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±3.4 MPa, 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±16 MPa with the deposition content of Al2O3 increasing.