K.K. Chawla/ Journal of the European Ceramic Society 28(2008)447-453 n Fig. 2. Schematic of periodic interfacial roughness. The fiber and matrix are indicated by f and m, respectively. A is the amplitude of the interfacial roughness. should be added algebraically to the radial thermal stress, 1.e the effect of surface roughness of the fiber along the debonded interface on the radial stress at the interface needs to be con- sidered together with the thermal stresses. The sense of the two e surface ro ughness of the avail- able oxide fibers can be varied significantly. In ceramic matrix composites, roughness-induced interfacial gripping, especially in the radial direction. will affect the interface debondin the sliding friction of debonded fibers, and the fiber pullout The objective of this paper is to show that by applying the principle of interfacial engineering in mullite fiber rein- forced mullite matrix composites, one can get fully dense but tough, damage tolerant composites. Thus, approaches involving a porous matrix or interlace are not considered here. Thin film coatings on fibers are used to modify and control the interface behavior in CMCs. In particular, BN can be a weak interphase between the mullite fiber and mullite matrix because of its graphite-type layer structure. 6- A major disadvantage of BN coatings is their poor oxidation resistance at high temperatures There are two ways around this problem. One possible way is to e a thick bn coating so a portion of the coating can be sacri- ficed during processing. The other possibility is to protect the B 2.5gm coating it by a second coating with better oxidation resistance, such as SiC. i.e.. use sic/bn double coating. That still retains a onoxide interphase in an oxide fiber/oxide matrix composite. It should be recognized here that both bn and sic/bn coatings are Fig 3. A mullite fiber/mullite matrix composite processed without an interphase nonoxides and thus susceptible to oxidation at high temperatures coating. The etched cross-section shows that mullite fiber in the center was lost after processingK.K. Chawla / Journal of the European Ceramic Society 28 (2008) 447–453 449 Fig. 2. Schematic of periodic interfacial roughness. The fiber and matrix are indicated by f and m, respectively. A is the amplitude of the interfacial roughness. should be added algebraically to the radial thermal stress, i.e., the effect of surface roughness of the fiber along the debonded interface on the radial stress at the interface needs to be con￾sidered together with the thermal stresses. The sense of the two can be the same or opposite. The surface roughness of the avail￾able oxide fibers can be varied significantly. In ceramic matrix composites, roughness-induced interfacial gripping, especially in the radial direction, will affect the interface debonding, the sliding friction of debonded fibers, and the fiber pullout length.1,3 The objective of this paper is to show that by applying the principle of interfacial engineering in mullite fiber rein￾forced mullite matrix composites, one can get fully dense but tough, damage tolerant composites. Thus, approaches involving a porous matrix or interlace are not considered here. Thin film coatings on fibers are used to modify and control the interface behavior in CMCs. In particular, BN can be a weak interphase between the mullite fiber and mullite matrix because of its graphite-type layer structure.6–8 A major disadvantage of BN coatings is their poor oxidation resistance at high temperatures. There are two ways around this problem. One possible way is to use a thick BN coating so a portion of the coating can be sacri- ficed during processing. The other possibility is to protect the BN coating it by a second coating with better oxidation resistance, such as SiC, i.e., use SiC/BN double coating. That still retains a nonoxide interphase in an oxide fiber/oxide matrix composite. It should be recognized here that both BN and SiC/BN coatings are nonoxides and thus susceptible to oxidation at high temperatures in air. Fig. 3. A mullite fiber/mullite matrix composite processed without an interphase coating. The etched cross-section shows that mullite fiber in the center was lost after processing