Microstructure and wear resistance of steel matrix composite coating reinforced by multiple ceramic particulates using SHS reaction of Al–TiO2–B2O3 system during plasma transferred arc overlay welding

The steel matrix (SHS-free) coating and its composite (SHS-produced) coating reinforced by multiple ceramic particulates were developed by plasma transferred arc (PTA) overlay welding. 5% and 10% weight percentages of mixtures of aluminum (Al), titanium dioxide (TiO2), and boron oxide (B2O3) powders by sequence weight ratio of 9:8:7 were used as precursors. Aluminothermic reduction of these oxides, being highly exothermic in nature, essentially leads to a self-propagating high-temperature synthesis (SHS) of multiple ceramic particulate reinforced steel matrix composite coatings. Composite coatings have been subsequently characterized by X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) attachment, transmission electron microscope (TEM), nanoindentation, and sliding wear measurement. The results show that the hypoeutectic microstructure exists in the steel matrix coating where it consists of γ(Fe, Ni), M7C3, and (Fe, Cr)2B phases. Adding mixtures of Al–TiO2–B2O3 by sequence weight ratio of 9:8:7 changes its microstructure into pseudoeutectic characteristic whose crystal growth is cell dendrite in 5% SHS-produced coating but dendrite in 10% SHS-produced coating. Not only the metastable Al2O3 with nanometer and TiB2, but the TiB and TiC can be formed in the SHS-produced coating, except for those phases that existed in the steel matrix coating. Hardness and sliding wear resistance of the SHS-produced coatings increase in comparison with that of the steel matrix coating. The best sliding wear resistance can be obtained in the 5% SHS-produced coating for its high ratio of hardness to elastic modulus.