Selective growth of diamond and carbon nanostructures by hot filament chemical vapor deposition

Diamond and carbon nanostructures have been synthesized selectively on differently pretreated silicon substrates by hot filament chemical vapor deposition in a CH₄/H₂ gas mixture. Under typical conditions for CVD diamond deposition, carbon nanotube and diamond films have been selectively grown on nickel coated and diamond powder scratched silicon surface, respectively. By initiating a DC glow discharge between the filament and the substrate holder (cathode), well aligned carbon nanotube and nanocone films have been selectively synthesized on nickel coated and uncoated silicon substrates, respectively. By patterning the nickel film on silicon substrate, pattern growth of diamond and nanotubes has been successfully achieved.     

Growth and adhesion of hot filament chemical vapor deposited diamond coatings on surface modified high speed steel

Diamond coatings were deposited by hot filament chemical vapor deposition on high speed steel substrates. Iron boride diffusion barrier and WC–Co layers were used as interlayers. At high deposition pressure, the quality of the diamond deposits is poor due to the extensive formation of graphitic deposits. At low pressure, diamond films of better quality were obtained, but their adhesion to the substrate was insufficient. A two-step deposition process at low pressure was developed. In a first deposition step performed at high methane percentage, a high nucleation density was achieved. In a second deposition step, the methane percentage was reduced to achieve continuous, dense, and adherent diamond layers on borided or WC–Co coated high speed steel substrates. Adhesion of these diamond layers on the surface modified high speed steel substrates was tested based on reciprocating sliding tests.     

Hot filament chemical vapour deposition and wear resistance of diamond films on WC-Co substrates coated using PVD-arc deposition technique

Different Cr- and Ti-base films were deposited using PVD-arc deposition onto WC-Co substrates, and multilayered coatings were obtained from the superimposition of diamond coatings, deposited on the PVD interlayer using hot filament chemical vapour deposition (HFCVD). The behaviour of PVD-arc deposited CrN and CrC interlayers between diamond and WC-Co substrates was studied and compared to TiN, TiC, and Ti(C,N) interlayers. Tribological tests with alternative sliding motion were carried out to check the multilayer (PVD + diamond) film adhesion on WC-Co substrate. Multilayer films obtained using PVD arc, characterised by large surface droplets, demonstrated good wear resistance, while diamond deposited on smooth PVD TiN films was not adherent. Multilayered Ti(C,N) + diamond film samples generally showed poor wear resistance.Diamond adhesion on Cr-based PVD coatings deposited on WC-Co substrate was good. In particular, CrN interlayers improved diamond film properties and 6 μm-thick diamond films deposited on CrN showed excellent wear behaviour characterised by the absence of measurable wear volume after sling tests. Good diamond adhesion on Cr-based PVD films has been attributed to chromium carbide formation on PVD film surfaces during the CVD process.     

High rate of diamond deposition through graphite etching in a hot filament CVD reactor

Although a hot filament diamond CVD reactor has many advantages over other processes, the relatively low growth rate of the films has been a crucial drawback. We developed a new hot filament process that uses no hydrocarbon gas for diamond deposition. The graphite plate was placed below the silicon substrate and only hydrogen was supplied during the process. We could achieve the growth rate of 9 μm/h, which is approximately 9 times higher than that of conventional hot filament CVD using a gas mixture of methane and hydrogen. In spite of the high growth rate, the quality of diamond films was not degraded. Besides, the diamond films consisted of small crystallites with a smooth surface while the conventional diamond films of the same thickness tend to have a columnar structure with a rough surface.     

Deposition of diamond/β-SiC composite gradient films by HFCVD: A competitive growth process

Surfaces featuring gradients of chemical composition and/or morphology allow high-throughput investigations and systematic studies in disciplines such as physics, chemistry, materials science, and biology. In this work, novel diamond/β-SiC composite films exhibiting a gradient composition were synthesized by a hot filament chemical vapor deposition (HFCVD) technique utilizing H₂, CH₄, and tetramethylsilane (TMS) as reaction gases. A specific filament-sample arrangement in the HFCVD chamber induced a gradation in chemical composition of the gas phase above the substrate surface, which, in turn, leads to a gradual change in the composition of the deposited films potentially ranging from pure diamond to pure β-SiC. It was possible to control the actual details of the diamond/β-SiC ratio in the gradient films by adjusting deposition pressure and TMS concentration. Aside from film characterization by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy were employed to determine the presence and quality of both diamond and β-SiC phases, respectively. The wealth of information provided by such diamond/β-SiC composite films allowed for a systematic investigation of the mechanism governing their growth. It turned out, that the growth process features nonequilibrium characteristics. It is dominated by a competition between a kinetic product (diamond) and kind of a thermodynamic product (β-SiC) to occupy any available positions on the substrate and the growing surface, respectively. With higher hydrogen radical concentration [H] and substrate temperature, the deposition is kinetically controlled, leading to diamond dominated films. On the other hand, a lower [H] and substrate temperature, consequently resulted in a predominantly thermodynamically controlled deposition, featuring a higher β-SiC content in the film.     

