Assessment of Tribological behavior of nickel-nano Si3N4 composite coatings fabricated by pulsed electroplating process

Abstract

Pure nickel and nano-composite nickel-silicon nitride (Si3N4) coatings were fabricated using pulsed current electrodeposition technique. A control sample for each coating was created and the effect of current density, duty cycle, and pulse frequency on the tribological properties of the electrodeposited coatings was investigated by altering these parameters. X-ray diffraction, scanning electron microscopy and pin-on-disk tests were performed to assess the properties of the fabricated coatings. X-ray diffraction results indicated that both pure and nano-composite coatings possess preferential growth of (111) crystallite planes, which results in the formation of binary symmetrical shaped grains. However, incorporating particles into the coating hindered the growth on other planes, specially (200) growth plane. Incorporation of Si3N4 particles into the coating reduced crystallite size and increased grain refinement; both of which resulted in the increased microhardness of the deposited coatings through dispersion strengthening mechanism. Complementary atomic force microscopy investigations indicated that the roughness of all nano-composite samples was higher than that for the pure nickel coating. After incorporation of Si3N4 particles the wear behavior of all coatings was improved as a result of the improved hardness. Measured wear rates indicate that increasing the current density, during electrodeposition, is the most influencing parameter in reducing the wear coefficient. Furthermore, implementing particles changes the wear mechanism from high plastic deformation, gouging, and delamination to ploughing. Although nano-composite coatings exhibit wider grooves on the surface compared to those on pure nickel coatings, they tend to adhere better to the substrate. The friction coefficient for all nano-composite coatings were marginally higher than that for pure nickel coating, to our understanding, as a result of inadequate load carrying effect of silicon nitride particles.