Ablation behavior and mechanism of SiC/Zr–Si–C multilayer coating for PIP-C/SiC composites under oxyacetylene torch flame

To improve the ablation resistance of PIP-C/SiC composites, SiC/Zr–Si–C multilayer coating was prepared by chemical vapor deposition (CVD) using methyltrichlorosilane (MTS) and hydrogen as the precursors and molten salt reaction using KCl–NaCl, sponge Zr and K₂ZrF₆, then the ablation capability of the coated composites was tested under oxyacetylene torch flame. The linear and mass ablation rates were much lower than those of uncoated samples. The linear and mass ablation rates of the three coating coated samples reached 0.0452 mm/s and 0.031 g/s, decreased by 27.3% and 27.1%, respectively. Moreover, the linear and mass ablation rates of the five coating coated samples reached 0.0255 mm/s and 0.0274 g/s, decreased by 59.0% and 35.5%. The gases released during ablation could take away a lot of heat, which was also helpful to the protection of the composites.     

The influence of diluent gas composition and temperature on SiC nanopowder formation by CVD

Crystalline cubic silicon carbide (3C-SiC) nanopowders were synthesized using hexamethyldisilane (HMDS) in a resistance heated chemical vapour deposition (CVD) reactor. The effects of different diluent gases on the synthesis of the SiC powder were also studied. The deposited powder was characterized using high-resolution X-ray diffraction (HRXRD) analysis, transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and BET surface area measurements. The crystallite size was estimated to be in the range of nanometer (10–20 nm) from XRD data and the particle size (∼10–30 nm) was obtained by TEM, HRTEM and BET. The growth condition was optimized in terms of crystallinity, chemical composition and deposition rate by varying different parameters such as the diluent gas (H₂/Ar ratio) and temperature.     

Microstructure and growth mechanism of SiC whiskers on carbon/carbon composites prepared by CVD

The SiC whiskers of good quality are expected to act as the reinforcing element in the ceramic coatings for C/C composites. Using CH₃SiCl₃(MTS) and H₂ as the precursors, SiC whiskers were prepared on the surface of C/C composites by chemical vapor deposition (CVD) at normal atmosphere pressure. XRD, SEM and TEM analyses show that the whiskers are β-SiC structure and their diameters are several-hundred nanometers. With the descending MTS concentration in the depositing room, the purity of the as-prepared whiskers increases and the diameters of the whiskers decrease. The investigation of growth mechanism shows the CVD-SiC whiskers grown up by the vapor–solid (VS) growth process.     

Chemical interaction during the chemical vapour deposition of silicon carbide on aluminosilicate fibres from halogenated precursors

Preliminary studies on the coating of silicon carbide (-SiC) on aluminosilicate fibres of the type Nextel by chemical vapour deposition (CVD), at atmospheric pressure, are reported. Classical CVD experiments were performed by using various precursor gases, such as silicon tetrachloride-methane-hydrogen, methyltrichlorosilane (MTS)-hydrogen and dimethyl dichlorosilane (DDS)-hydrogen mixtures. The deposition processes were studied by thermodynamic calculations. The SiC texture is dependent on the precursor used. It is shown that the best results are obtained from DDS-H₂ mixture; the deposit covers the filaments, but has a columnar growth commonly found in CVD materials. The mechanical properties of the different fibres, such as tensile strength and Young's modulus, were monitored at each stage before and after every coating. The decrease of _R is attributed to the high temperatures which modify the structure of the fibre and to attack the silicoaluminate substrate by the gas mixture with 1SS of AlCl₃, rather than the SiC coating.     

Structural changes of hot-wire CVD silicon carbide thin films induced by gas flow rates

Silicon carbide (SiC) thin films were prepared by hot-wire chemical vapor deposition in a CH₄ gas flow rate of 1 sccm, and the influence of the gas flow rates of SiH₄ and H₂ gases on the film structure and properties were investigated. In the case of a H₂ gas flow rate below 100 sccm, the SiC:H films obtained in SiH₄ gas flow rates of 3 and 4 sccm were amorphous. On the other hand, when the H₂ gas flow rate was above 150 sccm, SiH₄ gas flow rates of 4 and 3 sccm resulted in a Si-crystallite-embedded amorphous SiC:H film and a nanocrystalline cubic SiC film, respectively. It was found that gas flow rates were important parameters for controlling film structure.     

