Microstructural features and thermal stability of alumina-bonded nano-polycrystalline diamond synthesized by detonation sintering

In this work, alumina-bonded nano-polycrystalline diamond (NPD) was synthesized by detonation sintering in the temperature range of 3000–3500 K and pressure range of 15–25 GPa. The microstructures and thermal stability of the NPD detonation sintered at 3255.05 K and 24.51 GPa were studied, and are described herein. Transmission electron microscopy and electron diffraction revealed that the polycrystalline diamond has a unique formation process and no graphitization. Scanning electron microscopy indicated that the size of polycrystalline particles increased in samples 2, 3, and 4. And thermogravimetric analysis indicated that the thermal stability of the diamond particles was enhanced. The 18% mass loss of specimen corresponded to the oxidation and decomposition of the amorphous carbon and carbon-containing compounds synthesized by detonation. Finally, graphitization calculations showed that the graphitization probability of polycrystalline diamond produced at the temperature of 3255.05 K and pressure of 24.51 GPa was 15.04%.     

Physical–chemical model of processes at detonation synthesis of nanodiamonds

This article presents a principally new physical–chemical model of nanodiamond formation at explosion, which describes adequately all the existing experimental data on detonation synthesis of diamonds. According to this model, the detonation wave performs activation rapidly; then the reaction mixture composition keeps varying. In the diagram C – H – O this process results in continual motion of the point imaging the reaction mixture composition. The ratio of the diamond phase amount to the condensed carbon quantity in the explosion products is defined by the width of the section this point passes over in the diamond formation zone. Motion of the point in the area below the line H – CO results in decrease of the condensed carbon (CC) amount. Diamonds are formed by the free-radical mechanism in the unloading wave.     

Investigation on preparation of composite vitrified bonding ultrafine diamond grinding wheel and machining of PcBN inserts

The grinding of polycrystalline cubic boron nitride (PcBN) is hard owing to its high hardness and superior wear-resistance capacity. Machining of PcBN tools via vitrified diamond grinding wheels with a size above 10 μm may lead to brittle fracture instead of a ductile machining because of the poor toughness of cubic boron nitride. A uniformly dispersed M0.5/1.5 diamond grinding wheel with a composite vitrified bonding was fabricated to improve the surface roughness of PcBN inserts. It is demonstrated that the preparation of composite vitrified bonding with various additions of vitrified bonding produced by the melting-quenching technique (VB-MQ) has little effect on the performance of vitrified bonding, such as bending strength, CTE and phase and achieves the uniform dispersion of M0.5/1.5 diamond as the addition of VB-MQ is no greater than 50%. Both the grinding ratios and the surface roughness of PcBN inserts are enhanced.     

Selective Deposition of Mo2C-Containing Coatings on {100} Facets of Synthetic Diamond Crystals

An efficient way to improve the properties of metal–diamond composites (mechanical strength, wear resistance, thermal conductivity) is the preliminary modification of the diamond surface to improve its wettability by the metal matrix. In the present work, Mo₂C-containing coatings were deposited on the diamond crystals under different conditions: hot pressing (atmosphere of argon), spark plasma sintering (forevacuum), and annealing in air. The influence of the sintering parameters on the morphology and phase composition of the coatings deposited on diamond was studied. Mo₂C-containing coatings were selectively deposited on the facets of synthetic diamond microcrystals by annealing of the latter with a molybdenum powder. Experiments were carried out to deposit coatings under different conditions: during hot pressing (argon atmosphere), spark plasma sintering (forevacuum), and annealing in air. The process parameters were the temperature, holding time, and concentration of molybdenum in the initial mixture. Experiments with a pre-oxidized molybdenum powder were also conducted. The coated diamond crystals were investigated by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The deposition was enabled by the gas phase transport of molybdenum dioxide, MoO₂, contained in the starting powder. The following sequence of the coating formation stages was proposed. First, MoO₂ sublimes and is adsorbed mainly on the {100} facets of diamond. Then, it is reduced to metallic molybdenum by carbon of the diamond, which further reacts with carbon to form the Mo₂C carbide phase. These processes occurred during treatment of the mixtures in the hot press and the spark plasma sintering facility. When the mixture was annealed in air, no selective deposition was observed. During annealing, MoO₃ particles adhered to the diamond surface.     

