Effect of heat treatment on the hardness of Ti-Mo-N coating films deposited by RF reactive magnetron sputtering

Effects of heat treatment on microstructure and hardness of Ti-Mo-N coating films were investigated for various N contents. Ti-Mo-N films were deposited onto a stainless steel substrate by a reactive RF sputtering process in the mixture of argon (7.5 ccm) and nitrogen (0∼2.0 ccm) gases using Ti ₅₀Mo₅₀ target. X-ray results of as-deposited films indicated that the main phase of the Ti-Mo-N film was a bcc-(Ti, Mo) phase at a nitrogen flow rate (fN₂) ≤0.2 ccm and a δ-(Ti, Mo)N phase at fN₂ over 0.3 ccm. The Ti-Mo-N films so obtained were heat treated in argon atmosphere at 300∼1100°C for 30 minutes. Hardness was measured by a nanoindentation system. The hardness of the Ti-Mo-N films deposited at fN₂=2.0 ccm hardly changed with heat treatment. In contrast, the hardness of the films deposited at fN₂=0.2 and 0.3 ccm was drastically increased by heat treatment at temperatures higher than 900°C. In particular, the film deposited at fN₂=0.3 ccm showed the maximum hardness of about 35 GPa by heat treatment at 1000°C for 30 minutes. X-ray measurement and TEM observation indicated that the increment of hardness in the Ti-Mo-N film at fN₂ = 0.3 ccm is due to the formation of a bcc-(Ti, Mo) phase in a δ-(Ti, Mo)N phase.