Silicon carbide fibre

Commercially available non-oxide ceramic reinforcements are in three categories continuous, discontinuous, and whiskers. The great breakthrough in the ceramic fibre area has been the concept of pyrolysing polymers under controlled conditions, containing the desired species to produce high-temperature ceramic fibres. Silicon carbide fibre is a major development in the field of ceramic reinforcements. 

Ansorge F, Characterisation of carbon fibre/silicon carbide matrix composites, Naslain R, Lamon J, Doumeingts D eds.. High Temperature Ceramic Matrix Composites, Proceedings of 6th European Conference on Composite Materials, European Association for Composite Materials, American Ceramic Soc Inc, Ceramic Society of Japan, Bordeaux, 491-498, 20-24 Sep 1993. 

Turning now to other types of ceramic fibre, the most important material made by pyrolysis of organic polymer precursors is silicon carbide fibre. This is commonly made from a poly(diorgano)silane precursor, as described in detail by Riedel (1996) and more concisely by Chawla (1998). Silicon nitride fibres are also made by this sort of approach. Much of this work originates in Japan, where Yajima (1976) was a notable pioneer. 

More recently, Stanicioiu, Chinta Hartner (1959) attempted to reinforce the cement with glass fibres, but this was not successful. The most serious study on the reinforcement of dental silicate cement was made by J. Aveston (in Wilson et al., 1972). Silicon carbide whiskers, carbon fibres and alumina powder were introduced into the cement mix. Unfortunately, the glass powder/liquid ratio had to be reduced, and the strength gained by reinforcement was thereby lost. It is clear that dental silicate cement cannot be strengthened by fibre or particulate reinforcement. 

Although these polysilane by-products were not noticed at that time, they are now important main products. In the so-called "Yajima process" [10] (the usual reductive dehalogenating coupling of dimethyldichlorosilane with sodium) polysilanes were obtained and fibres could be formed from these which were then pyrolyzed to form silicon carbide fibres. 

Polysilanes. Following the first reports of soluble and processable polysilanes in the late 1970s, these macromolecules have attracted substantial interest from both fundamental and applied perspectives." The backbone of silicon atoms gives rise to unique electronic and optical properties as a result of the delocalisation of a-electrons. Several polysilanes have also been found to function as useful thermal precursors to silicon carbide fibres and these materials have also attracted attention with respect to microlithographic applications and as polymerisation initiators." ... 

An important application of polydimethylsilane is as a source of silicon carbide (SiC) fibres, which are manufactured under the trade-name Nicalon by Nippon Carbon in Japan. Heating in an autoclave under pressure converts polydimethylsilane to spinnable polycarbosilane (-Me2Si-CH2-) with elimination of methane. The spun fibres are then subjected to temperatures of 1200-1400 °C to produce silicon carbide fibres with very high tensile strengths and elastic moduli." As a result of their conductivity, polysilanes have also been used as hole transport layers in electroluminescent devices. In addition, the photoconductivity of polymethylphenylsilane doped with Cgo has been found to be particularly impressive. ... 

Figure 12.9(a) is a picture of a silicon carbide fibre (Nicalon) in a calcium-aluminosilicate glass matrix. This picture is taken from a detailed study of... 

Fig. 12.9. A silicon carbide fibre in a calcium-aluminosilicate glass matrix (a) ELSAM, 1.9 GHz, z= —3.2 pm (b) calculated contrast from eqns (12.2), (12.45), and (12.46), horizontal scale the same as (a), with tick spacing 2.5 pm this time the experimental parameters gave the best fit without any adjustment (Lawrence 1990).

Yajima, S., Hasegawa, Y., Hayashi, J., and Iimura, M. (1978). Synthesis of continuous silicon carbide fibre with high tensile strength and high Young s modulus. J. Mater. Sci. 13, 2569-76. [2, 221]... 

When a composite is subjected to external forces, the energy of the matrix is only transferred to the fibres when there is question of a proper attachment. For that reason fibres are some-times provided with a layer of another material. An example boron fibres in an aluminium matrix are provided with a silicon carbide coating and as a result the fibres are called borsic fibres. The thermal expansion coefficient of a fibre and its matrix must correspond. Figure 14.9 is a representation of what takes place when a crack in a fibre-reinforced matrix grows. 

Many researchers have used glass and glass-ceramic matrices for reinforcing with high-modulus graphite fibres [1, 2], silicon carbide fibres and silicon carbide mono-filaments [3-7], Very strong, tough and refractory composites were obtained from these efforts. 

Silicon carbide (SiC) deposited on a substrate of tungsten or carbon heated to about 1300°C [27] is called sigma fibre (BP Sigma). A detailed schematic of the process used by BP to make its Sigma fibre is shown in Fig. 3.8. Textron Specialty Material Co. has developed a series of surface-modified... 

Schematic of two Textron SCS-type silicon carbide fibres (reproduced by permission of Chapman Hall)56. 

Prewo, K.M. and Brennan, J J., High strength silicon carbide fibre reinforced glass-matrix composites , J. Mat. Sci., 15, 463 168 (1980). 

Brennan, J.J., Chyung, K. and Taylor, M.P., Reaction inhibited-silicon carbide fibre... 

Multiple matrix cracking perpendicular to the fibre axis due to thermal shock in UD Nicalon /CAS (reprinted from Journal of Materials Science 32(2) 1997, Thermal shock behaviour of unidirectional silicon carbide reinforced calcium aluminosilicate Blissett, Smith and Yeomans, Figure 2, with kind permission of Springer Science and Business Media). 

Blissett, M.J., Smith, P. A., Yeomans, J.A. (1997), Thermal shock behaviour of unidirectional silicon carbide fibre reinforced calcium aluminosilicate , J. Mater. Sci., 32, 317-325. Blissett, M.J., Smith, P.A., Yeomans, J.A. (1998), Flexural mechanical properties of thermally treated unidirectional and cross-ply Nicalon-reinforced calcium aluminosilicate composites , J. Mater. Sci., 33, 4181 —4190. 

According to the data in Table 25.5 and to Eq. (25.6) the compressive strength of filaments of refractory materials such as carbon and silicon carbide have compressive strengths about 10 times as large as those of organic fibres. This would seem to be a serious restriction to the use of organic polymers such as aramids in their application in composites. For most of the applications this restriction is of minor importance, however, since long before ac max is reached, instability in the construction will occur. The resistance of a column or a panel under pressure is proportional to the product of a load coefficient and a material efficiency criterion ... 

R. F. Cooper and K. Chyung, Structure and Chemistry of Fibre-Matrix Interfaces in Silicon Carbide Fiber-Reinforced Glass-Ceramic Composites An Electron Microscopy Study, J. Mater. Sci., 22, 3148-3160 (1987). 

K. M. Prewo, F. Johnson, and S. Starrett, Silicon Carbide Fibre-Reinforced Glass-Ceramic Composite Tensile Behaviour at Elevated Temperature, J. Mater. Sci., 24, 1373-1379 (1989). 

Fibre-reinforced silicon carbide, Fitzer and Gadow, 1986 [130]... 

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