Ultrahard stitching of nanotwinned diamond and cubic boron nitride in C2-BN composite

Materials combining the hardness and strength of diamond with the higher thermal stability of cubic boron nitride (cBN) have broad potential value in science and engineering. Reacting nanodiamond with cBN at moderate pressures and high temperatures provides a pathway to such materials. Here we report the fabrication of Cx-BN nanocomposites, measuring up to 10 mm in longest dimension, by reacting nanodiamond with pre-synthesized cBN in a large-volume press. The nanocomposites consist of randomly-oriented diamond and cBN domains stitched together by sp3-hybridized C-B and C-N bonds, leading to p-type semiconductivity. Dislocations near the sutures accommodate lattice mismatch between diamond and cBN. Nanotwinning within both diamond and cBN domains further contributes to a bulk hardness ~50% higher than sintered cBN. The nanocomposite of C2-BN exhibits p-type semiconductivity with low activation energy and high thermal stability, making it a functional, ultrahard substance.

Superhard and ultrahard materials, presently defined as having Vickers hardness (H V ) greater than 40 and 80 GPa, respectively, are sought after for their potential use as extreme abrasives and other applications in high-pressure science and technology [1][2][3][4] . Diamond is generally considered the hardest known substance, but due to graphitization in air at 800-900 K and reactivity with transition metals at high temperature, diamond has limited application in certain grinding environments. Cubic boron nitride (cBN), a diamond-structured compound, is widely used as an abrasive with much higher thermal stability than diamond (~1473 K) and low reactivity with steel, but the Vickers hardness of cBN   5,6 is only about half that of diamond. Because of their similar ionic radii, B, C, and N may form diamond-like compounds in solid solution that are expected to be ultrahard with higher thermal and chemical stability than diamond [7][8][9][10][11][12] . Based on theoretical predictions, experimental studies have attempted to synthesize a number of superhard B-C-N materials including diamond-like BC 2 N 13 , BC 4 N 10 , BC 3 14 and BC 5 15,16 . However, solid-solution B-C-N phases are generally considered metastable because syntheses usually lead to segregated carbon and boron compounds 17 . The extremely high pressures (20)(21)(22)(23)(24)(25) and temperatures (2000-2500 K) of many B-C-N syntheses ultimately limit production potentials and sample size 7,15,17 .
Recently, it was discovered that nanotwinned (nt) diamond 3 and nt-cBN 2,18 possess ultrahigh hardness and toughness, mitigating the so-called reverse Hall-Petch effect by shear strengthening from compressive forces across nanospaced twin boundaries 18 . Combining nt-diamond and nt-cBN to create an ultrahard nanocomposite would potentially lead to a material with optimized properties of each. Reacting diamond and cBN at high pressure and high temperature (HPHT) has been the typical route to synthesizing composites of diamond and cubic boron nitride (C x -BN) [19][20][21][22][23] , where x is the proportion of diamond relative to cBN. The conditions and mechanisms of stitching diamond and cBN together remains the primary challenge. Recently, an epitaxial cBN/diamond heterojunction was produced by growing single-crystal cBN on diamond seed crystals using a temperature gradient method 24 . A network of continuous stacking faults, arranged by hexagonal dislocation loops on the {111} heterointerface was found to accommodate the cBN/diamond lattice mismatch (~1.4 Å) 24 . In this study, we fabricated highly uniform C x -BN (x = 2, 4, 6, 8) nanocomposites at 7.5 GPa and 2273 K, which consist of randomly oriented, ~50 nm domains of diamond and cBN (~250 nm). The bulk samples, measuring up to 10-mm diameter by 5-mm in height, combine several novel features from recent syntheses in the diamond-cBN system 2,3,18,24 ; in particular, the new C 2 -BN composite possesses nanotwinning in both diamond and cBN domains, which are stitched by sp 3 -hybridized C-B and C-N bonds along randomly-oriented interfaces. Near the sutures, dislocations accommodate the lattice mismatch between diamond and cBN and, as recently predicted 24 , the C 2 -BN nanocomposite is a p-type semiconductor with low activation energy. Among the suite of new nanocomposites, C 2 -BN is the hardest, with Hv = 85(2) GPa, placing it in the ultrahard class of materials.
