Graphitization Of Diamond Research Articles

Effect of CeSi2 addition on microstructures of brazed layers and interfacial characteristics of brazed diamonds with Ni Cr filler alloy

The thermal damage of diamond is the key factor restricting the performance of brazed diamond tools with Ni-based filler. Hence, the Ni Cr filler alloy is modified by adding Ce element to reduce the graphitization and improve the mechanical properties of brazed diamond in this work. The results indicate that the microstructures of brazed layers with Ni Cr filler containing Ce element are obviously refined, and the brazed layer containing 2 wt.% CeSi 2 shows the better wear resistance with stable friction coefficient curve and low wear loss. Meanwhile, the brazed diamond with Ni Cr filler containing 2 wt.% CeSi 2 exhibits a higher exposure, and the Cr 3 C 2 on its surface is finer, which contributes to reduce the stress concentration and cracks formation at interface between diamond and filler. The Raman spectrum results show that the graphitization of brazed diamond is significantly weakened due to addition of CeSi 2 . The Ni Cr filler brazed diamonds containing 2% CeSi 2 remain relatively complete cutting edges and shapes after grinding test, showing the excellent wear resistance. Moreover, the calculation of elastic properties indicates that the in-situ formed CeNi 3 phase exhibits better ductility and plasticity, but lower hardness, and its formation is beneficial to improve the ductility and plasticity of brazed layer. • Diamond grits are brazed with Ni Cr filler containing different contents of CeSi 2 . • Microstructures of brazed layers containing CeSi 2 are refined. • Brazed layer containing 2 wt.% CeSi 2 exhibits the best wear resistance. • Graphitization of diamond brazed with 2 wt.% CeSi 2 -doped filler is weakened. • Brazed diamond with 2 wt.% CeSi 2 -doped filler presents superior mechanical performance.

Simulated Pressure-temperature Carbon Structure Map obtained through uniaxial compression of Bulk C60

Although the transformation from fullerene bulk into various functional carbon materials at high temperature and high pressure (HTHP) has been extensively explored experimentally, the understanding of the mechanism of structural transformations at atomic scale is still very poor. Based on the recently developed highly-accurate machine learning force field (MLFF), GAP-20, we performed a systematic study on the phase transformations of carbon structures. Various derived structures, such as C60 foams, sp2, sp3, and mixed sp2-sp3 amorphous carbon materials are formed at temperatures below 2500 K, while graphitic carbon, nano-graphitic carbon and diamond are formed at higher temperatures. These materials exhibit excellent mechanical properties and can be used for various applications. The activation energy of the transformation from amorphous carbon to diamond is found to be 2.42 eV, which explains the high stability of sp3 amorphous carbon materials observed recently [Nature, 2021, 599:599; Nature, 2021, 599:605]. The theoretical diagram obtained in this study provides a guidance for experimental synthesis of various functional carbon materials.

Reactive spark plasma sintering of SiC-TiC-diamond composites

SiC-TiC-diamond composites were fabricated via spark plasma sintering at hold temperatures of 1600 °C, 1625 °C, and 1650 °C from a powder blend containing by weight 30 % Si, 30 % Ti, and either 40 % uncoated or TiC-coated diamond. Due to the sintering conditions, sp3-diamond decomposed into thermodynamically favorable sp2‑carbon, reacting with Si and Ti to form SiC and TiC at T ≥ 1600 °C. As the hold temperature increased, the composite density reduced due to excess graphite formation for the uncoated diamond composites. However, at too low of a temperature the porosity fraction increased in the SiC-TiC matrix for the TiC-coated diamond composites. Overall, the results revealed that composites with ~97 % theoretical density and minimal (up to 5 vol%) graphite can be achieved by sintering Si-Ti-uncoated diamond at 1600 °C and Si-Ti-TiC-coated diamond at 1625 °C. X-ray diffraction, electron and x-ray microscopies with imaging analyses were carried out to quantify the volume fractions of each composite phase and used to explain the composite densification behavior. Conventional and energy filtered transmission electron microscopy, in addition to electron energy loss spectroscopy, were conducted to understand the nature of diamond graphitization with respect to the nucleation and growth of SiC, TiC, and nanoscale graphite.

