Durability of Cu–Al coating on Ti–6Al–4V substrate under fretting fatigue

The durability and degradation mechanisms of Cu–Al coating on Ti–6Al–4V substrate under fretting fatigue condition were examined. Fretting fatigue tests were conducted at several applied stress amplitudes with a constant normal contact load. Surface profile measurements on the contact zone showed a gradual coating wear with increasing fretting fatigue cycles at the low applied stress amplitudes, and the coating survived up to one million cycles. At the high applied stress amplitudes either the coating was separated from the substrate due to insufficient interfacial strength or the specimen fractured at locations away from the contact area. The static coefficient of friction (COF) was lower on the coated substrate than that on the un-coated substrate before the application of fretting fatigue loads. After the exposure to fretting fatigue cycles, the static COFs of the coated and the un-coated substrates were almost equal. The surface of the coated substrate thus became relatively rougher with cycling than that of the un-coated substrate from fretting action. Similar behavior was observed from the evolution of tangential force with cycling. The fretting loops between the tangential force and relative slip amplitude of the coated and un-coated substrates were similar, i.e. shapes were almost identical except for the difference in the magnitude of tangential force.