On the use of CrN/Cr and CrN interlayers in hot filament chemical vapour deposition (HF-CVD) of diamond films onto WC-Co substrates

CrN/Cr-based films were deposited using PVD-arc technique onto Co-cemented tungsten carbide (WC-Co) substrates and, then, seeded with diamond powder suspension or mechanically treated by Fluidized Bed Peening (FBP) of brittle diamond powders. Multilayered coatings were obtained from the superimposition of 4 µm-thick diamond coatings, deposited on the PVD interlayer using hot filament chemical vapour deposition (HFCVD). The effectiveness of fluidized bed peened CrN/Cr interlayers on the adhesion enhancement of diamond on WC-Co substrates was studied and compared to diamond coated WC-Co substrates with unpeened CrN/Cr or CrN interlayers, or pre-treated with two-step chemical etching (Murakami's reagent and Caro's acid, MC-treatment).In particular, growth, morphology, wear endurance and adhesion of the CVD deposited diamond films onto peened CrN/Cr interlayer were looked into. Diamond coatings on peened CrN/Cr interlayers exhibited a rougher surface morphology than as-prepared CrN/Cr films as a result of the surface roughening of the ductile Cr layer produced by the repeated impacts on it of the diamond powders during FBP. FBP was found to be a necessary step in improving the scarce adhesion of CVD diamond onto CrN/Cr-interlayer.However, the use of FB peened CrN/Cr interlayer did not represent the best way to pre-treat WC-Co substrates, as the unpeened single-layer CrN, or the use of MC pretreatment, was found to ensure better adhesion and wear endurance.     

Electron microscopy of interfaces in chemical vapour deposition diamond films on silicon

The structure of interfaces in diamond films grown on Si(100) has been investigated by transmission electron microscopy for the early stages of microwave-assisted chemical vapour deposition. Using conditions optimized for achieving so-called highly-oriented diamond films the depositions were performed in two steps, a bias-enhanced nucleation step and a subsequent growth step. Characteristic for the early deposition stages is the self-organized formation of regular arrays of predominantly {111}-facetted Si substrate surface grooves and islands elongated along [1̄10] and [110] directions. Subsequently, an interlayer of nanocrystalline β-silicon carbide islands forms, followed by the formation of epitaxially oriented diamond nanocrystals with high fractions of {111} interfaces. High-resolution electron microscopy of the interface regions depicts arrays of terminating {111} diamond planes at an average ratio of five diamond to four SiC lattice planes which corresponds to a remaining lattice mismatch of 2.3%. The orientation relationships between the lattices may be described by a coincidence site lattice model if the elastic lattice distortions are taken into account. Only small fractions of amorphous inclusions are present near interfaces, essentially consisting of amorphous carbon as could be deduced from analyses of the C K edge fine structure in electron energy loss spectra. The observations are compared with cases for which diamond nucleation directly on silicon has been obtained.     

Nucleation and growth of diamond films on aluminum nitride by hot filament chemical vapor deposition

Nucleation and growth of diamond films on aluminum nitride (ALN) coatings were investigated by scanning electron microscopy, Raman spectroscopy and scratch test. ALN films were grown in a magnetron sputtering deposition. The substrates were Si(111) and tungsten carbide (WC). Chemical vapor deposition (CVD) diamond films were deposited on ALN films by hot filament CVD. The nucleation density of diamond on ALN films was found to be approximately 10⁵ cm⁻², whereas over 10¹⁰ cm⁻² after negative bias pre-treatment for 35 min was −320 V, and 250 mA. The experimental studies have shown that the stresses were greatly minimized between diamond overlay and ALN films as compared with WC substrate. The results obtained have also confirmed that the ALN, as buffer layers, can notably enhance the adhesion force of diamond films on the WC.     

Comparative study of diamond films grown on silicon substrate using microwave plasma chemical vapor deposition and hot-filament chemical vapor deposition technique

Diamond films on the p-type Si(111) and p-type(100) substrates were prepared by microwave plasma chemical vapor deposition (MWCVD) and hot-filament chemical vapor deposition (HFCVD) by using a mixture of methane CH₄ and hydrogen H₂ as gas feed. The structure and composition of the films have been investigated by X-ray Diffraction, Raman Spectroscopy and Scanning Electron Microscopy methods. A high quality diamond crystalline structure of the obtained films by using HFCVD method was confirmed by clear XRD-pattern. SEM images show that the prepared films are poly crystalline diamond films consisting of diamond single crystallites (111)-orientation perpendicular to the substrate. Diamond films grown on silicon substrates by using HFCVD show good quality diamond and fewer non-diamond components.     

