Nanotribology of nacre: Anisotropic dissipation in a multiscale hybrid material

Sheet nacre (so-called mother–of–pearl) is a hybrid biocomposite with a multiscale structure including nanograins of calcium carbonate (97wt%–40nm in size) and two organic matrices: (i) the “intracrystalline” (mainly composed by silk-fibroin-like proteins), and (ii) the “interlaminar” one (mainly composed of β-chitin and proteins). Micro/nanotribological behaviour was investigated on sheet nacre displaying various configurations (so-called face-on and edge-on), by varying the orientation of the matrices vs. the sliding direction. Different levels of frictional dissipated energy were observed as a function of both: (i) the type of matrix which is involved in the dissipation mechanism (intracrystalline or interlaminar), and (ii) the orientation of the matrices themselves vs. the sliding direction. These various dissipative ways can involve either, multiscale wear processes entailing the both matrices, or irreversible deformation only, without any wear process. They have been identified and explained by considering the double composite structure of sheet nacre.