HFCVD Diamond Research Articles

Study on hydrogen retention resistance of diamond film by HFCVD

To explore the potential application of diamond coating on the surface modification of graphite-based plasma-facing components in nuclear fusion, the retention of hydrogen irradiated into diamond film deposited by HFCVD was studied. After being attacked with 200 eV hydrogen ions for 10 h at 200 °C, this diamond layer was observed to maintain the stability of crystal structure. The C-H absorption peaks in the FTIR spectrum that were conspicuous in the sole graphite were absent in the diamond surface subjected to the same quantity of hydrogen irradiation, suggesting that the implanted hydrogen was difficult to penetrate or persist in the diamond covering. Further computational studies have demonstrated that hydrogen atoms exhibit a low propensity for retention within the diamond (111), in contrast to reside stably in graphite or graphite-like structures. The above shows that HFCVD diamond film has strong resistance to hydrogen retention and structural stability under hydrogen irradiation.

Tribological properties of HFCVD diamond coatings over wide temperature range

Single and gradient multilayered diamond coatings, deposited on silicon nitride (Si3N4), were prepared by hot filament chemical vapour deposition. The tribological properties were evaluated in a wide temperature range (26 °C–300 °C), against Si3N4 ceramic balls, as the friction counterpart. The results showed that with the increase of temperature from 26 °C to 300 °C, the wear rate of NCD (nanocrystalline diamond) and GCD (gradient diamond) coatings rapidly increases. When the temperature reaches 300 °C, obvious cracks and detachment appear on the surface of the NCD coatings. Due to the special interlayer, the wear rate of the GCD coatings is lower than that of the NCD coatings. In the range of 26 °C to 100 °C, GCD exhibits excellent wear resistance and low friction coefficient due to its special gradient structure. When the temperature increases to 200 °C, NCD films with smaller grain sizes exhibit higher wear rates. From the wear morphology, it can be seen that cracks are generated in the NCD film, and as the temperature increases, the cracks continue to expand until the film spalling. GCD shows relatively good wear resistance at high temperatures. This study found that gradient multilayered diamond coatings exhibited superior wear resistance at high temperatures, compared with single-layer coatings tested in identical conditions.

Crystal orientation control of HFCVD diamond spherical film tools and tribological behavior on steel

(111)-oriented micron-crystalline diamond (111MCD), (110)-oriented micron-crystalline diamond (110MCD) and (110)-oriented nano-crystalline diamond (110NCD) spherical films were prepared on the surface of spherical WC-Co polishing tools by hot filament chemical vapor deposition technology. The microstructure, molecular structure, crystal orientation and surface roughness of spherical films were characterized by SEM, XRD, EBSD and White-light interferometer. The friction performance and material removal mechanisms were studied by friction experiments on steel. The results showed that both crystal orientation and grain size variation significantly influenced the tool properties. The roughness and friction coefficient of 111MCD, 110MCD, and 110NCD films decreased successively. During the friction process, the transformation of diamond to graphite reduced the friction coefficient but accelerated film delamination. 111MCD has the fastest material removal rate, but the graphitization rate and wear rate of the film are the highest, and there are a lot of abrasive chips in the friction interface. Diamond of (110) orientation can improve the wear resistance of the film and reduce the abrasive wear of the workpiece. Compared to 111MCD, 110NCD exhibited a 29.3 % reduction in friction coefficient, a 90 % improvement in wear resistance, a 49.4 % increase in surface smoothness of the processed workpiece, and the highest ID/IG value of 1.45 after film wear, demonstrating superior polishing performance and anti-graphitization properties. These results indicate that 110NCD tool is a highly efficient polishing tool with great potential.

Integration of Carbon Nanotubes in an HFCVD Diamond Synthesis Process in a Methane-Rich H2/CH4 Gas Mixture.

