On the oxidation behavior of cathodic arc evaporated Al-Cr-Fe and Al-Cr-Fe-O coatings. II. Transmission electron microscopy investigations of Al0.675Cr0.275Fe0.05-based films

Abstract

Detailed transmission electron microscopy analyses were performed to complement preceding investigations on Al0.675Cr0.275Fe0.05-based intermetallic coatings and substoichiometric oxides after ambient air annealing between 900 and 1300 °C. Oxidation of the intermetallic coating occurs uniformly via the formation of a metastable θ-Al2O3 scale at the surface and occasionally along the grain and defect boundaries deep into the coating, which then with higher oxidation temperature phase transform into the α structure. Substoichiometric films prepared with 50 sccm O2 per active source (p.a.s., ∼25 at. % O within the coatings; consisting of a nanocompositelike structure) show a complex elemental and structural separation with preferred oxidation of Al-rich regions to α-Al2O3 already at 900 °C. Cr- and Fe-containing domains remain mostly intermetallic up to Tox = 1300 °C, where coarse-grained solid solution α-(Al,Cr,Fe)2O3 phases are formed. Selective oxidation and phase separation are also observed for coatings having initially 40–50 at. % oxygen (prepared with 100 sccm O2 p.a.s.); however, the formation of α-structured (Al,Cr,Fe)2O3 grains is observed already at 1000 °C. The possibility to alter the phase formation sequence and microstructure of Al-Cr and Al-Cr-Fe coatings by adjusting the oxygen flow during synthesis (below the threshold to stoichiometric sesquioxides) was thus corroborated.