Stress and oxidation behaviours of rf-sputtered (Ti,Al)N films

(Ti,Al)N thin films were deposited onto A2 steel by reactive r.f. magnetron sputtering. The residual stress of as-deposited films was compressive for TiN and switched to be tensile for Ti0.62Al0.39N. After heating at 600 degrees C, the TiN film spalled off and iron oxides formed on the substrate surface, while there existed an unoxidized (Ti,Al)N layer adjacent to the substrate in the oxidized (Ti,Al)N films. The near surface of the (Ti,Al)N coating was a mixture of TiO2 and amorphous Al2O3 and the diffusion of iron from the substrate through the grain boundary or pinholes resulted in the formation of iron oxides. The (Ti,Al)N films with higher Al contents after oxidation maintained a red-blue colour. The addition of Al atoms promoted the oxidation resistance of the (Ti,Al)N coating in comparison with pure TiN. As the Al contents were increased, the anti-oxidation of the (Ti,Al)N films was enhanced, while the scale thickness and the diffusion of iron through the coating were significantly decreased.