Tissue phase affected fracture toughness of nano-columnar TiB₂₊z thin films

Structural imperfections such as nano-columnar tissue phases constitute a morphological design feature in protective thin films. Especially in the rising group of PVD transition metal diborides, excess Boron is known to form nm-sized B-rich precipitations in-between nano-crystalline domains. Here, we focus on super-stoichiometric TiB₂₊z thin films, varying the stoichiometry from TiB₂.₀₄ to TiB₄.₄₂ (B: 67–82 at.%). The tissue phase fraction and thickness is mainly governed by the B content and corresponding deposition conditions. A decreasing tissue phase width from ≈ 3–1 nm leads to an increasing critical stress intensity factor KIC of ≈ 40%.