Ceramic fibres thermal properties

SiC fibres (SiCf) are some of the more refractory ceramic fibres, and perform very satisfactorily in oxidising environments. A key problem in the development of SiC fibre composites is thermal degradation of the SiC fibre (Johnson et al 1987). The properties of SiC fibre start degrading above 600°C because... 

Ziegler G, Lucke J, Richter I, Suttor D, C-fibre reinforced composites with pol5nner-deiived matrix, microstructure, thermal properties, strength, Baxter J, Cot L, Fordham R, Gabis V, Hellot Y, Lefebvre M, Le Doussal H, Le Sech A, Naslain R, Sevagen A eds., European Ceramic Society, Euro Ceramics V, Part 3. Trans Tech Publications, Switzerland, 1870-1873, 1997 Key Eng Mater, 132-136. 

Although the uses of ceramic fibres in composite structures lie mainly in ceramic-matrix and metal-matrix composites, where their outstanding chemical and thermal resistance are important, there are a few applications in organic polymers. Their relevant properties are low thermal expansion, low electrical conductivity, low dielectric constant, high stiffness, good compressive strength, and in most cases complete resistance to combustion. On the other hand they are very brittle, hard to process, and mostly considerably more expensive than carbon and para-aramid fibres. They have, for example, been used in hybrid structures with carbon and para-aramid and in electronic circuit boards. The fibres available or potentially available include alumina, combinations of alumina with... 

The thermal properties of fibre-reinforced composites are anisotropic. Expansion in the direction of the fibre is usually very small or negative, but the thermal conductivity of some carbon fibre composites in the fibre direction can be high. Most polymers and several types of fibre are good electrical insulators. Carbon and some ceramic fibres have a much lower resistivity. It is thus possible to use composites for manufacturing radomes as well as for electromagnetic screening materials and systems which absorb radar signals. 

XA and HM carbon fibres are Courtaulds products note that Courtaulds have ceased making carbon fibres but information on their material has been included here since other manufacturers produce fibres with similar properties, see Figure 3.1, and the thermal properties of Courtaulds materials may be relevant to these other fibres. Aramid fibres used were Kevlar 29, Kevlar 49, Kevlar Hm (du Pont). E-, R- and D-glass fibres were used and Tyranno SiC and Nicalon SiC ceramic fibres (Ube Industries and Nippon Carbon, respectively). 934 (Fiberite) and MY720/HT976 (Ciba Geigy) epoxy resins were used (among others), as was polyester resin and acid cure phenolic resin. 

The impact of thermal shock on the properties of a ceramic or a CMC is assessed by means of both destructive and non-destructive testing methods. Flexural or tensile (mainly for CMCs) tests of suitably-sized thermally shocked specimens are usually employed to measure retained mechanical properties as a function of the temperature difference. The temperature differential for which a significant drop in property values is observed is the A A- For monolithic ceramics and particle- or whisker-reinforced CMCs the property under investigation is usually strength, whereas in fibre-reinforced CMCs a drop in Young s modulus is usually a better indication of the onset of damage. 

The low thermal expansion of C/SiC fibre ceramics in combination with their high rigidity and stability is utilized in low-expansion stractures. Not the high-temperature properties... 

Y.-M. Sung, S. Park, Thermal and Mecharrical Properties of Graphite Fibre-Reinforced Off-Stoichiometric Ba0.Al203.2Si02 Glass-Ceramic Matrix Composites, 7. Mat. Sci. Lett. 19, 315-317 (2000). 

Sung Y-M, Park S, Thermal and mechanical properties of graphite fibre-reinforced off-stoichiometric Ba0.Al203-2Si02 glass-ceramic matrix composites, J Mat Sci Lett, 19(4), 315-317, 2000. 

Polyimide characterised by thermal and thermal-oxidative stability at elevated temperatures, chemical resistance and good mechanical properties is relatively new in the family of polymer foams [3]. In some cases, depending on the nses, additional reinforcement can be inclnded. Examples are fibre reinforced foams and syntactic foams which are composites containing hollow glass, ceramic or plastic micro-spheres dispersed throughout the polymer matrix. 

In ceramic matrix composites (CMC), the fibres mainly serve to increase the fracture toughness. Because one important property of the fibres is their small defect size, it is possible to use the same material for fibre and matrix. The advantage of this is that the elastic properties of fibre and matrix are identical, avoiding the formation of stress concentrations, and that the coefficient of thermal expansion is also the same, so no residual stresses are generated during cooling. Chemical reactions do not occur as well. Possible fibre materials are mainly ceramics. 

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