In this work, we present experimental data on carbon nanotubes integration during diamond synthesis. Carbon nanotubes layers were preliminarily deposited on silicon and diamond substrates, after which the substrates were loaded into the HFCVD reactor for further growth of the diamond phase. The CVD process was held in an argon-free H2/CH4 working gas mixture without the use of a catalyst for carbon nanotubes growth. It is shown that in a wide range of studied working gas composition (CH4 concentration up to 28.6 vol.%) nanotubes etched from the substrate surface before the diamond growth process began.

Spatial distribution of thermally induced residual stresses in HF-CVD diamond coatings on microstructured steel surfaces

Spatial distribution of thermally induced residual stresses in HF-CVD diamond coatings on microstructured steel surfaces

Various HFCVD diamond coatings synergistically tuned using CH4 gas flow and working pressure and key merit evaluation of their coated tools

Diamond-coated tools can meet the service requirements of high precision, high efficiency and long life with multi-element coupling, but the milling characteristics of graphite materials with high spindle speed, feed speed and high force may require tools with a high number of micro-edges and good adhesion strength. In the present study, microcrystalline diamond (MCD), nanocrystalline diamond (NCD) and gradient composite diamond (GCD) coatings were deposited on commercial end mills subjected to distinct CH4 gas flows and working pressure by hot filament chemical vapor deposition. The adhesion strength, erosion resistance and milling performance of the diamond coatings in milling graphite materials were evaluated. Besides, the effect of CH4 gas flows and working pressure on the growth model of diamond coatings and the mechanism of crack propagation when subjected to stress concentration were studied. The results show that the combination of high CH4 gas flows and low working pressure achieves the re-nucleation of diamond particles, the smaller grain, more graphite/amorphous carbon in the boundaries and larger residual stresses lead to continuous crack propagation, causing poor adhesion strength, erosion resistance and milling performance in NCD and GCD coatings. Conversely, the low CH4 gas flows to ensure the epitaxial growth required for MCD coating, the larger grain, higher diamond purity, columnar structure and smaller residual stresses allow the pinning effect between coating and substrate, altering and increasing the path of crack propagation, making it suitable for milling conditions with high speed and high force. This study provides significant insights into the links between diamond morphology, structure, grain size, boundaries, and phase composition and the mechanical properties of adhesion strength, erosion resistance, and milling properties by synergistically tuning or alternating the CH4 gas flows and working pressure.

Effects of urea versus N2 addition on growth and mechanical properties of HFCVD diamond films on WC-Co substrates

Nitrogen doped diamonds with great application potentials can be synthesized by using different doping sources (i.e., urea and N2), 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 N2 (always lower than 0.04). The sufficient N2 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.

A simple procedure to obtain nanodiamonds from leftover of HFCVD system for biological application

Nanodiamonds (NDs) are amongst the most investigated carbon-based nanostructures due to their chemical stability and favorable mechanical properties. Despite the number of works on methods for NDs production, one of the main challenges is to achieve their colloidal stability in aqueous suspension. Additionally, NDs are normally obtained by expensive, complex and time-consuming process. Herein, it was presented a facile method to obtain NDs in aqueous suspension by using columnar structure diamond from Hot-Filament Chemical Vapour Deposition reactor (HFCVD). CVD diamond leftover thick film from CVDVale Company was used. Therefore, this method has the advantage of being not only practical but also cost-effective since it brings a profitable use of CVD diamond leftover. The Diamond thick film was submitted to ultrasonic cavitation in the presence and absence of ZrO2 microbeads in aqueous medium. The NDs hydrodynamic diameter and the stability in aqueous suspension were monitored by light scattering, size and morphology were analyzed by transmission electronic microscopy. Considering the wide application of NDs in biomedical devices, cytotoxicity of aqueous suspensions of NDs was evaluated against murine embryonic fibroblast cells. Furthermore, NDs were functionalized with hydrogen and carboxyl groups. NDs aqueous suspension of straight size distribution was obtained even in the absence of ZrO2 beads, indicating that they may be dispensable in order to decrease NDs size. NDs of average hydrodynamic diameter of 22 nm and − 35 mV of Zeta-potential were obtained after ultrasonic cavitation followed by 2 h of centrifugation, not demonstrating cytotoxicity to cells at very low (0.05–0.5 μg/mL) nor at higher concentrations (116 μg/mL). Nevertheless, NDs showed a moderate cytotoxicity at intermediary concentration range (0.5–2.2 μg/mL). From our knowledge, this is the first work that reports on a facile method for providing NDs aqueous suspension with high colloidal stability from HFCVD diamond leftover.

