Apparent Indentation Plasticity in Ceramic Coated Systems

Abstract

Modern surface engineering technologies, particularly the application of hard ceramic coatings to various substrates, are being increasingly used to provide enhancements in the hardness and tribological performance of a wide range of engineering components. At the simplest level, this is often viewed as attempting to confer some of the desirable properties of the coating material (eg. high hardness) onto the substrate. In order to optimise the performance of these systems, there is considerable need to measure and understand the origins of these enhancements, which, for thin film coated systems, are often out of all proportion to the volume of the coating material. Indentation testing is one of the most convenient techniques for assessing the properties of coatings in situ and can also be considered as a model for asperity-dominated abrasive wear. Traditionally, macro- and microindentation testing have been used, measuring a complex response involving substrate and coating and often cause cracking of the coating in and around indentations. Thus the only result that can be obtained from these tests is a hardness number, with all information on the deformation processes during loading and unloading being lost. Any perceived increase in penetration resistance has usually been attributed to the high yield stress of the coating. Indeed, many models for interpreting and predicting the hardness of a coated system have been based on a law of mixtures approach, weighted to the relative plastically deforming volumes (or areas) affected by the indentation in coating and substrate. However, this paper will demonstrate the misconception in this approach and show that it is the elastic properties of the coating that are often central in controlling apparent indentation plasticity.