Wear behavior and thermal stability of HPPMS (Al,Ti,Cr,Si)ON, (Al,Ti,Cr,Si)N and (Ti,Al,Cr,Si)N coatings for cutting tools

Abstract

During the hard machining of powder metallurgical high-speed steel, finely dispersed carbides in the steel expose tools to both thermal and mechanical load. PVD hard coatings are used to protect the tools from wear, oxidation and diffusion processes. With regard to mechanical and thermal properties, (Ti,Al,Cr,Si)N hard coatings are advantageous compared to (Ti,Al)N due to their nanocomposite coating architecture. In addition to monolithic (Ti,Al,Cr,Si)N, a bilayer coating with a (Ti,Al,Cr,Si)ON top layer is deposited in order to investigate the influence of oxygen on the interaction with the workpiece during machining. The coatings were deposited in an industrial scale coating unit using a hybrid technology consisting of direct current and high power pulse magnetron sputtering (dcMS/HPPMS). The influence of the oxygen/nitrogen and the aluminum/titanium ratio on the coating as well as compound properties of indexable inserts made from cemented carbide were investigated. Furthermore, the oxidation and the phase stability were investigated. Finally, the coating systems were examined in cutting tests during which powder metallurgical high-speed steel was milled using 6 mm cemented carbide milling tools. The coatings show a fine crystalline morphology with a cubic crystal structure and a smooth surface. For oxynitride coatings, both hardness and resistance against plastic deformation show increased values compared to the nitride coatings. The additional oxygen might lead to a more brittle deformation behavior. The coating with an increased titanium/aluminum ratio shows, on the one hand, the best compound properties. On the other hand, its oxidation and phase stability are lower compared to the coatings with a lower ratio. In contrast to titanium, aluminum forms protective oxide layers which increase thermal resistance. In the cutting tests, the nitride coating shows a slightly higher tool life compared to the oxynitride coating. The tool life of the nitride coating with an increased titanium/aluminum ratio is significantly increased compared to the other coatings.