Effect of borohydride concentration on tribological and mechanical behavior of electroless Ni-B coatings

Electroless Nickel boron (Ni-B) coatings were deposited over mild steel substrates to study the effect of borohydride concentration on tribological behaviour and mechanical properties. Three different types of coating bath solutions were used to deposit coatings. The as-deposited coatings were characterised based on surface morphology, composition and phase structure using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD), respectively. Friction and wear behaviour of the as-deposited coatings were evaluated by pin-on-disc tribo-tester. Coefficient of friction of the coatings decreases with the increase in borohydride concentration. Surface hardness, elastic modulus and scratch resistance behaviour of the coatings were evaluated using the data obtained by nano-indentation techniques. It is seen that the surface hardness and elastic modulus of the as-deposited coatings increase with the increase in borohydride concentrations. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation tests were conducted to analyse corrosion behaviour of the coatings against a corrosive environment of 3.5% NaCl solution. The corrosion test results displayed an increasing trend of corrosion resistance as the borohydride concentration increases in the coating bath solution.