Nucleation of diamond over nanotube coated Si substrate using hot filament chemical vapor deposition (CVD) system

We studied the use of carbon nanotubes as a seeding layer for the nucleation of diamond on Si (100) substrate by using a hot filament chemical vapor deposition (HFCVD) system. Prior to deposition, substrates were seeded with multi-wall carbon nanotube (MWCNT) powder which was prepared separately. MWCNTs were used as nucleation precursors. The diamond grains grew essentially over the nanotubes with a higher growth density in comparison with the un-seeded substrates. The scanning electron microscopy (SEM) image of surface morphology shows crystallites of cauliflower shaped grains. The micro Raman spectroscopic results show a sharp peak at 1,332 cm^-1 corresponding to diamond phase. X-ray photoelectron spectroscopic study show the presence of carbon (C_1s) phase. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Doping of vanadium to nanocrystalline diamond films by hot filament chemical vapor deposition

Doping an impure element with a larger atomic volume into crystalline structure of buck crystals is normally blocked because the rigid crystalline structure could not tolerate a larger distortion. However, this difficulty may be weakened for nanocrystalline structures. Diamonds, as well as many semiconductors, have a difficulty in effective doping. Theoretical calculations carried out by DFT indicate that vanadium (V) is a dopant element for the n-type diamond semiconductor, and their several donor state levels are distributed between the conduction band and middle bandgap position in the V-doped band structure of diamond. Experimental investigation of doping vanadium into nanocrystalline diamond films (NDFs) was first attempted by hot filament chemical vapor deposition technique. Acetone/H₂ gas mixtures and vanadium oxytripropoxide (VO(OCH₂CH₂CH₃)₃) solutions of acetone with V and C elemental ratios of 1:5,000, 1:2,000, and 1:1,000 were used as carbon and vanadium sources, respectively. The resistivity of the V-doped NDFs decreased two orders with the increasing V/C ratios.     

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.     

Negative-biased diamond nucleation on silicon at a wide range of methane concentrations in hot filament chemical vapour deposition

In order to enhance the production of hydrocarbon ions for bias-induced diamond nucleation, high CH₄ fractions up to 16% were applied in a hot filament chemical vapour deposition process. Filament temperatures between 2900 and 3000°C were used, at which the filament was kept clean throughout the process. On a negatively biased silicon substrate, diamond nucleation was observed to increase from 2×10⁷ to 10¹⁰ cm⁻² when the CH₄ fraction was increased from 2% to 8%. Diamond film grown on bias-induced nuclei exhibited a preferred orientation along [001], which is believed to be growth on the nuclei with the same orientation. By calculation, the production rate of hydrogen atoms is not pronounced by plasma chemistry compared with the supply of hydrogen atoms from a clean filament. This indicates further the importance of keeping the filament clean during the biasing.     

Fretting wear of thin diamond films deposited on steel substrates

The dominant wear mechanism of thin diamond films deposited onto steel substrates and the effect of film thickness on their lifetime under fretting conditions were studied by analyzing the running-in and the main period of the coatings wear life. Steel plate and steel ball specimens for the present study were both coated with diamond by chemical vapor deposition (CVD). The wear tracks resulting from the fretting tests were investigated by various surface analysis methods. The results showed that the dominant wear mechanism of the diamond coatings, when both surfaces are coated, is an abrasive form of fretting wear. Under these conditions, the lifetime of the diamond films increased with increasing film thickness. It was found that the wear rate during the main period is independent on the initial thickness of the diamond film and therefore its life depends on the residual thickness at the end of the running-in period.     

Large area deposition of boron doped nano-crystalline diamond films at low temperatures using microwave plasma enhanced chemical vapour deposition with linear antenna delivery

We report on the preparation and characterisation of boron (B) doped nano-crystalline diamond (B-NCD) layers grown over large areas (up to 50 cm × 30 cm) and at low substrate temperatures (< 650 °C) using microwave plasma enhanced linear antenna chemical vapour deposition apparatus (MW-LA-PECVD). B-NCD layers were grown in H₂/CH₄/CO₂ and H₂/CH₄ gas mixtures with added trimethylboron (TMB). Layers with thicknesses of 150 nm to 1 μm have been prepared with B/C ratios up to 15000 ppm over a range of CO₂/CH₄ ratios to study the effect of oxygen (O) on the incorporation rate of B into the solid phase and the effect on the quality of the B-NCD with respect to sp³/sp² ratio. Experimental results show the reduction of boron acceptor concentration with increasing CO₂ concentration. Higher sp³/sp² ratios were measured by Raman spectroscopy with increasing TMB concentration in the gas phase without CO₂. Incorporation of high concentrations of B (up to 1.75 × 10²¹ cm³) in the solid is demonstrated as measured by neutron depth profiling, Hall effect and spectroscopic ellipsometry.     

