On the oxidation behaviour of cathodic arc evaporated Al-Cr-Fe and Al-Cr-Fe-O coatings. I

Abstract

Arc evaporated Fe-alloyed intermetallic Al-Cr and corresponding substoichiometric oxides [by introducing either 50 or 100 sccm O₂ per active source (p.a.s.) during the deposition] were investigated with respect to their oxidation behavior after annealing in ambient air between 900 and 1300 °C for different durations. The authors could thereby show that depending on the initial coating morphology and chemical composition, significantly different microstructures evolve. The intermetallic coatings initially form metastable alumina scales, which transform into the thermodynamically stable corundum (α-Al₂O₃) structure during annealing in ambient atmosphere. Coatings prepared with 50 sccm O₂ p.a.s. show α-Al₂O₃ already after 3 h at T₀ₓ = 900 °C, whereas (Al₁₋ₓ₋ᵧCrₓFeᵧ)₂O₃ solid solutions are only detected after oxidation of the coating grown with 100 sccm O₂ p.a.s. for higher oxidation temperatures. Thereby, the presence of Fe (in particular, 5 at. %) leads to the formation of one (Al₁₋ₓ₋ᵧCrₓFeᵧ)₂O₃ solid solution without additional α-Al₂O₃ fractions.