Mechanical properties and microstructures of machinable silicon carbide

Mechanical properties and microstructures of machinable silicon carbide, fabricated by pressureless sintering of silicon carbide fine powder with the aid of polysilastyrene, have been examined. Drastic changes in microstrucyure and in mechanical properties between specimens sintered at below 1773 K and at above 1873 K were observed. By sintering at above 1883 K the macinable silicon carbide had a good strength of more than 200 Mpa with high reliability, which was maintained beyond 1773 K. Polysilastyrene was converted in -phase silicon carbide and ribbon carbon in the pores. The (001) plane of carbon is parallel to the (111) planes of -phase silicon carbide.     

Preparation and pressureless sintering of chemical vapour deposited SiC-B composite powder

SiC-B composite powder was prepared by chemical vapour deposition (CVD) using (CH₃)₂SiCl₂+B₂H₆+H₂ as source gases at 1673 K. The powder was -type polycrystalline silicon carbide containing several per cents of boron and carbon. The boron content increased from 0 to 7.7 mass % as the B₂H₆ gas concentration increased from 0 to 0.7 mol %. Boron and carbon in amorphous form dispersed homogeneously in the -SiC polycrystalline particles. The particles were spherical, non-agglomerated and uniform in size with an average particle size of about 50 nm. Sintering tests were performed with the resulting composite powder without applying pressure. Powder containing 1 mass % boron and 2 mass % carbon was sintered to a density of 3.16×10³kgm^–3 at 2273 K, and the Vickers hardness of the sintered body was 30 GPa. When the sintering temperature was higher than 2323 K, significant grain growth due to the phase transformation from to form occurred, which decreased bulk density and Vickers hardness.     

Characterization of carbon coatings on SiC monofilaments using Raman spectroscopy

The axial residual stresses in the carbon coatings deposited onto different silicon carbide monofilaments have been determined experimentally using Raman spectroscopy. The stress-dependent band shift for the carbon G-band at around 1600 cm⁻¹, due to symmetric in-plane stretching mode of graphite, has been found to be −1.6 cm⁻¹/GPa. Using this calibration, the axial residual stresses in carbon coatings can be estimated from measured band shifts between the broken end and middle of the monofilaments. It was found that the stresses in the coatings of all monofilaments were compressive and between −440 and −810 MPa. Modelling indicated that this was consistent with the coating stress arising from the difference in coefficients of thermal expansion of carbon and the underlying silicon carbide. The coating stress was measured as a function of distance from the broken monofilament end. It was found that the distance for the stress to build up varied greatly, from 40 μm in Ultra-SCS to 500 μm in SM1140+. This suggests there are significantly different shear stresses between the coatings and underlying silicon carbide in the different monofilaments.     

Sintering of nano crystalline α silicon carbide by doping with boron carbide

Sinterable nano silicon carbide powders of mean particle size (37 nm) were prepared by attrition milling and chemical processing of an acheson type alpha silicon carbide having mean particle size of 0.39 μm (390 nm). Pressureless sintering of these powders was achieved by addition of boron carbide of 0.5 wt% together with carbon of 1 wt% at 2050° C at vacuum (3 mbar) for 15 min. Nearly 99% sintered density was obtained. The mechanism of sintering was studied by scanning electron microscopy and transmission electron microscopy. This study shows that the mechanism is a solid-state sintering process. Polytype transformation from 6H to 4H was observed.     

Effect of structure of interfacial coating layer on mechanical properties of continuous fiber reinforced reaction sintered silicon carbide matrix composite

A dense silicon carbide matrix composite reinforced by Hi-Nicalon fibers CVD coated with boron nitride and silicon carbide was fabricated by slurry impregnation and subsequent reaction sintering with molten silicon. The effect of the structure and the thickness of the silicon carbide layer of the fiber coating on the mechanical properties of the composite were investigated. That is, three types of silicon carbide layers, namely a dense structure with a thickness of 0.15 m and two porous structures with a thickness of 0.15 m and 0.48 m, respectively, were investigated. As a result, excellent strength property of ceramic matrix composite (CMC) was obtained in the case of the dense silicon carbide (SiC) layer. The thickness effect of the SiC layer on the strength was smaller than that of the structure.     

Sintering of nano crystalline α silicon carbide doping with aluminium nitride

Sinterable silicon carbide powders were prepared by attrition milling and chemical processing of an acheson type α-SiC. Pressureless sintering of these powders was achieved by addition of aluminium nitride together with carbon. Nearly 99% sintered density was obtained. The mechanism of sintering was studied by scanning electron microscopy and transmission electron microscopy. This study shows that the mechanism is a solid state sintering process.     