Unusually tight aggregation in detonation nanodiamond: Identification and disintegration

A remarkable observation that detonation of oxygen-deficient explosives in an inert medium produces ultra-fine diamond particles having diameters of 4-5 nm was made four decades ago, but this novel form of diamond has never been isolated in pure form thereafter. The reason for the difficulty was that the core aggregates having a diameter range of 100-200 nm are extremely tight and could not be broken up by any known method of de-aggregation. After a number of futile attempts, we were able to obtain primary particles by using the recently emerging technique of stirred-media milling with micron-sized ceramic beads. The milled aqueous slurry of nanodiamond gave a stable, thick and dark-coloured colloidal solution. After light sonication, dynamic light scattering measurements gave a sharp distribution in the single-digit nano-range, and HRTEM indicated separate particles having diameters of 4-5 nm, which agreed with the X-ray value of 4.4 nm for the primary particles. A model is presented for the core aggregates, which resembles the well-known grape-shaped ‘aggregate structure’ of the hardest type of carbon black.     

A study of the initial stages of diamond deposition on ferrous substrates coated by a nitrided chromium interlayer and on nitrided polycrystalline chromium substrates

The use of a nitrided chromium interlayer has been found to improve the interfacial properties of diamond films deposited on ferrous substrates. This is achieved by hindering diffusion process of carbon and iron, good adhesion of the interlayer to the steel substrate, and very stable mechanical and chemical bonding between the interlayer and the diamond film. In the present study the initial stages of diamond deposition on steel substrates coated by a nitrided chromium interlayer and on nitrided polycrystalline chromium substrates are reported. Nitridation of chromium films deposited by electrochemical methods and polycrystalline chromium substrates resulted in the formation of two chromium nitrides phases, CrN and Cr₂N, and a rough surface morphology. The initial stages of diamond deposition were found to be accompanied by carburization of the substrates surface resulting in chromium carbide formation. The incubation time, diamond particle density and growth rate at the very initial stages of the deposition process were found to differ for these two substrates. It is suggested that these differences originate from different carburization rates of the two substrates. Phase transformation, recrystallization and diffusion processes in the near surface regions of both substrates resulted in very stable chemical bonding and good adhesion of the diamond film to the substrates. Raman spectra of the deposited films, on both substrates, show shift of the diamond peak position to higher wave numbers and split of the peak. These effects are associated with compressive stresses in the diamond film. Residual stresses in the deposited films were calculated from the shift of the diamond Raman peak. The residual stresses, as calculated from the Raman spectra, were found to increase with deposition time reaching values of 8.4 and 6.9 GPa for continuous diamond films on steel substrate coated with the nitrided chromium film and on nitrided chromium substrates, respectively. Based on a simple model it was estimated that thermal stress, arising from mismatch between the thermal expansion coefficient of diamond and the underlying substrates, is the major component of the compressive stress in the diamond films.     

Electrochemical characteristics of boron doped polycrystalline diamond electrode sintered by high pressure and high temperature

The bulk B-doped polycrystalline diamond (PCD) electrode in this study was prepared by high-pressure, high-temperature (HPHT) technology. The PCD was sintered under HPHT conditions, using B-doped diamond powders and a metal catalyst as raw materials, then the metal solvent phase was dissolved by aqua regia. The morphology and composition of the PCD were investigated with a scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersion spectrum (EDS). The results show that the sintered body possesses a polycrystalline structure with direct diamond–diamond bond and irregularly shaped pores of 1–10 μm distributed on the grain boundaries after the metal solvent phase was removed. The cyclic voltammogram and electrochemical impedance spectroscopy of this B-doped electrode have been investigated. The B-doped PCD electrode exhibits stable electrochemistry in a KCl support solution over a wide potential range. The quasi-reversible reaction occurs on the electrode for the [Fe(CN)₆]^3−/4− couples. The electrode process combines the diffusion-controlled mass transport plus the kinetic process. The electrochemical impedance spectroscopy (EIS) analysis shows the porous structure characteristic of the PCD electrode.     

Fabrication and microstructural characterization of the diamond@amorphous carbon nanocomposite core/shell structure via in-situ polymerization

A novel diamond@carbon core/shell structure, constituting diamond as a hard core and carbon as a soft shell, was synthesized from resole resin and nanodiamond as the starting materials via in-situ polymerization and subsequently high-temperature carbonization. The diamond@carbon nanocomposite was characterized using field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectra, and Fourier transform infrared spectroscopy (FT-IR). The results indicated that the shell material are comprised of amorphous carbon. The thickness of carbon shell was controlled from 502.5 nm to 27 nm by adjusting the concentration of the nanodiamond. As confirmed by TEM, FT-IR and XPS, the diamond@carbon nanocomposite revealed a stable structure, due to the formation of the chemical bonding between diamond and carbon shell after calcination process. Overall, the diamond@carbon nanocomposite abrasives could lead to a reducted surface roughness and damage of SiC wafer comparing with the nanodiamond abrasives, due to the spring-like effect coming from the elastic component of the amorphous carbon shell. Moreover, the as-prepared nanoparticles exhibited better dispersion stability than the pure diamond in the pH range from 8 to 11.     

