Effect of nanoscale surface roughness on sliding friction and wear in mixed lubrication

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 Sq 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 Sq 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.