Chemical state of (Ta, Si)N reactively sputtered coating on a high-speed steel substrate

The chemical state of (Ta, Si)N coating reactively sputtered on a high-speed steel substrate was studied by XRD, AES and XPS. Reactive sputtering was conducted by using Ta5Si3 target with the flux of N2 as the major variable, ranging from 4 to 20 sccm. In the as-deposited condition, the major phase was found to be TaN containing Si in solution and with the N content increasing with the flux of N2 during deposition. The chemical state of Ta was complex in that TaN, Ta2O5 and other ionic states of Ta co-existed in the oxide. Si, in addition to the existence in TaN, also formed an Si3N4 compound. When the reactively sputtered samples were annealing heat treated at 1000°C, an outward diffusion of C and Fe from the substrate changed the chemical composition of the coating. The concentration of C and Fe increased considerably across the whole thickness of the coating while that of Si showed a small increase at the interface of the coating and the substrate. In addition, formation of TaC was confirmed in the coating. The microhardness of the coating increased with the nitrogen flux and the annealing heat treatment that also increased the critical load in the adhesion test. Formation of the multiphase compounds in the coating and enrichment of the interface with Fe was suggested to be responsible for the improvement of the mechanical properties caused by the post-coating annealing heat treatment.