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

Pauline Kümmerl; Ganesh Kumar Nayak; Felix Leinenbach; Zsolt Czigány; Daniel Primetzhofer; Szilárd Kolozsvári; Peter Polcik; Marcus Hans; Jochen M. Schneider

paper

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

arXiv:2601.20691

Abstract

The composition-dependent thermal stability of (Cr $_{0.47 \mp 0.03}$ Al $_{0.53 \mp 0.03}$ ) $_{z}$ (O $_{y}$ N $_{1 - y}$ ) $_{1 - z}$ thin films 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 TEM and ERDA data, the thermal stability limit is extended to 1100 - 1150 °C. 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 $_{0.47 \mp 0.03}$ Al $_{0.53 \mp 0.03}$ ) $_{z}$ (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.