The Microstructure and Tribological Behavior of Ultrasonic Electroless Ni-P Plating on TC4 Titanium Alloy with Heat-Treatment

The ultrasonic electroless Ni-P plating is fabricated on the TC4 titanium alloy pretreated by zinc double immersion and alkaline pre-plating, and then is carried out heat-treatment at different temperature. The effects of heat-treatment on the microstructure and phase structure of the ultrasonic electroless Ni-P plating and their tribological behavior are investigated by SEM/EDS, XRD, a Vikers indenter, and a pin-on-disk wear testing machine. It is observed that the morphology of the ultrasonic electroless Ni-P plating has the typical cauliflower-like nodular structure and the average size of nodular is approximately 7 μm. EDS studies suggest that the phosphorous content of the plating is as high as 11 at%. The ultrasonic electroless Ni-P plating exhibits obvious amorphous structure characteristics by XRD analyzers. After heat-treatment at 400 °C, the structure of the ultrasonic electroless Ni-P plating changes from amorphous to crystalline state, and the interface of the nodular structure becomes clear and the nodular size is refined. It is because the dispersed Ni3P second-phase particles are uniformly and continuously distributed in the nickel solid solution. Compared with the TC4 substrate, the microhardness and the wear resistance of the ultrasonic electroless Ni-P plating are greatly improved, especially for the heat-treatment at 500 °C. The wear mechanism of the TC4 substrate is severe adhesion and abrasion wear, with slight oxidation wear, while the ultrasonic electroless Ni-P plating shows plastic deformation and microabrasion wear. After heat-treatment, the wear mechanism of the ultrasonic electroless Ni-P plating is slight abrasion wear.