Superhardness Induced by Grain Boundary Vertical Sliding in (001)-textured ZrB 2 and TiB 2 Nano Films

Superhardness up to 50 and 60 GPa has been achieved recently in orientationally (001)-textured ZrB 2 and TiB 2 films composed of nano columnar grains, which almost double those in common TMB 2 (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 Σ 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.