Multi-scale Synergistic Effects Enhanced Hierarchically Structured Polycrystalline Diamond for Exceptional Hardness and Toughness

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This long-standing trade-off can be resolved by architecting hierarchically microstructures as confirmed in metals, but this routine is normally forbidden in diamond due to the strong covalent structure. Starting from a rationally designed mixture of onion-like carbon and graphite, we exploit the precursor-directed high-pressure and high-temperature (HPHT) transformation to nucleate hierarchically interpenetrating network of nano/micron-crystalline diamond. Simultaneously, high stress caused by asynchronous phase transformation triggers high density of dislocations array in micrometer grains. The resulting hierarchical superhard polycrystalline diamond (HSPD) show exceptional Vickers hardness of 146.0 GPa along with a high fracture toughness of 14.1 MPa·m 0.5 , which is triple that of single-crystal diamond. The hierarchically microstructure allows multi-scales plastic deformation mechanisms to be activated concurrently, in which Hall-Petch strengthening from nanoscale domains restricts dislocation motion, whereas dislocations network together with coarse sub-micron grains activate crack deflection, branching and bridging to dissipates fracture energy. This multi-scale strategy, achieved through precursor-directed transformation, provides a general pathway to customize microstructure in covalent materials for fabricating damage-tolerant, ultrahard diamond and hard ceramics without sacrificing hardness. Physical sciences/Materials science/Structural materials/Ceramics Physical sciences/Materials science/Condensed-matter physics Hierarchical-structured polycrystalline diamond hardness and toughness high pressure and high temperature Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 INTRODUCTION Diamond, a quintessential ultrahard material (H V > 80 GPa), serves as a critical constituent in precision machining tools, ultrahard cutting implements, and extreme-environment protective coatings 1 , 2 , 3 . However, its covalently dominated structure inherently restricts dislocation slip systems, rendering it prone to brittle fracture under dynamic loading or thermal shock, which inevitably lead to sudden, catastrophic failure, and possess a challenge for application (e.g., high-speed cutting, geological drilling) 4 . By using binder of metal (Fe, Co, Ni) or inorganic material (Si), tough polycrystalline diamond can be realized but with lower hardness of 50–70 GPa and degraded application temperature below 700 ℃ 5, 6 . There are several promising improvement methods to toughness metal and ceramics, such as bioinspired lamination composition 7 , 8 , phase transition toughening 9 , gradient structure 10 , seeded dislocations 11 and bi-phase toughening methods 12 . However, researches on these methods for diamond is scarce due to the traditional method is forbidden in strongly covalent solids 13 . Nowadays, microstructural design, particularly tailoring the grain size distribution from nano to micron grains, have emerged as a compelling strategy to resolve the classical hardness-toughness conflict 3 . From angstrom-to-nanometer scale, the incorporation of nanotwins, defect-pinning, and coherent interfaces effectively impedes dislocation motion and crack propagation, leading to unprecedented levels of hardness while suppressing catastrophic failure 14 , 15 . At the nanometer scale, various of nanopolycrystalline diamond, cubic boron nitride, silicon nitride and various of ceramics have been enhanced with Hall-Petch effect, that surpass their single-crystal counterparts in both hardness and fracture toughness 2 , 16 . Stepping up to the micrometer scale, bio-inspired lamellar architectures, composite designs, and interlocking grain configurations have demonstrated promisingly dissipate mechanical energy, yielding substantial gains in damage tolerance without sacrificing hardness 17 . These findings underscore that the controlled defect and microstructure engineering, orchestrated across disparate length scales, in developing effective toughening mechanisms for structural materials. By seamlessly integrating the above different scales architectures, diamond-based materials are anticipated to be enhanced in hardness and toughness at the same time. This has been rationalized by the hierarchically structured diamond composition that exhibit exceptional toughness 13 . But the grains are in nanoscale and large range of hierarchically structured diamond has not been reported. Larger scale hierarchical structure materials, in which multi structurally or mechanically distinct populations are co-assembled within one body, have successfully dissolved the conventional strength-ductility trade-off in metals and ceramics, extending these principles to diamond promises analogous, transformative gains 10 . For example, fine or nano-scale grains deliver high strength through Hall-Petch strengthening, while coarse micro-scale grains accommodate plasticity via dislocation storage and crack blunting mechanisms 18 . In metal matrix composites, for example, nano-reinforcements (e.g. SiC / Al 2 O 3 ) strengthen the matrix via the Hall-Petch effect, while micrometer-scale phases enhance energy dissipation through crack bridging/deflection, yielding synergistic gains in both strength and ductility 19 , 20 , 21 . (In covalent ceramics, a micro-nano bimodal structure in hexagonal boron nitride (h-BN) enables anisotropy modulation and elevates fracture energy (from ~ 2.5 to ~ 4.5 MPa·m 0.5 ) 22 . If high density of dislocations is jointed into the hierarchical structure, defective hierarchically-structured polycrystalline material is anticipated to exhibit much better properties. The strength and ductility of forged GH5188 cobalt-based superalloy was enhanced by the gradient dislocations and dual-scale interlocked grains 23 . Such "multi-scale synergistic effects" have redefined the performance limits of structural material and has demonstrated remarkable advantages across various material systems including polymers, metals, ceramics, and composites 19 . However, such hierarchical schemes designs are almost undocumented in covalently bonded diamond, primarily due to the formidable challenge of precise microstructure control during high-pressure and high-temperature (HPHT) synthesis. In addition, unlike metals, diamond cannot be post-deformed to refine or texture microstructures; once sp 3 bonds form, dislocation multiplication and grain-boundary migration are essentially frozen. Furthermore, sintering hierarchical diamond powder to produce hierarchical polycrystalline diamond also failed, due to the heterogenous stress distribution within the sample because of the covalent feature. By selecting different carbon precursor materials and using HPHT method, precise design and regulation of complex microstructures were achieved, successfully preparing a series of innovative materials, such as nanograins 24 , 25 , lamellae diamond 2 , 26 , nanotwinned diamond, 14, 27 hexagonal diamond 28 , 29 , hierarchical diamond composite 13 , graphite-graphite diamond hybridize materials (Gradia) 15 , and amorphous diamond 30 , 31 . Low-density carbon allotropes (such as graphite, fullerenes, glassy carbon, etc.) 32 can be transformed into diamond-based materials with unique microstructures and properties under different HPHT conditions, providing rich possibilities for the design and development of multifunctional diamond materials. By deliberately mixing precursors of different size, shape, and stacking order, the conversion event can be forced to proceed asynchronously. Thus, precursor architecture-not post processing-becomes the enabling lever for hierarchical diamond that unite micron-scale lamellae with nanograins and a high density-dislocation substructure, all in single HPHT process. In this work, high-quality hierarchical superhard polycrystalline diamond (HSPD) bulks were fabricated by treating nano onion carbon with large graphite flake under HPHT conditions. These HSPDs exhibit simultaneous ultra-high hardness and exceptional fracture toughness. Transmission electron microscope reveals a multi-scale hierarchical structure, the size disparity between precursor carbons drives a hierarchical architecture, while transformation-induced stress field generate a dense, homogenous dislocation substructure. This controlled introduction high-density dislocations offers a generic route for microstructure engineering and establishes a new pathway toward damage-tolerant diamond and diamond-related composite for more superior performance. RESULTS AND DISCUSSION The artificial precursors are crucial for the final architecture of the synthesized polycrystalline diamond. Hierarchical structure in diamond was customized by regulating the ratio of precursor onion-like carbon (OLC) to micron-scale graphite during phase transformation under HPHT conditions. As show in Fig. S1 and S2, X-ray diffraction, scanning electron microscopy (SEM) images, and Raman spectrum confirm that the OLC particles (30–50 nm) are highly defective and nanocrystalline, whereas the micron-sized graphite flakes (2–3 µm) exhibit a highly ordered structure. These features are corroborated by the corresponding I D / I G ratios. Then the OLC, were evenly mixed with micron-scale graphite flakes in molar ratios of 10%, 30%, 50%, 70%, 90% OLC. As shown in Fig. S3, the micron graphite flakes are uniformly wrapped by OLC nanoparticles under ultrasonic mixing process, which transform into multi-scale microstructures under HPHT (P = 15 GPa, T = 2000 ℃). And the schematic diagram of the precursor, and synthesis process for