K.K. Chawla/ Journal of the European Ceramic Society 28(2008)447-453 gels are prepared from aluminum and silicon alkoxides or salts; 5. Results and discussion they have molecular-scale mixing because of a polymerized oxide network formed by hydrolytic condensation. Diphasic As expected, no fiber pullout was observed in the uncoated gels involve mixing of sols of boehmite and silica or mixing of mullite/mullite composites as shown in Fig. 3. A cross-section one colloidal component with alkoxide or salt of other. The two of the composite showing the mullite matrix(M) and mullite routes are quite different, mainly because of the different scales fiber(F) with the double coating of SiC and Bn in between of component mixing. The single-phase and diphasic gels show is shown in Fig. 4. We used two types of coatings, a thicker different types of mullite crystallization behavior during heat- Bn (1 um) and a BN/SiC double coating. The thicker coating ing. Single-phase gels have a very short interdiffusion distance allows for a part of the coating to be sacrificed by oxidation because of the molecular-scale mixing, and therefore, mullite during processing. The objective of using an SiC coating was crystallization can occur at temperatures as low as 1000C In to provide oxidation protection to Bn during processing. The diphasic gels, however, the diffusion distance is much longer, efficacy of BN coating to deflect an oncoming crack is shown in so mullite crystallization does not occur until above 1250C. Fig. 5. The crack, introduced by means of an indentation, can be Retardation of mullite crystallization in the diphasic gels pro- seen to deflect at the bn coating and go around the fiber rather vides a useful processing window o This is a key point. With than penetrate it. In the BN interphase, which is isostructural diphasic gels, >95%o of theoretical density (TD)can be obtained to C, the orientation of the basal planes parallel to the substrate through one of the following two methods. surface is attributed to low surface energy perpendicular to the basal planes. The orientation of(0002)BN basal planes parallel Sintering times of more than I h between 1200 and 1300oC. to the fiber surfaces is the favorable for the damage tolerance with carefully controlled heating of composites properties. It enables easy sliding along these Sintering for I h with a low heating rate(2 CImin)and a high planes thus producing a weak fiber/matrix interface In both cases, thick bn coating or Sic/bn double compaction pressure(441 MPa)for a green bod a noncatastrophic failure mode was observed. The stress- displacement curves obtained in a three-point bend tests for Compared to this, in single-phase gels, crystalline mullite composites containing interfacial coatings of BN and SiC/BN forms at very low temperatures, which makes densification diffi- coated as well as uncoated composites are shown in Fig 6a and cult because of the high degree of covalent bonding in crystalline b. Fig 7a shows the fracture surfaces of the composites contain- mullite. The result is that the densities obtained at the same ing l um BN-coated fibers and while Fig 7b shows the fracture hot pressing temperature are much lower than with the viscous- surface of a composite containing SiC/BN-coated fibers In both phase processing cases, the phenomenon of fiber pullout occurred; which led to a (a) 25m 100m Fig. 7. Fiber pullout(a)in thick BN-coated and(b) SiC/BN-coated mullite fiber/mullite matrix composite.K.K. Chawla / Journal of the European Ceramic Society 28 (2008) 447–453 451 gels are prepared from aluminum and silicon alkoxides or salts; they have molecular-scale mixing because of a polymerized￾oxide network formed by hydrolytic condensation. Diphasic gels involve mixing of sols of boehmite and silica or mixing of one colloidal component with alkoxide or salt of other. The two routes are quite different, mainly because of the different scales of component mixing. The single-phase and diphasic gels show different types of mullite crystallization behavior during heat￾ing. Single-phase gels have a very short interdiffusion distance because of the molecular-scale mixing, and therefore, mullite crystallization can occur at temperatures as low as 1000 ◦C. In diphasic gels, however, the diffusion distance is much longer, so mullite crystallization does not occur until above 1250 ◦C. Retardation of mullite crystallization in the diphasic gels pro￾vides a useful processing window.10 This is a key point. With diphasic gels, >95% of theoretical density (TD) can be obtained through one of the following two methods: - Sintering times of more than 1 h between 1200 and 1300 ◦C, with carefully controlled heating; - Sintering for 1 h with a low heating rate (2◦ C/min) and a high compaction pressure (441 MPa) for a green body. Compared to this, in single-phase gels, crystalline mullite forms at very low temperatures, which makes densification diffi- cult because of the high degree of covalent bonding in crystalline mullite. The result is that the densities obtained at the same hot pressing temperature are much lower than with the viscous￾phase processing. 5. Results and discussion As expected, no fiber pullout was observed in the uncoated mullite/mullite composites as shown in Fig. 3. 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 is shown in Fig. 4. We used two types of coatings, a thicker BN (1m) and a BN/SiC double coating. The thicker coating allows for a part of the coating to be sacrificed by oxidation during processing. The objective of using an SiC coating was to provide oxidation protection to BN during processing. The efficacy of BN coating to deflect an oncoming crack is shown in Fig. 5. The crack, introduced by means of an indentation, can be seen to deflect at the BN coating and go around the fiber rather than penetrate it. In the BN interphase, which is isostructural to C, the orientation of the basal planes parallel to the substrate surface is attributed to low surface energy perpendicular to the basal planes. The orientation of (0 0 0 2) BN basal planes parallel to the fiber surfaces is the favorable for the damage tolerance of composites properties.11 It enables easy sliding along these planes thus producing a weak fiber/matrix interface. In both cases, thick BN coating or SiC/BN double coating, a noncatastrophic failure mode was observed. The stress￾displacement curves obtained in a three-point bend tests for composites containing interfacial coatings of BN and SiC/BN￾coated as well as uncoated composites are shown in Fig. 6a and b. Fig. 7a shows the fracture surfaces of the composites contain￾ing 1m BN-coated fibers and while Fig. 7b shows the fracture surface of a composite containing SiC/BN-coated fibers. In both cases, the phenomenon of fiber pullout occurred; which led to a Fig. 7. Fiber pullout (a) in thick BN-coated and (b) SiC/BN-coated mullite fiber/mullite matrix composite.