Impact of Si on the high-temperature oxidation of AlCr(Si)N coatings

The resistance of wear protective coatings against oxidation is crucial for their use at high temperatures.Here,three nanocomposite AlCr(Si)N coatings with a fixed Al/Cr atomic ratio of 70/30 and a varying Si-content of 0 at.%,2.5 at.% and 5 at.% were analyzed by differential scanning calorimetry,thermogravimetric analysis and X-ray in order to understand the oxidation behavior depending on their Si-content.Additionally,a partially oxidized AlCrSiN coating with 5 at.%Si on a sapphire substrate was studied across the coating thickness by depth-resolved cross-sectional X-ray nanodiffraction and scanning trans-mission electron microscopy to investigate the elemental composition,morphology,phases and residual stress evolution of the oxide scale and the non-oxidized coating underneath.The results reveal enhanced oxidation properties of the AlCr(Si)N coatings with increasing Si-content,as demonstrated by a retarded onset of oxidation to higher temperatures from 1100℃ for AlCrN to 1260℃ for the Si-containing coatings and a simultaneous deceleration of the oxidation process.After annealing of the AlCrSiN sample with5 at.%Si at an extraordinary high temperature of 1400℃ for 60 min in ambient air,three zones developed throughout the coating strongly differing in their composition and structure:(i)a dense oxide layer comprising an Al-rich and a Cr-rich zone formed at the very top,followed by(ii)a fine-grained transition zone with incomplete oxidation and(iii)a non-oxidized zone with a porous structure.The varying elemental composition of these zones is furthermore accompanied by micro-structural variations and a complex residual stress development revealed by cross-sectional X-ray nanodiffraction.The results provide a deeper understanding of the oxidation behavior of AlCr(Si)N coatings depending on their Si-content and the associated elemental,microstructural and residual stress evolution during high-temperature oxidation.

Rapid solidification and metastable phase formation during surface modifications of composite Al-Cr cathodes exposed to cathodic arc plasma

A combination of both conventional and advanced high-resolution characterization techniques was applied to study the modified layers on the surface of three composite Al-Cr arc cathodes with identical nominal composition of Al-50 at.%Cr but varying powder grain sizes.The results revealed that the modified layers consist mainly of metastable phases such as Cr solid solution,high temperature cubic Al8 Cr5,supersaturated Al solid solution,and icosahedral quasicrystal.The metastable phase formation indicates that high cooling rates were involved during the solidification of molten material produced in the arc craters during cathode spot events.The average cooling rate was estimated to be 10^(6)K/s based on secondary dendrite arm spacing measurements and supporting phase-field based simulations.The formation mechanisms of the modified layers are discussed based on the obtained results and the current literature.