deposition techniques, which include Physical Vapour Deposition (PVD), chemical vapor deposition, and ion beam assisted deposition, can produce more homogeneous surfaces, but surface preparation and costs are higher [2, 3]. Molybdenum and its oxides have been studied as a good candidate for solid lubricant coating technology because of their higher hardness and relatively low wear coefficient when compared to uncoated bearing surfaces [4]. In recent studies by our research group, the wear and hardness of molybdenum-based coatings applied with PVD were evaluated from nanoto macroscopic scales [5, 6, 7], and it was shown that molybdenum can work in dry bearing surfaces under ambient conditions [8]. However, PVD presents limitations in coating thickness (approx. 2 µm) [3]. As an alternative, APS can provide a thicker coating (approx. 50 µm to 500 µm), leading to Science and Research 54 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0020 Introduction Solid lubricant coatings can be a valuable alternative to fluid lubricants when these can’t be used, particularly in extreme conditions like high vacuum, extreme temperatures, or contamination-sensitive environments [1]. Solid lubricant coatings face several challenges that must be addressed to ensure their effective functionality and performance. These include high friction coefficients and wear compared to elasto-hydrodynamic lubrication [1]. Additionally, once worn, solid lubricant coatings cannot usually regenerate, leading to the contacting materials being different from the initial ones and possibly leading to higher friction, wear, and consequently failure. To apply such coatings, different production processes can be used: thermal spraying methods and deposition methods [2]. Thermal spraying methods include, for instance, Atmospheric Plasma Spraying (APS) and flame spraying. Compared to deposition methods, thermal spraying methods are cost-effective, capable of delivering high deposition rates, and are suitable for producing thick coatings, but have the disadvantage of producing a more porous microstructure [2]. On the other hand, Molybdenum APS-Coatings: A Self-regenerative Solution for Wear Resistance in Dry-lubrication Applications Ricardo Crespo Martins, Dennis Konopka, Mareike Dukat, Florian Pape, Martin Nicolaus, Kai Möhwald, Gerhard Poll, Max Marian* Presented at GfT Conference 2025 Molybdenum coatings and their oxides are a promising technology to protect bearing surfaces from wear due to their good wear-resistance and their capability to regenerate by forming new protective and lubricious oxide layers. However, until recently, their applicability against higher hardness materials has not yet been studied. This study aims to explore Atmospheric Plasma Spraying (APS) to produce a molybdenum layer on bearing steel and compare the tribological behavior of coated and uncoated surfaces using a reciprocating pin-on-plate apparatus under dry-lubrication. The counter-body material was chosen to be alumina, given its inertness and high hardness. The tests were conducted at various distances and loads, comparing the friction and wear volumes of coated and uncoated specimens. Keywords Coatings, Wear-Resistant, Thermal Spraying, Solid Lubrication, DFG SPP 2074. Abstract * Ricardo Crespo Martins (corresponding author) 1 Dennis Konopka 1 Mareike Dukat 2 Dr. Florian Pape 1 Dr. Martin Nicolaus 2 Prof. Kai Moehwald 2 Prof. Gerhard Poll 1 Prof. Max Marian 1,3 1 Institute of Machine Design and Tribology, Leibniz Universität Hannover, An der Universität 1, 30823 Garbsen, Germany. 2 Institute of Materials Science, Leibniz Universität Hannover, An der Universität 2, 30823 Garbsen, Germany. 3 Department of Mechanical and Metallurgical Engineering, School of Engineering, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, 6904411 Macul, Región Metropolitana, Chile. longer dry-running times and lower maintenance costs. It also has the advantage of being able to regenerate due to the capability of molybdenum to oxidize with the ambient oxygen, when previous oxide layers wear out [9]. However, APS has shown lower hardness and higher porosity compared to PVD [3]. In the present work, the wear and friction behaviors of bearing steel and APS-sprayed molybdenum are compared under reciprocating conditions against a counter body made of alumina, at a wider range of loads and distances than in the previous study [3]. With this research, we aim to understand whether APS-applied molybdenum still outperforms bearing steel when a hard material is slid on its surface. Methodology The study was conducted using coated (hereafter, APS) and uncoated (hereafter, REF) specimens. The uncoated specimen and the substrate of the coated specimen were made of hardened 100Cr6 (AISI 52100) steel discs with a diameter of 40 mm and a height of 10 mm. To apply the APS coating, one of the flat surfaces of the