Tribological behaviour of nacre-Influence of the environment on the elementary wear processes

Nacre (so-called mother-of-pearl) is a natural organic/inorganic nanocomposite displaying exceptional high mechanical properties to weight ratio and a good biocompatibility with the human bones. Consequently, it becomes an attractive material for prosthesis design. These properties are ascribed to its highly ordered layered 'bricks and mortar' microstructure. This work studies the tribological behaviour of sheet nacre, rubbing on itself in dry conditions and in the presence of a liquid medium, in relation with its multiscale structure. The results show that the coefficient of friction, rather high under dry conditions (0.45), is even increasing under wet conditions (0.78). Aside a significant dissymmetry between pin and disc, the wear rates were shown to display the same trends. These changes are associated with marked differences in surface topography and in the extent of third body. The systematic study of the microstructure (by AFM, image analysis, SEM and TEM) of the worn surfaces and third bodies allows us to propose a particle detachment model where the role of the inter- and intracrystalline organic material and their interactions with the environment are emphasized.