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.     

Production of submicron SiC particles by d.c. thermal plasma: a systematic approach based on injection parameters

Aiming at producing high temperature structural ceramics, ultra-fine SiC powders were synthesized by the gas phase reaction of silicon tetrachloride with methane in a d.c. thermal plasma. The influence of parameters as the SiCl₄ feeding rate, C/Si and H₂/C molar ratios and internal pressure on the powder properties were investigated. The SiC powders were characterized by chemical analysis, Fourier transform infrared spectroscopy, X-ray diffraction and scanning electronic microscopy. The experimental set-up allows the production of β-SiC powders at a rate of 200 g h⁻¹ with particle size around 0.1 μm. The main impurities in the as-produced powder handled at ambient atmosphere are: oxygen (1.8–2.5%) and free carbon (3–4%). Interesting relationships were found between the SiCl₄ feeding rate and the H₂/C molar ratio and between the C/Si molar ratio and the internal pressure. The internal pressure plays a major role in controlling the particle size.     

Mechanism of production of ultra-fine silicon carbide powder by arc plasma irradiation of silicon bulk in methane-based atmospheres

The mechanism of production of ultra-fine SiC powder from a silicon bulk by arc plasma irradiation in either an Ar + CH₄ + H₂ or an Ar + CH₄ atmosphere was studied. Layer and island phases were newly formed in the silicon bulk upon irradiation, and it was revealed from scanning electron and Auger electron spectroscopy that these phases were composed of SiC. The intensity of the X-ray diffraction peaks due to the SiC phase increased with irradiation time almost in parallel to the carbon content involved in the silicon bulk. It is proposed that CH₄ is dissociated in the arc plasma and dissolved in the molten silicon bulk to produce the SiC phase, the sublimation of which is mostly responsible for the production of ultra-fine SiC powder.     

Pulse chemical vapour infiltration of SiC in porous carbon or SiC participate preform using an r.f. heating system

SiC was infiltrated into a porous carbon or an SiC particulate preform from a gaseous system of 6% CH₃SiCl₃-H₂ using a pulse chemical vapour infiltration apparatus and r.f. heating at 1273 to 1423 K. At 1273 K, the SiC matrix infiltrated the porous carbon initially to half the thickness of the substrate and finally over the full thickness. After 10000 pulses, three-point flexural strength saturated at about 120MPa. SiC particulate preform made from an average particle size of 4m was infiltrated by SiC. After 30000 pulses at 1273 K, the flexural strength of the composite increased to 200 to 220 M Pa.     

Pulsed chemical vapour infiltration of SiC to three-dimensional carbon fibre preforms

Three-dimensional carbon fibre preforms were infiltrated with silicon carbide from a gas system of CH₃SiCl₃-H₂ using a process of pressure pulsed chemical vapour infiltration. To infiltrate to a deep level, the temperature had to be lowered to 870–900°C, and the hold time per pulse below 1.0 s. Three-dimensional carbon fibre preforms partly filled with SiC fine powder were compared with those without filler. The weight of the preforms increased linearly with increasing number of pulses up to 10⁵ when no filler was present. However, the weight increase slowed down above 8×10⁴ pulses when the filler was used. Preforms with and without SiC filler showed three-point flexural strengths of 160 and 80 MPa after CVI of 10⁵ pulses, respectively. In order to improve the strength, a denser filling of SiC powder is necessary.     

The structure of chemical vapor deposited silicon carbide

The morphologies of SiC deposited by the thermal decomposition of CH₃SiCl₃ are presented. These are non-fluid bed deposits prepared in an induction-heated reactor. The morphologies of the deposits were found to vary systematically with the substrate temperature, chamber pressure and gas composition. Deposits at a low temperature, a moderate chamber pressure and with low H₂:CH₃SiCl₃ ratio are generally fine grained. Macrograins composed of finer micrograins are typical in intermediate temperature deposits. The surface contours of the macrograins are smoothly rounded. As temperatures or pressures are increased, the edges of grains become sharper, the surfaces become flatter and faceting becomes common. At very high temperatures and high H₂: CH₃SiCl₃ ratios, large single crystals are observed. Fracture surfaces containing faceted crystals no longer show the preponderance of transgranular fracture noted in deposits at lower temperature and lower H₂:CH₃SiCl₃ ratio. X-ray diffraction showed that these deposits are cubic β-SiC and disordered hexagonal 2H-SiC. The plane of preferred orientation was (111) for the low temperature and (220) for the higher temperature deposits. Microprobe analysis showed that carbon-rich, stoichiometric or silicon-rich SiC was deposited depending on the temperature, pressure and gas composition.     

