450 K.K. Chawla/Journal of the European Ceramic Sociery 28(2008)447-453 70 wt %o Al2O3, 30wt.% SiO, i.e., compared to Nextel 480 boria is Two of the most common interphases used in fiber com- 6Osites are carbon(C)and boron nitride(BN). These materials toughness. The graphitic form of c bon has easily cleavable basal planes. This makes it ideal as a weak interface for crack deflection. The problem with carbon is that it is readily oxidized at temperatures above 400Cin air. Boron nitride also has a layer structure like that of graphite Boron nitride, although a nonoxide is more oxidation resis- tant than carbon but its oxidation resistance is a function of its Double coatings of BN (inner)/SiC(outer)double layer were 200n produced by CVD An interfacial testing system with a fat-bottomed, diamond indenter was used to obtain the interface characteristics. Three Sic BN point flexural tests were done to examine the load-displacement characteristics of the composites composite showing the mullite matrix(M)and Mullite fiber/mullite matrix composites were made via a mullite fiber (F)with sle coating of sic and bn in between sol-gel route. Mullite gels can be single-phase or diphasic according to the scale of component mixing, depending upon 4. Materials and experimental procedure the nature of the alumina and silica precursors. Single-phase 3M Company has developed a series of oxide fibers. This Nextel 550/Mullite(HP) series of fibers, called the Nextel fibers, mainly consists of alumina and mullite-type fibers. Nextel ceramic oxide fibers Sic/BN coated Uncoated are continuous and polycrystalline. These fibers are typically transparent, nonporous, and have a diameter of 10-12 um. Two mullite-based fibers from this series. Nextel 480 and Nextel 550 were used. Nextel 480 fibers have 70 wt %o Al2 O3, 28 wt% SiO nd 2 wt %o B2O3 and are currently not produced. BN coatings on Nextel 480 were produced by CVD. Nextel 550 fibers have 0 0.1 0.2 0.3 Displacement, mm extel 480/Mullite(HP) (b)400 0.00.10203040.50.6 Fig. 6.(a)Stress vs displacement for a mullite fiber(Nextel 550)mullite matrix 5am the double coating shows a damage-tolerant behavior.(b) Stress vs displacement for a mullite fiber(Nextel 480)/mullite matrix comp h no coating and two composites with different coating thicknesses of BN. The uncoated composite Fig. 5. An indentation induced crack deflection at the BN coating in the as-made and the composite with 0.3 um BN coating show catastrophic, brittle fracture while the composite with 1 um bn coating shows a damage tolerant behavior.450 K.K. Chawla / Journal of the European Ceramic Society 28 (2008) 447–453 Fig. 4. A cross-section of the composite showing the mullite matrix (M) and mullite fiber (F) with the double coating of SiC and BN in between. 4. Materials and experimental procedure 3M Company has developed a series of oxide fibers.9 This series of fibers, called the Nextel fibers, mainly consists of alumina and mullite-type fibers. Nextel ceramic oxide fibers are continuous and polycrystalline. These fibers are typically transparent, nonporous, and have a diameter of 10–12m. Two mullite-based fibers from this series, Nextel 480 and Nextel 550 were used. Nextel 480 fibers have 70 wt.% Al2O3, 28 wt.% SiO2, and 2 wt.% B2O3 and are currently not produced. BN coatings on Nextel 480 were produced by CVD. Nextel 550 fibers have Fig. 5. An indentation induced crack deflection at the BN coating in the as-made composite. 70 wt.% Al2O3, 30 wt.% SiO2, i.e., compared to Nextel 480, boria is missing. Two of the most common interphases used in fiber com￾posites are carbon (C) and boron nitride (BN). These materials have intrinsic low fracture toughness. The graphitic form of car￾bon has easily cleavable basal planes. This makes it ideal as a weak interface for crack deflection. The problem with carbon is that it is readily oxidized at temperatures above 400 ◦C in air. Boron nitride also has a layer structure like that of graphite. Boron nitride, although a nonoxide, is more oxidation resis￾tant than carbon, but its oxidation resistance is a function of its stoichiometry. Double coatings of BN (inner)/SiC (outer) double layer were produced by CVD. An interfacial testing system with a flat-bottomed, diamond indenter was used to obtain the interface characteristics. Three￾point flexural tests were done to examine the load-displacement characteristics of the composites. Mullite fiber/mullite matrix composites were made via a sol–gel route. Mullite gels can be single-phase or diphasic according to the scale of component mixing, depending upon the nature of the alumina and silica precursors. Single-phase Fig. 6. (a) Stress vs. displacement for a mullite fiber (Nextel 550)/mullite matrix composite with no coating and double coating of SiC/BN. The composite with the double coating shows a damage-tolerant behavior. (b) Stress vs. displacement for a mullite fiber (Nextel 480)/mullite matrix composite with no coating and two composites with different coating thicknesses of BN. The uncoated composite and the composite with 0.3m BN coating show catastrophic, brittle fracture while the composite with 1m BN coating shows a damage tolerant behavior.