Improvement of ‘duplex’ PVD coatings for HSS cutting tools by ion mixing

This paper explores potential ways to improve ‘duplex’ coatings for cutting tool applications. These coatings are formed by a diffusion saturation process with nitrogen, commonly known as ion nitriding, followed by the application of a (Ti,Cr)N hard PVD coating. The coating includes an additional ion mixed layer applied to the previously nitrided surface of HSS prior to the deposition of a hard PVD coating. Such multi-layered coatings significantly increase, by a factor of 3–4 times, the wear resistance of cutting tools as compared to ‘duplex’ coatings. Three transition metals Ti, Zr, and W as well as Al were studied in combination with three nonmetallic elements N, C, and O. During the surface modification stage of this ‘triplex’ process these ions were mixed into the substrate surface. Tests were performed to compare the effects of the modified layers on the high-speed steel (HSS) cutting tool life. The highest wear resistance, after the triple surface treatment, was achieved using the process with the titanium that was ion mixed together with the nitrogen. The surface of the ion mixed layer before and after cutting was investigated using AES and SIMS methods. The coefficient of friction of the modified layer was studied vs. temperature. The improved ability of the ion mixed layer to accumulate the energy of elastic deformation was shown using nanoindentation tests. Ion mixing produces a thin surface layer with an amorphous-like structure that prolongs the stable wear stage, which considerably enhances the tool life as a result of self-organization process.