Structural, mechanical and thermal properties of CrAlNbN coatings

Alloying is an efficient way to improve or tailor the properties of CrAlN coatings, which have been extensively used as wear protective layers onto cutting and forming tools. Here, we have investigated the influence of Nb on the mechanical properties, thermal stability and oxidation resistance of CrAlN coatings. Cr0.48Al0.52N, Cr0.38Al0.57Nb0.05N, and Cr0.35Al0.55Nb0.10N coatings, deposited by cathodic arc evaporation, reveal a single-phase face-centered cubic structure with a hardness of 30.8 ± 0.9, 32.3 ± 0.8 and 32.3 ± 1.0 GPa, respectively. The addition of Nb improves the thermal stability of CrAlN via postponing the dissociation of CrN bonds from 1100 °C for CrAlN to 1200 °C for CrAlNbN, which endows Nb-containing coatings with the higher hardness upon annealed at elevated temperatures. Nevertheless, alloying with Nb decreases the proportion of protective (Cr, Al)2O3 layer and promotes the early formation of CrNbO4 and thus weakens the oxidation resistance of CrAlN coating. After oxidation at 1000 °C with a duration of 10 h, the Cr0.38Al0.57Nb0.05N and Cr0.35Al0.55Nb0.10N coatings exhibit a thicker oxide scale of ~0.7 and ~0.8 μm with Cr-rich/(Al, Nb)-rich bilayered structure, while the Cr0.48Al0.52N still have a dense oxide layer with a thickness of ~0.2 μm.