Exploring structural evolution and graphitization of the interface between tungsten and diamond (1 1 1) surface: A DFT study

In the ultra-precision machining field, tungsten (W) is a promising material as a polishing disc for machining diamond, which is easier to obtain high-quality diamond surfaces than cast iron. The removal mechanism of the diamond during high-speed polishing is considered surface graphitization. In this work, the diamond graphitization mechanism induced by tungsten is revealed by the density functional theory (DFT). The results show that the W adatoms at the fcc sites can activate the interlayer C-C bonds, causing the surface carbon atoms to separate. Notably, the electron transfer of W atoms and the structure contraction of the diamond surface cause the graphitization. Moreover, the movement of W adatoms intensifies the diamond surface structure transformation and graphitization process. It is worth noting that the W adatoms of the fcc and hcp sites have different contributions to the surface carbon atom separation during the interfacial sliding process. When the W atoms cross the hcp sites, the pulling force of the W-C bonds promotes the reconfiguration of the separated surface carbon atoms. The diamond-to-graphite transition is attributed to catalyst activation by W adatoms, diamond surface structure contraction, and mechanical tensile stress by the W-C bonds.

Wear of micro diamond tool in ultra-precision turning under dry and minimum quantity lubrication conditions

Minimum quantity lubrication (MQL) is an effective way to reduce the cutting temperature and tool wear. To reveal the effect of MQL on the wear of micro diamond tool, a calculation model for the cutting temperature of micro diamond tool under dry friction condition is established firstly by using the Fourier’s law of heat conduction. Regarding the boundary film as a layer of heat-conduction medium, a revised calculation model for the temperature distribution on tool rake face under MQL condition with different cutting fluids is further established. The predicted results indicate that low viscosity is beneficial to the wetting of cutting fluid on tool-chip contact interface, which can relieve the friction. The reduction of friction finally decreases the cutting temperature. Secondly, the cutting temperature–dependent wear volume of micro diamond tool is predicted by using the Usui wear rate model in response to different cutting fluids and different cutting distances. In dry cutting, the graphite wear of micro diamond tool prevails. However, the application of MQL can slow down the graphitization of diamond, so the wear of micro diamond tool visibly decreases. Finally, cutting experiments with different cutting fluids are performed to verify the established models. The experimental observations agree well with the theoretical prediction results. Such satisfactory consistency confirms that the cutting fluid with low viscosity can reduce the cutting temperature and inhibit tool wear effectively.

Picosecond laser ablation of polycrystalline CVD diamond

In the present work, the influence of the laser parameters on the surface characteristics of chemical vapor deposition (CVD) diamond are investigated. The temperature field, thermal stress field and phase field are established through simulation to explain the heat affected zone that results in the deterioration of diamond surface in the process of laser machining. The high temperature induced by laser and the local anisotropy of polycrystalline CVD diamond contribute to the inhomogeneous materials removal, which results in irregular gullies on the machined surface, and the thermal effect of picosecond laser leads to the gasification of a large amount of material, where less accumulated products in the processing area appear. The graphitization of diamond is induced in the groove, where less graphitization appears at the edge of the groove. In addition, the CVD diamond surface is oxidized and nitrided during the laser ablation. By comparing the surface morphology and the three-dimensional topography of the groove, a set of machining parameters are optimized to fabricate different surface microstructures on CVD diamond.

Battery-like flexible supercapacitors from vertical 3D diamond/graphite composite films on carbon cloth

