Surviving the surf: the tribomechanical properties of the periostracum of Mytilus sp.

We investigated the friction and wear behavior as well as the mechanical properties of the periostracum of Mytilus sp. Tribological properties were determined with a reciprocal sliding microtribometer, while mechanical characterization was performed using a nanoindenter. Measurements were performed in dry and wet conditions. On the dry periostracum we found a low friction coefficient of 0.078±0.007 on the young parts and a higher one of 0.63±0.02 on the old parts of the shell. Under wet, saline, conditions we only observed one average coefficient of friction of 0.37±0.01. Microscopic ex situ analysis indicated that dry periostracum wore rather rapidly by plowing and fatigue, while it exhibited a high wear resistance when immersed in salt water. The Young's modulus and hardness of the periostracum were also investigated in both dry and wet conditions. Under dry conditions the Young's modulus of the periostracum was 8±3GPa, while under wet conditions it was 0.21±0.05GPa. The hardness of dry periostracum samples was 353±127MPa, whereas the hardness of wet samples was 5±2MPa. It was found that, in the wet state, viscous behavior plays a significant role in the mechanical response of the periostracum. Our results strongly indicate that the periostracum can provide an important contribution to the overall wear resistance of Mytilus sp. shell.