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Corrosion of Ceramic and Composite Materials Part 12

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Tham khảo tài liệu 'corrosion of ceramic and composite materials part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 318 Chapter 7 MoSi2 is its oxidation resistance. Cook et al. 7.82 investigated the incorporation of 30 vol. TiB2 ZrB2 HfB2 and SiC as a reinforcement in hopes of developing a composite of greater oxidation resistance than the base MoSi2. Specimen were exposed to isothermal testing at 800 C 1200 C 1400 C and 1500 C for 24 hr in air in addition to a thermal cycle consisting of 55 min at 1200 C or 1500 C and then 5-min ambient cooling with subsequent reheating. All the boride-containing materials exhibited a greater deterioration than the silicon carbide-containing composite although none exhibited a greater oxidation resistance than the base MoSi2. See Sec. 5.2.3 on borides for a discussion of the oxidation of these materials. Although not generally thought of as metal matrix composites a relatively new class of materials called fibrous monolithic ceramics 7.83 actually may contain a metal as the matrix that surrounds cells of a fibrous polycrystalline ceramic. One example of such a material investigated by Baskaran et al. 7.84 contained fibrous polycrystalline alumina cells surrounded by nickel. The nickel cell boundary thickness varied from 1 to about 15 pm. Oxidation at 1200 C for 10 hr initially formed NiO that subsequently reacted with the alumina forming NiAl2O4. The formation of the aluminate was thought to provide protection toward additional oxidation. 7.5 POLYMER MATRIX COMPOSITES Two publications by ASTM discuss the environmental effects upon polymeric composites 7.85 7.86 . The largest amount of composites produced is probably of this type reinforced with glass fibers called glass-reinforced plastics polymers or polyesters GRP . Degradation in aqueous environments generally occurs by fiber matrix debonding. Since glass fibers are attacked by moisture which drastically reduces their strength glass fibers are given a protective coating. Graphite carbon fiber epoxy composites CFRP have seen some recent use in marine environments. In many cases they are .