Narrow Results

Composite coatings constitute a new class of materials which are mostly used for mechanical and tribological applications. The corrosion resistance of these composite coatings, however, has not been systematically studied and compared. In this study, electroless Ni–P composite coatings are formed on St37 steel through the addition of nano-scale SiC particles to the plating bath. This work aimed to investigate the corrosion characteristics of electroless nickel composite coatings using electrochemical measurements which include polarization and electrochemical impedance spectroscopy tests. The morphology and structure of the composite coatings were studied by scanning electron microscopy (SEM), energy dispersive spectrum (EDS) and X-ray diffraction (XRD). The results showed that both electroless nickel and electroless nickel composite coatings demonstrated significant improvement of corrosion resistance in salty atmosphere.

The present work investigates the effects of heat treatment on friction and wear behavior of electroless Ni–B coatings at elevated temperatures. Coating is deposited on AISI 1040 steel specimens and subjected to heat treatments at 350${}^{\circ }$C, 400${}^{\circ }$C and 450${}^{\circ }$C. Coating characterization is done using scanning electron microscope, energy dispersive X-Ray analysis and X-Ray diffraction analysis. Improvement in microhardness is observed for the heat treated deposits. Further, the effect of heat treatment on the tribological behavior of the coatings at room temperature, 100${}^{\circ }$C, 300${}^{\circ }$C and 500${}^{\circ }$C are analyzed on a pin-on-disc setup. Heat treatment at 350${}^{\circ }$C causes a significant improvement in the tribological behavior at elevated temperatures. Higher heat treatment temperatures cause deterioration in the wear resistance and coefficient of friction. The wear mechanism at 100${}^{\circ }$C is observed to be predominantly adhesive along with abrasion. While at 300${}^{\circ }$C, abrasive wear is seen to be the governing wear phenomenon. Formation of mechanically mixed layers is noticed at both the test temperatures of 100${}^{\circ }$C and 300${}^{\circ }$C for the coatings heat treated at 400${}^{\circ }$C and 450${}^{\circ }$C test temperature. The predominant wear mechanisms at 500${}^{\circ }$C are abrasive and fatigue for as-deposited and heat treated coatings, respectively.

The present work reports the deposition of a quaternary Ni-B-W-Mo coating on AISI 1040 medium carbon steel and its characterization. Quaternary deposits are obtained by suitably modifying existing electroless Ni-B bath. Composition of the as-deposited coating is analyzed by energy dispersive X-ray spectroscopy. The structural aspects of the as-deposited and coatings heat treated at 300${}^{\circ }$C, 350${}^{\circ }$C, 400${}^{\circ }$C, 450${}^{\circ }$C and 500${}^{\circ }$C are determined using X-ray diffraction technique. Surface of the as-deposited and heat-treated coatings is examined using a scanning electron microscope. Very high W deposition could be observed when sodium molybdate is present in the borohydride-based bath along with sodium tungstate. The coatings in their as-deposited condition are amorphous while crystallization takes place on heat treatment. A nodulated surface morphology of the deposits is also observed. Vickers’ microhardness and crystallite size measurement reveal inclusion of W and Mo results in enhanced thermal stability of the coatings. Solid solution strengthening of the electroless coatings by W and Mo is also observed. The applicability of kinetic strength theory to the hardening of the coatings on heat treatment is also investigated. Corrosion resistance of Ni-B-W-Mo coatings and effect of heat treatment on the same are also determined by electrochemical techniques.

Proper selection of bath additives is critical to the plating rate, phosphorus content and corrosion resistance of the nickel–phosphorus (Ni–P) alloy deposits. In this work, electroless Ni–P alloy deposition from an acidic bath using sodium hypophosphite as the reducing agent at relatively low temperatures (30°C and 60°C) was carried out in the presence and absence of urea additives. The effects of different concentrations of urea on plating rate and phosphorus content of the Ni–P deposits were studied. The results showed that the plating rate and the phosphorus content of the Ni–P deposits were largely dependent on the concentration of urea in the bath. The corrosion resistance of the Ni–P deposits was evaluated in aerated 3.5 wt.% sodium chloride (NaCl) solution using potentiodynamic polarization curves. It was found that the corrosion resistance of the deposits obtained from the bath solution containing 1 g/L of urea was the highest. In addition, cyclic voltammetry techniques were used to study the mechanism of electroless Ni–P alloy deposition in the bath. The results revealed that the addition of urea to the bath promoted the concentrations of adsorbed atomic hydrogen and sodium hypophosphite on mild steel surface, which markedly increased the phosphorus content of the Ni–P deposits. This work offers a new way that highlights the relation between the urea additives and phosphorus content.

This study focuses on the electroless deposition of Ni–P alloy over a copper substrate to minimize the corrosion rate of the substrate. Central Composite Design (CCD) has been performed using Design-Expert software for minimizing the corrosion rate of the coating. Along with that, CCD is also utilized to analyze the influence of various process parameters viz. concentration of Nickel Sulphate, the concentration of Sodium Hypophosphite and bath temperature. Potentiodynamic test has been employed to evaluate the corrosion rate of each of the coated substrates. In order to minimize the corrosion rate, optimization has been performed using particle swarm optimization (PSO). 21.59g/L of Nickel Sulphate, 26. 72g/L of Sodium Hypophosphite and 93.41°C as the bath temperature were the optimum conditions for the deposition of coating in order to achieve a corrosion rate value of 0.862mm/Yas obtained from the model analysis results. Further, Analysis of Variance (ANOVA) was implemented which corroborated that the parameter Nickel Sulphate along with the interaction between Sodium Hypophosphite and bath temperature were the significant ones in determining the corrosion rate of the coating deposited in optimized condition. Optical Microscopy, Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray analysis (EDX) were conducted to study the surface morphology and the elemental composition of the coated substrate respectively.