Al2O3 coatings were prepared using supersonic plasma spraying on the porous Si3N4 substrate at the spraying power of 52, 54, 58 and 60 kW, respectively. The effect of spraying power on the microstructures and properties of Al2O3 coating was investigated. The feedstock particles were sufficiently melted and Al2O3 coatings become more and more compact with the increasing spraying power. However, the defects, including the surface cracks, interlamellar cracks and needle-like particles, were produced within the coating when the spraying power was 60 kW due to the excessive power. As-received Al2O3 coatings sealed the surface pores of porous Si3N4, and decreased the water absorption obviously, but only with a slight decrease of the total porosity. Vickers hardness and flexural strength of porous Si3N4 with Al2O3 coating increased gradually as the spraying power increased from 52 to 58 kW, but it began to decrease when the spraying power was up to 60 kW due to the appearance of defects within the coating.
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