Nanoindentation of Pressure Quenched Fullerenes and Zirconium Metal from a Diamond Anvil Cell

Abstract

The sample size employed in high pressure diamond anvil cells is limited to a diameter of typically 25 to 150 microns. While this size is often sufficient for diagnostics using synchrotron x-ray diffraction and Raman scattering, ex-situ measurements of mechanical properties using conventional microhardness indentation techniques is not feasible. For some materials, the high pressure phase(s) can be quenched to ambient pressure allowing further characterization by other techniques. We make use of the very small probe volume allowed by nanoindentation to investigate the pressure-quenched structures of both C₇₀ fullerene and zirconium. For the case of C₇₀, we show that the amorphous phase established above 35 GPa can be quenched to ambient, and that it shows a largely elastic indentation loading behavior with a hardness of 30 GPa. We establish that this hard carbon phase contains a mixture of sp²- and sp³-bonded carbon and that it can be produced from C₇₀ fullerene by application of pressure at room temperature. With regard to zirconium metal, we confirm the irreversible transformation from the ambient hexagonal-close-packed phase to the simple hexagonal O-phase (AlB₂ structure) and document an 80% increase in hardness that may be attributed to the presence of covalent bonding based on sd²-hybridized orbitals forming graphite-like nets in the (0001) plane of the AlB₂ structure.