Investigation into the impact wear behaviour of ceramic coatings

Abstract

Ceramic coatings with good impact wear resistance, hardness, toughness and adhesion are required in many applications. Coatings deposited by physical vapor deposition processes (CrN over an ENiP interlayer, TiC x N y , TiN and Ti x% -diamond-like carbon) have been evaluated using a specially designed cyclic impact tester. The test allows the modelling of wear under repetitive high local loads, as encountered by machine parts subjected to impact or tools used in interrupted cutting or cold forming. To adjust the test conditions for different applications, the impact force and frequency are variable and each impact can be monitored using a piezo-force transducer. A comparison of the results of the tests on the coatings studied shows differences in the characteristics of the impact wear in each case. Scanning electron microscopy and optical inspections of the wear regions allowed the creation of a model to describe hard coating degradation under repeated impact loading. Three different zones were identified: a central zone of cohesive failure; an intermediate zone of cohesive-adhesive failure; and a peripheral zone with circular cracks. Furthermore, it was found that the wear increased at the macrocracks, depending on the number of impacts, before coating delamination or spallation started to occur. The TiC x N y coatings and, in particular, the TiC 0.75N 0.25 coatings were found to have better resistance to this kind of dynamic impact wear than was the case for the coatings with an ENiP interlayer. The Ti x%-diamond-like carbon coatings showed good wear resistance in the cavity of the indentation area until a high number of impacts was reached, but coating delamination at the high shear stress zones (at the edges and centre of the cavity) took place after a lower number of impacts.