The relationship between hardness and scratch adhession

Microhardness and scratch adhesion testing are the most commonly used techniques for assessing the mechanical properties of thin surface coatings such as physical vapour-deposited TiN. Both of these test methods utilize single-point contacts, a diamond pyramid for microhardness testing and usually a Rockwell C 120° diamond cone for scratch testing, and both induce plastic deformation in the substrate and coating. Clearly, static microhardness and scratch adhesion testing will have common features since, in both, the plastic deformation processes are likely to be similar. An analysis based on elastic-plastic identation theory has been developed that allows both the prediction of the hardness of a given coating-substrate system and the estimation of shear strains developed at the coating-substrate interface which, for weakly adhered films, leads to delamination of the coating around an indentation. The ideas embodied in the volume law-of-mixtures hardness model have also been applied to the scratch test, allowing estimates to be made of the interfacial shear strains present during testing since these contribute to the measured “critical load” Lc for coating failure. The role of substrate plastic deformation and other factors affecting Lc (such as friction, internal stress and coating thickness) will be discussed.