the HSPD is shown in Fig. 1 a. The X-ray diffraction patterns of HSPDs are shown in Fig. S3a. The main patterns peaks at 43.9˚, 75.4˚, and 91.4˚, which attribute to (111), (220), (311) planes of cubic diamond, indicates that the sample only consists of cubic diamond, without hexagonal diamond or graphite. The synthesis temperature is lower than that of graphite to cubic diamond 26 . This indicates that the OLC phase transition first to nucleate cubic diamond, thereby reducing the energy barrier for the phase transformation of graphite to cubic diamond and promoting the complete transformation of graphite, which is usually accompanied with hexagonal diamond even at 2000 ℃. Large graphite flakes also transform into diamond at lower temperature as that of other nano carbon precursors, such as C 60 , nano tubes and hydrogenated carbon onions 32 . Raman spectra (Fig. S3b) exhibit the cubic diamond TO vibration mode at 1332 cm − 1 (from micron grains) together with a broad band at 1400–1600 cm − 1 (from nano grains) characteristic of nanograined diamond, confirming a hierarchical grain-size distribution spanning nanometers to sub-micrometers. As shown by the transmission electron microscopy (TEM) in Fig. 1 b-g, there are pronounced hierarchical grain size distribution in all HSPDs. All the microstructure is characterized by interlocking with nano and submicron grains. The grain size statistics reveal that when the OLC content is 10%, the synthesized HSPD-10 sample exhibits a bimodal grain size distribution, primarily ranging from 30–80 nm and 300–400 nm (Fig. 1 b). Furthermore, dense grain boundaries are formed between the grains in the HSPD-10 (Fig. 1 c). When the OLC content in the precursor increases to 50%, the distribution range of submicron grains in HSPD-50 narrows, and well-defined coherent grain boundaries are formed between the grains (Fig. 1 d, e). As the OLC content in the precursor further increases to 70%, the grain size distribution in the HSPD-70 displays a nearly continuous gradient variation from 40–500 nm (Fig. 1 f). Simultaneously, a large number of crisscrossing stacking faults are formed within the submicron grains of the sample (Fig. 1 g). Notably, the microstructure of the nano grain domains is inverse of that reported for nano diamonds from OLC 32 , 33 , 34 . Nano-grains nucleated from OLC are almost dislocation-free, whereas the micron-sized grains derived from graphite contain a dense dislocation network. This inversion originates in the precursor geometry. Highly curved OLC particles provide abundant nucleation sites and transform first at slightly lower temperature, generating an internal stress field within the untransformed graphite flakes. When these flakes subsequently convert to diamond, the accumulated stress is injected as dislocations, yielding large grains that are rich in line defects. The result is a multi-length-scale architecture from dislocation, nano grains to micron grains, in which the coarse sub-micron grains, rather than the nano grains, carries high density of dislocation array, which is opposite to conventional microstructure and essential for the strain hardening mechanism 32 , 35 . Our strategy efficiently uses the moderate population of the large grains with high density of dislocations to achieve the synergetic hardening effect. The nano grains matrix is expected to impart high strength from an extrapolation of Hall-Petch effect, and the larger (softer) grains accommodate strains preferentially. Figure 2 a shows the Vickers hardness and toughness (indentation fracture method) of HSPDs with the function of OLC content. Vickers hardness and fracture toughness increase with OLC content, rising to a maximum at 70% OLC before declining. The optimal composition (HSPD-70) exhibits the maximum hardness value of H V = 146.0 GPa (F = 19.6 N), which exceeds the single crystal diamond (80–120 GPa) by 20%. Load-displacement nano-indentation curves (Fig. 2 b) yield Young’s modulus (E = 1152 GPa) higher than 1000 GPa. The nanoindentation hardness with depth curve is given in the Fig. S5, and it also gives hardness around 100 GPa. Based on the Young’s modulus results, the calculated fracture toughness is 14.1 MPa·m 0.5 , which is approximately threefold that of single-crystal diamond (SCD) (K IC = 3–5 MPa·m 0.5 ) 43, 45 . The hardness and toughness of the diamond sample were improved at the same time. As shown in Fig. 2 d, remarkably, the hardness and fracture toughness is better than that of traditional hard or superhard materials. The fracture toughness, surpass nature SCD, chemical-vapor-deposited and nanopolycrystalline diamond (NPD) 26 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 . Furthermore, HSPD exhibit even better hardness and fracture toughness than that of silicon or cobalt cemented polycrystalline diamond (Si-PCD, Co-PCD, Hv = 40–80 GPa, K IC = 6–10 MPa·m 0.5 ) 46, 47 . The simultaneous enhancement demonstrates that the long exploited hierarchical, multi-scale microstructure as metals and ceramics, can be translated to covalently bonded diamond, overturning its intrinsic hardness-toughness trade-off. To investigate the mechanism by which the hierarchically microstructure enhances the toughness of HSPD-70, micropillar compression experiments were conducted, as shown in Fig. 3 . As the load increases, the sample undergoes gradual compression (Fig. 3 c - g). At strains below 6.3%, the sample exhibits elastic deformation. With further increase in load, plastic deformation occurs in the strain range of 6.3% − 12.0%, which is impossible in brittle single crystal diamond. Subsequently, brittle fracture takes place, as show in Fig. 3 a. The total strain of the sample reaches 12.0%, which is rare in covalent bonding materials such as diamond. Remarkably, there is a region of plastic deformation, and give its plastic deformation ability, which is good for the high fracture toughness. This indicates that the multi-gradient microstructure in HSPD-70 enhances the fracture toughness of the sample by triggering plastic deformation as that in hierarchically cupper 18 . However, due to the small dimensions of the micropillar (L = 4.27 µm, D = 1.48 µm), its compressive strength is significantly lower than the theoretical compressive strength of single-crystal diamond. The microscopic morphology of cracks induced by Vickers indentation was observed using SEM as shown in Fig. S5. In HSPD-10, the cracks propagate almost linearly (Fig. S5a, b). When the nano grain content increase (HSPD-30 and HSPD-50), the crack branch, crack deflection and crack bridging appears, as shown in Fig. S5d - i. Due to the stress concentrate at the crack tip, less energy is consumed during the fracture process, and it exhibits a hard and brittle behavior with poor toughness. Compared to HSPD-30, HSPD-70 exhibits an increased crack deflection angle of nearly 90 degrees, and suggest multiple toughening mechanisms appear, which greatly dissipates the energy for crack propagation. The SEM-documented crack morphologies demonstrate that the hierarchical grain distribution fundamentally alters fracture dynamics in single crystal diamond and traditional nanopolycrystalline diamond. To illustrate the microscopic mechanism of different microstructure distributions in improving fracture toughness, post-mortem TEM test was performed on HSPD-30 and HSPD-70, along the cracks after the Vickers indentation test, as shown in Fig. 4 . Some cracks were observed to propagate along the intergranular interface between large and small grains for HSPD-30, as shown in Fig. 4 (a, b). Since the strength of grain boundaries is weaker compared to the strength of interatomic carbon bonds within diamond, cracks preferentially propagate along the grain boundaries, resulting in intergranular fracture model and almost linear with small zigzag path. Although this fracture mode can alter the crack propagation direction and disperse the stress at the crack tip, thereby contributing to toughening, the toughness improvement remains limited due to the relatively low energy consumed during the fracture of grain boundaries. However, by adjusting the ratio of nano and submicron grains and transgranular and intergranular fractures, the toughness can be further enhanced. As show in Fig. 4 d, e, when the crack propagation in the HSPD-70, there is more frequent obstruction and deflection between large and small grains during crack propagation. It makes the crack tip stress field strongly disturbed at the grain boundary, and each interfacial interaction consumes additional energy and changes the propagation direction to significantly extend the actual propagation path, in which the coarse grains provide bridging stress during crack opening displacement and inhibit crack tip propagation. At the coarse-grained/nanocrystalline interface, the grain size gradually varies from 480 to 240 nm. Zhang et al. have quantitatively analyzed that the gradient zone can increase the crack deflection frequency by 40% per unit thickness 48 . This gradient structure eliminates stress concentrations at the abrupt interface, forcing the crack to continuously adjust its propagation mode. This process cause crack deflection and prolong the crack propagation path, helping to improve toughness. Simultaneously, owing to the high strength of the coherent grain boundaries formed in the sample (Fig. 4 f), approaching that of the chemical bonds between (111) crystal planes, the proportion of intergranular fracture mode increases during crack propagation, as shown in Fig. 4 d, e and Fig. S6. Transgranular fracture consumes significantly more fracture energy compared to intergranular fracture, thereby enhancing toughness 49 . Compared to HSPD-30, HSPD-70 exhibits larger