Boron nitride (BN)-coated aluminoborosilicate (Nextel™ 312) bers, produced via ammonia nitridation, along with 'as-received' and 'desized' bers, were composited in a silicon oxycarbide (Blackglas™) matrix. The mechanical properties, failure properties, and ber-matrix interfacial chemistry of the composite were investigated. BN treated ber composites show a 90% improvement in exural strength and substantial increases in shear strength (short beam shear and Iosipescu) over the 'as-received' ber composite. The composite fabricated with 'desized' bers underwent spontaneous delamination during pyrolization, precluding mechanical testing. X-ray photoelectron spectroscopy of the starting materials and of composite fracture surfaces combined with scanning electron microscopy and energy dispersive X-ray spectroscopy indicate that the locus of failure of the BN-coated ber composite occurs at the matrix/ BN coating interface.

The most critical technical issue preventing large scale application of ceramic matrix composites is the cost-effective application of stable interface coatings on continuous ceramic fibers. Currently, an alumina-silica ceramic fiber containing up to 14 wt. % boria (Nextel 312TM) is composited at elevated temperatures to form a boron nitride (BN) coating on the fiber surface. This BN coating serves as a compliant layer facilitating crack deflection and producing a non-catastrophic failure mode. Continued development of these ceramic matrix composites requires a more complete understanding of the mechanistic paths involved in composite densification. The objective of this work is to investigate and more clearly describe the role of the BN coating and its relation to composite processing and properties of the densified Nextel 312TM fiber/BlackglasTM (silicon oxycarbide) composites.Three composites consisting of as-received fibers (coated with an organic sizing), desized fibers (sizing removed by heating), and boron nitride coated fibers were fabricated using BlackglasTM preceramic polymer 489C B-stage resin.