Sintering of ultrafine SiC powders prepared by plasma CVD

Ultrafine SiC powders with a nanometre particle size were synthesized by r.f. plasma chemical vapour deposition (CVD) using a chemical system of SiH₄−C₂H₄−Ar. The powder was also ultrapure with a grade of 99.999% purity. The product was polytype 3C−SiC and black in colour, in spite of its high purity, because of its ultrafine size. Silicon carbide is a difficult ceramic to sinter; it is possible to sinter it to full density with the aid of sintering additives. Ultrafine and ultrapure SiC powders were hot-pressed without sintering additives in the present study, in order to investigate the sintering behaviour. The CVD powders proved sinterable to 88% theoretical density without sintering additives. The present experiments revealed that powder treatment before firing was a key technology when using ultrafine powders as starting materials in the sintering process. The sintering behaviour of the powder was characterized by a large shrinkage. Phase transformation was negligible after hot pressing at 2200°C for 30 min.     

Properties of carbon nano-tubes–Cf/SiC composite by precursor infiltration and pyrolysis process

Carbon nanotubes (CNTs) were introduced into the precursor infiltration and pyrolysis (PIP) carbon fiber reinforced silicon carbide matrix (C_f/SiC) composite via the infiltration slurry. The weight fraction of CNTs in the composite was 0.765‰. The fiber–matrix interface coating was prepared through chemical vapor deposition (CVD) process using methyltrichlorosilane (MTS). Effects of the CNTs on mechanical and thermal properties of the composite were evaluated by three-point bending test, single-edge notched beam (SENB) test, and laser flash method. Attributed to the introduction of the small quantity of CNTs, flexural strength and fracture toughness of the C_f/SiC composite both increased by 25%, and thermal conductivity at room temperature increased by 30%.     

Characteristics of nano-crystalline diamond films prepared in Ar/H2/CH4 microwave plasma

Characteristics of nano-crystalline diamond (NCD) thin films prepared with microwave plasma chemical vapor deposition (CVD) were studied in Ar/H₂/CH₄ gas mixture with a CH₄ gas ratio of 1-10% and H₂ gas ratio of 0-15%. From the Raman measurements, a pair of peaks at 1140 cm^− 1 and 1473 cm^− 1 related to the trans-polyacetylene components peculiar to nano-crystalline diamond films was clearly observed when the H₂ gas ratio of 5% was added in Ar/H₂/CH₄ mixture. With an increase of H₂ gas content up to 15%, their peaks decreased, while a G-peak at roughly 1556 cm^− 1 significantly increased. The degradation of NCD film quality strongly correlates with the decrease of C₂ optical emission intensity with the increase of hydrogen gas contents. From the surface analysis with atomic force microscopy (AFM), it was found that grain sizes of NCD films were typically of 10-100 nm in case of 5% H₂ gas addition.     

Effect of soft substrate on the indentation damage in silicon carbide deposited on graphite

Indentation-induced damage is investigated in silicon carbide (SiC) deposited on graphite substrate. The SiC films have been grown by LPCVD (Low Pressure Chemical Vapor Deposition) method using MTS (CH₃SiCl₃) as a source gas and H₂ as a diluent gas to provide highly dense deposited layer and strong interfacial bonding. The elastic-plastic mismatch is very high to induce distinctive damages in the coating and the substrate layer. The specimens with various coating thicknesses are prepared by changing CVD condition or mechanical polishing. Indentation damages with different sizes are introduced by controlling indentation load in Nanoindentation, Vickers indentation and Hertzian indentation test. Basic mechanical properties such as hardness, toughness, elastic modulus are evaluated against coating thickness. Mechanical properties are sensitive to the indentation load and coating thickness. The results indicate that coating thickness has a vital importance on the design of hard coating/soft substrate system because the soft substrate affects on the mechanical properties.     