Fe-catalyzed synthesis of SiC nanofibers from methyltrichlorosilane

We have studied Fe-catalyzed chemical vapor deposition of silicon carbide nanofibers via thermal decomposition of methyltrichlorosilane, CH₃SiCl₃, in hydrogen at temperatures from 1100 to 1350°C and the effects of synthesis temperature and time and gas (CH₃SiCl₃ + H₂) flow rate on the growth rate of SiC nanofibers. In the temperature range 1100–1350°C, the activation energy for nanofiber growth is 120 kJ/mol. The SiC nanofibers have the stoichiometric composition and consist of single-crystal β-SiC (cubic structure).     

Reinforcement and antioxidizing of porous carbon by pulse CVI of SiC

In order to reinforce and antioxidize porous carbon, chemical vapour infiltration (CVI) of SiC was investigated using a repetitive cycle of evacuation of vessel and instantaneous source-gas filling. From a source gas of 5% CH₃SiCl₃-H₂, a temperature range of 1273 to 1373 K was considered to be suitable to infiltrate SiC into a deep level, and surface deposition was enhanced at above 1373 K which led to pore blockage. With 3000 pulses, flexural strength was improved from 35 to about 90 MPa. Several specimens were exposed to air at 1573 K for 1070h during which the specimens were cooled to room temperature between four and seven times. SiC felt was also obtained by oxidation of a carbon skelton after pulse CVI.     

Study on W/SiC interface of SiC fiber fabricated by chemical vapor deposition on tungsten filament

SiC fiber was fabricated by chemical vapor deposition on tungsten filament heated by direct current in a CH₃SiCl₃-H₂ gas system. Microstructure of W/SiC interfacial reaction zone in the fiber was identified by means of scanning electron microscope and transmission electron microscope. Results showed that the thickness of the interfacial reaction zone is between 350 and 390 nm, and two reaction products of W₅Si₃ and WC were formed during fabricating SiC fiber. Electron diffraction analysis and composition detection indicated that W₅Si₃ is adjacent to tungsten core and WC is adjacent to SiC sheath, and the W/SiC interface can be described as W/W₅Si₃/WC/SiC. Furthermore, the formation mechanism of the interfacial reaction zone is discussed.     

Preparation of silicon carbide powders by chemical vapour deposition of the (CH3)2SiCl2-H2 system

Silicon carbide (SiC) powders were prepared by chemical vapour deposition (CVD) using (CH₃)₂SiCl₂ and H₂ as source gases at temperatures of 1273 to 1673 K. Various kinds of SiC powders such as amorphous powder, -type single-phase powder and composite powder were obtained. The composite powders contained free silicon and/or free carbon phases of about a few nanometres in diameter. All the particles observed were spherical in shape and uniform in size. The particle size increased from 45 to 130 nm with decreasing reaction temperature and gas flow rate, as well as with increasing reactant concentration. The lattice parameter of the -SiC particles decreased with increasing reaction temperature. All the lattice parameters were larger than those of bulk -SiC.     

Fabrication and characterization of heat and plasma treated SiC/Al2O3-YSZ feedstocks used for plasma spraying

The SiC/Al₂O₃-YSZ (ZrO₂ + 8 wt.% Y₂O₃) powders with different SiC particle sizes were fabricated and treated from spray drying, heat treatment, and plasma spraying. The morphology, phase composition, flowability and density of powders were analyzed. The sphericity and flowability of powders treated by plasma flame are increased greatly, and the particle surface is very smooth. The flowability and density of powder with nano SiC were evident better than those of powder with submicron SiC. The optimum flowability and compactness of powder with submicron SiC is obtained when the critical plasma spray parameter is 341 and 325, respectively. For nano size SiC, the optimum flowability and the maximum compactness of powders are obtained with critical plasma spray parameter of 341. The grain size of powders is increased after heat treatment and plasma spraying. The SiC is oxidized to SiO₂ in the powders after heat treatment and plasma spraying. The Y₂O₃ dissolved from 8YSZ solid solution at higher critical plasma spray parameter. Besides, there is no phase transformation of ZrO₂ for powders. The metastable phase of Al₂O₃ appeared in feedstocks with submicron SiC, but no metastable phase was formed in feedstocks with nano SiC particles, which nano SiC can hinder the formation of Al₂O₃ metastable phase. The densification process and mechanism of reconstituted particles used for plasma spraying were analyzed from surface morphology, cross section and simulation.     

