Initiation of fretting fatigue scar of Ti-6Al-4V alloy dependent on the influence of its surface roughness

This paper reports on the experimental studies undertaken to detect the early stage when appears the fretting wear of the Ti-6Al-4V alloy used for the hip prostheses. Wear is a critical aspect for estimating the fretting fatigue. Studies were performed on special shape samples, in order to be able to study the influence of in contact surfaces roughness on the durability to fretting. Fretting pads, with roughness Ra of the contact surface of 0.015 μm and 0.045 μm, and Ti-6Al-4V samples with roughness of 0.045 μm, 0.075 μm and 0.19 μm, were used. Testing periods of 3 seconds, 1 minute and 5 minutes were selected to capture the moment of the fretting scar appearance, long before these initiate the eventual fretting cracking. Simultaneously with the surface fretting wear, the friction coefficient was also measured. From the in-time evolution determinations of the fretting wear, it resulted that, under the experimental conditions used, the minimum wear (Vu = 7.0 × 10-6 mm3) occurs at a certain value of the roughness (0.045 μm) and not at the minimum roughness (0.015 μm). Surprisingly, the minimum friction coefficient (μ = 0.038) does not coincide with the minimum fretting wear (Vu = 7.0 × 10-6 mm3).