discs was roughened by abrasive blasting with corundum of a grain size of 250 to 355 µm as blasting medium. The surface was then cleaned with isopropanol to remove contaminants. The coating was applied with atmospheric plasma spraying using a Delta-torch (GTV Verschleißschutz GmbH, Germany). The coating material was molybdenum powder (Mo with 99 % purity, powder particle size between 45 and 90 µm; GTV Verschleißschutz GmbH, Germany). The powder was melted using a plasma jet made of an ionized mixture of argon and hydrogen generated by an electric arc. The molten molybdenum was then projected to the substrate surface, solidifying and adhering to it. The APS process parameters are presented in Table 1. After the coating process, the surface was ground to a final thickness of 50 µm using silicon carbide as abrasive slurry. This thickness ensures that the maximum stress due to the non-conforming contact happens below the coating [10]. Before being tribologically stressed, the samples were cleaned using acetone and isopropanol. To evaluate the coated and uncoated systems, a ballon-plate apparatus (Milli-Tribometer, TRIBOtechnic, France) was used. With this tribometer, a loaded sphere (counter-body) was slid in a reciprocating motion against the specimen, and the friction force was measured. The conditions used in the tribological tests are given in Table 2. Both the load and sliding distance were varied, totaling nine sets of conditions for each of the specimen materials. For each test condition, three repetitions were carried out. This number of repetitions may only be of statistical relevance if the behavior (wear volume and friction coefficient) is similar between each repetition and the difference between conditions is considerable. The counter-body had a diameter of 6 mm, and was made of alumina (99.8 % Al 2 O 3 , G10; TK Linear GmbH, Germany). This material was chosen due to the particular high hardness, high wear resistance, and inertness. In previous studies [3], steel counter-bodies were used and the wear occurred on these. Although alumina is less common in rolling element bearings, it provides a mean to evaluate the self-regenerative properties in harsher conditions compared to the conditions provided by steel counter-bodies. Given the material properties and geometry of the contacting bodies, the initial Hertzian contact pressures resulted in 764, 963, and 1102 MPa, which respectively correspond to 1, 2, and 3 N of applied load. After being tribologically stressed, the samples were analyzed under a confocal laser scanning microscope (LSM; VKX-100, Keyence, Japan). This microscope enables the measurement of the surface profile and, consequently, the determination of the wear volume. To compare the wear behavior of the coated and uncoated samples, the surface profile of each wear scar was analyzed at three distinct locations. The wear area of each section was then determined and averaged per scar and test condition (totaling nine profile measurements per condition). This average wear area was then multiplied by the length of the scar (5 mm) to determine the wear volume. Results and Discussion The results of the friction coefficient corresponding to each distance at which the test was stopped are presented in Figure 1. As mentioned in the methodology, three distances were tested: 5, 50, and 100 meters, each one cor- Science and Research 55 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0020 1 3 62$27,#,$* 82.',* \#0*-)$*&I'))0,-& OWW&B& \#0*-)$*&F"30)& SW&; Z& B).",&2#"3&)+-0&& OW&cd: $,& U8%)".0,&2#"3&)+-0& >W&cd: $,& F"3%0)&200%&)+-0& I+@&EQ<QW&.d: $,& \9-0),+#& *""#$,.& "2& -40& / +: 7#0/ & B*-$10& L$/ -+,*0& (0-300,& ,"XX#0&+,%&/ '(/ -)+-0& >OW&: : & Table 1: Atmospheric Plasma Spraying parameters. 62$27,#,$ & 82.', & c"+%&& >6&E6&M&f& N+9@&/ #$%$,.&/ 700%& T&: : d/ & ! -)"; 0& Q&: : & ! #$%$,.&%$/ -+,*0& Q6&QW6>WW&: & I"',-0)<("%8&%$+: 0-0)& R&: : & Table 2: Test conditions used in the pin-on-plate tribometer. observed to increase. While the APS specimens demonstrated to have a higher friction coefficient at higher loads, the highest friction coefficient for REF was measured at the middle load (2 N), and the lowest was recorded at the highest load (3 N). In the bottom figure (Figure 1c), the same distinct behavior of increasing and decreasing friction values for REF and APS, respectively, is noticeable. However, unlike the tests stopped at 50 meters, the friction coefficient of REF was measured as the highest with the highest load, and vice versa. As for the APS, the friction coefficient values were shown to converge to a value of around 0.65. It was found that the longer the test, the lower the average standard deviation. While in the tests lasting 5 m the standard deviation was high, for the 100 m tests, the highest average standard deviation was recorded as 0.07 for REF at a load of 3 N. After each test, the