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

Deviations from the nominal stoichiometry are known to affect those properties of TiB2, 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 TiB2 films. Even for large B/Ti ratios, however, the films retain the AlB2 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 TiB2 nanograins. The results suggest a link between observed superhardness and B presence on grain surfaces.