Transition metal diborides are a favourable class of materials for the use as protective coatings, particularly TiB2±z is of great interest, as its hardness and elastic modulus are exceptionally high. Nevertheless, for the intrinsic fracture toughness KIC, the literature presents only little descriptions. So far it is known that the hardness and elastic modulus are highly dependent on the crystal orientation [1] and the amount of tissue phase present [2], which in turn can be influenced by the coating parameters. To figure out, if this also affects the fracture behaviour, we synthesized a broad stoichiometric variation from TiB2.07 up to TiB4.42 by DC magnetron sputtering and determined their structure-mechanical properties. By using in-situ microcantilever bending tests KIC was investigated. The presence of a B-rich tissue phase was confirmed by HR-TEM analysis. The dominance of the tissue phase is progressing with increasing B, also forcing smaller column sizes [3]. In summary, this study underlines the influence of stoichiometry and the potential of tissue phase engineering on the mechanical properties of TiB2+z based thin films. [1] H. Holleck, Material selection for hard coatings, J. Vac. Sci. Technol. A. 4 (1986) 2661–2669. [2] P.H. Mayrhofer et al., Self-organized nanocolumnar structure in superhard TiB2 thin films, Appl. Phys. Lett. 86 (2005) 131909. [3] C. Fuger et al., Revisiting the origins of super-hardness in TiB2+z thin films – Impact of growth conditions and anisotropy, Surf. Coat. Technol. (2022) 128806.