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For fabricating brazed inserts in machining difficult-to-machine materials, chemical vapor deposition (CVD) diamond thick films are potential candidates for polycrystalline diamond (PCD). In the present study, monolayer microcrystalline diamond (MCD), monolayer nanocrystalline diamond (NCD), and multilayer diamond thick films were respectively deposited on the Si3N4 substrates by the high-power density (HPD) microwave plasma CVD (MPCVD) technique, which were then brazed on WC–Co. Their microstructures, growth rates, and film quality were systematically studied. Within the multilayer film, in order to enhance the secondary nucleation on as-deposited MCD layers and promote the consecutive growth of NCD layers, increasing the nitrogen addition level was much more effective than increasing the methane/hydrogen ratio. The NCD interpositions could guarantee layer thickening, while suppressing the growing up of individual micro diamond grains. As revealed by Raman spectra, the multilayer structure could help restrain the accumulation of the residual stress with increasing the thickness. Besides, the multilayer diamond thick film also presented much enhanced erosive wear resistance, as compared with the monolayer MCD or NCD, because the NCD interlayers could prevent intergranular cracks from further propagating. Finally, cutting performances of various brazed inserts were evaluated, in the case of dry-turning the high-silicon aluminum alloy (40 wt.% Si), demonstrating that the multilayer structure could result in the enhancement of the impact toughness of cutting edges, and improve the machining quality. [ABSTRACT FROM AUTHOR]

Single crystalline diamond (SCD) grits are usually made by a high-pressure and high-temperature (HPHT) method and exhibit cuboctahedral shapes and smooth surfaces. In this paper, we synthesized new kinds of SCD grits and nanocrystalline diamond (NCD) grits by a chemical vapor deposition (CVD) method and explored their grinding performance by single grit grinding of SiC ceramics. Grinding by HPHT grits was also conducted for comparison. The CVD-SCD grits exhibit cuboctahedral shapes, but surfaces are covered by protruded twin crystals of several microns, while the NCD grits are spherical agglomerations of nano-grains. We demonstrated that these morphological features have profound influences on material removal behavior. Results show that the wear of NCD grits is dominated by wear flat and intergranular cracks. The worn NCD grits can exert intense rubbing and ploughing effects on the workpiece. In contrast, the wear of CVD-SCD grits is dominated by fracture of the protruded twin crystals. It is proved that the twins on the grit surface are effective cutting edges to remove the material in a micro scale and thus lead to the formation of fine grinding striations and smaller cracking pits, but this process has little influence on specific grinding energy. Besides, grits with rough twins, especially on the flank face, tend to exhibit lower grinding force ratios and thus better sharpness. Moreover, for grits with smooth rake faces, there exists an explicit critical undeformed chip thickness value around 0.4–0.6 μm, above which the material removal mechanism changes from ductile to brittle rapidly. However, for grits with rough twins on rake faces, this transition process becomes vague. [ABSTRACT FROM AUTHOR]