A microstructural study of silicon carbide fibres through the use of Raman microscopy

The microstructures of three different silicon carbide (SiC) fibres produced by CVD (chemical vapour deposition) have been examined in detail using Raman microscopy. Raman spectra were mapped out across the entire cross-sections of these silicon carbide fibres using an automated x-y stage with a spatial resolution of 1 m. The Raman maps clearly illustrate the variations in microstructure in such types of silicon carbide fibres. It appears that the SCS-type fibres contain carbon as well as SiC whereas the Sigma 1140+ fibre also contains free silicon. Furthermore, the differences in the detailed structures of the carbon and silicon carbide present in the fibres can also be investigated. Raman microscopy is demonstrated to be a very sensitive technique for characterising the composition and microstructure of CVD silicon carbide fibres prepared using different processing conditions.     

Oxidation behavior of green coke-based carbon–ceramic composites incorporating micro- and nano-silicon carbide

The oxidation resistance of the carbon–ceramic composites developed using green coke-based carbon and carbon black as carbon source, boron carbide, and micro- and nano-silicon carbide was carried out in the temperature range of 800 to 1,200 °C. Silicon carbide particulate as such and silicon carbide obtained by the reaction of green coke and silicon provided micro silicon carbide while silicon and carbon black and sol–gel silica and carbon black used as silicon carbide precursors led to the formation of nano-silicon carbide. The oxidation resistance of these composites at 800 to 1,200 °C for 10 h showed that the size of the silicon carbide influenced the oxidation resistance. The weight gain due to protective coating formed on oxidation was higher in composites containing nano-silicon carbide as compared to the composites containing micro silicon carbide.     

Carbothermal reduction synthesis of nanocrystalline zirconium carbide and hafnium carbide powders using solution-derived precursors

Zirconium carbide (ZrC) and hafnium carbide (HfC) powders were produced by the carbothermal reduction reaction of carbon and the corresponding metal oxide (ZrO₂ and HfO₂, respectively). Solution-based processing was used to achieve a fine-scale (i.e., nanometer-level) mixing of the reactants. The reactions were substantially completed at relatively low temperatures (<1500°C) and the resulting products had small average crystallite sizes (50–130 nm). However, these products contained some dissolved oxygen in the metal carbide lattice and higher temperatures were required to complete the carbothermal reduction reactions. Dry-pressed compacts prepared using ZrC-based powders with 100 nm crystallite size could be pressurelessly sintered to 99% relative density at 1950°C.     

注凝成型制备ZTA复相陶瓷

讨论了pH值、分散剂添加量、 ZrO₂(3Y)含量和固相体积分数对氧化锆增韧氧化铝(Zirconia toughened alumina, ZTA) 陶瓷注凝成型浆料粘度的影响,研究了注凝成型的烧结样品的性能和显微结构。结果表明,当pH值为8.5、分散剂添加量为0.9%时,可以制备固相体积分数达55%的低粘度ZTA (20%ZrO₂) 悬浮体。高固相悬浮体制备的烧结试样具有结构致密、ZrO₂分布较均匀和t-ZrO₂含量高等特征,其强度和断裂韧性分别达631.5 MPa和7.64 MPa · m1/2。      

颗粒整形对碳化硅陶瓷密封材料成型及烧结性能的影响

采用球磨对SiC粉体颗粒进行整形,并借助反应烧结制备SiC陶瓷密封材料,考察了颗粒整形对反应烧结SiC陶瓷成型、烧结性能、显微结构和力学性能的影响规律。结果表明,整形后的SiC颗粒的球形度高,粒径分布更为均匀;整形SiC粉体的振实密度和素坯密度明显提高,烧结体的显微结构更加均匀,主晶相为6H-SiC和Si,分布均匀,残炭很少;颗粒整形明显改善SiC陶瓷的成型性能及力学性能,当压力为15MPa时,整形后的SiC素坯密度为2.08g/cm~3,烧结体密度为3.06g/cm~3,抗弯强度和断裂韧性分别达到456MPa和3.87MPa·m1/2。     

Effects of promoters on the product quality of nanophase SiC/α-Si3N4 composite powders synthesized through carbothermal reduction nitridation

The effect of additives is investigated for the carbothermal reduction synthesis of nanophase silicon carbide/silicon nitride composite powders. Mixtures of silica, carbon, seed silicon nitride, and additive are reacted in a thermogravimetric analyzer. The mass loss information combined with compositional and spectroscopic analysis allows product quality (morphology, surface area, -Si₃N₄ and -SiC contents, oxygen content, etc.) information to be obtained. It was observed that all of the additives used in this study increased the reaction rate. Lithium carbonate produced a silicon nitride/silicon carbide composite that was not significantly different from experiments without promoter. However, the product quality was severely affected in other instances.     

