Thermal Diffusion Behavior and Internal Stress of Ni-P Coating on Titanium Alloy

Ni-P alloy coating on TC4 titanium alloy was prepared by an electroless plating method. The influence of annealing temperature on the adhesion of the Ni-P coating was investigated. Dynamic equation to describe the interface layers thickness, diffusion heat treatment temperature and holding time were established by SEM analysis. The product of the diffusion interface and the influence of different annealing temperatures on thermal stress of the coating were studied by X-ray diffraction. The results show that the Ni-P coatings has no diffusion layers when treated at 400 degrees C for 2 and 4 h, while the diffusion layers appear when treated above 400 degrees C. The dynamic equation, y=2.72exp(-6353/T)t, can be obtained according to the relations between the thickness of the diffusion layer and the time as well as temperature of heat treatment. The thermal stress of the coating increases significantly after heat treatment; the adhesion of the coating with heat treatment is much higher than that without heat treatment. The best adhesion of the coating can be achieved when the heat treatments are 400 degrees C x2 h, 400 degrees C x4 h and 500 degrees Cx4 h.