High temperature thermal stability of innovative nanostructured thin coatings for advanced tooling

Abstract

Tools for machining are made of hard steels and cemented carbide (WC-Co). For specialized applications, such as aluminium machining, diamond or polycrystalline cubic boron nitride are also used. The main problem with steel, isthat itexhibits a relatively low hardness (below 10 GPa) which strongly decreases upon annealing above about 600 K.Thus, the majority of modern tools are nowadays coated with hard coatings that increase the hardness, decrease the coefficient of friction and protect the tools against oxidation. A similar approach has been recently used to obtain a longer duration of the dies for aluminium die-casting. Multicomponent and nanostructured materials represent a promising class of protective hard coatings due to their enhanced mechanical and thermal oxidation properties. Surface properties modification is an effective way to improve the performances of materials subjected to thermomechanical stress. Three different thin hard nitrogen-rich coatings were mechanically, microstructurally, and thermally characterized: A 2.5 micron-thick CrN-NbN, a 11.7 micron-thick TiAlN, and a 2.92 micron-thick AlTiCrxNy. The CrN-NbN coating main feature is the fabrication by the alternate deposition of 4nm thick-nanolayers of NewChrome (new type of CrN, with strong adhesion and low coating temperature). All the three coatings can reach hardness and elastic modulus in excess of 20, and 250 GPa, respectively. Their main applications include stainless steel drawing, plastic materials forming and extrusion and aluminum alloys die-casting. The here studied TiAlN (SBN, super booster nitride) is one of the latest evolution of TiAlN coatings for cutting applications, where maximum resistance to wear and oxidation are required. The AlTiCrxNy combines the very high wear resistance characteristic of the Cr-coatings and the high thermal stability and high-temperature hardness typical of Al-containing coatings. All the coatings were deposited on a S600 tool steels. The coatings were subjected to 100 thermal cycles with 60 s dwelling time, respectively at the high- (573 to 1173 K) and at the roomtemperature. The investigated coatings showed a sufficient-to-optimal thermal response in terms of stability of hardness, elastic modulus, and oxidation behavior. The temperature induced hardness and elastic modulus coating variations were correlated to the morphology evolution and microstructure modification.