Nanodiamond growth on diamond by energetic plasma bombardment

The present work studies the growth of nanodiamond under prolonged DC glow discharge plasma bombardment of 1 μm thick polycrystalline CVD diamond. Using Raman spectroscopy, near edge X-ray absorption fine structure (NEXAFS) and secondary ion mass spectroscopy (SIMS) it is shown that nanodiamond formation on diamond starts directly, skipping the nucleation stage required to form the precursor material with the appropriate density and hydrogen content. On the other hand, the amorphous carbon/nanodiamond composite structure is substantial to the nanodiamond nucleation under energetic plasma bombardment and cannot be eliminated by starting with a micro-crystalline diamond substrate. The results give additional insight to the phenomena of nanocrystalline nucleation and growth under energetic particle bombardment relevant to a variety of systems including bias enhanced nucleation of diamond and cBN deposition by ion assisted methods.     

金刚石增强Na2O-B2O3-Al2O3-SiO2系陶瓷基复合材料的界面研究

以Na₂O-B₂O₃-Al₂O₃-SiO₂系低温玻璃为基础结合剂烧制金刚石增强陶瓷基复合材料,利用扫描电子显微镜、X射线能谱、X射线光电子能谱、拉曼光谱及力学性能测试仪等对其界面结合强度、界面处元素分布及界面化学键进行了表征。结果表明,Na₂O-B₂O₃-Al₂O₃-SiO₂系陶瓷结合剂与金刚石颗粒界面结合强度高,790 ℃煅烧时试样抗折强度达到77.82 MPa。Si、B、Na、Zn各元素在界面位置发生扩散,而Al元素没有明显扩散,元素扩散提升了结合剂对金刚石的把持力。陶瓷结合剂与金刚石在界面处形成C-O、C=O和C-B键,化学成键进一步增进界面结合。另外,790 ℃煅烧的复合材料中金刚石颗粒保存完好,而850 ℃煅烧时金刚石出现石墨化迹象。     

碳的雨贡纽状态方程及动压合成金刚石机理——动压合成金刚石之二

动压法已经成为纳米金刚石和多晶金刚石的主要合成方法,按照方法的不同又可以分成爆轰法、爆炸法和激光法等多种方法。文章论述了碳的雨贡纽状态方程和碳的高压相图。分析了爆轰法、爆炸法和激光法等几种不同方法合成金刚石的原理。估算出石墨直接转化金刚石所必须越过的势垒值为8.565×104J·mol-1,其激光源的功率密度不应低于2.534×105 W·cm-2。     

A stable suspension of single ultrananocrystalline diamond particles

As the result of successful disintegration of tight aggregates in detonation nanodiamond by stirred-media milling with microbeads, stable colloid of nanodiamond particles with a mean core size of 4 nm is obtained for the first time, but the colloid is colored deep black. X-ray diffraction, Raman scattering, HRTEM, UV–vis absorption spectra and viscosity data were used to characterize the colloid. It was suggested that the reason for the unexpected black color of the suspension is a result of graphitic partial surface (π-electrons formation of 4 nm diamond particles) induced by strong collision with beads during milling process. π-electrons are a reason of double electric layer formation and high viscosity of the suspension. Theoretical estimations fitted experimental data.     

金刚石表面电镀铬新工艺

在金刚石表面化学镀Ｎｉ－Ｗ－Ｐ做导电底层,然后采用三价铬镀液在金刚石表面电镀金属铬镀层,Ｘ－射线分析表明,在高温下金刚石表面镀层中有金属铬的碳化物生成。铬碳化物的生成增强了金刚石与胎体金属之间的浸润能力,从而增强了金刚石与胎体金属之间的结合强度。     

Preparation of Si–diamond–SiC composites by in-situ reactive sintering and their thermal properties

This article described a novel method of preparation of Si–diamond–SiC composites by in-situ reactive spark plasma sintering (SPS) process. The relative packing density of Si–diamond–SiC composite was 98.5% or higher in a volume fraction range of diamond between 20% and 60%. Si–diamond–SiC composites containing 60 vol% diamond particles yielded a thermal conductivity of 392 W/m K, higher than 95% the theoretical thermal conductivity calculated by Maxwell–Eucken's equation. Coefficients of thermal expansion (CTEs) of the composites are lower than the values of theoretical models, indicating strong bonding between the diamond particle and the Si matrix in the composite. The microstructures of these materials were studied by field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD). As a result of reaction between diamond and silicon, SiC phase formed.     

