Strengthening of porous mullite and zirconia CMC matrices by evaporation/condensation

A processing method using evaporation/condensation sintering in an HCl atmosphere was developed for strengthening porous materials without shrinkage. Strengthening without shrinkage is useful in preventing voids and cracks that might be formed during constrained densification, e.g., a porous matrix in a continuous fiber reinforced ceramic composite. Mixtures of mullite and zirconia (monoclinic, tetragonal (3 mol% Y2O3), and cubic (8 mol% Y2O3)) were studied and exposed to HCl vapor at temperatures up to 1300 degrees C. It was observed that the evaporation-condensation mass transport process produced a porous material with minimal shrinkage. As the crystal structure of the starting tetragonal and cubic zirconia powders did not change after extensive coarsening, it appeared that zirconium and yttrium were transported in the same proportion via evaporation/condensation. The process produced significant coarsening of the zirconia grains, which made the material resistant to densification when heated to 1200 degrees C in air. Because the sintering produced coarsening without shrinkage, the pores also coarsened and a porous microstructure was retained. Mixtures of mullite and zirconia were used because mullite does not densify under the processing conditions used here, namely, heat treatments up to 1300 degrees C. The mullite particles acted as a non-densifying second phase to further inhibit shrinkage when the mullite/zirconia composite was heated up to 1200 degrees C in air. The coarsened cubic zirconia plus mullite mixture had the least densification after heat treatments in air of 100 h at 1200 degrees C.