Freestanding diamond films are widely applied, but available fabrication techniques present obvious flaws. A new route to fabricate high-quality double-side-polished freestanding diamond film is proposed in this study. Typical MCD (Micro-crystalline diamond) or NCD (Nano-crystalline diamond) film is firstly deposited on pretreated SiC substrate by HFCVD technique. As-grown surfaces are smoothened by mechanical polishing. Diamond films are then separated from substrates by laser cutting. Finally, bottom surfaces, which are known as nucleated surfaces, are also polished. Compared with the Mo substrate used in the conventional route based on the natural peeling off of the diamond film, the thermal deformation and residual stress of the diamond film on the SiC substrate are smaller, mainly due to the reduced difference between thermal expansion coefficients of the diamond film and substrate. This is beneficial for avoiding the fracture of freestanding films in the cooling stage, and also beneficial for the subsequent polishing. Consequently, the interruptible repeated growth strategy with constant growth rates (1.0–1.2 μm/h for MCD and 1.3–1.5 μm/h for NCD) is feasible, and the long-duration operation of HFCVD device is dispensable. Compared with the Si substrate used in the route based on the Si etching, the SiC substrate is much more reusable in the present route. • A new route is proposed for preparing free-standing diamond film is proposed. • Selection of SiC substrate is beneficial for reducing the thermal residual stress. • The interruptible repeated growth strategy is feasible. • Separation of diamond film from substrate is accomplished by laser cutting. • SiC substrate can be recycled, which is environmentally-friendly and resource-conserving. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Monolayer diamond grinding tools are commonly made by electroplating or brazing. However, electroplated tools involve weak grain/matrix adhesion due to a lack of chemical bonds, and brazed diamonds often suffer thermal damage resulting from the high-temperature brazing process. This paper reports a new method to fabricate monolayer diamond grinding tools by chemical vapor deposition (CVD) technique. First, diamond powders at the range of 7–180 μm are chosen as seeds of the abrasive grains. The seeds are randomly distributed on SiC substrates by spraying the suspension containing the seeds toward the substrates in a spin coater machine, or orderly positioned on the substrates by a patterned mask. Then CVD diamond growth is conducted on both the substrates and the seeds simultaneously. As-grown diamond films on the substrates are regarded as bonding layers of the grinding tools, and the original seeds with irregular and crushed morphologies can grow into blocky cube-octahedral grains with smooth surfaces, which are supposed to have enhanced mechanical strength. Raman spectra demonstrate that as-grown diamonds on both the substrates and the seeds are of high quality and low levels of graphitization. Besides, relatively low residual stresses are detected in the diamond grains, because both the bonding layers and the grains are made of diamond. Grinding tests are conducted using as-fabricated CVD diamond grinding tools and electroplated/brazed diamond grinding tools as comparisons. The results suggest that the CVD diamond grinding tools possess the best wear resistance in terms of the fracture of the diamond grains. This is attributed to the epitaxial grain structure that restrains the crack propagations. In addition, the wear of the CVD diamond grains can generate tiny pyramidal crystalline tips and sharp edges, indicating the unique shelf-sharpness during the grinding. [ABSTRACT FROM AUTHOR]

Mechanical properties and solid particle erosion of chemical vapor deposition (CVD) diamond films are strongly influenced by the adopted carbon source in the deposition process. In the present study, methane, acetone, methanol and ethanol are respectively used as the carbon source to deposit micro-crystalline diamond (MCD) films on SiC substrates by the bias-enhanced hot filament CVD (BE-HFCVD) method. A standard air–sand erosion rig is employed to conduct erosion tests, using angular SiC sands of 320 meshes as impact abrasives. The impact velocity v e and impact angle α e are fixed as 100 m/s and 90°, respectively. The steady-state erosion rate ε s , film life t r and a unit film life t ru are defined to assess the erosion behaviors of as-deposited films, which have close relationships with the mechanical properties, as measured by the Rockwell hardness tester and the nanoindentation test. It's found that under the condition of the same film thickness, the methane-MCD film shows the lowest ε s and the longest t r , owing to the highest surface hardness and the most favorable film-substrate adhesion which is equivalent to the highest fracture strength, but the opposite is true for the methanol-MCD film. Moreover, due to the relatively higher growth rate, the acetone- and ethanol-MCD films both perform even longer t ru than the methane-one, indicating better “average” erosion behavior. Consequently, the optimal efficiency and economy can be achieved when adopting either the acetone or the ethanol to deposit MCD films of sufficient thickness on working surfaces of erosion resistant components suffering severe erosive wear, especially in the case of mass production. [ABSTRACT FROM AUTHOR]

