Room and high temperature failure mechanisms in solid oxide fuel cell electrolytes

The microstructural stability at elevated temperatures and the mechanical properties of 8 mol% yttria stabilised zirconia (YSZ) has been investigated. The YSZ was supplied by two manufacturers as 150 μm thick sheet suitable for the electrolyte in a planar solid oxide fuel cell (SOFC). The two materials had the cubic structure and this crystal structure was maintained up to 1100°C, which was the highest temperature investigated. However, there were differences, albeit small in most cases, in composition, density, grain size and surface finish between the products from the two manufacturers. Although the compositional and structural variations resulted in some differences in mechanical performance, the general trends shown by both materials were similar. The strength, as determined by biaxial flexure, fell by 23–30% on increasing the test temperature from room temperature (RT) to the SOFC operating temperature of 950°C and there was evidence for changes in crack initiation and propagation mechanisms at the higher temperature. The loading rate dependence of the RT strength yielded a low value for the stress exponent n in the standard lifetime equation, which indicated that sub-critical crack growth (sccg) was sensitive to applied stress. Constant load tests at 950–1000°C illustrated the sensitivity of sccg to stress and revealed that creep was also operative in this temperature range. It was concluded that the creep probably occurred by a grain boundary diffusion mechanism.