SIC FIBRE BOROSILICATE GLASS COMPOSITE .1. MATRIX DEVITRIFICATION/

The crystallization behaviour of borosilicate glass (Pyrex) with and w ithout a continuous SiC fibre reinforcement has been investigated as a function of heat treating time (10 min to 20h), temperatures (800 to 1050 degrees C) and particle sizes, The tendency to crystallize into c ristobalite was found to be very sensitive to the particle size of Pyr ex glass, Finer particles exhibited a higher amount of cristobalite th an coarse particles for the same heat treating conditions, indicating that crystallization of Pyrex resulted from heterogeneous nucleation a nd growth on the surface of each particle, For the SiC fibre/Pyrex com posite, the presence of fibres appeared not to contribute significantl y to the devitrification of matrix, since (1) nearly same degree of de vitrification occurred in the Pyrex powder compact alone compared with the degree of crystallization in the SiC fibre/Pyrex composite with s ame size of Pyrex particle, and (2) devitrification occurred randomly rather than occurring preferentially at the matrix-fibre interfaces, T he devitrification of Pyrex matrix to cristobalite under processing co nditions used for composites gave rise to extensive microcracking. The thermal mismatch stress originating from both a higher coefficient of thermal expansion of devitrified cristobalite and similar to 3.9% vol ume change at the phase transition temperature (similar to 250 degrees C) of cristobalite was calculated using the particle sphere model. Th e magnitude of stress calculated was high enough to cause cracking in and around a cristobalite particle surrounded by a Pyrex matrix.