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