Fabrication and properties of supersonic plasma sprayed Al₂O₃ coating on porous Si₃N₄ substrate

A dense Al₂O₃ coating was directly deposited on porous Si₃N₄ substrate by supersonic plasma spraying. The microstructure and interface bonding of supersonic plasma sprayed Al₂O₃ coating were investigated. The surface of coating displayed explicit splat morphologies with densely packed splats. The molten Al₂O₃ droplets infiltrated into the inner pores of porous Si₃N₄, and formed an intermediate layer with rod-like Si₃N₄ particles at the interface. As-received Al₂O₃ coating sealed the surface pores of porous Si₃N₄, and decreased the water absorption of porous Si₃N₄ obviously from 42.1% to 5.2%, but only with a slight decrease of the total porosity. The mechanical properties of porous Si₃N₄ were also improved due to Al₂O₃ coating. The flexural strength of porous Si₃N₄ increased slightly from 93.7±3.1 to 97.5±3.4MPa, although it decreased obviously at the beginning of spraying due to mechanical erosion of Al₂O₃ droplets. The Vickers hardness of porous Si₃N₄ increased gradually from 465±20 to 1023±16MPa with the deposition content of Al₂O₃ increasing.