Effects of particle distribution and calculation method on results of nano-indentation technique in heterogeneous nanocomposites-experimental and numerical approaches

Abstract

This study aims to investigate the effects of two testing parameters on the results of the nano-indentation experiment in the heterogeneous particle reinforced composites: (1) the particle distribution pattern, and (2) the methods for calculating the residual projected contact area of the sample under the indenter. For this purpose, several nano-indentation experiments were performed by using Berkovich indenter on the specimens prepared from Filtek Z350 XT dental nanocomposite as a particle reinforced composite with a volume fraction of 63.3 percent. Then, the nano-indentation procedure on the dental nanocomposite was simulated using the finite element (FE) method. In the FE simulation, several representative volume elements (RVEs) of the nanocomposite were modeled by modifying the Lubachevsky-Stillinger algorithm to distribute particles randomly with a volume fraction of more than 50 percent in a cubic volume. The hardness and elastic modulus of the nanocomposite were calculated from the results of the nano-indentation experiment and the simulation. In these calculations, the projected contact area of the indenter on the specimen was obtained using two different approaches of the Oliver-Pharr method and direct measurement of the area. The results indicated that the distribution of particles around the indenter has significant effects on the nano-indentation results. According to the simulation results, the heterogeneity of the material causes the projected contact area of the indenter on the sample to distort from its equilateral triangle shape, which is assumed in the Oliver-Pharr method. Therefore, the method of direct measurement of the indenter projected contact area on the specimen is proposed to obtain more precise results from the nano-indentation test on heterogeneous materials.