Microcrystalline Diamond Research Articles

Nitrogen-vacancy centers as a self-gauged micro-scale heater and its application for multi-modal sensing

The optical excitation of nitrogen-vacancy (NV) color centers in diamonds mostly results in fluorescence emission. During this process, a portion of the incident energy is transferred to phonon vibration, which heats the diamond crystal. For single NV color centers, the heat generated by the optical cycle is negligible, while for an ensemble of NV defects, the generated heat accumulates rapidly and heats the diamond. The temperature rise is rapid due to the high thermal conductivity of the diamond. In addition to the ability to be heated by light, the NV defect's unique properties also allow for the precise measurement of temperature using optically detected magnetic resonance. Here, we experimentally demonstrate that microcrystalline diamond containing NV center ensembles can be used as a self-gauged microheater. We attached a microcrystal diamond to an optical fiber in an endoscope configuration, evaluated its performance as a self-gauged heater under varied biologically relevant environments, and discussed its potential applications. In addition to the aforementioned capabilities, the NV defect enables the precise measurement of local magnetic fields. This provides a unique multimodal sensor to probe temperature-controlled magnetic phenomena at microscopic scales.

Periodic renucleation as an approach to improving the tribological properties of CVD diamond films

This study investigates an approach to enhancing the tribological properties of CVD diamond films by modifying their structure. The approach is based on periodic renucleation of the film during the synthesis process. A comparative analysis was conducted with films of a microcrystalline structure and a standard multilayer structure, consisting of alternating microcrystalline and nanocrystalline diamond layers. The results demonstrate that the periodically renucleated diamond film retains all the features of the multilayer structure, including low surface roughness of 311 nm, tensile residual stress of 2.8 GPa, low coefficient of friction of 0.08, and low wear rate of counter body of 2.19 × 10−6 mm3N−1m−1. Moreover, the periodically renucleated diamond film exhibits significantly improved wear resistance, with a 43 % lower wear rate compared to standard multilayer film.

Thermal stabilization enhancement of diamond films via boron doping and its antioxidant mechanism

Diamond films possess numerous extraordinary physical and chemical properties, but their high-temperature applications are limited by its poor thermal stability. Herein, we fabricated the undoped microcrystalline diamond (MCD) film and heavy boron-doped diamond (h-BDD-1, 2.94 × 1021 atoms·cm−3; h-BDD-2, 9.10 × 1021 atoms·cm−3) films to investigate the effect of boron doping on the thermal stability of diamond films. A series of evidences, including air annealing experiments, thermogravimetric-differential scanning calorimetry (TG-DSC) analysis, and in-situ ultra-high-temperature confocal laser-scanning microscope (CLSM), demonstrated that the thermal stability of diamond films was obviously enhanced by boron doping. Consequently, the h-BDD-2 film exhibited the best thermal stability with an initial oxidation temperature above 900 °C, which was much higher than that of MCD film (709 °C). By deconvolving the X-ray photoelectron spectra, we believed that the transition of B–C bonds to stable oxy-boron carbide complexes (B4O–C, B3O–C) is the key to improving the thermal stability. These results not only reveal a new antioxidant mechanism of boron-doped diamond films but also pave a way for the application of diamond in high-temperature environments.

Effect of Ti on the properties of diamond microcrystals synthesized in C–O–H supercritical fluid at high pressure and high temperature

Diamond micropowders 1–10 μm were synthesized at 7.5 GPa, 1300–1500 °C, and 3–6 s. Detonation nanodiamonds and pentaerythritol (C5H8(OH)4) with the addition of Ti were used as the reaction medium. The total nitrogen concentration, determined by electron paramagnetic resonance (EPR) and infrared (IR) spectroscopies, in diamonds synthesized without the addition of Ti was 1700 ppm. In diamonds synthesized with the addition of Ti, there are no 2N (A-center) and N3VH0 centers as well as oxygen-related functional groups, and the total concentration of nitrogen centers is reduced to 640 ppm. The addition of Ti leads to an increase in the synthesis temperature by 100–200 °C.Thermodynamics model calculations of the equilibrium compositions of the reaction medium at a pressure of 7.5. GPa and temperatures of 500, 1000 and 1500 °C were performed. The Gibbs free energy of possible reactions between the components of the reaction medium was calculated. It has been shown that under equilibrium conditions and a reaction medium corresponding to the synthesis conditions, the formation of TiN is thermodynamically possible. This leads to a decrease in the nitrogen concentration in diamonds. However, the formation of TiO2 is a competing reaction that can lead to a decrease in the concentration of oxygen-related functional groups.

