Effect of Contact Geometry on MoS 2 -based Dry Film Lubricants

Effect of Contact Geometry on MoS 2 -based Dry Film Lubricants | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of Contact Geometry on MoS 2 -based Dry Film Lubricants Samuel Leventini, Ashlie Martini This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8139383/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 13 Mar, 2026 Read the published version in Tribology Letters → Version 1 posted 11 You are reading this latest preprint version Abstract MoS₂-based dry film lubricants are widely used in aerospace mechanisms that have varied contact geometries. Standard tribotests for dry film lubricants simplify real geometries as either point or line contact to facilitate comparison of different materials or conditions. However, it remains unclear whether the results of a tribotest with one contact geometry can be generalized to tests or mechanisms with different contact geometries. To assess the effect of contact geometry, we measured friction and wear of three MoS₂-based dry film lubricants using block-on-ring and pin-on-disk tests with matched initial Hertzian contact pressure and linear velocity. Friction and wear magnitudes, as well as comparative trends among the dry film lubricants, differed between the two contact geometries. These findings encourage future studies to consider more than just point contact tribotests when evaluating MoS₂-based dry film lubricants. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Introduction Molybdenum disulfide (MoS 2 ) dry film lubricants (DFLs) are widely used for aerospace applications due to their superior tribological performance across a range of environmental and operating conditions. 1 – 9 Before deployment to space, however, DFLs are tested here on Earth. Such tribotests are used to measure friction coefficient, wear rate, and wear life of DFLs. Tribotests often have simple contact geometries to generate predictable Hertzian pressure distributions and ensure reproducible measurements. This approach enables direct comparison of different materials or conditions and facilitates interpretation of results. The two common contact geometries of tribotests are point and line contact. Most published studies that examine the tribological properties of MoS 2 -based DFLs use point contact-based tests. 1 – 4 Point contacts are typically achieved through pin-on-disc (POD) testing. In pin-on-disc tests, point contact is generated between the rounded end of the testing pin (or ball) against a flat surface, often a disc. This test set up can be roughly correlated with the contact between a ball and race in a ball bearing. Fewer studies exist on the properties of MoS 2 -based DFLs under line contact. 10 – 16 A common tribotest with line contact is the block-on-ring (BOR). This tribotest involves a flat block pressed against with a rotating ring. This geometry roughly approximates the line contacts formed in components such as roller bearings or meshing of gear teeth. Only a few previous studies have characterized DFLs in both point and line contact. One study examined the tribological properties of tungsten-doped hydrogenated diamond-like carbon (DLC), thin dense chrome, and polytetrafluoroethylene coatings in air with POD and BOR testing. 17 They found that the friction coefficient trends between coatings were the same in both POD and BOR and that in both tests the coating with the lowest friction coefficient also had the lowest wear. 17 However, in the POD tests, no wear was detected on the uncoated pin which was attributed to the lubricity provided by the coating and the higher hardness of the pin while, in the BOR tests, wear was observed on the uncoated block in severe testing conditions. Another study examined a sprayed-on MoS 2 coating using ball-on-coated flat and cylinder-on-coated flat tests in air and showed that the friction coefficient was lower for point contact tests than line contact. 18 The study also reported that the coating wear life was shorter in line contact than point contact. An investigation of MoS 2 -based DFLs from different manufacturers used POD and BOR to measure the friction coefficient and wear life. 19 In POD tests run in air, the Ti-doped MoS 2 DFLs had the longest wear life, but the lowest steady state friction coefficient was exhibited by undoped MoS 2 DFLs. 19 In the BOR tests run in dry nitrogen, the Sb 2 O 3 -doped MoS 2 DFLs exhibited the longest wear life, whereas the Al-doped MoS 2 DFLs had the shortest wear life. The lowest friction coefficient in dry nitrogen was exhibited by two undoped MoS 2 DFLs while the Al-doped MoS 2 DFLs exhibited the highest friction coefficient. 19 Lastly, a study reported that Ti-doped MoS 2 DFLs exhibited superior wear properties compared to chemical vapor deposited DLC, plasma sprayed Cu-Ni-In coatings and bare Ti-6Al-4V substrates in both POD and BOR tests. 20 Wear of the Ti-doped MoS 2 DFLs was difficult to quantify after the POD tests due to lack of visible wear while, in the BOR tests, wear volume was easily measured. The friction coefficient in POD tests was reported to be lowest for the Ti-doped MoS 2 DFLs compared to the other samples, but friction from the BOR tests was not reported. Thus, there are very few studies that have compared point and line contact tribotests and only three for MoS₂-based DFLs. 18–20 These studies show that both friction and wear trends may be the same or different between the two contact geometries, depending on the coatings and test conditions. This is important because it implies that the results of a tribotest with one contact geometry may not be useable for design of a lubricated mechanical component with a different contact geometry. One key limitation of the previous studies was that the Hertzian contact pressure and/or linear speed was not the same between the two different test set ups, which inhibited comparisons between them. To address this gap, we tested MoS 2 -based DFLs using POD for point contact and BOR for line contact. Tests were run under ambient air, room temperature conditions at the same initial Hertzian contact pressure and linear speed. We measured friction coefficient and wear from both tests. We then compared the trends between coatings within each contact type and compared point contact and line contact trends to each other. Materials and Methods Point contact testing was performed using a pin-on-disc setup. This setup involved an uncoated ball pressed against a coated disc with a fixed normal load. The POD setup is shown in Fig. 1 a. Line contact testing was performed using a block-on-ring setup, shown in Fig. 1 b. The BOR pressed an uncoated block against a coated ring at fixed load. Both testing modules were attached to an MFT 5000 RTEC tribometer. Tests were conducted at room temperature under ambient air, with relative humidity (RH) ranging from 25–45%. Normal loads were 2 N for point contact and 1,105 N for line contact. These loads resulted in a Hertzian contact pressure of 568 MPa for both testing configuration. Both tests used reciprocating motion. Tests were conducted at a linear velocity of 4 mm/s, corresponding to a rotational velocity of 2.18 RPM for the BOR. The total testing cycles were 52, 350, and 700. Some of the tests were run directly to 700 cycles with a 10 mm stroke length. Additional tests were then run using a stripe test approach where the stroke length decreased from 10 mm to 7.5 mm and then 5 mm, after 52, 350, and 700 cycles, respectively. Friction results from the direct 700 cycle tests were compared to those from the stripe tests after 700 cycles and the results were consistent. Each test was repeated 3 times. The DFLs were deposited onto 52100 steel 50.5 x 6.5 mm discs and 35 mm x 8.65 mm rings. Before deposition, both discs and rings exhibited an average roughness between 0.3 to 0.45 µm. Rings were tested as fabricated, and the discs were polished to have the same roughness. Both rings and discs had a Rockwell Hardness of 24C. The counterbodies for the point and line contact tests were 9.525 mm diameter balls and 15.7 × 6.35 × 10.2 mm