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V. Dalbauer, C.M. Koller, J. Ramm, S. Kolozsvári, P.H. Mayrhofer:
"Phase Formation of Cathodic Arc Evaporated AlxCr1-x and AlxCr1-xOδ Thin Films";
Talk: AVS 63rd Symposium & Exhibition, Nashville, TN, USA; 2016-11-06 - 2016-11-11; in: "Book of Abstracts", (2016), 125.

Aluminium-based oxides and oxide scales are highly valued for various demanding applications due to their outstanding thermo-mechanical properties as well as their superior resistance in oxidising and chemically hazardous environments. However, the polymorphic character of Al2O3 (and consequentially also (AlxCr1-x)2O3) synthesised at deposition temperatures lower than 800 °C is impeding its large-scale utilisation. Especially high Al containing films, being dominated by transient oxides, are susceptible for thermally-induced phase transformations, which are associated with the formation of crack networks. Therefore, the growth of the thermodynamically stable α-Al2O3 at deposition temperatures ~ 400-600 °C has been in the focus of research for many years. Although considerable advances were made, none of them proved to be applicable to industrial utilisation. A comprehensive and in-depth understanding of mechanisms, leading to the growth of transient oxides within the quasi-binary system Al2O3-Cr2O3 synthesised by cathodic arc evaporation is therefore still of major interest, as this knowledge is crucial for being able to grow coatings with dedicated crystallography and microstructure.
In the present work, we approach this issue by investigating the structural evolution of intermetallic AlxCr1-x and AlxCr1-xOδ films synthesised by arc evaporation. Depositions were carried out in non-reactive as well as reactive atmosphere, using low to intermediate O2 flow rates to examine its impact on film morphology and phase composition. In order to correlate the chemical aspect and process conditions (i.e., Cr-content and O2 partial pressure) with the accessible microstructure and crystallographic evolution, powder-metallurgically manufactured AlxCr1-x targets with different compositions were selected. By this, the significant phase regimes within the binary Al-Cr phase diagram are accessible.
In detail, arc evaporation of Al0.9Cr0.1, Al0.7Cr0.3, Al0.5Cr0.5, and Al0.25Cr0.75 targets leads to the formation of intermetallic films dominated by Al13Cr2, Al8Cr5 or AlCr2 phases, which well agrees with the equilibrium Al-Cr phase diagram. Chemical analyses demonstrate that differences in the Al/Cr-ratio between targets and films increase towards the Cr-rich side, which decrease by introducing O2 to the deposition process. Furthermore, the simultaneous broadening and intensity reduction of dominant intermetallic XRD peaks is accompanied by the emergence of weak broad signals indicative for the development of X-ray amorphous areas. Between 70 and 90 at.% Al we find a compositional window, which is characterised by a maximum target evaporation and minimum film growth rate.

AlCr intermetallics, (Al,Cr)2O3, oxides, alumina, cathodic arc evaporation

Project Head Paul Heinz Mayrhofer:
Christian Doppler Labors für anwendungsorientierte Schichtentwicklung