Simulation and experimental researches on the substrate temperature distribution of the large-capacity HFCVD setup for mass-production of diamond coated milling tools

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 Tf, 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 Tf 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.

Effect of mechanical pretreatment on nucleation and growth of HFCVD diamond films on cemented carbide tools with a complex shape

To enhance the mechanical pretreatment efficiency for WC-Co complex-shaped cutting tools, a self-made vibrating grinding equipment and mixed abrasives made up of walnut shell and diamond particles are proposed. The technique can abrade the dozens of tools simultaneously, modify surface properties of tools, and maintain sharpness of the cutting edges. Several experiments are performed to explore effects of the different types and proportions of walnut shell and diamond powders on the properties of nucleation, growth, and adhesion of diamond films deposited via a chemical vapor deposition method. Subsequently, Field Emission Scanning Electron Microscopy (FE-SEM), Atomic Force Microscopy (AFM,), Laser scanning micro-gauge, Micro-Raman Spectroscopy, Stylus Profiler, and Rockwell hardness tester are adopted to characterize the as-pretreated substrates and as-deposited diamond films. According to the results, a low residual cobalt content around 0.2 wt% and a markedly increased surface defects are detected on the pretreated milling tools when the preferred types and proportions of mixed abrasives are applied, which can help to significantly enhance the nucleation density and growth rate of diamond films, and strengthen the adhesion between the films and substrates.

Adherent HFCVD diamond on steels substrates using vanadium carbide intermediate layer

Investigations on CVD diamond deposition on steels for high performance tools development persisted over years. Literature makes out the need for an intermediate layer to improve adhesion and film quality. This work studied Thermo Diffused Vanadium Carbide (TDVC) coating deposited on AISI O1 steel by thermo reactive diffusion (TRD). Thick TDVC layers formed by varying the thermo diffusion time from 1 h to 12 h. Hot Filament Chemical Vapor Deposition reactor (HFCVD) growth conditions variation made possible diverse diamond films growth, from ultrananocrystalline to microcrystalline. X-ray diffraction, scanning electron microscopy with a field emission gun (FEG-SEM) and Raman spectroscopy were the characterization techniques used. The TDVC coating showed up as an excellent diffusional barrier and, also, a compressive stress reliever. The thicker high quality and adherent HFCVD diamond film grown over the TDVC had a thickness of 3 μm. A TDVC cross-section of 35 μm thickness could effectively mitigate diamond residual thermal compressive stress state after cooling.

Research on preparation of HFCVD diamond coated milling cutter and stone cutting performance

The study is on the properties of diamond coating prepared on milling cutter and its effect on the cutting performance of stone. The method of hot filament chemical vapour deposition HFCVD was used to deposit diamond film coatings on the surface of the cemented carbide milling cutter. The surface morphology, composition and grain orientation of the diamond films on the cemented carbide milling cutter were analysed by using scanning electron microscopy, Raman spectroscopy and X-ray diffraction; the prepared diamond coated milling cutter was used to process the stone experiment, and the cutting performance of the diamond layer cutter with different carbon sources was analysed. The prepared diamond-coated cutter has a small diamond grain size on the surface, typically <111> crystal face disappears, and the grain orientation presents nano-crystalline diamond. Compared to the uncoated cutter, the surface roughness decreases. After coating, the diamond coating on the surface of the cutter did not show obvious shedding, and the amount of wear on the back face of the uncoated cutter was consistent.

