Age hardening in harding coats

Thin transition metal nitrides and borides are the topic of many research works due to their physical, chemical and mechanical properties which make them applicable for several industrial fields. But especially increased hardness at elevated temperatures as a key-property for coatings in machining applications reveals a huge potential in increasing the life-time of machining tools. This can be achieved using age hardening mechanisms and spinodal decomposition of supersaturated phases which are well known processes, e.g., for precipitation-hardened Al-alloys, but only little is understood for hard coatings of refractory ceramics. Coatings within the model-systems Ti-Al-N and Ti-B-N are known to form supersaturated phases by vapour deposition (PVD), which separate during annealing into their stable constituents. Similar to the age hardening effect in Al-Cu alloys where different metastable phases appear during annealing, pre-investigations suggest that the formation route in Ti-Al-N and Ti- B-N coatings to build stable precipitates consists of various intermediate metastable Al-N or Ti-B phases or domains. These domains can be coherent or semi-coherent to the TiN matrix and cause the hardness to increase even at elevated temperatures. As soon as the stable equilibrium phases are obtained the hardness decreases which is known as overaging of such `alloys`. Therefore, during the research activities at the RWTH Aachen detailed ab- inito calculations as well as experimental investigations of such coating systems will be carried out to develop a concept for age-hardening in hard coatings with supersaturated phases. Based on the results of the project, tailored properties of hard coatings for individual applications shall be designed by modifying the kinetic and thermodynamic processes using different alloying elements.