Selective synthesis of diamond and CNT nanostructures directly on stainless steel substrates

Without surface pretreatment or applying additional interlayer, diamond films have been directly synthesized on an Fe-25Cr-5Al steel substrate by a hot filament chemical vapor deposition method from an H₂-1vol.% CH₄ gas mixture. Due to an effective removal of intermediate graphite phase from the diamond-steel interface, the coated diamond films were continuous and adherent well to the steel substrate. Aligned conical diamond structures were also achieved on this steel substrate by negatively biasing the substrate holder and inducing a glow discharge. The deposition behavior of carbon on Fe-Cr-Ni steel substrate was different. A graphite-rich carbon film incorporated with diamond particles grew in the absence of biasing, then aligned carbon nanotube bundles were formed in the presence of negative biasing and glow discharge. The different deposition behavior of carbon on the two kinds of steel substrates was addressed in terms of the effect of their chemical compositions.     

Analysis of optical emission spectroscopy in diamond chemical vapor deposition

We used optical emission spectroscopy (OES) to study the gas phase chemistry in hot filament chemical vapor deposition (HFCVD) diamond processes. The results show that the methane concentration strongly influenced the intensity ratios of CH, CH⁺ and H_γ to H_β, and the effects of the pressure and filament temperature on the relative concentrations of the species were also analyzed. Spatially resolved OES implied that a relative high concentration of atomic H existed near the substrate surface, which is favorable for diamond film growth. Although the relatively high concentrations of CH and CH⁺ to atomic H are beneficial to increasing diamond nucleation density, they are very harmful to the growth of diamond films.     

Interplay between adhesion and interfacial properties of diamond films deposited on WC-10%Co substrates using a CrN interlayer

In this study, the effect of deposition temperature on the adhesion of diamond films deposited on WC-10%Co substrates with a Cr-N interlayer is investigated. Diamond films were deposited at different temperatures (550, 650 and 750 °C), using a hot filament chemical vapor deposition reactor. It was found that the optimal adhesion is obtained for the film deposited at 650 °C. The interplay between carbon interfacial diffusion and the adhesion of diamond films deposited at different deposition temperatures were investigated. The combined use of different characterization techniques (Indentation tests, SIMS, XPS, XRD and SEM) shows that the adhesion strength depends on the thickness of Cr-C layer formed at the interface during diamond deposition, which is strongly influenced by the deposition temperature. It is suggested that at the optimum deposition temperature, thickness of the Cr-C layer is too low to introduce a large thermal stress at the interface and sufficiently thick enough to withstand the propagation of indentation induced cracks.     

Enhancement of (100) texture in diamond films grown using a temperature gradient

Diamond films with dominant (100) texture were grown with a temperature gradient across the Si (100) substrates using hot filament chemical vapor deposition technique. Deposition was carried out with 0.8% CH₄ in balance hydrogen at an average substrate temperature of 880 °C. The deposition pressure was varied between 20–120 torr. Films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). XRD shows very strong (400) reflection in all the samples. SEM results show a smooth diamond surface comprised of (100) platelets. As the (100) diamond plates were grown on top of the (100) oriented silicon substrate the faces are more or less aligned parallel with the substrate surface, resulting in a relatively smoother diamond surfaces. FTIR studies show novel features in the films. Quantitative analysis was carried out to measure the H content in the films.     

Graphitized filament plasma enhanced CVD deposition of nanocrystalline diamond

A novel approach to the deposition of polycrystalline diamond is presented. The technique is based on the hot filament chemical vapour deposition technique (HFCVD). While it is similar to a high plasma power “bias enhanced growth” HFCVD, it relies on a graphite filament rather than on a metal one. It was found that with an appropriate choice of the growth parameters, 4–9% CH₄ in H₂, filament temperature > 2200 °C, 25 mBar gas pressure, plasma power > 500 W, a long filament lifetime can be achieved, when a simultaneous deposition of graphitic carbon on the hot graphite filament and of nanocrystalline diamond on a substrate facing the filament assembly is realized. In this paper the growth of nanocrystalline diamond films and their characterization (SEM, XRD, AFM) are presented. While the technique is promising for low cost, large area deposition of nanocrystalline diamond films, also the growth of microcrystalline diamond has been observed.     

