Mechanical Behavior of SiC(f)/SiC Composites and Correlation to in situ Fiber Strength at Room and Elevated Temperatures

Abstract

Mechanical properties of Nicalon‐fiber‐reinforced silicon carbide matrix composites were evaluated, in flexure, at various temperatures ranging from ambient to 1300°C. First matrix cracking stress ranged from 250 to 280 MPa and was relatively insensitive to test temperature. The measured ultimate strength showed a small increase from a room‐temperature value of 370 to 460 MPa at 800°C. Beyond 800°C, however, strength dropped to as low as 280 MPa at 1300°C. This decrease in ultimate strength at elevated temperatures is believed to be partly due to degradation of in situ Nicalon fiber strength. Scanning electron microscopy was employed to evaluate the in situ Nicalon fiber strengths via fracture mirror size measurements. Degradation of Nicalon fiber strength is attributed to thermal damage and to structural changes to the fiber at elevated temperatures. Measured values of ultimate strength of the composites were compared with predictions made on the basis of in situ fiber strength characteristics and an available analytical model.