Meenu Srivastava

Electrodeposition of Ni and Co coatings from ionic liquid

Abstract The aim of the present study is to compare the structure and properties of nickel, cobalt and nickel–cobalt alloy coatings obtained from ionic liquid (IL) with those obtained from aqueous electrolyte. Choline chloride, a cheap and non-toxic IL, has been chosen for electrodeposition. The coatings obtained from IL displayed higher microhardness compared with those obtained from aqueous electrolyte. A difference in the surface morphology was observed. X-ray diffraction studies showed that the nickel rich coatings exhibited face centred cubic crystal structure, whereas those rich in cobalt displayed hexagonal close packed structure. Comparison in the corrosion behaviour of the coatings showed that the Ni coating possessed better corrosion resistance irrespective of the electrolyte adopted.

SiC particle dispersion in nanocrystalline Ni matrix: Restriction to thermal grain growth and its effect on mechanical properties

Silicon carbide (SiC) particles were dispersed in nanocrystalline Ni (nc-Ni) matrix and thermal grain growth was studied. Microstructural characterization revealed that while the average grain size of as prepared foils of both nc-Ni and Ni-SiC nanocomposite was found to be ∼ 60 nm, presence of SiC particles restricted thermal grain growth remarkably. While the grain size of nc-Ni annealed at 300°C was found to be 210 nm that of Ni-SiC nanocomposite was only ∼100 nm. Differential Scanning Calorimetry (DSC) results exhibited higher heat flow in case of nc-Ni as compared to the Ni-SiC nanocomposite which signifies rapid thermal grain growth in the former one. Tensile tests carried out on both the materials showed that yield strength (YS) of the nanocomposite was always higher than nc-Ni. However, the ultimate tensile strength (UTS) of the former one was lower up to 100°C which might be due to premature brittle fracture initiated along SiC particle interface. At 200°C and above both YS and UTS were found to be higher for nanocomposite due to easier plastic flow being enabled at elevated temperature.

FESEM and XPS studies of ZrO2 modified electrodeposited NiCoCrAlY nanocomposite coating subjected to hot corrosion environment

The aim of this study is to investigate the influence of Co and reactive element oxide, i.e. ZrO2, addition on the hot corrosion behavior of NiCrAlY electrodeposited nanocomposite coatings. The hot corrosion behavior of the coatings was investigated in a medium comprising 75% Na2SO4 and 25% NaCl and expressed as weight change. The addition of ZrO2 resulted in a reduction of Co content from 40 wt% to 20 wt%, as shown by EDX. The hot corrosion studies revealed that the addition of cobalt and yttrium to the NiCrAl coating enhanced its corrosion resistance. However, the effect of Y was more pronounced in the case of NiCrAl compared to NiCoCrAl coatings. The introduction of small amounts of ZrO2 further resulted in a reduction in weight gain. The formation of oxide of Ni on the surface of NiCrAlY and that of Co on the surface of NiCoCrAlY coatings was confirmed by FESEM and XPS analysis. XPS analysis revealed the presence of oxide of Al as well as small amounts of oxide of Cr on the surface and near surface of NiCrAlY and NiCrAlY(Z) coatings, while only oxide of Cr was observed in NiCoCrAlY and NiCoCrAlY(Z) coatings and this was responsible for their improved hot corrosion resistance.