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.     

Preparation of SiC foams by CVI-R technique with SiCl4/H2/CH4 system

Silicon carbide foams were prepared by the chemical vapor infiltration-reaction (CVI-R) of SiCl₄/H₂/CH₄ with carbon foam derived from mesophase pitch (MP), which had not only high bending strength but also low bulk density. The influence of the CH₄/SiCl₄ ratio in reaction atmosphere on the properties of as-prepared silicon carbide foams was investigated in detail. As the CH₄/SiCl₄ ratio was 0.25, resultant foam possessed the highest bending strength of 17.13 MPa. At the same time, correlations between properties and microstructure are also discussed.     

Pressure dependence of morphology and phase composition of SiC films deposited by microwave plasma chemical vapor deposition on cemented carbide substrates

SiC films were deposited on cemented carbide substrates by employing microwave plasma chemical vapor deposition method using tetramethylsilane (Si(CH₃)₄) diluted in H₂ as the precursor. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and scratching technique were used to characterize morphology, composition, phases present and adhesion of the films. Experimental results show that the deposition pressure has great influence on morphologies and phase composition of the films. In sequence, SiC films with a cauliflower-like microstructure, granular films with terrace-featured SiC particles coexisting with Co₂Si compound and clusters of nanometer SiC nanoplatelets appear as a function of the deposition pressure. In terms of plasma density and substrate temperature, this sequential appearance of microstructures of SiC films was explained. Adhesion tests showed that among the three types of films studied, the films with the terrace-featured SiC particles have relatively higher adhesion. Such knowledge will be of importance when the SiC films are used as interlayer between diamond films and cemented carbide substrates.     

Deposition process of Si-B-C ceramics from CH3SiCl3/BCl3/H2 precursor

Si-B-C coatings have been prepared by chemical vapour deposition (CVD) from CH₃SiCl₃/BCl₃/H₂ precursor mixtures at low temperature (800-1050 °C) and reduced pressures (2, 5, 12 kPa). The kinetics (including apparent activation energy and reaction orders) related to the deposition process were determined within the regime controlled by chemical reactions. A wide range of coatings, prepared in various CVD conditions, were characterized in terms of morphology (scanning electron microscopy), structure (transmission electron microscopy, Raman spectroscopy) and elemental composition (Auger electron spectroscopy). On the basis of an in-situ gas phase analysis by Fourier transform infrared spectroscopy and in agreement with a previous study on the B-C system, the HBCl₂ species was identified as an effective precursor of the boron element. H_xSiCl_(4−x), SiCl₄ and CH₄, derived from CH₃SiCl₃, were also shown to be involved in the homogeneous and the heterogeneous reactions generating silicon and carbon in the coating. A correlation between the various experimental approaches has supported a discussion on the chemical steps involved in the deposition process.     

Synthesis and characterization of silicon nitride powders produced in a d.c. thermal plasma reactor

Ultra-fine silicon nitride powder was synthesized from the SiCl₄-NH₃-H₂-Ar system using a d.c. plasma torch reactor (production rate 150–400 g h⁻¹). The powder produced is pure white, fluffy and amorphous. The particles are spheroidal in shape with a mean diameter between 30–60 nm forming aggregates of 0.1–0.4 μm depending on the operational conditions. Chemical analysis on the crude powder handled at ambient atmosphere revealed: N(−NH₄Cl):37–39%, O:3–5% and Cl:2–3%. The amorphous powder can be crystallized around 1500 °C under nitrogen to give an α-phase content in excess of 90%. Infrared spectra can be used to semi-quantitatively determine the NH₄Cl content of the crude powder. That proportion is between 2.5 and 4%. The influence of some process parameters e.g. (N/Si and H₂/N molar ratios, internal pressure) on powder properties was also investigated. The N/Si molar ratio was found to be the most important parameter for the powder composition whereas the internal pressure plays a major role on the powder morphology.     