Nitrogen doped diamonds with great application potentials can be synthesized by using different doping sources (i.e., urea and N 2), effects of which on growth behavior and properties of diamond films on WC-Co substrates were presented in this paper. The doping efficiency of urea was always higher than 0.12, much higher than that of N 2 (always lower than 0.04). The sufficient N 2 addition could increase growth rate, and induce apparent grain nanocrystallization, by modifying reactant gas phase chemistry. The grain nanocrystallization contributed more to the reduction of the surface roughness, and degradation of the diamond purity and mechanical properties. On the contrary, the urea doping resulted in much less degradation of the diamond purity and mechanical properties, while providing sufficient N incorporations into the diamonds. Besides, urea doping promoted the formation of the diamond (220) planes on the polycrystalline diamond surfaces, which had a close relationship with the actual N doping concentration in the diamond. The controllable adjustment of the growth and properties of diamond films, by selecting the doping source and optimizing the doping ratio, could help to meet distinctive application requirements, and balance the machining efficiency and application performance of diamond coated components. [Display omitted] • Urea and N 2 of different concentrations are adopted as nitrogen doping sources. • Doping efficiency by using urea is higher. • Two doping sources deteriorate diamond quality and mechanical properties by different means. • Influences of urea on diamond quality and mechanical properties are much less severe. • Mechanical properties can be adjusted by optimizing N doping source and ratio. [ABSTRACT FROM AUTHOR]

Abstract: Recently, shaped drawing dies with complicated hole shapes have attracted intensive interests to produce shaped wires. In order to produce high-quality wires, the surface smoothness of the drawing dies is a major requirement and as important as the wear resistance. In the present investigation, the deposition parameters in diamond films growth on the interior-hole surfaces of the WC–Co rectangular-hole shaped drawing dies are optimized by orthogonal simulations, for which homogeneous substrate temperature (dT <15°C) and gas density fields are obtained. Thereafter, boron-doped diamond (BDD) films are fabricated and characterized, showing that high-quality films with uniform thickness (24–25μm) have been deposited on the whole interior-hole surface. Compared with the undoped diamond film, the BDD one presents much better adhesion and long-tested wear resistance. Moreover, the BDD film is also much easier to be polished to the appropriate surface smoothness due to the grain refinement and hardness reduction. Particularly, by comparison to the WC–Co drawing die, the working lifetime of the BDD coated one increases by a factor of above eight and in the course of processing, copper rectangular wires with higher surface quality and better dimensional precision are obtained. [Copyright &y& Elsevier]

Abstract: LiNbO3 films were grown on nanocrystalline diamond (NCD)/Si substrates with amorphous SiO2 buffer layer by pulsed laser deposition technique at various oxygen pressures ranging from 10Pa to 80Pa. The stoichiometric LiNbO3 ceramic target was used. Significant effects of oxygen pressure on c-axis orientation, stoichiometry, crystallinity and surface morphology of LiNbO3 films were investigated. It was found that the above properties are strongly dependent on the oxygen pressure. Completely c-axis oriented LiNbO3 films with the average surface roughness of 9.0nm could be achieved under an optimum oxygen pressure of 40Pa. Surface acoustic wave devices were fabricated and characterized utilizing the structure of LiNbO3/SiO2/NCD/Si. [Copyright &y& Elsevier]

Abstract: In the present study, adopting the optimized deposition parameters determined by orthogonal simulations, high-quality boron-doped and undoped composite diamond (BD–UCD) films with uniform thickness are deposited on both the inner hole and conical surfaces of the SiC nozzles used in the spray drying equipment. The erosion behavior of the novel diamond film is firstly evaluated in an air–sand erosion rig, with single-layer boron-doped diamond (BDD) and undoped diamond films as comparisons. Thereafter, the erosive wear performance of the BD–UCD coated nozzles is further studied under the industrial condition. The results show that the BD–UCD film exhibits an apparent increase in the erosion resistance over the other two types of diamond films. The mechanism responsible is found to be that the surface undoped diamond layer in the BD–UCD film has outstanding hardness, and boron doping can improve the adhesion between the underlying BDD layer and the substrate. Moreover, under the industrial condition, the ethylene catalytic cracking catalyzers produced with the BD–UCD coated nozzles have much more stable quality than those produced with the uncoated nozzles owing to the aperture stability of the inner hole, further proving the high erosion resistance of the BD–UCD protecting coatings. [Copyright &y& Elsevier]

