To enhance the restricted oxidation resistance of transition metal diboride (TMB) ceramics, alloying with Si and disilicide phases is an effective method, resulting in the formation of highly dense and protective SiO₂ scales. This phenomenon has been well documented in the context of bulk ceramics, and recent studies have also corroborated its occurrence in thin-film TMBs, including CrB2, HfB2, and TiB2. The incorporation of Si, TaSi₂ or MoSi₂ into TiB₂ results in a significant reduction in oxidation kinetics, while exhibiting only minor effects on the mechanical properties. In the case of quaternary TiB₂-based coatings, hardness values of 36 GPa (TaSi₂) and 27 GPa (MoSi₂) have been achieved, in comparison to approximately 38 GPa for the binary system. All of the aforementioned coatings exhibited α-AlB₂ crystal structure, with a preferred (0001) orientation being a key factor in achieving the highest hardness. Nevertheless, the fracture characteristics of these Si-alloyed TMBs remain largely unexplored. The objective of the present study is to elucidate the fracture characteristics, particularly KIC, of these Si-containing TMBs at elevated temperatures up to 850 °C through the application of in-situ micromechanical testing techniques. Accordingly, a series of Ti-TM-Si-B₂±z coatings was deposited via non-reactive DC magnetron sputtering using a variety of composite targets, including TiB₂, TiB₂/TiSi₂ (90/10 & 80/20 mol%), TiB₂/TaSi₂ (90/10 & 80/20 mol%), and TiB₂/MoSi₂ (85/15 & 80/20 mol%). To gain a deeper understanding, additional detailed structural investigations were conducted using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and elastic recoil detection analysis (ERDA). In comparison to the binary TiB₂+z and the quaternary Ti-Ta-Si-B₂±z, the Si and MoSi₂-containing coatings exhibited a distinct onset of plastic deformation at approximately 600 °C. This phenomenon can be attributed to the precipitation of silicon-containing phases, which underlines the significance of conducting material testing at temperatures relevant to their intended applications.