Long-term behavior of polycrystalline oxide fibers at elevated temperatures

Abstract

Ceramic matrix composites based on oxide materials have gained more attention in the last decades, because of their chemical stability, strength and considerably high toughness. The development of this class of material was only possible due to the appearance of suitable oxide fibers. Currently available fibers are based either on alumina for high strength, or on mullite for better long-term performance. However, it is well known that these fibers are prone to loss of their mechanical properties above 1000 degree celsius. Although high, these temperatures are easily reached during processing and in-field applications of composites. Therefore, the aim of this work is to study the mechanical behavior and the degradation mechanisms of oxide fibers at elevated temperatures. For that, two mullite fibers were evaluated: the well-established mullitea alumina NextelTM 720, and the fully crystalline mullite fiber CeraFib 75. Both fibers were analyzed before and after heat treatments at temperatures ranging 1000-1400 degree celsius for 25 h. The characterization approach included microstructural analyses, as well as creep and tensile tests at room and high temperatures. For comparison, the same procedure was conducted with the alumina fibers NextelTM 610 and CeraFib 99. As-received NextelTM 720 fibers present a microstructure of mullite grains with smaller a-alumina grains, whereas the microstructure of CeraFib 75 consists basically of mullite with traces of y-alumina. The higher amount of mullite in CeraFib 75 resulted in lower room-temperature strength. Still, CeraFib 75 showed higher strength retention than NextelTM 720 at temperatures above 1200AdegreeC, while the measured creep rates were in the same order of magnitude. With the thermal treatments performed, two microstructural changes were observed: grain growth and dissociation of the mullite phase. The kinetics of these reactions were quantified and related to the mechanical performance of the fibers. Thus, a strength decrease was observed for all oxide fibers mainly due to grain growth. On the other hand, the phase transformations caused by the thermal exposures improved the thermal stability of the fibers. As a consequence, the treated fibers were more resistant to creep, i.e., the creep rates decreased. In summary, this work presents a more detailed analysis on the long-term behavior of oxide fibers at high temperatures. Based on these results, it is suggested that a fiber with a chemical composition near to the stoichiometric 3/2 mullite would have higher thermal stability.