K Nubian et al. / Journal of the European Ceramic Society 20(2000)537-544 sified and the porous matrix is only partly in contact to the fiber interfaces. Obviously, there exist radial stress gradients around the fibers with high stress concentra- tion in hot-pressing direction [Fig. 8(b)]. We believe that the processing conditions(e.g phase combination of the matrix material, temperature and the way how the pres- sure was applied, phase transformation occurring in the matrix and the interphase material, etc ) may have great nfluence on the possible formation of strong bonding at the interface. The slurry infiltrated pre-composite con- tains as a matrix amorphous SiO2-rich phase which soft ens at relatively lower temperatures. During hot- pressing, the amorphous phase in the matrix facilitates sintering, versus a viscous flow of ?-Al2O3-particles before transforming to mullite at 1250C. The sintering experiments show that mullite formation does occur at temperatures >1250C only. This means that the amor- phous phase is present throughout the whole processing ne and may lead to strong bonding with the interface 3.3. C/ZrOr-interfaces A further approach in order to achieve damage toler ant composites, weakening of fiber/matrix-bonding and elimination of the process-related stresses between fibers and matrix has been established by using the fugitive coating concept. These fiber/matrix-interfaces consist of C/ZrO2 double layers and are deposited by CVD. Since Fig 9. Scanning electron image of interface in CIZror-coated mullite hot-pressing was performed in argon atmosphere, the atrix composites (a) and 3-point-bending stress-str urve of this composite carbon layer acted as a buffer zone and that way pre- vented exaggerated interfacial fiber/matrix bonding and Iso the formation of compressive stresses at the matrix/ sipating crack deflection process to take place. There fiber-interfaces in hot-pressing direction. TEM investi- fore, the gap thickness should be at the same order as gations on as-hot-pressed samples confirmed the exis- surface roughness of fiber and matrix. tence of thin(10-100 nm)amorphous carbon layers surrounding fibers. After burn-out of the carbon layers in air thin gaps between fiber and zro2-layer are forme formed 4 Conclusions [Fig 9(a)]. Scanning electron microscopy observations and stress-strain curves of the C/ZrO2-interface comp The ZrO2 deposition was described by a first order sites demonstrate that an extensive fiber pull-out, thus reaction. Deposition temperatures about 300C was damage tolerant behavior is achieved [Fig 9(b)]. Crack found to be optimum for ZrO2-coating on fiber fabrics, deflection occurs at the gap, while the ZrO2-layer compared to higher temperatures used with dense remains attached to the matrix. Apparently, the thick wafers. Use of Zr-acetylacetonate resulted in porous ness of the fugitive coating plays a major role for the coatings after hot-pressing the oxided oxide fiber-rein damage tolerant behavior of the composite. a broad forced composites. C/ZrO2-double coating are achieved gap surrounding the fiber can not work, since the by successive CVD-coating with propane and Zr-acet required load-transfer between matrix and fibers does ylacetonate not occur On the other hand, if the gap is too narrow, Composites are fabricated by infiltration of coated many local contact points between the matrix and fiberfiber yarns with pre-mullite slurry and consequently develop, leading to a strong fiber/matrix bonding and hot-pressing the infiltrated prepegs. Porous ZrOz-coat an associated brittle fracture behavior of the composite. ing at interface of mullite/aluminosilicate fiber-rein ur present studies have shown that carbon layers with forced composites displayed no fiber-pull-out. C/ZrO2, thickness ranging between about 10 and 100 nm are in turn, resulted in damage tolerant fracture of the suitable for fugitive coatings. Obviously the ideal gap composites. The mechanical and microstructural obser- thickness is that which enables the fiber to remain vations of the composites at RT and at 1200C after 2 h uncompressed, at the same time, allows energy dis- heat-treatment showed that a thickness of 10 nm for