TRIBO-MECHANICAL CHARACTERIZATION OF ENB ALLOY COATINGS: EFFECT OF HEAT-TREATMENT TEMPERATURE AND SODIUM BOROHYDRIDE CONCENTRATION

Previously electroless Ni-B (ENB) coatings were analyzed and optimized based on various coating parameters. However, variation of nano-indentation behaviour like nano-hardness, elastic modulus and scratch hardness variation with bath composition and heat treatment temperature has not been reported earlier. An attempt has been made to explore the same in the present study. ENB coating layers are deposited on AISI 1040 steel specimen with varying concentration of sodium borohydride (NaBH4) and heat-treated at 350°C, 450°C and 550°C to investigate the related effects. Nano-hardness and elastic modulus of as-coated specimens are found to improve with NaBH4 concentration due to increased boron content and nodule size. Both nano-hardness and elastic modulus are observed to improve further upon heat treatment because of incorporation of various boride phases leading to compact morphology and increased size of the nodules. Scratch hardness value also increases with NaBH4 concentration and it improves further upon heat treatment and reaches to its maximum at 450°C due to presence of compact and hard Ni2B phase. Compact homogeneous surface morphology enhances the friction and wear behaviour of the heat-treated coatings even though surface roughness deteriorates after heat treatment.