angles crack deflection, accompanied by an increase in crack branching and transgranular fracture. The synergistic effect of these mechanisms results in a 30% improvement in toughness for HSPD-70 compared to HSPD-30, making it nearly three times as tough as single-crystal diamond. So, the proportion of transgranular fracture and intergranular fracture was tuned by the hierarchically structure with nano/micron grain ratio. Figure 5 presents HRTEM images of the crack propagation across the sub-micrometer grains of HSPD-70 sample. Crack deflect at the interface of between nano grain regions and sub-micron grains and propagate into the large micron grains. Compared with that of single-crystal diamond, the common transgranular fracture mode involves cracks propagating straight along the (111) crystal plane, which gives catastrophic fracture in single diamond crystal. However, unlike single-crystal diamond, the cracks in this sub-micron grains undergo multiple deflections within the crack path, as show in Fig. 5 a - d. The primary reason is the interlaced distribution of stacking faults within the grain. These larger grains accumulated large number of dislocations, twin boundaries, and sub-grain boundaries. When the crack is parallel to the dislocations, it propagates along the direction of the dislocations. When the crack is perpendicular to the dislocations, crack deflected at the dislocation sites. It encounters obstacles and deflects, dispersing the tip stress and increasing the energy consumption during fracture, thereby enhancing toughness. Each deviation disperses the crack-tip stress intensity and consumes the fracture energy, converting the normally brittle transgranular path into a tortuous, energy-dissipating trajectory that underpins the three-fold toughness increase. The above-mentioned multi-scale toughening mechanism, spanning nanoscale defects, nanograins, and micrograins, is summarized in Fig. 5 e. All the microstructures played corresponding roles in enhancing toughness, achieving a comprehensive improvement in the mechanical properties of hierarchical diamonds. At the nanoscale, high density of dislocations in sub-micron grains and high density of grain boundary in nanocrystalline regions promotes crack bifurcation, transforming a single dominant crack into multiple subcritical branches (energy dissipation). The gradient from nano- to micrograins further induces crack branching and deflection through maximized boundary density. Meanwhile, micron grains act as bridging ligaments that exert crack-wake traction. By strategically partitioning nanocrystalline (hardness-enhancing) and micron-scale (toughness-promoting) regions, hierarchical-structured achieves extrinsic toughening superior to monomodal structures, validating hierarchical design as a paradigm for transcending hardness-toughness trade-offs in ultrahard materials. This work not only establishes a universal microstructural design paradigm (“nanograins for strengthening, micrograins for toughening”) but also provides deeper insights into the fracture dynamics of covalent materials, laying the groundwork for the development of next-generation ultrahard materials. In addition, by further improving the sintering quality, the mechanical limit of the above-mentioned diamond materials is anticipated to further enhance, solving the problem of brittleness of diamond materials. CONCLUSION By programming an onion-carbon-graphite precursor to transform under HPHT (15 GPa and 2000°C), we have produced HSPD that simultaneously exhibit Vickers hardness of 146.0 GPa and fracture toughness of 14.1 MPa·m 0.5 , which is threefold that of single-crystal diamond. The precursor-directed reaction self-assembles a hierarchical grain scaffold, nano grain domains interlocked with submicron grains, with high density of dislocations. Nanoscale domains provide Hall-Petch strengthening while the coarse fraction plus dislocation walls activate crack deflection, branching and bridging that greatly dissipate fracture energy. This synthesis route demonstrates that hierarchical, damage-tolerant architectures can be engineered even in the most strongly covalent solids, offering a general strategy for next-generation ultra-hard ceramics that no longer trade hardness for toughness. METHODS Samples synthesis. In this study, high-purity graphite powder (purity 99.95%, grain size 2–3 µm, Shanghai Aladdin Biochemical Technology Co., Ltd.) and onion carbon powder (purity 99.5%, grain size 30–50 nm) were used as starting materials. Onion carbon with molar ratios of 10 mol %, 30 mol %, 50 mol %, 70 mol % and 90 mol % were mechanically milled with graphite, in an agate mortar, at room temperature for 2 hours. The mixtures were cold-compacted into cylinders with a diameter of 2.2 mm and a height of 2 mm in a cubic large-volume press (Guiye company, SPD 6 × 600T) at 5 GPa for 15 min, yielding densified precursors ready for subsequent high-pressure and high temperature synthesis. The Kawai-type large-volume press experiments were performed using a high-pressure apparatus (Egret-10) at Ningbo University and LVPECF-1 at the B1 station, Synergetic Extreme Condition User Facility (SECUF). The pretreated raw materials were loaded into the standard 10/5 components in Fig. S7 50 . Magnesium oxide octahedrons doped with 5% Cr 2 O 3 were used as the pressure transmitting medium (PTM). Rhenium foil served as the heater and lanthanum chromate (LaCrO 3 ) sleeve provided the thermal insulation. The chamber pressure was pre-calibrated at room temperature using the phase-transition pressures of Bi, ZnTe, ZnS, GaAs, and GaP. The temperature was in-situ monitored with W 5% Re 26% -W 26% Re 5% thermocouple (type C) with an accuracy of ± 50 ℃. The different raw material was sintered at 15.0 GPa at 2000°C. Samples were heated to the target temperature at a rate of 50°C / min and held for 20 min, then characterized by quenching to room temperature with the power turned off and returning to ambient pressure. Then, bulk samples with size of 1.8 mm in diameter and 1.5 mm in height were obtained for subsequent characterizations. The corresponding samples were labelled as HSPD, HSPD-10, HSPD-30, HSPD-50, HSPD-70 and HSPD-90. XRD and Raman spectroscopy. Phase identification and lattice parameters were obtained X-ray diffraction with Cu Kα radiation (Rigaku D / Max 2550 V / PC, Rigaku Corporation, Japan, λ = 1.5418 Å)) and Mo Kα radiation (Rigaku FR-X, Rigaku Corporation, Japan, λ = 0.7093 Å). Raman spectra of the samples were measured using a Mono Vista CRS + 500 spectrometer equipped with the CCD detector, with a 532 nm laser serving as the excitation source. SEM, TEM and HRTEM measurements. The microstructure and phase composition uniformity were characterized using scanning electron microscopy (SEM, HITACHI, SU-70, Japan). A thin cross-section transmission electron microscopy (TEM) foil was prepared using a dual-beam focused-ion-beam (FIB, Nova nano lab 200) along the cracks. A transmission electron microscope (TEM, Thermo Fisher, FEI Talos F200X, 200 kV) was used to generate selected area electrons diffractions (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images. Grain-size distributions were extracted by analyzing TEM images. Hardness, fracture toughness and micropillar compression measurements. Vickers hardness was measured using a pyramid-shaped diamond indenter by continuously varying the loading force between 0.98 N and 19.6 N. This method provided reliable hardness values under stable loading rates. The exact diagonal length of each indentation was measured using an optical microscope. H V was calculated by the formula (H V = 1.8544 × F/d 2 ) 37 , 51 , where F(N) is the applied loading force, and d (µm) is the average length of the two diagonals. The indentations, created on the polished surface accompanied by the emergence of cracks at the corners of the imprints was post examined with SEM. Nano indenter G200 instrument with continuous stiffness measurement (CSM) mode was used to measure microhardness and Young’s modulus 52 . To calculate the fracture toughness, the equation K IC = 0.016(E/H V ) 0.5 (F/D 1.5 ) was utilized 53 , with H V being the hardness, and E representing the Young's modulus derived from the nanoindentation testing, where F = 19.6 N is the loading force and D (µm) is the length from the center of the indentation to the edge of the crack. The cracks formed at all four indent corners and were imaged by optical microscopy and SEM to calculate the fracture toughness. Micropillars, approximately 1.28 µm in diameter and 4.27 µm in height, were prepared by FIB with an ion beam voltage of 30 keV. The micropillar compression tests were performed on a nanoindentation instrument (NMT04, Oxford, UK) equipped with a diamond flat punch indenter. The equipment operates with a displacement-controlled mode with a strain rate of 5–110 nm/s. Note that these pillar compression tests performed outside the SEM are useful to rule out any electron beam effect on the deformation behavior. Declarations Declarations of interest The authors declare that they have no conflict of interest. Acknowledgement This work was supported by the National Key Research and Development Program of China (Grant No. 2023YFA1406200), Open Project of Synergetic Extreme Condition User Facility-Jilin Branch (No. SECUF013). 