Properties of CVD SiC, CVI SiC/SiC, and NITE — SiC/SiC have been reported and discussed. The CVI SiC/SiC composite combines the advantageous properties of CVD SiC and the benefits of reinforce­ment by SiC-based continuous fibers. In particular, CVI SiC/SiC exhibits damage tolerance, limited sen­sitivity to flaws and notch, high load-carrying capac­ity, and improved reliability. As opposed to earlier generations of SiC/SiC that were reinforced with Nicalon fibers, the database for SiC/SiC reinforced with the advanced near-stoichiometric fibers is incomplete. The properties of Nicalon-reinforced SiC/SiC should provide a useful baseline. The main features of the mechanical behavior of SiC/SiC composites have been described, and the relationships between the microstructure and properties have been discussed. They have been established on CVI SiC/SiC composites reinforced with Nicalon fibers. They should be reproduced on those CVI SiC/ SiC composites reinforced with near-stoichiometric Hi-Nicalon S fibers. The inherent surface roughness of Tyranno-SA3 fibers is an issue.

Precise knowledge of the mechanisms that govern the mechanical behavior is useful for proper use or design with SiC/SiC. Damage and ultimate fracture of CVI SiC/SiC involve load transfer from matrix to fibers at various length scales defined by the 2D woven structure and the tow microstructure. At high tempera­tures, additional load transfers are driven by the local stress relaxation induced by temperature and/or envi­ronment. The ultimate fracture and delayed failure are dictated by the tows. Scale effects and scatter in strength data are limited when compared to monolithic ceramics. Fiber-matrix interfaces and interphases play a significant role in damage tolerance and load­carrying capacity. Interfaces resistant to crack extension are beneficial to composite performances. However, this scheme is invalid when the fiber surface is rough.

Properties of CVI SiC/SiC can be tailored via engi­neering of the interfaces and the use of advanced fibers.