Assessment of oxygen in silicon carbide fibres

Five different types of SiC fibre produced by chemical vapour deposition were analysed using Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and X-ray (WDX) analysis. Fibres studied include SCS0, SCS6, Sigma SM1240 and two types of SiC fibres denoted SAM1 and SAM2, produced in the Commonwealth of Independent States (Ukraine, former USSR). Fibres were fracturedin situ in the Auger spectrometer. For each fibre, the oxygen, carbon and silicon yields were measured and qualitative assessment of oxygen was performed. Results suggest that the SCS0 fibre contains less oxygen than other SiC fibres. It was revealed that the SAM1 fibre (120 m diameter) has a duplex SiC and carbon coating deposited over a 20 m tungsten core prior to the main SiC deposition, to decouple mechanically the tungsten core from the main SiC deposition.     

Thermal diffusivity of chemically vapour deposited silicon carbide reinforced with silicon carbide or carbon fibres

The thermal diffusivity of chemically vapour deposited silicon carbide reinforced with either Nicalon SiC yarn or PAN-precursor carbon fibres was measured by the laser-flash method during various time-temperature treatments. The diffusivity was found to depend on the degree of densification, the direction of heat flow with respect to the fibre orientation, and the thermal history. Structural modifications, confirmed by X-ray diffraction, produced large permanent changes in the thermal properties of the SiC-SiC composites when heated above 1200° C, while only minor changes were seen in C-SiC composites heated above 1500° C. On sabbatical leave of absence from the Société Européenne de Propulsion, Bordeaux, France.     

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.     

Transmission electron microscopic characterization of zirconia reinforced with silicon carbide fibres

The microstructures of zirconia reinforced with silicon carbide fibres prepared by the sol–gel process have been examined using a transmission electron microscope. The characteristic feature of highly oriented grains of monoclinic zirconia, with its unique b-axis as well as c-axis of the tetragonal structure nearly all parallel to the hot pressing plane, shows the formation of matrix texture. Twinning in the monoclinic phase was well developed and highly dominated by twinning with the (1 0 0) plane as the interface. Alternative twinning with the interfaces parallel to the (0 0 1) plane has also been revealed and a possible model was suggested based upon the basic structure to be coincident with the [1 0 0] rotation twinning. Strong intergrowth of the tetragonal and monoclinic phases was frequently found and the orientation relationship was determined. The possible orientation variants resulting from the tetragonal to monoclinic phase transformation were proposed based upon such orientations of the lattices. Two of them with the misorientation angle of 9°15′ and 80°45′, respectively, were also found to coexist with the two kinds of twinning. Such texture configuration may have a close relation to the improvement of toughness. This revised version was published online in November 2006 with corrections to the Cover Date.     

Reactivity and molecular structure of silicon carbide fibres derived from polycarbosilanes

The thermal behaviour of Nicalon NML 202 fibres was carried out in the absence and in the presence of oxygen. The gaseous species produced and the oxygen balance were determined with a mass spectrometer. Three samples were studied: the desized initial fibre, this fibre after treatment with hydrofluoric acid, and a polycrystalline silicon carbide powder. It was shown that (i) the initial fibre has still an organic character, (ii) it contains a silica-rich layer, and (iii) beneath the silica-rich sheath the bulk of the fibre contains free carbon embedded in a silicon carbide-based network.     

The development of composite carbon fibres of large diameter—macro fibres

This paper describes the development of a process for manufacturing carbon fibres of larger diameter than those normally available from the “RAE process” [1] or other routes which have so far been described (e.g. the hot-stretching of pitch [2] or cellulose-based [3] fibres). The method consists of impregnating a bundle of carbon fibres or their precursors with resin and after curing, subjecting the composite fibre to a controlled carbonization. The end-product is a thin rod or fibre of carbon-fibre reinforced carbon, which can, in principle, have any desired cross-sectional shape or area [4]. Experiments have been conducted to discover the effect of various processing variables on the properties of the composite fibre. Having chosen a suitable resin for impregnation the principal variables are (a) the degrees of pyrolysis of the initial fibre before impregnation, (b) the rate of cure of the resin to form the “intermediate” and (c) the rate of temperature rise and the maximum temperature during the carbonization process. Fibres of reasonable properties (Young's modulus 76.8 GN m^?2 and ultimate tensile strength of 267 MN m^?2) have been produced but improvements should be obtainable by optimization of this basic process as fibres with strengths up to 400 MN m^?2 have been observed.     

Factors determining the diameter of silicon carbide whiskers prepared by chemical vapor deposition

Since the whisker diameter is one of the important parameters for determining the characteristics of whisker-related systems, an understanding of the factors that affect its size is of great value for whisker preparation. In this study, chemical vapor deposition (CVD) of silicon carbide (SiC) whiskers using a gas mixture of methyltrichlorosilane and hydrogen has been conducted in a hot-wall reactor on graphite plates coated with Ni as a liquid-forming agent. The deposited SiC whiskers are then characterized by scanning electron microscopy (SEM) to determine their nucleation and growth behavior. Experimental results show that the diameter of SiC whiskers is determined by both the vapor—liquid—solid (VLS) mechanism and vapor—solid (VS) radial deposition, where the former is affected by the area of the solid—liquid interface from which the crystal precipitates and the latter by the thickening kinetics of vapor-deposited SiC on the lateral face. However, a comparison of the two factors indicates that an appropriate choice of the diameter of liquid droplets for VLS whisker growth is more effective than radial VS deposition for obtaining whiskers of desired diameters.     

