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GuB - 2015
Christian Koller (C.M. Koller)
"Structure and Morphology of Cathodic Arc Evaporated (Cr,Al)2O3 - based Multilayer Coatings"
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Aluminium based oxides are prominent representatives of functional coatings for protective application. Depending on the crystallographic constitution and growth conditions, a broad spectrum covering chemical, mechanical, and thermal performance can be attained. Particularly, the thermodynamically stable corundum phase attracts attention and is of great interest for industrial application. Typically synthesised by CVD, an increasing demand for low temperature PVD deposition is given. However, the growth of single-phased α-Al2O3 coatings at deposition temperatures lower than 600 °C turns out to be challenging. On this account the alloying of Cr to form a solid solution (Al,Cr)2O3, or the utilisation of a Cr2O3 and Cr-rich (Cr,Al)2O3 as seed layer are promising approaches, which, however, are not entirely satisfactorily for high Al-concentrations.
In this work we extend the concept of a bilayer architecture to an alternating multilayer arrangement. The (Cr,Al)2O3 seed layer processed from powder metallurgically produced Cr0.75Al0.25 targets is followed by either (Al,Cr)2O3 or (Al,Cr,Fe)2O3 layers for which we used Al0.7Cr0.3 and Al0.675Cr0.275Fe0.05 targets, respectively. Variations in the layer periodicity is realised by the arrangement of mounted cathode materials as well as slight variations in cathode powering.
X-ray diffraction analyses suggest the growth of multi-phased films consisting of a hexagonal and cubic phase. However, in contrast to the Al-rich (Al,Cr)2O3 coating—representing our benchmark system—the amount of cubic phase fractions can be significantly reduced. Thereby, an alternating arrangement of Cr0.75Al0.25 and Al0.7Cr0.3 (or Al0.675Cr0.275Fe0.05)2O3, hence smallest bilayer period, shows the most pronounced XRD peaks with only marginal cubic reflections. Supplementary transmission electron microscopy investigations confirm the XRD results and reveal a predominant and well-defined hexagonal morphology. Predominant metastable cubic phase fractions could only be determined at the interface to the substrate, hence where initial film growth occurs.

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