surface profile of each wear scar was analyzed. The results of the average wear volume per test are shown in Table 3. Whenever it was possible to measure a wear area, the values are given, otherwise, a zero value is presented. The data indicate that wear was always measured for REF, regardless of the distance or Science and Research 56 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0020 responding respectively to the figures a), b), and c). Each line corresponds to the average friction coefficient per test condition. The running-in behavior of the two specimens at three different loads is depicted in Figure 1a). It is shown that the friction coefficient of both specimens starts at a low value, around 0.2, and it is shown to increase steadily. While REF was observed to take more time to reach this behavior, less time was required by the APS specimen, and after around 0.5 meters of the test, its friction coefficient is already increasing less abruptly. In these first meters, it is observed that the friction coefficient of REF is already higher at higher loads (2 and 3 N), but lower at the lowest tested load (1 N). It should be noted that the running-in behavior had a large standard deviation (from the six lines of the graph, the highest average standard deviation across the whole test was 0.13 for REF at a load of 3 N), making the comparison between each condition less reliable. In a second phase of the test (tests that lasted 50 meters), it was observed that the friction coefficients of the specimens exhibited different behaviors: while APS tests were noted to decrease with the distance, REF tests were ! ! "# $"# %"# Figure 1: Friction coefficient evolution with distance of a 6 mm diameter reciprocating ball-on-plate tests at 8 mm/ s. Each line corresponds to the mean of three repetitions. load, except at the lowest load and distance. However, for APS, at lower loads, the wear was found to be very low or negligible at any distance. Except for the tests that lasted 50 meters with a load of 2 N, APS has always shown to have a lower wear volume than REF. Exemplary images of the APS and REF surfaces taken using the LSM are given in Figure 2a) and 2b), respectively. After the tests, the counter-bodies were analyzed under a microscope and it could be concluded that no wear occurred on these, as it is possible to see in Figure 2c) and 2d). However, for the APS tests, a layer on the surface of the sphere was noticeable, possibly molybdenum and its oxides. As the tests had three different distances, it was possible to evaluate the friction and wear at the end of each distance. After a running-in phase during the first meters of test, the contacting materials were the ones expected (steel and its oxides with alumina; molybdenum and its oxides with alumina). Given the differences in hardness between the molybdenum (3.8 GPa) and steel surfaces (typically 8 GPa), and the counter-body (typically 15-16 GPa), it would be expected that the wear volume of the molybdenum would be higher. However, even at lower distances and a low amount of repetitions, it was observed that this was not the case, and the coated specimens presented lower wear. It was also evident that at low loads (1 N), regardless of the sliding distance, wear was not observed on the coated specimen, a phenomenon that was only seen at low distances and loads with the uncoated specimen. This is of particular relevance, given the high hardness of the counter-body. This phenomenon of the molybdenum coating points to a high dependence Science and Research 57 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0020 Figure 2: Surfaces of the specimens (a and b) and counter-bodies (c and d) after a 100 m test with a load of 1 N at 8 mm/ s. The direction of sliding is shown by the red arrow. Table 3: Average wear volume of the specimen in x10 -6 mm 3 . d) Surface of the counter-body after the test against the uncoated specimen (REF). c) Surface of the counter-body after the test against the coated specimen (APS). a) Wear scar of the coated specimen (APS). b) Wear scar of the uncoated specimen (REF). Mo-based solid lubricant coatings deposited by APS as a PVD alternative: Mechanical and tribological performance,” Tribology Transactions, just-accepted, pp. 1-18, 2025, doi: 10.1080/ 10402004.2025.2519334 [4] M. Marian, D. Berman, A. Rota, R. L. Jackson, and A. Rosenkranz, “Layered 2d nanomaterials to tailor friction and wear in machine elements - a review,” Advanced Materials Interfaces, vol. 9, no. 3, p. 2101622, 2022, doi: 10.1002/ admi.202101622 [5] B.-A. Behrens, G. Poll, K. Möhwald, S. Schöler, F. Pape, D. Konopka, K. Brunotte, H. Wester, S. Richter, and N. Heimes, “Characterization and modeling of nano wear for molybdenum-based lubrication layer systems,” Nanomaterials, vol. 11, no. 6, p. 1363, 2021, doi: 10.3390/ nano11061363 [6] N. Heimes, F. Pape, G. Poll, D. Konopka, S. Schöler, K. Möhwald, and B.