Adhesion of titanium nitride coatings on high-speed steels

Abstract

The adhesion is of vital importance for the performance of tools coated with titanium nitride. In view of this, we have studied the effects of substrate temperature and pretreatment on the adhesion of TiN coatings. Since high-speed steels are among the most used substrate materials for tool applications we have chosen to deposit the films onto eight different high speed steels. The TiN coatings were deposited using reactive dc-magnetron sputtering, and the adhesion was measured using the scratch test. In order to study the film–substrate interface Auger electron spectroscopy was used in combination with ion beam depth profiling. On most of the steels, the adhesion of the films increased with substrate temperature, reaching a maximum between 400 and 500 °C. The increased adhesion is associated with changes in the oxide layer on the steel substrate. As the temperature increases, Fe2O3 and F3O4 decompose to FeO. Sputter etching of the substrate prior to deposition improves the film adhesion even though a complete removal of the oxide layer is not achieved. The use of an intermediate layer of pure titanium increases the adhesion for films deposited at temperatures below 400 °C. Above this temperature a thin TiC layer is formed in the titanium–steel interface reducing the adhesion. The composition and distribution of the carbides particles in the high-speed steels are also found to be important for the film adhesion. High adhesion is obtained when the steel contains a high density of MC (M, mainly vanadium). The results are discussed in terms of interfacial energies. A low interfacial energy and thus a high adhesion is e.g., obtained when the structural matching between the substrate and the film is good. In the case of TiN this can be achieved if a thin layer of FeO exists on the high-speed steel matrix, or if the surface density of MC carbides is high. Both FeO and MC have the same structure as TiN with only a small lattice mismatch.