Annealing studies and oxidation tests of a hybrid multilayer arrangement of cathodic arc evaporated Ti–Al–N and reactively sputtered Ta–Al–N coatings

• Growth of fcc Ti1−xAlxNarc/Ta1−yAlyNrsd multilayers with 20 nm bilayer period
• Enhanced thermo-mechanical properties as compared to monolithic coatings
• Significantly increased oxidation resistance compared to Ti1−xAlxNarc/Ta1−yAlyNarc

A multilayer architecture of cathodic arc evaporated Ti1−xAlxN and reactively sputtered Ta1−yAlyN layers is investigated in terms of thermal stability, thermo-mechanical properties, and oxidation resistance. In a recent study, Ti1−xAlxN/Ta1−yAlyN multilayers demonstrated increased hardness and stability upon annealing treatments as compared with their monolithically grown counterparts. However, the respective coatings prepared by arc/arc configuration were deficient in oxidation tests. The present work emphasises on an improvement in oxidation resistance taking advantage of a hybrid processes using cathodic arc evaporation for Ti1−xAlxN layers and reactive sputter deposition for Ta1−yAlyN. Variations in Ta1−yAlyN layer thicknesses are realised by varying the Ta0.75Al0.25 cathode powering between 3, 4, and 5 kW, respectively. Multilayers of thin arc evaporated Ti1 − xAlxN (supersaturated cubic structure, ~ 18 nm layer thickness) and reactively sputtered Ta1−yAlyN (supersaturated cubic structure, ~ 2 nm layer thickness) layers exhibit as-deposited hardness values of ~ 30.7–34.6 GPa. Upon vacuum annealing a hardness increase to peak values of ~ 33.6–36.4 GPa at 800 °C is triggered by spinodal decomposition of the matrix as well as slight intermixing of the individual layer interfaces. The hardness is of still ~ 31.4–33.5 GPa even after annealing at 1000 or 1100 °C. For all Ti1−xAlxN/Ta1−yAlyN multilayers, 80–90% of the initial ~ 2.6 µm thin multilayer remains unaffected even after 20 h exposure to ambient air at 850 °C, highlighting the significant increase in oxidation resistance with respect to monolithically grown Ti1−xAlxN and Ta1−yAlyN coatings.