Structural and phase transformation behaviour of electroless Ni-W-Cr-P alloy coatings on stainless steel

Ni-W-Cr-P alloy coatings were prepared by electroless deposition on stainless steel. The effects of heat-treatment on the structure and phase transformation behavior, microhardness of the Ni-W-Cr-P alloy coatings were mainly investigated. XRD analysis shows that as-deposited Ni-W-Cr-P coatings were microcrystalline; the precipitation of Ni₃P was observed at the annealing temperature of 400°C; after heat-treatment at 500°C the crystallization of Ni₃P and Ni was near completion; at 600°C new phase Cr_1.12Ni_2.88 was observed and Ni₃P began to decompose. Cr_1.07Fe_18.93 and Ni₁₇W₃ were formed when heated at 700°C, and Ni₃P was not found. With increasing temperature to 800°C, FeNi₂P and Cr₄Ni₁₅W were the only two dominant phases. The experimental results reveal that with an increase in the annealing temperature, micro-hardness of the Ni-W-Cr-P alloy coatings increased, reached the maximum value at 700°C, and then decreased slightly. Annealing temperature dependence of the microhardness and corrosion resistance of the coatings was also observed. The related strengthening mechanism in the electroless deposited Ni-W-Cr-P alloy coatings is also discussed.