Paper number 147


Narottam P. Bansal and Jeffrey I.

Eldridge National Aeronautics and Space Administration, Lewis Research Center Materials Division,Cleveland, Ohio 44135, U.S.A.

Summary To evaluate the effects of fiber coatings on composite mechanical properties, unidirectional celsian matrix composites reinforced with uncoated Hi-Nicalon fibers and those precoated with a dual BN/SiC layer in two separate batches (batch 1 and batch 2) were tested in three-point flexure. The uncoated-fiber reinforced composites showed catastrophic failure with strength of 210 +-35 MPa and a flat fracture surface. In contrast, composites reinforced with coated fibers exhibited graceful failure with extensive fiber pullout and showed significantly higher ultimate strengths, 904 and 759 MPa for the batch 1 and 2 coatings, respectively. Fiber push-in tests and microscopic examination indicated no chemical reaction at the uncoated or coated fiber-matrix interfaces that might be responsible for fiber strength degradation. Instead, the low strength of composite with uncoated fibers was due to degradation of the fiber strength from mechanical damage during composite processing. Despite identical processing, the first matrix cracking stresses (sigma mc) of the composites reinforced with fibers coated in batch 1 and batch 2 were quite different, 436 and 122 MPa, respectively. The large difference in sigma mc of the coated-fiber composites was attributed to differences in fiber sliding stresses (tau friction), 121.2 +- 48.7 and 10.4 +- 3.1 MPa, respectively, for the two composites as determined by the fiber push-in method. Such a large difference in tau friction for the two composites was found to be due to the difference in the compositions of the interface coatings. Scanning Auger microprobe analysis revealed the presence of carbon layers between the fiber and BN, and also between the BN and SiC coatings in the composite showing lower friction. This resulted in lower sigma mc, in agreement with the ACK theory. The ultimate strengths of the two composites depended mainly on the fiber volume fraction and were not significantly effected by tau friction values, as expected. The poor reproducibility of the fiber coating composition between the two batches was judged to be the primary source of the large differences in performance of the two composites.
Keywords oxide matrix composite, fiber-matrix interface, interphase composition, fiber coatings, fiber push-in, mechanical properties, microstructure, silicon carbide fiber.

Theme : Ceramic Matrix and C/C Composites ; Interface and Interphase

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