| Multilayer coatings of TiN/ZrN and Ta/TaN were deposited on a high-speed tool-steel substrate by a reactive sputtering method, and their mechanical properties were evaluated in terms of processing parameters, such as the nitrogen flux, the compositional modulation wavelength, and the annealing temperature. The hardness and the adhesion strength of the TiN/ZrN multilayer coatings increased with the nitrogen flux, but excessive nitrogen flux caused a deterioration of the properties due to the formation of microcracks. A flux ratio, N₂/Ar, of 0.2 was found to be the optimum condition for the mechanical properties. The two properties of the coatings increased with the annealing temperature also, although temperatures higher than 400℃ were detrimental for the mechanical properties because of oxidation. A smaller compositional modulation wavelength resulted in both higher hardness and better adhesion, which agreed with previous observation on the effect of the coherency strains of the TiN/ZrN interface on the hardness and the adhesion. The TaN film deposited with the F_(N2)/F_(Ar) ratio of 0.4 in the reactive sputtering process exhibits the highest crystallinity, and the highest hardness and the adhesion strength of the Ta/TaN multilayers. The hardness and adhesion strength of the Ta/TaN multilayers becomes deteriorated with increasing the annealing temperature in the heat treatment right after depositing the layers. Therefore, post-annealing treatments are not desirable for the Ta/TaN multilayers from the standpoint of mechanical properties. Also the hardness of Ta/TaN multilayers increases with decreasing the compositional modulation wavelength, but the adhesion property of the layers is nearly independent of the wavelength. |
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