Effects of interlayer interactions on the nanoindentation behavior and hardness of 2:1 phyllosilicates

Six non-expandable, hydrous 2:1 phyllosilicate minerals with different layer charges, including pyrophyllite and talc from the pyrophyllite-talc group and biotite, two muscovites, and margarite from the mica group, were nanoindented to probe their nanoscale deformation behavior and hardness. Remarkably different deformation responses were obtained, which are highly dependent upon the type and strength of interlayer interactions controlled by Coulomb or van der Waals forces. Concave load–displacement curves were observed on the micas with stronger Coulomb forces as interlayer interaction, while convex curves on the other group with weaker van der Waals attractions as interlayer interaction. The hardness increases with layer charge both within each group and between the two groups, ranging from 3.86 GPa for talc to 16.34 GPa for margarite, with the highest being four times greater than the lowest. Owing to their layered structure that favors kink band formation and layer delamination, pop-ins of varied extensions occur randomly in the load–displacement curves. An observed apparent indentation size effect is attributed to the pop-in events. The dependence of hardness on interlayer interactions between 2:1 layers instead of the atomic bonds within the 2:1 layer suggests that the interlayer interactions can be used as a hardness or plastic signature for phyllosilicate minerals.