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R. Hollerweger, D. Holec, M. Arndt, R. Rachbauer, P. Polcik, J. Paulitsch, P.H. Mayrhofer:
"Chemical and Structural Design Concepts for Increasing the Oxidation Resistance of Ti-Al-N based Coatings";
Talk: International Conference on Metallurgical Coatings and Thin Films, ICMCTF 2014, San Diego, CA, USA; 2014-04-27 - 2014-05-02; in: "Abstract Book ICMCTF 2014", (2014).

Ti1-xAlxN is a typical protective coating to increase the lifetime of machining and forming tools especially during high temperature applications and under demanding tribological conditions. Due to thermal decomposition and severe oxidation at temperatures above ~800 °C the field of their applications is limited. To counteract these tendencies quaternary systems like Ti1-x-yAlxTayN were developed and successfully implemented. However, especially the mechanisms for increased oxidation resistance and the ideal chemical composition for an optimized behavior are still not clarified and understood.
Therefore, we have reactively deposited Ti1-x-yAlxTayN coatings with a Ti/Al ratio of 51/49 and 35/65 and Ta contents of 0, ~8, and ~16 at%. By using isothermal Differential Scanning Calorimetry combined with Thermal Gravimetric Analysis we observe for single cubic phased Ti0.32Al0.60Ta0.08N a mass gain of only ~5% after 5h at 950 °C in synthetic air, whereas Ti0.35Al0.65N is completely oxidized after 15 min (mass gain ~24 %). Structural investigations by X-Ray Diffraction and Scanning Electron Microscopy reveal anatase-to-rutile phase transformations with increasing oxidation time and a porous scale for Ta-free Ti0.35Al0.65N. Contrary, Ti0.32Al0.60Ta0.08N exhibits a highly dense and rutile dominated and protective scale.
Density Functional Theory simulations of the phase stabilities throughout the ternary system rutile (R) and anatase (A) (Ti,Al,Ta)O2, corundum () type (Al,Ta,Ti)2O3, and orthorhombic (Ta,Ti,Al)2O5 show that in the case of Ti0.35Al0.65N the transformation A +   A + R +   R +  occurs. If Ta is alloyed a rutile phase field opens even at 0 K which allows for the direct formation of an R +  scale. This indicates that the phase transformation - which is accompanied by a volume change of 5-10% leading to the formation of pores and cracks within the scale - can be avoided for Ti0.32Al0.60Ta0.08N.
Based on these results we can conclude that for increased oxidation resistance the coatings chemical composition has to be optimized to the respective oxide phase diagram to allow for alumina and a Ti-oxide-based phase without or minimal anatase to rutile phase transformation up to the application temperature of the coating.