Theoretical and experimental studies of mirror-bright Au coatings deposited from a novel cyanide-free thiosulphate-based electroplating bath

Abstract

The study describes the electrodeposition of mirror-bright Au coatings from a cyanide-free thiosulphate-based bath. The optimized composition of the electroplating bath contains chloroauric acid, sodium thiosulphate, cetyltrimethylammonium bromide (CTAB), 1,4-diazabicyclo[2.2.2]octane (DABCO), and nicotinic acid (NA). Quantum chemical calculations are carried out to study the electronic properties of various potential bath additives that can be used for the formulation of Au electroplating bath. Based upon these studies DABCO, and NA are selected as bath additives. The electrochemical impedance spectroscopy (EIS) study shows the electrochemical behaviour of the bath and the correlation of impedance values with the current density used. Deposition of Au is carried out from the formulated bath via pulse galvanostatic method at different current densities (10, 50, 100 and 200 mA/cm2) at a pulse frequency of 100 Hz and 50 % duty cycle. The XRD and EDS studies of the coatings confirm the presence of gold in its elemental form. The SEM studies, used for studying the effect of current density on the coating morphology, confirm that the coatings have a smooth and compact microstructure. The XPS study shows the atomic composition of the Au coatings. The nano-hardness tests show that the coating hardness decreases with an increase in current density. The coating deposited at 10 and 200 mAcm−2 has a hardness of 294 ± 11.3 and 190 ± 7.2 VHN, respectively.