We control the stoichiometry in C x -BN, by varying the proportions of starting powders of diamond (average grain size 50 nm) and cBN (average grain size 250 nm). Synthesis was conducted at 7.5 GPa while holding constant temperature of 2273 K for two hours ( Table 1). The components of C 2 -BN remain diamond-structured; however, the recovered C x -BN composites with C 4 -BN, C 6 -BN, and C 8 -BN contained diamond and cBN together with appearance of graphitic phase (Fig. S1). The presence of minor amounts of graphite decreases with increasing ratio of cBN added to the starting material, indicating that cBN plays a role as catalyst in stitching diamond and cBN domains while protecting the diamond from graphitization at the high temperatures of synthesis (2273 K).
Since the aim of this study is to synthesize and characterize nanopolycrystalline C x -BN composite free of graphite, here we focus on the physical properties of C 2 -BN, which was the only sample not exhibiting the presence of graphite. Figure 1a shows a representative X-ray diffraction (XRD) pattern of the recovered C 2 -BN composite. With increasing synthesis temperature, the diffraction lines become broader and the peaks of diamond and cBN from the 2273 K synthesis have considerable overlap (Fig. S1). Raman spectra (Fig. 1b) of C 2 -BN produced at 7.5 GPa and 2273 K are dominated by one sharp Raman peak at 1332 cm −1 and two weak peaks at about 1055 and 1305 cm −1 , corresponding to diamond and cBN longitudinal optical and transverse optical modes, respectively.  The Fourier transform infrared (FTIR) spectrum ( Fig. S2) shows two main peaks at 1065 and 1318 cm −1 together with one weak shoulder at 1208 cm −1 , which correspond to stretching of the sp 3 B-N bonds in cBN 21,22 , C-N bonds within the sutures of diamond and cBN domains 25 and C-C bonds in diamond 26 , respectively. This characterization shows that the C 2 -BN samples consist of diamond and cBN domains, consistent with XRD data (Fig. 1a). The recovered samples from 7.5 GPa and 1500-2273 K were black and opaque, while transparent samples were achieved by subsequent annealing at 15-18 GPa and 2100 K (Fig. S3). Scanning electron microscopy (SEM) confirms the C 2 -BN composite is uniform and well sintered (Fig. 2a). Thin foils of C 2 -BN synthesized at 7.5 GPa and 2273 K were prepared by focused-ion beam (FIB) techniques for transmission electron microscopy (TEM). Characteristic TEM and high-resolution TEM (HRTEM) images are shown in Fig. 2b-e. The TEM images in Fig. 2b,c show the absence of porosity in the C 2 -BN sample, unlike many sintered diamond samples from which catalysts have been removed. The samples exhibit a nanopolycrystalline structure, consisting of hybridized, randomly distributed diamond and cBN domains (Figs S4 and S5). Furthermore, HRTEM images reveal that nanotwinned structures have been produced during HPHT treatment (Fig. 2d,e). These multiple twins in the diamond and cBN domains are 1.5-2 nm and 6-8 nm wide, respectively. HRTEM images in Fig. 3 show that the two main domains of diamond and cBN have been stitched together by high-pressure sintering. The rigid lattice misfit between diamond and cBN domains is accommodated by the presence of partial dislocations and stacking faults at the interface between the two domains ( Fig. S6) 24 . The sutures between diamond-cBN domains are typically 1-2 nm thick. HRTEM images and selected area electron diffraction (SAED) patterns (Fig. 3c,d) from the diamond-cBN junctions reveal that cBN domains twin to conjoin the adjacent diamond. Stacking faults and dislocations are observed in cBN in close proximity to the interfaces (Fig. S6). Through this misfit accommodation mechanism 24 , strain between diamond and cBN domains is released, leading to stability of the C 2 -BN composite.
