High temperature mechanical and thermal stability of silicate matrix composites

Abstract

Two silicate matrix composites, Pyrex/Nicalon and BaO-MgO-Al₂O₃-SiO₂ (BMAS)/ Tyranno, have been used to study composite stability with respect 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 properties. Tensile tests have been carried out from room temperature up to 1200°C on the BMAS material, and it was found that a steady degradation in strength occurred from 500 to 1100°C, with a small but significant increase up to 1200°C. Creep experiments have been performed on both the Pyrex and BMAS materials, it was found that Pyrex has a creeping matrix and elastic fibres below the matrix softening point, whereas the BMAS composite showed creep in both components, though at long times the creep rate was shown to be fibre controlled. 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 fibres. 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.