On the surpassing fracture toughness of TiB2+z thin films

Abstract

The hexagonal transition metal diboride family offers highly promising material systems for the purpose of protective thin films. Here, we focus on DC magnetron sputtered TiB2+z which was re-investigated, aiming for a broad variation in B content, from almost stoichiometric TiB2.07 (B: 67 at %) up to super-stoichiometric TiB4.42 (B: 82 at.%). The experiments revealed a correlation of deposition pressure and preferred 0001 crystal orientation, which is essential for reaching maximum film hardness of ~40 GPa. In contrast, H decreases for > 10 GPa for 10-11 & 1000 oriented coatings, underlining the pronounced anisotropy of the TiB2+z material system. Furthermore, an increase in excess B is not only leading to a broadening of a B-rich tissue phase, but to decreasing (< 5 nm) grain sizes (grain boundary fraction increases). Here, the reverse/inverse Hall-Petch effect contributes to a drop in H of ~5GPa. The decreasing grain size also influences the intrinsic KIC, determined by micro-cantilever bending experiments. A slightly declining KIC = 3.02 ± 0.13 MPa√m for TiB2.43 to KIC = 2.51 ± 0.14 MPa√m for TiB4.42 was measured, attributed to the decreasing grain size and hence cohesive grain boundary strength due to excess B. Moreover, solid particle erosion tests evinced no detectable mass loss of the TiB3.06 thin film up to 300 s erosion time.