Multimethod cross-sectional characterization approaching the limits of diamond monophase multilayers

Chemical vapour deposited diamond draws significant scientific interest due to its wide range of mechanical and functional properties, which can be easily tuned by the applied deposition conditions. Here, hot filament chemical vapour deposition was used to synthesize four monophase multilayer diamond thin films with individual layer thickness down to ∼ 300 nm. Extensive structural characterization by scanning electron microscopy and Raman spectroscopy on the cross-section confirmed the different diamond morphologies originating from the different applied process parameters of the individual sublayers, where Raman spectroscopy could give a clear distinction between nano- and microcrystalline diamond down to a layer thickness of ∼ 1 μm. Cross-sectional X-ray nanodiffraction identified exclusively the diamond crystal structure throughout the cross-section of the multilayered diamond thin films. Cross-sectional phase and FWHM analysis allowed to discriminate between the diamond morphologies down to ∼ 500 nm, thus identifying the limitations of monophase diamond multilayers. While the microcrystalline diamond sublayers all exhibit pronounced structural gradients expressed in (i) increasing intensity of the 111 Debye-Scherrer ring, (ii) ⟨110⟩ fibre texture, and periodic variations of (iii) grain size and (iv) residual stress, the nanocrystalline diamond sublayers showed no pronounced cross-sectional variations. In summary, the experimental results provide insights into the cross-sectional evolution of microstructure and residual stress and the limitations of monophase multilayered diamond thin films.

Corrosion resistance of diamond films with different grain sizes

Titanium alloys are widely used in the aerospace industry due to their excellent comprehensive properties. However, as the application becomes more and more extensive, higher requirements are put forward for the corrosion resistance of titanium alloys. Due to its natural chemical stability, diamond has become a good corrosion-resistant material. Diamond films with three different grain sizes were prepared on the surface of titanium alloy by hot-filament chemical vapor deposition (HFCVD). The protective efficiency of the three kinds of films to the substrate is above 90 %. The corrosion resistance of the nanocrystalline diamond (NCD) films is obviously better than that of the microcrystalline diamond (MCD) films. For NCD films, as the diamond grain size decreases, a cluster structure of particle stacking is formed, which hinders the arrival of corrosive substances to the substrate and further improves the corrosion resistance.

Superior tribological and sealing performance of micro-crystalline diamond coated silicon carbide seals under dry friction and high load

Silicon carbide seals suffer severe wear under the conditions of dry friction and high load, posing a great challenge in achieving a stable performance and a long lifetime. In this work, micro-crystalline diamond coatings were deposited on silicon carbide sealing rings to prevent the failure caused by wear. The deposited micro-crystalline diamond coatings exhibit high-quality crystallinity and strong adhesive strength with the silicon carbide substrate. In sealing tests, the micro-crystalline diamond coated silicon carbide sealing pair demonstrates superior tribological and sealing properties under dry friction and high load of 1800 N, featuring a low coefficient of friction of 0.16, a low wear rate of 4.00 × 10−7 mm3·N−1·m−1 and a high friction limit of 5.24 MPa·m·s−1. Characterizations of morphology, chemical composition and microstructure confirm that the smoothening effect on the contact face and the formation of graphite during the sealing test synergistically contribute to the outstanding tribological and sealing performance.

Turning the optical properties of microcrystalline diamond films by boron ion implantation and annealing

We investigated the correlation between microstructures and optical properties of boron ion implanted microcrystalline diamond (B-MCD) thin films annealed at different temperatures. Ellipsometry characterization of B-MCD films was performed in the wavelength range of 380–1000 nm and the disperse parameters of the films such as single oscillator energy and dispersive energy were uniquely determined by using a Gaussian model. The results show that the refractive index increases from 2.18 to 2.21 to 2.28–2.29 with annealing temperature increasing from 700 to 800 °C and then it slightly decreases to 2.25–2.28 with annealing temperature further increasing to 900 °C, implying that the annealing temperature has a significant impact on the optical properties of B-MCD films. Raman spectra reveal that the full width at half maximum of diamond peak significantly decreases after 800 °C annealing, indicating a better quality of diamond crystalline. X-ray photoelectron spectroscopy results show that this sample contains higher sp3 carbon content, lower sp2 and C–O contents on the film surface after 800 °C annealing, implying that fewer defects exist. The results are of great significance for the preparation of excellent boron ion implanted microcrystalline diamond for waveguide based Mach-Zehnder Interferometer liquid sensing.

