Ti-Al-N/Mo-Si-B multilayers: An architectural arrangement for high temperature oxidation resistant hard coatings

Abstract

State of the art mechanical engineering is in need of high temperature and oxidation resistant materials. Major aspects in the design of highly oxidation resistant coatings are the formation of a continuous, non-volatile oxide scale, the adherence of this scale as well as retarded transport mechanisms within the scale. Considering these criteria combined with the industrial widespread requirements for excellent mechanical properties (hardness≥25GPa), we present a sophisticated design concept of extremely oxidation resistant multilayered Ti-Al-N/Mo-Si-B coatings. Through the variation of the bilayer period (from 7 to 237nm for our Ti0.57Al0.43N/Mo0.58Si0.28B0.14 multilayers), hardness, growth morphology, thermal stability and oxidation resistance are adjusted. Detailed high resolution transmission electron microscopy, as well as X-ray diffraction investigations of as deposited and annealed coatings highlight, that coatings with a bilayer period of 37nm (composed of 6nm thin Mo-Si-B and 31nm thick Ti-Al-N layers) are superior to the other variations. Only if the individual Ti-Al-N layers are thinner than 21nm the hardness of our Ti0.57Al0.43N/Mo0.58Si0.28B0.14 multilayers is below that of Ti0.57Al0.43N coatings without a multilayer architecture. Isothermal thermo-gravimetric analysis in oxidative atmosphere for 420min at 700, 800, and 900°C, yield to growth rates (parabolic growth rate constant, kp*) of 8.2·10−5, 8.6·10−4, and 3.2·10−3 (mg/mg)2·s−1 for the 37-nm-bilayer multilayer, respectively. Above 1000°C, the significant formation of volatile Mo-based oxides, lead to a rapid degradation of these coatings. Based on our results we can conclude that the combination of very thin high-temperature stable Mo-Si-B with Ti-Al-N layers represents an alternative concept to gain oxidation resistant coatings up to 1000°C, while keeping the well-known mechanical properties of Ti-Al-N.