Co evolutions for WC–Co with different Co contents during pretreatment and HFCVD diamond film growth processes

Abstract Systematical researches were accomplished on WC–Co with different Co contents (6%, 10% and 12%, mass fraction). Based on the XPS and EDX, from orthogonal pretreatment experiments, it is indicated that the acid concentration, the time of the acid pretreatment and the original Co content have significant influences on the Co-removal depth (D). Moreover, the specimen temperature, original Co content and Co-removal depth dependences of the Co evolution in nucleation, heating (in pure H2 atmosphere) and growth experiments were discussed, and mechanisms of Co evolutions were summarized, providing sufficient theoretical bases for the deposition of high-quality diamond films on WC–Co substrates, especially Co-rich WC–Co substrates. It is proven that the Co-rich substrate often presents rapid Co diffusion. The high substrate temperature can promote the Co diffusion in the pretreated substrate, while acts as a Co-etching process for the untreated substrates. It is finally found that the appropriate Co-removal depth for the WC–12%Co substrate is 8–9 μm.

Simulation and experimental researches on HFCVD diamond film growth on small inner-hole surface of wire-drawing die with no filament through the hole

Based on the traditional deposition technique with the filament through the hole (DTFTH), the growth of high-quality diamond films on inner hole surfaces, especially those with small apertures, is rather difficult and laborious. In the present research, the deposition technique with no filament through the hole (DTNFTH) is proposed, and its feasibility and limitations are discussed on the basis of the radiation computation, temperature simulation, analyses on mean free paths of various active radicals, and the experimental verification of the “shading and distance effects”. It is indicated that the ratio of the hole depth to the hole diameter should be smaller than four. Afterwards, the drawing die with an aperture of 2 mm is designed, and a case study focusing on a single die and the mass production of diamond coated dies by the DTNFTH are both accomplished, using optimized deposition parameters. It is demonstrated that the film thicknesses are in the range of 4.8–6 μm (case study) and 3.5–5 μm (mass production), showing acceptable uniformity. Uncoated, DTFTH- and DTNFTH-diamond coated dies are all submitted to field tests, indicating that the lifetime of the DTFTH-diamond coated die is approximately eight times more than that of the uncoated one, while the DTNFTH-diamond film prolongs the lifetime of the uncoated die by a considerable factor of five.

TiB2 barrier interlayer approach for HFCVD diamond deposition onto cemented carbide tools

The deposition of diamond coatings is often impaired by poor adhesion on hard metal substrates. To enhance the adhesion of diamond coatings on Co-cemented tungsten carbide (WC-Co), TiB2 thin films (100 nm to 1 μm in thickness) were deposited as interlayers by magnetron sputtering. Diamond coatings were continuously, densely and quickly deposited on the TiB2 interlayers. The adhesion of the diamond coatings evaluated by Rockwell C indentation, was enhanced by utilizing TiB2 interlayer with appropriate thickness compared to that without interlayer. The barrier function of the interlayer versus cobalt is dependent on the thickness of the TiB2 interlayer. As observed by scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS), the cobalt diffusion was effectively hindered by utilizing TiB2 interlayers with a thickness greater than 200 nm, whereas the strongest film adhesion was obtained for the TiB2 interlayer with thickness of ca. 200 nm. Furthermore, the thickness of the deposited diamond film was 1.7 times higher than that of the diamond film grown without TiB2 interlayer, indicating an accelerated growth. Raman spectroscopy and X-ray diffraction (XRD) analyses demonstrated the existence of TiB2 and high quality of diamond films. This work indicates that thin TiB2 interlayers are promising candidates for achieving adherent diamond coated cutting tools, such as micro-drills and milling tools.

