Structure of [110] tilt grain boundaries in zirconia bicrystals.

Cubic stabilized zirconia bicrystals with [110] symmetric tilt grain boundaries were fabricated by diffusion bonding of two single crystals with the composition of ZrO2-9.6mol%Y2O3. The structures of symmetric tilt small angle grain boundary and two types of symmetric tilt sigma3 grain boundaries with different grain boundary planes were observed by transmission electron microscopy (TEM). High-resolution transmission electron microscopy (HREM) observations clarified that the [110] small angle tilt grain boundary consists of periodic array of b = a/2[110] type edge dislocations. This result is consistent with Frank's dislocation model for small angle grain boundary. HREM observation also revealed that the 70.5 degrees sigma3 grain boundary shows atomically coherent grain boundary structure with the boundary plane of [111], while the 109.5 degrees sigma3 grain boundary accompanies grain boundary facets taking [111]/[115] asymmetric grain boundary plane. Because of the very low surface energy of [111] plane and/or high lattice matching of [111] and [115] type planes, the grain boundary faceting may be preferred in spite of increasing grain boundary area to about 6%. TEM-energy-dispersive X-ray spectroscopy (EDS) analyses were performed on both sigma3 grain boundaries, and the segregation of yttrium ions to the boundaries was detected in both cases. The amount of segregation is about the same in both sigma3 boundaries. It can be concluded that the segregation of yttrium ions to sigma3 grain boundary exists in cubic zirconia.