The Effect of Electroless Bath Parameters and Heat Treatment on the Properties of Ni-P and Ni-P-Cu Composite Coatings

Ni-P-(Cu) composite coatings were applied on a St37 steel in an acidic bath of hypophosphite composite. The effect of the concentration of the Cu particles and pH of the solutionon the amount of Ni and P of the coatings, their morphology, and hardness of the coatings were explored. Some of the coated samples were also heat treated at 400ºC for 1 hour. The phases formed, the microstructure and the amount of the alloying elements of the coatings were analyzed by X-Ray powder diffraction (XRD), Scanning Electron Microscope (SEM) equiped with Energy Dispersive Spectroscopy (EDS). Results show that although adding Cu particles reduced the hardness of Ni-P coating (from 482.4 to 351.2 VH in 1 gr/lit Cu), within Ni-P-Cu composite coatings the hardness of Ni-P-Cu composite coating increased from 351.2 to 380.7 VH by increasing the Cu particles from 1 to 7 gr/lit. The structure of the coating was crystalline during the heat treatment making the coating harder. With increasing the pH of the solution from 4.5 to 7 and then to 9, the weight percent of P and the particle of Cu were reduced.