blocks, respectively. Both were made of 440C stainless steel and had an average surface roughness of 0.28 to 0.3 µm. Balls were tested as fabricated. Blocks were polished to match the ball roughness, and heat treated to achieve similar hardness of Rockwell 60C. The MoS 2 -based coatings tests were Esnalube 382, Everlube 620C, and Everlube 811. These are commercially available coatings. Esnalube 382 used MoS 2 as a lubricating pigment, a silicate binder and water as a carrier. Everlube 620C used MoS 2 as a lubricating pigment, a phenolic epoxy binder, and solvent as a carrier. Everlube 811 used MoS 2 and graphite as a lubricating pigment, used a silicate binder, and used water as a carrier. Esnalube 382 and Everlube 811 had a minimum film thickness of five µm, while Everlube 620C had a minimum thickness of eight µm. All coatings had a maximum film thickness of thirteen µm. Application specs were AS1701F Class VI, AS5272 Type 1, and MIL-PRF-81329E for Esnalube 382, Everlube 620C, and Everlube 811, respectively. Representative photos of the coated rings and disks are shown in Fig. 2 . Before tribotesting, the average line roughness of the unworn portions of the DFL coated discs and rings was measured using a Bruker Dektak contact profilometer. Cutoff lengths following ASME B46.1 were used to obtain accurate roughness measurement. Three measurements were taken per coated substrate for each coating. The friction coefficient traces from each test were recorded and exported after testing for analysis. The friction coefficient per cycle was processed to account for any directional bias and exclude the static friction that occurred when the sliding direction changed. 21 Wear of the DFL for the point and line contact tests was measured using line profiles obtained from the Bruker Dektak profilometer. Three line scans were performed on the wear scar from each test, examining the left edge, center, and right edge. For point contact tests, the wear scar width was also measured using white light interferometer attached to the tribometer, and a Leica optical microscope at 2.5x or 10x magnification. Results Prior to testing, the average roughness of unworn discs and rings was measured using contact mode profilometry. The results are shown in Fig. 3 . The average roughness values for Esnalube 382 and Everlube 620C were found to be statistically similar for both the discs and rings. Everlube 811 was found to be significantly rougher than the other two coatings for both the discs and rings. The friction coefficient traces for the 700 cycle tests are shown in Fig. 4 . In the POD tests, all the coatings exhibited an increase in friction coefficient during the initial testing cycles. The rate of increase slowed with cycles and most POD tests reached steady state by around 300 cycles. This was not seen in most of the BOR tests where, instead, the friction coefficient dropped in the first few cycles and then stayed around a constant value for the remainder of the test. The friction coefficient for Esnalube 382 in POD exhibited the most instability among the tests run. Everlube 620C showed unstable friction in both the POD and BOR tests. The friction coefficient for Everlube 811 was the most stable among the coatings in either test. The friction coefficient was averaged over the three 700 cycle tests per coating per contact configuration, and the results are shown in Fig. 5 . In the POD tests, Esnalube 382 exhibited the highest average friction coefficient, followed by Everlube 620C and Everlube 811. Everlube 620C and Everlube 811 had statistically similar average friction coefficients. In BOR testing, Everlube 620C exhibited higher average COF than Esnalube 382 and Everlube 811. Comparing the two contact geometries, the friction coefficient was higher in POD compared to BOR for all DFLs. Initially, all wear scars from the 700 cycle tests were measured using contact mode profilometry. Line profiles obtained from profilometry were exported to be linearly fit for wear scars on both discs and rings. Representative wear tracks for both discs and rings are shown in Fig. 6 . This analysis revealed a limitation of contact mode profilometry for measuring wear in the POD tests. Particularly, as shown in Fig. 6 a-c, the depth of the wear scars was comparable to the roughness of the coating such that the dimensions of the wear scar could not be determined unambiguously. This issue was most severe for Everlube 811 which had the roughest pre-test surface. Further, the results in Fig. 6 are from the 700 cycle tests for which the wear scars were deepest. For the 350 and 52 cycle tests, the POD wear scars were often unidentifiable relative to the roughness of the coatings. To overcome this limitation, we considered other approaches for measuring POD wear, specifically white light interferometry and optical microscopy. We measured the wear scar widths for Esnalube 382 from tests run to 52, 350, and 700 cycles using each measurement instrument. The results are shown in Fig. 7 . For 350 and 700 cycles, the largest width was obtained with the interferometer. This trend was not observed at 52 cycles, but the wear scars from these tests were smallest and defining the extents of the wear tracks was the most ambiguous. Importantly, for all three measurement methods, the wear width increased with number of sliding cycles and the magnitudes were roughly comparable. Therefore, we decided to use optical microscopy, as it provided consistent results with an efficient measurement approach. The wear scars for each coating from 52, 350, and 700 cycle POD tests were measured using optical microscopy. The average wear scar width measurements from the three tests at each testing cycle for each coating are plotted in Fig. 8 . An inset in the top left shows a representative microscope image with the wear track identified for Esnalube 382. Everlube 811 exhibited the largest average wear scar at any number of cycles. Esnalube 382 and Everlube 620C exhibited statistically similar wear scar diameters at 52 and 350 cycles while, at 700 cycles, Everlube 620C had a smaller average wear scar diameter than Esnalube 382. For all three coatings, the wear scar width increased approximately linearly with number of cycles. Assuming Archard’s wear law, this gives wear rates of 0.0083, 0.0035, and 0.0122 mm/N*m for Esnalube 382, Everlube 620C, and Everlube 811, respectively. In contrast to the wear scars from POD, the BOR wear scars were distinct and readily measurable using contact mode profilometry (Fig. 6 ). Since the width of the wear scars were the same for all tests due to the contact geometry, here we quantified wear based on depth. However, it was found that, in some cases, the wear depth was greater than the thickness of the DFLs, indicating that the coating had completely been removed. Wear beyond this point corresponded to material removed from the substrate of the ring. In these cases, the reported wear depth was set to the average film thickness of the coating. The results in Fig. 9 show that the coating was completely worn away for Esnalube 382 and Everlube 811 even after only 52 cycles. In the case of Everlube 620C, the wear depth did not increase significantly with an increase in testing cycle and was lower overall than the other two tested coatings. Discussion We first compared the three DFLs to each other based on results from both contact geometry tests. In terms of friction, as shown in Fig. 5 , the average friction coefficient was highest for Esnalube 382 when measured using POD but was highest for Everlube 620C when measured using BOR. In both POD and BOR, Everlube 811 had the lowest or tied for lowest friction coefficient. This good performance could be explained by the fact that Everlube 811 contains graphite which is known to provide low friction in air conditions. 