Difference in the structure and morphology of CVD diamond films grown on negatively charged and grounded substrate holders: Optical study

Microcrystalline diamond films were grown by plasma-enhanced chemical vapor deposition from a CH4/H2 gas mixture on Si single-crystalline substrates placed on negatively charged and grounded substrate holders. The obtained diamond films had the (100) predominant faceting of microcrystals. The film structure and morphology were analyzed by scanning electron microscopy, photoluminescence, Raman and FTIR spectroscopies. The main physical factor causing the difference in the structure of the diamond films grown on the grounded and charged substrate holders was found to be the flow of low-energy (up to 200 eV) Si+, N2+, H, O ions in the latter holder. These ions predominantly embedded into the structure of the diamond films grown on the charged substrate holder leading to appearance of residual mechanical stress up to 2 GPa. Ion bombardment led to increase in the volume fraction of non-diamond carbon component in the film grain boundaries, decrease in sp3-bonded carbon fraction and reduction of the diamond microcrystals lateral size. Larger amount of grain boundaries in the diamond films grown on the charged substrate holder promoted diffusion of Si atoms from the substrate to the plasma and growing film surface, inducing formation of SiV centers in the diamond microcrystals even in the 150…200 μm thick films. The concentration of Si-related defects was much smaller in the films grown using the grounded substrate holder. These films had substantially smaller volume fraction of graphite-like carbon in the grain boundaries and were more homogeneous.

Nano- and micro-crystalline diamond film structuring with electron beam lithography mask

Direct current plasma enhanced chemical vapor deposition (CVD) was employed to create polycrystalline diamond films from CH4/H2 gaseous mixture at 98 mbar pressure and various substrate temperatures between 720 °C and 960 °C. The Si chips with patterns of periodic masked and open seeded zones were used as substrates. The mask free seeded areas evolved into polycrystalline diamond films after CVD process. The diamond crystallites of the films featured single crystal ordering individually with distinct cubic (100) or octahedral (111) facets on the film surfaces. Notably, specific growth conditions were determined for obtaining diamond films composed of the crystallites of nanometre and micrometre scale. These conditions are differing from those observed for non-pattern-prepared Si substrates. The nano-crystalline diamonds emerged within the 4.5–5 A current range, with growth conditions involving 3% CH4/H2 mixture at 98 mbar. The micro-crystalline diamonds (MCDs) predominantly characterized by well-developed rectangular (100) crystal faces on the film surface were successfully grown with current settings of 5.5–6 A, under 3% CH4/H2 mixture at 98 mbar. Furthermore, MCDs characterized by entirely crystalline (111) diamond faces forming CVD film surface were attained within a growth parameter range of 4.5–5.8 A, employing 3% CH4/H2 mixture for certain samples, or alternatively, utilizing 5 A with a 1.5% CH4/H2 mixture for others. Upon thorough evaluation, it was established that SiO2, TiO2, and Cr masks are well-suited materials for the planar patterning of both nano- and micro-crystalline diamond films, and the bottom-up approach can pave the way for the production of diamond planar structures through CVD, facilitated by electron beam lithography (EBL).

Micromechanical properties of micro- and nanocrystalline CVD diamond thin films with gradient microstructures and stresses

Chemical vapor deposition produced diamonds attract considerable scientific and industrial interest due to their exceptional mechanical and functional properties. Here, hot filament (HF) chemical vapor deposition was used to synthesize two diamond thin films with different cross-sectional microstructure and residual stresses. Structural characterization by scanning electron microscopy, Raman spectroscopy, and cross-sectional synchrotron x-ray nanodiffraction revealed different diamond morphologies. While the microcrystalline diamond film exhibits pronounced microstructural gradients expressed by gradually increasing (i) intensities of the 111 Debye-Scherrer rings, (ii) ⟨110⟩ fiber texture sharpness, (iii) grain size, and (iv) slightly oscillating residual stress at the level of −0.5 GPa, the nanocrystalline diamond film showed no pronounced cross-sectional variation of microstructure above the nucleation zone of ∼0.5 μm and a steady stress level of 0.25 GPa. In situ micromechanical cantilever bending tests revealed highly different mechanical properties of the two films. In detail, Young's modulus of 830 ± 53 and 459 ± 53 GPa, fracture stress of 12.4 ± 0.8 and 7.8 ± 1.0 GPa, and fracture toughness values of 6.9 ± 0.4 and 3.6 ± 0.3 MPa⋅m½ were evaluated for the micro- and nanocrystalline diamond films, respectively. In summary, this study provides valuable insights into the microstructure-residual stress correlation in micro- and nanocrystalline diamond films, especially illuminating their influence on micromechanical properties.

