Laser surface engineering of B4C/Fe nano composite coating on low carbon steel: Experimental coupled with computational approach

Abstract

Laser surface engineering was successfully used to engineer a nano carbide composite coating on a low carbon steel substrate. Two powder precursors of B4C and B4C+ 4 wt% Fe were utilized with different laser powers and processing speeds. A smooth crack free coating layer was achieved with B4C+ 4 wt% Fe powder precursor. The phase evolution and microstructure characterization of the coating layer were conducted using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Microscopy observations revealed a fine dendritic microstructure consisting of α-Fe phase dendrites with nanosized B4C precipitates, while the interdendritic regions contained mainly Fe2B and Fe3C phases. The retention of B4C in the coating layer was observed only at high laser power with high scanning speed. This is due to the lower stability of B4C with respect to Fe2B and Fe3C phases as confirmed with density functional theory based thermodynamic calculations. The hardness increased nearly seven fold for the nano-composite coating compared to the steel substrate due to evolution of high hardness ceramic phases, in addition to the refinement of the microstructure. This laser coating has a high potential in advance engineering applications where a thick coating layer with high hardness is required. Keywords: Laser surface processing, Composite coatings, Boron carbide, Thermodynamic modeling