dry-lubrication, the circumferential speed is held for t = 15 min under no-load to determine the no-load power loss of the test gearbox P L0,dry : (2) In the subsequent test under load, the load-dependent power loss P LP,dry is determined by subtracting the power loss of the transmission gearbox (P L0,cal + P LP,cal ) and the no-load power loss of the test gearbox P L0,dry . This results in the load-dependent loss of the test gearbox P LP,dry : (3) The SKF method in [23] method is used to determine the load-dependent bearing power loss P LBP of the test gearbox, which can also be subtracted to finally derive the load-dependent gear power loss P LGP,dry of the test gearbox: (4) The load-dependent gear power loss P LGP acc. to equation (1) can be rewritten by the assumption of a mean , = − , ! , = − , − ( , ! + , ! ) , = , − " coefficient of gear friction µ mz acc. to ISO/ TR 14179-2 [4]: (5) Subsequently, the mean coefficient of friction can be determined from the load-dependent gear power loss P LGP,dry using P in and H VL . 2.4 Analysis of Tribofilm and Transfer film To analyze the tooth flank of the considered test gears, a single tooth is separated due to geometric limitations of the characterization techniques. This is achieved by cutting the gears between two teeth. This is done fluid-free with an angle grinder to prevent removing any tribofilm or transfer film. Compressed air cooled the gear, and temperature is monitored during cutting. Subsequently, a surface analysis after mechanical testing of the coated gear flanks and uncoated pinion flank is conducted by means of confocal laser scanning microscopy (CLSM), VK-X210, Keyence, NeuIsenburg, Germany. For the characterization of the chemical composition of tribofilm on wheel and pinion flanks after the efficiency test, an energy-dispersive X-ray spectroscopy (EDX) detector integrated into a Phenom XL G2 desktop scanning electron microscope (SEM) by ThermoFischer Scientific, Dreieich, Germany, is used. The employed accelerating voltage is E V = 15 kV with a working distance of W D = 8 mm, and the mapping resolution range is set to 480 x 480 pixel with a pixel dwell time range of t = 10 ms/ pixel. A systematic chemical analysis of tribofilm on the coated gear and uncoated pinion is conducted by means of Raman spectroscopy employing a Renishaw InVia Reflex, Renishaw GmbH, Pliezhausen, Germany. The parameters for analysis are shown in Table 3. The laser is calibrated before the measurement by using a silicon reference sample. Representative spectra are chosen for evaluation and fitted by OriginPro Software using a Lorentz fitting filter as recommended in [24]. = #$ ⋅ 1 ⋅ ( ̅ , ) ⋅ ( ̅ , ) ̅ ̅ %&&&&&&&&&&&'&&&&&&&&&&&* -. ⋅/ 02 Science and Research 23 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0009 Parameter Value Laser wavelength λ in nm 532 Laser power P in mW 2.6 Accumulations n in - 5 Grating in 1/ mm 1,800 Exposure time t in s 15 Objective lens 5x Table 3: Raman acquisition parameters ϑ M = 200 °C already at N 1 = 5,273 and N 1 = 6,295. One CrAlN+Mo: W: S/ uncoated test reaches the abort criteria of ϑ M = 200 °C at N 1 = 19,212, while another CrAlN+Mo: W: S/ uncoated test is stopped at N 1 = 5,284 to analyze the gear flanks at the corresponding state. A further repetition test shows a similar trend and is stopped at N 1 = 41,680 due to no significant changes in the operating behavior. The load-dependent gear power loss P LGP , dry in Figure 3a and mean coefficient of gear friction µ mz in Figure 3b shows a steep increase from the very beginning for the tests uncoated/ uncoated. A plateau is finally reached at a high level of power loss and mean coefficient of gear friction. Consequently, this results in a steep and Science and Research 24 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0009 3 Results and Discussion This section presents the results of the tests conducted on the FZG efficiency test rig and an analysis of the gears. 