1 , 22 Regardless, the difference between the DFLs in terms of friction was relatively small and all DFLs three exhibited lower friction than for steel-on-steel contact, indicating they all provided effective lubrication in both contact geometries. The DFLs differed from each other more significantly in terms of wear. In POD testing, wear depth was small relative to the coating thickness for all three coatings, as shown in Fig. 6 . However, the comparison of the wear scar widths measured from optical microscope in Fig. 8 showed that Everlube 811 exhibited the most wear. This may be because it was the roughest coating (Fig. 3 ) and rougher coatings have been reported to exhibit higher wear. 23 In BOR testing, the coatings were removed completely for Esnalube 382 and Everlube 811 but remained relatively low at any cycle for Everlube 620C, as shown in Fig. 9 . The superior performance of Everlube 620C may be attributable partially to the binder used for this DFL which was a phenolic epoxy as compared to the silicate binder of the other two coatings. It has been reported that the binder used in DFLs can affect the tribological performance, with phenolic epoxy and epoxy resins being used in the majority of bonded solid lubricants. 1 , 24 – 26 DFLs using silicate binders have been reported to be softer and to provide less adhesion to the substrate than DFLs using phenolic epoxy binders. 1 , 26 Everlube 620C also had the largest maximum film thickness, which may have contributed to its better wear performance. 23 Next, we evaluated the effect of test geometry by comparing results from POD and BOR. For all three coatings, the friction coefficient was higher in POD than BOR, as shown in Fig. 5 . This trend is opposite to that observed in a previous study that compared sprayed-on MoS 2 coatings using ball-on-coated flat and cylinder-on-coated flat tests. 18 However, in that study, the flat was coated in the line contact tests while the ring was coated in our study. Another study reported that the relative magnitude of friction measured using POD and BOR was a function of both the applied load and coating roughness, although the Hertzian pressure was not matched in the two tests. 17 For tests with thin dense chrome coatings, which had roughness similar to our MoS 2 -based DFLs, the friction coefficient was comparable in 1 GPa POD and 56 MPa BOR tests, but the friction coefficient was much higher in 1 GPa POD tests than in 156 MPa BOR tests. 17 Wear results from our point and line contact tests cannot be quantitatively compared since we reported width from optical microscope for POD and depth from profilometer for BOR. However, wear depths for POD can be approximated from the profilometry data in Fig. 6 and were less than 0.01 in all cases, which is an order of magnitude lower than the wear depths for BOR in Fig. 9 . This trend is consistent with previous studies. In Ref 17 , no wear was detected on the uncoated pin in POD tests under any conditions while, in BOR tests, wear was observed on the uncoated block in severe testing conditions. In Ref. 18 , coating wear life was consistently shorter in line contact than point contact, indicating a faster wear rate in line contact. The difference was attributed to the larger contact area in line contact that promotes water bonding to the surface more than in point contact. Water absorbing to the surface of the DFL is detrimental as water inhibits the easy-shear properties of MoS 2 2,18,27–29 Lastly, in Ref. 20 , wear of the Ti-doped MoS 2 DFLs was difficult to quantify in POD tests due to lack of visible wear while, in the BOR tests, wear volume was easily measured which is, again, consistent with our results. Lastly, we considered the observation from our tests that friction was higher in POD vs. BOR, while wear was lower for POD vs. BOR. This opposite trend for friction and wear has been reported in previous studies with POD testing where the DFL with the lowest average COF exhibited the highest coating volume wear or wear rate. 17 , 30 This opposite trend can be explained in terms of the fundamental lubrication mechanism of MoS 2 -based DFL coatings. Particularly, during sliding, MoS 2 layers are oriented in the direction of shear and then transferred from the DFL to the uncoated counterbody, creating what is referred to as a transfer film. 16 , 31 – 34 The formation of the basally oriented lamella and the transfer film enable the easy shear ability of MoS 2 . 35 Therefore, faster material transferred from the DFL, i.e., more wear, may facilitate sliding and lead to lower friction. We evaluated this mechanism with qualitative analysis of the countersurfaces after testing, shown in Fig. 10 . In these images, there is evidence of transfer films in both POD and BOR but there is clearly much more material transferred on the block than the ball. Therefore, in BOR testing, the severe DFL wear led to formation of significant transfer film, which then facilitated low friction. In contrast, in POD testing, there was less wear and very little transfer film such that the friction was higher. Conclusions We evaluated friction and wear of three commercial MoS₂-based dry film lubricants using POD (point contact) and BOR (line contact) tests that had the same initial Hertzian contact pressure and linear velocity. For all three coatings, the average coefficient of friction was higher in point contact than in line contact. In terms of wear, BOR testing resulted in substantially more wear (in many cases complete coating loss) than POD for all coatings. The relative performance of the coatings also changed with geometry. Everlube 620C exhibited superior wear resistance in BOR, consistent with its phenolic epoxy binder and greater film thickness, while Everlube 811 produced the largest wear in POD, likely due to its higher initial roughness. The opposing trends of friction and wear were rationalized by transfer film formation. Specifically, severe wear in BOR promoted extensive transfer films that lowered friction, whereas limited transfer-film formation in POD corresponded with higher friction despite lower wear. From a practical standpoint, the results demonstrate that reliance on a single tribotest geometry, especially widely used point-contact tests, can be problematic since coating performance depends on contact geometry. Selection and specification of MoS₂-based DFLs for aerospace components should therefore include tribotesting representative of the target contact type. Future studies could be performed to more fully characterize differences in DFL performance in point and line contact tests. Particularly, since the tests here were conducted in ambient air at room temperature, line and point contact should be compared in conditions with varied humidity and temperature, as well as different counterbody materials that may alter transfer-film formation. Variation of surface roughness, film thickness, binder chemistry, and loading conditions will further help decouple the mechanisms that drive geometry-dependent friction and wear. Declarations Funding This work was supported by Blue Origin. Competing Interests The authors have no relevant financial or non-financial interests to disclose. Author Contribution S.L. performed the experiments and data analysis for testing conducted in this project. S.L. and A.M. wrote the main manuscript text. S.L. prepared the figures for the manuscript. All authors reviewed the manuscript. Acknowledgement We would like to acknowledge Brian Dykas and his team at Blue Origin for providing the samples used in this study and their collaboration on this project. Data Availability All data used for this manuscript is shared in the text and figures. The raw data files generated from the study are available from the authors upon reasonable request. References Lince, J.R.: Effective Application of Solid Lubricants in Spacecraft Mechanisms. Lubricants. 8 , 74 (2020) Faiyad, A., Miliate, D., Leventini, S., Lince, J.R., Martini, A.: Temperature–dependent friction, wear, and life of MoS₂ dry film lubricants for space mechanisms: A comprehensive review. Tribol Lett. 73 , 114 (2025) Vazirisereshk, M.R., Martini, A., Strubbe, D.A.: Baykara, M. Z. Solid Lubrication with MoS2: A Review. Lubricants. 7 , 57 (2019) Scharf, T.W., Prasad, S.V.: Solid lubricants: a review. J. Mater. 