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Journal of Physics: Conference Series 215 Li B et al (2022) Heterogeneous Diamond-cBN Composites with Superb Toughness and Hardness. Nano Lett 22:4979–4984 Mazhnik E, Oganov AR (2019) A model of hardness and fracture toughness of solids. J Appl Phys 126 McKie A, Herrmann M, Sigalas I, Sempf K, Nilen R (2013) Suppression of abnormal grain growth in fine grained polycrystalline diamond materials (PCD). Int J Refract Met Hard Mater 41:66–72 Peng C et al (2022) Spark plasma sintering of WC-VC 0.5 composites with exceptional mechanical properties and high-temperature performance. Mater Sci Engineering: A 831 Yan C-s, Mao H-k, Li W, Qian J, Zhao Y, Hemley RJ (2004) Ultrahard diamond single crystals from chemical vapor deposition. Phys status solidi (a) 201:R25–R27 Yang B et al (2021) Strengthening effects of penetrating twin boundary and phase boundary in polycrystalline diamond. Diam Relat Mater 117 Drory MD, Dauskardt RH, Kant A (1995) R.O.Ritchie. Fracture of synthetic diamond. JApplPhys 78:3083–3088 Gomon D, Auriemma F, Antonov M (2019) Assessment of abrasive powder behaviour during impact-abrasive wear of PCD elements. Wear 426–427:151–161 Liu S, Han L, Zou Y, Zhu P, Liu B (2017) Polycrystalline diamond compact with enhanced thermal stability. J Mater Sci Technol 33:1386–1391 Zhang ZH, Wei CW, Han JJ, Cao HJ, Chen HT, Li MY (2020) Growth evolution and formation mechanism of η′-Cu 6 Sn 5 whiskers on η-Cu 6 Sn 5 intermetallics during room-temperature ageing. Acta Mater 183:340–349 Lian M et al (2024) Enhancing the fracture toughness of polycrystalline diamond by adjusting the transgranular fracture and intergranular fracture modes. Int J Refract Met Hard Mater 118 Leinenweber KD et al (2012) Cell assemblies for reproducible multi-anvil experiments (the COMPRES assemblies). Am Mineral 97:353–368 Anton RJ, Subhash G (2000) Dynamic Vickers indentation of brittle materials. Wear 239:27–35 Oliver WC, Pharr GM (2011) An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 7:1564–1583 Sagar KG, Suresh PM (2039) A critical evaluation of indentation techniques for measuring the hardness vs toughness: A review. AIP Conference Proceedings 020073 (2018) Additional Declarations There is NO Competing Interest. Supplementary Files Supplementaryinformation.docx Supplementary information TableS1.docx Table S1 Fig.S1.jpg Fig.S2.jpg Fig.S3.jpg Fig.S4.jpg Fig.S5.jpg Fig.S6.jpg Fig.S7.jpg Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8711962","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":589732439,"identity":"2a7046de-1ce9-469d-ba50-da8f7b0161c7","order_by":0,"name":"Tian Cui","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxklEQVRIiWNgGAWjYFACxgaGBAYGOQYJEIeNOC2NDUAtxqRoAVnDwJDYQLQW/hnJ7Q8e1NxJ3y7dY8DwoewwA//sBvxaJG4kAh127FnuzjlnDBhnnDvMIHHnAH4tBhIgLWyHczfcyDFg5m07DBRJIEbLv8PpBiAtf4nWkth2OAGshZEYLRJnHjbOSOw7bLjhzrGCgz3n0nkkbhDQwt+e/uDjj2+H5Q1uN2988KPMWo5/BgEtKOAAEPOQoH4UjIJRMApGAS4AAK2ASG3NiSQ5AAAAAElFTkSuQmCC","orcid":"","institution":"Ningbo University","correspondingAuthor":true,"prefix":"","firstName":"Tian","middleName":"","lastName":"Cui","suffix":""},{"id":589732440,"identity":"7f698985-e8d7-4108-8f69-9c1b4d89fcbe","order_by":1,"name":"Shuailing Ma","email":"","orcid":"","institution":"Ningbo University","correspondingAuthor":false,"prefix":"","firstName":"Shuailing","middleName":"","lastName":"Ma","suffix":""},{"id":589732441,"identity":"cc871884-6ee2-4510-a5bb-88286901e4dc","order_by":2,"name":"Yunfeng She","email":"","orcid":"","institution":"Ningbo University","correspondingAuthor":false,"prefix":"","firstName":"Yunfeng","middleName":"","lastName":"She","suffix":""},{"id":589732442,"identity":"31a06ea8-0f79-47ee-a46b-5ceb02dddedb","order_by":3,"name":"Min Lian","email":"","orcid":"","institution":"Institute of High-Pressure Physics, School of Physical Scientific and Technology, Ningbo University, Ningbo, 315211, China.","correspondingAuthor":false,"prefix":"","firstName":"Min","middleName":"","lastName":"Lian","suffix":""},{"id":589732443,"identity":"07cfa1f2-cafe-4cc0-bfbd-0777df6d129e","order_by":4,"name":"Xiao Ma","email":"","orcid":"","institution":"Ningbo University","correspondingAuthor":false,"prefix":"","firstName":"Xiao","middleName":"","lastName":"Ma","suffix":""},{"id":589732444,"identity":"9f00e8d5-d2ca-4510-8096-797efc13cba2","order_by":5,"name":"Shuo Yang","email":"","orcid":"","institution":"Ningbo University","correspondingAuthor":false,"prefix":"","firstName":"Shuo","middleName":"","lastName":"Yang","suffix":""},{"id":589732445,"identity":"9eefae9a-5d3f-48b5-a926-68b69ed0f838","order_by":6,"name":"Xingbin Zhao","email":"","orcid":"","institution":"Ningbo University","correspondingAuthor":false,"prefix":"","firstName":"Xingbin","middleName":"","lastName":"Zhao","suffix":""},{"id":589732446,"identity":"623b6ad2-8197-492d-98fc-8c1ca6ca49d5","order_by":7,"name":"Cun You","email":"","orcid":"","institution":"Synergetic Extreme Condition User Facility, Jilin University","correspondingAuthor":false,"prefix":"","firstName":"Cun","middleName":"","lastName":"You","suffix":""},{"id":589732447,"identity":"7e829789-6519-424f-9aa2-280eed703eef","order_by":8,"name":"Pinwen Zhu","email":"","orcid":"https://orcid.org/0000-0002-3377-0816","institution":"Jilin University","correspondingAuthor":false,"prefix":"","firstName":"Pinwen","middleName":"","lastName":"Zhu","suffix":""}],"badges":[],"createdAt":"2026-01-27 15:06:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8711962/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8711962/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":102472308,"identity":"e3e38bd5-3b8d-435b-ad25-fa7094240ce2","added_by":"auto","created_at":"2026-02-12 04:17:46","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":5483063,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic of the bimodal precursor (onion-like carbon + graphite flakes), high pressure and high temperature synthesis route and resultant dual-size diamond distribution. The TEM micrographs and corresponding particle size histograms for different samples (b, c) 30 mol % OLC; (d, e) 50 mol % OLC; (f, g) 70 mol % OLC.\u003c/p\u003e","description":"","filename":"Fig.1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/e2e6dec0077b1847198458b3.jpg"},{"id":102472315,"identity":"9d7acded-4c51-4f7f-81fb-347abbc28d5e","added_by":"auto","created_at":"2026-02-12 04:17:46","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":2239784,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Vickers hardness and indentation fracture toughness as a function of molar ratios of OLC under 19.6 N; (b) The representative nano-indentation load-displacement and (c) Vickers hardness changes with loading force curves for HSPD-70; Inset gives optical micrographs of the Knoop indentation under a load of 19.6 N.(d) Vickers hardness and fracture toughness value for different hard/superhard materials, including nanopolycrystalline diamond, synthetic and nature diamond single crystal and other hard materials\u003csup\u003e26, 36, 37, 38, 39, 40, 41, 42, 43, 44\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Fig.2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/7ed08948dd0a57ed325e50c0.jpg"},{"id":102472306,"identity":"ca1b792d-b841-496a-95d3-7d282a52ffa9","added_by":"auto","created_at":"2026-02-12 04:17:46","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2500053,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Stress - strain curve of the HSPD-70 micropillar compression experiment, (b) SEM image of the micropillar before compression, (c - g) photographs of the sample during the compression process.\u003c/p\u003e","description":"","filename":"Fig.3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/91b2b1abec54f0c27bb10bc0.jpg"},{"id":102746000,"identity":"25f83f8f-1dc5-4777-af2b-ce0f240670c6","added_by":"auto","created_at":"2026-02-16 08:55:08","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":5531091,"visible":true,"origin":"","legend":"\u003cp\u003eThe TEM and HRTEM show the effect of different particle size distributions on crack deflection and grain boundaries. (a-d) 30 mol % OLC; (e-h) 70 mol % OLC.\u003c/p\u003e","description":"","filename":"Fig.4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/81a49e09cedfa2da30135bb3.jpg"},{"id":102745710,"identity":"2b92092e-87a6-4fd9-8f1e-5e468a44e2e6","added_by":"auto","created_at":"2026-02-16 08:53:30","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":5621359,"visible":true,"origin":"","legend":"\u003cp\u003e(a-c) TEM micrographs of HSPD-70 (70 mol % OLC) showing dislocation-mediated crack deflection; the crack path is repeatedly titled and branched where it intersects dislocation walls. (d) HRTEM images revealing a high density of dislocation network within sub-micron grains. (e) Schematic illustrating how nanoscale grains, sub-micrometer grains and the dislocation array cooperate to impede crack propagation by simultaneous crack deflection, branching and energy dissipation.