Mechanical and microstructure characterization of hard nanostructured N-bearing thin coating

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, is that it exhibits
a relatively low hardness (below 10GPa) which strongly decreases upon annealing above about 600K. 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. Multi-component 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 thermo-mechanical 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-nanolayer 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 two different thermal cycling tests: one
for 100 thermal cycles consisting of 60 s dwelling time, respectively at the high- (573 to 1173 K) and at the room-temperature,
a second for 100 thermal cycles consisting of 115s dwelling time, at same temperatures of the first test, followed by 5s
dwelling at room-temperature. 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 measured by nanoindentation.