The effect of residual stress and gradient nanostructure on the fretting fatigue behavior of plasma electrolytic oxidation coated Ti–6Al–4V alloy

The influence of plasma electrolytic oxidation (PEO) treatment and ultrasonic surface rolling process (USRP) pretreatment followed by PEO treatment on the fretting fatigue (FF) performance of Ti–6Al–4V alloy was investigated. PEO treatment yielded effective improvement in the FF life of the alloy at low stress levels, which was attributed to that the coating with high hardness suppressed crack initiation. The FF life of USRP-PEO-coated specimens was significantly improved and by at least a factor of 81 at a maximum alternating stress of 650 MPa compared to the uncoated ones. This resulted mainly from the fact that the high-hardness coating retarded the FF crack initiation, and the synergistic effect of the induced large deep-distributed compressive residual stress and gradient nanostructure retarded early FF crack propagation. The compressive residual stress played the dominant role in the FF life improvement. For the PEO-coated or USRP-PEO-coated specimens, the FF cracks were initiated at the interface of stick zone and slip zone in the fretting wear contact zone of the coating and then propagated through the coating and coating/substrate interface into the substrate. During fretting, the wear debris of the coating moved into the cracks and promoted early crack propagation as wedge effect.