Samples of high-speed cutting steel, vacuum heat treated and ion-nitrided in various nitriding agents, showed in the layer different phase-compositions. With short-time nitridation the layer was formed by intermetallic compound Fe4N, and with long-time nitridation by multiphase layer containing Fe4N, Fe2.5N and Fe3N. Layer microhardness after short-time nitridation was 1532 HV 0.05 and with long-time nitridation, where layer was thicker, it was only 1037 HV 0.05. A part of samples treated by ion-nitridation and even without it, ws TiN coated with 2-mu-m thickness. Coating adhesion on the monophase layer was almost equivalent to the original surface with TiN coating, while in the multiphase layer adhesion was insufficient. By TiN coating the wear of high-speed cutting steel was significantly moderated and further improvement was attained by combination of ion-nitrided layer with subsequent coating. With wear tests the thickness of ion-nitrided layer had a more significant influence than coating adhesion to the layer. By coating treatment at temperature of 550-degrees-C in a vacuum, significant diffusion of iron into the layer was proved. From the beginning the diffusion proceeds faster, and after occupying free places it slows down, which was verified in the time from 10 minutes to 10 hours.
