J. Kimmel et al. Journal of the European Ceramic Society 22(2002)2769-2775 Mullite was used in the intermediate layer because its shows an example of a surface asperity that correlated thermal expansion coefficient is close to that of SiC, and with localized spallation. The surface asperity in the its higher temperature capability compared with BSAs CFCC with SiC seal coat causes vertical cracking in the However, the stability of mullite in the combustor EBC that exists either as-processed or after thermal environment appears to be an issue. The mullite phase cycles from engine start/stops Once the crack is formed, rom both liners separated into silica and alumina pha- accelerated oxidation of the silicon layer occurs raising ses. When the bsas top layer recessed, the silica in the the middle and top layers, thus, causing the coating to intermediate layer was preferentially lost leaving behind buckle and eventually spall. The CFCC liner fabrication porosity(Fig. I1) process is being modified to minimize surface asperities As previously discussed, pinholes formed at many In addition, smoothing of the EBC is being evaluated in locations where surface asperities occurred. Fig. 12 Keiser rig testing 0.5mm (a) 0.5 mm Fig. 12. Pinhole formation on the outer liner at the location of surface asperities. The stages of EBC spallation include(a) vertical cracking of the mullite BSAs intermediate and BSAS top layers, and (b) accelerated oxidation of the silicon layer raising the middle and top layers and, thus causing the coating to buckleMullite was used in the intermediate layer because its thermal expansion coefficient is close to that of SiC, and its higher temperature capability compared with BSAS. However, the stability of mullite in the combustor environment appears to be an issue. The mullite phase from both liners separated into silica and alumina pha￾ses. When the BSAS top layer recessed, the silica in the intermediate layer was preferentially lost leaving behind porosity (Fig. 11). As previously discussed, pinholes formed at many locations where surface asperities occurred. Fig. 12 shows an example of a surface asperity that correlated with localized spallation. The surface asperity in the CFCC with SiC seal coat causes vertical cracking in the EBC that exists either as-processed or after thermal cycles from engine start/stops. Once the crack is formed, accelerated oxidation of the silicon layer occurs raising the middle and top layers, thus, causing the coating to buckle and eventually spall. The CFCC liner fabrication process is being modified to minimize surface asperities. In addition, smoothing of the EBC is being evaluated in Keiser rig testing. Fig. 12. Pinhole formation on the outer liner at the location of surface asperities. The stages of EBC spallation include (a) vertical cracking of the mullite+BSAS intermediate and BSAS top layers, and (b) accelerated oxidation of the silicon layer raising the middle and top layers and, thus, causing the coating to buckle. 2774 J. Kimmel et al. / Journal of the European Ceramic Society 22 (2002) 2769–2775