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. (C) 2003 Elsevier B.V. All rights reserved.