Synthesis of nanocrystalline diamond films using microwave plasma CVD

Diamond is one of the most valuable materials for the industrial applications because of its excellent properties including high hardness, with good electrical insulation and thermal conductivity. Mechanical polishing processes of diamond are difficult and very costly. To limit those costs, it is reasonable to think that the surface roughness of the as-grown diamond film should be as small as possible. In this study, a nanocrystalline diamond film was synthesized on a 4-inch Si wafer at 923 K and methane concentration of 10 vol.%, (H₂/CH₄=100/10 sccm) using a microwave plasma CVD system. In order to increase the nucleation density, the substrate was pretreated by dry scratch method with diamond powder of two sizes (250 nm and 5 nm). The nucleation density was approximately 1×10¹¹ cm⁻². The grown diamond films were analyzed by Raman spectroscopy and X-ray diffraction (XRD). The grain size was observed to be approximately 10 nm by FE-SEM observation. Surface roughness was measured as Rms=8.4 nm by atomic force microscope (AFM). The as-grown properties of those nanocrystalline diamond films were almost efficient for tribological and the optical applications.     

Boron doped diamond deposited by microwave plasma-assisted CVD at low and high pressures

Boron doped diamond is deposited over a range of pressures and chemistries including pressures from 35–120 Torr and gas chemistries including hydrogen–methane–diborane and argon–methane–hydrogen–diborane mixtures. The diamond deposition system is a 2.45 GHz microwave resonant cavity system. Diborane (B₂H₆) gas chemistry has been utilized with flow rates of 2.5–100 ppm. At low pressures of 35 Torr polycrystalline films are deposited using a feed gas mixture of hydrogen and 0.5% methane. At moderate pressures of 95 Torr, diamond films are grown using 60% Ar, 39% H₂ and 1% CH₄. For the high pressure experiments of 120 Torr, polycrystalline films are deposited using 98% H₂ and 2% CH₄. The deposition rate ranges from 0.3 to 1.6 μm/h. This investigation describes the relationship of the diborane flow rate and pressure versus the resulting film morphology, electrical properties, and morphology of the deposited films. The deposition of boron-doped polycrystalline diamond is done on 5 cm diameter silicon and silicon dioxide coated substrates. The resistivity spatial variation across the wafer was ± 5% indicating a good uniformity.     

Tribological properties of partly polished diamond coatings

Extremely low friction coefficient was achieved with “partly polished diamond coatings”. Diamond coatings were deposited onto Si substrates by MWCVD with the mixture of CH₄ and H₂. Deposited films were characterized by X-ray diffraction (XRD), Raman spectroscopy and Electron Spectroscopy for Chemical Analysis (ESCA). Sharp peak derived from polycrystalline diamond was observed by XRD. Whereas Raman profile of partly polished diamond coatings was close to that of ta-C. This result suggests that small diamond grains were surrounded by amorphous carbon structure in the diamond coatings. Deposited diamond coating was polished with each other. Surface roughness R_a was reduced to 0.3, 0.2 and 0.08 μm, respectively. The hardness of the polished diamond coatings investigated by Nanoindentation technique was approximately 40.8 GPa, which was relatively lower value compared with conventional as-deposited CVD diamond coatings. For the tribological properties, we examined the effect of surface roughness using flat-ended pin-on-disk apparatus and ball-on-disk apparatus with bearing ball (SUJ2) and stainless steel (SUS304). Diamond coatings were deposited onto flat-ended pin and disk, and they were polished to R_a = 0.3, 0.2 and 0.08 μm. After the 6000 cycle process extremely low friction coefficient, μ = 0.05, was achieved with the pair of R_a (flat-ended pin, disk) = R_a (0.08, 0.3) in flat-ended pin-on-disk apparatus. In order to clarify the effect of surface roughness, ball-on-disk was carried out with different surface roughness, R_a = 1.7, 0.3, 0.2 and 0.08 μm. Here as-deposited diamond coating, R_a = 1.7 μm, was used as a reference point. Friction coefficient of μ = 0.09 was obtained for both balls. After the tribological tests balls were analyzed by scanning electron microscope (SEM) and energy dispersed X-ray spectrometer (EDX).     

