High temperature oxidation of Cr-Al-O-N thin films: Complex interplay of oxide scale formation and kinetically limited decomposition

The oxidation behavior of (Cr 0.47±0.03 Al 0.53±0.03 ) z (O y N 1- y ) 1-z thin films (z = 0.475 ± 0.015) was investigated as a function of the O concentration with y = 0, 0.15, and 0.40 at 1000 °C to 1200 °C. Three regions were identified after oxidation at 1000 °C and 1100 °C: an Al-rich oxide scale (I) with a Cr-rich oxynitride gradient interlayer (II) is formed, and O levels decrease linearly within the interlayer towards the remaining film (III). Increasing the O concentration in the film reduces the amount of Cr in the oxide scale and significantly increases the stability of the remaining film during oxidation, which can be readily understood by O-induced enhancement of the activation energy of migration. Non-metal mobility is required for the decomposition of (Cr,Al)(O,N) into hexagonal Cr 2 N, and the formation of O vacancies is inhibited due to their high vacancy formation energy. All film compositions show a passivating oxidation behavior with comparable oxide scale thickness (I) of ~130 ± 30 nm after oxidation at 1000 °C for 16 h. In contrast, at 1100 °C, (Cr 0.48 Al 0.52 ) 0.48 (O 0.15 N 0.85 ) 0.52 exhibits the thinnest combination of oxide scale and oxynitride interlayer thickness (I+II), while it is 27% and 42% thicker for (Cr 0.50 Al 0.50 ) 0.49 N 0.51 and (Cr 0.44 Al 0.56 ) 0.46 (O 0.40 N 0.60 ) 0.54 , respectively. Oxidation kinetics are reduced for the oxynitride films, however, cracks are formed at the interlayer of (Cr 0.44 Al 0.56 ) 0.46 (O 0.40 N 0.60 ) 0.54 , eventually causing delamination. Consequently, (Cr 0.48 Al 0.52 ) 0.48 (O 0.15 N 0.85 ) 0.52 offers the highest oxidation resistance while exhibiting lower oxidation kinetics than Cr 2 AlC at 1100 °C. • Formation of Al-rich oxide scale and oxynitride gradient interlayer • O incorporation reduces the amount of Cr in the oxide scale • O incorporation increases the stability of the remaining film by enhancing the activation energy of migration • At 1000 °C, passivating oxidation behavior is observed • At 1100 °C, (Cr 0.48 Al 0.52 ) 0.48 (O 0.15 N 0.85 ) 0.52 offers lower oxidation kinetics than Cr 2 AlC