Combined laser and flame surface coating of refractory ceramics: phase and microstructural characteristics

Abstract

Hybrid flame spraying, combined with CO2 laser irradiation, has been developed to improve the structural integrity and performance of alumina coatings deposited on an alumina-based refractory substrate. The alumina coatings, over 400 μm in thickness, were prepared using various process parameters. The phases and microstructures of the coatings were investigated with X-ray diffraction, optical microscopy, scanning electron microscopy and an image analysis method. It was found that, with the hybrid spraying, the extent of densification was increased and the β-Al2O3 content of the coatings was decreased, with increase in laser power level and decrease in workpiece traverse velocity. A larger laser beam size could decrease and, even, eliminate the intermediate metastable γ-Al2O3, δ-Al2O3 and θ-Al2O3 phases in the corresponding coating. The above results suggest that the β-Al2O3 content of the coatings should reflect the degree of melting of the starting alumina powder. A larger laser beam would be useful for decreasing the cooling and solidifying rates of the melted alumina.