Effects of Environment on Creep Behavior of Two Oxide-Oxide Ceramic Matrix Composites at 1200 degs C

摘要

Previous studies by the advisor and graduate students examined creep behavior of the Nextel720/Alumina CMC in air and in 100% steam environments at 1200 and 1330 deg C. Results showed that while this oxide/oxide system exhibits an exceptionally high fatigue limit at 1200 deg C it also experiences substantial strain accumulation under sustained loading conditions. Furthermore, these earlier investigations revealed a significant degrading effect of 100% steam environment on material performance under both static and cyclic loadings. The present effort will investigate creep rupture behavior of Nextel720/Alumina composite in the inert gas environment. In addition, creep rupture behavior of Nextel720/Aluminosilicate CMC will be investigated in both inert gas and in 100% steam environments. Combined with existing data, results of this research will fully reveal effects of progressively more oxidizing environment on creep resistance of these CMCs. In addition, degradation of Nextel720 fibers under load in oxidizing environments will be assessed. The study followed a systematic plan. Baseline tensile tests were performed to verify the at-temperature basic properties and to guide selection of the creep stress levels. Creep-rupture tests were carried out at different stress levels at 1200 deg C. In order to examine combined effects of temperature and exposure to oxidizing environment on the creep response, creep-rupture tests were performed in the inert gas and in 100% steam environments. For selected creep stress levels, creep tests in laboratory air were performed as well. As a result of this effort creep-rupture curves, as well as families of creep curves were established. Degradation of creep resistance due to increasing moisture exposure were assessed. Composite microstructure, as well as damage and failure mechanisms were examined.