On the oxidation behaviour of cathodic arc evaporated Al[sbnd]Cr and Al[sbnd]Cr[sbnd]O coatings

Abstract

This study reports on the temperature-induced, ambient-air-exposure related structural and morphological evolution of various AlCr and AlCrO thin films prepared by cathodic arc evaporation with 20 sccm Ar, 50 sccm O2, or 100 sccm O2 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 °C.The high affinity of Al to O promotes the formation of essentially pure α-Al2O3 during oxidation starting between 900 and 1050 °C of the coatings prepared with 20 sccm Ar or 50 sccm O2 p.a.s. and cathodes with Al content ≥50 at.%. However, the AlCrO coatings prepared with 100 sccm O2 p.a.s. form solid solution α-(Al,Cr)2O3 oxides with Al/Cr ratios according to the used AlCr 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 α-(Al,Cr)2O3 oxides, otherwise the high affinity of Al to O would rather lead to the formation of Al2O3.