Oxide fibers glass matrix composites

Oxide fibers find uses both as insulation and as reinforcements. Glass fibers, based on silica, possess a variety of compositions in accordance with the characteristics desired. They represent the biggest market for oxide fibers. Unlike other oxide fibers, glass fibers are continuously spun from the melt and are not used at temperatures above 250°C. Short oxide fibers can be melt blown whilst other aluminasilicate and alumina based continuous fibers are made by sol-gel processes. Initial uses for these fibers were as refractory insulation, up to 1600°C, but they are now also produced as reinforcements for metal matrix composites. Continuous oxide fibers are candidates as reinforcements for use up to and above 1000°C. 

Figure 7-54. Two types of oxidation of SiC fiber/SiC matrix composites with BN fiber coatings (a) Borosili-cate glass formation after oxidation at 816 C in oxygen for 100 h. (b) Volatilization of BN due to water vapor interactions after oxidation at 500 °C in humid air. (Jacobson et al., 1999b).

When carbon fiber is added to a glass matrix, it would be expected to form a composite, with improvements in toughness, strength (when using continuous fiber), increased thermal conductivity and a decreased coefficient of thermal expansion. The glass matrix would not, however, protect the carbon fiber from oxidation in air at about 500°C [76] and the fiber would then have little bonding to the glass matrix. 

The interface between HM carbon fiber and a Pyrex borosilicate glass matrix was Analyzed by Bleay and Scott [111,112] and found to be some 100 nm thick and believed to comprise Na enriched silicon oxycarbide, showing that some reaction had taken place during fabrication. Measurement of the interlaminar shear strength of the composite indicated that this layer was not a source of weakness. Substantial fiber pull-out had occurred, however, exposing clean fiber surfaces and smooth sockets. It was concluded that the interfacial shear process was confined to the outer layer of the fiber. Heat treatment of the composite in air caused preferential oxidation of the fiber, the rate being higher parallel to the fiber axis than perpendicular to it. 

Oxide fibers include glass fibers, mullite fibers, zirconia fibers and alumina fibers. Of these, a-alumina-based fibers have been used intensively for ceramic matrix composites. Fiber FP, manufactured by Du Pont in 1979, was the first wholly a-alumina fiber produced [34]. At present, Almax (Mitsui Mining Material Co. Ltd., Japan) and Nextel 610 (3M Co., USA) are commercially available a-alumina fibers. Almax contains 99.5% alumina and has an elastic modulus of 330 GPa, and Nextel 610 has a tensile strength of 2.4 GPa and an elastic modulus of 380 GPa [35]. 

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