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Supplementary Files ToCGraphicFinal.png Cite Share Download PDF Status: Published Journal Publication published 13 Mar, 2026 Read the published version in Tribology Letters → Version 1 posted Editorial decision: Revision requested 22 Jan, 2026 Reviews received at journal 22 Jan, 2026 Reviews received at journal 03 Jan, 2026 Reviews received at journal 02 Jan, 2026 Reviewers agreed at journal 29 Dec, 2025 Reviewers agreed at journal 25 Dec, 2025 Reviewers agreed at journal 04 Dec, 2025 Reviewers invited by journal 01 Dec, 2025 Editor assigned by journal 19 Nov, 2025 Submission checks completed at journal 19 Nov, 2025 First submitted to journal 17 Nov, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8139383","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":553571403,"identity":"6065650c-cca9-4112-913f-fe16b0db48a1","order_by":0,"name":"Samuel Leventini","email":"","orcid":"","institution":"University of California Merced","correspondingAuthor":false,"prefix":"","firstName":"Samuel","middleName":"","lastName":"Leventini","suffix":""},{"id":553571405,"identity":"f2c3142e-7157-465f-9c61-d53459af651f","order_by":1,"name":"Ashlie 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10:50:38","extension":"png","order_by":22,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":68057,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/c4cdfd1148e458ff6313634a.png"},{"id":97339050,"identity":"c44b6dde-bbed-4615-9be2-d0462d4a5fbb","added_by":"auto","created_at":"2025-12-03 10:50:38","extension":"png","order_by":23,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":56190,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/e35fcf797202f19d542fba67.png"},{"id":97339048,"identity":"46099932-ee70-448e-b597-0f78e3a68295","added_by":"auto","created_at":"2025-12-03 10:50:38","extension":"xml","order_by":24,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":78839,"visible":true,"origin":"","legend":"","description":"","filename":"9c3a5290f9b845b1b81ccbbf3f92afdf1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/d6bcd09d3b294f743e900188.xml"},{"id":97339058,"identity":"e6bc5a69-bf4e-4866-87b0-ba003e1ab9db","added_by":"auto","created_at":"2025-12-03 10:50:39","extension":"html","order_by":25,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":86568,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/1e03b7470200d4cb48468fd7.html"},{"id":97339035,"identity":"d0708cf2-e85d-4bdd-9bc9-2d8adb30d150","added_by":"auto","created_at":"2025-12-03 10:50:38","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":469554,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Point contact testing configuration using pin on disc (POD) test setup, and (b) line contact testing configuration using block on ring (BOR) test setup. In the point contact test, an uncoated ball is pressed down on a MoS\u003csub\u003e2\u003c/sub\u003e-coated disc that moves in linear reciprocating motion. In the line contact test, an uncoated block is pressed down on a ring that rotates in an oscillatory motion.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/3d78d1225d663838436613c8.png"},{"id":97370839,"identity":"656ef76d-3d21-43da-ad16-db914a301702","added_by":"auto","created_at":"2025-12-03 16:27:59","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":450926,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative images of the DFL-coated (a) rings and (b) discs. The font color for the names of the coatings will be used throughout the manuscript: Red for Esnalube 382, blue for Everlube 620C, and green for Everlube 811.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/2d27df2029d88ff5ed0e3e1b.png"},{"id":97370248,"identity":"0e8c7ec4-4ad5-43b9-9180-0c6ef2ef438e","added_by":"auto","created_at":"2025-12-03 16:26:59","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":113730,"visible":true,"origin":"","legend":"\u003cp\u003eCoated substrate measured roughness for the (a) discs and (b) rings. The average roughness of the Esnalube 382 and Everlube 620C samples are similar for both ring and disk sample. However, the Esnalube 382 coated rings and disks are rougher than the samples with the other two coatings. Error bars were set as the maximum and minimum values of the average roughness values measured from the coated samples.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/354db6d45fe25724916cc420.jpeg"},{"id":97339034,"identity":"922a9573-80a3-44c9-868c-88269abf29c8","added_by":"auto","created_at":"2025-12-03 10:50:38","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":281294,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient traces from the 700 cycle tests for each coating measured using (a-c) POD and (d-f) BOR for each coating: (a, d) red for Esnalube 382, (b, e) blue for Everlube 620C, and (c, f) green for Everlube 811. Results are shown for three tests per coating and contact geometry.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/cb226350443a40d1e5375f3a.png"},{"id":97370355,"identity":"6ee77129-83d0-4a0f-a517-517b08b26128","added_by":"auto","created_at":"2025-12-03 16:27:11","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":176370,"visible":true,"origin":"","legend":"\u003cp\u003eCoefficient of friction (COF) averaged over the three 700 tests for each coating for (a)POD and (b) BOR testing. Error bars were set as the maximum and minimum of the average COF values for each coating in each testing configuration.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/75a266d36c329c9981e099ed.png"},{"id":97370886,"identity":"51c85d51-ec21-4da6-a8ec-1501833c9dc3","added_by":"auto","created_at":"2025-12-03 16:28:06","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":325526,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative images of coated samples after testing and corresponding surface topography profiles of the wear scars from point and line contact testing at 700 cycles. Wear profiles were recorded using contact mode profilometry for both (a-c) discs and (d-f) rings for each coating: (a, d) red for Esnalube 382, (b, e) blue for Everlube 620C, and (c, f) green for Everlube 811. Note the axis scales differ between the POD and BOR results.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/06e7277f67216817f15d9536.png"},{"id":97339038,"identity":"a7eff0e2-4251-4ba8-a253-733373899a51","added_by":"auto","created_at":"2025-12-03 10:50:38","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":81479,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of methods to analyze the wear scar width for POD testing of Esnalube 382. Error bars reflect the maximum and minimum values of the measurements from three tests per coating at each total cycle number. The trend of increasing wear with number of cycles was captured by all three methods but optical microscopy was determined to be the most consistent and reliable method.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/db9e63b2422f266d0a60be66.png"},{"id":97339041,"identity":"2d4ef24d-0781-4325-8004-d27df436db76","added_by":"auto","created_at":"2025-12-03 10:50:38","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":383908,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eAverage wear scar width on the disk for all three coatings at each POD testing cycle measured using optical microscopy. The error bars are the maximum and minimum of the average wear scar width for each coating and cycle number. A representative optical microscope image of a 700-cycle wear scar for the Esnalube 382 coating is shown as an inset, where dashed red lines represent the edges of the wear track used for measuring the wear scar width.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/778c999306ab85793c09abe8.png"},{"id":97339043,"identity":"d34b20d8-95ca-46c6-b8a3-2f5ac063425c","added_by":"auto","created_at":"2025-12-03 10:50:38","extension":"jpeg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":235206,"visible":true,"origin":"","legend":"\u003cp\u003eWear depth on the ring from BOR testing cycles measured using contact mode profilometry. If the depth of the wear scar was found to be larger than the coating thickness, the wear depth was set as the average film thickness of the coating. The error bars are the maximum and minimum values of the average wear depths for Everlube 620C. The inset is a representative wear scar cross section for Esnalube 382 for which the wear depth is greater than the DFL thickness indicated by a dashed horizontal line.