Their unique material characteristics make transition metal diboride-based thin films to perfect candidates for replacing state of the art protective and functional coatings. Not only machining tools but also high-precision components (e.g. turbine blades) within aircrafts and turbines used for energy production demand for surface improving materials. Mechanical properties like hardness, Young’s modulus and fracture toughness of the thin films are essential to protect the components from impacting mechanical stresses, especially against sudden impacts at high stress levels. Well-known for their superior hardness, various TMB2 exhibit enhanced resistance against fracture exceeding KIC values of well-established nitride-based coating materials (e.g. TiN or Ti1-xAlxN) and are therefore perfect aspirants for various industrial applications. Here, we focus on magnetron sputtered non-stoichiometric TiB2±z exhibiting outstanding mechanical properties. Beside super hardness of 45.90 ± 1.20 GPa and Young’s modulus of 524.27 ± 14.10 GPa the coatings exhibit a fracture toughness of KIC = 4.79 ± 0.57 MPa√m – tested via different micro-mechanical testing procedures. Detailed TEM and TEM-EELS investigations elucidate, that the distinct excess of boron predominates the constitution of the precipitating tissue phases around the columnar growth morphology. Due to covalently bonded boron-boron bonds the cohesive grain boundary strength is enhanced, impeding severe intercolumnar crack growth. The study highlights the great potential of TiB2±z for new applications in the field of high-performance components and reveal the importance of a detailed understanding of the grain boundary strength for fracture tough thin films.