THE EFFECT OF STATIC AND CYCLIC TENSILE STRESS AND TEMPERATURE ON FAILURE FOR PRECRACKED HI-NICALON/BN/CVD SIC MINICOMPOSITES IN AIR

摘要

Single tow Hi-Nicalon, BN-mterphase, CVI SiC matrix minicomposites were tested in tension for a variety of stress/temperature conditions. These included, stress-rupture, thermal fatigue, and cyclic fatigue in air for precracked minicomposites. The time-dependent mechanical properties were compared to the time-dependent mechanical properties expected for the fibers alone. The intermediate temperature range of approx 900 to 1100 deg C was found to result in the most severe degradation judged relative to the fiber mechanical properties for the same test conditions. Cyclic stress was found to be the most severe loading condition for these intermediate temperature tests. The proposed mechanisms for intermediate temperature degradation were fiber degradation combined with stress concentrations placed on the fibers. Fiber degradation was considered to be caused by the formation of a liquid in the interphase region which reacts with the fiber. The stress concentrations result from a strong bonding glass formed in the interphase region due to solidification of the originally low viscosity B_2O_3-SiO_2 liquid as the B-content of the liquid decreased due to hydrolysis and Si-content of the liquid increased due to reaction with the fiber and matrix. For higher temperature and lower stress conditions, the fiber creep-rupture properties dominated.