\u003c/p\u003e","description":"","filename":"Fig.5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/95ef80692fbbae6524bd3cdb.jpg"},{"id":107705361,"identity":"8b17dd31-fa98-4188-b8d6-6c52849386a7","added_by":"auto","created_at":"2026-04-24 09:11:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":21642741,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/178ea6fb-bb4a-4f13-a6df-22b96398a140.pdf"},{"id":102472319,"identity":"4c5969c2-dca8-45ca-abb1-021fd3aa42a7","added_by":"auto","created_at":"2026-02-12 04:17:46","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2003081,"visible":true,"origin":"","legend":"Supplementary information","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/7a355562cc57bd3ec130669a.docx"},{"id":102746082,"identity":"e4a08824-cb98-4eda-aa86-25c428bf3209","added_by":"auto","created_at":"2026-02-16 08:55:37","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":18048,"visible":true,"origin":"","legend":"Table S1","description":"","filename":"TableS1.docx","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/a19c25ea7ae0362468ac3fb2.docx"},{"id":102472311,"identity":"dfe0bcaa-3c9d-4804-9541-6c3cef6025b8","added_by":"auto","created_at":"2026-02-12 04:17:46","extension":"jpg","order_by":8,"title":"","display":"","copyAsset":false,"role":"supplement","size":2102338,"visible":true,"origin":"","legend":"","description":"","filename":"Fig.S1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/ece2a9df9dcee46d8489668a.jpg"},{"id":102472310,"identity":"712b0ccb-8bc4-4524-ba7e-950779b1cae9","added_by":"auto","created_at":"2026-02-12 04:17:46","extension":"jpg","order_by":9,"title":"","display":"","copyAsset":false,"role":"supplement","size":2298073,"visible":true,"origin":"","legend":"","description":"","filename":"Fig.S2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/ccf7e6d8839901879deb2fed.jpg"},{"id":102472313,"identity":"a818bde9-48da-4a90-9303-2db5f8375396","added_by":"auto","created_at":"2026-02-12 04:17:46","extension":"jpg","order_by":10,"title":"","display":"","copyAsset":false,"role":"supplement","size":1257146,"visible":true,"origin":"","legend":"","description":"","filename":"Fig.S3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/44f8ca0d4f6626f9ffed3854.jpg"},{"id":102472318,"identity":"3c72521a-077d-4a6e-b6aa-6304e68f992e","added_by":"auto","created_at":"2026-02-12 04:17:46","extension":"jpg","order_by":11,"title":"","display":"","copyAsset":false,"role":"supplement","size":1036211,"visible":true,"origin":"","legend":"","description":"","filename":"Fig.S4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/27d981c844f085b14663ae1b.jpg"},{"id":102746520,"identity":"59b644d0-ec56-40a4-a961-cfafdbf8374d","added_by":"auto","created_at":"2026-02-16 08:58:02","extension":"jpg","order_by":12,"title":"","display":"","copyAsset":false,"role":"supplement","size":4779053,"visible":true,"origin":"","legend":"","description":"","filename":"Fig.S5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/9564ea617ed8ddead29eb448.jpg"},{"id":102746080,"identity":"190e726d-1508-4aa6-9c24-aeef1a80affb","added_by":"auto","created_at":"2026-02-16 08:55:35","extension":"jpg","order_by":13,"title":"","display":"","copyAsset":false,"role":"supplement","size":8100767,"visible":true,"origin":"","legend":"","description":"","filename":"Fig.S6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/2692f53fe74421f130cf873a.jpg"},{"id":102746079,"identity":"c14616b8-7dd9-4443-88e0-7ae96d978d63","added_by":"auto","created_at":"2026-02-16 08:55:35","extension":"jpg","order_by":14,"title":"","display":"","copyAsset":false,"role":"supplement","size":541163,"visible":true,"origin":"","legend":"","description":"","filename":"Fig.S7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8711962/v1/61ea6af8890055a72cac2774.jpg"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Multi-scale Synergistic Effects Enhanced Hierarchically Structured Polycrystalline Diamond for Exceptional Hardness and Toughness","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eDiamond, a quintessential ultrahard material (H\u003csub\u003eV\u003c/sub\u003e \u0026gt; 80 GPa), serves as a critical constituent in precision machining tools, ultrahard cutting implements, and extreme-environment protective coatings\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. However, its covalently dominated structure inherently restricts dislocation slip systems, rendering it prone to brittle fracture under dynamic loading or thermal shock, which inevitably lead to sudden, catastrophic failure, and possess a challenge for application (e.g., high-speed cutting, geological drilling)\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. By using binder of metal (Fe, Co, Ni) or inorganic material (Si), tough polycrystalline diamond can be realized but with lower hardness of 50\u0026ndash;70 GPa and degraded application temperature below 700 ℃\u003csup\u003e5, 6\u003c/sup\u003e. There are several promising improvement methods to toughness metal and ceramics, such as bioinspired lamination composition\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, phase transition toughening\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, gradient structure\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, seeded dislocations\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e and bi-phase toughening methods\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. However, researches on these methods for diamond is scarce due to the traditional method is forbidden in strongly covalent solids\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eNowadays, microstructural design, particularly tailoring the grain size distribution from nano to micron grains, have emerged as a compelling strategy to resolve the classical hardness-toughness conflict\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. From angstrom-to-nanometer scale, the incorporation of nanotwins, defect-pinning, and coherent interfaces effectively impedes dislocation motion and crack propagation, leading to unprecedented levels of hardness while suppressing catastrophic failure\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. At the nanometer scale, various of nanopolycrystalline diamond, cubic boron nitride, silicon nitride and various of ceramics have been enhanced with Hall-Petch effect, that surpass their single-crystal counterparts in both hardness and fracture toughness\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. Stepping up to the micrometer scale, bio-inspired lamellar architectures, composite designs, and interlocking grain configurations have demonstrated promisingly dissipate mechanical energy, yielding substantial gains in damage tolerance without sacrificing hardness\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. These findings underscore that the controlled defect and microstructure engineering, orchestrated across disparate length scales, in developing effective toughening mechanisms for structural materials. By seamlessly integrating the above different scales architectures, diamond-based materials are anticipated to be enhanced in hardness and toughness at the same time. This has been rationalized by the hierarchically structured diamond composition that exhibit exceptional toughness\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. But the grains are in nanoscale and large range of hierarchically structured diamond has not been reported.\u003c/p\u003e \u003cp\u003eLarger scale hierarchical structure materials, in which multi structurally or mechanically distinct populations are co-assembled within one body, have successfully dissolved the conventional strength-ductility trade-off in metals and ceramics, extending these principles to diamond promises analogous, transformative gains\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. For example, fine or nano-scale grains deliver high strength through Hall-Petch strengthening, while coarse micro-scale grains accommodate plasticity via dislocation storage and crack blunting mechanisms\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. In metal matrix composites, for example, nano-reinforcements (e.g. SiC / Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) strengthen the matrix via the Hall-Petch effect, while micrometer-scale phases enhance energy dissipation through crack bridging/deflection, yielding synergistic gains in both strength and ductility \u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. (In covalent ceramics, a micro-nano bimodal structure in hexagonal boron nitride (h-BN) enables anisotropy modulation and elevates fracture energy (from ~\u0026thinsp;2.5 to ~\u0026thinsp;4.5 MPa\u0026middot;m\u003csup\u003e0.5\u003c/sup\u003e)\u003csup\u003e22\u003c/sup\u003e. If high density of dislocations is jointed into the hierarchical structure, defective hierarchically-structured polycrystalline material is anticipated to exhibit much better properties. The strength and ductility of forged GH5188 cobalt-based superalloy was enhanced by the gradient dislocations and dual-scale interlocked grains\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Such \"multi-scale synergistic effects\" have redefined the performance limits of structural material and has demonstrated remarkable advantages across various material systems including polymers, metals, ceramics, and composites\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eHowever, such hierarchical schemes designs are almost undocumented in covalently bonded diamond, primarily due to the formidable challenge of precise microstructure control during high-pressure and high-temperature (HPHT) synthesis. In addition, unlike metals, diamond cannot be post-deformed to refine or texture microstructures; once sp\u003csup\u003e3\u003c/sup\u003e bonds form, dislocation multiplication and grain-boundary migration are essentially frozen. Furthermore, sintering hierarchical diamond powder to produce hierarchical polycrystalline diamond also failed, due to the heterogenous stress distribution within the sample because of the covalent feature. By selecting different carbon precursor materials and using HPHT method, precise design and regulation of complex microstructures were achieved, successfully preparing a series of innovative materials, such as nanograins\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e, lamellae diamond\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, nanotwinned diamond,\u003csup\u003e14, 27\u003c/sup\u003e hexagonal diamond\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, hierarchical diamond composite\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e, graphite-graphite diamond hybridize materials (Gradia)\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, and amorphous diamond\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. Low-density carbon allotropes (such as graphite, fullerenes, glassy carbon, etc.)