Atomic-scale interlayer friction of gibbsite is lower than brucite due to interactions of hydroxyls

To investigate the role of atomic-scale structure on the frictional properties of gibbsite, a dioctahedral-type aluminum hydroxide, we calculated the atomic-scale interlayer shear properties using the first-principles method based on density functional theory. We found that the presence of vacant sites within the octahedral sheet of gibbsite enables hydroxyls to move to more stable positions and reduce the repulsive force, leading to a lower atomic-scale shear stress of gibbsite compared with brucite, a trioctahedral-type magnesium hydroxide. We also estimated the macroscopic single-crystal friction coefficient of gibbsite with the assumption that only the atomic-scale interlayer friction controls macroscopic friction. The estimated single-crystal friction coeficient for gibbsite is 0.36(6), which is clearly lower than the experimentally obtained friction coeficient of the powdered gouge of gibbsite (0.74). This diference between the interlayer friction coeficient and gouge friction coeficient suggests the presence of additional mechanisms that afect the frictional strength, such as microstructures within a fault gouge.

Automated summary

This study investigates the influence of atomic-scale structure on the frictional properties of gibbsite, a dioctahedral-type aluminum hydroxide. By employing the first-principles method based on density functional theory, the atomic-scale interlayer shear properties of gibbsite are calculated. The results reveal that the presence of vacant sites within the octahedral sheet of gibbsite allows the hydroxyls to move to more stable positions and reduce the repulsive force, resulting in a lower atomic-scale shear stress compared to brucite, a trioctahedral-type magnesium hydroxide. Furthermore, the macroscopic single-crystal friction coefficient of gibbsite is estimated, assuming that only the atomic-scale interlayer friction influences the macroscopic friction. The estimated single-crystal friction coefficient for gibbsite is 0.36(6), which is significantly lower than the experimentally obtained friction coefficient of the powdered gouge of gibbsite (0.74). This disparity suggests the existence of additional mechanisms, such as microstructures within a fault gouge, that affect the frictional strength.