HIGH-TEMPERATURE MECHANICAL AND THERMAL-STABILITY OF SILICATE MATRIX COMPOSITES

Two silicate matrix composites, Pyrex/Nicalon and BaO-MgO-Al2O3-SiO2 ( BMAS)/Tyranno, have been used to study composite stability with respec t to time at temperature, and under applied stress. Samples aged in an oxidizing atmosphere have been tested in flexure at room temperature, and also by fibre ''push-down'' to investigate the interfacial proper ties. Tensile tests have been carried out from room temperature up to 1200 degrees C on the BMAS material, and it was found that a steady de gradation in strength occurred from 500 to 1100 degrees C, with a smal l but significant increase up to 1200 degrees C. Creep experiments hav e been performed on both the Pyrex and BMAS materials, it was found th at Pyrex has a creeping matrix and elastic fibres below the matrix sof tening point, whereas the BMAS composite showed creep in both componen ts, though at long times the creep rate was shown to be fibre controll ed. A simple model for the development of strain with time is reported and used to obtain values for the creep rate of both the matrix and f ibres. Activation energies were calculated for the creep processes in both matrix and fibres. The values obtained were: Pyrex, 256 kJ mol(-1 ), BMAS matrix, 300 kJ mol(-1) and the Tyranno fibres, 495 kJ mol(-1).