Carbon, the most abundant element on earth, has lots of allotropes in different crystal structures. One of the exotic forms of carbon is called glassy carbon (GC), which has been reported as a precursor to synthesize nano-diamonds in both hexagonal and cubic form in many works by high pressure and/or high temperature treatment. GC consists of only carbon atoms and is a predominately sp2 bonded disordered material with graphene-like layers entangle randomly by sp3 bonds in its structure. It is isotropic and considered to have prototypical super-elastic mechanical properties.

In this study, GC has been compressed and recovered from a series of pressure values ranging from ~0 to ~80 GPa using a diamond anvil cell (DAC). By nanoindentation on the recovered materials to approach their mechanical responses and molecular dynamics (MD) simulations, we found that GC starts to lose its super-elasticity after compression to ~6 GPa, and its deformation level is at the maximum at ~30 GPa. After this pressure threshold (~30 GPa), the material starts to become anisotropic, with the graphene-like layers starts to rearrange and align perpendicular to the compression axis. In samples recovered from pressures lower than ~60 GPa, almost all of the sp3 bonds generated under pressure convert back to sp2 bonds after pressure release. This observation changes on the samples recovered from pressures higher than ~60 GPa, that the sp3 bonds generated under pressure start to remain in the material after releasing the pressure to ambient. As the pressure goes higher to ~80 GPa, we observed the formation of both nano- lonsdaleite (hexagonal diamond) and nano- cubic diamond. The exciting property about nanocrystalline hexagonal diamond is that it has been reported to be even harder than cubic diamond via first principles calculation, which needs further investigation in experiments and is undergoing in our study via nanoindentation, atomic forced microscopy (AFM) and finite element analysis (FEA).

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