Bias Voltage for Avoiding Fatigue Strength Degradation Due to TiAlN Coating on Ti-Alloy Substrates

Abstract

Fatigue strength tests of TiAlN coated Ti-alloys were carried out to investigate the coating process condition for avoiding the degradation of fatigue strength and ensuring the enhanced fatigue strength of TiAlN coated Ti-alloys. At the same applied stress amplitude, fatigue lives became shorter than the uncoated specimen with decreasing bias voltage, while they became longer than the uncoated specimen with increasing bias voltage. This phenomenon results mainly from that the compressive residual stress of the coating layer increases with increasing bias voltage. From the present results, the critical relationship between applied stress and bias voltage to give the boundary between shorter and longer lives was obtained. It was found from this relationship that at fatigue limit level, fatigue life of the TiAlN coated Ti-alloy specimen would be not degraded but enhanced compared to the uncoated specimen regardless of bias voltage. While, the higher bias voltage was required for avoiding the degraded fatigue life at stress amplitudes higher than the fatigue limit.