Revisiting the origins of super-hardness in TiB2+z thin films – Impact of growth conditions and anisotropy

Abstract

Hexagonal transition metal diborides embody promising material systems for the purpose of protective thin films. Here, we focus on DC magnetron sputtered TiB2+z coating materials, comprehensively revisiting the impact of the stoichiometry on the structure-mechanical properties, from nearly stoichiometric TiB2.07 (B: 67 at. %) up to super-stoichiometric TiB4.42 (B: 82 at. %). The structural analysis confirmed the apparent correlation between the deposition pressure and the preferred {0001} orientation, which is essential to gain super-hardness (>40 GPa). In contrast, the hardness decreases for >10 GPa for 101¯1 and 1000 oriented thin films, underlining the pronounced anisotropy of TiB2+z. The broad stoichiometry variation revealed no predominant hardness effect based on a B-rich tissue phase. The excess B contributes to a decreasing column size correlating with a decreasing hardness of ≈ 7 GPa (B/Ti ratios >2.5) due to column boundary sliding events. Micro-cantilever bending experiments proved a declining fracture toughness from 3.02 ± 0.13 MPa√m for TiB2.43 to 2.51 ± 0.14 MPa√m for TiB4.42 to be column size dependent.