On the correlation of substrate microhardness with the critical load of scratch adherence for hard coatings

Substantial experimental evidence confirms that the critical load of adherence determined from scratch measurements for hard thin films varies linearly with the substrate hardness, for materials of a uniform cross-sectional hardness distribution. For substrates that have a non-uniform cross-sectional hardness distribution, such as plasma-nitrided materials used for duplex coating processes, a linear correlation is not observed, except at relatively high hardness indentation load values. The reasons for this behavior were investigated experimentally for TiN films of identical characteristics deposited by plasma CVD on uniform hardness substrates of different materials as well as on plasma-nitrided M2 substrates with different effective diffusion layer thicknesses. It is concluded that only when the indentations are performed in the fully plastic deformation regime is it possible to establish a unique and representative linear correlation between the critical load of scratch adherence and the substrate hardness, for hard thin films deposited on uniform and plasma-nitrided materials.