A qualitative understanding of high intensity mechanical shearing and exfoliation of graphite nanoplatelets in a three-body contact using molecular dynamic simulations
W Nishad and S Subbiah and N Swaminathan, JOURNAL OF MANUFACTURING PROCESSES, 71, 645-652 (2021).
Graphite and graphene-based lubricants are increasingly investigated for manufacturing processes such as forming and machining. Their effectiveness depends on trapping graphite particles in between the tribologically interacting surfaces and the consequent shearing and exfoliation of the graphitic layers. The mechanism by which such shearing and exfoliation occurs is however difficult to elucidate through experiments. To address the aforementioned difficulty, we have carried out molecular dynamics studies of graphene exfoliation by simulating a three-body rubbing contact scenario, with graphite nanoplatelet trapped between iron and diamond. The iron is given simultaneous normal and shear velocities towards the stationary diamond surface. High intensity continuous rubbing, simulated via large sliding distances and diminishing gaps between the iron and diamond surfaces, is used to understand the extent of exfoliation and the damage incurred in the exfoliated layers. Simulations show that under certain conditions, layer shearing and exfoliation result in graphene layers being produced. Increasing normal velocity and decreasing the shear velocity increases the number of fragments produced. Several phenomena observed in the simulations, such as locking-in of layer orientations, tearing of edges, amorphization, and iron contamination, agree qualitatively with experimental observations. The insights from this study can be used to optimize and control the process parameters of friction induced mechanical exfoliation processes and also to understand the usable life of graphite-based lubricants.
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