Preparation of aligned silicon carbide whiskers from porous carbon foam by silicon thermal evaporation

Aligned silicon carbide whiskers were prepared from porous carbon foams by thermal evaporation of silicon. High-density silicon carbide whiskers were vertically deposited on the surface of siliconizing carbon foam. The whiskers were straight and hexagon-shaped with diameter of 1-2 μm and length of about 40 μm. They consisted of a single-crystalline zinc blende structure crystal in the [111] growth direction. The pore structure of carbon foam played an important role in determining distribution of the whiskers on the surface of siliconizing carbon foam. When carbon foam with higher porosity and larger pore size was employed, distributions of the whiskers were more ordered and more intensive. The whiskers were grown by the vapor-solid (VS) mechanism.     

Chemical vapour deposition of zirconium carbide and silicon carbide hybrid whiskers

Zirconium carbide and silicon carbide hybrid whiskers were codeposited by chemical vapour deposition using methyl trichlorosilane, zirconium chloride, methane and hydrogen as the precursors. The zirconium carbide and silicon carbide whiskers were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The results indicate that the codeposition process is more effective in the presence of methane than in the absence of methane. The codeposition process and the growth of zirconium carbide in the whiskers can be accelerated at high temperature in the presence of methane. A growth model was proposed based on the deposition model of carbon, zirconium carbide and silicon carbide.     

Plasticizing and wettability-enhancing coatings on carbon,silicon carbide and boron fibers

This paper is concerned with two groups of coatings on high modulus and high strength fibers for reinforcing composite materials with metal and polymer matrices.The first group includes coatings for improving and stabilizing the strength of fibers through a plasticizing effect. For carbon fibers the best results were obtained by chemically applying a nickel coating 1 μm thick with subsequent annealing at 1000°C to form a solid solution of carbon in nickel. In the case of boron and silicon carbide fibers, strength stabilization is achieved by applying coatings, 0.5–2 μm thick, of aluminum and some aluminum alloys by pulling the fibers through a melt. Analysis of the physicochemical interaction of the fibers with the coatings indicates a selective dissolution of the atoms of the fiber material in the coating at stress concentrators, with the result that the stress concentrators are smoothed out. In addition, the plasticizing effect is promoted by the relaxation of stresses in the coating at the stage of microplastic strain of the fibers.The second group includes coatings that enhance the wettability of the fibers by metal melts. An essential role in the improvement of the impregmation of carbon fiber strands and fabrics is played by the highly dispersive surface structure of the coatings, which exhibit a capillary effect as the melt spreads. The best results are obtained in double-layer coatings in which the first layer, silicon carbide, is protective and the second, molybdenum, enhances wettability.     

Elaboration of aluminum-matrix composite materials reinforced with inorganic fibers

This article is a literature survey concerning the different methods presently known for producing fiber-reinforced aluminum-matrix composites. The elaboration problems are discussed in terms of wettability between aluminum or aluminum alloys and inorganic fibers. The fibers involved are continuous monofilaments about 100 μm diameter such as boron or silicon carbide fibers, and tows of 500–6000 endless thinner filaments 5–15 μm as graphite fibers, alumina fibers and Nicalon silicon carbide fibers. The typical mechanical properties (strength, modulus, fatigue) attained with these reinforcing fibers are also reported.     

R-curve measurement of silicon nitride ceramics using single-edge notched beam specimens

R-curve behaviour of three kinds of silicon nitride-based ceramics has been studied using the single-edge notched beam (SENB) technique. If the notch is deep enough, the specimen shows stable fracture during the bending test, even when the sample is a brittle material. The conditions required to obtain stable fracture in the bending test are clarified by the analysis. The crack length of the specimen was also calculated from the changing load during the fracture test. In this study, coarse-grained silicon nitride shows a large increase of theR-curve. On the other hand, silicon nitride with silicon carbide whiskers shows noR-curve increase. The rise of theR-curve should be related to the microstructure of the ceramics, and especially to the grain size of the specimen, because silicon carbide whiskers are not large compared to the silicon nitride grains, and silicon carbide can reduce the grain growth of silicon nitride during sintering.     

Microstructural characterization of a silicon carbide whisker reinforced 2124 aluminium metal matrix composite

The microstructure of a silicon carbide whisker (SiC_w) reinforced 2124 aluminium metal matrix composite was characterized using scanning transmission electron microscopy (STEM). The SiC whiskers ranged in length from approximately 2 to 10 µm, and demonstrated good bonding to the aluminium matrix. In a few cases, the interface between SiC whiskers and the aluminium matrix exhibited wavy characteristics. The size of subgrains in the aluminium matrix was found to be dependent upon that of SiC whiskers. In addition, two types of intermetallic compounds were observed in the composite.     

