Water-based gelcasting of surface-coated silicon nitride powder

A layer of Y2O3-Al2O3, used as a sintering aid, was coated onto the surface of Si3N4 particles by the precipitation of inorganic salts from a water-ba sed solution containing Al(NO3)(3), Y(NO3)(3), and urea. The electrokinetic and colloidal characteristics of the Si3N4 powder were changed significant ly by the coating layer. As a result, dispersion of the Y2O3-Al2O3- coated Si3N4 powder was significantly greater than that of the original powder. Fu rthermore, the Y2O3-Al2O3 coating layer prevented the hydrogen-gas-discharg ing problem that occurred during gelcasting of the original Si3N4 powder be cause of reaction between the uncoated powder and the basic aqueous solutio n in suspension. Surface coating, as well as the gelcasting process, signif icantly improved the microstructure, room-temperature bending strength, and Weibull modulus of the resulting ceramic bodies.