Studies on autocatalytic deposition of ternary Ni–W–P alloys using nickel sulphamate bath

An attempt has been made to deposit ternary Ni–W–P coatings autocatalytically using nickel sulphamate and sodium tungstate as nickel and tungsten sources, respectively, and sodium hypophosphite as a reducing agent. These coatings were deposited using an alkaline citrate based bath and were compared with plain Ni–P coatings. Both deposits were characterized for their structure, morphology, crystallization behavior and microhardness. Results obtained from EDX analysis showed that binary Ni–P alloy contains 11.0 wt.% of phosphorus. Incorporation of tungsten (2.7 wt.%) had reduced the phosphorus content to about 3.5 wt.% in ternary Ni–W–P deposits. Apart from phosphorus and nickel contents, a trace of sulphur was noticed in ternary Ni–W–P deposit. Structural examination carried out by XRD studies revealed the presence of a broad peak with a calculated grain size of 1.2 nm for binary alloy, where as a sharp peak with a grain size of 10 nm is obtained for ternary Ni–W–P alloy. Optical micrographs of as-plated binary Ni–P alloys exhibited slightly nodular structure compared to coarse nodular structure of ternary Ni–W–P deposits. Etched metallographic cross-sections revealed a lamellar/banded structure in both deposits. Studies on phase transformation behavior carried out by DSC showed that the crystallization temperature for ternary alloys was higher compared to binary alloys. Calculated activation energies for Ni–P and Ni–W–P deposits were 229 and 289 kJ/mol, respectively. Microhardness measurements made on as-plated and heat-treated (400 °C for 1 h) deposits showed higher hardness values for ternary Ni–W–P deposits compared to binary deposits.