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The effect of relative density on the hardness and fracture toughness of Al‐substituted cubic garnet Li 6.19 Al 0.27 La 3 Zr 2 O 12 ( LLZO ) was investigated. Polycrystalline LLZO was made using solid‐state synthesis and hot‐pressing. The relative density was controlled by varying the densification time at fixed temperature (1050°C) and pressure (62 MP a). After hot‐pressing, the average grain size varied from approximately 2.7–3.7 μm for the 85% and 98% relative density samples, respectively. Examination of fracture surfaces revealed a transition from inter‐ to intragranular fracture as the relative density increased. The Vickers hardness increased with relative density up to 96%, above which the hardness was constant. At 98% relative density, the Vickers hardness was equal to the hardness measured by nanoindentation 9.1 GP a, which is estimated as the single‐crystal hardness value. An inverse correlation between relative density and fracture toughness was observed. The fracture toughness increased linearly from 0.97 to 2.37 MP a√m for the 98% and 85% relative density samples, respectively. It is suggested that crack deflection along grain boundaries can explain the increase in fracture toughness with decreasing relative density. It was also observed that the total ionic conductivity increased from 0.0094 to 0.34 mS /cm for the 85%–98% relative density samples, respectively. The results of this study suggest that the microstructure of LLZO must be optimized to maximize mechanical integrity and ionic conductivity.
Kim et al. (Mon,) studied this question.