Interfacial studies of refractory glass-ceramic-matrix/advanced-SiC-fiber-reinforced composites

Abstract

The main objective of this study was to characterize the chemistry and structure of new advanced small diameter silicon based fibers and to determine how these factors influence the nature of the fiber-matrix interface in refractory glass-ceramic matrix composites. It is the nature of this interface that then determines to a great degree the thermal, environmental, and mechanical properties of the composite. The fibers to be discussed in this paper include the experimental polymer derived crystalline SiC fibers Dow Corning Corp., the SiNCO “Black” fibers from Textron Specialty Materials, and the new low oxygen radiation cured Nicalon SiC type fibers from Nippon Carbon Co. Since the availability of all of these fibers was extremely limited, emphasis was placed on the mechanical (ultimate tensile strength), chemical (scanning Auger), and microstructural (scanning and transmission electron microscopies) characterization of the fibers, as well as their fracture behavior, bonding characteristics, and interfacial compatibility with various glass-ceramic matrix materials. It was found that the Dow Corning SiC fibers exhibited a varied microstructure and chemistry that ranged from primarily large grains (300–400 nm) of β-SiC to a mixture of finer grain size (100–150 nm) SiC surrounded by extremely fine grained (less than 10 nm) graphite. When incorporated into a lithium aluminosilicate or barium-magnesium alumino-silicate (BMAS) glass-ceramic matrix composite, a very thin (approximately 12 nm) graphitic carbon-rich layer was found to have formed at the fiber-matrix interface, resulting in debonding between fiber and matrix during composite fracture. In a very refractory barium aluminosilicate matrix, the fibers did not appear to form the carbon-rich interface. The Textron “Black” fibers were found to be very similar to Dow Corning's HPZ fibers in structure and composition, except they contained more carbon (approximately 27 at.%) than HPZ (approximately 19%). From reaction couples with a BMAS glass-ceramic matrix, it was found that, like HPZ fibers, a strongly bonded reaction zone of Si 2N 2O was formed at the fiber-matrix interface. Fiber coatings would be necessary to impart a weakly bonded interface for fracture tough composites to be realized. The low oxygen Nicalon fibers were found to be about 38% stronger and 42% stiffer than comparable commercially available ceramic grade Nicalon fibers, and to be much less prone to degradation on exposure to high temperature (1300 °C). Like ceramic grade Nicalon fibers, the low oxygen fibers formed a weakly bonded carbon-rich interface when incorporated into a BMAS glass-ceramic matrix.