Determination of film-only hardness by eliminating the effects of elastic deformation of substrate

A method is developed for the determination of the film-only hardness of thin films deposited on substrates by nanoindentation. The method is suitable for tests in low load range where the substrate only deforms elastically. The basic assumption is that the difference between the displacement recorded during unloading at a certain load and that recorded during loading at the same load only contains the displacement due to the elastoplastic deformation in the film, whereas the displacements due to the elastic deformation of the substrate during unloading and loading are equal and eliminated. With this assumption, one can derive the unloading curve of a hypothetical bulk model made of the film material, from which the hardness of the film material can be evaluated through conventional Oliver and Pharr's method. The validity of the approach was examined by applying the method to hafnium boride thin films deposited on silicon and fused quartz substrates.