Microstructural, surface and electrochemical properties of pulse electrodeposited Ni-W/Si3N4 nanocomposite coating

In this study, Ni-W/Si3N4 nanocomposite coatings were fabricated through pulse electrodeposition from an aqueous ammonia-citrate bath containing suspended silicon nitride (Si3N4) nanoparticles. The microstructural, surface and electrochemical properties were evaluated using SEM, AFM, EDS, XRD, XPS and EIS techniques. The effects of Si3N4 nanoparticles on the compositions and properties of the deposits were investigated. The results demonstrated that the electrodeposited Ni-W/Si3N4 coating had a compact, crack-free and homogeneous microstructure, containing 1.3-3.0 wt% Si3N4. The average roughness value (R-a) and the root mean square roughness value (R-q) of the coatings obtained at 10 g L-1 Si3N4 with the surface area of 9 mu m(2) were 17.4 nm and 22.34 nm, respectively. As for the coating obtained at 20 g L-1 Si3N4, the R-a and R-q value were 25.35 nm and 35.1 nm, respectively. XRD analysis indicated that the coatings prepared at different Si3N4 concentration had the same diffraction angle with the Ni phase except for the diffraction intensity. The Si3N4 particle concentration in bath influences the Si3N4 and W content, hardness, roughness, wear rate as well as corrosion resistance of the composite coating. The composite coating obtained at 15 g L-1 Si3N4 in as-deposited state acquires the maximum microhardness value of 953 +/- 10 HV, meanwhile showed the least weight loss, thereby displaying the best wear resistance. XPS spectra illustrate that the coating surface consists of Ni, W, Si, N, O and C elements. The composites electrodeposited at 10 g L-1 Si3N4 has a higher content of metallic tungsten. With the increase of Si3N4 concentration, the R-c and R-p value gradually increased. The metallic Ni and W were reduced and deposited from the form of [(Ni)(HWO4)(Cit)](2-) complex, including four consecutive steps, i.e., transportation, adsorption, reduction and incorporation.