K Nubian et al. / Journal of the European Ceramic Society 20(2000)537-544 the porous medium, x is the distance to bundle surface. isolated ZrO, grains(Fig. 7). Furthermore, the pores do In order to improve the constancy of the layer thickness not present the open channel-type pore formation typi according to Fig. 6 the deposition temperature should cal for volatilization processes. Therefore, we believe be as low as possible. Experiments show, however, that that crystallization processes as well as burn-out can be no deposition is possible at temperatures below 300 c held responsible because the reaction velocity is too slow. So deposition In spite of their relatively high interface porosity temperatures at about 310C was found optimum composites with ZrO2 fiber/matrix- interfaces show no X-ray diffraction data of the fibers CVD-coated at fiber pull-out [Fig 8(a)] and display stress-strain curves 370C showed that the coatings just after the CVD- corresponding to brittle fracture behavior. Scanning process were amorphous ZrO2. After a heat-treatment electron microscopy investigations on the fracture sur at 1250'C or during hot-pressing of composites at faces reveal that the fiber/matrix bonding, despite the 1250.C, the coatings were converted to monoclinic presence of a porous Zro2 layer, was strong. Especially ZrO2 On determination of mass changes of the coated in hot-pressing direction, the matrix at the contact points fibers after heat-treatment at 700oC in oxygen, a weight to the fibers is highly densified and is in intense contact to loss of approximately 15 wt% was found, indicating the the interfaces. The area around the fibers, perpendicular presence of residual carbon in the as-coated form, due to the hot-pressing direction on the contrary are less den to the incomplete decomposition of the Zr(acac)4. This value is in good agreement with the literature data of einbeck e 3. 2. ZrO, and C/ZrO interfaces in mullite/ aluminosilicate fiber-reinforced composites ZrOrinterfaces Transmission electron microscopic investigations of the composites with ZrO2 fiber /matrix- interface yielded porous Zro2 layers with a thickness of about 200-500 nm(Fig. 7). In a first approach, we assumed that pore formation was mainly caused by burn-out process of the residual carbon which was in the order of 15%. more detailed inspection of the microstructure in ZrO2-layer showed that the porous Zro2 after hot-pressing can not be 8m150kU492E39434979 due only to burn-out. This was derived from the observa tion that the interphase displays an open porosity which is homogeneously distributed between well-rounded and Hot-Press Direction cnse an ss Banc Dense area] on microscopic image of ZrO2 fiber/matrix drawing of interfacial conditions after hot-pressing in unidir terraces in mullite ma omposite. The interfaces consist of porous aluminosilicate fiber-reinforced/porous Zro2 interphase and ZrO2 layers with a th of about 500 nm matrIx compositesthe porous medium, x is the distance to bundle surface. In order to improve the constancy of the layer thickness according to Fig. 6 the deposition temperature should be as low as possible. Experiments show, however, that no deposition is possible at temperatures below 300C because the reaction velocity is too slow. So deposition temperatures at about 310C was found optimum. X-ray di€raction data of the ®bers CVD-coated at 370C showed that the coatings just after the CVD￾process were amorphous ZrO2. After a heat-treatment at 1250C or during hot-pressing of composites at 1250C, the coatings were converted to monoclinic ZrO2. On determination of mass changes of the coated ®bers after heat-treatment at 700C in oxygen, a weight loss of approximately 15 wt% was found, indicating the presence of residual carbon in the as-coated form, due to the incomplete decomposition of the Zr(acac)4. This value is in good agreement with the literature data of Brenn¯eck et al. 6 3.2. ZrO2 and C/ZrO2-interfaces in mullite/ aluminosilicate ®ber-reinforced composites ZrO2-interfaces Transmission electron microscopic investigations of the composites with ZrO2 ®ber/matrix-interface yielded porous ZrO2 layers with a thickness of about 200±500 nm (Fig. 7). In a ®rst approach, we assumed that pore formation was mainly caused by burn-out process of the residual carbon which was in the order of 15%. More detailed inspection of the microstructure in ZrO2-layer showed that the porous ZrO2 after hot-pressing can not be due only to burn-out. This was derived from the observa￾tion that the interphase displays an open porosity which is homogeneously distributed between well-rounded and isolated ZrO2 grains (Fig. 7). Furthermore, the pores do not present the open channel-type pore formation typi￾cal for volatilization processes. Therefore, we believe that crystallization processes as well as burn-out can be held responsible. In spite of their relatively high interface porosity, the composites with ZrO2 ®ber/matrix-interfaces show no ®ber pull-out [Fig. 8(a)] and display stress±strain curves corresponding to brittle fracture behavior. Scanning electron microscopy investigations on the fracture sur￾faces reveal that the ®ber/matrix bonding, despite the presence of a porous ZrO2 layer, was strong. Especially in hot-pressing direction, the matrix at the contact points to the ®bers is highly densi®ed and is in intense contact to the interfaces. The area around the ®bers, perpendicular to the hot-pressing direction on the contrary are less den￾Fig. 7. Transmission electron microscopic image of ZrO2 ®ber/matrix interfaces in mullite matrix composite. The interfaces consist of porous ZrO2 layers with a thickness of about 500 nm. Fig. 8. (a) Scanning electron microscopic image and (b) schematic drawing of interfacial conditions after hot-pressing in unidirectionally aluminosilicate ®ber-reinforced/porous ZrO2 interphase and mullite matrix composites. 542 K. Nubian et al. / Journal of the European Ceramic Society 20 (2000) 537±544