Graphitization effects of CH4 addition on NCD growth by first and second Raman spectra and by X-ray diffraction measurements

Nanocrystalline diamond (NCD) films were grown on silicon substrates by hot filament chemical vapour deposition in Ar/H₂/CH₄ gas mixtures. In the current study, the methane volume concentration varied from 0.5 to 3.5 vol.% in order to estimate its effect on nanodiamond morphology and structure. Film micrograph obtained from scanning electron microscopy showed diamond grain agglomerate, also called ballas diamond, which presented the grain size variation as a function of methane concentration increase. The transition from diamond agglomerate to graphite structure was also observed when CH₄ concentration is higher than 2.0 vol.%, confirmed from second order Raman measurements. The film local stress was estimated from the G-peak shift analyses and showed critical values necessary for the graphite/diamond phase formation. Structural investigations carried out by X-ray diffraction measurements for films deposited from 0.5 up to 2.0 vol.% CH₄ presented characteristic diamond diffraction peaks corresponding to 〈111〉, 〈220〉 and 〈311〉. A preferential orientation, changed from 〈110〉 to 〈111〉, was observed during NCD film deposition as a function of the methane concentration.     

NEXAFS characterization of nanocrystalline diamond thin films synthesized with high methane concentrations

Diamond thin films were deposited on silicon in gas mixtures of methane and hydrogen with different methane concentrations ranging from 1% to 100% using microwave plasma assisted chemical vapor deposition. Both Raman spectroscopy and synchrotron near edge extended X-ray absorption fine structure spectroscopy (NEXAFS) were used to characterize the electronic structure and chemical bonding of the synthesized films. The NEXAFS spectra of the nanocrystalline diamond (NCD) films exhibit clear spectral characteristics of diamond. Close observation reveals that the films (10% CH₄ or above) exhibit a slightly broadened exciton transition with a 0.25 eV blue shift. With the increase in methane concentration, the growth rate, the surface smoothness, and the sp² carbon concentration of the films increase while the grain size decreases. Well-faceted microcrystalline diamond films were synthesized with a methane concentration of 5% or lower, while NCD films were formed with a methane concentration of 10% or higher. Diamond thin films with low surface roughness and fine nanocrystalline structure have been synthesized with high methane concentrations (50% or above). It has been observed that the diamond growth rate increases with methane concentration. The growth rate at 100% methane concentration is approximately 10 times higher than at 1%.     

Microwave plasma chemical vapor deposition of cone-like structure of diamond/SiC/Si on Si (100)

Diamond deposition on 1 × 1 cm² Si (100) substrates with bias was carried out by microwave plasma chemical vapor deposition (MPCVD). Distribution of deposited diamonds has been significantly improved in uniformity over all the Si substrate surface area by using a novel designed dome-shaped Mo anode. The deposits were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman analysis. SEM observations show that there is a high density of cone-like particles uniformly deposited on the surface of the substrate in short bias nucleation period. The average diameter, height and density of cone-like structure were increased with methane concentration in the bias stage. TEM reveals that the cone-like structure is actually composed of Si conic crystal covered with diamond. Between Si and diamond, a thin layer of cubic SiC is found in epitaxy with Si. Furthermore, for 3% CH₄ concentration, the range of diameter of cone-like structure was about 20–90 nm and the size of diamond was about 10–60 nm.     

Hybrid diamond/graphite films: Morphological evolution,microstructure and tribological properties

Diamond films were deposited on silicon substrate by microwave plasma enhanced chemical vapor deposition (MPCVD) using H₂ and CH₄ gas mixture. The morphological evolution process was characterized systematically by means of field-emission scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. Special attention has been paid to the influence of the methane concentrations on the microstructures of diamond films, which shows a gradual transition from nanocrystalline to microcrystalline films, and finally displays a hybrid diamond-graphite nanostructure with the length of a few micrometers. Finally, the friction behaviour of the hybrid films was studied. The value of the friction coefficient of the hybrid films is 0.10 and the corresponding wear resistance is below 1.9 × 10^− 7 mm³/N·m in diamond composites/Al₂O₃ sliding system in ambient atmosphere under dry sliding conditions. It is a convenient path to synthesize hybrid diamond/graphite nanostructures by MPCVD depending on higher methane concentrations in the absence of nitrogen or argon. The structure is appropriate for the potential applications as high efficient mechanical tools.     

