HI-NICALON FIBER-REINFORCED CELSIAN MATRIX COMPOSITES - INFLUENCE OF INTERFACE MODIFICATION

Unidirectional celsian matrix composites having 42-45 vol % of uncoate d or BN-SiC coated Hi-Nicalon fibers were tested in three-point bend a t room temperature. The uncoated fiber-reinforced composites showed ca tastrophic failure with strength of 210 +/- 35 MPa and a flat fracture surface. In contrast, composites reinforced with coated fibers exhibi ted graceful failure with extensive fiber pullout. Values of first mat rix cracking stress and strain were 435 +/- 35 MPa and 0.27 +/- 0.01%, respectively, with ultimate strength as high as 960 MPa. The elastic Young modulus of the uncoated and coated fiber-reinforced composites w ere 184 +/- 4 GPa and 165 +/- 5 GPa, respectively. Fiber push-through tests and microscopic examination indicated no chemical reaction at th e uncoated or coated fiber-matrix interface. The low strength of compo site with uncoated fibers is due to degradation of the fiber strength from mechanical damage during processing. Because both the coated-and uncoated-fiber-reinforced composites exhibited weak interfaces, the be neficial effect of the BN-SiC dual layer is primarily the protection o f fibers from mechanical damage during processing.