Nanotwinned Diamond Research Articles

Coherent Interface Migration Toughens Diamond

AbstractOvercoming the hardness‐toughness trade‐off in diamonds attracts much interest in physics, chemistry, materials science, and engineering. Recently synthesized nanotwinned diamond composite exhibits massive enhancement in fracture toughness without sacrificing its unprecedented Vickers hardness [Y. Yue et al., Nature 582, 370 (2020)]. Several mechanisms for the toughness enhancement are unveiled based on the edge‐cracked models while the mechanism from Vickers indentation has remained elusive. Here, the energy of nanotwinned diamonds, diamond polytypes, and diamond composites is systematically investigated from Vickers indentation simulation. The results show diamond structures dissipate energy by interface migration, accompanied by the phase transformation from diamond polytypes to the cubic diamond (3C diamond). By tuning the density and distribution of interfaces, the dissipated energy of the diamond is increased to more than twice that of a single‐crystal 3C diamond. This work complements the established mechanisms and provides a universal strategy for toughening diamonds and related materials.

Interactions between shuffle-set (001) dislocation and (111) twin boundary in nanotwinned diamond

Recent studies have revealed unprecedented fracture toughness in synthesized nanotwinned diamond (nt-diamond) due to the obstruction of dislocations by twin boundary. However, the atomic-scale interactions between dislocations and twin boundary in diamond remain elusive and unclear. In this work, we take advantage of the recently published interatomic potential for diamond's dislocation activity to study the interactions between (001) dislocation and (111) twin boundary in molecular dynamics (MD) simulations. The twin boundaries show obvious hindrance for (001) dislocation. In the penetrations, the shuffle-set dislocation will climb to the upper gliding plane and leave a vacancy tube behind. With the gradual increase of applied shear stress, the vacancy left on the twin boundary could launch another shuffle-set dislocation in the opposite direction in matrix. The mechanism revealed in this work show obvious difference between the interaction between twinning and dislocation in metals. This study will provide a fundamental understanding of the dislocation behaviors and their strengthening and toughing mechanisms in nt-diamond materials and the related materials.

Self-healing of fractured diamond.

Materials that possess the ability to self-heal cracks at room temperature, akin to living organisms, are highly sought after. However, achieving crack self-healing in inorganic materials, particularly with covalent bonds, presents a great challenge and often necessitates high temperatures and considerable atomic diffusion. Here we conducted a quantitative evaluation of the room-temperature self-healing behaviour of a fractured nanotwinned diamond composite, revealing that the self-healing properties of the composite stem from both the formation of nanoscale diamond osteoblasts comprising sp2- and sp3-hybridized carbon atoms at the fractured surfaces, and the atomic interaction transition from repulsion to attraction when the two fractured surfaces come into close proximity. The self-healing process resulted in a remarkable recovery of approximately 34% in tensile strength for the nanotwinned diamond composite. This discovery sheds light on the self-healing capability of nanostructured diamond, offering valuable insights for future research endeavours aimed at enhancing the toughness and durability of brittle ceramic materials.

A compressive-shear biaxial stress field model for analyzing the anisotropic mechanical behavior of nanotwinned diamond

Synthetic nanotwinned diamond (Nt-D) with unprecedented hardness and toughness is an ideal material for manufacturing ultra-precision cutting tools. It is urgent to investigate the surface formation mechanism of Nt-D under complex stress fields. Here, we developed a compressive-shear biaxial stress field model to study the extreme anisotropic mechanical behavior of Nt-D. It was found that graphitization occurred in all shear directions of the Nt-D (0001) plane through the microstructure evolution of 2H-diamond, 4H-diamond, 6H-diamond, and 9R-diamond, in which the stress release mechanism was accompanied by partial slips in certain specific crystal directions such as 〈1010〉. In addition, the generalized stacking fault energy calculation indicates that the partial slip in 6H- and 9R-diamonds is mainly attributed to the low slip energy barrier between adjacent interlayer. These results expand our understanding of the surface formation mechanism of Nt-D and will contribute to the manufacturing of new-generation ultra-precision tools.