Gradual transitions in morphology of diamond films grown by using N2 admixtures of CH4+H2 gas in a hot filament assisted chemical vapour deposition system

A study of the evolution of morphology of diamond films grown as a function of N₂ gas additions to the CH₄+H₂ precursor in an HF-CVD system is presented. With the increase of admixture of N₂ fraction, in contrast to earlier studies, the morphology was observed first to gradually change from {111}-faceted crystallites texture to that of an intermediate cubo-octahedral crystallite texture and then gradually but finally to transform completely into that of {100}-faceted crystallites. The threshold nitrogen concentration, [N₂]^thr, required to bring about the said transition in morphology was much larger than it was reported previously. Moreover, the morphology transition required a larger [N₂]^thr when a large fraction of methane was employed. Further additions of nitrogen, that just exceeded the [N₂]^thr, resulted in growth of films containing slightly bigger {100}-multi-layered grains or isolated planar {100}-platelets. For extremely large nitrogen additions, the growth of nanocrystalline or amorphous carbon films was observed. The N₂ additions more than 50 vol.% did not yield any deposition. Raman scattering and photoluminescence measurements were used respectively for characterizing the quality and nitrogen doping in the films. These results are attributed to the possible catalytic role of atomic nitrogen at the growing surface.     

Thermoluminescence properties of nitrogen containing chemical vapour deposited diamond films

The behaviour of centres due to nitrogen impurities introduced during the growth process into the chemical vapour deposited (CVD) diamond lattice is reported for the first time by thermoluminescence (TL) studies between 300 and 670 K after 200–400 nm ultraviolet (deuterium lamp) illumination at 300 K (RT) in air. TL curves exhibit some glow peaks at 490, 520 and 620 K, characterized by activation energies of approximately 1.2, 1.4 and 1.9 eV, respectively. Spectral analysis of these peaks which reveals some differences due to nitrogen content in the films shows well defined emission bands and interesting features leading to a better knowledge of the broad red photoluminescence (PL) band observed in our films. Nitrogen addition during the growth of the CVD material leads to the quenching of both of the 1.68 eV line related to Si centre and the broad green band centred at 2.25 eV which were observed for the high quality film.     

Low temperature growth of highly transparent nanocrystalline diamond films on quartz glass by hot filament chemical vapor deposition

Highly transparent and hard nanocrystalline diamond (NCD) films were prepared on quartz glass by hot filament chemical vapor deposition (HFCVD). The effects of total gas pressure, substrate's temperature, and concentration of CH₄ on the grain size, surface's roughness and hardness, growth rate, as well as the optical properties of NCD films were investigated. The results indicated that with a low total gas pressure and high CH₄ concentration, high frequency of secondary nucleation can be obtained. In addition, low substrate temperature can increase the rate of the hydrogen atom etched sp² graphite carbon in the film, yielding a smooth surface of NCD films and very high sp³ content. Under optimized conditions, the hardness can be enhanced up to 65 Gpa, with 80% maximum transmittance in the visible light region. The aforementioned reaction platform outcomes a 1.2 μm thickness of NCD coating with a low root-mean-square (r.m.s.) surface roughness around 12–13 nm and a high growth rate around 1 μm/h. The influences of the total gas pressure, substrate's temperature, and CH₄ concentration for growing NCD films were also discussed in this paper.     

Diamond deposition on chromium, cobalt and nickel substrates by microwave plasma chemical vapour deposition

Only after a relatively long incubation time (which is necessary to saturate the substrate and its surface with carbon by diffusion or formation of an intermediate layer) did diamond nucleation and deposition occur on Cr, Co and Ni, Also, prior to the onset of the diamond formation, non-diamond carbon layers can be formed with too high a concentration of CH₄. However, most of the experimental facts observed during the diamond depositions on Cr, Co and Ni surfaces can be explained by interactions occurring between the reaction gases and the substrates.

Chromium substrates form an intermediate carbide layer prior to diamond deposition. Diamond nucleation did not occur readily. Cobalt has only a low solubility for C. At low CH₄ concentrations, diamond was deposited on pure Co. No deposition of amorphous carbon was observed. Nickel has a certain C solubility. Diamond nucleation occurred only after the substrate and its surface had been carbon saturated. The length of the interval until saturation was reached depended on the substrate thickness.

During the time needed to cover the substrate fully with a diamond layer, the metal vapour from substrate interacted with the diamond growth. Large growth steps developed on the diamond crystal facets. Also refractory metal substrates placed near to the Cr, Co or Ni substrates were contaminated and their diamond coatings exhibited the same growth step features.     

