In-situ XRD studies of arc evaporated Al-Cr-O coatings during oxidation

Abstract 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 Al 0.90 Cr 0.10 , Al 0.75 Cr 0.25 , Al 0.70 Cr 0.30 , Al 0.50 Cr 0.50 , or Al 0.25 Cr 0.75 cathodes and using either 20 standard cubic centimetres per minute (sccm) Ar, 50 sccm O 2 , or 100 sccm O 2 per active source (p.a.s.). Generally, the crystallinity of the as-deposited coatings decreases with increasing O 2 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) 2 O 3 phases at ~900 °C during annealing in O 2 atmosphere. Those, prepared from the Cr-rich cathode (Al 0.25 Cr 0.75 ) with 20 sccm Ar or 50 sccm O 2 p.a.s., exhibit the formation of α-(Al,Cr) 2 O 3 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) 2 O 3 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 O 2 p.a.s., however, show the formation of α-(Al,Cr) 2 O 3 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 Al 2 O 3 ) 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) 2 O 3 could be detected for cathodes with Al ≥ 50 at.% prepared with 20 sccm Ar or 50 sccm O 2 p.a.s. Coatings prepared with Al 0.75 Cr 0.25 , Al 0.70 Cr 0.30 , and Al 0.50 Cr 0.50 cathodes and 100 sccm O 2 p.a.s. show the formation of θ-(Al,Cr) 2 O 3 already in the as-deposited state. α-(Al,Cr) 2 O 3 could only be detected for as-deposited coatings when preparing them with Al 0.25 Cr 0.75 and 100 sccm O 2 p.a.s.