Topography orientation effects on friction and wear in sliding DLC and steel contacts, part 3: Experiments under dry and lubricated conditions

Surface topography can affect the tribological performance in many applications. The influence of surface roughness and topographic orientation on friction and wear in dry and oil lubricated steel vs steel and diamond-like carbon (DLC) coated steel vs DLC coated steel sliding contacts was investigated. The surfaces had a centerline average roughness (Ra) between 0.004 and 0.11 μm and a topographic groove orientation of 0°, 45° and 90° with respect to the sliding direction. Tests conducted showed that the strongest effect of the orientation of the topographic directions occurs in dry sliding DLC vs DLC contacts. A super-lubricious DLC surface layer was efficiently formed, exhibiting low friction coefficients of 0.04 for smooth surfaces, and rough surfaces with grooves oriented along the sliding direction. The process of surface layer formation was most severely disturbed for average surfaces roughness with grooves oriented at 45° to the sliding direction. For these surfaces, the coefficient of friction was about five times higher (0.21) as compared to smooth samples (0.04). In dry sliding steel contacts, the coefficient of friction decreased with increasing surface roughness. On the other hand, the coefficient of friction increased with surface roughness in oil lubricated steel contacts. Adding a synthetic oil into the DLC vs DLC sliding contact prevented the formation of the super-lubricious surface layer. In tests with rotational steel vs steel sliding the friction coefficient was about 10% lower than in test with linear reciprocal sliding but was higher in DLC vs DLC contacts.