Flexible supercapacitors (SCs) based on carbon nanomaterials are one of the promising energy storage devices for wearable electron devices. However, their low energy density and cycle stability severely restrict further practical applications. The development of unique capacitor electrodes is expected largely to overcome these challenges. Herein, vertical 3D diamond/graphite (DG) composite films are directly grown on carbon cloth (CC) using a microwave plasma chemical vapor deposition (MPCVD) technique. Thin, straight nanosheets, consisted of diamond as core and graphite as shell, are interlaced with each other to form a 3D porous structure. As a binder- and additive-free electrode material, the DG/CC electrodes are used to construct electrical double layer capacitors (EDLCs) in diluted H2SO4 solution and pseudocapacitors (PCs) using Fe(CN)63−/4- redox electrolytes. Both EDLCs and PCs exhibit large specific capacitance and high capacitance retention. When assembled as all-solid-state flexible symmetric SCs, they offer high full-cell capacitances of 30.4 and 143.6 mF cm−2 at 10 mV s−1 for EDLCs and PCs, respectively. The power densities are 6.3 μW cm−2 and 3.0 mW cm−2 for EDLCs and PCs, together with their energy densities of 3.7 and 86.2 μWh cm−2, respectively. As flexible SCs, the specific capacitance can be completely recovered for EDLCs and has only about 6.7% loss for PCs even after the U-shape bending state. Such vertical 3D porous DG films are thus promising for the construction of high-performance battery-like SCs for an intelligent wearable energy supplier.

Ultraviolet-ozone assisted ultrasonic vibration cutting for extending diamond tool life

Ultrasonic vibration cutting (UVC) has proven its effectiveness in suppressing the severe chemical wear of diamond tools in machining of metals with strong chemical affinity to carbon, and the formation of inert oxide film on the freshly cut metal surface is expected to further extend the tool life due to the oxide’s passivation effect. Based on the existing tool wear suppression mechanism to slow the diamond graphitization, this paper proposed ultraviolet-ozone assisted UVC to further enhance the oxide formation and improve the diamond tool life. A numerical model for calculating the oxide formation rate is developed to study the influence of supplying gases with different oxidizing capabilities. Meanwhile, an in-situ ultraviolet-ozone assisted UVC system is in-house built, and the diamond tool wear for cutting of stainless steel and tungsten are studied. Experimental results show that, compared to air, oxygen and ozone, ultraviolet-ozone assisted UVC demonstrates the highest oxide growth rate and the longest tool life accordingly, which further verifies the theory that metal oxidation performs an important role in suppressing the chemical wear of diamond tools.

Influence of Diamond Grain Size on the Basic Properties of WC-Co/Diamond Composites Used in Tools for Wood-Based Materials Machining.

The paper presents the effect of diamond particle size (varying between 2.5 µm and 20 µm) on the microstructure, density and hardness of WC-Co/diamond composites. The obtained materials contained 30% vol. diamond. The advanced sintering method Pulse Plasma Sintering (PPS) was used for the production of composites. The sintering process was carried out in two stages at a pressure of 50 and 100 MPa and a temperature of 1050 °C. Depending on the size of the diamond particles, composites with a density of 91–99% were obtained. Microstructure studies were performed employing scanning electron microscopy, along with an analysis of the chemical composition in micro-areas. Additionally, the phase composition was investigated by means of X-ray diffraction. In addition, hardness tests were performed. It was found that the size of the diamond particles significantly influenced the microstructure of the tested materials, as well as the density and hardness. As a result of PPS sintering of composites containing the finest diamond particles (2.5–5 µm), the presence of a metastable type of diamond—graphite was found.

Cu-induced enhancement of interfacial bonding for brazed diamond grits with Ni[sbnd]Cr filler alloys

Restraining diamond graphitization is still the potential problem for brazed diamond tools with NiCr filler alloys. Herein, we systematically investigate the interfacial bonding and mechanical performance of brazed diamond tools with Cu-doping NiCr filler alloys through multi-scale atomistic simulations and experiments. First-principles calculations show that the Cu-doping not only inhibits the structural damage of diamond, but also enhances the metallurgical bonding between diamond and filler alloys. The tensile simulations of Ni-Cr-Cu/diamond interface further verify the strengthening effects of Cu-doping since these bonds of CC within diamond and Ni-Cr-C at interface can withstand greater tensile strains. Combined with molecular dynamics simulations, Cu-doping effectively weaken the graphitization of diamond, which is confirmed through experimental Raman spectroscopy detection. More importantly, the brazed diamond tools by NiCr filler doped with 40 wt% Cu alloy exhibit excellent mechanical grinding performance. These results shed a new light on improving the mechanical properties of brazed diamond tools.