3.1 Experimental Results Figure 3 shows the measured load-dependent gear power loss P LGP,dry , mean coefficient of gear friction µ mz and gear bulk temperature ϑ M of the pinion and wheel over the load cycles at the pinion N 1 . Test results are shown for uncoated/ uncoated and CrAlN+Mo: W: S/ uncoated configurations. The two uncoated/ uncoated tests show similar behavior and reach the abort criteria of Figure 3: Load-dependent gear power loss P LGP (a), mean coefficient of gear friction µ mz (b) and gear bulk temperature ϑ M (c) over the load cycles at the pinion for tests with uncoated/ uncoated and coated/ uncoated gears of type C mod (T 1 = 94.1 Nm, v t,C = 5 m/ s) 1.5 1 0.5 0 0 2 4 6 8 10 12 14 16 18 20 Load cycles at pinion N 1 in 1,000 test uncoated/ uncoated 1 test uncoated/ uncoated 2 test CrAlN+Mo: W: S/ uncoated (N 1 =5,284) test CrAlN+Mo: W: S/ uncoated (N 1 =19,212) test CrAlN+Mo: W: S/ uncoated (N 1 =41,680)* 0.5 0.3 0.2 0 0 2 4 6 8 10 12 14 16 18 20 0.4 0.1 Load cycles at pinion N 1 in 1,000 200 150 100 20 0 2 4 6 8 10 12 14 16 18 20 50 Pinion Wheel Load cycles at pinion N 1 in 1,000 *stopped intentionally (a) (b) (c) continuous increase in the bulk temperatures of the pinion and wheel (cf. Figure 3c). Both bulk temperatures are very close to each other. Once the abort criteria of ϑ M = 200 °C is reached at N 1 = 6,295 for the test uncoated/ uncoated 1 and at N 1 = 5,273 for the test uncoated/ uncoated 2, the tests are aborted. In contrast, the tests CrAlN+Mo: W: S/ uncoated in Figure 3 show a decrease of the load-dependent gear power loss P LGP,dry and the mean coefficient of gear friction µ mz immediately after the beginning. A minimum mean coefficient of gear friction µ mz of 0.048 and 0.064 and minimum load-dependent gear power loss P LGP,dry of 0.105 and 0.144 kW is reached. This means a reduction of 84 and 88 % compared to the test uncoated/ uncoated 1, which is achieved by the CrAlN+Mo: W: S coating applied. The low mean coefficients of gear friction indicate a functioning friction-reducing tribofilm and thus the formation of the solid lubricant. Observing the CrAlN+Mo: W: S/ uncoated (N 1 = 19,212) test, there is a steep increase in the mean coefficient of gear friction μ mz and load-dependent gear power loss P LGP,dry at around 10,000 load cycles (cf. Figure 3a and b). From 12,000 load cycles on, this increase becomes more moderate but continues to increase until the abort criterion is reached. This indicates that the effectiveness of the tribofilm is no longer sufficient at the point of steep increase. The second test CrAlN+Mo: W: S/ uncoated (N 1 = 41,680) shows a similar trend from beginning until it reaches a second plateau from 14,000 load cycles. To analyze the coating and transfer film in the minimum plateau, the test CrAlN+Mo: W: S/ uncoated (N 1 = 5,284) is stopped intentionally. As illustrated in Figure 3c, the bulk temperatures of the tests CrAlN+Mo: W: S/ uncoated exhibits a characteristic pattern. A slight increase in bulk temperature ϑ M is observed in the low friction plateau region. As friction and power loss increase, the bulk temperature ϑ M continues to rise as well. The test CrAlN+Mo: W: S/ uncoated (N 1 = 19,212) reaches the abort criterion while the test CrAlN+Mo: W: S/ uncoated (N 1 = 41,680) stabilizes due to the second friction plateau after 14,000 load cycles. It is notable that there is a considerable difference in the bulk temperatures ϑ M of the uncoated pinion and the coated wheel. The findings of [25] indicate that a coating can function as a thermally insulating layer, which results in diminished heat dissipation into the substrate. 3.2 Analysis of Tribofilm and Transfer Film Formation To gain an overview of the gear tooth flanks and identify the critical wear areas, CLSM overview images are obtained, as illustrated in Figure 4. To accurately determine the presence of the coating, an EDX mapping is conducted along the coated wheel tooth flank. In this mapping, the elements Mo, W and S are represented in yellow, with no difference among them due to the overlapping of the characteristic Mo and S EDX-peaks [26], whereas interlayer elements such as Al and Cr are depicted in blue. The substrate material Fe is shown in green, to provide a clear contrast between the coating and the substrate. As illustrated in Figure 4a, the uncoated wheel from the test uncoated/ uncoated 1 exhibits significant wear along its tooth flank in the areas of high sliding, with a maximum wear depth of z = 20 µm. Negligible wear is observed in the vicinity of the pitch point C, where rolling friction is predominated. The overall high wear value on the uncoated gear tooth flanks correlates with the rapid increase in power loss and bulk temperature observed from the beginning of the