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V. Dalbauer, J. Ramm, S. Koloszvári, C.M. Koller, P.H. Mayrhofer:
"On the Thermal Stability of cathodic arc evaporated (Al1-xCrx)2O3 thin films";
Talk: 45th International Conference on Metallurgical Coatings and Thin Films (ICMCTF) 2018, San Diego, CA; 2018-04-23 - 2018-04-27; in: "45th ICMCTF", (2018), 68.

The thermo-mechanically excellently performing α-alumina (corundum-type) is a perfect candidate to protect tool or component surfaces suffering from mechanical loads in hazardous atmospheres. Thus α-alumina protective coatings significantly extend the tool-lifetime especially in oxidising environment and at high temperatures. However, a major concern is the formation of amorphous phase fractions and/or metastable Al2O3 polymorphs during low-temperature physical vapour deposition, which can effectively be counteracted by alloying with Cr, where the phase composition of (Al1-xCrx)2O3 coatings strongly depends on the Cr content.
With respect to industrial application, the knowledge about structure-property-relationships of (Al1-xCrx)2O3 as a result of thermal exposure is of utmost importance. We therefore study the structural evolution of arc evaporated (Al1-xCrx)2O3 coatings, which have been prepared by Al0.75Cr0.25, Al0.70Cr0.30, Al0.50Cr0.50, or Al0.25Cr0.75 cathodes.
The Cr-rich (Al0.49Cr0.51)2O3 and (Al0.23Cr0.77)2O3 coatings crystallise in a single-phase corundum-type structure (α-(Al,Cr)2O3) with pronounced columnar and facetted growth. Contrary, the Al-rich (Al0.72Cr0.28)2O3 and (Al0.69Cr0.31)2O3 coatings are multi-phased with a large metastable cubic-structured phase fraction and α-(Al,Cr)2O3.
Upon annealing to 800 and 950 °C, the metastable phases transform into a γ-type phase-with only minor indications for an intermediate θ-structure- and further to an α-type solid solution for temperatures above ~1080 °C. This structure stays stable up to the highest temperature tested, 1500 °C. The accompanied formation of bcc Cr phases indicates the decomposition of metallic droplets with-depending on the annealing conditions-subsequent oxidation of Al. Annealing within the spinodal-regime up to 6 h did not result in any phase separation towards α-Al2O3 and α-Cr2O3.
Thermo-mechanical properties of (Al1-xCrx)2O3 show a stronger dependence on the microstructure than on the crystal structure of the as-deposited coatings. Although exhibiting a multi-phase constitution, Al-rich coatings demonstrate higher hardness than the single-phased α-(Al0.23Cr0.77)2O3 coating, which consists of tapered crystallites. Highest H and E values of ~22 GPa and ~300 GPa are obtained for (Al0.49Cr0.51)2O3, which combines a dense microstructure with a dominant α-character. Upon vacuum annealing-and therewith associated structural transformation and densification-H and E of the Al- and Cr-rich coating compositions converge with peak values of H ~27 GPa and E ~450 GPa at 1050 °C.