Au–Pd catalyzed growth of carbon nanofibers mat

Carbon nanofibers (CNFs) mat was synthesized from Au–Pd catalyst at 700 °C for 60 min in 10 kPa of the gas mixture of C₂H₂, H₂ and Ar by thermal chemical vapor deposition (CVD). Fine Au–Pd catalyst as admixture on the growth substrate of SiO₂ was adopted for the growth of CNFs mat. Fine Pd particles without Au particles were detected at the top/inside of as-grown CNFs by TEM-EDS analysis. The Au component within Au–Pd particles with ca. 1.5 nm did not contribute to the CNF growth. On the other hand, the growth scale of individual CNF was ranged from 5 to 150 nm in diameter and less than 100 μm in length, by controlling H₂ or Ar concentration in carbon source gas of C₂H₂/Ar or C₂H₂/H₂/Ar. It seemed that Ar gas accelerated the growth of the CNFs mat and the length of each CNF was increased with increasing the supplying amount of H₂. In contrast, H₂ induced the lower growth density of CNFs mat and the decrease in the diameter of those. The CNFs mainly grow through adsorption and decomposition of acetylene on the surface of Au–Pd catalyst and precipitation of carbon on the surface of those.     

Microstructure and growth of SiC film by excimer laser chemical vapour deposition at low temperatures

The formation of SiC film by photo-chemical vapour deposition (CVD) using ArF excimer laser with a wavelength of 193 nm has been studied to establish a low-temperature and mild synthetic process for ceramic films. Photo-CVD was conducted using Si₂H₆ and C₂H₂ as source gases on quartz glass, molybdenum and graphite substrates at 508–623 K. The pressure ratio of Si₂H₆ to C₂H₂ was changed from 0.06 to 1.2. The laser was irradiated at 50 mJ cm^–2 at 25 Hz. The films were identified using transmission electron microscopy and infrared spectrometry, and the chemical composition was determined using Auger electron specroscopy. The growth rate of SiC was measured with a step-height profilometer. ß-SiC, with almost stoichiometric composition, was formed in the pressure ratio range, Si₂H₆/C₂H₂, of 0.06–1.2 at 508–623 K. Crystallization of the SiC film was observed with increasing Si₂H₆/C₂H₂ ratio. The film growth rate linearly depended on Si₂H₆ pressure but was independent of C₂H₂ pressure. The rate-determining step of film formation is assumed to be photolysis of Si₂H₆. The cross-section of photolysis calculated from the film growth rate was about 10^–18 cm² for the ArF laser.     

Thermodynamic study on co-deposition of ZrB2-SiC from ZrCl4-BCl3-CH3SiCl3-H2-Ar system

Thermodynamics phase diagram of ZrB₂-SiC co-deposited from precursors of ZrCl₄-BCl₃-CH₃SiCl₃ (methyltrichlorosilane, MTS)-H₂-Ar has been investigated in detail by using the FactSage code and its embedded database (130 species being involved). The yields of condensed phases in the co-deposition process have been examined as the functions of the inject reactant ratios of BCl₃ / (BCl₃ + MTS) and H₂ / (ZrCl₄ + BCl₃ + MTS), and the temperature at a fixed pressure of 5 kPa. The results show that their yields strongly depend on the molar ratios of the inject reactants and the temperature. Consequently, the pure ZrB₂-SiC composite without free C, B₄C, ZrC and ZrSi can be co-deposited under the ideal condition by adjusting the reactant ratios and the temperature. The gas-phase equilibrium concentration distribution shows that the high input amount of H₂ is favorable for the co-deposition of ZrB₂ and SiC at a fixed ratio of ZrCl₄:BCl₃:MTS:Ar. In the end, the theoretical results can lay down guidelines for increasing the experimental yields of ZrB₂ and SiC.     