\u003c/p\u003e","description":"","filename":"floatimage9.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/9d0a233d92c4e6d599d421cf.jpeg"},{"id":97369688,"identity":"f68279d0-853c-480e-ad60-53a4ac351a25","added_by":"auto","created_at":"2025-12-03 16:25:33","extension":"jpeg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":236071,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative images of transfer films on the initially uncoated countersurfaces after 700 cycles: (a) optical microscope image of the ball after POD tests, and (b) photos of the block after BOR tests.\u003c/p\u003e","description":"","filename":"floatimage10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/42b15063465bdfa61420f064.jpeg"},{"id":104739381,"identity":"f69a0bb0-2b2f-458f-bdcb-bb60c70f862d","added_by":"auto","created_at":"2026-03-16 16:05:18","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3238434,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/77203312-ffa0-49af-884c-780815120f6b.pdf"},{"id":97371306,"identity":"d0771b34-b201-4031-91d2-9bed6bcbfc55","added_by":"auto","created_at":"2025-12-03 16:28:42","extension":"png","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":279165,"visible":true,"origin":"","legend":"","description":"","filename":"ToCGraphicFinal.png","url":"https://assets-eu.researchsquare.com/files/rs-8139383/v1/45e5a3488772337811075a72.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of Contact Geometry on MoS 2 -based Dry Film Lubricants","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMolybdenum disulfide (MoS\u003csub\u003e2\u003c/sub\u003e) dry film lubricants (DFLs) are widely used for aerospace applications due to their superior tribological performance across a range of environmental and operating conditions.\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5 CR6 CR7 CR8\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e Before deployment to space, however, DFLs are tested here on Earth. Such tribotests are used to measure friction coefficient, wear rate, and wear life of DFLs. Tribotests often have simple contact geometries to generate predictable Hertzian pressure distributions and ensure reproducible measurements. This approach enables direct comparison of different materials or conditions and facilitates interpretation of results. The two common contact geometries of tribotests are point and line contact.\u003c/p\u003e\u003cp\u003eMost published studies that examine the tribological properties of MoS\u003csub\u003e2\u003c/sub\u003e-based DFLs use point contact-based tests.\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e Point contacts are typically achieved through pin-on-disc (POD) testing. In pin-on-disc tests, point contact is generated between the rounded end of the testing pin (or ball) against a flat surface, often a disc. This test set up can be roughly correlated with the contact between a ball and race in a ball bearing. Fewer studies exist on the properties of MoS\u003csub\u003e2\u003c/sub\u003e-based DFLs under line contact.\u003csup\u003e\u003cspan additionalcitationids=\"CR11 CR12 CR13 CR14 CR15\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e A common tribotest with line contact is the block-on-ring (BOR). This tribotest involves a flat block pressed against with a rotating ring. This geometry roughly approximates the line contacts formed in components such as roller bearings or meshing of gear teeth.\u003c/p\u003e\u003cp\u003eOnly a few previous studies have characterized DFLs in both point and line contact. One study examined the tribological properties of tungsten-doped hydrogenated diamond-like carbon (DLC), thin dense chrome, and polytetrafluoroethylene coatings in air with POD and BOR testing.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e They found that the friction coefficient trends between coatings were the same in both POD and BOR and that in both tests the coating with the lowest friction coefficient also had the lowest wear.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e However, in the POD tests, no wear was detected on the uncoated pin which was attributed to the lubricity provided by the coating and the higher hardness of the pin while, in the BOR tests, wear was observed on the uncoated block in severe testing conditions. Another study examined a sprayed-on MoS\u003csub\u003e2\u003c/sub\u003e coating using ball-on-coated flat and cylinder-on-coated flat tests in air and showed that the friction coefficient was lower for point contact tests than line contact.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e The study also reported that the coating wear life was shorter in line contact than point contact. An investigation of MoS\u003csub\u003e2\u003c/sub\u003e-based DFLs from different manufacturers used POD and BOR to measure the friction coefficient and wear life.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e In POD tests run in air, the Ti-doped MoS\u003csub\u003e2\u003c/sub\u003e DFLs had the longest wear life, but the lowest steady state friction coefficient was exhibited by undoped MoS\u003csub\u003e2\u003c/sub\u003e DFLs.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e In the BOR tests run in dry nitrogen, the Sb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-doped MoS\u003csub\u003e2\u003c/sub\u003e DFLs exhibited the longest wear life, whereas the Al-doped MoS\u003csub\u003e2\u003c/sub\u003e DFLs had the shortest wear life. The lowest friction coefficient in dry nitrogen was exhibited by two undoped MoS\u003csub\u003e2\u003c/sub\u003e DFLs while the Al-doped MoS\u003csub\u003e2\u003c/sub\u003e DFLs exhibited the highest friction coefficient.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Lastly, a study reported that Ti-doped MoS\u003csub\u003e2\u003c/sub\u003e DFLs exhibited superior wear properties compared to chemical vapor deposited DLC, plasma sprayed Cu-Ni-In coatings and bare Ti-6Al-4V substrates in both POD and BOR tests.\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e Wear of the Ti-doped MoS\u003csub\u003e2\u003c/sub\u003e DFLs was difficult to quantify after the POD tests due to lack of visible wear while, in the BOR tests, wear volume was easily measured. The friction coefficient in POD tests was reported to be lowest for the Ti-doped MoS\u003csub\u003e2\u003c/sub\u003e DFLs compared to the other samples, but friction from the BOR tests was not reported.\u003c/p\u003e\u003cp\u003eThus, there are very few studies that have compared point and line contact tribotests and only three for MoS₂-based DFLs. \u003csup\u003e18\u0026ndash;20\u003c/sup\u003e These studies show that both friction and wear trends may be the same or different between the two contact geometries, depending on the coatings and test conditions. This is important because it implies that the results of a tribotest with one contact geometry may not be useable for design of a lubricated mechanical component with a different contact geometry. One key limitation of the previous studies was that the Hertzian contact pressure and/or linear speed was not the same between the two different test set ups, which inhibited comparisons between them. To address this gap, we tested MoS\u003csub\u003e2\u003c/sub\u003e-based DFLs using POD for point contact and BOR for line contact. Tests were run under ambient air, room temperature conditions at the same initial Hertzian contact pressure and linear speed. We measured friction coefficient and wear from both tests. We then compared the trends between coatings within each contact type and compared point contact and line contact trends to each other.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cp\u003ePoint contact testing was performed using a pin-on-disc setup. This setup involved an uncoated ball pressed against a coated disc with a fixed normal load. The POD setup is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. Line contact testing was performed using a block-on-ring setup, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. The BOR pressed an uncoated block against a coated ring at fixed load. Both testing modules were attached to an MFT 5000 RTEC tribometer.