Kink bands, nonlinear elasticity and nanoindentations in graphite Original Research Article

Herein we report on the response of graphite single crystals––loaded parallel to their c-axis––to a 13.5 μm radius spherical diamond nanoindenter. Up to loads of 5 mN, corresponding to stresses of ≈0.5 GPa, fully reversible hysteresis loops are observed. At stresses >0.5 GPa, the first loops are slightly open; subsequent loops, in the same location, are fully reversible and harder than the first. Simple compression experiments on polycrystalline cylinders yielded qualitatively similar results. Our results, together with much of the literature on the mechanical properties of graphite, can be explained by invoking the formation of incipient kink bands, IKBʹs, that give way to mobile dislocation walls that, in turn, coalesce into kink boundaries with increasing stress. The IKBʹs are fully reversible; the dislocation walls result in plastic deformation, and the kink boundaries explain the hardening. Since the dislocations are confined to the basal planes, they cannot entangle and can thus move reversibly over relatively large distances resulting in the dissipation of substantial amounts (up to 100 MJ/m3) of energy during each cycle. At stresses >1.5 GPa, massive pop-ins––of the order of 60 μm––are observed. Examination of the craters formed provided direct evidence for kink bands and the formation of a multitude of subgrains under the indenter. Based on this work, it is clear that graphite is a member of a larger class of solids––kinking nonlinear elastic solids––that includes the Mn+1AXn phases, layered silicates, nonlinear mesoscopic elastic solids, among others.