Behaviors of diamond coated Ti-related parts are significantly affected by carbonized interlayers, so it is essential to determine or predict the carbonization depth during diamond growth. Following the Fick's law of diffusion with carbon concentration-dependent diffusion coefficients, it is roughly predicted that carbonization depths in Ti and Ti–6Al–4V substrates hardly reach 30 μm within a long duration of 16 h. After trying some characterization methods (XPS, EDS, metallographic analysis and ToF-SIMS), the EDS line scanning is selected as the practical one with sufficient accuracy and convenience. Influences of some critical factors, including growth duration, substrate temperature, reactant pressure and acetone (carbon source) concentration, are studied and discussed. Considering all of the factors, especially those related to the diamond coverage that can suppress carbon diffusion, the modified diffusion models for describing the carbon diffusion, are constructed, defining carbon concentration and diamond coverage (duration, pressure, carbon concentration)-dependent diffusion coefficients. Such the models can be used for well predicting the carbonization depths under most conditions, which are valuable for controlling the carbonized layers during fabrications of diamond coated Ti-related components facing to various applications. Image 1 • Carbon diffusions into Ti substrates when depositing CVD diamond films are studied. • Influences of several critical factors are systematically studied. • Modified diffusion models are constructed, which can be adopted for predictions. • Diffusion coefficients are defined as carbon concentration and diamond coverage dependent. • Present results are valuable for controlling carbonized layers when depositing diamond films. [ABSTRACT FROM AUTHOR]

H-, O-, OH- and F-terminated N-doped diamond surfaces are calculated by the density functional theory in the present study. Combined effects of terminating species and substitutional N dopants in the 1st or 2nd carbon layers on several critical features (i.e., bond length, total electron density, bond population, atomic charge, Fukui function, and density-of-states) are systematically studied and discussed. The N dopant presents clear, but local, effects for all the terminating species and dopant positions. The adsorption of any individual species to the N dopant in the 1st carbon layer results in the reduction of adsorption energy (less negative), most probably owing to the full shell structure of this N dopant. The H, OH and F each provides one unpaired electron to occupy the antibonding states within the N–H, N–OH and N–F bonds, making the bonds elongated, reducing the corresponding electron densities and bond populations. On the contrary, the N dopant in the 2nd carbon layer induces slight increment of the adsorption energy for O, OH and F, attributed to the positive atomic charges of neighboring topmost C atoms. Amongst the four adsorbates, the O shows the highest reactivity to either an electrophilic or a nucleophilic attack. Besides, N dopants have slight influences on the reactivity of the adjacent adsorbates. The present results can help to understand some related chemical processes (such as the oxidation and electrochemical analysis) on N-doped diamond films. • H, O, OH and F adsorption on N-doped diamond surfaces are focused on. • H, OH and F-terminated N-doped diamond surfaces are calculated for the first time. • Adsorption energy, bond information, Fukui function and DOS are calculated. • Combined influences of terminating species and N dopants are clarified. • Calculation results can help for understanding many related chemical processes. [ABSTRACT FROM AUTHOR]

