Microstructure and oxidation behaviour of silicate coating on Si3N4: comparison with native oxide layers

Abstract The microstructures of silicate layers on Si3N4 ceramics formed by oxidation or by spray coating with SiO2 and subsequent annealing were compared in order to clarify the effects of oxidation on pore formation and crystallisation in the different surface layers. In samples containing Al2O3/Y2O3, intensive separation and dissolution of sialon grains from the bulk into the oxidised layer was observed, and this is likely to be one of the reasons for the lower oxidation resistance of Si3N4 sintered with Y2O3/Al2O3 than with MgO/Y2O3. Oxygen supplied from the atmosphere plays an important role in bubble formation and in the crystallisation of cristobalite in the silicate layer. The silicate coating layer inhibits the oxidation of Si3N4 effectively up to 1200°C. At 1300°C, uncoated samples gained 0·75 by weight after oxidation for 500 min, whereas the weight gain of spray coated Si3N4 was 0·4% under the same conditions.