Developing Interfacial Carbon‐Boron‐Silicon Coatings for Silicon Nitride‐Fiber‐Reinforced Composites for Improved Oxidation Resistance

Abstract

C‐B‐Si coatings were formed on a Si3N4 fiber using chemical vapor deposition and embedded in a Si‐N‐C matrix using polymer impregnation and pyrolysis. The boron‐containing layer was anticipated to form borosilicate glass and seal oxygen‐diffusion passes. Two types of C‐B‐Si coatings were tested on the fiber–matrix interface, and they improved the oxidation resistance of the composite. The first coating was multilayered: a crystalline sublayer composed of B‐Si‐C was sandwiched between two graphitelike carbon sublayers. The second coating was a graphitelike carbon layer containing a small amount of boron and silicon. The carbon (sub)layer of both coatings weakened the fiber–matrix bonding, giving the composites a high flexural strength (1.1 GPa). The composites retained 60%–70% of their initial strength, even after oxidation at 1523 K for 100 h. The mechanism for improved oxidation resistance was discussed through the microstructure of the interface, morphology of the fracture surface, and oxygen distribution on a cross section of the oxidized composite.