Comparative study of titanium carbide films deposited by plasma-enhanced and conventional magnetron sputtering at various methane flow rates

Titanium carbide (TiC_x) films were deposited by reactively sputtering Ti target with varied methane (CH₄) flow rates using an unbalanced magnetron sputtering equipment operating in conventional (CMS) and plasma-enhanced (PEMS) mode. The microstructure and properties of the TiC_x films were mainly scrutinized with X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, nano indentation, and ball-on-disk tribo-test. As the CH₄ flow rate is increased from 4 to 16 sccm, x in the TiC_x films deposited by PEMS and CMS is gradually increased from 0.28 to 0.96 and 0.28 to 0.73, respectively; and the concentration of TiC phase in the films rises. A densified microstructure with strong preferred growth is observed in the PEMS-deposited films, while a loose columnar microstructure with almost random growth exists in the CMS-deposited films. When the CH₄ flow rate is increased, the hardness of the films deposited by PEMS is firstly increased and then decreased, and reaches a maximum value of 29.6 GPa; but that of CMS-TiC_x film is fluctuated below 10.6 GPa. The friction coefficient of TiC_x-coated samples against pure aluminum is gradually increased with increasing CH₄ flow rate from 4 to 14 sccm, accompanied by a reduction of the aluminum adhesion. The wear rate of the TiC_x-coated samples as a function of the CH₄ flow rate is firstly decreased and then increased. The lowest wear rate of the PEMS-prepared samples is 7.2 × 10⁻¹⁶ m³/(N·m) obtained at a CH₄ flow rate of 14 sccm, while that of the CMS-prepared samples is 9.3 × 10⁻¹⁵ m³/(N·m) obtained at 12 sccm. Therefore, PEMS is a promising method to prepare TiC_x films with a good combination of the mechanical properties and wear-resistance against pure aluminum.     

Mechanical properties of MWPECVD diamond coatings on Si substrate via nanoindentation

The mechanical properties of polycrystalline diamond coatings with thickness varying from 0.92 to 44.65 μm have been analysed. The tested samples have been grown on silicon substrates via microwave plasma enhanced chemical vapour deposition from highly diluted gas mixtures CH₄-H₂ (1% CH₄ in H₂). Reliable hardness and elastic modulus values have been assessed on lightly polished surface of polycrystalline diamond films.The effect of the coating thickness on mechanical, morphological and chemical-structural properties is presented and discussed. In particular, the hardness increases from a value of about 52 to 95 GPa and the elastic modulus from 438 to 768 GPa by varying the coating thickness from 0.92 to 4.85 μm, while the values closer to those of natural diamond (H = 103 GPa and E = 1200 GPa) are reached for thicker films (> 5 μm). Additionally, the different thickness of the diamond coatings permits to select the significance of results and to highlight when the soft silicon substrate may affect the measured mechanical data. Thus, the nanoindentation experiments were made within the range from 0.65% to 10% of the film thickness by varying the maximum load from 3 to 80 mN.     

Mechanical characterization of ultra-thin fluorocarbon films deposited by R.F. magnetron sputtering

Fluorocarbon films were deposited on silicon substrate by R.F. magnetron sputtering using a polytetrafluoroethylene (PTFE) target. Structure of the deposited films was studied by X-ray photoelectron spectroscopy (XPS). Hardness, elastic modulus and scratch resistance were measured using a nanoindenter with scratch capability. -CF_x (x = 1, 2, 3) and C-C units were found in the deposited fluorocarbon films. The hardness and elastic modulus of the films are strongly dependent on the R.F. power and deposition pressure. The film hardness is in the range from 0.8 GPa to 1.3 GPa while the film elastic modulus is in the range from 8 GPa to 18 GPa. Harder films exhibit higher scratch resistance. Differences in nanoindentation behavior between the deposited fluorocarbon films, diamond-like carbon (DLC) films and PTFE were discussed. The fluorocarbon films should find more applications in the magnetic storage and micro/nanoelectromechanical systems.     

