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. h films promote a preferred [001] crystallographic hcp orientation which has a large impact on the resistance to crack initiation and then the amount of debris generated by friction. Ti-rich coatings show better tribological properties due to a higher toughness and a higher elastic modulus. For Ti0.50Al0.50N, abrasive wear debris are generated after several friction cycles, leading to the progressive destruction of the coating. The main direction of Ti-rich films is along the fcc [100] axis, though some [111] crystallised domains grow in Ti0.62Al0.38N and Ti0.50Al0.50N. The former film which has less volume of [111] crystallised domains shows more satisfactory wear behaviour than Ti0.50Al0.50N.