Adhesion improvement of diamond coatings on cemented carbide with high cobalt content using PVD interlayer

At present, diamond coating is usually deposited on cemented carbide (WC-Co) tool with low Co content (Co ≤ 6 wt.%). It is more difficult to deposit diamond coating on WC-Co with high Co content because of the strong catalytic effect of Co. However, WC-Co tools with high Co content (Co ≥ 6 wt.%) are more widely used in difficult-to-cut materials machining because of their higher strength and better ductility. In this paper, the research was carried out on the adhesion performance of diamond coating on WC-Co (Co 10 wt.%). The deposition of diamond coating was conducted in hot filament chemical vapor deposition (HFCVD) system with the presence of the strong carbon-forming metallic interlayer (Nb, Cr or Ta), which was prepared using physical vapor deposition (PVD) on WC-Co substrate after chemical etching through a two-step process (Murakami solution and Caro's acid), which is a general way to treat the WC-Co substrate before growth of diamond coating. The results showed that the diamond films grown on the above treated WC-Co substrate have higher nucleation density, purity and adhesion strength than those on WC-Co substrates pretreated only using PVD interlayer or chemical etching. The PVD interlayer restrains the diffusion of Co as a result of high substrate temperature during the diamond film deposition, and consequently prevents the formation of the loosened layer induced by the removal of Co binder phase in the WC-Co substrate. The results also indicated that Nb interlayer leads to the most adhesion improvement of diamond films on the WC-Co inserts among the Nb, Ta and Cr interlayers.     

Synthesis of nanocrystalline diamond films by DC plasma-assisted argon-rich hot filament chemical vapor deposition

Continuous nanocrystalline diamond (NCD) films were grown in an argon-rich gas atmosphere with relatively high growth rates by sustaining a low power (5 W) DC plasma in a hot filament chemical vapor deposition system (HFCVD). The parameter window for the synthesis of NCD films was studied as a function of argon, methane and hydrogen concentrations, as well as substrate temperature and DC bias. The results are consistent with reports indicating that the DC plasma induces re-nucleation by ion bombardment during the initial growth step and helps to maintain the atomic H and hydrocarbon species near the growing surface. It was found that DC plasma-assisted HFCVD enables high NCD growth rates and expands the parameter window, rendering it unnecessary to heat the filament above 2800 K.     

Improvement in adhesion of diamond films on cemented WC substrate with Ti–Si interlayers

Diamond films were deposited on the cemented WC+(3–5)% Co substrates by a microwave plasma chemical vapor deposition system. The substrates were pretreated with various processing steps before diamond deposition, including: polishing, etching for Co removal, Ti coating by DC sputter, and amorphous Si coating by E-gun. The residual stress of the films was determined by both Raman shift and low incident beam angle X-ray diffraction (LIBAD) methods. The adhesion of the films was evaluated by indentation adhesion testing. The film morphology and film–substrate interface structure were examined by SEM and Auger electron spectroscopy, respectively. The results show that Ti–Si can be a good interlayer to improve film adhesion and inhibit diffusion of Co to the substrate surface on diamond nucleation. This is due to the formation of strong TiC and SiC bonding to enhance film adhesion; Si acts as a promoter for diamond nucleation, and the residual stress with application of interlayer is much lower than that interlayer-free. The results also show the existence of an optimum Ti thickness for the best film adhesion.     

Growth and Adhesion Enhancement of Diamond Films Deposited on Steel Substrates by a Cr–N Interlayer

Diamond film deposition onto iron-based substrates by chemical vapor deposition methods is complicated by the formation of black carbon or graphitic soot on the substrate surface prior to diamond nucleation and growth, by fast diffusion of carbon into the iron substrate, and by poor adhesion of the deposited film. These complications suggested the use of a buffer layer between the deposited diamond film and the iron-based substrate. We review different methods used to improve the adhesion of diamond film to steel substrates. In particular we describe in detail our own studies which involve the use of a Cr-N interlayer. The use of a chromium nitride interlayer has been found to improve significantly the adhesion of diamond films deposited on ferrous substrates. This is achieved by hindering diffusion processes of carbon and iron, very stable mechanical and chemical bonding between the interlayer and the diamond film, and good adhesion of the interlayer to the steel substrate. We also report on our studies related to residual stress present in the films, as well as a correlation between the interlayer properties and adhesion strength of deposited films.     

