Determining hardness of thin films in elastically mismatched film-on-substrate systems using nanoindentation

Nanoindentation has developed as a practical tool for determining the mechanical properties of thin films. In this paper, we develop a method for determining the hardness of thin films on substrates based on measurements of the contact stiffness vs. depth of indentation and prior knowledge of the elastic properties of the film and substrate. The method utilizes an analysis of contact stiffness vs. contact area for purely elastic indentation to correct for the effects of surface roughness and pile-up/sink-in on the contact area during elastic–plastic indentation of elastically mismatched film/substrate systems. We find the hardness to be essentially depth independent for indentation depths approaching the thickness of the film for several thin film/substrate systems. This new technique is described in detail and is applied to the nanoindentation of compliant films on stiff substrates (Al/Si and Al/sapphire) and stiff films on compliant substrates (W/Si and W/glass).