Influence of the substrate nature on the properties of nanocrystalline diamond films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films have been deposited by microwave plasma CVD from CH₄/N₂ mixtures on a variety of substrates such as polycrystalline diamond, cubic boron nitride, silicon, titanium nitride, and Ti–6Al–4V. The study aimed to investigate the influence of the chemical nature of the substrate, the surface roughness, and the pretreatment of the substrate on the nucleation, the bulk structure, and the mechanical and tribological properties of the NCD/a-C films. The present paper is especially devoted to the bulk structure of the films. By means of X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) it is shown that the bulk properties of the films are not affected by the properties of the substrate although these have a strong influence on the nucleation behaviour. XRD measurements show that – irrespective of the substrate used – the films contain diamond nanocrystallites of 3–5 nm diameter. From the Raman spectra it can be inferred that the crystallite/matrix ratio does not vary. The XPS measurements, finally, show that there are no great changes in the sp²/sp³ ratio of the matrix. These findings are discussed in view of possible growth mechanisms of NCD/a-C nanocomposite films.     

Effects of nitrogen addition on morphology and mechanical property of DC arc plasma jet CVD diamond films

Nitrogen-doped diamond films have been synthesized by 100 KW DC arc plasma jet chemical vapor deposition using a CH₄/Ar/H₂ gas mixture. The effect of nitrogen addition into the feed gases on the growth and surface morphology and mechanical property of diamond film was investigated. The reactant gas composition was determined by the gas flow rates. At a constant flow rate of hydrogen (5000 sccm) and methane (100 sccm), the nitrogen to carbon ratio (N/C) were varied from 0.06 to 0.68. The films were grown under a constant pressure (4 KPa) and a constant substrate temperature (1073 K). The deposited films were characterized by scanning electron microscopy, Raman spectroscopy and X-ray diffraction. The fracture strength of diamond films was tested by three point bending method. The results have shown that nitrogen addition to CH₄/H₂/Ar mixtures had led to a significant change of film morphology, growth rate, crystalline orientation, nucleation density and fracture strength for free-standing diamond films prepared by DC arc plasma jet.     

Deposition of cBN films by ion sputtering of SiB6 and AlB12 boron-base materials

Films of cubic boron nitride were deposited on to Mo, Si, WC–Co substrates heated up to 600–800 °C in a three-electrode d.c. device with a discharge localization by magnetic field. During SiB₆ and AlB₁₂ sputtering in Ar–N₂ and Ar–N₂–H₂ atmospheres, the low-frequency (LF) substrate bias ranged from 50 to 500 V (20 kHz), and the total pressure was 1.5 mTorr. The effects of gas atmosphere, LF substrate bias and ionization energy on structure formation and composition of the films being deposited have been studied. The film structure and composition were examined using IR spectroscopy and X-ray diffraction analysis. The deposited films have polycrystal structures with a hexagonal, wurtzitic or cubic symmetry. The predominated quantity of cubic phase is observed in films deposited using SiB₆ in Ar–N₂ atmosphere at a LF substrate bias of 500 V. By introducing hydrogen into the gas atmosphere at a ratio of three parts of N₂ to seven parts of H₂, a wurtzitic phase forms in the film. The cubic BN phase is observed in films obtained using AlB₁₂ if the LF substrate bias is between 200 and 300 V. The Vickers microhardness of the films produced reaches 30.0–35.0 GPa.     

