To improve the high-temperature (T > 1000 °C) oxidation resistance of transition metal diborides TMB2, alloying approaches with Si, as a strong oxide-forming element, have proven to be successful [1-3]. Both ternary (TM-Si-B) and quaternary (e.g., TM-Mo-Si-B by alloying TMB₂ with MoSi₂) systems have shown pore formation due to phase changes and diffusion processes during annealing above 1100 °C. Regarding long-term (t > 1000 h) applications, pores significantly weaken the protective function of the coatings and pose a problem. This study investigates PVD-synthesized ternary and quaternary TM diborides regarding the effect of different TM/B stoichiometries as well as alloying elements influencing pore formation. Phase transformations, as well as diffusion processes and the pore formation itself, were examined in detail using high-resolution techniques such as transmission electron microscopy (TEM), elastic recoil detection analysis (ERDA), atom probe tomography (APT), and Rutherford backscattering spectrometry (RBS). References [1] T. Glechner, H.G. Oemer, T. Wojcik, M. Weiss, A. Limbeck, J. Ramm, P. Polcik, H. Riedl, Influence of Si on the oxidation behavior of TM-Si-B2±z coatings (TM = Ti, Cr, Hf, Ta, W), Surf. Coat. Technol. 434 (2022) 128178. [2] L. Zauner, A. Steiner, T. Glechner, A. Bahr, B. Ott, R. Hahn, T. Wojcik, O. Hunold, J. Ramm, S. Kolozsvári, P. Polcik, P. Felfer, and H. Riedl, Role of Si segregation in the structural, mechanical, and compositional evolution of high-temperature oxidation resistant Cr-Si-B2±z thin films, 10.2139/ssrn.4251252. [3] A. Bahr, S. Richter, R. Hahn, T. Wojcik, M. Podsednik, A. Limbeck, J. Ramm, O. Hunold, S. Kolozsvári, H. Riedl, Oxidation behaviour and mechanical properties of sputter-deposited TMSi2 coatings (TM = Mo, Ta, Nb), Journal of Alloys and Compounds 931 (2023) 167532.