To determine the local bonding characteristics across the diamond-cBN sutures, we carried out inelastic X-ray scattering (IXS; beam size 20 μ m) and electron energy-loss spectroscopy (EELS; beam size 1 nm) on the C 2 -BN sample. The IXS spectra in Fig. S7 show that all three elements, B, C and N, are sp 3 hybridized along the sutures. In contrast, pure diamond sintered under identical conditions (7.5 GPa and 2273 K) exhibits both sp 2 and sp 3 bonds (Fig. S8). This confirms that graphitization of sintered diamond can be effectively minimized or eliminated in C 2 -BN by choosing the optimal synthesis conditions. EELS was conducted to analyze the interface between diamond and cBN from the selected area in the C 2 -BN sample from 2273 K (Fig. 4a). Figure 4b shows the EELS spectrum and all the three bonds corresponding to the characteristic K-shell ionization edges of B, C and N in X-ray photoelectron spectroscopy (XPS) was performed to study the stoichiometry of the hybridized diamond-cBN junctions. Figures 4c-e show XPS spectra of B, N and C, respectively. The shape and position of the spectra for all three elements are different from previous studies on pure diamond 27 , cBN 28 , and B-C-N compounds 29,30 . The C 1s peak is located at 285.2 eV (Fig. 4c), which is similar to the value of C 1s in sp 3 C-C bonds observed in diamond 31 . The full width at half-maximum (FWHM) is about 2.3 eV, suggesting different valence states of carbon are present. Two smaller peaks at higher (285.9 eV) and lower (284.5 eV) binding energy are due to C 1s in sp 3 C-N and C-B bonds, respectively 32 . In addition, a higher binding energy at 289.0 eV is observed from the contribution of C 1s in C-O bonds 29,33 . The main peak of the B 1s spectrum is located at 190.7 eV, very close to that of B 1s (191.0 eV) in pure cBN 28,29 , suggesting the main bonding configuration in the C 2 -BN composite produced at 7.5 GPa and 2273 K is similar to that of cBN, where one N atom is surrounded by four B atoms. However, a small shoulder at lower binding energy implies a contribution of B and C, because of the lower electronegativity of C atoms than N 34 . A higher binding energy at 192.4 eV implies a contribution from the configuration of B 1s in B-O bonds 35 . The N 1s binding energy is located at 398.4 eV (Fig. 4e), similar to the position of N 1s in cBN 28,29 . However, two small shoulders are observed at higher binding energy of 400.3 and 402.1 eV, which are due to N 1s in C-N and C-O, respectively 29,30 . All the C 1s, B 1s and N 1s spectra indicate that the main configuration for C, B and N atoms is sp 3 B-N bonding, together with a contribution from the bonding configuration in the boundary region between diamond and cBN domains. In addition to sp 3 -hybridized C-N and C-B bonds produced during HPHT reaction, some C-O, B-O, and N-O bonds remain from the starting materials exposed to air. Vickers hardness measurements were conducted on the C x -BN nanocomposites using a standard square-pyramidal diamond indenter. We imaged and compared the indented surfaces using an optical microscope, SEM, and well-calibrated 3D optical microscope to determine the H V value. Figure 5a shows the hardness-load curve of C 2 -BN. The asymptotic region is reached at loads above 4.9 N and shows an ultrahard value of 82(3) GPa at 19.6 N. In a second C 2 -BN sample, the indentation at 19.6 N load corresponded to H V = 85(2) GPa, confirming the ultrahard value of our nanocomposite C 2 -BN. For other compositions of C x -BN as well as cBN and sintered diamond, we studied indentations at 19.6 N load and found a peak hardness for the composition C 2 -BN (Fig. 5b). H V of the sintered diamond sample is lower than sintered cBN because of the presence of a graphitic phase on the diamond surface (Fig. S8).
The compressibility of C 2 -BN was studied under quasi-hydrostatic pressures up to 40 GPa at 300 K in a diamond-anvil cell using synchrotron X-ray diffraction at Sector 16 (HPCAT) of the Advanced Photon Source (APS). Figure S10 shows the compression data of diamond and cBN domains in the bulk C 2 -BN sample, together with the previously reported equations of state of single-crystal diamond 36 and ultrahard nanopolycrystalline (NP) cBN 17 . The P-V data (Table S2) were fitted to a third-order Vinet equation of state, finding for diamond V 0 = 45.33 (± 0.01) Å 3 and K T0 = 436.4 (± 2.3) GPa when K 0 ' = 3.0 is fixed to the value from a previous study 36 to allow for better comparison of K T0 . Our value of K T0 is close to the adiabatic bulk modulus derived from recent ultrasonic measurements, K T0 = 441.8 (± 0.8) for single-crystal diamond and K T0 = 442.5 (± 0.5) GPa for NP-diamond 37 . For the cBN domains, we obtained V 0 = 47.30 (± 0.01) Å 3 and K T0 = 398.6 (± 2.5) GPa when K 0 ' = 2.3 is fixed to the value of a previous study 17,38 . When K 0 ' is allowed to refine, we obtain V 0 = 47.33 (± 0.01) Å 3 , K T0 = 377.4 (± 7.1) GPa, and K 0 ' = 3.6 (± 0.4) (Fig. S10). Using K T0 = 398.6 (± 2.5) GPa for the cBN domains, we obtain a theoretical nanopolycrystalline average K T0 value of the C 2 -BN composite of 417.5 (± 2.4) GPa.
