As-deposited electroless Ni–P coatings were heated in air and subsequently treated by microwave-assisted hydrogen plasma with various powers. The phase evolution and microstructure of the Ni–P coatings were studied in detail. The results indicate that the amorphous Ni–P coatings were first transformed into Ni, Ni3P, and the residual amorphous phase after heating at 300 °C in air. The NiO, Ni2P, and Ni12P5 phases with a half size of an island at the as-deposited state were formed after heating at 400 °C for 3 h. Furthermore, the 600 W or higher hydrogen plasma power was required to obtain the Ni and Ni3P phases. A short hydrogen plasma treatment of 1 min at 600 W can reduce NiO into Ni and transform Ni2P and Ni12P5 into larger N3P grains. The NiO phase was first transformed into the Ni phase, which subsequently reacted with the Ni2P into Ni3P phases. In addition, no phosphorus loss was observed. For longer plasma treatment (10 min), the nickel phase dissolved into the lattice of the Ni3P phase, and the surface morphology of the Ni–P coatings changed considerably because of the occurrence of phase transformation and partial melting.