test. The tooth flank root and tip area does not contact the pinion surface (cf. Figure 2), thus the coating remains unaffected. For the coated wheel from the test CrAlN+Mo: W: S/ uncoated (N 1 = 5,284) in Figure 4b), less substrate is exposed when compared to the wheel after completion, and the elements of the interlayer are minimally present. Wear along the tooth flank is barely noticeable with a maximum wear depth of z = 3 µm. However, the CrAlN+Mo: W: S coating with a coating thickness of d = 3 µm is already partly removed from the surface. Due to the lead crowning C β , a wear concentration in the midsection can be observed. For the coated wheel from the test CrAlN+Mo: W: S/ uncoated (N 1 = 19,212), the delamination of the coating’s solid lubrication elements on the upper part of the tooth flank becomes more evident in Figure 4c, indicated by increased measurements of Al and Cr content. Moderate wear can be observed in the upper part of the tooth flank, while wear in the lower part of the tooth flank is much more pronounced. This result suggests that the toplayer and the graded function layer containing the solid lubricant elements is consumed to achieve lubrication along the tooth flank, creating MoS 2 and reducing the wear. As a result, the power loss is reduced compared to uncoated gears, and a reduction in wear is achieved, as determined by the profile measurement. In order to determine the triboactive transformation of the coating on the wheel of the test CrAlN+Mo: W: S/ uncoated (N 1 = 5,284), Raman measurements on the remaining coating are conducted. The results of an exemplary spectrum found on the analyzed wheel tooth flank are presented in Figure 5. The characteristic double peak of MoS 2 is clearly observed at ω E2g = 385 cm -1 and ω A1g = 405 cm -1 corresponding to the E 2g and the A 1g band respectively, as measured in [27]. Tillmann et al. report similar results Science and Research 25 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0009 and W oxides are also found on the tooth flank surface, which might contribute to the lubricating effect. To analyze the formation of a transfer film on the pinion, the same characterization techniques are employed. The formation of a transfer film on the pinion tooth flank surface is analyzed using EDX. However, due to the scattering of triboactive elements along the pinion flank, no Raman measurement of the triboactive elements is pos- Science and Research 26 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0009 where formation of a lubricating tribofilm contributes to low friction for non-synchronized, dry running twinscrew machines [28]. Due to this mechanical activation of the solid-lubricant elements Mo, W and S, a MoS 2 tribofilm is obtained. The characteristic WS 2 peak is observable at ω E2g = 350 cm -1 , while the second characteristic WS 2 peak is reported in [29] to overlap with the MoS 2 peak at around ω A1g = 405 cm -1 . This suggests the presence of WS 2 in the resulting tribofilm. Various Mo Figure 4: CLSM images, EDX mapping and profile measurements of wheel tooth flanks of the tests (a) uncoated/ uncoated 1, (b) CrAlN+Mo: W: S/ uncoated (N 1 = 5,284) and (c) CrAlN+Mo: W: S/ uncoated (N 1 = 19,212) (a) (b) (c) Science and Research 27 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0009 Figure 5: Raman spectra on wheel of test CrAlN+Mo: W: S/ uncoated (N 1 = 5,284) Figure 6: CLSM images, EDX mapping and flank profile from the tests (a) uncoated/ uncoated 1, (b) CrAlN+Mo: W: S/ uncoated (N 1 = 5,284) and (c) CrAlN+Mo: W: S/ uncoated (N 1 = 19,212) (a) (b) (c) 3. On the coated wheel, from a test stopped after N 1 = 5,284 pinion load cycles, EDX mapping measured remaining triboactive elements. Raman spectroscopy identified them as solid lubricants MoS 2 and WS 2 . Furthermore, various Moand W-oxides were found on the tooth flanks. This demonstrates that solid lubricants are formed in situ under the tribological conditions found in gear contacts. 