Mobility transition at grain boundaries in two-step sintered 8mol% yttria-stabilized zirconia

Stagnation of grain growth is often attributed to impurity segregation, which becomes more severe as the grain size grows. In this respect, there is no evidence for segregation-induced slowdown in the grain growth of yttria-stabilized cubic zirconia, which obeys the parabolic law when the size increases by more than ten times. However, lowering the temperature below 1300 degrees C triggers an abrupt slowdown, constraining the average grains to grow by less than 0.5m in 1000hours despite a relatively large driving force imparted in the fine grains of similar to 0.5m. Yet isolated pockets of abnormally large grains, and even most remarkably, pockets of abnormally small grains, emerge in the same latter sample. Such extreme bifurcation of microstructure has never been observed before, and can be explained by an inhomogeneous distribution of immobile four-grain junctions. The implications of these findings for two-step sintering are discussed.