Perchlorosilanes and perchlorocarbosilanes as precursors for SiC synthesis

Homologues with the general stoichiometry a(SiCl₄): bSi: cC: d(SiC) are shown to be potential precursors for the low-temperature gas-phase synthesis of silicon carbide. Thermal decomposition of these precursors yields the chemically stable gaseous species SiCl₄ and condensed Si, C, SiC, SiC + Si, or SiC + C. We use thermodynamic modeling of the thermal decomposition of octachlorotrisilane, Si₃Cl₈, to analyze the key features of the thermolysis of perchlorosilanes with the general stoichiometry a(SiCl₄): bSi. The equilibrium compositions of reaction products in the Si₃Cl₈-CO system are determined. This reaction system enables low-temperature (400–1200 K) formation of silicon carbide.     

Formation of microcrystalline SiC films by chemical transport with a high-pressure glow plasma of pure hydrogen

The formation of microcrystalline 3C-SiC films on Si substrates by the plasma-enhanced chemical transport method was investigated using a pure hydrogen glow plasma at 0.027 MPa. In this method, no source gas was necessary. Instead, the erosion products of a sintered 3C-SiC plate in a hydrogen plasma were used as the deposition source. By Fourier transform infrared (FT-IR) absorption gas analysis, the species generated by the hydrogen etching of sintered SiC were found to be SiH₄ and CH₄, which can serve as precursors for SiC film formation. The etch rate of sintered SiC by hydrogen plasma decreased with increasing source temperature. The maximum etch rate of the sintered SiC was 450 nm/min at an input power of 47 W/cm². Films prepared by this method at substrate temperatures (T_sub) of 600 and 1073 K were analyzed by FT-IR absorption spectroscopy. An absorption peak at 800 cm^- 1 related to Si-C bonds was clearly observed, but no significant hydrogen-related absorption peaks, such as C-H and Si-H, were observed in the prepared films. The deposition rate of SiC was about 8 nm/min, independent of T_sub. The SiC films had a columnar structure, and their surface morphologies revealed faceted growth. With decreasing T_sub, the lateral grain size became large. The current-voltage characteristics of a prepared SiC/Si heterojunction np diode showed rectifying behavior, demonstrating that the doping of an SiC film can be achieved without a doping gas source. The dopant distribution near the SiC/Si interface deduced from capacitance-voltage measurements suggests that the precise control of the initial growth stage is important to obtain a good SiC/Si interface.     

Powder effects in SiC matrix layered structures fabricated using selective area laser deposition vapor infiltration (SALDVI)

Silicon carbide has been deposited by laser-induced chemical vapor infiltration from the gas precursor tetramethylsilane, Si(CH₃)₄, into loosely packed powder layers of SiC, ZrO₂-Y₂O₃, or Mo. The goal is to produce dense layered structures of arbitrary shape by computer controlled laser scanning where the pore spaces between the powder particles are filled with solid material deposited from the gas phase using the selective area laser deposition vapor infiltration (SALDVI) process. Layered samples were fabricated for each powder material using both single line (bar) and multiple line (slab) laser scan patterns and 10 Torr Si(CH₃)₄, 2.5 m/s scan speed, 1000°C target temperature, and 120 m layer thickness. Samples of SiC and ZrO₂-Y₂O₃ are prone to surface cracking in the bar geometry, and cracking and delamination of layers in the slab geometry. Samples fabricated with Mo powder have no cracks or delamination defects in either bar or slab geometry as well as a better surface appearance.     

Pressure-pulsed chemical vapour infiltration of SiC to porous carbon from a gas system SiCl4-CH4-H2

SiC matrix was deposited into porous carbon from a gas system SiCl₄-CH₄-H₂ in the temperature range 900–1200 °C using pressure-pulsed chemical vapour infiltration (PCVI) process. At 1000 °C, silicon single phase, a mixed phase of (Si + SiC), and SiC single phase, were detected by X-ray diffractions for specimens obtained with the reaction time per pulse of 1, 2–3, and 5 s, respectively. At 1100 °C, SiC single phase was obtained with a reaction time of only 0.3s. Between 1050 and 1075 °C, deposition rate accelerated suddenly. The increase of SiCl₄ concentration increased the deposition rate linearly up to 4%–6%. The residual porosity decreased from 29% to 6% after 2×10⁴ pulses of CVI at 1100 °C, and the flexural strength was 110 MPa.