Effect of P codeposition on the structure and microhardness of Co–W coatings electrodeposited from gluconate bath

Abstract

Effects of addition of P to Co-W coatings from gluconate bath using direct current (DC) and pulse current (PC) methods have been investigated in this study. Co-W-P coatings with different P concentrations are prepared by varying hypophosphite concentration in the bath. Current efficiency of the Co-W-P electrodeposition is lower than that for Co-W coatings. Increase in NaH 2 PO 2 concentration increases the cobalt content significantly and decreases the tungsten content drastically. Co-W-P coatings display 'cauliflower-like' morphology and roughness of the coatings increases with increasing P content. As-deposited Co-W-P deposits are amorphous while heat treatment at different temperatures has rendered them crystalline with the precipitation of stable species like, Co 3 W, Co 2 P, etc. Unlike Co-W coatings, Co-W-P shows two-step crystallization in differential scanning calorimetry (DSC) and on heat treatment, which is similar to the behavior of Co-P electrodeposited from gluconate baths. Moreover, inclusion of phosphorous and heat treatment have led to significant increase in microhardness of the Co-W-P coatings. X-ray photoelectron spectroscopy (XPS) studies provide a detailed insight into the nature of Co, W and P species in as-deposited and sputtered coatings. Microhardness of the heat-treated coatings is higher than the as-deposited counterparts and is comparable with that of hard chromium.