Al-Si/B₄C composite coatings on Al-Si substrate by plasma spray technique

Plasma-sprayed coatings of Al-Si/B₄C have been prepared on Al-Si piston alloys for diesel engine motors. The Al-Si/B₄C composite powders including 5-25 wt% B₄C were prepared by mixing and ball-milling processes. These powders were deposited on Al-Si substrate using an atmospheric plasma spray technique. The coatings have been characterised with respect to phase composition, microstructure, microhardness, bond strength and thermal expansion. It was found that Al, Si, B₄C and Al₂O₃ phases were determined in the coatings with approximately 600 μm thick by using X-ray diffraction analysis. Scanning electron microscope observation revealed that boron carbide particles were uniformly distributed in composite coatings and B₄C particles were fully wetted by Al-Si alloy. Also, no reaction products were observed in Al-Si/B₄C composite coatings. It was found that surface roughness, porosity, bond strength and thermal expansion coefficient of composite coatings decreased with increasing fraction of the boron carbide particle. It was demonstrated that the higher the B₄C content, the higher the hardness of coatings because the hardness of B₄C is higher than that of Al-Si.