Partially stabilized zirconia substrate for chemical vapor deposition of free-standing diamond films

In this work we investigated the use of partially stabilized zirconia (PSZ) as the substrate for deposition of CVD diamond films. The polycrystalline PSZ substrates were sintered at high temperatures and the results showed that this material has unique properties which are very appropriated for the growth of free-standing diamond films. The diamond nucleation density on PSZ is high, even without seeding, and the CVD diamond film was totally released from the substrate after the deposition process, without cracking. Micro-Raman analysis revealed that the free-standing diamond film had a good crystallinity on both surfaces with practically no stress in the structure. The same PSZ substrate can be reutilized for the deposition of a large number of diamond films. The average growth rate is about 5–6 μm/h in a microwave plasma reactor at 2.5 kW. The deposition process causes the reduction of ZrO₂, producing ZrC. The high mobility of oxygen in the zirconia matrix at high temperature would probably help to etch the interface region between the substrate surface and the diamond film, decreasing the adhesion strength and eliminating some defects in the film structure related to non-diamond carbon phases.     

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.     

Analysis of diamond nucleation on molybdenum by biased hot filament chemical vapor deposition

The nucleation and initial growth of diamond on molybdenum using biased hot filament chemical vapor deposition were investigated by scanning electron microscopy, Raman spectroscopy and adhesion force tests. The studies showed that the negative biased pre-treatment greatly enhanced the nucleation density and adhesion force of diamond films on molybdenum. The experimental evidence was confirmed that there is large stress near the interface between the diamond and the Mo substrate, which were originated from the disordered graphite phases and molybdenum carbide near the interface. This may play an important role during nucleation stage. However, larger stress can cause the degradation of the adhesion force of diamond films on Mo substrate. However, the adhesion force was enhanced with increasing bias voltage. The theoretical relationship between the adhesion force and the bias voltage is given by theoretical calculation.     

Optimal parameters to produce high quality diamond films on 3D Porous Titanium substrates

Diamond films grown on three-dimensional (3D) porous titanium substrate were obtained by hot filament chemical vapor deposition (HFCVD) technique. The growth parameters strongly influenced the film properties during this complex film formation process. The pressure inside the reactor as well as the methane concentration showed their influence on the film morphology, quality, and growth rate. The substrates were totally covered by a diamond coating including deeper planes leading to a 3D porous diamond/Ti composite material formation. The sp²/sp³ ratio as “purity index” (PI) and the “growth tendency index” (GTI), evaluated from Raman and X-ray spectra respectively, were obtained for these composite materials as a function of their growth parameters.     

Diamond nucleation suppression in chemical vapor deposition process

A systematic study of the effect of different pre-treatments of the Si substrate surface in suppressing diamond nucleation was performed to investigate the nature of the nucleation centers in chemical vapor deposition (CVD) of diamond. The Si substrates were initially scratched with diamond powder and then submitted to one of the following pre-treatments: thermal annealing in high vacuum and in air, deposition of an amorphous silicon film, and ⁸⁴Kr⁺ ion implantation. The pre-treated substrates were used in a hot filament CVD diamond process, and the diamond films obtained were analyzed by different techniques. The results suggest that the observed nucleation reduction under certain pre-treatment conditions is related to modifications induced on the original topographical features of the scratched substrate surface, which would be responsible for the CVD diamond nucleation. The dimensions of these surface features are estimated to be of the order of 5 nm.     

Diamond coating of coloured Si3N4 ceramics

The effect of Si₃N₄ secondary phases on chemical vapour deposition (CVD) diamond film growth was analyzed. Silicon nitride substrates were obtained by pressureless sintering, placing the green samples inside a powder bed of Si₃N₄/BN. Local variations in the sintering atmosphere led to samples with different grey colouration as well as chemical and physical characteristics, determined by X-ray diffraction and thermal conductivity tests, which affected the diamond film growth. A complete characterization of the films, including thickness, average crystal size, surface roughness, texture and adhesion, was done. The Si₃N₄ substrate with glassier phase gave thicker diamond films, with smaller crystal sizes and better film adhesion to the substrate than the diamond films grown on ceramic substrates with less vitreous phase.