To improve the precision and homogeneity for mass-production of HFCVD diamond coated milling tools, the simulation of substrate temperature field generated by the HFCVD setup was conducted, adopting a 3D simulation model well in accordance with the actual equipment. Firstly, the deviation of the simulation model is assessed by comparing the simulated temperature and measured temperature based on a validation test. The comprehensive influences of various critical setup parameters on the fabrication of diamond coatings are illustrated systematically according to the Taguchi simulation scheme (L16, five factors and four levels), including the filament diameter d, the filament temperature T f , the separation between two adjacent filaments D, the filament height H, and the material of sample holder M, in order to obtain an optimal HFCVD setup. The results indicate that filament diameter d and the separation between two adjacent filaments D can effectively contribute to adjusting the average temperature; the distance between filaments and tool tips H, and the material of sample holder M exhibit remarkable effects on the temperature uniformity; the filament temperature T f shows obvious influences on both the average temperature and the temperature uniformity. Furthermore, the optimal HFCVD setup has been proposed, based on which a more uniform temperature field around all the tool tips can be acquired, contributing to the mass-production of high-quality and high-precision diamond coated milling tools. Afterwards, the mass-production experiments of depositing MCD and NCD diamond coatings on WC-6% Co milling tools were conducted in the actual HFCVD reactor, and tool-tip temperature were measured, revealing that the optimal setup can produce an extremely uniform temperature field. Moreover, the characterizations and cutting performances of MCD and NCD diamond coatings deposited on milling tools were evaluated, indicating that high-quality and high-precision diamond coatings with quite satisfactory homogeneity of thickness, surface and cross-sectional morphologies, chemical compositions and phases can be acquired during mass-production of diamond coated milling tools on the base of the optimal HFCVD setup. Unlabelled Image • Large-capacity HFCVD setup is used in mass-production of diamond coated milling tools. • A set of simulations on the temperature distribution are conducted and the optimal HFCVD setup parameters are proposed. • The influences of HFCVD setup parameters on diamond coatings are illustrated according to Taguchi simulation scheme. • Experiments of mass-production of diamond coated milling tools are conducted based on the optimal HFCVD setup parameters. [ABSTRACT FROM AUTHOR]

The pulsed laser processing has been used for machining ultrahard diamonds. However, structural evolutions, along with microscopic mechanisms, have hardly been clarified. In the present study, a molecular dynamics model for describing the pulsed laser processing of diamonds was developed, and the calculation results were verified through corresponding experiments, by using the typical nanosecond pulsed laser processing. At first, the temporal evolution of the temperature field was calculated, establishing the correlation between the temperature distribution and material ablation phenomena. Besides, intricacies of dislocation generation mechanism were elaborately explained, indicating that stress waves played prominent roles in formation and propagation of dislocations. In order to understand the phase transition phenomena, including the graphitization, vaporization and solidification, variations in coordination numbers based on calculations, together with Raman and XPS characterizations on diamond samples before and after laser processing, were examined and compared. Moreover, different effects of the pulsed laser processing on polycrystalline and monocrystalline diamonds were compared. It was found that the polycrystalline diamond shows lower rate of temperature fall and lower velocity of stress waves. This study provides valuable insights into the microscopic mechanisms governing pulsed laser processing of diamonds, thereby broadening the scope for optimizing laser machining processes. [Display omitted] • MD calculations are conducted for clarifying microscopic mechanisms of laser machining diamond. • Stress waves serve as the primary trigger for the generation of dislocations. • Diamond rapidly transforms into a non-diamond solid structure, followed by melting and vaporization. • Upon ceasing laser irradiation, liquid and gaseous carbon tend to condense into amorphous carbon. • Stress waves change direction when propagating between grains in polycrystalline diamonds. [ABSTRACT FROM AUTHOR]

Reactive ion etching (RIE) is a widely applied technique for high-precision machining of demanding diamond devices. However, the persistent challenge of widely distributed dislocation-related etch pits on processed surfaces remains poorly understood. In this study, the material removal and pit formation during the etching of the dislocation region by hydrogen and oxygen were investigated by reactive force-field (ReaxFF) molecular dynamics simulation combined with experimental verification. The results unveiled the pivotal role of crystal structure disruption and symmetrically distributed stress, induced by dislocations, in promoting bond-breaking processes and triggering the formation of etch pits. The etching parameter optimized simulations revealed that once the impact effect of oxygen atoms counterbalances the promotion of dislocation to the bond breaking, the formation of dislocation pits can be effectively avoided, which is convincingly validated by experimental results. Besides, our research further verified that pure hydrogen etching is an inefficient surface post-treatment process with a more significant pit-forming problem, and exists two distinct pit-forming mechanisms dependent on the incident energy. Additionally, substrate temperature variation has global but negligible influences on pits formation within the 400 K range. These findings carry significant implications for the optimization of processing parameters and the large-scale implementation of RIE in diamond processing, demonstrating the potential to address pit-forming issues with commonly used gases. [Display omitted] • A feasible scheme for studying the etching process of diamond with extended defect. • Structure disruption and stress concentration cause the dislocation etch pits. • Incident energy of etching particles has significant effects on processing quality. • Oxygen with high incident energy can effectively alleviate pits formation. • Pure hydrogen etching is an inefficient treatment process with poor surface quality. [ABSTRACT FROM AUTHOR]