We also determined the thermal stability (in air) of the C x -BN samples by thermoanalytical analysis (TG/DTG) from 400 K to 1620 K (Fig. 5c). The synthetic C 2 -BN sample shows remarkably higher stability than either single-crystal boron-doped diamond (~1000 ppm) with the onset temperature of oxidation in air, T OX ~1027 K, or nanopolycrystalline diamond (T OX ~950 K) 17 . The synthetic nanocomposite of C 2 -BN remains stable up to 1183 K. Moreover, the oxidation rate for C 2 -BN composite decreases substantially between 1183 K and 1425 K. To investigate the electrical properties of our synthetic C x -BN nanocomposites, Hall measurements were performed with four electrodes at room temperatures ( Table 1). The sintered diamond (sample S6) and C x -BN materials with high carbon concentration (sample S5) show good n-type conductivity with a low resistivity that can be due to the graphitization under HPHT treatment. Both C 2 -BN and C 4 -BN nanocomposites synthesized at 2273 K and 7.5 GPa exhibit p-type semiconductivity. Resistivity of the p-type C x -BN is observed to decrease and the carrier concentration increases with increasing synthetic temperature condition up to 2273 K (sample S9 and S10). The p-type C x -BN synthesized at 2273 K (sample S2-4) shows low electrical resistivity, and the electron and hole mobility is in a wide range of 2.7-365 cm 2 V −1 s −1 . The carrier concentration of the C 2 -BN composite can reach 1.4387 × 10 19 cm −3 at 300 K. This value is comparable with that of B-doped diamond 39 . These results suggest that we can control the electrical properties of our C x -BN materials from insulator to highly conducting by adjusting synthetic P-T or tuning the ratio between the C and BN with high flexibility and applications in electronics.
Temperature-dependent electrical measurements on our C 2 -BN nanocomposite (Fig. 5d) show a significant increase in resistance by more than 7-fold on cooling from room temperature (ρ = 21Ω) to 1.8 K, a typical semiconducting behavior. The value of the activation energy is calculated on the basis of a linear Arrhenius plot of the logarithm of the resistance ln (R) versus the inverse temperature T −1 in the temperature range between from 100 K and 50 K (See Fig. 5d, inset). The calculated activation energy is 6.21 meV and is similar to hybridized BN-C graphene 30 . Since both pure diamond and cBN are good insulators, the p-type semiconductivity of the C x -BN composite must derive from the sp 3 -hybridized C-B and some B-B bonds within the sutures of diamond and cBN domains 30,40 .
This study introduces a new strategy and direction in the search for novel conductive ultrahard materials. Nanocomposite C x -BN materials will enable the design of particular physical properties in diamond-based structures. Previously, B-doping has been the most effective method used to control the electrical properties of diamond. However, it is difficult to synthesize high-quality diamond crystals with heavy B-doping because boron is typically heterogeneously distributed within the crystal and it is difficult to avoid graphitic defects 41,42 . The current synthesis and characterization of p-type bulk nanopolycrystalline C x -BN with low activation energy suggests that stable and uniform diamond-based semiconductors can be formed in large quantities and in a reproducible way. In addition, the well-sintered nanopolycrystalline bulk pieces possess no cleavage, which will allow C x -BN composite materials to be fabricated into arbitrary shapes for industrial applications such as machine parts. Combining the high thermal conductivity and radiation resistance of diamond with the toughness of cBN at high temperatures, the ultrahard C x -BN bulk nanocomposites have potential applications in electronics under extreme thermal and pressure conditions.
