Microstructure and Properties of Ni-P@WC Coatings Prepared by Electroless Composite Plating on A356 Aluminum Alloy Surface

Ni-P and Ni-P@WC coatings were prepared on the surface of A356 aluminum alloy substrate by electroless plating, and their microstructure and properties were investigated. Results showed that the obtained Ni-P and Ni-P@WC coatings possessed uniform distribution of elements. The surface of Ni-P coatings was flat and dense, while that of Ni-P@WC coating was rough with small tumors. Both coatings were composed of cell-like amorphous structure with a medium-high phosphorus composition, and nano-WC particles were dispersed in the Ni-P matrix. Moreover, the microhardness of the A356 substrate was significantly increased after coated with Ni-P or Ni-P@WC, which were 636.5 HV1 N and 917.8 HV1 N, respectively. The introduction of nano-WC particles to the Ni-P matrix could improve the resistance of plastic deformation of coatings and further intensify the wear resistance. Furthermore, the Tafel polarization curve test results displayed that the self-corrosion potential of Ni-P and Ni-P@WC coatings shifted to positive values significantly in comparision of that of the A356 substrate, and their self-corrosion current densities reduced apparently. Compared with the Ni-P coating, the Ni-P@WC coating possessed a lower self-corrosion current density（3.73 μA/cm2）, a higher polarization resistance（20.30 kΩ·cm2） and a lower corrosion rate, presenting the superior corrosion resistance to the A356 substrate.