A series of deformation experiments were conducted on selected polycrystalline diamonds (PCD) and nano-polycrystalline diamond (NPD) using the deformation DIA (D-DIA) apparatus [1] at the Advanced Photon Source (APS), Argonne National Laboratory.  Well-sintered samples, 0.5 mm in diameter and 0.8 mm in length, were laser-cut from corresponding bulk materials. While PCDs contained Co or Si as binding agents, NPD samples were synthesized from high-purity graphite without binding material [2]. Samples were deformed at pressures up to 12 GPa, and temperatures up to1673 K under essentially identical strain rates of ~1.5x10^-5 s^-1. Under such conditions, PCDs are expected to deform by dislocation creep [3]. Monochromatized synchrotron radiation up to 60 keV was used for both radiographic imaging to measure plastic strain and 2D x-ray diffraction to measure differential stress. Maximum attainable axial strains for the PCDs were about 18%. Differential stresses in the samples were quantified based on distortion of the (111) lattice planes as a function of azimuth angle. All binder-containing PCDs exhibit strengths lower than that of single-crystal diamond under similar P and T conditions [4]. TEM revealed high density of dislocation tangles in grains (~1 – 20 microns in size) of the PCDs, consistent with dislocation-controlled mechanism [3] and continued strain hardening observed in the stress-strain curves. For the NPD samples, all experiments at 1273 K ended with fractures at strains below 2%. Plastic deformation was achieved only above 1473 K, with no strain hardening. Samples deformed up to ~10% strain showed no dislocation development in NPD (grainsize ~50 nm). In fact, the recovered NPD samples showed microstructure non-distinguishable from that prior to deformation. Strength of NPD is ~50% higher that that of single-crystal diamond. We contribute the much greater strength of NPD to dislocation-nucleation controlled mechanisms. 

References:

[1] Wang, Y., et al., The deformation-DIA: A new apparatus for high temperature triaxial deformation to pressures up to 15 GPa. Rev. Sci. Instrum., 2003. 74(6): p. 3002-3011.

[2] Irifune, T., et al., Ultrahard polycrystalline diamond from graphite. Nature, 2003. 421: p. 599-600.

[3] Devries, R.C., Plastic-deformation and work-hardening of diamond. Mat. Res. Bull., 1975. 10: p. 1193-1200.

[4] Weidner, D.J., Y. Wang, M.T. Vaughan, Strength of diamond, Science, 1994. 266: p. 419-422.