Progress in high temperature nanomechanical testing of coatings for optimising their performance in high speed machining

Abstract

Frictional heating results in very high operating temperatures in high speed machining but the nanoindentation tests used to evaluate novel PVD coating systems for improved cutting performance are invariably performed at room temperature. If nanomechanical measurements are to be used reliably in the optimisation of coatings for high speed machining then it is much better that the measurements are performed at the relevant temperature. High temperature nanoindentation data are reviewed for a wide range of nitride-based hard coatings on cemented carbide and design rules suggested for coating optimisation for different machining applications. The importance of high temperature mechanical properties and microstructure on the adaptive and multifunctional behaviour in improving tool life is investigated. The coatings studied show large differences in how their hardness, modulus and H/E vary with increasing temperature which have a significant influence on their behaviour in high temperature mechanical contact applications which have differing requirements in terms of hot hardness and plasticity. In continuous high-speed turning operations the high temperature hardness is paramount and coatings with high hot hardness display longer tool life. In interrupted cutting conditions toughness and plasticity are at least as important, and in end milling of hardened steels for example, high hot hardness should be combined with improved plasticity for longer tool life. Multilayer AlTiCrSiYN/AlTiCrN coatings have shown improved performance compared to the state of the art monolayer coatings. The reasons for this improved performance are discussed. Overall, the high temperature nanoindentation data show excellent correlation to coating life in high speed machining applications.