\u003csup\u003e32\u003c/sup\u003e can be transformed into diamond-based materials with unique microstructures and properties under different HPHT conditions, providing rich possibilities for the design and development of multifunctional diamond materials. By deliberately mixing precursors of different size, shape, and stacking order, the conversion event can be forced to proceed asynchronously. Thus, precursor architecture-not post processing-becomes the enabling lever for hierarchical diamond that unite micron-scale lamellae with nanograins and a high density-dislocation substructure, all in single HPHT process.\u003c/p\u003e \u003cp\u003eIn this work, high-quality hierarchical superhard polycrystalline diamond (HSPD) bulks were fabricated by treating nano onion carbon with large graphite flake under HPHT conditions. These HSPDs exhibit simultaneous ultra-high hardness and exceptional fracture toughness. Transmission electron microscope reveals a multi-scale hierarchical structure, the size disparity between precursor carbons drives a hierarchical architecture, while transformation-induced stress field generate a dense, homogenous dislocation substructure. This controlled introduction high-density dislocations offers a generic route for microstructure engineering and establishes a new pathway toward damage-tolerant diamond and diamond-related composite for more superior performance.\u003c/p\u003e"},{"header":"RESULTS AND DISCUSSION","content":"\u003cp\u003eThe artificial precursors are crucial for the final architecture of the synthesized polycrystalline diamond. Hierarchical structure in diamond was customized by regulating the ratio of precursor onion-like carbon (OLC) to micron-scale graphite during phase transformation under HPHT conditions. As show in Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e and S2, X-ray diffraction, scanning electron microscopy (SEM) images, and Raman spectrum confirm that the OLC particles (30\u0026ndash;50 nm) are highly defective and nanocrystalline, whereas the micron-sized graphite flakes (2\u0026ndash;3 \u0026micro;m) exhibit a highly ordered structure. These features are corroborated by the corresponding I\u003csub\u003eD\u003c/sub\u003e / I\u003csub\u003eG\u003c/sub\u003e ratios. Then the OLC, were evenly mixed with micron-scale graphite flakes in molar ratios of 10%, 30%, 50%, 70%, 90% OLC. As shown in Fig. S3, the micron graphite flakes are uniformly wrapped by OLC nanoparticles under ultrasonic mixing process, which transform into multi-scale microstructures under HPHT (P\u0026thinsp;=\u0026thinsp;15 GPa, T\u0026thinsp;=\u0026thinsp;2000 ℃). And the schematic diagram of the precursor, and synthesis process for the HSPD is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe X-ray diffraction patterns of HSPDs are shown in Fig. S3a. The main patterns peaks at 43.9˚, 75.4˚, and 91.4˚, which attribute to (111), (220), (311) planes of cubic diamond, indicates that the sample only consists of cubic diamond, without hexagonal diamond or graphite. The synthesis temperature is lower than that of graphite to cubic diamond\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. This indicates that the OLC phase transition first to nucleate cubic diamond, thereby reducing the energy barrier for the phase transformation of graphite to cubic diamond and promoting the complete transformation of graphite, which is usually accompanied with hexagonal diamond even at 2000 ℃. Large graphite flakes also transform into diamond at lower temperature as that of other nano carbon precursors, such as C\u003csub\u003e60\u003c/sub\u003e, nano tubes and hydrogenated carbon onions\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Raman spectra (Fig. S3b) exhibit the cubic diamond TO vibration mode at 1332 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (from micron grains) together with a broad band at 1400\u0026ndash;1600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (from nano grains) characteristic of nanograined diamond, confirming a hierarchical grain-size distribution spanning nanometers to sub-micrometers.\u003c/p\u003e \u003cp\u003eAs shown by the transmission electron microscopy (TEM) in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb-g, there are pronounced hierarchical grain size distribution in all HSPDs. All the microstructure is characterized by interlocking with nano and submicron grains. The grain size statistics reveal that when the OLC content is 10%, the synthesized HSPD-10 sample exhibits a bimodal grain size distribution, primarily ranging from 30\u0026ndash;80 nm and 300\u0026ndash;400 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Furthermore, dense grain boundaries are formed between the grains in the HSPD-10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). When the OLC content in the precursor increases to 50%, the distribution range of submicron grains in HSPD-50 narrows, and well-defined coherent grain boundaries are formed between the grains (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed, e). As the OLC content in the precursor further increases to 70%, the grain size distribution in the HSPD-70 displays a nearly continuous gradient variation from 40\u0026ndash;500 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef). Simultaneously, a large number of crisscrossing stacking faults are formed within the submicron grains of the sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg).\u003c/p\u003e \u003cp\u003eNotably, the microstructure of the nano grain domains is inverse of that reported for nano diamonds from OLC\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. Nano-grains nucleated from OLC are almost dislocation-free, whereas the micron-sized grains derived from graphite contain a dense dislocation network. This inversion originates in the precursor geometry. Highly curved OLC particles provide abundant nucleation sites and transform first at slightly lower temperature, generating an internal stress field within the untransformed graphite flakes. When these flakes subsequently convert to diamond, the accumulated stress is injected as dislocations, yielding large grains that are rich in line defects. The result is a multi-length-scale architecture from dislocation, nano grains to micron grains, in which the coarse sub-micron grains, rather than the nano grains, carries high density of dislocation array, which is opposite to conventional microstructure and essential for the strain hardening mechanism\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Our strategy efficiently uses the moderate population of the large grains with high density of dislocations to achieve the synergetic hardening effect. The nano grains matrix is expected to impart high strength from an extrapolation of Hall-Petch effect, and the larger (softer) grains accommodate strains preferentially.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea shows the Vickers hardness and toughness (indentation fracture method) of HSPDs with the function of OLC content. Vickers hardness and fracture toughness increase with OLC content, rising to a maximum at 70% OLC before declining. The optimal composition (HSPD-70) exhibits the maximum hardness value of H\u003csub\u003eV\u003c/sub\u003e = 146.0 GPa (F\u0026thinsp;=\u0026thinsp;19.6 N), which exceeds the single crystal diamond (80\u0026ndash;120 GPa) by 20%. Load-displacement nano-indentation curves (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) yield Young\u0026rsquo;s modulus (E\u0026thinsp;=\u0026thinsp;1152 GPa) higher than 1000 GPa. The nanoindentation hardness with depth curve is given in the Fig. S5, and it also gives hardness around 100 GPa. Based on the Young\u0026rsquo;s modulus results, the calculated fracture toughness is 14.1 MPa\u0026middot;m\u003csup\u003e0.5\u003c/sup\u003e, which is approximately threefold that of single-crystal diamond (SCD) (K\u003csub\u003eIC\u003c/sub\u003e = 3\u0026ndash;5 MPa\u0026middot;m\u003csup\u003e0.5\u003c/sup\u003e)\u003csup\u003e43, 45\u003c/sup\u003e. The hardness and toughness of the diamond sample were improved at the same time. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed, remarkably, the hardness and fracture toughness is better than that of traditional hard or superhard materials. The fracture toughness, surpass nature SCD, chemical-vapor-deposited and nanopolycrystalline diamond (NPD) \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Furthermore, HSPD exhibit even better hardness and fracture toughness than that of silicon or cobalt cemented polycrystalline diamond (Si-PCD, Co-PCD, Hv\u0026thinsp;=\u0026thinsp;40\u0026ndash;80 GPa, K\u003csub\u003eIC\u003c/sub\u003e = 6\u0026ndash;10 MPa\u0026middot;m\u003csup\u003e0.5\u003c/sup\u003e)\u003csup\u003e46, 47\u003c/sup\u003e. The simultaneous enhancement demonstrates that the long exploited hierarchical, multi-scale microstructure as metals and ceramics, can be translated to covalently bonded diamond, overturning its intrinsic hardness-toughness trade-off.