A comprehensive study of electroless composite coatings developed with sodium borohydride as a reducing agent

Abstract

Electroless alloy and composite coatings are fabricated over steel (AISI1025) substrates using an electroless solution containing sodium borohydride as a reducing agent. First, changes in morphological features, deposition rates with respect to immersion time, and deposition mechanisms of electroless Ni-B-W coatings are investigated thoroughly along with the assessment of their tribological behavior. Certain specific characteristics can make electroless Ni-B-W coatings a potential replacement for electrodeposited hard chromium (EHC) coatings. It is noteworthy that metal finishing industries are desperately searching for such a replacement owing to concerns raised on the environmental ramifications of popularly used EHC plating. Next, with minor changes in the deposition method, silicon carbide (SiC) particles are successfully incorporated into matrices of electroless ternary Ni-B-W alloy coatings. Variable amounts of SiC particles are added to the electroless bath solution while fabricating Ni-B-W-based electroless composite coatings of different compositions. These composite coatings are then subjected to heat treatment at different temperatures to study its effect on several microstructural, mechanical and tribological characteristics. Variable heat treatment temperatures govern the magnitude of crystallization, degree of dispersion and sizes of precipitates within coatings’ matrices differently leading to major changes in mechanical and tribological characteristics of developed coatings.