Mechanisms of coating failure as demonstrated by scratch and indentation testing of TiN coated HSS

Abstract

In situ monitoring of acoustic emission during scratching and indentation in combination with post-test light optical and scanning electron microscopy were used in order to evaluate the dominant coating failure mechanisms for TiN coated high speed steel. During scratching, six different coating failure mechanisms could be distinguished, i.e. cracks parallel with the scratch channel, semicircular cracks within the scratch channel, external transverse cracks, cohesive chipping, adhesive spalling, and complete breakthrough of the coating within the scratch channel resulting in substrate exposure. During indentation, four different types of coating failure could be distinguished, i.e. circular cracks within the indentation, radial cracks outside the indentation, cohesive chipping, and adhesive spalling. The influence of coating thickness, substrate hardness, and substrate surface topography on the coating failure mechanisms is very complex and depends on the type of contact, i.e. static or dynamic, contact load, etc. However, the results show that indentation testing enables a more appropriate method for the evaluation of the intrinsic fracture resistance of the coating material and coating/substrate adhesion as compared with scratch testing. In contrast, scratch testing, simulating the contact condition between an asperity sliding on a flat, is preferably used for evaluating the load carrying capacity of a coating/substrate composite. It can also be used in order to find the optimum substrate surface roughness for coated components used in various sliding applications.