Physical and chemical vapor deposited protective hard coatings are commonly used to enhance the lifetime and performance of cutting tools. These protective coatings need to fulfill certain requirements such as high hardness, good wear resistance and fracture toughness as well as a high thermal stability and oxidation resistance in order to withstand extreme conditions during machining applications. The aim of the present work was to develop an in-depth understanding of the structure-property relationship of the Ti(Al,Si)N hard coating system by applying a wide range of advanced characterization techniques. In a first step, the microstructure of the Ti(Al,Si)N coating system was investigated in detail and correlated with the chemical composition and mechanical properties including fracture toughness and fracture stress. Addition of Si led to a fine-grained and nanocomposite structure for both TiSiN and TiAlSiN coatings, which exhibited an identical high hardness of ~40 GPa. Moreover, for the TiAlSiN coating with an Al metal fraction of 14 % the fracture stress and toughness were significantly enhanced compared to the Al-free coating. Since also the oxidation resistance of protective hard coatings is essential for their performance in cutting applications, in a next step, the oxidation mechanism of TiSiN coatings was illuminated in detail by conducting in-situ synchrotron X-ray diffraction measurements and high-resolution scanning transmission electron microscopy studies. It was found that during oxidation of the nanocomposite TiSiN coatings - consisting of nanocrystalline TiN grains embedded in an amorphous SiNx (a-SiNx) phase - both, rutile and anatase TiO2 are formed up to a temperature of ~1020 °C. Above this temperature, the transformation of the metastable anatase TiO2 phase to the stable rutile TiO2 modification starts. Deposition of a three-layer model system consisting of SiNx/TiN/SiNx by magnetron sputtering allowed to gain further insight into the influence of the thickness of the a-SiNx tissue phase on the oxidation resistance of TiSiN coatings. Here, it was shown that a higher thickness of the SiNx layers correlates with an enhanced oxidation resistance of the crystalline TiN layer, the former being significantly more resistant to oxidation than the crystalline TiN phase. Moreover, the effect of the addition of small amounts of Al on the oxidation resistance of TiSiN coatings was evaluated by assessing the oxidation mechanism of two powdered TiAlSiN coatings with different Al contents by in-situ X-ray diffraction. A higher Al content could be related to higher fractions of anatase TiO2, being a result of suppressed grain growth during oxidation. In summary, this thesis establishes the relationship between chemical composition, microstructure and mechanical properties of the Ti(Al,Si)N coating system and further provides a comprehensive understanding of the oxidation mechanism of Ti(Al)SiN coatings. Additionally, the importance of the combinatorial application of advanced characterization techniques for the detailed analysis of protective hard coatings is highlighted.