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTests were conducted at room temperature under ambient air, with relative humidity (RH) ranging from 25\u0026ndash;45%. Normal loads were 2 N for point contact and 1,105 N for line contact. These loads resulted in a Hertzian contact pressure of 568 MPa for both testing configuration. Both tests used reciprocating motion. Tests were conducted at a linear velocity of 4 mm/s, corresponding to a rotational velocity of 2.18 RPM for the BOR. The total testing cycles were 52, 350, and 700. Some of the tests were run directly to 700 cycles with a 10 mm stroke length. Additional tests were then run using a stripe test approach where the stroke length decreased from 10 mm to 7.5 mm and then 5 mm, after 52, 350, and 700 cycles, respectively. Friction results from the direct 700 cycle tests were compared to those from the stripe tests after 700 cycles and the results were consistent. Each test was repeated 3 times.\u003c/p\u003e\u003cp\u003eThe DFLs were deposited onto 52100 steel 50.5 x 6.5 mm discs and 35 mm x 8.65 mm rings. Before deposition, both discs and rings exhibited an average roughness between 0.3 to 0.45 \u0026micro;m. Rings were tested as fabricated, and the discs were polished to have the same roughness. Both rings and discs had a Rockwell Hardness of 24C. The counterbodies for the point and line contact tests were 9.525 mm diameter balls and 15.7 \u0026times; 6.35 \u0026times; 10.2 mm blocks, respectively. Both were made of 440C stainless steel and had an average surface roughness of 0.28 to 0.3 \u0026micro;m. Balls were tested as fabricated. Blocks were polished to match the ball roughness, and heat treated to achieve similar hardness of Rockwell 60C.\u003c/p\u003e\u003cp\u003eThe MoS\u003csub\u003e2\u003c/sub\u003e-based coatings tests were Esnalube 382, Everlube 620C, and Everlube 811. These are commercially available coatings. Esnalube 382 used MoS\u003csub\u003e2\u003c/sub\u003e as a lubricating pigment, a silicate binder and water as a carrier. Everlube 620C used MoS\u003csub\u003e2\u003c/sub\u003e as a lubricating pigment, a phenolic epoxy binder, and solvent as a carrier. Everlube 811 used MoS\u003csub\u003e2\u003c/sub\u003e and graphite as a lubricating pigment, used a silicate binder, and used water as a carrier. Esnalube 382 and Everlube 811 had a minimum film thickness of five \u0026micro;m, while Everlube 620C had a minimum thickness of eight \u0026micro;m. All coatings had a maximum film thickness of thirteen \u0026micro;m. Application specs were AS1701F Class VI, AS5272 Type 1, and MIL-PRF-81329E for Esnalube 382, Everlube 620C, and Everlube 811, respectively. Representative photos of the coated rings and disks are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBefore tribotesting, the average line roughness of the unworn portions of the DFL coated discs and rings was measured using a Bruker Dektak contact profilometer. Cutoff lengths following ASME B46.1 were used to obtain accurate roughness measurement. Three measurements were taken per coated substrate for each coating.\u003c/p\u003e\u003cp\u003eThe friction coefficient traces from each test were recorded and exported after testing for analysis. The friction coefficient per cycle was processed to account for any directional bias and exclude the static friction that occurred when the sliding direction changed.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e Wear of the DFL for the point and line contact tests was measured using line profiles obtained from the Bruker Dektak profilometer. Three line scans were performed on the wear scar from each test, examining the left edge, center, and right edge. For point contact tests, the wear scar width was also measured using white light interferometer attached to the tribometer, and a Leica optical microscope at 2.5x or 10x magnification.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003ePrior to testing, the average roughness of unworn discs and rings was measured using contact mode profilometry. The results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The average roughness values for Esnalube 382 and Everlube 620C were found to be statistically similar for both the discs and rings. Everlube 811 was found to be significantly rougher than the other two coatings for both the discs and rings.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe friction coefficient traces for the 700 cycle tests are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. In the POD tests, all the coatings exhibited an increase in friction coefficient during the initial testing cycles. The rate of increase slowed with cycles and most POD tests reached steady state by around 300 cycles. This was not seen in most of the BOR tests where, instead, the friction coefficient dropped in the first few cycles and then stayed around a constant value for the remainder of the test. The friction coefficient for Esnalube 382 in POD exhibited the most instability among the tests run. Everlube 620C showed unstable friction in both the POD and BOR tests. The friction coefficient for Everlube 811 was the most stable among the coatings in either test.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe friction coefficient was averaged over the three 700 cycle tests per coating per contact configuration, and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. In the POD tests, Esnalube 382 exhibited the highest average friction coefficient, followed by Everlube 620C and Everlube 811. Everlube 620C and Everlube 811 had statistically similar average friction coefficients. In BOR testing, Everlube 620C exhibited higher average COF than Esnalube 382 and Everlube 811. Comparing the two contact geometries, the friction coefficient was higher in POD compared to BOR for all DFLs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eInitially, all wear scars from the 700 cycle tests were measured using contact mode profilometry. Line profiles obtained from profilometry were exported to be linearly fit for wear scars on both discs and rings. Representative wear tracks for both discs and rings are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. This analysis revealed a limitation of contact mode profilometry for measuring wear in the POD tests. Particularly, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea-c, the depth of the wear scars was comparable to the roughness of the coating such that the dimensions of the wear scar could not be determined unambiguously. This issue was most severe for Everlube 811 which had the roughest pre-test surface. Further, the results in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e are from the 700 cycle tests for which the wear scars were deepest. For the 350 and 52 cycle tests, the POD wear scars were often unidentifiable relative to the roughness of the coatings.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo overcome this limitation, we considered other approaches for measuring POD wear, specifically white light interferometry and optical microscopy. We measured the wear scar widths for Esnalube 382 from tests run to 52, 350, and 700 cycles using each measurement instrument. The results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. For 350 and 700 cycles, the largest width was obtained with the interferometer. This trend was not observed at 52 cycles, but the wear scars from these tests were smallest and defining the extents of the wear tracks was the most ambiguous. Importantly, for all three measurement methods, the wear width increased with number of sliding cycles and the magnitudes were roughly comparable. Therefore, we decided to use optical microscopy, as it provided consistent results with an efficient measurement approach.