Mechanisms of elastic softening in highly anisotropic carbons under in-plane compression/indentation

Abstract

We present a combined experimental and computational study of the elastic behavior of a series of highly anisotropic pyrocarbons, with crystallite sizes La in the 2–10 nm range, under a-axis compressive load. The materials include a rough laminar and a regenerative laminar pyrocarbon, as-prepared by chemical vapor deposition and after various heat treatments up to 2600 °C, for which a-axis nanoindentation experiments have been performed, showing a significant decrease in the indentation modulus and hardness with increasing La (or heat treatment temperature). To rationalize this behavior, molecular dynamics simulations of the uniaxial compression of accurate atomistic models of the materials as well as pristine graphite were performed, unraveling significant out-of-plane deformations in the models with increasing compressive strain, leading to elastic softening. More precisely, significant kinks were observed around extended screw dislocation-like defects in the most disordered pyrocarbon at rather large strain levels (∼ 3%). Conversely, graphite rather shows the formation of extended buckles, starting at very low strain values. Finite element modelling shows that such kinking/buckling transitions should take place in a large area under the indenter tip within usual nanoindentation conditions. Both finite element calculation and analytical approximation of the indentation modulus predict the correct trend of decreasing modulus with increasing La when applied with the elastic tensors computed after the buckling/kinking transitions, certainly proving the importance of the latter in the observed experimental indentation moduli.