Multicomponent Alloying for Improved Hard Coatings

Abstract

The cover image shows an energy dispersive elemental map of a Ti-Cr-Al-N coating heated at 1100 ºC for 2 hours, Ti (red), Cr (blue) and Al (green). The domain size is around 20 nanometers.© Rikard Forsén, unless otherwise stated ISSN: 0345-7524 Printed by LiU-Tryck, Linköping, Sweden, 2014 AbstractCoatings are vital to protect and to increase the productivity of cutting tools in high speed and dry cutting applications. During the cutting operation the temperature may exceed 1000 ºC it is therefore necessary that the coatings withstand high temperatures. A lot of development and research has been carried out during the last 30 years on finding new coating material systems providing enhanced properties such as adhesion, hardness and oxidation resistance at elevated temperatures. This thesis is based on multicomponent alloying of quaternary transition metal nitride hard coatings with a main focus on Ti-CrAl-N coatings. Many different coatings and compositions have been deposited using an industrial scale cathodic arc evaporation deposition system. All deposited coatings contain Al as this element is known to increase the hardness and the oxidation resistance of nitride coatings. The deterioration of the hardness in Al-containing nitride coatings is generally attributed to the transformation of cubic Al-N into hexagonal Al-N and the consequent domain coherency relaxation. This thesis investigates these phenomena on an atomic level providing a deeper understanding of and a way to engineer improved hard nitride coatings. The essence of this thesis is that by adding a third metal to a ternary nitride material system, for example one of the most frequently used Ti-Al-N, it is possible to tune and engineer the thermal stability of the cubic structure and the coherency strain which in turn affects the hardness and the oxidation resistance. The key point is that new intermediate phases in the decomposition process are generated so that the eventual detrimental phases are suppressed and delayed. More specifically, when Cr is added to the Ti-Al-N material system the coatings exhibit an age hardening process up to 1000 ºC caused by spinodal decomposition into coherent TiCr-and AlCr-rich cubic TiCr-Al-N domains. This means that the unstable cubic Ti-Cr-Al-N phase decomposes via yet another unstable cubic Cr-Al-N phase before the detrimental hexagonal transformation of Al-N takes place. The hardness is therefore retained up to a higher temperature compared to Ti-Al-N coatings.By utilizing multicomponent alloying through addition of Ti to Cr-Al-N coatings the hardness is retained after annealing up to 1100 ºC. This is a dramatic improvement compared to Cr-Al-N coatings. Here the Ti addition promotes the competitive spinodal decomposition into TiCr-and Al-enriched domains suppressing the detrimental hexagonal Al-N formation.To investigate the effect of multicomponent alloying for other material systems with different mixing free energies and atomic sizes, Zr-containing, Zr-Cr-Al-N and Zr-Ti-Al-N, quaternary nitride coatings ...