Alumina atomic layer deposition nanocoatings on primary diamond particles using a fluidized bed reactor

Ultra-thin alumina films are successfully deposited on primary micron-sized diamond particles in a scalable fluidized bed reactor. The studies of fluidization at reduced pressure show that micron-sized diamond particles can be fluidized with the assistance of vibration. Alumina films are grown at 177 °C by atomic layer deposition (ALD) using sequential exposures of Al(CH₃)₃ and H₂O. The deposited alumina films are characterized by X-ray photoelectron spectroscopy, transmission and scanning electron microscopy, inductively coupled plasma-atomic emission spectroscopy, and surface area. The results indicate that the alumina films are conformally coated on the primary diamond particle surface, and the growth rate of alumina is 0.12 nm per coating cycle.     

Sputtered DLC-TiB2 multilayer films for tribological applications

This study focuses upon the deposition of diamond-like carbon thin films for tribological applications. Carbon consists of mainly sp² bonds, having a low Hardness and low coefficient of friction. By depositing carbon in a methane (CH₄)-rich atmosphere, the energetics of the process favours the formation of diamond-like bonds, namely sp³. To improve the tribological properties of a multilayer system, a refractory metal ceramic TiB₂, has been multilayered with carbon and deposited on titanium (Ti) substrates to a total thickness of 3 μm. A multilayer stack of 10 bi-layers has been deposited, and the volume fraction of carbon in the coating has been varied by changing the thickness of the individual layers. This study employs pulsed-dc sputtering combined with Ar 7.5% CH₄ to deposit both carbon and TiB₂.The Raman spectroscopy data shows that the carbon deposited was amorphous in nature with an I_D / I_G ratio of 1.25. TiB₂ sputtered in Ar 7.5% CH₄ formed Ti-B-C, containing both a hard TiB₂ phase and a lubricating diamond-like carbon phase.Three volume fractions of carbon have been investigated: 25%, 50% and 75%. From nanoindentation studies, the Hardness varies from 5 to 3 GPa for the 75% and 25% carbon-containing coatings, respectively, measured at a penetration depth of approximately one-third of the coating. This increase in Hardness as a function of percentage of carbon has been attributed to the coatings forming load-bearing properties.From wear studies, friction coefficients of around 0.3 have been measured. Thus, these multi-layered TiB₂/C coatings are both load bearing and lubricious.     

Investigation in the role of hydrogen on the properties of diamond films grown using Ar/H2/CH4 microwave plasma

The transition of diamond grain sizes from micron- to nano- and then to ultranano-size could be observed when hydrogen concentration is being decreased in the Ar/CH₄ plasma. When grown in H₂-rich plasma (H₂ = 99% or 50%), well faceted microcrystalline diamond (MCD) surface with grain sizes of less than 0.1 μm are observed. The surface structure of the diamond film changes to a cauliflower-like geometry with a grain size of around 20 nm for the films grown in 25% H₂-plasma. In the Ar/CH₄ plasma, ultrananocrystalline diamond (UNCD) films are produced with equi-axed geometry with a grain size of 5-10 nm. The H₂-content imposes a more striking effect on the granular structure of diamond films than the substrate temperature. The induction of the grain growth process, either by using H₂-rich plasma or a higher substrate temperature increases the turn-on field in the electron field emission process, which is ascribed to the reduction in the proportion of grain boundaries.     

Tribological properties and cutting performance of boron and silicon doped diamond films on Co-cemented tungsten carbide inserts

Boron and silicon doped diamond films are deposited on the cobalt cemented tungsten carbide (WC-Co) substrate by using a bias-enhanced hot filament chemical vapor deposition (HFCVD) apparatus. Acetone, hydrogen gas, trimethyl borate (C₃H₉BO₃) and tetraethoxysilane (C₈H₂₀O₄Si) are used as source materials. The tribological properties of boron-doped (B-doped), silicon-doped (Si-doped) diamond films are examined by using a ball-on-plate type rotating tribometer with silicon nitride ceramic as the counterpart in ambient air. To evaluate the cutting performance, comparative cutting tests are conducted using as-received WC-Co, undoped and doped diamond coated inserts, with high silicon aluminum alloy materials as the workpiece. Friction tests suggest that the Si-doped diamond films present the lowest friction coefficient and wear rate among all tested diamond films because of its diamond grain refinement effect. The B-doped diamond films exhibit a larger grain size and a rougher surface but a lower friction coefficient than that of undoped ones. The average friction coefficient of Si-doped, B-doped and undoped diamond films in stable regime is 0.143, 0.193 and 0.233, respectively. The cutting results demonstrate that boron doping can improve the wear resistance of diamond films and the adhesive strength of diamond films to the substrates. Si-doped diamond coated inserts show relatively poor cutting performance than undoped ones due to its thinner film thickness. B-doped and Si-doped diamond films may have tremendous potential for mechanical application.     