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe wear scars for each coating from 52, 350, and 700 cycle POD tests were measured using optical microscopy. The average wear scar width measurements from the three tests at each testing cycle for each coating are plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. An inset in the top left shows a representative microscope image with the wear track identified for Esnalube 382. Everlube 811 exhibited the largest average wear scar at any number of cycles. Esnalube 382 and Everlube 620C exhibited statistically similar wear scar diameters at 52 and 350 cycles while, at 700 cycles, Everlube 620C had a smaller average wear scar diameter than Esnalube 382. For all three coatings, the wear scar width increased approximately linearly with number of cycles. Assuming Archard\u0026rsquo;s wear law, this gives wear rates of 0.0083, 0.0035, and 0.0122 mm/N*m for Esnalube 382, Everlube 620C, and Everlube 811, respectively.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn contrast to the wear scars from POD, the BOR wear scars were distinct and readily measurable using contact mode profilometry (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Since the width of the wear scars were the same for all tests due to the contact geometry, here we quantified wear based on depth. However, it was found that, in some cases, the wear depth was greater than the thickness of the DFLs, indicating that the coating had completely been removed. Wear beyond this point corresponded to material removed from the substrate of the ring. In these cases, the reported wear depth was set to the average film thickness of the coating. The results in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e show that the coating was completely worn away for Esnalube 382 and Everlube 811 even after only 52 cycles. In the case of Everlube 620C, the wear depth did not increase significantly with an increase in testing cycle and was lower overall than the other two tested coatings.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eWe first compared the three DFLs to each other based on results from both contact geometry tests. In terms of friction, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the average friction coefficient was highest for Esnalube 382 when measured using POD but was highest for Everlube 620C when measured using BOR. In both POD and BOR, Everlube 811 had the lowest or tied for lowest friction coefficient. This good performance could be explained by the fact that Everlube 811 contains graphite which is known to provide low friction in air conditions.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e Regardless, the difference between the DFLs in terms of friction was relatively small and all DFLs three exhibited lower friction than for steel-on-steel contact, indicating they all provided effective lubrication in both contact geometries.\u003c/p\u003e\u003cp\u003eThe DFLs differed from each other more significantly in terms of wear. In POD testing, wear depth was small relative to the coating thickness for all three coatings, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. However, the comparison of the wear scar widths measured from optical microscope in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e showed that Everlube 811 exhibited the most wear. This may be because it was the roughest coating (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) and rougher coatings have been reported to exhibit higher wear.\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e In BOR testing, the coatings were removed completely for Esnalube 382 and Everlube 811 but remained relatively low at any cycle for Everlube 620C, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. The superior performance of Everlube 620C may be attributable partially to the binder used for this DFL which was a phenolic epoxy as compared to the silicate binder of the other two coatings. It has been reported that the binder used in DFLs can affect the tribological performance, with phenolic epoxy and epoxy resins being used in the majority of bonded solid lubricants.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e DFLs using silicate binders have been reported to be softer and to provide less adhesion to the substrate than DFLs using phenolic epoxy binders.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e Everlube 620C also had the largest maximum film thickness, which may have contributed to its better wear performance.\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eNext, we evaluated the effect of test geometry by comparing results from POD and BOR. For all three coatings, the friction coefficient was higher in POD than BOR, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. This trend is opposite to that observed in a previous study that compared sprayed-on MoS\u003csub\u003e2\u003c/sub\u003e coatings using ball-on-coated flat and cylinder-on-coated flat tests.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e However, in that study, the flat was coated in the line contact tests while the ring was coated in our study. Another study reported that the relative magnitude of friction measured using POD and BOR was a function of both the applied load and coating roughness, although the Hertzian pressure was not matched in the two tests.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e For tests with thin dense chrome coatings, which had roughness similar to our MoS\u003csub\u003e2\u003c/sub\u003e-based DFLs, the friction coefficient was comparable in 1 GPa POD and 56 MPa BOR tests, but the friction coefficient was much higher in 1 GPa POD tests than in 156 MPa BOR tests.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eWear results from our point and line contact tests cannot be quantitatively compared since we reported width from optical microscope for POD and depth from profilometer for BOR. However, wear depths for POD can be approximated from the profilometry data in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and were less than 0.01 in all cases, which is an order of magnitude lower than the wear depths for BOR in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. This trend is consistent with previous studies. In Ref \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, no wear was detected on the uncoated pin in POD tests under any conditions while, in BOR tests, wear was observed on the uncoated block in severe testing conditions. In Ref.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, coating wear life was consistently shorter in line contact than point contact, indicating a faster wear rate in line contact. The difference was attributed to the larger contact area in line contact that promotes water bonding to the surface more than in point contact. Water absorbing to the surface of the DFL is detrimental as water inhibits the easy-shear properties of MoS\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e2,18,27\u0026ndash;29\u003c/sup\u003e Lastly, in Ref.\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e, wear of the Ti-doped MoS\u003csub\u003e2\u003c/sub\u003e DFLs was difficult to quantify in POD tests due to lack of visible wear while, in the BOR tests, wear volume was easily measured which is, again, consistent with our results.\u003c/p\u003e\u003cp\u003eLastly, we considered the observation from our tests that friction was higher in POD vs. BOR, while wear was lower for POD vs. BOR. This opposite trend for friction and wear has been reported in previous studies with POD testing where the DFL with the lowest average COF exhibited the highest coating volume wear or wear rate.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e This opposite trend can be explained in terms of the fundamental lubrication mechanism of MoS\u003csub\u003e2\u003c/sub\u003e-based DFL coatings. Particularly, during sliding, MoS\u003csub\u003e2\u003c/sub\u003e layers are oriented in the direction of shear and then transferred from the DFL to the uncoated counterbody, creating what is referred to as a transfer film.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan additionalcitationids=\"CR32 CR33\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e The formation of the basally oriented lamella and the transfer film enable the easy shear ability of MoS\u003csub\u003e2\u003c/sub\u003e.