The wear and erosion resistance of hard PVD coatings

Abstract

The abrasive and erosive wear behaviour of titanium nitride coatings prepared using physical vapour deposition has been studied and the mechanism(s) by which the coatings fail identified and contrasted with those of uncoated material similarly treated. For abrasive wear it is not sufficient to consider only hardness as a guide to coating selection; it is also important to consider the load-bearing capacity of the coating/substrate system. Under mild abrasive wear, thin coatings eventually fail by localized detachment of the film at the intersections of the plastic grooves formed by the abrasive particles, whilst thicker coatings support the contact stresses elastically and degrade by a microchipping or polishing mechanism with little or no deformation of the underlying substrate. In contrast, under severe abrasive wear, thick coatings fail by a cohesive fracture mechanism. Under erosive conditions it is found that thick coatings are more resistant to angular particle erosion whilst thin coatings have longer lifetimes when exposed to blunt erodents. Whilst no correlation can be found between erosion resistance and the scratch-adhesion critical load for failure Lc, good correlations between the failure mechanism(s) found in erosion and scratch testing can be made. The role of internal stress and substrate type in erosion and scratch behaviour has also been studied, high internal stresses being found to induce spalling in both erosion and scratch tests.