GRADED COMPOSITIONS AND MICROSTRUCTURES BY INFILTRATION PROCESSING

Mullite/alumina particulate composites were fabricated by infiltrating porous alumina preforms with an SiO2-containing sol followed by a heating step to cause mullite formation and densification. Electron microprobe analysis was performed to obtain concentration profiles across sections of the sintered composites. These analyses revealed the existence of concentration gradients, the mullite content decreasing with increasing distance from the surface of the bodies. Analyses with a scanning electron microscope also indicated a microstructural effect; the alumina grain size in composite bodies tended to increase with distance from the surface of the sample. These two effects (microstructural and compositional) have been related and it has been concluded that while the presence of mullite limits grain growth in alumina, the mullite content must be at least approximately 5 wt % in order for grain growth to occur in a controlled fashion. The results point out the potential of the infiltration approach as a means for tailoring the composition and microstructure of ceramic bodies.