HARDNESS MEASUREMENTS OF THIN FILMS-DETERMINING THE CRITICAL RATIO OF DEPTH TO THICKNESS USING FEM

The finite element method is used to simulate the indentation process of a wedge-shaped indenter (136 degrees) into Al/Si, Si/Sapphire and TiN/HSS film/substrate systems. The relationship between the mechanical properties of both film and substrate on the shape and size of the plastic deformed zone is studied in relation to the ratio of depth of penetration to film thickness. A key conclusion of this study is that the general rule not to exceed 10-20% of the film thickness in indentation experiments to obtain a true film hardness value is not a universal law. The critical ratio of depth of penetration to film thickness (D/t) varies sensitively with the coated systems. Since the influence of the interlayer and the substrate on the hardness of composite systems grows just above the critical indentation depth and the permissible error of microhardness measurements is in general 5-10%, in the case of a very soft him on a hard substrate, because of the confinement of the plastic deformed volume by lateral spreading within the soft film, the critical ratio of Dit is found to be greater than theoretically expected. For example, in the case of an aluminum film on silicon a relative indentation depth of more than 30% will still give negligible deviations from the true film hardness. On the contrary, in the case of a hard film it seems that one has to pay particular attention to the value of relative indentation depth, because the plastic deformation easily extends into the substrate.