Insight into the tribological performances of coconut shells as a potential natural water lubrication material

To better understand the tribological performance of coconut shell and to demonstrate its potential as a wearresistant material for use in a water-lubricated environment, the microstructure of coconut shell was hereby characterized, and its hardness and water absorption properties were correspondingly measured. The tribological characteristics of coconut shells rubbing against a ZrO2 ceramic ball in dry and wet environments were studied, and the wear mechanism was revealed. The measurement characterization and experimental results indicate that the long axis of coconut shell ellipsoid cells grows along the direction of the long axis pointing towards the coconut shell ellipsoid, but this directionality has a relatively small impact on the tribological properties. The average sliding friction coefficient of coconut shell rubbing against ZrO2 ceramic balls is about 0.52 and 0.095, respectively under dry and wet friction conditions. Under dry friction conditions, the wear of coconut shells is mainly because of cell tearing, detachment and abrasive wear caused by alternating friction forces. Mainly considering the combined effects of the high hardness, high strength and good water absorption of coconut shells, as well as the water film at the friction interface, the friction force significantly reduces and makes it difficult to damage the cell structure, leading to a wear rate decrease by more than 100 times under wet friction conditions, much lower than that of lignum vitae. Overall, this study indicates the potential of coconut shell as a water lubricated friction pair material featuring excellent performance.