Synthesis And Properties Of Nano-scale Twin Ni-Co Alloy Coatings

Nano-scaled Ni-Co alloy coatings with twins were synthesized by using pulsed current (PC) and pulsed reversal current (PRC) electrodeposition technique. The morphology, composition and microstructure of these coatings were characterized by SEM (with EDXS), XRD and TEM, respectively. Their corrosion behavior was investigated by potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS) technique. The hardness and modulus of the passive film on the coatings were tested by nanoindentation technique. The mechanical properties of the coatings were evaluated by microhardness and friction characterization. The results demonstrated that the Co contents markedly decreased in the PC and PRC samples (approximately 10.5 at.%) compared with the DC sample (about 15.8 at.%). SEM observation indicated that the surface morphology transformed from pyramidal crystallites for DC sample to spherical-shaped grains for PC and PRC samples, and the cluster size of the PRC samples appeared more uniform compared with the PC sample. All the samples formed a solid solution of Co in Ni with fcc structure, and a preferred orientation was also observed from (200) to (111). Samples obtained by PC method possessed the finest crystalline (about 100 nm) and showed a mass of distribution of nano-scale twins on the whole structure. Only farthing twins were observed in DC coatings. Bidirectional twins and dislocations were introduced in the PRC coatings. Compared with sample by DC electrodeposition method, the corrosion resistance, wear resistance of the nanotwin samples and nano-mechanical properities of the passive film on the nanotwin samples were all apparently improved. The similar improvements were observed in the bidirectional nanotwin sample, but the improvements were not so notable due to the existence of the dislocation in the sample, which would counteract the favorable effect of the nano-scale twins. All the enhanced properties have been discussed in terms of the microstructure modification of the samples.In order to further decrease the grain size and improve the twins density, phytic acid and saccharin were added to the electrolyte as additives to prepare the Ni-Co alloy coatings. The effect of these additives on the corrosion resistance of the samples was studied by various electrochemical measurements. The results demonstrated that the addition of phytic acid had little effect on the grain size and density of twins in the samples. However, it greatly enhanced the corrosion resistance of the samples. Addition of saccharin could refine the grain size and improve the surface brightness of the samples, but inhibit the formation of twins, leading to the deterioration of the corrosion resistance. In addition, the effect of saccharin addition (0.01 g/l,0.05 g/l,0.1 g/l,1 g/l,2 g/l,5 g/1) on the corrosion resistance was investigated. The results demonstrated that the corrosion resistance increased with the increase of saccharin addition from 0.01 g/l to 0.05 g/l, and then decreased when the saccharin addition exceeded 0.05 g/l. Therefore, the samples with the mostly enhanced corrosion resistance could be obtained by the saccharin addition of 0.05 g/l.