Oxide fiber reinforced alumina matrix composites

SILICON CARBIDE AND OXIDE FIBER REINFORCED ALUMINA MATRIX COMPOSITES... 

Tests on tin oxide fiber coatings in model composite systems indicated some crack deflection at the coating-fiber interface (Siadati et al., 1991 Venkatesh and Chawla, 1992). However, tensile tests of tin oxide coated alumina fiber-reinforced alumina matrix composites demonstrated a decrease in the extent of fiber pullout as the density of the matrix phase was increased. This led to increasingly brittle fracture behavior in these composites (Goettler, 1993). Tin oxide also has thermal stability problems at elevated temperatures (Norkitis and Hellmann, 1991). For example, in the presence of air at temperatures above 1300°C (2,372°F), tin oxide (solid) decomposes into SnO (gas) and Oj (gas). This decomposition occurs at even lower temperatures when the partial pressure of oxygen in the test environment is reduced. 

Two all-oxide composites were studied. A sol-gel derived alumina matrix composite produced by COI Ceramics that is reinforced with Nextel 720 (3M Company) fibers. The matrix porosity level is 40% and the fiber volume fraction is 47%. The processing route is shown in Figure 1. A sintering process between 1800 2100 °F controls porosity levels. The second ceramic composite was a mullite/alumina matrix composite developed by the University of California at Santa Barbara with the same reinforcing fibers. The porosity level for this material is 40% with a fiber volume fraction of 37%. The processing route is shown in Figure 2. A repeated precursor-impregnation process is responsible for the porosity. 

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. 

The aforementioned fibers are combined with various matrix materials to produce oxide-oxide CMCs. Coatingless, porous-matrix CMCs based on fabric reinforcement are the most fully developed and examples of these materials are given in Table 2. General Electric s (GE) GEN-IV was one of the earliest porous matrix, all-oxide fiber composites, while COI Ceramics (COI ) is currently the most prominent commercial supplier of oxide-oxide composites. The matrix materials are primarily aluminosilicate (ex. manufacturers GE, COI), alumina-mullite (University of California, Santa Barbara [UCSB]), and/or alumina (COI). 

The material studied was Nextel 720/Alumina (N720/A), an oxide-oxide CMC (manufactured by COl Ceramics, San Diego, CA) consisting of a porous alumina matrix reinforced with Nextel 720 fibers. There is no fiber coating. The damage tolerance of N720/A is enabled by the porous matrix. The composite was supplied in a form of 2.8-mm thick plates comprised of 12 0°/90°... 

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