Experimental assessment of the elastic properties of thin TiN/AlN superlattice and nano-multilayer coatings

Nanoindentation is widely used for the mechanical assessment of coatings and thin films. Whereas this can lead to reliable assessments for coating-only properties for short range properties like plasticity the longer range of elastic interactions means that measured data always contains a contribution from the substrate. This paper describes a simple model which can be used to assess the extent of these interactions and predict how changing the elastic properties of individual layers in a complex multilayer or superlattice coating may influence the contact modulus measured by nanoindentation. The model highlights the effect of the transition from hexagonal to cubic AlN as the bilayer period is reduced in TiN/AlN nano-multilayer coatings and how the elastic anisotropy of the layer materials can also be significant in textured coatings. These effects may be masked by the effects of tip bluntness on nanoindentation data from very thin (submicron) films and a coating thickness of at least 3 μm is required for reliable coating data to be obtained for hard coatings on a stiff substrate tested with a typical used Berkovich indenter.