Investigation of mechanical and tribological properties of electroless Ni-P-B ternary coatings on steel

Abstract

The Ni–P-B ternary coating was prepared on steel specimens through electroless deposition and their microstructural and tribo-mechanical properties were investigated. Heat treatment of the as plated coating was carried out at 390 °C to realize its effect on microstructural, mechanical, tribological properties of the coating. Microstructural analysis was carried out to study phase structure, morphology, topography, and coating compositions by using XRD, FESEM, AFM, and EDS. Contact angle measurement was also performed to evaluate the wetting behavior of the coatings. The as-plated coating exhibited hydrophobic character with a contact angle of ∼ 96.7°, whereas the heat-treated coating resulted in hydrophilic character with a contact angle of ∼ 79.5°. Mechanical and tribological properties were evaluated by using Vickers microhardness, nanoindentation, and dry sliding friction test. It is observed that the heat treatment process has an excellent impact on mechanical and tribological properties. Heat-treated ternary coating resulted in higher hardness (6 GPa) than as-plated coating (5 GPa). Similarly, higher young’s modulus was also achieved in the heat-treated coating (232 GPa) during nanoindentation experiments. Profound improvement in tribological performance is executed in both coatings over steel specimens. Maximum up to ∼ 33% and ∼ 45% decrement in COF is observed for as-plated and heat-treated Ni-P-B ternary coatings respectively. Due to greater hardness in heat-treated samples, the formation of the protective bilayer is observed along wear tracks as harder third particles created a strong adhesive bond by plastic deformation. The non-uniformity of the protective layer in the as-plated coating leads to higher COF and wear depth compared to heat-treated coating. Residual third particles behaved as micro cutters for as-plated coating while rolling effect is observed in case of heat-treated ternary coating.