Polymer‐based nucleation for chemical vapour deposition of diamond

Chemical vapour deposition of diamond on foreign substrates is hindered due to its high surface energy. Therefore, nucleation treatment has to be employed to initialize the formation of diamond crystals. This article deals with diamond growth on silicon substrates coated with three types of polymers: (i) polystyrene (PS), (ii) polylactic‐co‐glycolic acid (PLGA), and (iii) polyvinyl alcohol (PVA) were applied in different forms, i.e., microspheres (PS, PLGA), monolayers (PLGA), multilayers (PLGA, PLGA/PS), and composites with embedded diamond nanoparticles (PLGA, PVA). Thin polymers and microsphere monolayers did not contribute to the diamond nucleation and/or growth. A thicker continuous polymer film (>750 nm) or thin polymer/microsphere layer led to a homogeneous and dense formation of diamond grains. In the case of nucleation using polymer composites, where the thin polymer film serves as a 3D carrier matrix for embedded diamond nanoparticles, a comparable nucleation density to the well‐established ultrasonic seeding method was achieved. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43688.     

Mechanical properties of nanocrystalline diamond/amorphous carbon composite films prepared by microwave plasma chemical vapour deposition

Nanocrystalline diamond films (NCD) have been deposited by microwave plasma chemical vapour deposition from CH₄/N₂ mixtures with varying methane content. They consist of diamond nanocrystallites with sizes of 3–5 nm embedded in an amorphous matrix with grain boundary widths of 1–1.5 nm. The CH₄ content in the gas phase has almost no influence on the microscopic structure but a strong effect on the macroscopic structure and morphology. The mechanical and tribological properties of these films have been investigated by nanoindentation, nano tribo tests, and nano scratch tests. The hardness of a 4-μm-thick film deposited with 17% methane was about 40 GPa, the indentation modulus 387 GPa, and the elastic recovery 75%. Ball-on-disk tests against an Al₂O₃ ball revealed, after initially higher values, a friction coefficient of ≤0.1. Tribo tests and scratch tests proved a strong adhesion and a protective effect on silicon substrates. Finally, the correlations between the macroscopic structure of the films and their mechanical and tribological properties are discussed.     

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.     

Magnetoresistance of boron-doped chemical vapor deposition polycrystalline diamond films

The electrical properties and magnetoresistance of boron-doped polycrystalline diamond films grown on p-typed Si (100) by hot filament chemical vapor deposition have been investigated. As the atomic boron concentration increases from 3×10¹⁷ to 3×10¹⁹ cm⁻³, and grain size from 5 to 15 μm, the quality of diamond is improved, which causes the carrier mobility μ and longitudinal magnetoresistance change rate Δρ_///ρ₀ to increase. For a magnetic field (B) of 20 tesla and temperature 300 K, the longitudinal resistance change rate Δρ_///ρ₀ is up to 20%. Meanwhile, Δρ_///ρ₀ is proportional to μ²B² in a low field and proportional to μ^1.5B in a high field. It is the first time that a result is obtained in a high field.     

Simulation of H---C---S containing gas mixtures relevant to diamond chemical vapour deposition

Mole fractions of 24 species present within x%H₂S/1%CH₄/H₂ (x=0–1%) and 1%CS₂/H₂ gas mixtures have been calculated using the CHEMKIN computer package in conjunction with a mechanism based on the composite conversion: CH₄+2H₂SCS₂+4H₂. Arrhenius parameters for each elementary reaction involving S-containing species are presented, along with associated thermodynamic properties for each species. Molecular beam mass spectrometric measurements of species mole fractions in microwave activated x%H₂S/1%CH₄/H₂ (x=0–1%) mixtures agree well with the model calculations, if we assume a (reasonable) gas temperature of 1630 K. The agreement between similar measurements of both 0.5%H₂S/1%CH₄/H₂ and 1%CS₂/H₂ hot filament activated gas mixtures is less good, but the calculations succeed in reproducing many of the observed trends in species mole fraction with change in filament temperature.     

Chemical vapour deposition of diamond on nitrided chromium using an oxyacetylene flame

In this paper we present our first preliminary results on chemical vapour deposition (CVD) of diamond onto nitrided chromium using an oxyacetylene flame. Polycrystalline diamond films were obtained after deposition at very low substrate temperatures (<400°C). At these low temperatures there was extremely weak bonding, or no bonding at all, between the deposited layer and the substrate. To obtain stronger bonding, four growth experiments were carried out at initially higher substrate temperatures (700–1000°C). Whilst growth continued, the substrate temperatures were lowered step by step to 250°C. It was observed that on lowering the substrate temperature by more than about 500°C from the initial temperature, delamination occurred, suggesting that the thermal stresses exceeded the bonding strength. Subsequently, adherent diamond coatings were grown on the freshly exposed substrate surface whilst further lowering the substrate temperature. These diamond coatings were characterized using scanning electron microscopy and the adhesion of the diamond coatings to the substrates was assessed by means of the scotch tape test.