Chemical vapour deposition of silicon carbide on hollow and C-shaped carbon fibres

The thermally induced chemical vapour deposition of silicon carbide from methyltrichlorosilane (MTS) on isotropic pitch based carbon fibres with hollow and C-shaped cross sections was investigated. The uncoated and coated fibres were characterized by their mechanical properties (tensile strength, Young's modulus and torsional rigidity modulus) and surface energetic analysis. The oxidation behaviour of the carbon fibres before and after coating was discussed in terms of the weight change measured in a thermal balance during heating in an air flow at a constant heating rate.     

A new modified Weibull distribution function for the evaluation of the strength of silicon carbide and alumina fibres

The strength distributions of silicon carbide and alumina fibres have been evaluated using a modified Weibull distribution function. The function provides an upper and lower strength limit and is characterized by two shape and location parameters. The sum of squares was used as a measure of fit between the distribution function and the data. The result showed good agreement between the two. In addition, the strength distribution and the average value at a different gauge length were extrapolated from the parameters estimated at the original gauge length. In this case also, the proposed function accurately predicted the data points.     

The growth of small diameter silicon nanowires to nanotrees

In this work we have studied a way to control the growth of small diameter silicon nanowires by?the vapour-liquid-solid (VLS) mode. We have developed a method to deposit colloids with good density control, which is a key point for control of the nanowire (NW) diameter. We also show the high dependence of the allowed growth diameter on the growth conditions, opening the door to the realization of as-grown 2?nm silicon NWs. Finally we have developed a smart way to realize nanotrees in the same run, by tuning the growth conditions and using gold on the sidewall of nanowires, without the need for two catalyst deposition steps.     

Improved accuracy of the laser diffraction technique for diameter measurement of small fibres

The accuracy of a laser diffraction technique has been improved for small fibre diameter measurement by a calibration with SEM-measured internal standards. The calibration showed that the laser diameter is about 5% larger than the corresponding SEM diameter. After correction for systematic error via accurate calibration, the laser technique was found to be fast and capable of attaining a diameter measurement accuracy of 0.1m.     

Compressive failure of unidirectional hybrid fibre-reinforced epoxy composites containing carbon and silicon carbide fibres

The compressive failure of unidirectional hybrid fibre-reinforced epoxy matrix composites containing carbon (C) and silicon carbide (SiC) fibres has been investigated. In contrast to the case of flexural testing previously investigated by the authors, no significant increase in compressive strength, elastic modulus, or work of fracture was noted for the case of composites containing a mixture of C and SiC fibres. The specific compressive strength and elastic modulus generally decreased with increasing SiC fibre content due to the higher density of these fibres. Failure modes of tested specimens were classified into two main groups, namely compressive shear and compressive crushing, with the presence of fibre kinking and longitudinal splitting being noted in both cases.     

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.     

2014 and 6061 aluminium alloy-based powder metallurgy composites containing silicon carbide particles/fibres

The present investigation covers the processing and mechanical properties of 2014 and 6061 Al alloy-based power metallurgy composites containing up to 8 vol% of SiC in either particle or continuous fibre form. For consolidation of the green compacts, liquid phase sintering under vaccum at 635°C was adopted. The addition of reinforcement imparted improved densification such that particulate composites were better densified than the fibrous ones. Relatively higher work hardening rates were observed in fibrous composites than in the particulate ones. The ductility values of obtained porous particulate composites were similar to those of the fully dense ones reported in the literature.     

Effects of coating treatment on mechanical properties of carbon fibres and reinforced silicon carbide composites

Abstract

Three-dimensionally braided carbon fibre reinforced SiC matrix composites have been fabricated and the effects of coating treatment on the mechanical properties have been investigated. It has been found that pyrocarbon coating can improve the strength of the heat treated carbon fibres. When the coating thickness was 0.5 m, the composites had better mechanical properties: a flexural strength of 643 MPa and a fracture toughness of 17.9 MPa m12. The composites also exhibited a toughening fracture mode.     

The evaluation of the strength distribution of silicon carbide and alumina fibres by a multi-modal Weibull distribution

The strengh distributions of silicon carbide and alumina fibres have been evaluated by a multimodal Weibull distribution function. This treatment is based on the concept that the fracture of the fibre is determined by competition among the strength distributions of several kinds of the defect sub-population. Since those fibres were observed to have two types of fracture mode, the evaluation of a bi-modal Weibull distribution was performed in comparison with the single Weibull distribution usually employed. The accuracy of the fit for these two distributions was judged from maximum logarithm likelihoods and cumulative distribution curves. The result showed that the logarithm likelihood calculated using the bi-modal Weibull distribution function gave a larger value, as compared with those using the single Weibull distribution function. The curve predicted from the former function was also in good agreement with the data points. In addition, the strength distribution and the average value at a different gauge length were extrapolated from the Weibull parameters estimated at the original gauge length. In this case, also, the bi-modal Weibull distribution gave a more accurate prediction of the data points.     

Aluminium nitride coatings on silicon carbide fibres,prepared by pyrolysis of a polymeric precursor

Metallic aluminium was anodically dissolved in an organic electrolyte. A viscous solution, containing a polyiminoalane precursor was obtained. Nicalon (SiC) fibres were thermally pretreated and then coated with this fluid. The coated fibres were dried and calcined at 900 ° C in anhydrous ammonia. Thermal pretreatment of the fibres in nitrogen at 1400 ° C and dip coating with solutions of relatively low aluminium concentrations resulted in dense, nearly uncracked aluminium nitride layers.