To examine the possibility of plastic deformation upon loading, we conducted FEM simulations using elastic and elastic-plastic constitutive relations to model both the ex situ diamond spherical indentation as well as the tungsten microprobe used in the operando SEM experiments. Considering a LLZO hardness of about 8 GPa , we expect plastic deformation to occur at a yield strength of between 5-8 GPa . Both elastic and elastic-plastic FEM simulations of diamond indentation result in a poor fit to the experimental results. This suggests that plastic deformation alone cannot explain the observed deviations from elastic behavior in the load curve. It is likely that micro/nano-scale fracture events are instead responsible for the decrease in slope. Other damage modes such as phase transformation may also influence the loading curve, but experimental data is lacking to understand how large this effect may be. See Figure for more load-displacement curves.