thesi®ed and the porous matrix is only partly in contact to the ®ber interfaces. Obviously, there exist radial stress gradients around the ®bers with high stress concentra￾tion in hot-pressing direction [Fig. 8(b)]. We believe that the processing conditions (e.g. phase combination of the matrix material, temperature and the way how the pres￾sure was applied, phase transformation occurring in the matrix and the interphase material, etc.) may have great in¯uence on the possible formation of strong bonding at the interface. The slurry in®ltrated pre-composite con￾tains as a matrix amorphous SiO2-rich phase which soft￾ens at relatively lower temperatures. During hot￾pressing, the amorphous phase in the matrix facilitates sintering, versus a viscous ¯ow of g-Al2O3-particles before transforming to mullite at 1250C. The sintering experiments show that mullite formation does occur at temperatures 51250C only. This means that the amor￾phous phase is present throughout the whole processing line and may lead to strong bonding with the interface. 3.3. C/ZrO2-interfaces A further approach in order to achieve damage toler￾ant composites, weakening of ®ber/matrix-bonding and elimination of the process-related stresses between ®bers and matrix has been established by using the fugitive coating concept. These ®ber/matrix-interfaces consist of C/ZrO2 double layers and are deposited by CVD. Since hot-pressing was performed in argon atmosphere, the carbon layer acted as a bu€er zone and that way pre￾vented exaggerated interfacial ®ber/matrix bonding and also the formation of compressive stresses at the matrix/ ®ber-interfaces in hot-pressing direction. TEM investi￾gations on as-hot-pressed samples con®rmed the exis￾tence of thin (10±100 nm) amorphous carbon layers surrounding ®bers. After burn-out of the carbon layers in air thin gaps between ®ber and ZrO2-layer are formed [Fig. 9(a)]. Scanning electron microscopy observations and stress±strain curves of the C/ZrO2-interface compo￾sites demonstrate that an extensive ®ber pull-out, thus damage tolerant behavior is achieved [Fig. 9(b)]. Crack de¯ection occurs at the gap, while the ZrO2-layer remains attached to the matrix. Apparently, the thick￾ness of the fugitive coating plays a major role for the damage tolerant behavior of the composite. A broad gap surrounding the ®ber can not work, since the required load-transfer between matrix and ®bers does not occur. On the other hand, if the gap is too narrow, many local contact points between the matrix and ®ber develop, leading to a strong ®ber/matrix bonding and an associated brittle fracture behavior of the composite. Our present studies have shown that carbon layers with thickness ranging between about 10 and 100 nm are suitable for fugitive coatings. Obviously the ideal gap thickness is that which enables the ®ber to remain uncompressed, at the same time, allows energy dis￾sipating crack de¯ection process to take place. There￾fore, the gap thickness should be at the same order as surface roughness of ®ber and matrix. 4. Conclusions The ZrO2 deposition was described by a ®rst order reaction. Deposition temperatures about 300C was found to be optimum for ZrO2-coating on ®ber fabrics, compared to higher temperatures used with dense wafers. Use of Zr-acetylacetonate resulted in porous coatings after hot-pressing the oxided/oxide ®ber-rein￾forced composites. C/ZrO2-double coating are achieved by successive CVD-coating with propane and Zr-acet￾ylacetonate. Composites are fabricated by in®ltration of coated ®ber yarns with pre-mullite slurry and consequently hot-pressing the in®ltrated prepegs. Porous ZrO2-coat￾ing at interface of mullite/aluminosilicate ®ber-rein￾forced composites displayed no ®ber-pull-out. C/ZrO2, in turn, resulted in damage tolerant fracture of the composites. The mechanical and microstructural obser￾vations of the composites at RT and at 1200C after 2 h heat-treatment showed that a thickness of 10 nm for the Fig. 9. Scanning electron image of interface in C/ZrO2-coated mullite ®ber mullite matrix composites (a) and 3-point-bending stress-strain curve of this composite. K. Nubian et al. / Journal of the European Ceramic Society 20 (2000) 537±544 543