SEM images post-indentation with a peak load of 100 mN. In (A), (B), and (D) the indentation direction was perpendicular to the aragonite c-axis, in (C) multiple crystals in coral skeleton have unknown orientations. A) Geologic aragonite crystals indented perpendicular to c-axis, B) synthetic spherulites (inset shows additional toughening mechanisms: crack deflection, microcracking, and crack branching), C) coral skeleton, and D) Pmnacre (indented perpendicular to c-axis, that is, with nacre tablet layers horizontal in the image). Hrfnacre has a behavior qualitatively similar to Pmnacre (Figure S6C, Supporting Information). SEM images are used to extract the radius of the residual indent RI and of the entire damaged area RD by fitting the smallest possible circle centered on the indent. E) Typical indentation curves at a peak load of 100 mN: synthetic and coral aragonites show characteristics pop-in events (highlighted by arrows), associated with sudden damage beneath the contact surface. Geologic aragonite also shows such discontinuities but at higher loads, Pmnacre and Hrfnacre do not (see inset), at either lower or higher loads, indicating a progressive damage accumulation but no abrupt pop-in events. F) Ratio of mean damage radius (RD) and indentation radius (RI) plotted versus indentation peak load (trend lines are locally weighted scatterplot smoothing, LOWESS, regression fits with a smoothing factor of 0.8). This metric quantifies whether damage tends to localize around the indent (as in Pmnacre and Hrfnacre) or to propagate (as in geo aragonite). Both coral and synthetic spherulites have intermediate behavior. G) Experimentally measured indentation fracture toughness (abbreviated as “Toughness”), obtained by fitting the correlation between damage radius (RD, a measure of crack length) and indentation load (see Methods in Supporting Information, and Figure S7, Supporting Information).[40,60] Data are presented as mean ± SD over multiple locations within the same sample, except for the spherulites, where 4 different spherulites were measured. Comparison of all the samples demonstrates that slight misorientations, present in synthetic spherulites, coral skeleton, and nacre but absent from geologic aragonite, correspond to greater toughness.
