The effects of matrix microcracking on the oxidation behaviour of carbon-fibre/glass-matrix composites
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Carbon-fibre/glass-matrix composites were fabricated using Fortafil fibres and two different glass matrices: a sodium-borosilicate glass (CGW 7740), and a calcium-aluminosilicate glass (CGW 1723). Upon cooling from the hot-pressing temperature used to fabricate the composites (approximately 1250°C), the glass matrices cracked due to differences in the coefficients of thermal expansion between the fibres and the matrix. At elevated temperatures these cracks serve as short-circuit diffusion paths for oxygen transport, and the majority of the weight loss from the cracked samples was caused by oxygen diffusing along these microcracks and reacting with the fibres. Because of the relatively large diameter of these cracks compared to the mean free path for diffusing oxygen, traditional gas kinetics can be applied to the various transport processes occurring in the oxidation reactions, and there is no need to allow for capillary size or to apply Knüdsen diffusion. The composites made of 1723 glass exhibited linear relationships between specific-mass loss (Δ mass/initial exposed surface area of carbon fibres) and time at all oxidation temperatures (450, 500, 550 and 600 °C). With the 7740 composites, a parabolic relationship between specific-mass loss and time was obtained. As the oxidation temperature approached or exceeded the glass-transition temperature, Tg, for the 7740 composites (560 °C), this parabolic relationship became more pronounced. Microstructural evidence revealed that at temperatures near or exceeding the Tg for the 7740 glass the microcracks in the matrix heal, thereby decreasing the amount of fibre surface area available for chemical reaction. Because the rate of oxidation is directly proportional to the amount of available fibre-surface area, the weight-loss data appear parabolic with time. Additionally, the mechanism for the oxidation of the carbon fibres does not appear to change once the fibres are placed in a glass matrix. The apparent activation energy for oxidation remained constant at approximately 174 kJ mol−1.
KeywordsCarbon Fibre Apparent Activation Energy Oxidation Temperature Exposed Surface Area Glass Matrice
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- 6.K. M. Prewo, Mater. Sci. Res. 20 (1986) 529.Google Scholar
- 7.E. Fitzner, A. Gkogkidis and M. Heine, High Temperatures-High Pressures 16 (1984) 363.Google Scholar
- 8.K. K. Chawla, J. Met. 35(3) (1983) 82.Google Scholar
- 10.D. J. Johnson, Chemistry and Physics of Carbon, Vol.20, edited by P. A. Thrower (Marcel Dekker, New York, 1987) p. 1.Google Scholar
- 11.B. L. Butler and R. J. Diefendorf, in Papers of the 9th Conference on Carbon, June 16–20, 1969, Boston, MA (American Carbon Society, University Park, PA, 1969) p. 45.Google Scholar
- 13.N. P. Bansal and R. H. Doremus, “Handbook of glass properties” (Academic Press, Orlando, 1986) pp. 32–37.Google Scholar
- 14.K. M. Prewo, J. J. Brennan and G. K. Layden, Ceram. Bull. 65(5) (1986) 305.Google Scholar
- 15.B. H. Eckstein, “The weight loss of carbon fibres in circulating air”, 18th International SAMPE Biennial Conference, 7–9 October 1986, Seattle, Washington (SAMPE, Covina, CA, 1986) p. 149.Google Scholar
- 16.J. B. Barr and B. H. Eckstein, “The oxidation of carbon fibres in air”, Extended Abstract of the 18th Biennial Conference on Carbon (Worcester, MA, 1987).Google Scholar
- 17.Clyde H. Sheppard, “Thermal and oxidative stability of carbon fibres and composites”, 18th International SAMPE Technical Conference, 7–9 October, 1986, Seattle, Washington (SAMPE Covina, CA, 1986) pp. 142–148.Google Scholar
- 19.G. H. Geiger and D. R. Poirier, “Transport phenomena in metallurgy” (Addison-Wesley, Reading, MA, 1973) pp. 464–469.Google Scholar