A structural study of metastable tetragonal zirconia in an Al2O3-ZrO2-SiO2-Na2O glass ceramic system

The structural and microstructural properties (crystalline system at the beginning of crystallization, lattice disorder and crystallite size) of metastable zirconia have been studied by an X-ray line broadening analysis using simplified methods based on suitably assumed functions describing the diffraction profiles. Metastable tetragonal zirconia has been crystallized at 970, 1000 and 1050° C, respectively, starting from an Al₂O₃-ZrO₂-SiO₂Na₂O glassy system with a chemical composition very close to that of well known electromelted refractory materials. In the present work we have definitely shown the presence, inside the crystallized zirconia phase, of internal microstrains having values ranging approximately between 2 and 4×10^–3. Moreover, we have confirmed the peculiar smallness in size of precipitated zirconia crystallites ( 200 Å). Therefore, in the present system, the stabilization of the tetragonal form of ZrO₂ with respect to the stable monoclinic one can be explained in terms of a contribution to the amount of free energy due to strain energy, in addition to the previously hypothesized surface energy. The observed strong line broadening for some samples treated at lower temperatures (970 and 1000° C) gives rise to an apparent cubic lattice pattern; but the asymmetry of each apparent single line masks unequivocally a tetragonal doublet. This latter conclusion disagrees with some hypotheses on the existence of a cubic metastable form of ZrO₂ which could originate at the beginning of zirconia crystallization.