-A. Behrens, “Characterisation of selfregenerative dry lubricated layers on Mo-basis by nano mechanical testing,” in Production at the leading edge of technology: Proceedings of the 9 th Congress of the German Academic Association for Production Technology (WGP), September 30th-October 2nd, Hamburg 2019. Springer, 2019, pp. 139-148, doi: 10.1007/ 978-3-662-60417-5_14 [7] S. Schöler, M. Schmieding, N. Heimes, F. Pape, B.-A. Behrens, G. Poll, and K. Möhwald, “Characterization of molybdenum based coatings on 100Cr6 bearing steel surfaces,” Tribology Online, vol. 15, no. 3, pp. 181-185, 2020, doi: 10.2474/ trol.15.181 [8] D. Konopka, F. Pape, N. Heimes, B.-A. Behrens, K. Möhwald, and G. Poll, “Functionality investigations of drylubricated molybdenum trioxide cylindrical roller thrust bearings,” Coatings, vol. 12, no. 5, p. 591, 2022, doi: 10.3390/ coatings12050591 [9] Maier, Hans, Kai Möhwald, Gerhard Poll, and Florian Pape. “Dry Lubrication of Rolling Bearings.” DE Patent 102023101922A1, filed August 2024. [10] T. Coors, F. Pape, and G. Poll, “Comparing the influence of residual stresses in bearing fatigue life at line and point contact,” Residual Stresses 2018: ECRS-10, vol. 6, p. 215, 2018, doi: 10.21741/ 9781945291890-34 [11] B. Hwang, J. Ahn, and S. Lee, “Effects of blending elements on wear resistance of plasma-sprayed molybdenum blend coatings used for automotive synchronizer rings,” Surface and Coatings Technology, vol. 194, no. 2-3, pp. 256-264, 2005, doi: 10.1016/ j.surfcoat.2004.07.072 [12] S. Manjunatha and S. Basavarajappa, “Effect of powder particle size on wear resistance of plasma sprayed molybdenum coating,” Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, vol. 228, no. 7, pp. 789-796, 2014, doi: 10.1177/ 1350650114531744 [13] I. K. Piltaver, I. J. Badovinac, R. Peter, I. Saric, and M. Petravic, “Modification of molybdenum surface by lowenergy oxygen implantation at room temperature,” Applied surface science, vol. 425, pp. 416-422, 2017, doi: 10.1016/ j.apsusc.2017.07.029 [14] J. Khedkar, A. Khanna, and K. Gupt, “Tribological behaviour of plasma and laser coated steels,” Wear, vol. 205, no. 1-2, pp. 220-227, 1997, doi: 10.1016/ S0043-1648(96)07291-2 Science and Research 58 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0020 of the wear on the load, as confirmed by [11, 12]. The lower wear of the APS-coated specimen is justified by the formation of a tribofilm composed of molybdenum oxides. These oxides are formed in the presence of ambient oxygen near the wear scar and the higher temperature generated by the friction energy, which would allow for better conditions for the oxides to form [13]. These oxides, when formed, can serve as a lubricant due to their high hardness and 2D layered structure that provides low-shear films and thus lower friction [14]. It is postulated that when these oxides are removed from the surface, new ones are formed, self-regenerating the surface with the lower friction material. Conclusions and Outlook Molybdenum coatings applied using APS show good wear-resistant properties and a lower friction coefficient than the uncoated bearing steel, when a hard and inert counter-body is slid on their surface. This coating is a good candidate as a technology that can be used in highly loaded contacts and to enhance the tribological behavior of rolling element bearings under dry lubrication, even when hard counter-bodies are used. Although it is more common for steel spheres to be used, these conditions can be seen in hybrid rolling element bearings when the fluid lubrication system fails and no lubricant is available to separate the surfaces, as well as in conditions where fluid lubrication is not suitable due to extreme environmental conditions. Given that the amount of molybdenum necessary to apply the coating is small and the time to prepare and apply the APS coating is also short when compared with other coating production technologies (PVD, for instance), this coating should be evaluated as a technology to be used in bearing raceways. In future work, the behavior of this coating technology shall be evaluated under various ambient conditions to better understand the dependence of oxide formation with temperature, humidity, and pressure. Tests with rolling element bearings featuring molybdenum coated surfaces will help assess the viability of the coating and the method of its application. References [1] E. Omrani, P. K. Rohatgi, and P. L. Menezes, Tribology and applications of self-lubricating materials. CRC Press, 2017. [2] V. Kumar and B. Kandasubramanian, “Processing and design methodologies for advanced and novel thermal barrier coatings for engineering applications,” Particuology, vol. 27, pp. 1-28, 2016, doi: 10.1016/ j.partic.2016.01.007 [3] D. Konopka, R. Crespo Martins, M. Dukat, F. Pape, K. Möhwald, G. Poll, and M. Marian, “Self-regenerative