It was recalled that superplasticity was a macroscopic phenomenon which was very sensitive to slight differences in atomic structure and chemical bonding at grain boundaries. The mechanism of grain boundary sliding in superplastic ceramic was described by the movement of boundary dislocations at clean grain boundaries in so-called pure materials. The small impurity content often formed a thin glassy film with an equilibrium thickness (1nm) at grain boundaries. The thin film became a supercooled liquid, and grain boundary sliding was described by the movement of steps.

F.Wakai, N.Kondo, H.Ogawa, T.Nagano, S.Tsurekawa: Materials Science Forum, 1997, 243-245, 337-44