Molecular dynamics study of thermal conductivities of cubic diamond, lonsdaleite, and nanotwinned diamond via machine-learned potential

Diamond is a wide-bandgap semiconductor with a variety of crystal configurations, and has the potential applications in the field of high-frequency, radiation-hardened, and high-power devices. There are several important polytypes of diamonds, such as cubic diamond, lonsdaleite, and nanotwinned diamond (NTD). The thermal conductivities of semiconductors in high-power devices at different temperatures should be calculated. However, there has been no reports about thermal conductivities of cubic diamond and its polytypes both efficiently and accurately based on molecular dynamics (MD). Here, using interatomic potential of neural networks can provide obvious advantages. For example, comparing with the use of density functional theory (DFT), the calculation time is reduced, while maintaining high accuracy in predicting the thermal conductivities of the above-mentioned three diamond polytypes. Based on the neuroevolution potential (NEP), the thermal conductivities of cubic diamond, lonsdaleite, and NTD at 300 K are respectively 2507.3 W⋅m−1⋅K−1, 1557.2 W⋅m−1⋅K−1, and 985.6 W⋅m−1⋅K−1, which are higher than the calculation results based on Tersoff-1989 potential (1508 W⋅m−1⋅K−1, 1178 W⋅m−1⋅K−1, and 794 W⋅m−1⋅K−1, respectively). The thermal conductivities of cubic diamond and lonsdaleite, obtained by using the NEP, are closer to the experimental data or DFT data than those from Tersoff-potential. The molecular dynamics simulations are performed by using NEP to calculate the phonon dispersions, in order to explain the possible reasons for discrepancies among the cubic diamond, lonsdaleite, and NTD. In this work, we propose a scheme to predict the thermal conductivity of cubic diamond, lonsdaleite, and NTD precisely and efficiently, and explain the differences in thermal conductivity among cubic diamond, lonsdaleite, and NTD.

Polishing of ultrahard nanotwinned diamond tool for producing nearly damage-free tool surface and highly sharp cutting edge

Polishing of ultrahard nanotwinned diamond tool for producing nearly damage-free tool surface and highly sharp cutting edge

Simulations of plasticity in diamond nanoparticles showing ultrahigh strength

We use molecular dynamics (MD) simulations to deform single crystal spherical carbon nanoparticles (NP), 4–45 nm diameter, with a hard, flat indenter, compressing along the [001] direction. There is no clear amorphization nor phase change in the NP, but there is significant deformation, with bent crystalline planes, and many atoms that retain sp3 coordination, but are no longer recognized as having diamond structure by different structure-identification methods. Machine-learning is used to improve diamond-structure identification. The NP deforms laterally, and volumetric strain is ~0.1 when the uniaxial strain is ~0.5. Poisson's ratio increases with strain, and the elastic limit is reached at 0.2–0.3 strain, at a contact pressure of ~150 GPa. For NPs above 5 nm, dislocations appear and are mostly (1/2)<110>{111} full dislocations, with a few partial dislocations for larger nanoparticles, without twinning. These results agree with the recent observation of plastic deformation in diamond nanopillars. Small NP display elastic modulus, yield stress and hardness increasing with NP size, but NPs with diameter larger than 25 nm display an approximately constant dislocation and dislocation junction density, which leads to a plateau in the hardness versus NP size, at ~150 GPa, close to bulk diamond. Diamond nanoparticles could provide high strength thin coatings, lighter than full-density nanotwinned diamond but with nearly the same strength.

Toughening and Crack Healing Mechanisms in Nanotwinned Diamond Composites with Various Polytypes.

As an emerging ceramic material, recently synthesized nanotwinned diamond composites with various polytypes embedded in nanoscale twins exhibit unprecedented fracture toughness without sacrificing hardness. However, the toughening and crack healing mechanisms at the atomic scale and the associated crack propagation process of nanotwinned diamond composites remain mysterious. Here, we perform large-scale atomistic simulations of crack propagation in nanotwinned diamond composites to explore the underlying toughening and crack healing mechanisms in nanotwinned diamond composites. Our simulation results show that nanotwinned diamond composites have a higher fracture energy than single-crystalline and nanotwinned diamonds, which originates from multiple toughening mechanisms, including twin boundary and phase boundary impeding crack propagation, crack deflection and zigzag paths in nanotwins and sinuous paths in polytypes, and the formation of disordered atom clusters. More remarkably, our simulations reproduce more detailed crack propagation processes at the atomic scale, which is inaccessible by experiments. Moreover, our simulations reveal that crack healing occurs due to the rebonding of atoms on fracture surfaces during unloading and that the extent of crack healing is associated with whether the crack surfaces are clean. Our current study provides mechanistic insights into a fundamental understanding of toughening and crack healing mechanisms in nanotwinned diamond composites.

