Excess of boron in TiB₂ superhard thin films: a combined experimental and ab initio study

Abstract

Deviations from the nominal stoichiometry are known to affect those properties of TiB 2 , such as high hardness and metallic character that favour its employment in various technological applications. Here we use a combination of experimental and theoretical approaches to elucidate the dependence of B concentration on controlled growth conditions and the atomic-scale details of excess B incorporation. We find a monotonic increase in B/Ti ratio with the increase in substrate voltage during magnetron sputtering growth of nanocrystalline TiB 2 films. Even for large B/Ti ratios, however, the films retain the AlB 2 hexagonal structure, albeit with increased lattice constants. Using first-principles calculations we attribute these structural features to incorporation of a portion of excess B as bulk interstitials, while the remaining overstoichiometric B atoms agglomerate on B-rich surfaces of the TiB 2 nanograins. The results suggest a link between observed superhardness and B presence on grain surfaces.