Development of a Wöhler-like approach to quantify the Ti(CxNy) coatings durability under oscillating sliding conditions

The selection of a proper material for the particular engineering application is a complex problem, as different materials offer unique properties and it is not possible to gather all useful characteristics in a single one. Hence, employment of different surface treatment processes is a widely used alternative solution. In many industrial applications, coating failure may be conducive to catastrophic consequences. Thus, to prevent the component damage it is essential to establish the coating endurance and indicate the safe running time of coated system. To this study PVD TiC, TiN and TiCN hard coatings have been selected and tested against polycrystalline alumina smooth ball. The series of fretting tests with reciprocating sliding at the frequency 5 Hz have been carried out under 50-150 N normal loads and under wide rage of constant as well as variable displacement amplitudes from 50 μm to 200 μm at a constant value of relative humidity of 50% at 296 K temperature. To quantify the loss of material a dissipated energy approach has been applied where the wear depth evolution is referred to the cumulative density of friction work dissipated during the test. Different dominant damage mechanisms have been indicated for the investigated hard coatings, which is debris formation and ejection in case of TiC coating and progressive wear accelerated by cracking phenomena in case of TiN and TiCN coatings. Energy-Wöhler wear chart has been introduced, in which the critical dissipated energy density corresponds to the moment when the substrate is reached after a given number of fretting cycles. Two different methods to determine the critical dissipated energy density are introduced and compared. The Energy-Wöhler approach has been employed not only to compare the global endurance of the investigated systems but also to compare the intrinsic wear properties of the coatings. It has been shown that the fretting wear process is accelerated by the stress-controlled spalling phenomenon below a critical residual thickness and a severe decohesion mechanism is activated. Finally the applicability of the investigated method to other coated systems subjected to wear under sliding conditions is discussed and analyzed. The perspectives of this new approach are elucidated.