Influence of Nitrogen Content on the Microstructure Evolution and Oxidation Resistance toward Ambient Air of CrAlSiN Coatings Deposited by FCVAD Technique

In this study, CrAlSiN coatings with nitrogen contents ranging from approximately 42 to 54 at. % were deposited and subsequently exposed to temperatures of 700 or 900 °C for 2 h in ambient air to investigate the impact of nitrogen content on the microstructure evolution and high-temperature oxidation resistance. It was found that the CrAlSiN coating with nitrogen content of 42-45 at. % exhibited an amorphous/nanocrystalline hybrid structure, comprising pure metallic Cr and (Al,Cr)N phases within the matrix, as determined by the TEM, XRD, and XPS analysis. In contrast, as the nitrogen content exceeded 52 at. %, the coatings transformed to a dominantly columnar structure featuring solely Cr(Al)N phase. The CrAlSiN coating with a nitrogen content of ∼52 at. % retained amorphous fraction of around 20% but demonstrated superior oxidation resistance with the lowest parabolic rate constant of 1.1 × 10^-14 cm²/s at 900 °C compared to other coatings. This enhanced performance was primarily attributed to the high stability of Cr(Al)N phase and formation of a fine-columnar/amorphous microstructure devoid of metallic Cr, resulting in a significantly thin (∼60 nm) yet dense Al₂O₃-rich monolayer atop the coating surface during the oxidation. Conversely, substoichiometric CrAlSiN coatings with N content ≤45 at. % would form a thick Cr₂O₃ outer layer over an underlying Al₂O₃ layer. Additionally, elemental segregation of Cr, Si, and Al was observed in all CrAlSiN coatings after exposure to 900 °C for 2 h, which was induced by the spinodal decomposition of the ternary nitride phases.