Phase transformation behavior of nanocrystalline Ni–W–P alloys containing various W and P contents

Abstract

In the present investigation, electroless (EL) ternary Ni–W–P coatings were prepared using hypophosphite based alkaline bath by varying sodium tungstate as tungsten source (5–80g/L). Maximum amount of W incorporation (8.2±1wt.%) was obtained when the bath contained about 20g/L of tungsten source. At very high concentrations of W source in the bath the deposit contained about 4wt.% W and 2wt.% P. All the as-deposited ternary coatings exhibited nodular surface morphology. X-ray diffractograms (XRD) obtained for as-deposited EL NiWP alloys indicated that crystallinity of the coatings increased with decrease in phosphorus content. Calculated grain size for the deposits varied from 1.2 to 12.7nm when the tungsten source varied from 5 to 80g/L in the bath. Higher crystallization temperatures were obtained due to W codeposition in NiP matrix. Presence of metastable phases such as Ni5P2 and NiP apart from stable Ni and Ni3P was identified for the heat treated deposits (400°C/1h) containing lower amount of W and higher amount of P. Whereas other ternary deposits after the heat treatment predominantly revealed face centered cubic (f.c.c.) Ni (111) peak. Activation energy for the crystallization of all the alloys has been carried out by modified Kissinger method. Microhardness measurements were carried out on all the deposits isothermally heat treated at 600°C for 1h.