MICROSTRUCTURES OF LASER-TREATED AL2O3-ZRO2-CEO2 COMPOSITES

This work uses room temperature X-ray diffraction and electron microscopy to investigate the effect of CO2 laser treatments (800-2000 W, 10-15 s) on phase transformations and microstructural modification of sintered (1600 degrees C, 4 h) disks A(x)Z(y)C(z) (where A, Z and C denote Al2O3, ZrO2 and CeO2 respectively, and the subscripts represent molar ratios). Laser treatments under these conditions caused melting and more or less sublimation of all the samples; subsequent solidification and condension (predominant for CeO2-rich composition) resulted in different microstructures between the samples. Dendritic and cellular domain structures due both to eutectic growth of alpha-Al2O3 and ZrO2-CeO2 solid solution were found in specimens A(30)Z(63)C(7) and A(70)Z(27)C(3) respectively. In the CeO2-rich specimen A(40)Z(12)C(48) a drastic effect of condensation caused dendritic clusters of CeO2 cubes (fluorite structure with minor amount of ZrO2 and Al2O3 in solid solution) which overlaid on an Al2O3-rich matrix, predominantly CeAlO3. Condensation through a rapidly solidifying liquid caused incorporation of Al2O3 in the fluorite structure and hence deviation from the octahedral shape predicted by the periodic bond chain model. Solute (Al2O3) trapping also suppressed the martensitic transformation of tetragonal (t-) to monoclinic (m-) ZrO2 as manifested by the dendritic t-ZrO2 cooled from the Al2O3-ZrO2 eutectic. Laser treatment increased the Ce2+/Ce4+ ratio and hence darkened the samples.