Evolution of microstructure and local thermal conditions during directional solidification of A356-SiC particle composites

Solidification microstructures of aluminium silicon alloy (A-356) containing 0, 10, 15 and 20 vol % silicon carbide particles formed during directional solidification from a chill have been studied and compared with the structures obtained during solidification of the base alloy under similar mould and chill conditions. Columnar dendritic structure was observed during solidification of the base alloy at all distances from the chill. In the case of composites, the presence of silicon carbide particles disturbs the orderly aligned arrangement of dendrites observed in the base alloy, under similar solidification conditions, except near the chill surface where a particle-free zone is observed due to probable pushing of particles by the macroscopic solidification front with cell spacings finer than the particle size. During the entire range of solidification conditions studied in this work, the silicon carbide particles are pushed by growing dendrites of -aluminium into the last freezing eutectic liquid. The observations on pushing of silicon carbide particles have been examined in relation to existing models on particle pushing by planar solidification fronts. Even in the regions away from the chill, where silicon carbide particles are present, there are large regions covering several dendrite arm spacings where there are no particles representing another form of macrosegregation of particles. It is observed that the secondary dendrite arm spacings (DAS)of -aluminium are related to cooling rate by an equation DAS =b (T) ⁿ for the base alloy as well as for the composite. The coefficientb is generally higher for composites than for base alloy, and it is found to be a function of particle content. The value ofn for the composite is close to the value of the base alloy and is not significantly influenced by the presence of particles. Cooling rate, temperature gradients and the rate of advancement of the solidification front have been experimentally measured for the base alloy as well as for the composites during unidirectional solidification. The study indicates that the presence of particles themselves alters the cooling rates, temperature gradients and growth rate of the macroscopic solidification front under identical thermal surroundings during solidification. The possible influences of these alterations in growth condition on the solidification microstructure due to the presence of particles are discussed together with the other possible direct influences of particles on dendritic growth of aluminium-silicon alloys.     

The wettability of silicon carbide by liquid aluminium: the effect of free silicon in the carbide and of magnesium,silicon and copper alloy additions to the aluminium

Results from the sessile-drop method are reported for the effects on wetting angle, , of free silicon in the silicon carbide substrate and of alloy additions of silicon, copper or magnesium to the aluminium drop for the temperature range 700–960 or 1040 °C in a titanium-gettered vacuum (10^–4/10^–5 torr; 1 torr=133.322 Pa). Wetting angle, , was reduced by a factor as large as 2.8 for pure aluminium on reaction-bonded, compared with sintered silicon carbide, attributable to partial dissolution by the aluminium of the 18 wt% free silicon present in the reaction-bonded material. For wetting of reaction-bonded silicon carbide, the addition of 5 wt% silicon, copper or magnesium to the aluminium gave contact angles that decreased in the sequence SiCuMg, with the magnesium addition being the only one to result in wetting (i.e. <90 °) for all conditions studied. These results may have implications for design of conditions for joining or promotion of infiltration of silicon carbide parts, preforms or arrays with aluminium alloy melts.     

Fabrication of fibre reinforced green bodies by electrophoretic deposition of silicon powder from aqueous suspensions

Fine silicon powders with a mean particle size of 0.5 m were used to prepare aqueous suspensions for electrophoretic deposition (EPD). A cellulose film membrane was proved to be a cost-efficient alternative material to the dialysis hoses with excellent properties for the EPD-process. Several organic additives were tested to stabilise the suspension against sedimentation effects, to increase the electrophoretic mobility of particles and to optimise the mechanical properties of the dried green bodies. Mainly the electrokinetic sonic analysis (ESA) was used to characterise the influence of the additives and several parameters like solids content and ionic strength on the thickness of the electrical double layer. A computer-controlled and monitored EPD led to a dense and homogeneous packing of the powder and contributes to the understanding of deposition mechanisms. Even severe machining of the dried green bodies was possible. Using the planetary milled silicon powder the small spaces between the carbon (C-) or silicon carbide (SiC-) monofilaments of the fibre fabrics were infiltrated homogeneously by the silicon particles.     

