Nanoindentation, micropillar compression and nanoscratch testing of ZrB2 grains

The mechanical response under nanoindentation, micropillar compression and nanoscratch tests of ultra-high temperature ZrB2 ceramic grains were investigated. The tests were carried on selected surface areas where the grain orientations were mapped by electron backscatter diffraction (EBSD) prior to the measurements (Fig.1a). Instrumented indentation were applied for compression using flat punch tip for nanoindentation and nanoscratch tests Berkovich tips were used. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and additional EBSD were performed to study the surface morphology and to characterize the deformations. Strong influence of crystal orientation was observed during micropillar compression while nanoindentation and nanoscratch tests revealed smaller anisotropy. Considerable plastic deformation is revealed under pillar compression, as it is shown in Fig. 1b,c, but indentation and scratch tests showed detectable plasticity, as well. Uniformly, basal oriented grains exhibited higher hardness, yield stress and rupture stress values compared to the prismatic orientation. The elastic anisotropy showed reversed tendency with lower indentation and Young’s modulus values corresponding to the basal orientation in comparison with the prismatic. To describe the elastic anisotropy, the Vlassak-Nix model and finite element model (FEM) calculations were performed based on the single crystal elastic constants of ZrB2. To explain the obtained hardness anisotropy, a theoretical model was proposed in which the critical force for slip activation is determined as a function of crystal orientation, based on the possible slip systems of materials. The calculated results shows similar tendency as the experimental values.