Fretting wear of a TiN PVD coating under variable relative humidity conditions—development of a ‘composite’ wear law

Fretting is defined as a small oscillatory displacement between two contacting bodies. The interface is damaged by debris generation and its ejection from the contact area. The application of hard coatings is an established solution to protect against fretting wear. For this study a TiN hard coating manufactured by a PVD method was selected, and tested against a smooth polycrystalline alumina ball. A fretting test programme was carried out at a frequency of 5 Hz, 100 N normal load, 100 μm displacement amplitude and at five values of relative humidity: 10, 30, 50, 70 and 90% at a temperature of 296 K. The intensity of the wear process is shown to be significantly dependent on the environmental conditions. A dissipated energy approach was employed in this study to quantify the wear rates of the hard coating. The approach gives stable prediction of wear kinetics under constant medium relative humidity. An increase of relative humidity is shown to promote the formation of hydrated structures at the interface and modify the third body rheology. This phenomenon was characterized by the evolution of wear kinetics associated with a significant variation of the corresponding energy wear coefficient. Hence, a ‘composite’ wear law, integrating the energy wear coefficient as a function of relative humidity, is introduced. It permits a prediction of wear under variable relative humidity conditions from 10 to 90% within a single fretting test. The stability of this approach is demonstrated by comparing various variable relative humidity sequences.