The amount of difficult to machine materials has increased disproportionately in the last decade. This development is driven by innovative industry sectors like automotive, aerospace and energy sector or medical engineering which need new materials with better properties like less weight or higher thermal stability. For example, the use of austenitic stainless steels and duplex steels has increased steadily so that they nowadays represent an amount of 30 % of all used steel materials. Unfavourable for cutting these materials is their high toughness, their low thermal conductivity, tendency to strain hardening and bad chip formation. Another fast growing material group is the group of superalloys which are used in turbine manufacturing. Due to their low thermal stability, their high warm hardness and their tendency to sticking these materials also belong to the group of difficult to cut materials.To machine these materials oxide PVD or CVD coatings deposited on the tools are interesting. In the last two decades, with the introduction of the pulsed sputtering technique, the deposition of crystalline alumina coatings by means of MSIP (Magnetron Sputter Ion Plating) became focus of many researches. Crystalline PVD Al2O3-coatings promise high hot hardness, high oxidation resistance at elevated temperatures and reduced adhesive wear. Alumina exists in different crystallographic phases. a-Al2O3 appears to be the only thermodynamically stable phase at all common temperatures and pressures. Another promising candidate is ?-Al2O3 which is more fine-grained than a-Al2O3 and can be deposited at lower temperatures. At high temperatures ?-Al2O3 transforms into a-Al2O3, which limits the application temperature. But until now it is not clearly proved, up to which temperatures ?-Al2O3 thin films are stable and which mechanisms influence the stability. In the present work different ?-Al2O3 coatings are deposited on cemented carbides by means of Magnetron Sputter Ion Plating (MSIP). To improve adhesion two different bond coats, (Ti,Al)N and (Cr,Al)N, and their influence on compound properties as well as thermal stability is investigated. In a next step the influence of Silicon doping is examined. The synthesized coatings were analyzed regarding coating and compound properties, thermal stability and tribological performance. Finally cutting tests were carried out.