Ab initio supported development of Nb- and Ta-alloyed (Ti,Al)N thin films with improved thermal stability

Metastable (Ti,Al)N thin films with face-centered cubic (fcc) structure are widely applied to protect tools and components used in the machining industry for their high hardness and exceptional thermal stability. Alloying is a simple but powerful strategy to further improve the performance of (Ti,Al)N thin films. Here, the combination of experiments and ab initio calculations allowed us to further improve the mechanical properties, thermal stability, and oxidation resistance of arc-evaporated (Ti,Al)N films through the addition of Nb and Ta. The developed Ti0.40Al0.53Nb0.07N, Ti0.34Al0.52Nb0.14N, Ti0.40Al0.53Ta0.07N, and Ti0.34Al0.50Ta0.16N thin films show a single-phase fcc structure like Ti0.46Al0.54N. While the hardness (H) of as-deposited (Ti,Al,Nb)N thin films is similar to that of Ti0.46Al0.54N (30.1 ± 0.4 GPa), it increases to 33.2 ± 0.7 and 34.1 ± 1.1 GPa by alloying with 7 and 16 at.% Ta (metal fraction), respectively. Both alloying elements allow to retard the spinodal decomposition of (Ti,Al)N, as suggested by X-ray diffraction of vacuum-annealed samples. The (Ti,Al)N and (Ti,Al,Nb)N samples exhibit the onset of wurtzite (w-) AlN formation at ~1050 °C, which is postponed to ~1200 °C for the (Ti,Al,Ta)N thin films. Ab initio calculations suggest that age hardening of (Ti,Al)N is improved by Nb and Ta, with Nb being more effective than Ta. This is verified by experiments showing that Ti0.40Al0.53Nb0.07N (Ti0.34Al0.52Nb0.14N) and Ti0.40Al0.53Ta0.07N (Ti0.34Al0.50Ta0.16N) experience an increase of H to 34.7 ± 0.5 GPa (36.7 ± 0.7 GPa) and 35.8 ± 1.0 GPa (37.0 ± 1.1 GPa) when annealed at 1000 °C, compared to the 32.4 ± 0.7 GPa of Ti0.46Al0.54N annealed at 900 °C. The relative H increase is thus higher for (Ti,Al,Nb)N than for (Ti,Al,Ta)N. Furthermore, Nb and Ta also improve the oxidation resistance of (Ti,Al)N with Ta being more effective than Nb. Based on these results we can conclude that both Nb and Ta elements improve hardness, thermal stability, and oxidation resistance of (Ti,Al)N, with Nb providing more relative hardness gain but Ta providing higher absolute values.