4. The presence of a transfer film on the pinion tooth flanks, as observed by EDX analysis, confirmed the formation of a transfer film, which may have contributed to the measured reduction in friction. Considering these findings, the next step should be a comprehensive parameter study to evaluate the solid lubricant and its lifetime under different operating conditions. This further study should identify limits and potential areas of application. Acknowledgement The authors acknowledge the financial support of the German Research Foundation, Deutsche Forschungsgemeinschaft (DFG), within the priority program SPP 2074 and the research project “Fluid-less lubrication systems with high mechanical load” (No. BO 1979/ 66-2 / STA 1198/ 17-2). References [1] C. Donnet, Advanced solid lubricant coatings for high vacuum environments, Surface and Coatings Technology 80 (1996) 151-156. https: / / doi.org/ 10.1016/ 0257-8972(95)02702-5. [2] I.M. Allam, Solid lubricants for applications at elevated temperatures, J Mater Sci 26 (1991) 3977-3984. https: / / doi.org/ 10.1007/ BF00553478. [3] C. Donnet, A. Erdemir, Solid Lubricant Coatings: Recent Developments and Future Trends, Tribology Letters 17 (2004) 389-397. https: / / doi.org/ 10.1023/ B: TRIL.0000044487.32514.1d. [4] ISO-International Organization for Standardization, Gears - Thermal capacity -Part 2: Thermal load-carrying capacity: ISO/ TR 14179-2: 2001, ISO-International Organization for Standardization, 2001. [5] T.W. Scharf, Transition Metal Dichalcogenide-Based (MoS 2 , WS 2 ) Coatings, in: G.E. Totten (Ed.), Friction, Lubrication, and Wear Technology, ASM International, 2017, pp. 583-596. [6] R. Michalczewski, M. Kalbarczyk, W. Piekoszewski, M. Szczerek, W. Tuszyński, The rolling contact fatigue of WC/ C-coated spur gears, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 227 (2013) 850-860. https: / / doi.org/ 10.1177/ 1350650113478179. Science and Research 28 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0009 sible. Figure 6 presents a summary of the results, comparing the uncoated pinion flanks from the tests uncoated/ uncoated 1, CrAlN+Mo: W: S/ uncoated (N 1 = 5,284) and CrAlN+Mo: W: S/ uncoated (N 1 = 19,212). Due to the presence of Cr in both the coating interlayer and the 16MnCr5 substrate, a mapping including this element cannot differentiate whether the interlayer has been transferred from the coated wheel or originates from the pinion substrate. Consequently, the Cr content is excluded from this analysis. At the uncoated pinion from the test uncoated/ uncoated 1 in Figure 6a, a high wear of up to z = 21 µm is noticeable. On the tooth flank surface of the pinion from the test CrAlN+Mo: W: S/ uncoated (N 1 = 5,284) in Figure 6b, adhesions containing solid lubricant elements can be observed. This confirms the transfer of the tribofilm from wheel to pinion. After the test CrAlN+Mo: W: S/ uncoated (N 1 = 19,212) (cf. Figure 6c), no transfer film can be measured, only elements prevenient of the interface like Al. Higher wear of up to z = 11 µm can be measured, especially in the region of the tooth flank root area. This reinforces the theory that the tribofilm on the wheel is consumed with increasing load cycles to lubricate the dry contact. This process reduces power loss and controls the bulk gear temperature. Due to the consumption of the triboelements after higher pinion load cycles, the coating failure increases the friction, power loss and bulk temperature. 4 Conclusion This study investigated the tribological behavior of CrAlN+Mo: W: S coated wheels paired with uncoated pinions under dry lubrication. To investigate the relationship between tribofilm formation and pinion load cycles, gear tests were carried out at a pinion torque T 1 = 94.1 Nm and circumferential speed v t,C = 5 m/ s. The following conclusions can be drawn: 1. The deposition of a CrAlN+Mo: W: S coating on the wheel has been shown to result in a significant reduction in the mean coefficient of gear friction µ mz when compared to uncoated tests. The reduction was approximately 84 % to 88 %, resulting in values ranging from µ mz = 0.048 to µ mz = 0.064. This result validates the friction reduction with CrAlN+Mo: W: S coatings in gear contacts. 2. The detection of interlayer elements such as Al and Cr on the wheel tooth flanks after reaching the abortion criteria ϑ M = 200 °C confirms the partial consumption of the CrAlN+Mo: W: S-toplayer and the subsequent wear of the functional coating. This observation highlights the coating’s active contribution in the tribological contact and demonstrates a correlation between coating wear and operational lifetime. [7] J.V. Pimentel, T. Polcar, M. Evaristo, A. 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