Fe-based amorphous/nanocrystalline composite coating by plasma spraying: Effect of heat input on morphology, phase evolution and mechanical properties

Abstract

Fe-based amorphous/nanocrystalline composite coatings with a lean composition of Fe−2.5Cr−6.7Si−2.5B−0.7C (wt%) were synthesized by atmospheric plasma spraying (APS) onto a mild steel substrate. The effects of plasma power on the morphology and the phase content of the coatings were systematically investigated. Denser coatings with better inter-splat bonding were obtained at a higher plasma power, which was attributed to higher degree of powder melting. The retention of amorphous phase and formation of various nanocrystalline Fe-borides in the amorphous matrix was decided by the variation in plasma power, which in turn affected the mechanical properties of the coatings. Increasing plasma power resulted in higher hardness and elastic modulus of the coatings, which is attributed to the compact microstructure of the coatings containing amorphous matrix with nanocrystalline intermetallics (Fe23B6, Fe2B, and/or Fe3B) distributed. The nanoscratch results indicated that the increased plasma power resulted in uniform scratch profiles. Moreover, dry sliding wear test showed that both coefficient of friction and wear rate decreased with increasing plasma power. An analytical model was used to correlate mechanical and tribological properties of the coatings, which insinuated that the coating prepared at a plasma power of 35.5 kW exhibited significantly high shear strength than other coatings deposited at lower plasma power and approximately 3.6 times greater than that of the mild steel substrate.