Low-pressure growth of single-crystal silicon carbide

We report on a low-pressure, reproducible process for producing large-area device-quality single-crystal silicon carbide (SiC) on 〈100〉 and 〈111〉 p-type silicon substrates. The process has been used to produce epitaxial layers up to 20 μm thick, and 2″ in diameter. The unintentional doping (n-type) was in the 10¹⁷ cm⁻³ range with mobilities of several hundred cm²/V s.     

Structure,morphology, and elemental composition of arc discharge evaporation products obtained with a graphite cathode and a composite anode

This paper presents a study of arc discharge sputtering products obtained with metal-graphite anodes. The sputtering products are shown to contain carbides of all the metals studied, except for nickel and copper carbides. Most of the sputtering products contain spheroidal metal or carbide particles, fullerenes, nanotubes, and filaments up to 2 μm in diameter. The surface of the particles is typically covered with carbon multilayers. In some cases, multilayer graphene scrolls are present, which is tentatively attributed to the catalytic effect of the metals or metal carbides. In the case of the C-Nb anode, the sputtering products contain faceted niobium carbide microcrystals up to 300 μm in size.     

Factors influencing the crystallization of ultrafine plasma-synthesized silicon nitride as a single powder and in composite SiC-Si3N4 powder

A study of various parameters affecting the crystallization process of amorphous silicon nitride produced by plasma gas-phase reaction was undertaken to determine the conditions under which whiskers are formed. This process is influenced by the ammonium chloride content of the starting powder and the presence of nitrogen in the furnace atmosphere. This last parameter is also influential on the / phase ratio, along with other factors like the silica content, temperature and duration of the thermal treatment. Heat treatment at 1500°C for 30 min under argon produced well-defined -Si₃N₄ crystals with a hexagonal cross section, a mean length around 0.8 m, and no sign of agglomeration. Under the same conditions, crystallization of silicon nitride in SiC-Si₃N₄ composite did not give crystals, but Si₃N₄ whiskers. Therefore silicon carbide plays a major role in their formation.     

Synthesis of silicon carbide whiskers using the vapour-liquid-solid mechanism in a silicon-rich droplet

SiC whiskers can be produced from 1350–1500 °C by carbothermal reduction of the silica in a fixed bed percolated by a hydrogen flow. At 1450 °C and above, the addition of iron to the silica-carbon mixture leads to the formation of submicrometre whiskers in the bed, ending with a silicon-rich droplet. The iron has evaporated and condensed at a lower temperature, a few centimetres downstream from the bed, allowing the formation of silicon carbide whiskers ending with an iron droplet according to the vapour-liquid-solid (VLS) mechanism. Submicrometre whiskers are also obtained without iron over a broader range of temperatures. Silicon carbide whisker production in a fixed bed is then possible using a (VLS) mechanism in a silicon-rich droplet and may be controlled without the addition of transition metals, thus improving the purity.Supported by Pechiney Electrometallurgie and the European Communities, Brite project RIIB.A267C.     

Nitridation of whisker-reinforced reaction bonded silicon nitride ceramics

Ceramic matrix composites were fabricated from silicon carbide whisker-reinforced reaction bonded silicon nitride. Optimal dispersion of the SiC whiskers in the silicon powder slip was achieved by milling and pH control; a pH range of 4–5 giving the best results. Only a slight drop in green density was observed for a 30 wt% addition of SiC whiskers. The effects of the whisker additions on the nitridation kinetics of reaction bonding were investigated and the additions were found to increase the induction period before nitridation and to slightly decrease the nitridation rate but green density and temperature were still found to be the main factors controlling nitridation. Modulus of Rupture measurements for the composites showed a decrease in strength compared to the monolithic material.     

Increase in the yield of silicon carbide whiskers from rice husk

Favourable conditions for the growth of good quality silicon carbide (SiC) whiskers from rice husk have been discussed in the light of available evidence on the probable growth mechanism and the theoretical understanding of the same. Preliminary results indicate an increase in whisker yield at lower temperatures and coarsening of whiskers with longer duration of conversion.     

原位合成莫来石晶须增强碳化硅泡沫陶瓷

以碳化硅粉、氧化铝、高岭土为主要原料,采用有机泡沫浸渍法制备出碳化硅泡沫陶瓷坯体,经原位合成反应法在碳化硅泡沫陶瓷内生成莫来石晶须,研究反应温度对莫来石晶须合成的影响,以及莫来石的理论设计含量对泡沫陶瓷的抗压强度和抗热震性能的影响。结果表明:在1 450℃下形成的莫来石晶须直径约为0.5~1.8μm,长径比约为8~30。当莫来石理论设计质量分数为25%时,泡沫陶瓷的抗压强度为1.76MPa,抗热震性能为15次。     