Laser-Induced Graphitization of Diamond Under 30 fs Laser Pulse Irradiation.

The degree of laser-induced graphitization from a sp3-bonded to a sp2-bonded carbon fraction in a single crystal chemical vapor deposited (CVD) diamond under varying fluence of an ultrashort pulsed laser (30 fs, 800 nm, 1 kHz) irradiation has been studied. The tetrahedral CVD sp3 phase is found to transition to primarily an sp2 aromatic crystalline graphitic fraction below the critical fluence of 3.9 J/cm2, above which predominantly an amorphous carbon is formed. A fractional increase of fluence from 3.3 to 3.9 J/cm2 (∼20%) results in a substantially (∼3-fold) increased depth of the sp2 graphitized areas owing to the nonlinear interactions associated with a fs laser irradiation. Additionally, formation of a C═O carbonyl group is observed below the critical threshold fluence; the C═O cleavage occurrs gradually with the increase of irradiation fluence of 30 fs laser light. The implications for these findings on enhancement of fs driven processing of diamonds are discussed.

Formation and growth characteristics of nanostructured carbon films on nascent Ag clusters during room-temperature electrochemical CO2 reduction.

Synthesis of carbon nanostructures at room temperature and under atmospheric pressure is challenging but it can provide significant impact on the development of many future advanced technologies. Here, the formation and growth characteristics of nanostructured carbon films on nascent Ag clusters during room-temperature electrochemical CO2 reduction reactions (CO2RR) are demonstrated. Under a ternary electrolyte system containing [BMIm]+[BF4]−, propylene carbonate, and water, a mixture of sp2/sp3 carbon allotropes were grown on the facets of Ag nanocrystals as building blocks. We show that (i) upon sufficient energy supplied by an electric field, (ii) the presence of negatively charged nascent Ag clusters, and (iii) as a function of how far the C–C coupling reaction of CO2RR (10–390 min) has advanced, the growth of nanostructured carbon can be divided into three stages: Stage 1: sp3-rich carbon and diamond seed formation; stage 2: diamond growth and diamond–graphite transformation; and stage 3: amorphous carbon formation. The conversion of CO2 and high selectivity for the solid carbon products (>95%) were maintained during the full CO2RR reaction length of 390 min. The results enable further design of the room-temperature production of nanostructured carbon allotropes and/or the corresponding metal-composites by a viable negative CO2 emission technology.

High-temperature evolution of diamond-SiC composites

Diamond-SiC composites are attractive for improving the catastrophic fracture behaviour of SiC. However, fundamental knowledge is missing about the structure of this system and the mechanism of diamond graphitization. We used spark plasma sintering to study the diamond-Si-SiC system between 1600 and 2000?C in the function of nanocrystalline (ND) and microcrystalline (MD) diamond addition as well as the quantity of Sibonding phase. Increasing sintering temperature induces intense graphitization and formation of nano-onions, few-layered graphene and well-ordered graphite in the prepared composites at elevated temperature. High resolution transmission electron microscopy study demonstrates the occurrence of the previously erroneously identified 5H-SiC polytype in the samples prepared at 2000?C. Regardless of Si and diamond contents, SiC formation is not confirmed even at high temperature.

Theoretical Insight Into Diamond Doping and Its Possible Effect on Diamond Tool Wear During Cutting of Steel

Natural diamond tools experience wear during cutting of steel. As reported in our previous work, Ga doping of diamond has an effect on suppressing graphitization of diamond which is a major route of wear. We investigate interstitial and substitutional dopants of different valence and different ionic radii (Ga, B, and He) to achieve a deeper understanding of inhibiting graphitization. In this study, ab initio calculations are used to explore the effects of three dopants that might affect the diamond wear. We consider mechanical effects via possible solution strengthening and electronic effects via dopant-induced modifications of the electronic structure. We find that the bulk modulus difference between pristine and doped diamond is clearly related to strain energies. Furthermore, boron doping makes the resulting graphite with stable sp2 hybridization more perfect than diamond, but Ga-doped diamond needs 2.49 eV to form the two graphene-like layers than only one layer, which would result in the suppressed graphitization and reduced chemical wear of the diamond tool.

