Impact of O concentration on the thermal stability and decomposition mechanism of (Cr,Al)N compared to (Ti,Al)N thin films

The composition-dependent thermal stability 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) with O concentrations of y = 0, 0.15, and 0.40 is investigated up to 1200°C and then compared to (Ti 0.56 Al 0.44 ) z (O y N 1-y ) 1- z . X-ray diffraction reveals a thermal stability limit of 1150°C independent of the O concentration, as witnessed by the formation of decomposition products, namely h-Cr 2 N for (Cr 0.50 Al 0.50 ) 0.49 N 0.51 and c-Cr for both (Cr 0.48 Al 0.52 ) 0.48 (O 0.15 N 0.85 ) 0.52 and (Cr 0.44 Al 0.56 ) 0.46 (O 0.40 N 0.60 ) 0.54 . Based on transmission electron microscopy and elastic recoil detection analysis data, the thermal stability limit is extended to 1100°C – 1150°C for all films. Chemical environment-dependent DFT calculations indicate that bond breaking limits the thermal stability. In (Cr,Al)N, N has the lowest activation energy for migration. Furthermore, the O vacancy formation energy is highest in (Cr,Al)(O,N). It has to be overcome to enable diffusion on the non-metal sublattice, which is necessary for forming decomposition products like w-AlN or c-Cr. However, once Cr-N bonds break, decomposition into h-Cr 2 N and subsequent c-Cr together with N 2 is triggered. This results in N evaporation, generating sufficient non-metal vacancies that greatly enhance diffusion and render the extensive vacancy formation energies for non-metals irrelevant. This reduction of the activation energy for mass transport on the non-metal sublattice to the migration barrier causes the similar thermal stability in (Cr 0.47±0.03 Al 0.53±0.03 ) z (O y N 1- y ) 1- z . In contrast, Al bonds break first without creating non-metal vacancies in (Ti,Al)(O,N). Thus, the high O vacancy formation energy in (Ti,Al)(O,N) significantly increases the thermal stability compared to (Ti,Al)N as well as the here investigated films.