Structural and Morphological Transformations in Aqueous Graphite-Based Lubricant Subjected to Extreme Tribological Interactions of Low-Speed Intermittent Machining

We study morphological and structural changes in graphite flakes present in aqueous graphite-based lubricants when repeatedly subjected to extreme tribological interactions at the tool–chip interface in low speed, high depth of cut intermittent machining, a process involving combined high normal and shear stress. Orthogonal tube turning experiments are conducted on a specially fabricated mild steel workpiece completely submerged in an aqueous graphite solution. Surface profiler, electron microscopy and Raman spectroscopy studies on initial and sheared graphite flakes indicate crushing followed by layer sliding, rolling, edge curling, tearing and exfoliation of layers. Raman spectroscopy studies highlight formation of low amorphous carbon by structural disorder and edge defects formed while crushing and tearing with no new basal defects in the sheared graphite layers. Some regions of the flake thin down to highly disordered few-layer graphene with a thickness reduction from about 30 micrometres to about 1–2 nanometres with an I _2D /I _G value of around 0.7. When reused back into the tribological region, these morphological changes can significantly influence the continued lubrication process by the generation of few-layer graphene.