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ICMCTF - 2014
Robert Hollerweger (R. Hollerweger)
"Chemical and Structural Design Concepts for Increasing the Oxidation Resistance of Ti-Al-N based Coatings"
R. Hollerweger1, D. Holec2, M. Arndt3, R. Rachbauer3, P. Polcik4, J. Paulitsch1,5, P. H. Mayrhofer1,5

1 Christian Doppler Laboratory for Application Oriented Coating Development at the Institute of Materials Science and Technology, Vienna University of Technology, A-1040 Vienna, Austria
2 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700 Leoben, Austria
3 OC Oerlikon Balzers AG, LI-9469 Balzers, Principality of Liechtenstein
4 Plansee Composite Materials GmbH, D-86983 Lechbruck am See, Germany
5 Institute of Materials Science and Technology, Vienna University of Technology, A-1040 Vienna, Austria
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.

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