Microstructural evolution in reaction-bonded silicon carbide

A detailed microstructural investigation of reaction-bonded silicon carbide has been performed using both fully-bonded and quenched samples and other specially prepared specimens containing large original single crystals of known crystallographic habit. The development of the epitaxial SiC overgrowth on the original SiC particles has been followed and found to proceed by the progressive growth and coalescence of identically-oriented nuclei. This epitaxial layer grows with a habit characteristic of (cubic) SiC and then transforms to-SiC in the high temperature region behind the reaction front. The formation of faceted grain boundaries is explained by this growth morphology. Furthermore, SiC:SiC grain boundaries, SiC:SiC epitaxial boundaries and SiC:Si interfaces have all been characterized by TEM techniques. The grain boundaries are of particular interest since they usually comprise a thin (1 nm) layer of amorphous SiC with occasional suicide and graphite inclusions. The general cleanliness of the vast majority of interfacial area is a result of the removal of the impurities insoluble in SiC by liquid silicon moving through the sample. The overall distribution of impurities is discussed. Other microstructural features have been characterized and texturing due to original particle alignment during fabrication of the green compact investigated. The control of the mechanical properties of reaction-bonded silicon carbide by these various microstructural features is discussed. A basis for an explanation of the interesting trace-impurity-controlled contrast seen in secondary electron images of these materials is also established.     

Effect of frequency on ambient temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers has been undertaken on room temperature fatigue during static and dynamic loading at constant ΔK. It is shown that sub-critical crack growth rates are lower when the material experiences sustained far field loading than during cyclic far field loading. The increased crack growth rate during cyclic loading is attributed to a wedging effect within the crack wake causing an increase in the tensile stress and resultant increased micro-cracking ahead of the crack tip. This additional micro-structural damage leads to enhanced sub-critical crack growth rates during cyclic loading. The asperities that are responsible for the wedging effect are attributed to the isolation of small portions of material due to branching of small cracks and by degradation of the bridging SiC whiskers and Si₃N₄ grains within the crack wake.     

Polypropylene reinforced with silicon carbide whiskers

Thermal, crystallization and mechanical behaviour of isotactic polypropylene (iPP) reinforced with advanced silicon carbide whiskers (SiC_w) has been investigated. It is well established that the existence of chemical and physical interactions between the matrix and the reinforcement enhances the cohesive strength at the interphase thus improving the mechanical performance of the composite. In order to improve chemico-physical interactions between the components of the inorganic–organic composite system, their affinity has been enhanced in two ways: by coating the whiskers with a thin layer of acrylate-grafted polydivinylbenzene and by using as matrix a chemically modified polypropylene. The mechanical properties of the resulting composite materials have been compared and related to the dispersion grade of the whiskers within the matrices and the morphology of the samples. This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

热蒸发法碳化硅纳米晶须阵列的合成与表征

在1600℃不同真空度下, 采用热蒸发硅的方法, 在石墨基板和聚丙烯腈(PAN)炭纤维两种碳源基体原位生长具有一定取向的碳化硅纳米晶须——垂直于石墨片表面森林状和试管刷状碳化硅纳米晶须阵列。通过X射线衍射及场发射扫描电镜, 发现晶须为3C-SiC, 直径约100nm, 长度约50μm。炭纤维表面的产物顶端多为针尖状, 而石墨片表面的产物多为六方棱柱状。因其纳米尺寸效应, 在380nm波长的光激发下, 所制晶须在波长为468nm 附近出现光致发光峰。透射电镜、 多点衍射电子衍射图表明, 所制得的3C-SiC晶须为单晶, 其生长方向为3C-SiC的[111]方向。基于反应过程中硅熔体与碳源分离的事实, 讨论了3C-SiC晶须阵列生长的气固反应机理。      

Hot-pressing behaviour of silicon carbide powders with additions of aluminium oxide

The hot-pressing behaviour of different silicon carbide powders (average particle sizes ranging from ～ 0.5 to 9 μm) with aluminium oxide additions ranging from 0.01 to 0.15 volume fractions was investigated. Using powders with an average particle size < 3 μm, densities ≥ 99% theoretical could be achieved at 1950° C (1 h) with 28 MN m^?2 for volume fractions of AI₂O₃ \(\bar > \) 0.02. A liquid phase forms at high temperatures which dissolves the silicon carbide particles to promote densification by a solution-reprecipitation mechanism.     

Alignment of silicon carbide whiskers in polymer matrix

Polyacrylonitrile (PAN) loaded with up to 70% by weight of silicon carbide whiskers has been used as a model system to demonstrate the potential of whisker alignment by a technique similar to the conventional fibre spinning. Continuous fibres containing high percentages of whiskers can be readily produced using a laboratory scale conventional wet-spinning apparatus. Excellent whisker alignment along the fibre axes was found even for the as-extruded filaments without drawing. However, an improved alignment was obtained through optimizing whisker loading and by drawing the as-spun fibres. Drawing tends to improve the fibre mechanical properties considerably, rendering them easily handable or even useful as a “composite fibre” reinforcement in their own right. Although whisker alignment generally gets better with drawing, over-stretching leads to irregular fibre diameters. Preliminary experiments show that after some minor modifications, this technique may be applied to fabricate whisker reinforced ceramic composites. This revised version was published online in November 2006 with corrections to the Cover Date.