Indentation size effect in the hardness measurements of high entropy carbides

The indentation load-size effect was investigated during hardness testing of high entropy carbides with different hardness, applying indentation loads from 50 mN to 10 N. The experimental systems were recently developed (Hf-Ta-Zr-Nb-Ti)C and (Mo–Nb–Ta–V–W)C high-entropy carbides, prepared by ball milling and a spark plasma sintering, with single-phase and high relative density of 99.4% and 99.0%, respectively. The load dependence of hardness was analysed using the traditional Meyer's law, the proportional specimen resistance model and the modified proportional specimen resistance model. The best correlation (R2 > 0.99) between the measured values and the used models was achieved using the modified proportional specimen resistance model. This resulted in the so-called load-independent hardness of (Hf-Ta-Zr-Nb-Ti)C and (Mo–Nb–Ta–V–W)C systems 21.96 and 14.81 GPa in the case of analyses for 50 mN–10 N load range and 25.42 GPa and 14.13 GPa in the case of 50 mN–1 N load range, respectively. The different deformation and damage mechanisms detected as a potential reason for the origin of the load-size effect were microcracks at grain boundaries during the micro/macro-indentation and plastic deformation/nanocrack formation at the indents during nano/micro-indentation.