Ni-silicide precursor for gate electrodes

Ni-silicide film was deposited at a low temperature of 160 °C by CVD using a Ni(PF₃)₄/Si₃H₈ gas system. Injecting Si₃H₈ during the Ni deposition does not affect the deposition rate, but the step-coverage quality deteriorates at high growth temperatures. At high growth temperatures, the Ni/Si ratio of the film deposited on the sidewall varies as the distance from the open area increases. High step-coverage quality and a constant Ni/Si ratio independent of the location of the deposition are strongly required to fabricate a three-dimensional device. These requirements were achieved with this CVD by depositing the Ni-silicide film below 180 °C.     

SiC fibre by chemical vapour deposition on tungsten filament

A CVD system for the production of continuous SiC fibre was set up. The process of SiC coating on 19 μ m diameter tungsten substrate was studied. Methyl trichloro silane (CH₃SiCl₃) and hydrogen reactants were used. Effect of substrate temperature (1300–1500°C) and concentration of reactants on the formation of SiC coating were studied. SiC coatings of negligible thickness were formed at very low flow rates of hydrogen (5 × 10⁻⁵ m³/min) and CH₃SiCl₃ (1.0 × 10⁻⁴ m³/min of Ar). Uneven coatings and brittle fibres were formed atvery high concentrations of CH₃SiCl₃ (6 × 10⁻⁴ m³/min of Ar). The flow rates of CH₃SiCl₃ and hydrogen were adjusted to get SiC fibre with smooth surface. The structure and morphology of SiC fibres were evaluated.     

Deposition of device quality silicon nitride with ultra high deposition rate (> 7 nm/s) using hot-wire CVD

The application of hot-wire (HW) CVD deposited silicon nitride (SiN_x) as passivating anti-reflection coating on multicrystalline silicon (mc-Si) solar cells is investigated. The highest efficiency reached is 15.7% for SiN_x layers with an N/Si ratio of 1.20 and a high mass density of 2.9 g/cm³. These cell efficiencies are comparable to the reference cells with optimized plasma enhanced (PE) CVD SiN_x even though a very high deposition rate of 3 nm/s is used. Layer characterization showed that the N/Si ratio in the layers determines the structure of the deposited films. And since the volume concentration of Si-atoms in the deposited films is found to be independent of the N/Si ratio the structure of the films is determined by the quantity of incorporated nitrogen. It is found that the process pressure greatly enhances the efficiency of the ammonia decomposition, presumably caused by the higher partial pressure of atomic hydrogen. With this knowledge we increased the deposition rate to a very high 7 nm/s for device quality SiN_x films, much faster than commercial deposition techniques offer [S. von Aichberger, Photon Int. 3 (2004) 40].     

超音速等离子体喷涂MoSi2涂层工艺研究

涂层技术是C/C复合材料高温抗氧化与抗烧蚀的有效手段,单一的SiC涂层很难为C/C复合材料提供有效的长寿命保护。金属间化合物MoSi2高温时会形成一层致密的SiO2保护膜,具有特别优异的高温抗氧化性能,常作为C/C复合材料的高温抗氧化涂层。本文采用超音速等离子喷涂法在带SiC涂层的C/C复合材料表面制备了MoSi2涂层,主要研究了喷涂功率、主气(Ar)流量对粉料表面温度、飞行速度、沉积率以及对涂层表面微观结构和结合强度的影响。结果表明:喷涂功率在47.5~52.5 kW之间,既能使粒子有较高的速度和温度,还能保证粉末不过熔,在喷涂功率为50 kW时,粉料的沉积率最高,氧化不高,涂层表面致密性好,截面结合紧密,结合强度高;Ar流量为65 L/min时,能够保证MoSi2粉末有较高的表面温度与较快飞行速度,沉积率最高,氧化不高,涂层表面致密,几乎没有孔隙与裂纹。因此,调控超音速等离子体喷涂工艺参数能够在带SiC涂层的C/C复合材料表面得到致密且结合良好的MoSiO2涂层。     

