安定化ジルコニアの微構造とジルコニア部分安定化への一考察

抄録

Microstructure of partly stabilized zirconia from Norton Company and perfectly stabilized zirconia was examined by means of polarization microscope and X-ray diffraction. A mechanism of the partial stabilization of zirconia was inferred from the results obtained with the aid of the phase diagram of the binary system CaO-ZrO₂.
Norton's partly stabilized zirconia is an aggregate of 1000-1500μ massive grains, each consisting of optically anomalous cubic zirconia showing birefringence, 5-10μ in diameter, and of monoclinic zirconia, a few or sometimes 6-10μ in diameter, all of which arranged nealy parallel to some crystallographic directions. Stabilized zirconia consists of cubic zirconia and a little amounts of monoclinic zirconia. It contains further a few glassy matrix and crystallites of a kind of gehlenite-akermanite solid solutions, i.e., melilite group minerals deposited in it.
Lime content of the Norton's sample is lower than that of the stabilized zirconia, but lime solubility in the cubic zirconia seems to be much larger in the former than in the latter. These results were discussed with the aid of phase diagram of the system CaO-ZrO₂, and it was concluded that the main process of the partial stabilization of zirconia was the exolution of tetragonal zirconia into cubic and monoclinic modifications. The exolution occurring in solid state is very difficult to reach the equilibrium, and is governed strongly by cooling condition. The faster the cooling rate, the larger the deviation from the equilibrium and the lower the solubility of lime into the cubic zirconia formed. Therefore, on the one hand the properties of the partially stabilized zirconia can be controlled with cooling conditions, on the other hand the zirconia grains seems to contain some unstable parts. If it is the case, some problems seem to exist in the use of the partially stabilized zirconia and of lime as the stabilizer of cubic zirconia.