EFFECTS OF INTERFACE MODIFICATION ON MECHANICAL BEHAVIOR OF HI-NICALON FIBER- REINFORCED CELSIAN MATRIX COMPOSITES

摘要

Unidirectional celsian matrix composites having~42 volume percent of unconated or BN/SiC-coated Hi-Nicalonfibers were tested in three-point bend at room temperature. Theuncoated fiber-reinforced composites showed catastrophicfailure with strength of 210+-35MPa and a flat fracture surface.In contrast, composites reinforced with BN/SiC-coated fibersexhibited graceful failure with extensive fiber pullout. Values offirst matrix cracking stress and strain were 435+-35MPa and0.27+-0.01%, respectively, with ultimate strength as high as960MPa. The elastic Young's modulus of the uncoated andBN/SiC-coated fiber-reinforced composites were measured as184+-4GPa and 165+-5GPa, respectively. Fiber push-throughtests and microscopic examination indicated no chemicalreaction at the uncoated or coated fiber-matrix interface. Thelow strength of the uncoated fiber-reinforced composite isprobably due to degradation of the fibers from mechanicalsurface damage during processing. Because both the coated anduncoated fiber reinforced composites exhibited weak interfaces,the beneficial effect of the BN/SiC dual layer is primarily theprotection of fibers from mechanical damage during processing.