Water‐Based Gelcasting of Surface‐Coated Silicon Nitride Powder

A layer of Y 2 O 3 –Al 2 O 3 , used as a sintering aid, was coated onto the surface of Si 3 N 4 particles by the precipitation of inorganic salts from a water‐based solution containing Al(NO 3 ) 3 , Y(NO 3 ) 3 , and urea. The electrokinetic and colloidal characteristics of the Si 3 N 4 powder were changed significantly by the coating layer. As a result, dispersion of the Y 2 O 3 –Al 2 O 3 ‐coated Si 3 N 4 powder was significantly greater than that of the original powder. Furthermore, the Y 2 O 3 –Al 2 O 3 coating layer prevented the hydrogen‐gas‐discharging problem that occurred during gelcasting of the original Si 3 N 4 powder because of reaction between the uncoated powder and the basic aqueous solution in suspension. Surface coating, as well as the gelcasting process, significantly improved the microstructure, room‐temperature bending strength, and Weibull modulus of the resulting ceramic bodies.