Abstract: It is generally considered that heat treatments have a negative impact on the mechanical properties of nacre due to thermal
decomposition of the organic matrix. However, the present work investigated the microindentation behavior on fresh
and heat-treated nacres from two orthogonal directions, and the results demonstrate that both hardness value and damage
tolerance can remain almost unchanged on the cross-section with the organic matrix degeneration, despite a significant
deterioration on the platelet surface. Theoretical analyses suggest that the anisotropic response of indentation behavior to
heat treatment in nacre is primarily caused by its structural orientation. Specifically, compared with a single layer of irregular
interplatelet interfaces in cross-sectional specimens, the multiple layers of parallel interlamellar interfaces in in-plane
specimens exhibit a much greater ability to impede indenter-triggered destruction, and heat treatments would reduce the
in-plane hardness but nearly have no effect on the cross-sectional hardness. Moreover, the deeper embedding of platelets
in cross-sectional specimens enhances their resistance to interface cracking caused by organic matrix degradation at high
temperatures, leading to a reduced sensitivity to damage. Therefore, the indentation behavior of nacre shows different
tendencies in response to variations in the organic matrix state along normal and parallel directions.

Key words: Nacre · Structural orientation · Heat treatment · Indentation behavior · Organic matrix