Influence of heat treatment and fiber orientation on the damage threshold and the fracture behavior of Nextel fiber-reinforced Mullite-SiOC matrix composites analysed by acoustic emission monitoring

In the present study, we elucidate the influence of oxidative heat exposures at 1000 and 1200 °C on an alumina fiber-reinforced polymer-derived ceramic matrix composite containing small residual amounts of carbon. Therefore, we investigated the flexural performance and fracture toughness of on- (0°/90°) and off-axis (45°) reinforced samples. Acoustic emission was used to monitor the internal damage and its progression during loading. At 1000 °C, a moderate reduction of strength and fracture toughness is found while after exposure to 1200 °C a dramatic decrease down to 50 % is observed. For all composites, a reduction of the damaged volume was found after heat treatments indicating a decrease of crack deflection. However, especially at 1000 °C, composites reinforced in 0°/90° direction seemed to be more affected, as no detrimental effect on the mechanical performance was found for the 45° composites. Remarkably, the oxidation-induced silica formation increases the absolute and relative damage thresholds of all composites. A Griffith-like linear relationship between strength and toughness is found. These findings are pivotal for designing and engineering next generation CMCs toward long-term applications.