The Effect Of Nitride Coatings On The Oxidation And Corrosion Behaviour Of Ti-6Al-4V

Mono-layered Ti_0.7Al_0.3N, Ti_0.5Al_0.5N and Ti_0.6Al_0.3Si_0.1N films, double-layered Ti_0.5Al_0.5N/Ti_0.7Al_0.3N and Ti_0.6Al_0.3Si_0.1N/Ti_0.7Al_0.3N films were deposited on Ti6Al4V alloy by the arc ion plating （AIP） method. The isothermal and cyclic oxidation resistance at 550-750oC and corrosion resistance at 450-550oC of the coated specimens were investigated.It was found that the mass gain of the Ti6Al4V alloy was significantly decreased by the films at 650oC and 750oC during isothermal oxidation. All the films exhibited good oxidation resistance and provided effective protection for the substrate at 650oC. At 750°C, a good isothermal oxidation resistance still remained for the Ti_0.6Al_0.3Si_0.1N-coated and Ti_0.6Al_0.3Si_0.1N/Ti_0.7Al_0.3N-coated Ti6Al4V but not for the Ti_0.5Al_0.5N/Ti_0.7Al_0.3N-coated and Ti_0.5Al_0.5N-coated alloy. The reason is that the addition of Si can assists Al selective oxidation in forming an Al₂O₃ layer.It was found that the deposition of all the films significantly improved cyclic oxidation resistance of the Ti6Al4V alloy at 550°C and 650°C, due to the formation of a thin and continuous Al₂O₃ scale. At 750°C, a good cyclic oxidation resistance still remained for the Ti_0.6Al_0.3Si_0.1N-coated and Ti_0.6Al_0.3Si_0.1N/Ti_0.7Al_0.3N-coated Ti6Al4V but not for the Ti_0.5Al_0.5N/Ti_0.7Al_0.3N-coated and Ti_0.5Al_0.5N-coated alloy. The better cyclic oxidation performance of the Ti_0.6Al_0.3Si_0.1N film probably resulted from the addition of Si, which may increase the Al activity and thusly promoted in the formation of an Al₂O₃ layer during oxidation.Serious corrosion occurred on the Ti6Al4V alloy when oxidized at 450oC in the presence of solid NaCl deposit and water vapor. The deposition of Ti_0.5Al_0.5N、Ti_0.5Al_0.5N/Ti_0.7Al_0.3N、Ti_0.6Al_0.3Si_0.1N and Ti_0.6Al_0.3Si_0.1N/Ti_0.7Al_0.3N films effectively hinder the accelerated corrosion induced by solid NaCl deposit and water vapor and provided effective protection for Ti6Al4V at 450°C. Ti0.5Al0.45Si0.05N-coated Ti6Al4V exhibited good corrosion resistance when oxidized at 550°C in the presence of solid NaCl deposit and water vapor.