Effects of processing variables on the mechanical properties of Ta/TaN multilayer coatings

The Ta/TaN multilayer structure with repeated layers of a poly-crystalline Ta layer of high ductility and a TaN layer of high hardness is expected to exhibit toughness. This paper reports the results on the hardnesses and the adhesion strengths of the Ta/TaN multilayer and the compositionally gradient Ta/TaN layer deposited on the high speed steel substrate by reactive sputtering as a function of annealing temperature. The TaN film deposited with the N2/Ar ratio of 0.4 in the reactive sputtering process exhibits the highest crystallinity leading to the highest hardness and adhesion strength of the Ta/TaN multilayer. The hardness and adhesion strength of the Ta/TaN multilayer becomes deteriorated with increasing the annealing temperature in the heat treatment conducted right after the deposition of the layer. Therefore, the post-annealing treatment is not desirable in the case of the Ta/TaN multilayer from the standpoint of mechanical properties. Also the hardness of the Ta/TaN multilayer increases with decreasing the compositional modulation wavelength, but the adhesion property of the layers is nearly independent of the wavelength. On the other hand, the compositionally gradient Ta/TaN film exhibits the highest hardness and adhesion strength for the post-annealing temperatures of 200 and 400°C, respectively. This result for the compositionally gradient Ta/TaN film differs from that of the Ta/TaN multilayer.