Superhardness Induced by Grain Boundary Vertical Sliding in (001)-textured ZrB2 and TiB2 Nano Films

Superhardness up to 50 and 60 GPa has been achieved recently in orientationally (001)-textured ZrB2 and TiB2 films composed of nano columnar grains, which almost double those in common TMB2 (TM-Zr, Ti). But the atomic origin, especially the role of shear stresses under indentation on grain boundaries (GBs), remains largely unknown. In this work, we report the identification of the experimentally observed GBs as Sigma(13) [001] tilt GB by first-principles calculations. The calculated formation energies of the GBs and TM vacancies on the GBs explain well the experimental results. Under shear stresses beneath indentors, the GB motion transforms the accumulated shear stresses parallel to (001) plane into a vertical sliding of the nano grains in [001] direction, while maintaining the GB structure unchanged. This special mode of shear plastic deformation correlates their hardness to the compressive strengths in [001] direction, instead of to the shear strengths on (001) plane as in bulk crystals, with the calculated compressive strengths well accounting for the measured superhardness in these (001)-textured nano ultra-high temperature ceramics. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.