On the oxidation behaviour of cathodic arc evaporated Al-Cr and Al-Cr-O coatings

This study reports on the temperature-induced, ambient-air-exposure related structural and morphological evolution of various Al-Cr and Al-Cr-O thin films prepared by cathodic arc evaporation with 20 sccm Ar, 50 sccm O-2, or 100 sccm O-2 per active source (p.a.s. Detailed cross-sectional scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies are conducted on as-deposited coatings and samples that have been oxidised for 3 h at 900, 1050, and 1200 degrees C. The high affinity of Al to O promotes the formation of essentially pure a-Al(2)O3 during oxidation starting between 900 and 1050 degrees C of the coatings prepared with 20 sccm Ar or 50 sccm O-2 p.a.s. and cathodes with Al content >= 50 at.%. However, the Al-Cr-O coatings prepared with 100 sccm O-2 p.a.s. form solid solution alpha-(Al,Cr)(2)O-3 oxides with Al/Cr ratios according to the used Al-Cr cathodes upon thermal exposure to ambient air. The phase composition (prior to oxidation) strongly determines the oxide phase formation. A sufficient intermixing of Al,Cr, and O (with about 60 at.% O) is needed to form alpha-(Al,Cr)(2)O(3 )oxides, otherwise the high affinity of Al to O would rather lead to the formation of Al2O3.