\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e Therefore, faster material transferred from the DFL, i.e., more wear, may facilitate sliding and lead to lower friction. We evaluated this mechanism with qualitative analysis of the countersurfaces after testing, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. In these images, there is evidence of transfer films in both POD and BOR but there is clearly much more material transferred on the block than the ball. Therefore, in BOR testing, the severe DFL wear led to formation of significant transfer film, which then facilitated low friction. In contrast, in POD testing, there was less wear and very little transfer film such that the friction was higher.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eWe evaluated friction and wear of three commercial MoS₂-based dry film lubricants using POD (point contact) and BOR (line contact) tests that had the same initial Hertzian contact pressure and linear velocity. For all three coatings, the average coefficient of friction was higher in point contact than in line contact. In terms of wear, BOR testing resulted in substantially more wear (in many cases complete coating loss) than POD for all coatings. The relative performance of the coatings also changed with geometry. Everlube 620C exhibited superior wear resistance in BOR, consistent with its phenolic epoxy binder and greater film thickness, while Everlube 811 produced the largest wear in POD, likely due to its higher initial roughness. The opposing trends of friction and wear were rationalized by transfer film formation. Specifically, severe wear in BOR promoted extensive transfer films that lowered friction, whereas limited transfer-film formation in POD corresponded with higher friction despite lower wear.\u003c/p\u003e\u003cp\u003eFrom a practical standpoint, the results demonstrate that reliance on a single tribotest geometry, especially widely used point-contact tests, can be problematic since coating performance depends on contact geometry. Selection and specification of MoS₂-based DFLs for aerospace components should therefore include tribotesting representative of the target contact type. Future studies could be performed to more fully characterize differences in DFL performance in point and line contact tests. Particularly, since the tests here were conducted in ambient air at room temperature, line and point contact should be compared in conditions with varied humidity and temperature, as well as different counterbody materials that may alter transfer-film formation. Variation of surface roughness, film thickness, binder chemistry, and loading conditions will further help decouple the mechanisms that drive geometry-dependent friction and wear.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eThis work was supported by Blue Origin.\u003c/p\u003e\u003cp\u003eCompeting Interests\u003c/p\u003e\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eS.L. performed the experiments and data analysis for testing conducted in this project. S.L. and A.M. wrote the main manuscript text. S.L. prepared the figures for the manuscript. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe would like to acknowledge Brian Dykas and his team at Blue Origin for providing the samples used in this study and their collaboration on this project.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eAll data used for this manuscript is shared in the text and figures. The raw data files generated from the study are available from the authors upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLince, J.R.: Effective Application of Solid Lubricants in Spacecraft Mechanisms. Lubricants. \u003cb\u003e8\u003c/b\u003e, 74 (2020)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFaiyad, A., Miliate, D., Leventini, S., Lince, J.R., Martini, A.: Temperature\u0026ndash;dependent friction, wear, and life of MoS₂ dry film lubricants for space mechanisms: A comprehensive review. 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Tribol Lett. \u003cb\u003e64\u003c/b\u003e, 18 (2016)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFusaro, R.L.: Effect of Substrate Surface Finish on the Lubrication and Failure Mechanisms of Molybdenum Disulfide Films. S L E Trans. \u003cb\u003e25\u003c/b\u003e, 141\u0026ndash;156 (1982)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFayeulle, S., Ehni, P.D., Singer, I.L., Paper, V. (eds.): (ii) Role of transfer films in wear of MoS2 coatings. in Tribology Series, vol. 17, pp. 129\u0026ndash;138. Elsevier (1990)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWiner, W.O.: Molybdenum disulfide as a lubricant: A review of the fundamental knowledge. Wear. \u003cb\u003e10\u003c/b\u003e, 422\u0026ndash;452 (1967)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHoffman, E.E., Marks, L.D.: Soft interface fracture transfer in nanoscale MoS2. Tribol Lett. \u003cb\u003e64\u003c/b\u003e, (2016)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8139383/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8139383/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMoS₂-based dry film lubricants are widely used in aerospace mechanisms that have varied contact geometries. Standard tribotests for dry film lubricants simplify real geometries as either point or line contact to facilitate comparison of different materials or conditions. However, it remains unclear whether the results of a tribotest with one contact geometry can be generalized to tests or mechanisms with different contact geometries. To assess the effect of contact geometry, we measured friction and wear of three MoS₂-based dry film lubricants using block-on-ring and pin-on-disk tests with matched initial Hertzian contact pressure and linear velocity. Friction and wear magnitudes, as well as comparative trends among the dry film lubricants, differed between the two contact geometries. These findings encourage future studies to consider more than just point contact tribotests when evaluating MoS₂-based dry film lubricants.\u003c/p\u003e","manuscriptTitle":"Effect of Contact Geometry on MoS 2 -based Dry Film Lubricants","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-03 10:50:33","doi":"10.21203/rs.3.rs-8139383/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-01-22T17:44:32+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-22T08:27:45+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-03T20:58:00+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-02T05:12:26+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"229010615158779473273945636056719962286","date":"2025-12-29T13:26:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"295520786868050170761528063490814553040","date":"2025-12-25T09:01:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"213262576189997966693284240895091451161","date":"2025-12-04T23:49:18+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-01T16:20:59+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-19T05:19:11+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-19T05:17:25+00:00","index":"","fulltext":""},{"type":"submitted","content":"Tribology Letters","date":"2025-11-17T23:11:43+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"b3a0fa76-05e2-41ec-9cec-bd284efff90b","owner":[],"postedDate":"December 3rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-03-16T16:02:15+00:00","versionOfRecord":{"articleIdentity":"rs-8139383","link":"https://doi.org/10.1007/s11249-026-02129-w","journal":{"identity":"tribology-letters","isVorOnly":false,"title":"Tribology Letters"},"publishedOn":"2026-03-13 15:58:02","publishedOnDateReadable":"March 13th, 2026"},"versionCreatedAt":"2025-12-03 10:50:33","video":"","vorDoi":"10.1007/s11249-026-02129-w","vorDoiUrl":"https://doi.org/10.1007/s11249-026-02129-w","workflowStages":[]},"version":"v1","identity":"rs-8139383","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8139383","identity":"rs-8139383","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}