Oxidation of arc-evaporated Al1−xCrxN coatings

Abstract

The recently introduced Al1−xCrxN coatings are characterized by superior thermal stability and oxidation resistance. Although several studies on the microstructure and mechanical properties of these coatings have been published, the oxidation behavior and mechanisms were only investigated for Al1−xCrxN with Cr∕Al ratio bigger than 1. Within this work, the oxidation of arc-evaporated Al1−xCrxN coatings was investigated as a function of the Al content. Coatings were deposited onto cemented carbide substrates using an industrial-scale arc evaporation system (Balzers rapid coating system) and alloyed Al∕Cr targets yielding Al1−xCrxN coatings ranging from x=0.29 to x=0.79. Heat treatments in air were conducted in the temperature range between 900 and 1100°C. For comparison, samples were also annealed in Ar between 900 and 1200°C. Elemental distributions within the coating and the oxide layers were determined by secondary ion mass spectroscopy. After annealing in Ar, Cr was found to segregate to the coating surface following the decomposition of the face-centered cubic Al1−xCrxN phase into hcp AlN and, under release of N2, Cr2N and later on Cr. Simultaneously, Al depletion was observed within the Cr enriched near-surface zone. In contrast, the decomposition of the cubic Al1−xCrxN phase is severely hindred during annealing in air. Outward diffusion of Cr and Al, N depletion, and inward diffusion of O were observed, manifesting diffusion mechanisms depending on the initial coating composition. A protective mixed Cr2O3–Al2O3 scale, at the coating surface mainly dominated by Cr2O3, was formed for all coating compositions investigated. This protective oxide scale prevents the decomposition of the cubic Al1−xCrxN phase during high temperature exposure.The recently introduced Al1−xCrxN coatings are characterized by superior thermal stability and oxidation resistance. Although several studies on the microstructure and mechanical properties of these coatings have been published, the oxidation behavior and mechanisms were only investigated for Al1−xCrxN with Cr∕Al ratio bigger than 1. Within this work, the oxidation of arc-evaporated Al1−xCrxN coatings was investigated as a function of the Al content. Coatings were deposited onto cemented carbide substrates using an industrial-scale arc evaporation system (Balzers rapid coating system) and alloyed Al∕Cr targets yielding Al1−xCrxN coatings ranging from x=0.29 to x=0.79. Heat treatments in air were conducted in the temperature range between 900 and 1100°C. For comparison, samples were also annealed in Ar between 900 and 1200°C. Elemental distributions within the coating and the oxide layers were determined by secondary ion mass spectroscopy. After annealing in Ar, Cr was found to segregate to the coating sur...