Deposition of Ni-W-Mo ternary alloy coating on copper via pulsed current electrodeposition with enhanced corrosion resistance and mechanical properties

Abstract

Nickel-tungsten-molybdenum (Ni-W-Mo) alloy coatings have been used extensively in micro-electro-mechanical systems (MEMS) because of their thermal stability, superior toughness, and corrosion resistance in extreme environments. The aim of the present study is to characterize the corrosion resistance and mechanical performance of Ni-W-Mo ternary alloy coating on Cu substrate. Meanwhile, the Mo element was co-deposited in the Ni-based plating to enhance the toughness of the coating layer owing to its self-lubricant performance. The Ni-W-Mo coating was successfully fabricated on Cu substrate by pulse electro-deposition via pulse currents of 9.0 and 4.5 A/dm2. X-ray diffraction (XRD) and Scanning electron microscopy (SEM) equipped with X-ray energy dispersive spectroscopy (EDS) were used to investigate the microstructure, morphology, rupture surface, and chemical composition of the Ni-W-Mo ternary alloy coating. A higher Mo content (27 wt%) and presence of nodules were obtained at a pulse current of 9.0 A/dm2. The nodules of the Ni-W-Mo ternary alloy coatings were observed using SEM, and a crack-free coating was confirmed to be completely bonded with the Cu substrate via scribe-grid testing. The corrosion resistance of the Cu substrate with and without the Ni-W-Mo ternary alloy coatings was evaluated in brine (3.5 wt% NaCl) via potentiodynamic polarisation testing in ambient temperature. The results showed that the alloy coating increased the corrosion resistance of the Cu substrate in brine. The quasi-crystal structure of the proposed alloy coating enhanced the micro-hardness and tensile resistance of the Cu substrate, which were evaluated in the ambient air and brine using the slow strain rate test (SSRT). The tensile strength of the specimen with the Ni-W-Mo ternary alloy coating (current density 9.0 A/dm2) increased by approximately 22 MPa compared with that of the Cu substrate in ambient air. These results confirm that the corrosion resistance and toughness of the Ni-W-Mo ternary alloy coatings deposited using the electrodeposition technique with pulsed-current power are superior to those of the Cu substrate.