Microstructure evaluation and strengthening mechanism of Ni–P–W alloy coatings

Ni–P–W alloy coatings were deposited onto AISI420 steel substrates by both RF magnetron sputtering and electroless plating techniques. The coating hardness was investigated through the microhardness testing with a Knoop indenter. The strengthening mechanism of the Ni–P–W coatings in both as-deposited and heat-treated states were described with respect to compositional distribution and microstructural evolutions. In the as-deposited state, all the coatings exhibited amorphous/nanocrystalline structure. After heat treatment, the amorphous phase of Ni–P–W coatings transformed into Ni3P precipitates and Ni(W) solid solution. The coatings were strengthened by the precipitation of Ni–P compounds and dissolving of W in the crystallized Ni matrix. With the aid of microstructure, the strengthening mechanism for the Ni–P–W coatings were proposed. Quantitative analysis for the strengthening effect of the Ni–P–W coatings was performed based on the elemental concentrations, Ni–P compound precipitation and Ni(W) matrix ratio. The heat-treated Ni76.7P15.9W7.4 coating showed the highest peak hardness of 17.5 GPa when the maximum W solution in the Ni matrix was achieved.