Glancing angle intermediate layer deposition approach for CVD diamond coating of high-speed steel substrates

This work demonstrates a new approach to the synthesis of highly adhesive diamond coatings on high-speed steel (HSS) substrates using a glancing angle deposition (GLAD) technique. The parameters of deposition of a molybdenum intermediate layer with a high level of surface roughness at a minimum layer thickness are determined. A maximum roughness of 38.1 nm with a minimum Mo layer thickness of 2 μm was achieved at an incidence angle of 15° and three sample positions. The duration of Mo sputtering at each rotation was 15, 10, 30 min. It is shown that a 2 μm Mo interlayer deposited by GLAD technique increases microcrystalline diamond film adhesion to HSS substrate compared to normally deposited Mo interlayer of the same thickness. The average delimitation area of the diamond film synthesized on a normally deposited molybdenum layer exceeds the corresponding parameter for the coating synthesized on the GLAD Mo layer by more than 8 times. Additionally, the process of diamond-Mo interlayer mechanical interlocking is also considered.

Deposition and Characterisation of a Diamond/Ti/Diamond Multilayer Structure

In this work, a diamond/Ti/diamond multilayer structure has been fabricated by successively following thin-film CVD and PVD routes. It has been found that a combined pre-treatment of the silicon base substrate, via argon plasma etching for creating surface roughness and, thereafter, detonation nanodiamond (DND) seeding, helps in the nucleation and growth of well-adherent CVD diamond films with a well-defined Raman signal at 1332 cm−1, showing the crystalline nature of the film. Ti sputtering on such a CVD-grown diamond surface leads to an imprinted bead-like microstructure of the titanium film, generated from the underlying diamond layer. The cross-sectional thickness of the titanium layer can be found to vary by as much as 0.5 µm across the length of the surface, which was caused by a subsequent hydrogen plasma etching process step of the composite film conducted after Ti sputtering. The hydrogen plasma etching of the Ti–diamond composite film was found to be essential for smoothening the uneven as-grown texture of the films, which was developed due to the unequal growth of the microcrystalline diamond columns. Such hydrogen plasma surface treatment helped further the nucleation and growth of a nanocrystalline diamond film as the top layer, which was deposited following a similar CVD route to that used in depositing the bottom diamond layer, albeit with different process parameters. For the latter, a hydrogen gas diluted with PH3 precursor recipe produced smaller nanocrystalline diamond crystals for the top layer. The titanium layer in between the two diamond layers possesses a very-fine-grained microstructure. Transmission electron microscopy (TEM) results show evidence of intermixing between the titanium and diamond layers at their respective interfaces. The thin films in the composite multilayer follow the contour of the plasma-etched silicon substrate and are thus useful in producing continuous protective coatings on 3D objects—a requirement for many engineering applications.

A new route to fabricate high-quality double-side-polished freestanding diamond film

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.

Study on tribology and cutting performance of boron doped diamond composite coated tool

Microcrystalline diamond (MCD), nanocrystalline diamond (NCD), boron doped microcrystalline diamond (BDMCD), and boron doped microcrystalline-nanocrystalline diamond (BDM-NCD) films were prepared by hot filament chemical vapor deposition (HFCVD) technique. Scanning electron microscopy (SEM), Raman spectroscopy and Rockwell indention testes were used to characterize the microstructure and properties of the coatings, and friction experiments were carried out to study the tribological properties of diamond films and GCr15 steel balls under different lubrication conditions. At the same time, the cutting performance of different diamond coated tools was studied by cutting 30% SiCp/Al composites. The results show that the residual compressive stress of the NCD film is the largest, which is −1.88 GPa. Due to the mitigation effect of the underlying BDMCD film, the BDM-NCD film has the smallest residual stress, which is −0.37 GPa. It can be seen from the friction test that the average coefficient of friction of NCD film is lower than that of MCD film due to the smooth surface. The average friction coefficient of BDM-NCD film is the lowest, which is 0.326. At the same time, the wear resistance of BDM-NCD film is the best, and the value is 0.53×10−6 mm3/Nmm. From the wear morphology, it can be seen that the amount of metastatic material adhered to the wear surface of the diamond films is less under water lubrication than under dry friction condition. Compared with dry friction, the wear scar depth of BDM-NCD film under water lubrication is smaller, showing the better tribological properties. Cutting experiments show that BDM-NCD film has the best wear resistance and the best cutting performance.

