Wear mechanisms of titanium and aluminium nitride coatings: A microtribological approach

Microtribology experiments were carried out on a set of protective nanostructured Ti1−xAlxN (0≤x≤1) coatings, deposited by radio frequency magnetron reactive sputtering onto steel substrates and Si(100). Tests were carried out at room temperature using low applied loads and sliding velocities to prevent from surface oxidation. The surfaces were in contact against alumina to avoid the sticking of the counterpart, using a ball-on-disc reciprocating tribometer. Thus, these conditions allow the determination of the wear behaviour of the nitride layer itself. Film wear mechanisms were investigated from the evolutions of the friction coefficient and scanning electron microscopy observations. Moreover, two different models were used to characterise the coating according to x Al content: calculations of film fracture toughness KIC from microindentation tests and crack propagation resistance CPRs from scratch experiments. By X-ray diffraction, growth directions of the crystallised domains of the nanostructured films are analysed. Combining the results obtained from the different mechanical tests, the film damages caused by friction stresses are presented as a function of composition and micro-and nanostructure of the films, which play a crucial role in the functionality of coatings. The amount of wear debris generated by friction is directly linked to the coating crack initiation resistance. The nature of wear debris, i.e. ductile or brittle, acting as a third body, has a major influence on the evolution of the thin film damage.