\u003c/p\u003e \u003cp\u003eTo investigate the mechanism by which the hierarchically microstructure enhances the toughness of HSPD-70, micropillar compression experiments were conducted, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. As the load increases, the sample undergoes gradual compression (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec - g). At strains below 6.3%, the sample exhibits elastic deformation. With further increase in load, plastic deformation occurs in the strain range of 6.3% \u0026minus;\u0026thinsp;12.0%, which is impossible in brittle single crystal diamond. Subsequently, brittle fracture takes place, as show in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea. The total strain of the sample reaches 12.0%, which is rare in covalent bonding materials such as diamond. Remarkably, there is a region of plastic deformation, and give its plastic deformation ability, which is good for the high fracture toughness. This indicates that the multi-gradient microstructure in HSPD-70 enhances the fracture toughness of the sample by triggering plastic deformation as that in hierarchically cupper\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. However, due to the small dimensions of the micropillar (L\u0026thinsp;=\u0026thinsp;4.27 \u0026micro;m, D\u0026thinsp;=\u0026thinsp;1.48 \u0026micro;m), its compressive strength is significantly lower than the theoretical compressive strength of single-crystal diamond.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe microscopic morphology of cracks induced by Vickers indentation was observed using SEM as shown in Fig. S5. In HSPD-10, the cracks propagate almost linearly (Fig. S5a, b). When the nano grain content increase (HSPD-30 and HSPD-50), the crack branch, crack deflection and crack bridging appears, as shown in Fig. S5d - i. Due to the stress concentrate at the crack tip, less energy is consumed during the fracture process, and it exhibits a hard and brittle behavior with poor toughness. Compared to HSPD-30, HSPD-70 exhibits an increased crack deflection angle of nearly 90 degrees, and suggest multiple toughening mechanisms appear, which greatly dissipates the energy for crack propagation. The SEM-documented crack morphologies demonstrate that the hierarchical grain distribution fundamentally alters fracture dynamics in single crystal diamond and traditional nanopolycrystalline diamond.\u003c/p\u003e \u003cp\u003eTo illustrate the microscopic mechanism of different microstructure distributions in improving fracture toughness, post-mortem TEM test was performed on HSPD-30 and HSPD-70, along the cracks after the Vickers indentation test, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Some cracks were observed to propagate along the intergranular interface between large and small grains for HSPD-30, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (a, b). Since the strength of grain boundaries is weaker compared to the strength of interatomic carbon bonds within diamond, cracks preferentially propagate along the grain boundaries, resulting in intergranular fracture model and almost linear with small zigzag path. Although this fracture mode can alter the crack propagation direction and disperse the stress at the crack tip, thereby contributing to toughening, the toughness improvement remains limited due to the relatively low energy consumed during the fracture of grain boundaries. However, by adjusting the ratio of nano and submicron grains and transgranular and intergranular fractures, the toughness can be further enhanced.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs show in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, e, when the crack propagation in the HSPD-70, there is more frequent obstruction and deflection between large and small grains during crack propagation. It makes the crack tip stress field strongly disturbed at the grain boundary, and each interfacial interaction consumes additional energy and changes the propagation direction to significantly extend the actual propagation path, in which the coarse grains provide bridging stress during crack opening displacement and inhibit crack tip propagation. At the coarse-grained/nanocrystalline interface, the grain size gradually varies from 480 to 240 nm. Zhang \u003cem\u003eet al.\u003c/em\u003e have quantitatively analyzed that the gradient zone can increase the crack deflection frequency by 40% per unit thickness\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. This gradient structure eliminates stress concentrations at the abrupt interface, forcing the crack to continuously adjust its propagation mode. This process cause crack deflection and prolong the crack propagation path, helping to improve toughness.\u003c/p\u003e \u003cp\u003eSimultaneously, owing to the high strength of the coherent grain boundaries formed in the sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef), approaching that of the chemical bonds between (111) crystal planes, the proportion of intergranular fracture mode increases during crack propagation, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, e and Fig. S6. Transgranular fracture consumes significantly more fracture energy compared to intergranular fracture, thereby enhancing toughness\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. Compared to HSPD-30, HSPD-70 exhibits larger angles crack deflection, accompanied by an increase in crack branching and transgranular fracture. The synergistic effect of these mechanisms results in a 30% improvement in toughness for HSPD-70 compared to HSPD-30, making it nearly three times as tough as single-crystal diamond. So, the proportion of transgranular fracture and intergranular fracture was tuned by the hierarchically structure with nano/micron grain ratio.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e presents HRTEM images of the crack propagation across the sub-micrometer grains of HSPD-70 sample. Crack deflect at the interface of between nano grain regions and sub-micron grains and propagate into the large micron grains. Compared with that of single-crystal diamond, the common transgranular fracture mode involves cracks propagating straight along the (111) crystal plane, which gives catastrophic fracture in single diamond crystal. However, unlike single-crystal diamond, the cracks in this sub-micron grains undergo multiple deflections within the crack path, as show in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea - d. The primary reason is the interlaced distribution of stacking faults within the grain. These larger grains accumulated large number of dislocations, twin boundaries, and sub-grain boundaries. When the crack is parallel to the dislocations, it propagates along the direction of the dislocations. When the crack is perpendicular to the dislocations, crack deflected at the dislocation sites. It encounters obstacles and deflects, dispersing the tip stress and increasing the energy consumption during fracture, thereby enhancing toughness. Each deviation disperses the crack-tip stress intensity and consumes the fracture energy, converting the normally brittle transgranular path into a tortuous, energy-dissipating trajectory that underpins the three-fold toughness increase.\u003c/p\u003e \u003cp\u003eThe above-mentioned multi-scale toughening mechanism, spanning nanoscale defects, nanograins, and micrograins, is summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee. All the microstructures played corresponding roles in enhancing toughness, achieving a comprehensive improvement in the mechanical properties of hierarchical diamonds. At the nanoscale, high density of dislocations in sub-micron grains and high density of grain boundary in nanocrystalline regions promotes crack bifurcation, transforming a single dominant crack into multiple subcritical branches (energy dissipation). The gradient from nano- to micrograins further induces crack branching and deflection through maximized boundary density. Meanwhile, micron grains act as bridging ligaments that exert crack-wake traction. By strategically partitioning nanocrystalline (hardness-enhancing) and micron-scale (toughness-promoting) regions, hierarchical-structured achieves extrinsic toughening superior to monomodal structures, validating hierarchical design as a paradigm for transcending hardness-toughness trade-offs in ultrahard materials. This work not only establishes a universal microstructural design paradigm (\u0026ldquo;nanograins for strengthening, micrograins for toughening\u0026rdquo;) but also provides deeper insights into the fracture dynamics of covalent materials, laying the groundwork for the development of next-generation ultrahard materials. In addition, by further improving the sintering quality, the mechanical limit of the above-mentioned diamond materials is anticipated to further enhance, solving the problem of brittleness of diamond materials.\u003c/p\u003e"},{"header":"CONCLUSION","content":"\u003cp\u003eBy programming an onion-carbon-graphite precursor to transform under HPHT (15 GPa and 2000\u0026deg;C), we have produced HSPD that simultaneously exhibit Vickers hardness of 146.0 GPa and fracture toughness of 14.1 MPa\u0026middot;m\u003csup\u003e0.5\u003c/sup\u003e, which is threefold that of single-crystal diamond. The precursor-directed reaction self-assembles a hierarchical grain scaffold, nano grain domains interlocked with submicron grains, with high density of dislocations. Nanoscale domains provide Hall-Petch strengthening while the coarse fraction plus dislocation walls activate crack deflection, branching and bridging that greatly dissipate fracture energy. This synthesis route demonstrates that hierarchical, damage-tolerant architectures can be engineered even in the most strongly covalent solids, offering a general strategy for next-generation ultra-hard ceramics that no longer trade hardness for toughness.\u003c/p\u003e"},{"header":"METHODS","content":"\u003cp\u003e \u003cb\u003eSamples synthesis.\u003c/b\u003e In this study, high-purity graphite powder (purity 99.95%, grain size 2\u0026ndash;3 \u0026micro;m, Shanghai Aladdin Biochemical Technology Co., Ltd.) and onion carbon powder (purity 99.5%, grain size 30\u0026ndash;50 nm) were used as starting materials. Onion carbon with molar ratios of 10 mol %, 30 mol %, 50 mol %, 70 mol % and 90 mol % were mechanically milled with graphite, in an agate mortar, at room temperature for 2 hours. The mixtures were cold-compacted into cylinders with a diameter of 2.2 mm and a height of 2 mm in a cubic large-volume press (Guiye company, SPD 6 \u0026times; 600T) at 5 GPa for 15 min, yielding densified precursors ready for subsequent high-pressure and high temperature synthesis.