CRYSTALLIZATION DURING FIBER EXTRACTION OF ZRO2-AL2O3-BASED MATERIALS

Three eutectic compositions in the systems of ZrO2-Al2O3, ZrO2-Al2O3-S iO2 and ZrO2-Al2O3-TiO2, designated ZA, ZAS, and ZAT respectively, wer e melt extracted into fibers using a rotating Mo wheel, Extraction was performed at four velocities and resulted in fine uniform-diameter fi bers at a speed of less than or equal to 1.6 m/s, but thicker fibers e xhibited Rayleigh waves at velocities of greater than or equal to 3.2 m/s. Fine uniform-diameter fibers experienced high cooling rates (appr oximate to 10(5) k/s) whereas the cooling rates decreased as the fiber s became thicker. X-ray diffraction of the uniform diameter fibers ext racted at 1.6 m/s revealed that ZAS and ZAT fibers were amorphous wher eas ZA fibers were slightly crystalline, containing metastable cubic Z rO2 and delta-Al2O3. At extraction speeds of 4.6 and 11.5 m/s, tetrago nal ZrO2 and various modifications of Al2O3 were detected in the rapid ly solidified fibers. Differenzial thermal, analysis showed that with increasing fiber thickness, the amorphous volume fraction decreased su ch that the ZA fibers produced at 11.5 m/s contained almost no amorpho us phase. In general, extracted fibers exhibited featureless surfaces with the exception of ZA and ZAT fibers at a wheel velocity of 11.5 m/ s. By contrast, due to the lower local cooling rates, these fibers exh ibited fine dendritic morphology, especially on the Rayleigh waves.