Intersectional nanotwinned diamond-the hardest polycrystalline diamond by design

The hardness of nanotwinned diamond (nt-diamond) is reported to be more than twice that of the natural diamond, thanks to the fine spaces between twin boundaries (TBs), which block dislocation propagation during deformation. In this work, we explore the effects of additional TBs in nt-diamond using molecular dynamics (MD) calculations and introduce a novel intersectional nanotwinned diamond (int-diamond) template for future laboratory synthesis. The hardness of this int-diamond is predicted by first analyzing individual dislocation slip modes in twinned grains and then calculating the bulk properties based on the Sachs model. Here we show that the hardness of the int-diamond is much higher than that of nt-diamond. The hardening mechanism of int-diamond is attributed to the increased critical resolved shear stress due to the presence of intersectional TBs in nt-diamond; this result is further verified by MD simulations. This work provides a new strategy for designing new super-hard materials in experiments.

Mechanical polishing of ultrahard nanotwinned diamond via transition into hard sp2-sp3 amorphous carbon

Ultrahard nanotwinned diamond (nt-D) is an ideal material for next-generation high-precision, high-efficiency cutting tools, due to its high hardness, enhanced fracture toughness, and increased oxidation resistance temperature, when compared with natural diamond. However, a critical problem that limits the application of such material is the grinding and polishing of nt-D material into suitable shapes. In this study, we confirmed the feasibility of classical mechanical polishing for nt-D through amorphization transition. The effect of mechanical loading and the catalysis of Fe nano-particles work together, promoting the transformation of nt-D into a hard sp2-sp3 amorphous carbon. The surface of the amorphous carbon layer and the interface between amorphous carbon and nt-D were both smooth after polishing. The results are valuable for the mechanical processing and further applications of ultrahard nanotwinned diamond.

Strengthening and toughening by partial slip in nanotwinned diamond

It was reported recently that synthesized nanotwinned diamond (nt-diamond) possesses unprecedented hardness and ductility, however, the role of nanotwins in strengthening and toughening mechanisms remains unclear. In this article, we compared the atomic reconfiguration patterns of nt-diamond and nt-Si obtained with first principles calculations. We found that the detwinning in nt-diamond, which can be achieved by partial slip on the glide-set plane, could account for the strengthening of nt-diamond. Such continuous partial slip could lead to an extremely large increase in strain range and intragranular deformation resistance for nt-diamond. In contrast, the stress of nt-Si could be released by the slip on the shuffle-set plane, which could not induce detwinning. Different responses in nt-diamond and nt-Si can be accounted for with their generalized stacking fault energies (GSFEs). The glide-set plane in diamond has lower GFSE, therefore, the atoms would slide preferentially along the glide-set plane; while in Si, the shuffle-set plane has lower GFSE. These results can provide insights into the atomic reconfiguration as well as the unprecedented strength and ductility of nt-diamond under shear deformation.

Extreme static compression of carbon to terapascal pressures

Recent reports of record-setting ultrahigh static pressures achieved in diamond anvil cells (DACs, 495 GPa) and double-stage DACs (ds-DACs, 1 TPa) raise fundamental questions about mechanical response of diamond and related carbon structures under extreme compression. Here we present results from a combined first-principles calculation and finite-element analysis that unveil the mechanisms responsible for the greatly enhanced compressive strengths of these carbon structures by lateral confining stresses concentrated near anvil tips, stemming from structural deformations in compressed DACs and further strengthened by additional confining pressures in ds-DACs. Our results indicate that diamond anvils oriented in the [001] direction with a flat culet diameter of 20 μm can sustain peak pressures above 500 GPa, vastly exceeding its pure compressive strength of about 200 GPa. Among nano-carbon structures with enhanced shear strengths by nano- or grain-boundaries, we find that nano-twinned diamond possesses the highest compressive strength, reaching 1 TPa in ds-DAC settings, in agreement with experimental observations. The present findings establish key benchmarks and expand the realm of understanding of diamond and related carbon structures under extreme loading conditions; these results offer crucial insights for rational design of advanced and novel DAC devices.

