Mechanical strength of zirconia and hafnia phases

Using high-resolution synchrotron-based x-ray powder diffraction and density-functional theory, we investigate the high-pressure behavior of transition metal dioxides zirconia (ZrO2) and hafnia (HfO2), which have been previously suggested as hard, potentially superhard, candidate materials. We determine the mechanical strength of each high-pressure dioxide phase as well as their respective equations of state (EOS) and phase diagram. In this work, new experimental EOSs are provided for each phase observed, some of which are notably different from previous studies; in particular, for the ambient-pressure monoclinic MI-ZrO2 and MI-HfO2 phases. Scaling relations for both dioxides indicate that the OII phase of both ZrO2 and HfO2, while dense and quenchable, have a low mechanical hardness (H) of ~10 GPa and thus do not qualify as superhard materials (where H > 40 GPa). The other low-pressure phases (MI and OI) show similar mechanical strength values. In order to evaluate correlations between hardness and elastic indicators we also compute the shear moduli and elastic constants for each phase. Investigating the EOS, phase stability, and the mechanical strength provides a better understanding of the relationship between phase and its mechanical properties. We find that the shear modulus better correlates with hardness between phases than the bulk modulus.