Microstructural characteristics and mechanical properties of diamond grinding wheel brazed with modified Ni–Cr–W alloy using continuous tunnel furnace

A novel brazing technology using a continuous tunnel furnace with modified Ni–Cr–W filler alloy is presented to achieve brazed diamond grinding wheels with high efficiency and low thermal damage. Mechanical characterization confirms that the static pressure strength and impact toughness of the diamond brazed using the new brazing technology are 22.2% and 10.5% higher, respectively, than those of diamond brazed using conventional vacuum brazing technology, respectively. Raman spectroscopy reveals that the degree of graphitization of diamond brazed with the novel brazing technology is decreased. In addition, residual stress is reduced by 23% after brazing with new brazing technology. Scanning electron microscopy is used to observe interfacial microstructure of brazed diamond. The elemental distribution and phase composition of brazed joint are analyzed by energy dispersive spectroscopy and X-ray diffraction. The results demonstrate that metallurgical bonds are formed between diamond and modified alloy, endowing strong bonding force to diamond abrasives. Tungsten (W) can react with diamond and modified alloy to form W–C compounds and bonding phases. Besides, W combines with certain amount of Ni element to reduce graphitization degree, improve integrity and decrease residual stress of brazed diamond. Compared with commonly utilized vacuum brazed grinding wheel, grinding test indicates that service life of grinding wheel, prepared by new brazing technology, is improved. The machining efficiency is increased by 42.4% and the abrasive invalidation rate is reduced by 33.7%. In conclusion, the diamond grinding wheel, prepared by new brazing technology, exhibits superior performance.

Bottom-Up Evolution of Diamond-Graphite Hybrid Two-Dimensional Nanostructure: Underlying Picture and Electrochemical Activity.

The diamond-graphite hybrid thin film with low-dimensional nanostructure (e.g., nitrogen-included ultrananocrystalline diamond (N-UNCD) or the alike), has been employed in many impactful breakthrough applications. However, the detailed picture behind the bottom-up evolution of such intriguing carbon nanostructure is far from clarified yet. Here, the authors clarify it, through the concerted efforts of microscopic, physical, and electrochemical analyses for a series of samples synthesized by hot-filament chemical vapor deposition using methane-hydrogen precursor gas, based on the hydrogen-dependent surface reconstruction of nanodiamond and on the substrate-temperature-dependent variation of the growth species (atomic hydrogen and methyl radical) concentration near substrate. The clarified picture provides insights for a drastic enhancement in the electrochemical activities of the hybrid thin film, concerning the detection of important biomolecule, that is, ascorbic acid, uric acid, and dopamine: their limits of detections are 490, 35, and 25nm, respectively, which are among the best of the all-carbon thin film electrodes in the literature. This work also enables a simple and effective way of strongly enhancing AA detection.

The mechanisms of carbon nano-flake balls growth by laser ablation and microwave plasma chemical vapor deposition

The role of laser ablation (LA) for synthesis of carbon nano-flake balls (CNFBs) in microwave plasma chemical vapor deposition (MPCVD) has been investigated. In this article, a systematic study in frequency, speed, pulse width and processing time of LA were conducted in order to identify the role of LA and find an optimized condition window of the growth process. In addition, the study also demonstrates the nucleation and growth mechanism of CNFBs through Optical Emission Spectroscopy (OES), high-resolution transmission electron microscopy (HRTEM), and electron energy loss spectroscopy (EELS) observation on the spacing between graphite and diamond and the SiC interface between Si and diamond site. The defect areas, which are caused by Hα etching, react with carbon atoms to form a layer of SiC film, then followed by the increasing amount of CH and CN-C2 radicals, forming diamond nanoparticle and graphite, respectively. In other words, the growth of CNFBs is the near simultaneity of the formation of diamond nanoparticle and the growth process (diamond and graphite). It is discovered that the existence of defects on the silicon substrate and the silicon carbide (SiC) layer after LA are the key factors of growing CNFBs.

