The structural evolution of cathodic arc evaporated Al-Cr-O coatings with hugely different Al, Cr, and O contents was investigated by in-situ XRD measurements upon annealing in oxygen up to 1200 °C. Different chemical compositions are achieved by arc evaporation of Al0.90Cr0.10, Al0.75Cr0.25, Al0.70Cr0.30, Al0.50Cr0.50, or Al0.25Cr0.75 cathodes and using either 20 standard cubic centimetres per minute (sccm) Ar, 50 sccm O2, or 100 sccm O2 per active source (p.a.s.). Generally, the crystallinity of the as-deposited coatings decreases with increasing O2 flow rate during arc evaporation, while their Al/Cr ratio increases significantly (with respect to the cathode composition). The coatings prepared from Al-rich cathodes show first indications for the formation of corundum structured α-(Al,Cr)2O3 phases at ~900 °C during annealing in O2 atmosphere. Those, prepared from the Cr-rich cathode (Al0.25Cr0.75) with 20 sccm Ar or 50 sccm O2 p.a.s., exhibit the formation of α-(Al,Cr)2O3 phases already at 700–800 °C. Their corresponding coatings prepared with the highest oxygen flow rate of 100 sccm p.a.s., exhibit a considerable phase fraction of α-(Al,Cr)2O3 already in the as-deposited state.
Upon oxidation at 1200 °C, most coatings are basically composed of α-oxides, with a wide range of their Al/Cr-ratio. The coatings prepared with 100 sccm O2 p.a.s., however, show the formation of α-(Al,Cr)2O3 with a rather narrow Al/Cr-ratio, which is due to the sufficient mixing of Al, Cr, and O species already in the as-deposited state. Furthermore, selective oxidation (preference for Al2O3) leads in most cases to the formation of Cr, which is detectable up to 1200 °C.
Between 950 and 1050 °C, the formation of θ-(Al,Cr)2O3 could be detected for cathodes with Al ≥ 50 at.% prepared with 20 sccm Ar or 50 sccm O2 p.a.s. Coatings prepared with Al0.75Cr0.25, Al0.70Cr0.30, and Al0.50Cr0.50 cathodes and 100 sccm O2 p.a.s. show the formation of θ-(Al,Cr)2O3 already in the as-deposited state. α-(Al,Cr)2O3 could only be detected for as-deposited coatings when preparing them with Al0.25Cr0.75 and 100 sccm O2 p.a.s.