Temperature-induced fibre/matrix interactions in porous alumino silicate ceramic matrix composites

The thermal stability of alumino-silicate fibre (Nextel 720)/porous mullite matrix composites was investigated in the temperature range between 1300 and 1600°C. In the as-prepared state the fibres consist of mullite plus α-Al2O3, while the porous mullite matrix includes minor amounts of a SiO2-rich glass phase. Temperature-controlled reactions between the silica-rich glass phase of the matrix and α-Al2O3 at the rims of the fibres to form mullite have been observed. At the end of this process, virtually all glass phase of the matrix is consumed. Simultaneously, alumina-free layers about 1 μm thick are formed at the periphery of the fibres. The mullite forming process is initiated above about 1500°C under short time heat-treatment conditions (2 h) and at much lower temperature (1300°C) under long-term annealing (1000 h). Subsequent to annealing below the thermal threshold, the composite is damage tolerant and only minor strength degradation occurs. Higher annealing temperatures, however, drastically reduce damage tolerance of the composites, caused by reaction-induced gradually increasing fibre/matrix bonding. According to this study, the thermal stability of alumino silicate (Nextel 720) fibre/mullite matrix composites ranges between 1500°C in short-term and 1300°C in long-term heat-treatment conditions.