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High temperature oxidation and wear properties of magnetron sputtered AlTiTaN based hard coatings

par Khetan, Vishal
Président du jury Godet, Stéphane
Promoteur Delplancke, Marie-Paule
Co-Promoteur Choquet, Patrick P.
Publication Non publié, 2016-01-27

Thèse de doctorat

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Résumé :

Hard nanostructured TiAlN coatings have gained high importance in the field of protective tribological coatings. Nevertheless, their use regarding high temperature (>800°C) applications such as dry high speed machining still remains a challenge. Addition of elements such as Ta or Y has shown a beneficial impact on these properties. But for a better performance of these coatings, an in-depth understanding of their oxidation and wear mechanisms over a wider range of temperatures is needed which is currently unavailable in the literature. This work investigated the wear and oxidation properties of AlTiTaN hard coatings deposited by reactive magnetron sputtering at a substrate temperature of 250°C. Depending on process conditions, coatings with a preferential crystallographic orientation of cubic {111} or {200} with a columnar microstructure were observed. The oxidation and wear mechanisms for these coatings were investigated between 700°C and 950°C in air for various test durations. Further, the influence of Y doping in AlTiTaN coating was also studied.By combining Dynamic-Secondary Ion Mass Spectrometry , X-ray diffraction (XRD) and Transmission Electron Microscopy measurements, it was demonstrated that a single amorphous oxide layer comprising of Ti, Al and Ta oxides formed at 700°C became a bilayer composed of a crystalline Al rich layer (protective Al2O3) and a Ti/Ta rich oxide layer at 900°C. The oxidation mechanism was governed primarily by inward diffusion of O at 700°C while from 800°C onwards outward diffusion of Al and inward diffusion of O controlled the reaction rate. A correlation between the oxidation kinetics and wear mechanism of AlTiTaN coatings, investigated at 700°, 800° and 900°C, was established.