Influence of silicon carbides on the structure and properties of composite nickel-phosphorus coating

The authors studied the structure, properties and corrosion resistance in various acids of nickel-phosphorus coatings with dispersed silicon carbides after crystallization annealing under various modes. Temperatures of the beginning of crystallization after heating at speeds of 1, 5, 20 °С/min and the percentage of crystalline phases formed under isothermal conditions (nickel phosphide Ni3P and nickel) were determined. High microhardness of more than 1000 HV is achieved in a composite nickel-phosphorus coating with dispersed particles of silicon carbides during prolonged low-tempera­ture annealing, accompanied by crystallization with formation of already insignificant (10 %) amounts of Ni3P. The revealed dispersed Ni3P located both in the body and along boundaries of the grain, make the main contribution to the increment of microhardness. The yield strength and ultimate strength of coatings increase during crystallization annealing by only 12 – 15 MPa, and the elongation drops to zero, which is due to the formation of brittle Ni3P compounds. Annealing with short soaking at crystallization temperatures leads to the fact that silicon carbides exhibit a barrier effect, reducing the intensity of formation of crystalline Ni3P and corrosion resistance, while long soaking at lower crystallization temperatures forms about 70 % Ni3P, contributing to consistently high hardness and improved corrosion resistance. Corrosion resistance of composite coatings Ni-P + silicon carbides, regardless of the heat treatment modes, is maximum in acetic and orthophosphoric acids at 70 % nickel phosphide and minimum in nitric acid and its mixtures with other acids.