Synthesis of Vanadium Interface for HFCVD Diamond Deposition on Steel Surface

Synthesis of Vanadium Interface for HFCVD Diamond Deposition on Steel Surface

Comparisons of HFCVD diamond nucleation and growth using different carbon sources

The differences between the nucleation, growth and basic characteristics of bias-enhanced hot filament chemical vapor deposition (BE-HFCVD) microcrystalline diamond (MCD) films fabricated under the different carbon source (methane, acetone, methanol and ethanol) environments are studied in this article, adopting SiC chips as substrates. With the same commonly-used nucleation parameters, acetone and ethanol can provide either higher nucleation density (ND) or larger nuclei size (NS). Besides, under the same commonly-used growth conditions, relatively higher growth rate (R) of the MCD film is obtained adopting methanol than methane, and both acetone and ethanol can further increase it. Moreover, as characterized by the X-ray Diffraction (XRD) and Raman spectroscopy, the MCD film fabricated using methane shows much lower total residual stress (σ), better film quality and lower (220)/(111) ratio compared with those fabricated using all the other carbon sources. The above results can be attributed to the presence of oxygen elements, the different dissociation energies of chemical bonds in the carbon sources, the different quantities of CH3 radicals generated from the carbon sources or the different amounts of defects in as-deposited diamond films. Based on the present study, it is suggested that the carbon source should be reasonably chosen in order to, respectively, improve the film quality or obtain high ND, NS and R.

Simulation of Temperature Distribution in HFCVD Diamond Films Growth on the Multitudinous Micro End Mills

In the process of HFCVD diamond film growth on the multitudinous micro end mills, the uniformity and stability of the temperature distribution have a vital importance on the quality of film. So a new method by using the finite volume is proposed to analyze the importance of different disposition parameters on the uniformity of substrate temperature field. These parameters are filament diameter (d), filament-substrate distance (H), filament separation (S) and filament length (L). The mono-factor method are used to optimize the best parameter combination. The simulation results show that the optimized parameters are d=0.65mm, H=10mm, S=27mm and L=160mm.

SIMULATION-BASED OPTIMAL DESIGN OF HFCVD EQUIPMENT ADOPTED FOR MASS PRODUCTION OF DIAMOND FILMS ON INNER-HOLE SURFACES

HFCVD diamond films have been extensively applied on inner-hole surfaces of some wear resistant components as protective coatings, the mass production of which puts forward a high request to the HFCVD equipment. In this paper, adopting a typical kind of drawing die as the substrate, several different substrate configurations are proposed to expand the capacity of the equipment. Temperature distributions on inner-hole surfaces of the substrates are respectively predicted and compared by the finite volume method (FVM) simulations. In summary, for parallel configurations, proper-designed auxiliary devices (e.g. the heat-insulated plate, the water-cooled plate and the heat-insulated cushion block) can provide sufficient effects on the uniformity of the substrate temperature distribution. Moreover, without any auxiliary devices, homogeneous substrate temperature distributions can also be obtained using circular pattern configurations (radial, hexagon and triangular). Accordingly, the HFCVD equipment referring to the design concept of the triangular configuration is trial-manufactured. In the verification experiments, similar substrate temperature values are detected in three different modules, and as-deposited microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) films both present uniform grain size, thickness and quality, well validating the rationality and correctness of the simulation-based optimal design.

Analysis of residual stresses and wear mechanism of HF-CVD diamond coated cemented carbide tools

Chemical vapour deposition (CVD) diamond coated tools have demonstrated their potential for the machining of difficult to machine materials in the last 10 years. The lack of adequate coating adhesion in many cases remains an issue and often leads to spontaneous coating delamination and sudden tool failure however. The work described in this paper was undertaken with the aim of understanding the influence of residual stresses and coating quality on the coating adhesion during the machining of aluminium alloys. Residual stresses were determined using Raman spectroscopy as well as X-ray diffraction analysis and a comparison of the results obtained using the two methods is given. Raman spectroscopy was also implemented to analyse the coating quality. Furthermore, the failure mechanism of CVD diamond coated tools was studied using cross-sections prepared by focused ion beam sputtering.