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Surface roughness is significant to friction and wear. However, the influence remains elusive, especially at the nanoscale. This study aimed to explore the effect of the nanoscale surface roughness on sliding friction and wear. For bearing steel tribo-pairs, the surface roughness can affect the lubrication state transition. The lower the surface roughness is, the easier it is to enter mixed lubrication and full film lubrication. Moreover, reducing the surface roughness can decrease friction and wear in mixed lubrication. At 94 mm/s sliding speed, as the surface roughness S q decreases from 769.7 nm to 21.1 nm, the average friction coefficient first remains unchanged and then decreases by 20%, while the wear rate decreases by 85%. However, further reducing the surface roughness may result in friction failure, raising friction and wear. At 10 mm sliding radius, about 20∼40 nm S q can lead to the minimal friction and wear. For the mechanism, as the surface roughness decreases, the asperity contact diminishes. Therefore, friction and wear decrease. However, when the surface roughness passes the critical point, asperities cannot fully accommodate the interfacial wear debris. Consequently, friction failure occurs. The findings may provide an enhanced understanding of the role of surface roughness in tribology. [Display omitted] • The surface roughness can affect the lubrication state transition. • Reducing the surface roughness can decrease friction and wear in mixed lubrication. • Too low surface roughness may result in friction failure, raising friction and wear. [ABSTRACT FROM AUTHOR]

The friction and wear behavior were studied for three materials typically found in industrial bearing applications: a Sn-based Babbitt, polytetrafluoroethylene (PTFE) and a polyether ether ketone (PEEK) composite. Test conditions were unidirectional sliding with a ball-on-disc geometry where the counter surface material was a 440C stainless steel ball (6.35 mm dia.). The tests were also in the presence of a lubricant, ISO46 oil, with the sample and contact submerged throughout the test. The relative effects of load and sliding velocities on the wear rates were assessed by means of regression analysis. Connections between wear and the materials mechanical properties, which were measured by instrumented indentation, were also made. The ranking of wear performance of the materials correlated best to the ratio of H/(E*)0.447, where H is the test material's hardness and E* is the reduced modulus for the contact. The power applied to the reduced modulus is that used in previous studies to estimate the oil film thickness for an isoviscous, elastic lubrication regime. • Friction and wear rates of bearing materials are measured in oil-lubricated sliding against a stainless steel ball. • The mechanical properties were assessed by instrumented indentation at different loads and loading rates. • Good correlation between the wear rates and the ratio between the materials' hardness H and reduced moduli E*0.447. • The H/E*0.447 ratio is proposed as a new means to rank the wear resistance of materials in boundary lubricated contacts. [ABSTRACT FROM AUTHOR]