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.

Automated summary

The indentation load-size effect was studied in the hardness testing of high entropy carbides. The (Hf-Ta-Zr-Nb-Ti)C and (Mo-Nb-Ta-V-W)C high-entropy carbides were used with indentation loads ranging from 50 mN to 10 N. The load dependence of hardness was analyzed using different models, and the modified proportional specimen resistance model showed the best correlation with the measured values. This resulted in load-independent hardness values of 21.96 GPa and 14.81 GPa for (Hf-Ta-Zr-Nb-Ti)C, and 25.42 GPa and 14.13 GPa for (Mo-Nb-Ta-V-W)C, depending on the load range.