Nanostructured superhard materials

Superhard materials play irreplaceable roles in the industry areas including mechanical processing, oil exploration, geological exploration, etc., as well as in the scientific fields such as earth science and high pressure science. Developing high-performance superhard materials with extraordinary hardness, fracture toughness, and thermal stability has been a long-lasting goal for both scientific and industrial communities. Natural diamond has been thought of the hardest material in nature since it was discovered in ancient India more than 6000 years ago. Ever since the successful synthesis of diamond in the laboratory in 1955, finding artificial materials with hardness exceeding that of the natural diamond has been a pursued dream of human beings. However, this is truly a challenge to materials science, and many researchers have argued that it is impossible to fulfill this dream. In 2003, we proposed a microscopic understanding of the indentation hardness as the combined resistance of chemical bonds in a material to indentation, and established a microscopic hardness model for covalent single crystals. This model reveals several key factors, such as short and strong chemical bonds, high valence electron density or high bond density, and strongly directional bonds, which are beneficial to enhance the hardness. On the base of the hardness model for covalent single crystals, we systematically investigated the hardening mechanisms in polycrystalline covalent materials, and established a hardness model for polycrystalline covalent materials. Two main hardening effects in polycrystalline covalent materials are identified, namely the Hall-Petch effect and the quantum confinement effect, both of which contribute increasingly to hardness with decreasing microstructural characteristic size. As a result, polycrystalline covalent materials can be continually hardened with the microstructural characteristic size down to the deep nanoscale, which is significantly different from the nanostructured metals and designates a brand-new direction to greatly enhance the hardness of covalent materials. Nanograining and nanotwinning are two popular strategies to minimize the microstructures. Twin boundaries possess excess energy typically one order of magnitude lower than that of grain boundaries. Nanowinning thus provides a more effective mechanism to achieve smaller microstructural characteristic size compared with nanograining. We proposed a novel idea to significantly enhance the performance of diamond and cubic boron nitride (cBN) through forming ultrathin nanotwinned microstructures, and synthesized nanotwinned diamond and cBN bulks from onion-structured carbon and boron nitride precursors, respectively, via martensitic phase transformations under high pressure and high temperature. Nanotwinned diamond and cBN both shows greatly enhanced hardness, fracture toughness and thermal stability compared with the corresponding single crystals due to the formation of ultrathin nanotwins. The hardness of nanotwinned diamond reaches 200 GPa, twice as high as that of natural diamond and turning the dream of synthesizing a material harder than natural diamond into reality. In addition, there is no hardness anisotropy or cleavage features typical for single crystals due to the randomly oriented nanograins and nanotwins. The successful synthesis of nanotwinned ultrahard materials is a great promotion to the high-performance superhard materials research. With these advantages in performance, nanotwinned ultrahard materials can produce technological innovations in industry and high pressure science.

Revealing the formation mechanism of ultrahard nanotwinned diamond from onion carbon

Controlled nanotwinning is an effective way to enhance the mechanical properties of materials. Recent discovery of nanotwinned diamond converted from carbon nano-onions with high-density defects reveals that the presence of nanotwinned structures can increase the hardness of the product to exceed that of natural diamond by a surprisingly large margin. To understand the mechanism of nanotwinning, the microscopic transformation pathway from carbon nano-onions to nanotwinned diamond was investigated in the present study. We carried out a direct high-pressure high-temperature synthesis of nanotwinned diamond from onion carbon without high-density defects. The obtained nanotwinned diamond possesses an exceptionally high Vickers hardness of 215 GPa at 4.9 N. The transformation path was analyzed using aberration-corrected transmission electron microscopy (TEM) which suggests a martensitic process strongly influenced by the pressure-temperature conditions. Specifically, the appearance of {111} nanotwinned structure and stacking faults was determined by the characteristics of the onion shells, while the accumulation of the stress due to the sliding of the shells cause the crystal to re-align along the shear direction. These findings not only clarify the direct transformation mechanism from onion-like precursors to nanotwinned diamond, but also have broad implications for further exploration of new materials with exceptional properties.

Understanding the mechanical characteristics of nanotwinned diamond by atomistic simulations

As is known to all, diamond is the hardest natural substance. The synthesis of harder materials than the natural diamond is very favorable but challenging. It has been found that the introduction of nanotwinned (NT) structure into diamond could be an effective approach to improve the hardness of diamond. Nevertheless, the fundamental understanding of the strengthening characteristics by NT structure is still ambiguous. In this work, we unveil the effect of NT structure on the deformation mechanism and mechanical properties of diamond at the atomic scale by virtue of atomistic simulations. Interestingly, the hardness measured by nanoindentation shows a positive correlation to the twin spacing instead of the inverse Hall-Petch effect extensively discovered in many traditional metal materials. However, it is not the smallest twin spacing achieving the highest tensile strength, indicating the existence of twin-spacing-induced inverse Hall-Petch effect in NT diamond during tensile deformation. The specific deformation mechanisms are investigated which are capable of providing an explicit explanation for the observed mechanical characteristics.

