Temporary reinforcement of proton-conductive ceramics by introducing a graded carrier

Proton-conductive yttrium-doped barium zirconate ceramics were hydrated under various conditions and their mechanical strength and Vickers hardness were evaluated. The hydration rate and lattice constants of surfaces, shallow-scraped surfaces, and deep-scraped surfaces were measured under each condition. When the water vapor partial pressure was high and the hydration temperature was low, mechanical strength increased by up to 1.9-fold that before treatment. When the strength increased, the Vickers hardness rose as the test load decreased, implying the formation of a surface compression layer. An examination of the correlation between strength and the lattice constant revealed that the greater the difference in the lattice constant between the surface and shallow-scraped surfaces, and the smaller the difference between shallow-scraped and deep-scraped surfaces, the greater the mechanical strength. Such strengthening may reflect the fact that the surface remains hydrated, and compressive stress thus acts on a volume-expanded surface layer of the internal bulk, which has not changed in overall volume. Although specimens exhibiting large increases in strength underwent rapid local hydration, there was no mechanical damage, and the ionic conductivity was the same as that of specimens hydrated under mild conditions.