Postannealing Temperature Effects on the Etching Behavior and Field Emission Properties of Cu‐Implanted Diamond

To investigate the postannealing temperature effects on the surface etching behavior and electron field emission (EFE) properties of the Cu‐implanted microcrystalline diamond (MCD) films, they are subjected to furnace annealing in a temperature range of 600–900 °C under N2 atmosphere at a rapid annealing rate. The etching behavior happens when the samples are annealed at 700 °C and above. The EFE performances enhance first and then decrease with the increasing annealing temperature. The optimum EFE property with a low turn‐on field of 6.42 V μm−1 is obtained after the Cu‐implanted MCD film is annealed at 700 °C. The enhancement in EFE properties originates from the presence of nanographitic phase and the formation of electron‐rich Cu nanoparticles, which encourage a higher efficient electron transport. In contrast, the structural studies reveal that when the samples annealed at 800 °C and above, Cu nanoparticles evaporate from the diamond surface and the sp2‐bonded carbon decomposes. Consequently, etch pits appear on the surface of the MCD sample. Moreover, it demonstrates that graphitization is the dominant mechanism of the diamond surface etching behavior and the enhancing of EFE properties.

A competitive-adsorption chemical vapor deposition method: Diamond coated on WC-Co substrate without acid-alkali pretreatment by adding graphene oxide particles

Detrimental components existed in substrate surface are regarded as a severe barrier of growing the chemical vapor deposition (CVD) coating. In this work, a novel competitive-adsorption CVD method was proposed to deposit the diamond coating on Co-cemented tungsten carbide (WC–Co) substrate without traditional acid-alkali pretreatment. Graphene oxide (GO) particles were adopted as the adsorbent to introduce the competitive adsorption with Co element during deposition. Four different GO concentrations were conducted to evaluate the effect of GO particles on the coating. The results shown that the WC-Co substrate with GO particles and without pretreatment can be covered by the integrated microcrystalline diamond (MCD) coating (GO-MCD) that presented the similar surface morphology to the MCD coated on WC-Co substrate with acid-alkali pretreatment (P-MCD). Furthermore, Raman spectra, AFM, XPS and XRD patterns only presented a slight difference between the GO-MCD coating and the P-MCD coating. Indentation test suggested that GO particles can enhance the adhesion strength and crack resistance of diamond coating. Meanwhile, GO particles did not deteriorate the tribological property of diamond coating. Hence, our proposed method might provide a promising approach to deposit the desired coating on the given substrate even with some detrimental components.

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.

Effect of the interfacial microstructure on hardness of multi-layer diamond coatings

Multi-layer diamond coatings have been widely employed in cutting tools due to their superior mechanical performance, which mainly stems from their complex multi-layer structures and versatile interfaces. However, it is still lack of investigation on how these interfaces may be tuned to enhance the mechanical properties. Herein, the multi-layer diamond coatings with different modulation periods (ʌ = 1.70, 0.87, 0.29 μm), containing microcrystalline diamond (MCD), sub-microcrystalline diamond (SMCD) and nanocrystalline diamond (NCD) as sub-layers, were synthetized by hot filament chemical vapor deposition (HFCVD). The interfacial microstructures were characterized by scanning transmission electron microscopy (STEM). The hardness (H) and Young's modulus (E) of the coatings were evaluated by nanoindentation tests with two different modes. The planar defects including stacking faults (SFs) and twinning boundaries (TBs) are observed at both MCD/SMCD and NCD/MCD interfaces but absent at SMCD/NCD interfaces. Owing to more interfaces, SFs and TBs as barriers inhibiting the dislocation motion effectively, the H and E of the coatings are enhanced with the decrease of ʌ. Hence, the coating with ʌ of 0.29 μm, possessing more SFs, TBs and more sharp interfaces, obtains the highest hardness (78.39 ± 1.10 GPa). In summary, the mechanical properties of the multi-layer diamond coatings can be greatly improved by the elaborately designed interfacial microstructure, which is of great significance to promote the far-ranging application of diamond coatings.

Primary irradiation damages and tribological property evolutions of heavy-ion radiated microcrystalline diamond films grown by MPCVD

Primary irradiation damages and tribological property evolutions of heavy-ion radiated microcrystalline diamond films grown by MPCVD