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Previous reports of rapid phase transformation above 18 GPa [Erskine and Nellis, Nature 349, 317 (1991)] and large elastic waves below 18 GPa [Lucas et al., J. Appl. Phys. 114, 093515 (2013)] for shock-compressed ZYB-grade highly oriented pyrolytic graphite (HOPG), but not for less oriented ZYH-grade HOPG, indicated a link between the orientational order dependence of the HOPG response above and below the phase transformation stress. To gain insight into this link and into the mechanical response of HOPG shocked to peak stresses approaching the phase transformation onset, the compressibility of ZYB- and ZYH-grade HOPG in the shocked state was examined using front surface impact experiments. Particle velocity histories and sound velocities were measured for peak stresses reaching 18 GPa. Although the locus of the measured peak stress-particle velocity states is indistinguishable for the two grades of HOPG, the measured sound velocities in the peak state reveal significant differences between the two grades. Specifically (1) the measured sound velocities are somewhat higher for ZYH-grade HOPG compared with ZYB-grade HOPG; (2) the measured sound velocities for ZYH-grade HOPG increase smoothly with compression, whereas those for ZYB-grade HOPG exhibit a significant reduction in the compression dependence from 12 GPa to 17GPa and an abrupt increase from 17GPa to 18 GPa; and (3) the longitudinal moduli, determined from the measured sound velocities, are smaller than the calculated bulk moduli for ZYB-grade HOPG shocked to peak stresses above 15 GPa, indicating the onset of an elastic instability. The present findings demonstrate that the softening of the longitudinal modulus (or elastic instability) presented here is linked to the large elastic waves and the rapid phase transformation reported previously—all observed only for shocked ZYB-grade HOPG. The elastic instability in shocked ZYB-grade HOPG is likely a precursor to the rapid phase transformation observed above 18 GPa for this HOPG grade. [ABSTRACT FROM AUTHOR]

The effect of tip radius on atomic force microscopy (AFM) nanoindentation is investigated through indentation on the (111) face of a gold single crystal. The hardness is derived using two different methods: by measuring directly the projected area of the residual indent with AFM images and by measuring the cross-sectional area of the indenter before and after each nanoindentation test. The hardness values obtained from the cross-sectional area of the indenter are comparable with those obtained from images of the residual indent scanned with a sharp tip. Two AFM tips of average radii of 70±12 and 112±26 nm are used to indent the sample to various depths ranging from 4 to 50 nm. For depths above 30 nm, hardness values remain constant around 500 MPa for both indenters. For depths below 30 nm, the hardness increases as the indent depth decreases for the sharp and blunt indenters, and the indent depth dependence is observed over a wider depth range for the sharp indenter. For depths below 30 nm, the hardness values obtained with the sharp indenter are also consistently higher than those obtained with the blunt indenter. The results confirm a size scale effect during nanometer scale indentation for both varying penetration depth and tip radius, both of which influence the volume of material sampled during deformation. [ABSTRACT FROM AUTHOR]

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