Thermal stability and oxidation resistance of arc evaporated TiAlN, TaAlN, TiAlTaN, and TiAlN/TaAlN coatings

Phase stability and oxidation resistance are main objectives when synthesising hard and protective coatings for applications requiring also high thermal stability. Even though TiAlN is a well-studied and nowadays widely used high performance coating, the demand for further optimisation is omnipresent. Recent investigations on quaternary compounds demonstrate that the alloying of Ta to TiAlN films not only results in enhanced phase stability, but also in a significantly increased oxidation resistance. In this study we address thermal investigations of reactive cathodic arc evaporated coatings and elucidate the diverse performance of monolithically grown TiAlN, TaAlN, TiAlTaN, and a multilayered architecture of TiAlN and TaAlN layers. Subtle variations of the bilayer period between 30 and 38 nm were realised by varying the arc current at the TaAl cathode. search demonstrates that the quaternary Ti0.45Al0.36Ta0.19N and the multilayered TiAlN/TaAlN coatings feature enhanced mechanical properties and thermal stability as compared with their monolithically grown constituents Ti0.54Al0.46N and Ta0.89Al0.11N. All coatings synthesised exhibit as-deposited hardness values of 32 ± 1 GPa, but only the quaternary Ti0.45Al0.36Ta0.19N and the multilayered TiAlN/TaAlN coatings demonstrate pronounced age-hardening with peak hardness values of 37 ± 2 and 35 ± 2 GPa for annealing temperatures of ~ 1000 and 1100 °C, respectively. Complementary X-ray diffraction and differential scanning calorimetry confirm their enhanced phase stability. Even though, also the multilayered design shifts the formation of wurtzite-structured AlN to higher temperatures, only quaternary Ti0.45Al0.36Ta0.19N could withstand ambient air oxidation at 850 °C for 20 h. This is based on the ability of forming a nearly single-phased dense protective mixed oxide scale, having an outermost Al-rich composition. Approximately 70% of this quaternary nitride remained unaffected from oxidation. With the present study we conclusively demonstrate that thermomechanical properties of cathodic arc deposited TiAlN coatings can significantly be enhanced by either forming a quaternary compound or sophisticated architectural design with tantalum, both allow for wide-ranged industrial applications.