Hot sodium sulphate corrosion of a Nicalon silicon carbide fibre-reinforced lithium aluminosilicate glass-ceramic matrix composite

The corrosion products arising from the exposure of a Nicalon silicon carbide fibre-reinforced lithium aluminosilicate glass-ceramic matrix composite to molten sodium sulphate at 900 °C for 100 h in both oxygen and argon atmospheres were studied by X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM respectively). The microstructure of the as-received composite plates was found to be similar to that reported by other workers. The matrix consisted of grains of close to stoichiometric mullite and -spodumene and a high silica glass with 20–50 nm wide fibre-matrix interfaces comprising a layer of turbostratic carbon and amorphous silica. The effects of hot sodium sulphate corrosion were found to be very similar in both argon and oxygen but proceeded at a much greater rate in the latter case where it had progressed 100 m into the composite and consumed many fibres. XRD studies indicated that mullite had virtually disappeared in the corroded region and this was confirmed by SEM. TEM studies of thin sections cut from near the end of the corroded zone also showed that the matrix had become a very fine mixture of glass and -spodumene grains and that the fibre-matrix interface region had grown to ca. 600–800 nm wide. The microstructure of this corroded interface comprised several alternating layers of turbostratic carbon, mixed carbon and amorphous silica and pure carbon, each with widths varying between ca. 100 and 200 nm. This layered structure apparently developed as a result of oxidation of the silicon carbide fibre in the presence of a gradient of oxygen partial pressure that decreased from the matrix across the interface to the fibre.     

Silicon carbide transport during carbothermic reduction of SiO<Subscript>2</Subscript>: Thermodynamic evaluation and experimental study

Mass-transfer processes during the high-temperature carbothermic reduction of silicon dioxide have been studied using thermodynamic modeling. The chemical vapor transport of silicon carbide has been investigated using SiO₂ + xSiC mixtures—major reaction products in the SiO₂-C system—as examples. Thermodynamic modeling results indicate that the vapor transport of silicon carbide is possible at temperatures from 1300 to 1500°C, and that the major gaseous species involved are Si and CO. Vapor transport processes have been studied experimentally. It is shown that the thermal reaction between carbon monoxide and silicon leads not only to direct conversion of silicon particles to silicon carbide but also to the growth of silicon oxycarbide fibers. The synthesized material has been characterized by x-ray diffraction and high-resolution optical microscopy.     

Microstructure and oxidation resistance of SiC coated carbon-carbon composites via pressureless reaction sintering

The effects of processing parameters on the microstructure and oxidation resistance of silicon carbide (SiC) coated carbon-carbon (C-C) composites were investigated. C-C composites were made from plain woven carbon cloths and phenolic derived carbon matrices in the laboratory. Pressureless reaction sintering has been used to apply SiC coating to C-C composites using epoxy resin and silicon powder as the precursor. Results showed that the oxidation resistance of C-C composites was enhanced by coating with SiC. The pressureless reaction sintering process exhibits good processability. -SiC was formed after heat treatment at 1800 °C and the -SiC formed after heat treatment at 2200 °C. The SiC coated C-C composites exhibit good oxidation resistance at 1000 °C for 100 h under the test conditions.     

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.     

Preparation of silicon carbide powders by chemical vapour deposition of the SiH4-CH4-H2 system

Chemical vapour deposition (CVD) of the SiH₄ + CH₄ + H₂ system was applied to synthesize-silicon carbide powders in the temperature range 1523 to 1673 K. The powders obtained at 1673 K were single-phase-SiC containing neither free silicon nor free carbon. The powders obtained below 1623 K were composite powders containing free silicon. The carburization ratio (SiC/(SiC + Si)) increased with increasing reaction temperature and total gas flow rate, and with decreasing reactant concentration. The average particle sizes measured by TEM ranged from 46 to 114nm, The particle size increased with the reaction temperature and gas concentration but decreased with gas flow rate. The-SiC particles obtained below 1623 K consisted of a silicon core and a-SiC shell, as opposed to the-SiC particles obtained at 1673 K which were hollow. Infrared absorption peaks were observed at 940 and 810 cm^–1 for particles containing a silicon core; whereas a single peak at about 830 cm^–1 with a shoulder at about 930 cm^–1 was observed for the-SiC hollow particles. The lattice parameter of-SiC having a carburization ratio lower than 70 wt%, was larger than that of bulk-SiC and decreased with the increasing carburization ratio. However, when the carburization ratio exceeded 70 wt%, the lattice parameter became approximately equal to that of bulk-SiC.