Nanoindentation on soft film/hard substrate and hard film/soft substrate material systems with finite element analysis

In this paper, the cone nanoindentation on soft film/hard substrate and hard film/soft substrate material systems was systematically investigated with the FE method. Based on the elastic–perfectly plastic material model, two typical sets of mechanical parameters (i.e., Young’s modulus and Poisson’s ratio) and initial yield stress were configured in such a way that the piling-up and sinking-in phenomena during nanoindentation can be observed. The mechanical responses of soft film/hard substrate and hard film/soft substrate material systems to the indenter’s maximum displacement were obtained by changing the ratio between the indenter’s maximum displacement and thin-film thickness, hmax/t, from 0.05 to 0.45 (at 0.05 increments) and by comparing FE analysis results with the corresponding bulk materials’ mechanical responses. Finally, the effect of substrate properties on the calculated mechanical properties of thin films and the piling-up and sinking-in phenomena were studied. It was found that, when Young’s modulus of a thin film was smaller than that of its substrate, the calculated Young’s modulus of the thin film increased with the indenter’s maximum displacement; and that, when Young’s modulus of a thin film was larger than that of its substrate, the calculated value decreased with the indenter’s maximum displacement. For the configured hard film/soft substrate material system, the calculated hardness of the thin film started to decrease only when hmax/t was equal to 0.40. It is concluded that the calculated Young’s modulus of thin films responds more sensitively to the mechanical properties of substrates than does the calculated hardness. Also, the relationship of the indenter’s contact depth and its maximum displacement deviates from the linear relationship noted for thin-film materials. The piling-up of the soft film/hard substrate material and the sinking-in of the hard film/soft substrate material are more severe than those of the corresponding soft and hard bulk materials.