Figure 4. Nanoindentation load-displacement response from nacre's aragonite platelets and geologic aragonite monocrystal along [002].

For the geologic aragonite monocrystal, at the nanoindentation displacement of approximately 6.6 nanometers, a sudden displacement pop-in was found, which is consistent with the plasticity onset at the stress level of the theoretical strength. In comparison, no discernible pop-in event can be detected for nacre's aragonite platelets, implying the stress underneath the nanoindenter was not accumulated to the theoretical strength. Its lower loading slope and more apparent viscoelastic effect at the peak indentation holding segment jointly indicate the existence of biopolymer matrix, buffering stress concentration within individual aragonite platelets.