Strong oxide formers such as silicon or Si-based phases are added to improve the oxidation resistance of transition metal diborides (TMBs), forming highly dense and protective SiO2 scales. Alloying TiB2 with pure Si (ternaries) and disilicides such as TaSi2 and MoSi2 (quaternaries) has been proven to significantly improve oxidation resistance not only for bulk ceramics [1] but also for PVD-synthesized thin films [2]. In more detail, alloying of TaSi2 and MoSi2 into TiB2 thin films reduces the amount of Si needed to provide excellent oxidation resistance and minorly influences the mechanical properties. For quaternary TiB2 based coatings, hardness values of 36 GPa (TaSi2) and 27 GPa (MoSi2) compared to around 38 GPa of the binary TiB2 system have been achieved. All coatings crystallize in the α-AlB2 structure type with a preferred 0001 orientation. In contrast to the oxidation resistance of such Si and disilicides alloyed TMBs, the fracture characteristics have not been described in detail. It is known that the fracture toughness (KIC) of binary TiB2±z thin films highly depend on grain boundary phases and the amount present of such tissue phases. Therefore, in the present study, a set of different coating compositions has been deposited by DC magnetron sputtering using a broad set of composite targets TiB2, TiB2/TiSi2 (90/10 & 80/20 mol%), TiB2/TaSi2 (90/10 & 80/20 mol%) and TiB2/MoSi2 (85/15 & 80/20 mol%). After detailed structural and chemical analyses using XRD, SEM, TEM and ERDA/RBS, the fracture characteristics, in particular the KIC of these coatings have been determined using in-situ micromechanical testing techniques at room temperature as well as at elevated temperatures. [1] GB. Raju, et al,. J Am Ceram Soc. 2008;91(10):3320–3327 [2] A. Bahr, et al., Materials Research Letters. 11 (2023) 733–741