Near-surface structure and residual stress in as-machined synthetic graphite

B Marz and K Jolley and R Smith and HZ Wu, MATERIALS & DESIGN, 159, 103-116 (2018).

DOI: 10.1016/j.matdes.2018.08.041

We have used optical and electron microscopy and Raman spectroscopy to study the structural changes and residual stress induced by typical industrial machining and laboratory polishing of a synthetic graphite. An abrasion layer of up to 35 nm in thickness formed on both machined and polished surfaces, giving the same I-D/I-G ratios evidencing graphite crystal refinement from an La of similar to 110 nm down to an average of 21 nm, but with different residual compression levels. For the as-polished sample, structural change was limited to the near surface region. Underneath the as-machined surface, large pores were filled with crushed material; graphite crystals were split into multi- layered graphene units that were rearranged through kinking. Graphite crystal refinement in the subsurface region, measured by L-a, showed an exponential relationship with depth (z) to a depth of 35-40 mu m. The positive shift of the G band in the Raman spectrum indicates a residual compression accompanied by refinement with the highest average of similar to 2.5 GPa on top, followed by an exponential decay inside the refined region; beyond that depth, the compression decreased linearly down to a depth of similar to 200 mu m. Mechanisms for the refinement and residual compression are discussed with the support of atomistic modelling. (C) 2018 The Authors. Published by Elsevier Ltd.

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