[Display omitted] • Calculation of activation energies of acetone and methane precursors. • Decomposition paths confirmation of three liquid-doped molecules. • Growth rate comparison of diamond films fabricated using various precursors. • Distribution of doped elements detected by TOF-SIMS. Fabricating diamond films using liquid carbon source precursors in hot filament chemical vapor deposition (HFCVD) is an emerging technology. Although some experiments have demonstrated the feasibility of depositing doped diamond films using liquid carbon source precursors such as acetone, the theoretical explanation still needs to be elucidated. In this study, the density functional theory (DFT) method was used to analyze the decomposition and adsorption processes of acetone and methane on the surface of a diamond (1 1 1) models and to clarify the advantages of liquid carbon source precursors. The decomposition and adsorption reaction processes of trimethyl borate, urea, and ethyl orthosilicate on the surface of diamond (1 1 1) model were also analyzed to elucidate the doping mechanisms of B, N, and Si elements. Additionally, undoped diamond films were fabricated using acetone and methane precursors, illustrating that liquid precursors have higher growth rates than gas precursors. B-, N-, and Si-doped diamond films were also fabricated using liquid-doped carbon source precursors. Element detection by TOF-SIMS illustrates that all three elements had been doped into the diamond films and were uniformly distributed. Conversely, no doping elements were detected in N-doped diamond film fabricated with nitrogen and methane precursors. [ABSTRACT FROM AUTHOR]

Recently, specially-designed printed circuit board (PCB) milling cutters with specific cutting edge structures have been extensively applied. Hot filament chemical vapor deposition (HFCVD) diamond coatings uniformly deposited on cemented carbide (WC-Co) cutters can effectively improve their performance. To improve the coating uniformity, the complex cutters were precisely modeled and the temperature field generated by the HFCVD setup was simulated, using the finite volume method (FVM), taking spiral milling cutters as a reference for comparison. Temperature measurements revealed deviations between simulated and measured temperatures smaller than 4%, verifying the accuracy of the constructed model. Effects of some critical HFCVD setup parameters, including the filament height (H f), the material and height of the sample holder (M s and H s), the material and dimensions of the heat insulating block (M h , H h and A h), and the surface convection coefficient (h), were studied by the control variable method. Under the optimized conditions, the axial temperature deviation on cutting edges dropped from 193.3 °C to 64.3 °C for a PCB milling cutter, and from 53.3 °C to 29.6 °C for a spiral cutter, ensuring appropriate temperature ranges for fabricating high-quality diamond films. Besides, the two-dimensional plane temperature field and received radiation field of a cutter were both calculated by MATLAB. The results showed that heat conduction hindrance and heat convection reduction could effectively reduce energy consumption and improve the uniformity of substrate temperature. With the aim to study the specific cutting-edge structures, cross-sectional temperature distributions were further examined by presenting both simulations and numerical calculations. It was seen that the temperature distribution for PCB milling cutters was complicated, which was attributed to the densely-distributed cutting edges and long flute. Lastly, under the optimized HFCVD setup parameters, it was found that the grain size and thickness uniformity of diamond coatings, at different positions along the axial direction, were significantly improved, revealing the effectiveness of simulation as an optimization method. [Display omitted] [ABSTRACT FROM AUTHOR]