Molecular dynamics simulations for responses of nanotwinned diamond films under nanoindentation

We performed molecular dynamics (MD) simulations for the responses of single crystal (SC) and nanotwinned (nt) diamond films under nanoindentation, respectively, aimed to uncover the effects of twin boundary (TB) and twin thickness (δ) on hardness (H) and the corresponding deformation mechanisms. We found the Hall-Petch type relationship between H and δ. We also found that the inelastic deformation of SC-diamond under indentation could mainly be attributed to the nucleation and propagation of 〈110〉{111} dislocation loops. It showed that dislocation blockage and pile up at the TBs may induce additional hardening of the nt-diamond, while the softening of the material could be attributed to: (i) the formation and movement of the dislocation loops parallel with the surface, and (ii) the breakage of TBs, which may serve as new sites for dislocations nucleation.

Nanotwinned diamond synthesized from multicore carbon onion

Nanopolycrystalline diamond (NPD) and nanotwinned diamond (NtD) were successfully synthesized in a multianvil, high-pressure apparatus at high temperatures by using the precursors of carbon onion. It was found that distinct carbon onions with hollow or multicore microstructures lead to the formation of different diamond products, namely NPD or NtD, respectively. The Vickers hardness of high-quality NtD with an average twin thickness of 6.8 nm reached as high as 180 GPa, which was measured by indentation hardness experiment. The existence of stacking faults other than various defects in the carbon onion was found to be crucial for the formation of twin boundaries in the product. The origin of the extraordinarily high Vickers hardness in the NtD sample is attributable to the high concentration of twin boundaries. Our work directly supports the argument that pursuit of nanotwinned microstructure is an effective strategy to harden materials, which is in good agreement with the well-known Hall-Petch effect.

Extreme Mechanics of Probing the Ultimate Strength of Nanotwinned Diamond.

Recently synthesized nanotwinned diamond (NTD) exhibits unprecedented Vickers hardness exceeding 200GPa [Q. Huang etal., Nature (London) 510, 250 (2014)]. This extraordinary finding challenges the prevailing understanding of material deformation and stress response under extreme loading conditions. Here we unveil by first-principles calculations a novel indenter-deformation generated stress confinement mechanism that suppresses the graphitization or bond collapse failure modes commonly known in strong covalent solids, leading to greatly enhanced peak stress and strain range in the indented diamond lattice. Moreover, the twin boundaries in NTD promote a strong stress concentration that drives preferential bond realignments, producing a giant indentation strain stiffening. These results explain the exceptional indentation strength of NTD and offer insights into the extreme mechanics of the intricate interplay of the indenter and indented crystal in probing ultrahard materials.

Nanotwinned diamond with unprecedented hardness and stability.

Although diamond is the hardest material for cutting tools, poor thermal stability has limited its applications, especially at high temperatures. Simultaneous improvement of the hardness and thermal stability of diamond has long been desirable. According to the Hall-Petch effect, the hardness of diamond can be enhanced by nanostructuring (by means of nanograined and nanotwinned microstructures), as shown in previous studies. However, for well-sintered nanograined diamonds, the grain sizes are technically limited to 10-30 nm (ref. 3), with degraded thermal stability compared with that of natural diamond. Recent success in synthesizing nanotwinned cubic boron nitride (nt-cBN) with a twin thickness down to ∼3.8 nm makes it feasible to simultaneously achieve smaller nanosize, ultrahardness and superior thermal stability. At present, nanotwinned diamond (nt-diamond) has not been fabricated successfully through direct conversions of various carbon precursors (such as graphite, amorphous carbon, glassy carbon and C60). Here we report the direct synthesis of nt-diamond with an average twin thickness of ∼5 nm, using a precursor of onion carbon nanoparticles at high pressure and high temperature, and the observation of a new monoclinic crystalline form of diamond coexisting with nt-diamond. The pure synthetic bulk nt-diamond material shows unprecedented hardness and thermal stability, with Vickers hardness up to ∼200 GPa and an in-air oxidization temperature more than 200 °C higher than that of natural diamond. The creation of nanotwinned microstructures offers a general pathway for manufacturing new advanced carbon-based materials with exceptional thermal stability and mechanical properties.