Abstract
Diamonds may not be forever, but research interest in diamond has never ebbed. Owing to its highly symmetric crystal structure and strong covalent C–C bonds, diamond possesses an exceptional combination of physical properties. Its hardness and thermal conductivity are the highest among covalent materials. It also has a large bandgap and electric breakdown field, as well as optical transparency over a wide range of wavelengths. All of these are essential for a wide range of applications in both industrial and scientific areas. Despite these outstanding advantages, however, diamond is extremely brittle, with inferior toughness and poor deformability. These shortcomings have caused undesired tool breakage and have imposed severe constraints on technological innovations. To surmount these intrinsic deficiencies, tremendous research effort has been dedicated to developing advanced diamond products, with great progress being achieved in the past few years.
Highlights
The Hall–Petch effect provides a general pathway to harden diamond by decreasing the characteristic length scale of the continuous crystal lattice
With suitably selected carbon precursors such as carbon onion nanoparticles, nanotwinned diamond with an average twin thickness of ∼5 nm has been synthesized at high pressure and high temperature (HPHT) and exhibits an unprecedented hardness of up to 200 GPa
The microstructure of nt-diamond can be further tailored at a nanoscale through optimizing HPHT conditions, providing further enhancement of the mechanical properties of diamond-related materials
Summary
In addition to the mechanical performance enhancements achieved through microstructure engineering, progress has been made in clearing up several long-standing puzzles about diamond, such as its poor deformability, its low tensile strength, and the existence of room-temperature plasticity,4–6 thanks to advances in sample preparation and characterization techniques.7 By reducing the dimensions of diamond crystals to minimize the influence of internal defects, high elastic tensile strain and tensile strength approaching the theoretical elasticity and Griffith theoretical strength (122 GPa) of diamond can be achieved in single-crystal diamond nanoneedles.4,5 For example, a 〈100〉-oriented 60-nm-diameter diamond nanoneedle deforms elastically to a tensile strain of 13.4% with a tensile strength of 125 GPa.5 This astonishing performance can be further improved with better control of minute surface defects such as steps caused by discontinuous atomic layers. The hardness of diamond can be enhanced via extrinsic hardening effects from grain or twin boundaries and dislocations.
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