Raman scattering,AFM and nanoindentation characterisation of diamond films obtained by hot filament CVD

In this work, structure and mechanical properties of diamond films fabricated by HFCVD on silicon substrates with nanodiamond seeding were investigated. Raman spectroscopy was used to characterise the diamond phase content, crystalline quality and source of stresses in these films. Topography, hardness and Young's modulus were studied by scanning force microscopy (SFM) and nanoindentation methods. It has been ascertained that for the diamond films grown on silicon substrates with nanodiamond seeding hardness and crystalline quality is higher than for films on scratched silicon. The diamond films demonstrate Raman upshift with respect to natural diamond, indicating presence of internal compressive stress. It was shown that various types of impurities and defects induce compressive stresses in the diamond grains.     

Microstructural investigation of diamond-SiC composites produced by pressureless silicon infiltration

Superhard silicon carbide-bonded diamond materials were synthesized by liquid silicon infiltration of diamond-containing preforms. The properties of the materials were strongly influenced by the strength of the interfaces between the diamond and the silicon carbide. Interface formation was investigated through local analysis of the microstructure in the interface regions using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) as well as X-ray diffraction (XRD). The results of these experiments revealed a pronounced orientation relationship between SiC and diamond at their interfaces and, as a result, strong bonding of the diamond particles to the ceramic matrix. There was also an orientation relationship between the nano-sized SiC grains, which were embedded in residual silicon near the diamond interfaces, and diamond. Additionally, the different morphologies and phenomena occurring in the microstructures of the diamond-SiC composites and their dependence on the infiltration temperature were studied.     

金刚石镀Ti对金刚石锯片性能的影响研究

研制了一套简单易行的镀膜工艺,并将镀膜金刚石应用于胎体配方中制得金刚石锯片,并与相同胎体配方含未镀金刚石的锯片作比较,考察其对锯片性能的影响,并从金刚石和胎体间的界面反应对界面粘结强度的影响及金刚石的损伤程度上对其机理进行了微观分析。     

Influences of WC-Co hard metal substrate pre-treatments with boron and silicon on low pressure diamond deposition

Diamond coatings were produced on WC-Co hard metal substrates. To improve the adhesion between the diamond coating and the substrate a substrate surface pre-treatment with boron or with silicon vapor was applied. This surface pre-treatment resulted in an increase in both the diamond nucleation density and the diamond growth rate. Simple adhesion tests confirmed an improved adhesion of thin diamond layers as compared with those on untreated hard metal substrates.Secondary ion mass spectroscopy (SIMS) depth profiles revealed an enrichment of B or of Si at the substrate-diamond interface due to the pre-treatment procedure. The correlation of the Co and W depth profiles in samples coated for 12 and 24 h supports the theory of diamond dissolution into the substrate. Co was detected only in the interface regions and on the surface of the diamond layers but not in the bulk of the thick layers. The SIMS results confirm X-ray examinations of the hard metal Co binder phase.     

金刚石线锯切割设备现状分析

随着光伏、半导体等高精端产业的发展,硬脆性材料,如晶体硅、蓝宝石、特种陶瓷等材料的切割加工显得尤为重要。近年来,世界各国大力投入研究开发,新工艺,新设备不断涌现。金刚石线锯切割成为研究和发展的主流趋势,文章介绍国外切割设备研究的最新进展以及国内发展状况,分析其性能优势,展望金刚石线切割设备应用前景。     

Consequences of strong and diverse electrostatic potential fields on the surface of detonation nanodiamond particles

Discovery of strong electrostatic fields on the surface of primary particles of detonation nanodiamond by Barnard and Sternberg not only provided a highly likely explanation on the long pending origin of agglutination in the crude detonation product, but also marked the first recognition of a new type of interfacial interaction that can be as strong as C–C covalent bonding. The sign and potential distribution of the electrostatic field are specific to the crystallographic indices of facets and size of particles, thus the new electrostatic feature could well be unique to nanocrystals. This article interprets various enigmatic behaviors of nanodiamond particles that we have so far been unable to understand in terms of surface electrostatics. A mechanism of self polarization in the energy-minimized nanodiamond crystals in terms of orbital interactions through space and bond is presented.