Mechanical and Microstructural Properties of Nextel™ 720 Relating to its Suitability for High Temperature Application in CMCs

Nextel™ 720 fibers and their suitability for high-temperature (HT) applications in CMCs were analyzed at temperatures ranging from 900° to 1300°C. The thermal stability of the Nextel™ 720 fibers was characterized by single-fiber tensile tests at various temperatures after heat treatments up to 1300°C. In addition to bulk fiber grain growth resulting from heat treatment at T > 1200°C, a locally-catastrophic degradation of the fiber material occurred during heat treatment at 1300°C. This degradation was caused by locally enriched impurities and had a mean spacing of 135mm [1]. The subcritical crack growth (SCG) was determined using the dynamic fatigue method. At 900°C no SCG could be detected while at 1000°C (N=18) and at 1100°C (N=9.4) intensive SCG was found. Nevertheless the locally catastrophic degradation as well as the intense SCG has only a minor influence on the use of Nextel™ 720 fibers in a ceramic composite; this due to low values (between 0.34 and 15.2 mm) of the fiber length relevant to composite failure.