In summary, we synthesized C x -BN composites with dimensions up to 10 mm × 5 mm at 7.5 GPa and 2273 K. The composite of nanotwinned diamond and cBN in a 2:1 ratio (C 2 -BN) shows ultrahard properties by Vickers indentation tests, low compressibility, high thermal stability, and p-type semiconductivity -a combination of features that have not previously been found together in pure diamond-like B-C-N materials. Subsequent annealing at 15-18 GPa and 2100 K improves its transparency. The nanotwinned domains of diamond and cBN are stitched by sp 3 -hybridized C-B and C-N bonds with dislocations near the grain boundaries to accommodate the rigid lattice mismatch of diamond and cBN, thus combining the features of a variety of recently reported ultrahard materials [1][2][3]18,[23][24] . The moderate synthesis P-T conditions and rapid annealing time (2 hours) to form ultrahard nanopolycrystalline C 2 BN composite make it a promising material for machine tooling with other practical benefits. The unique combination of properties in C 2 -BN provides a pathway for fabrication of diamond-based electronics applicable to multifunctional devices operating in extreme environments.

Methods
Sample synthesis and characterization. HP-HT synthesis experiments were carried out using a Chinatype cubic high-pressure apparatus (SPD-6× 1200) at the State Key Laboratory of Superhard Materials, Jilin University. High-purity mixtures were prepared of nanodiamond (50 nm) and cBN (250 nm) with varying ratios (x) of diamond to cBN (i.e. C x -BN), including BN, C 2 -BN, C 4 -BN, C 6 -BN, C 8 -BN, C 10 -BN, C 12 -BN and C. The starting materials were prepared by mechanically mixing for five hours at ambient conditions and treated by hot sulfuric acid/diluted hydrochloric acid to remove impurities introduced during the milling progress. Synthesis temperatures were measured using a Pt-30% Rh/Pt-6%Rh thermocouple junction placed within 0.5 mm of the sample. Pressure was pre-calibrated at high temperatures by the graphite-diamond transition with different catalysts. Samples were first subjected to pressures of 7.5 GPa, heated to temperatures of 1500-2273 K with holding times of 2 hours before rapidly cooling to room temperature in 2 minutes. The C x -BN cylinders (10-mm diameter × 5-mm height) are black in color. One sample of C 2 -BN was re-annealed at 15-18 GPa and 2100 K for 2 hours using the 1000-ton multi-anvil press at GSECARS (Sector 13) of the APS, Argonne National Laboratory (ANL), producing a transparent and colorless run product (Fig. S3). Run products were initially studied using an optical microscope, SEM, a powder XRD diffractometer, and Raman spectroscopy. Then we employed HRTEM, EELS, IXS, and XPS measurements on the produced C x -BN composites to determine their chemical compositions and crystal structures.
Hardness and thermal stability measurement. Vickers Hardness measurements were performed in the Optical Microscopy & Metallography (OMM) Facility at Northwestern University. A microhardness tester (KB 5 BVZ) was used to measure H V with a diamond Vickers indenter. H V was determined from H V = 1,854.4F/d 2 , where F (in Newtons) is the applied load and d (in μ m) is the arithmetic mean of the two diagonals (d 1 and d 2 ) of the Vickers indentation (Fig. 5a). For C 2 -BN, the average of at least seven hardness data points for each load value was used to determine the H V from the asymptotic intercept. To study the effect of cracking around the projected area on the inferred indentation dimensions, we used a combination of optical microscopy, SEM, and a high-resolution 3D microscope (Bruker, ContourGT Optical Profiler) to analyze the indented surface. The dynamic TG/DTG studies were performed in air using the NETZSCH STA 449 C thermoanalyser operating at a heating rate of 10 K min −1 in the temperature range from 200 to 1650 K. These bulk samples were crushed into powders of micron sizes before the thermoanalytical studies.
Compressibility. The compressibility measurement was carried out in diamond-anvil cell using neon as the pressure medium. High-pressure X-ray diffraction patterns were collected up to 40 GPa, with exposure times of 5-10 min, at room temperature at beamline 16BM-D (HPCAT) of the APS. The beam size was ~5 × 10 μ m at FWHM. Pressure was measured using the ruby fluorescence method 43 . A fragment of C 2 -BN (50 μ m diameter, 8-10 μ m thick) was placed into the 160-μ m diameter hole of a rhenium gasket, pre-indented to 25 μ m thickness.
Electrical measurement. The van der Pauw method was used for electrical transport measurements, in which four electrodes were used in the resistivity measurements. The electrical characteristics were measured with a transport properties measuring system (East Changing ET 9000 Series). The resistivity measurements were carried out at room temperature (300 K) and normal humidity level (40% RH). The temperature dependence of the resistance was measured using a standard four-probe method (PPMS, Quantum Design).