\u003c/p\u003e \u003cp\u003eThe Kawai-type large-volume press experiments were performed using a high-pressure apparatus (Egret-10) at Ningbo University and LVPECF-1 at the B1 station, Synergetic Extreme Condition User Facility (SECUF). The pretreated raw materials were loaded into the standard 10/5 components in Fig. S7\u003csup\u003e50\u003c/sup\u003e. Magnesium oxide octahedrons doped with 5% Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e were used as the pressure transmitting medium (PTM). Rhenium foil served as the heater and lanthanum chromate (LaCrO\u003csub\u003e3\u003c/sub\u003e) sleeve provided the thermal insulation. The chamber pressure was pre-calibrated at room temperature using the phase-transition pressures of Bi, ZnTe, ZnS, GaAs, and GaP. The temperature was in-situ monitored with W\u003csub\u003e5%\u003c/sub\u003eRe\u003csub\u003e26%\u003c/sub\u003e-W\u003csub\u003e26%\u003c/sub\u003eRe\u003csub\u003e5%\u003c/sub\u003e thermocouple (type C) with an accuracy of \u0026plusmn;\u0026thinsp;50 ℃. The different raw material was sintered at 15.0 GPa at 2000\u0026deg;C. Samples were heated to the target temperature at a rate of 50\u0026deg;C / min and held for 20 min, then characterized by quenching to room temperature with the power turned off and returning to ambient pressure. Then, bulk samples with size of 1.8 mm in diameter and 1.5 mm in height were obtained for subsequent characterizations. The corresponding samples were labelled as HSPD, HSPD-10, HSPD-30, HSPD-50, HSPD-70 and HSPD-90.\u003c/p\u003e \u003cp\u003e \u003cb\u003eXRD and Raman spectroscopy.\u003c/b\u003e Phase identification and lattice parameters were obtained X-ray diffraction with Cu Kα radiation (Rigaku D / Max 2550 V / PC, Rigaku Corporation, Japan, λ\u0026thinsp;=\u0026thinsp;1.5418 \u0026Aring;)) and Mo Kα radiation (Rigaku FR-X, Rigaku Corporation, Japan, λ\u0026thinsp;=\u0026thinsp;0.7093 \u0026Aring;). Raman spectra of the samples were measured using a Mono Vista CRS\u0026thinsp;+\u0026thinsp;500 spectrometer equipped with the CCD detector, with a 532 nm laser serving as the excitation source.\u003c/p\u003e \u003cp\u003e \u003cb\u003eSEM, TEM and HRTEM measurements.\u003c/b\u003e The microstructure and phase composition uniformity were characterized using scanning electron microscopy (SEM, HITACHI, SU-70, Japan). A thin cross-section transmission electron microscopy (TEM) foil was prepared using a dual-beam focused-ion-beam (FIB, Nova nano lab 200) along the cracks. A transmission electron microscope (TEM, Thermo Fisher, FEI Talos F200X, 200 kV) was used to generate selected area electrons diffractions (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images. Grain-size distributions were extracted by analyzing TEM images.\u003c/p\u003e \u003cp\u003e \u003cb\u003eHardness, fracture toughness and micropillar compression measurements.\u003c/b\u003e Vickers hardness was measured using a pyramid-shaped diamond indenter by continuously varying the loading force between 0.98 N and 19.6 N. This method provided reliable hardness values under stable loading rates. The exact diagonal length of each indentation was measured using an optical microscope. H\u003csub\u003eV\u003c/sub\u003e was calculated by the formula (H\u003csub\u003eV\u003c/sub\u003e = 1.8544 \u0026times; F/d\u003csup\u003e2\u003c/sup\u003e) \u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e, where F(N) is the applied loading force, and d (\u0026micro;m) is the average length of the two diagonals. The indentations, created on the polished surface accompanied by the emergence of cracks at the corners of the imprints was post examined with SEM. Nano indenter G200 instrument with continuous stiffness measurement (CSM) mode was used to measure microhardness and Young\u0026rsquo;s modulus\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e. To calculate the fracture toughness, the equation K\u003csub\u003eIC\u003c/sub\u003e = 0.016(E/H\u003csub\u003eV\u003c/sub\u003e)\u003csup\u003e0.5\u003c/sup\u003e(F/D\u003csup\u003e1.5\u003c/sup\u003e) was utilized\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e, with H\u003csub\u003eV\u003c/sub\u003e being the hardness, and E representing the Young's modulus derived from the nanoindentation testing, where F\u0026thinsp;=\u0026thinsp;19.6 N is the loading force and D (\u0026micro;m) is the length from the center of the indentation to the edge of the crack. The cracks formed at all four indent corners and were imaged by optical microscopy and SEM to calculate the fracture toughness. Micropillars, approximately 1.28 \u0026micro;m in diameter and 4.27 \u0026micro;m in height, were prepared by FIB with an ion beam voltage of 30 keV. The micropillar compression tests were performed on a nanoindentation instrument (NMT04, Oxford, UK) equipped with a diamond flat punch indenter. The equipment operates with a displacement-controlled mode with a strain rate of 5\u0026ndash;110 nm/s. Note that these pillar compression tests performed outside the SEM are useful to rule out any electron beam effect on the deformation behavior.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclarations of interest \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflict of interest.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e \u003cp\u003eThis work was supported by the National Key Research and Development Program of China (Grant No. 2023YFA1406200), Open Project of Synergetic Extreme Condition User Facility-Jilin Branch (No. SECUF013). Some of the Kawai-type large-volume press experiments were performed at the B1 station, Synergetic Extreme Condition User Facility (SECUF).\u003c/p\u003e\u003ch2\u003eData Availability Statement\u003c/h2\u003e \u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBundy FP, Hall HT, Strong HM (1955) .Wentorf. Man-Made Diamond. Nature 176:51\u0026ndash;55\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eIrifune T (2003) Ultrahard polycrystalline diamond from graphite. \u003cem\u003eNature\u003c/em\u003e 421, 599\u0026thinsp;\u0026ndash;\u0026thinsp;560\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNie A, Zhao Z, Xu B, Tian Y (2025) Microstructure engineering in diamond-based materials. 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J Mater Res 7:1564\u0026ndash;1583\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSagar KG, Suresh PM (2039) A critical evaluation of indentation techniques for measuring the hardness vs toughness: A review. \u003cem\u003eAIP Conference Proceedings\u003c/em\u003e 020073 (2018)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Hierarchical-structured, polycrystalline diamond, hardness and toughness, high pressure and high temperature","lastPublishedDoi":"10.21203/rs.3.rs-8711962/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8711962/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe unrivalled hardness of diamond is offset by an intrinsic brittleness that severely limits its technological applications. This long-standing trade-off can be resolved by architecting hierarchically microstructures as confirmed in metals, but this routine is normally forbidden in diamond due to the strong covalent structure. Starting from a rationally designed mixture of onion-like carbon and graphite, we exploit the precursor-directed high-pressure and high-temperature (HPHT) transformation to nucleate hierarchically interpenetrating network of nano/micron-crystalline diamond. Simultaneously, high stress caused by asynchronous phase transformation triggers high density of dislocations array in micrometer grains. The resulting hierarchical superhard polycrystalline diamond (HSPD) show exceptional Vickers hardness of 146.0 GPa along with a high fracture toughness of 14.1 MPa\u0026middot;m\u003csup\u003e0.5\u003c/sup\u003e, which is triple that of single-crystal diamond. The hierarchically microstructure allows multi-scales plastic deformation mechanisms to be activated concurrently, in which Hall-Petch strengthening from nanoscale domains restricts dislocation motion, whereas dislocations network together with coarse sub-micron grains activate crack deflection, branching and bridging to dissipates fracture energy. This multi-scale strategy, achieved through precursor-directed transformation, provides a general pathway to customize microstructure in covalent materials for fabricating damage-tolerant, ultrahard diamond and hard ceramics without sacrificing hardness.\u003c/p\u003e","manuscriptTitle":"Multi-scale Synergistic Effects Enhanced Hierarchically Structured Polycrystalline Diamond for Exceptional Hardness and Toughness","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-12 04:17:36","doi":"10.21203/rs.3.rs-8711962/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"d528f24a-522b-4d73-ae98-c0c71f258a0e","owner":[],"postedDate":"February 12th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":62752268,"name":"Physical sciences/Materials science/Structural materials/Ceramics"},{"id":62752269,"name":"Physical sciences/Materials science/Condensed-matter physics"}],"tags":[],"updatedAt":"2026-04-17T07:20:17+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-12 04:17:36","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8711962","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8711962","identity":"rs-8711962","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}