On ripplocations and the deformation of graphite

The micromechanism for the deformation of layered crystalline solids (LCSs) such as graphite, mica, and the MAX phases has long been assumed to be basal dislocations alone. The latter, however, cannot explain several important observations made when graphite is deformed such as non-linear elasticity, the stress-strain curves' strong sensitivity to confining hydrostatic pressures, evidence for c-axis strain, among other observations. In 2015, the term ripplocation-best described as an atomic layer ripple-was coined and since then we have been making the case that atomic layers in LCSs - like any other layered systems - buckle. Using molecular dynamics simulations on graphite we showed that buckling leads to the nucleation of multiple, oppositely signed ripplocation boundaries (RBs) that rapidly propagate, wavelike, into the bulk to form standing waves. It is the to-andfro motion of RBs and the associated friction between the layers that accounts for the energy dissipation observed when LCS/graphite are cyclically loaded. We also make the case that RBs are not atomically sharp and can be precursors of kink boundaries, KBs. The difference between the two is fundamental: RBs, if not pinned, are fully reversible because they are highly elastically strained. KBs are irreversible and their energy is in the boundary itself. We show that the hallmarks of deformation by ripplocations are: i) nano-ligaments observed in transmission electron microscope microgrpahs; ii) large pileups around spherical indentations when the layers are indented along the c-axis and delamination cracks when they are indented edge-on, and iii) fully reversible hysteretic stress-strain curves.

Automated summary

The micromechanism for the deformation of layered crystalline solids (LCSs) has long been assumed to be basal dislocations alone. However, observations made during graphite deformation cannot be explained by basal dislocations alone. This study introduces the concept of ripplocation, where atomic layers buckle and form ripple-like structures. These ripplocation boundaries (RBs) propagate and cause energy dissipation during cyclic loading. The study also suggests that RBs can be precursors of kink boundaries (KBs), which are irreversible. The characteristics of deformation by ripplocations include nano-ligaments, pileups around spherical indentations, and fully reversible stress-strain curves.