Temperature-dependent structure-property relations in continuous mullite-based ceramic fibers

Mullite-based fibers are promising candidates for reinforcement fibers for ceramic matrix composites. This paper summarizes the high-temperature properties of continuously produced mullite and zirconia-toughened mullite (ZTM) fibers. Special attention was devoted to the continuous sintering of fibers with complete mullite crystallization while avoiding grain growth. The fibers were annealed between 1000 and 1500 °C for 25 h in air and analyzed at room temperature. The addition of ZrO2 did not significantly reduce the average matrix grain size. With increasing annealing temperature, notable changes in the phase composition occurred. Pristine fibers contained mullite and intermediary γ-Al2O3 and an amorphous phase, leading to an ongoing mullite crystallization up to 1200 °C. Grain growth and changes in the phase composition led to a decrease in Weibull strength. ZTM fibers showed a lower relative strength retention than mullite fibers, but contained up to 76 % (1310 MPa) of their initial strength at 1300 °C.