Full-dense nanopolycrystalline diamond is stronger than single-crystal diamond
A group of researchers from Osaka University and Ehime University demonstrated the Hugoniot elastic limit (HEL) of full-dense nanopolycrystalline diamond (NPD).
HEL is defined as the longitudinal pressure (stress) at which a solid undergoes a transition from reversible elastic deformation to irreversible plastic deformation upon dynamic compression.
Grain size influences the strength of a polycrystalline material, i.e. smaller-grained metals are stronger, but only a little is known about how the grain size affects the HEL and Hugoniot of polycrystalline materials, mainly due to the difficulties of synthesizing homogeneous and full-dense polycrystalline samples with different grain sizes.
The researchers’ main issue was whether the grain boundary strengthening effect (GBSE) is valid for diamond under high strain rate ( ∼109 s−1) shock compression.
They performed experiments at the GEKKO XII laser facility at the Institute of Laser Engineering, Osaka University to investigate the HEL of full-dense NPD up to 1600 GPa.
By applying ultra-high pressure to grains of 10-20 nm (which is known to have the largest GBSE), they deformed NPD, which had been synthesized to have a density of 3.514 (±0.003) g/cm3, the same as that of a single-crystal diamond (laser pulse duration: 2.5 ns). They recorded velocity profiles in real time by a line-imaging velocity interferometer system for any reflector (VISAR). The HEL of full-dense NPD was 208 (±14) GPa, which was more than twice as high as that of single-crystal diamond and the highest HEL of any materials ever measured.
NPD with an average grain size about several tens of nanometers were made in Japan. This group demonstrated that the GBSE was also effective against high strain-rate dynamic compressions, so NPD is expected as promising hard materials.
This group’s achievements will affect research and development of materials that require high strength, such as structural materials for extreme conditions and high-performance ceramics.
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The article, “Shock Response of Full Density Nanopolycrystalline Diamond,” was published in Physical Review Letters at DOI: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.185701.