High temperature surface graphitization of CVD diamond films and analysis of the kinetics mechanism

A plasma arc generator was used to heat treat the free-standing CVD diamond films at 1450–1900 °C. An Arrhenius curve was fit according to trends in the measured Raman peak intensity ratio of diamond to graphite as a function of heat treatment temperature, and the curve was extrapolated linearly. It was found that the extrapolation lines intersected at the Debye temperature, ΘD = 2021 K. By comparing the activation energies of diamond graphitization in literature and from different measurement modes in our study, it was found that the graphitization mechanisms were different on both sides of Debye temperature due to differences in the excited lattice vibration modes. When the heat treatment temperature was just below the Debye temperature, the graphitization process was promoted by generating vacancies or the breaking of single CC bonds, and the apparent activation energy was 366 ± 60 kJ·mol−1. When the heat treatment temperature was above the Debye temperature, the graphitization process was promoted by the simultaneous breaking of multiple CC bonds, and the apparent activation energy was 887 ± 141 kJ·mol−1. In combination with the morphology changes on the surface of the CVD diamond film after high temperature heat treatment, the mechanism of the micro-cyclic phase transition on the surface of the diamond film at temperatures lower than Debye temperature was summarized as follows. First, CC bonds on the diamond surface were activated and broken at high temperatures, and some diamond carbon atoms dissociated from solid phase sites to the gas phase by forming hydrocarbon free radicals. Then, the carbon atoms from these hydrocarbons entered the graphite lattice step by step to complete the spontaneous deposition of sp2 clusters.

Binder jetting additive manufacturing of copper/diamond composites: An experimental study

This study demonstrates the feasibility of fabricating copper/diamond (Cu/Dia) composite samples using binder jetting additive manufacturing for the first time. Binder jetting fabricates parts in two steps: forming green parts with a binder at a low temperature and consolidating the green parts by sintering. The low processing temperature makes binder jetting an excellent candidate to create complex-shaped Cu/Dia parts without causing diamond graphitization. In this study, mixtures of copper-coated diamond powder and pure copper powder were used to print Cu/Dia samples on a binder jetting printer. The printed samples were then sintered under a medium vacuum environment with flowing forming gas (95% nitrogen and 5% hydrogen). The effects of diamond volume fraction (10 vol% and 50 vol%) and sintering temperature (800 °C and 900 °C) on density and porosity, phase composition, and microstructure of Cu/Dia samples were investigated. As the diamond volume fraction increased from 10 vol% to 50 vol%, the sinterability of the powder mixture decreased. Increasing sintering temperature from 800 °C to 900 °C significantly improved sintered bulk density for samples from each powder mixture. When sintering temperature increased from 800 °C to 900 °C, the microstructure evolved from a granular structure to a network structure. In all sintered samples, the distribution of diamond was uniform and there was no sign of diamond graphitization.

Catalytic graphitization of single-crystal diamond

Diamond and graphene are carbon allotropes with starkly different physical characteristics. Their combination into graphene-on-diamond heterostructures could benefit from the complementary properties of both components. Graphitization of single-crystalline diamond surfaces is a promising synthesis route, but a clear understanding of the growth of graphene or graphite from solid carbon sources is so far missing. Using aberration-corrected transmission electron microscopy, Raman spectroscopy, and electrical transport measurements, we provide detailed insight in the mechanisms of structural changes of nickel-catalyzed graphitization of diamond. We propose competing atomistic processes occurring at contact sites of diamond and Ni, depending on diamond surface terminations. One-dimensional etching process dominates on (111) diamond surfaces that remain almost atomically flat during graphitization. Two-dimensional etching of (110) and (100) diamond surfaces results in Ni drilling into the diamond substrate. Our findings also provide evidence on the reaction rates of the catalysis. The most reactive diamond surface in the (100) orientation is covered with the largest amount of well-crystallized graphite, whereas the (111) surface shows the highest stability against catalytic etching. In the latter case, only a thin disordered graphite layer is formed, yielding the lowest electric conductance. By clarifying these etching mechanisms, our results can improve the synthesis of graphene-on-diamond heterostructures.