Liquid precursor infiltration processing of powder compacts .1. Kinetic studies and microstructure development

The kinetics of infiltrating a solution precursor into Si3N4 powder compacts were studied using either water or an aqueous solution of Zr-nitrate and Y-nitrate that formed a crystalline Zr(Y)O-2 (3 mol% Y2O3) solid solution during pyrolysis. When the powder compact contained air, the infiltration involved two steps: (1) relatively rapid intrusion of liquid via flow due to capillary pressure and (2) diffusion of entrapped gas to the surface as its pressure became equal to the capillary pressure, The kinetics of both processes are described with different parabolic rate laws-Darcy's law and Fick's law, respectively, When the intruded precursor is converted to an inorganic during heat treatment, the void space is partially filled with pyrolyzed precursor without shrinkage of the Si3N4 powder. The kinetics of subsequent cycles depend on the permeability of the pyrolyzed precursor, which depends on its microstructural development during heat treatment subsequent to pyrolysis. Surface cracks can form within the powder compact during either precursor drying or pyrolysis; they can be avoided by strengthening the powder compact by forming small necks between touching particles via evaporation-condensation. Precursor molecules were found to concentrate near the surface as the precursor solvent was removed by drying. The movement of precursor molecules was prevented by gelling prior to drying, viz,, by soaking the infiltrated bodies in an aqueous NH4OH solution. Microstructures developed during cyclic precursor infiltration and pyrolysis were characterized to show that cracklike voids are produced within the pyrolyzed precursor due to its large volume change during pyrolysis and densification; the size distribution of the cracklike voids is proportional to the size distribution of the voids within the initial powder compact.