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

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 ID/IG ratios evidencing graphite crystal refinement from an La of ~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 sub-surface region, measured by La, showed an exponential relationship with depth (z) to a depth of 35–40 μm. The positive shift of the G band in the Raman spectrum indicates a residual compression accompanied by refinement with the highest average of ~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 ~200 μm. Mechanisms for the refinement and residual compression are discussed with the support of atomistic modelling.