THE EFFECTS OF MATRIX MICROCRACKING ON THE OXIDATION BEHAVIOR OF CARBON-FIBER GLASS-MATRIX COMPOSITES

Carbon-fibre/glass-matrix composites were fabricated using Fortafil fi bres and two different glass matrices: a sodium-borosilicate glass (CG W 7740), and a calcium-aluminosilicate glass (CGW 1723). Upon cooling from the hot-pressing temperature used to fabricate the composites (ap proximately 1250-degrees-C), the glass matrices cracked due to differe nces in the coefficients of thermal expansion between the fibres and t he matrix. At elevated temperatures these cracks serve as short-circui t diffusion paths for oxygen transport, and the majority of the weight loss from the cracked samples was caused by oxygen diffusing along th ese microcracks and reacting with the fibres. Because of the relativel y large diameter of these cracks compared to the mean free path for di ffusing oxygen, traditional gas kinetics can be applied to the various transport processes occurring in the oxidation reactions, and there i s no need to allow for capillary size or to apply Knudsen diffusion. T he composites made of 1723 glass exhibited linear relationships betwee n specific-mass loss (A mass/initial exposed surface area of carbon fi bres) and time at all oxidation temperatures (450, 500, 550 and 600-de grees-C). With the 7740 composites, a parabolic relationship between s pecific-mass loss and time was obtained. As the oxidation temperature approached or exceeded the glass-transition temperature, T(g), for the 7740 composites (560-degrees-C), this parabolic relationship became m ore pronounced. Microstructural evidence revealed that at temperatures near or exceeding the T(g) for the 7740 glass the microcracks in the matrix heal, thereby decreasing the amount of fibre surface area avail able 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 f or oxidation remained constant at approximately 174 kJ mol-1.