Plastic Deformation And Strengthening Mechanisms Of Nanopolycrystalline Diamond

Bulk nanopolycrystalline diamond (NPD) samples were deformed plastically within the diamond stability field up to 14 GPa and above 1473 K. Macroscopic differential stress Delta sigma was determined on the basis of the distortion of the 111 Debye ring using synchrotron X-ray diffraction. Up to similar to 5(2)% strain, Debye ring distortion can be satisfactorily described by lattice strain theories as an ellipse. Beyond similar to 5(2)% strain, lattice spacing d(111) along the Delta sigma direction becomes saturated and remains constant with further deformation. Transmission electron microscopy on as-synthesized NPD shows well-bonded grain boundaries with no free dislocations within the grains. Deformed samples also contain very few free dislocations, while density of {111} twins increases with plastic strain. Individual grains display complex contrast, exhibiting increasing misorientation with deformation according electron diffraction. Thus, NPD does not deform by dislocation slip, which is the dominated mechanism in conventional polycrystalline diamond composites (PCDCs, grain size >1 mu m). The nonelliptical Debye ring distortion is modeled by nucleating 1/2(110) dislocations or their dissociated 1/6(112) partials gliding in the {111} planes to produce deformation twinning. With increasing strain up to , similar to 5(2)%, strength increases rapidly to , similar to 20(1) GPa, where d(111) reaches saturation. Strength beyond the saturation shows a weak dependence on strain, reaching similar to 22(1) GPa at >10% strain. Overall, the strength is similar to 2-3 times that of conventional PCDCs. Combined with molecular dynamics simulations and lattice rotation theory, we conclude that the rapid rise of strength with strain is due to defect-source strengthening, whereas further deformation is dominated by nanotwinning and lattice rotation.