Alloying with silicon or Si-based phases is an efficient approach to improve the oxidation resistance of transition metal diborides (TMBs). It is well established for bulk ceramics [1,2] but was also recently verified for thin film TMBs such as CrB2, HfB2, or TiB2 [3,4]. Adding these strong oxide formers to diborides results in highly dense and protective SiO2-based scales, whereas the amount of Si required differs between ternaries (pure Si addition) and quaternaries (alloying via disilicides), respectively. In more detail, alloying of TaSi2 and MoSi2 into TiB2 thin films not only reduces the amount of Si needed to provide excellent oxidation resistance but also 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 system have been achieved. Interestingly, for ternary Ti-Si-B2±z films the mechanical properties vary in a wide range ceasing 20 GPa while exhibiting similar oxidation stabilities. All these coatings crystalized in the α-AlB2 structure type with a preferred 0001 orientation being decisive for highest hardness. Meanwhile the oxidation behavior and mechanical properties have been thoroughly described for ternary and quaternary TMBs so far [3,4] the fracture characteristics of these coating materials are rather unexplored. Based on a recent study [4] the fracture toughness (KIC) of binary TiB2±z is known to be highly dependent on the amount of tissue phase present at the grain boundaries. Therefore, this study aims to unravel the fracture resistance of Si containing ternary and quaternary TMBs, focusing on alloying routes with disilicides. A set of different coating compositions has been deposited by non-reactively 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%). The elastoplastic behaviour involving also fracture characteristics such as KIC are evaluated by micro-mechanical testing methods, such as cantilever bending or pillar compression testing, as well as nanoindentation. For an in-depth understanding, these results are correlated with detailed structure-morphological investigations using XRD, SEM, TEM, or ERDA/RBS.