Hot-wire chemical vapor deposition of silicon nanoparticles on fused silica

Silicon nanoparticles on fused silica have potential as recombination centers in infrared detectors due quantum confinement effects that result in a size dependent band gap. Growth on fused silica was realized by etching in HF, annealing under vacuum at 700-750 °C, and cooling to ambient temperature before ramping to the growth temperature of 600 °C. Silicon particles could not be grown in a thermal chemical vapor deposition (CVD) process with adequate size uniformity and density. Seeding fused silica with Si adatoms in a hot-wire chemical vapor deposition (HWCVD) process at a disilane pressure of 1.1 × 10^− 5 Pa followed by thermal CVD at a disilane pressure of 1.3 × 10^− 2 Pa, or direct HWCVD at a disilane pressure of 2.1 × 10^− 5 Pa led to acceptable size uniformity and density. Dangling bonds at the surface of the as-grown nanoparticle were passivated using atomic H formed by cracking H₂ over the HWCVD filament.     

Chemical vapour deposition of superconductors

Chemical vapour deposition processes (CVD) can produce metastable fine-grained materials as well as epitaxial coatings and can have a very large throwing power depending on the process parameters. Therefore, CVD is an prospective method to deposit high-temperature superconducting materials withT _c⩾10 K. One of the first superconductors which were produced was Nb₃Sn on tapes and single wires. This superconducting material is, however, today produced by metallurgical methods. Since the detection of Nb₃Ge, CVD has become for these coatings the main method of production for the following reasons: high deposition rates, possibility to dope the material by addition of further doping gases to the CVD-process, continuous process. These coatings were deposited on tapes. For the first time the large throwing power of the CVD process was utilized for the deposition of B1 -NbC _x N _y , on carbon fibre bundles. This opens the possibility to produce multifilamentary structures used for magnetic applications. The structure of the coating can be varied by changing the gas properties, by addition of further gases, by an ultrasonic field, by ignition of a gas discharge and by multi-layering. CVD could also be a prospective method for producing the new class of superconductors withT _c⩾30 K.     

Transient stages during the chemical vapour deposition of silicon carbide from CH3SiCl3/H2: impact on the physicochemical and interfacial properties of the coatings

Transient chemical vapour deposition experiments were produced from MTS/H₂ mixtures by varying the deposition temperature or the gas flow rates (Q_MTS or Q_H2) versus time. The gas phase, deposition rates and properties of the transient coating (φ_Tr) were investigated and adhesion assessments of SiC/φ_Tr/SiC bilayers were performed by scratch testing. Transient stages resulting from a decrease of Q_MTS or temperature lead to silicon co-deposition, but do not affect interfacial properties. Transient stages resulting from a decrease of Q_H2 eventually lead to carbon co-deposition. Thick and continuous carbon interlayers lead to a poor adhesion whereas thin and discontinuous layers do not.     

The effect of argon dilution on deposition of microcrystalline silicon by microwave plasma enhanced chemical vapor deposition

Microwave plasma-enhanced chemical vapor deposition (PECVD) is a very promising method for industrial scale fabrication of microcrystalline silicon solar cells since the technique is well applicable for large areas, and high deposition rates can be obtained. We have investigated the effect of Ar dilution on the growth process and the material properties of microcrystalline silicon. The major benefit of Ar addition in the MWPECVD process, using H₂ and SiH₄ as reactant gases, is an improved stabilization of the plasma, in particular at low pressure and MW power. We show, however, that material properties of the microcrystalline silicon layers deteriorate if we partly substitute H₂ by Ar during the deposition. The density of the layers - as expressed by the refractive index - decreases, and the defect density (measured by Fourier transform photocurrent spectroscopy) increases with increasing Ar flow. Investigation of the plasma by optical emission study shows that Ar atoms play a very active role in the dissociation processes of H₂ and SiH₄. Substitution of H₂ by Ar decreases the SiH^? emission and increases the Si^? emission. On the other hand, the H_α/H_β ratio increases upon substitution of H₂ by Ar. The latter effect shows that Ar addition does not lead to higher electron temperatures and we conclude that the changes of SiH^? and Si^? emissions are due to dissociation of SiH₄ by Ar^? (quenching reactions). The precise role of Ar in MWPECVD of microcrystalline silicon needs further investigation, but we conclude that the usage of this gas should be minimized in order to maximize the quality of the silicon layers.