On the oxidation behaviour 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 T₀ₓ = 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.