Nanoindentation induced deformation anisotropy in WC, β-Si3N4 and ZrB2 crystals

The influence of crystal orientation on elastic and plastic response of WC, β-Si3N4 and ZrB2 ceramic grains is important to understand, model and enhance its composite mechanical properties. In order to investigate this, nanoindentation testing was carried out using Berkovich tip on selected surface areas which were mapped by electron backscatter diffraction (EBSD) prior to the tests. To study the surface morphology after nanoindentation and to characterize the resulted deformation fields around the imprints additional EBSD, atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were performed. Considerable elastic and plastic anisotropy was found is WC and β-Si3N4 (Fig. 1a,b) crystals while the orientation dependence of ZrB2 grains exhibited slight influence on hardness and indentation modulus. The measured indentation modulus, as the elastic response, was compared with the model proposed by Vlassak and Nix and our finite element model (FEM) calculations using single crystal elastic constants, as it is shown for β-Si3N4 in Fig. 1c. To explain the obtained hardness anisotropy, as the plastic response, a theoretical model is 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 predictions of the applied models describing both elastic and plastic behaviors are in good agreement with the experimental results, (for β-Si3N4 see in Fig. 1d

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