Enhanced sealing performance with CVD nanocrystalline diamond films in self-mated mechanical seals

Nanocrystalline diamond (NCD) films were evaluated as protective tribo-coatings on silicon nitride mechanical seal rings. The NCD films were deposited by microwave plasma assisted chemical vapour deposition (MPCVD) method from a CH₄/H₂/N₂ gas mixture. The sealing performance and friction behaviour of self-mated NCD films were assessed using the ring-on-ring tribological test in planar configuration varying the rotating speed and the applied load. Water sealing conditions were obtained in the P · V (P, the effective pressure and V, the linear speed) range of 0.5–4.8 MPa ms^− 1. The high hardness and smoothness of the NCD films resulted in a very low and stable friction coefficient value of 0.01, without any measurable wear.     

Growth of microcrystalline and nanocrystalline diamond films by microwave plasmas in a gas mixture of 1% methane/5% hydrogen/94% argon

Well-faceted microcrystalline diamond (MCD) films were deposited along with nanocrystalline diamond (NCD) films on the same substrate by a microwave plasma in the gas mixture of 1% CH₄+5% H₂+94% Ar. This was achieved by forcing a microwave plasma ball generated at 170 torr gas pressure to touch a silicon substrate that was pre-seeded by nanocrystalline diamond powder resulting in a high concentration of atomic hydrogen on the surface of growing diamond. Previously reported compositional mapping of the argon–methane–hydrogen system for MCD and NCD growth was not valid in this process parameter space. The non-uniform concentrations of atomic hydrogen and carbon containing radicals such as C₂ as well as varied local substrate temperature resulted in the simultaneous deposition of well-faceted MCD films in some areas with nanograined NCD films in others. Dilution of methane/hydrogen microwave plasmas by as much as 94% of argon alone could not suppress the growth of MCD.     

Synthesis and mechanical wear studies of ultra smooth nanostructured diamond (USND) coatings deposited by microwave plasma chemical vapor deposition with He/H2/CH4/N2 mixtures

Ultra smooth nanostructured diamond (USND) coatings were deposited by microwave plasma chemical vapor deposition (MPCVD) technique using He/H₂/CH₄/N₂ gas mixture. The RMS surface roughness as low as 4 nm (2 micron square area) and grain size of 5–6 nm diamond coatings were achieved on medical grade titanium alloy. Previously it was demonstrated that the C₂ species in the plasma is responsible for the production of nanocrystalline diamond coatings in the Ar/H₂/CH₄ gas mixture. In this work we have found that CN species is responsible for the production of USND coatings in He/H₂/CH₄/N₂ plasma. It was found that diamond coatings deposited with higher CN species concentration (normalized by Balmer H_α line) in the plasma produced smoother and highly nanostructured diamond coatings. The correlation between CN/H_α ratios with the coating roughness and grain size were also confirmed with different set of gas flows/ plasma parameters. It is suggested that the presence of CN species could be responsible for producing nanocrystallinity in the growth of USND coatings using He/H₂/CH₄/N₂ gas mixture. The RMS roughness of 4 nm and grain size of 5–6 nm were calculated from the deposited diamond coatings using the gas mixture which produced the highest CN/H_α species in the plasma. Wear tests were performed on the OrthoPOD®, a six station pin-on-disk apparatus with ultra-high molecular weight polyethylene (UHMWPE) pins articulating on USND disks and CoCrMo alloy disk. Wear of the UHMWPE was found to be lower for the polyethylene on USND than that of polyethylene on CoCrMo alloy.