FF Lange et al. Materials Science and Engineering A195(1995)145-150 The size distribution of the crack-like voids pro- fiber preform and packed by filtration under the capil duced within a pyrolyzed precursor is proportional to lary pressure provided by the plaster of Paris. After the size distribution of the voids within the initial complete powder consolidation, the bodies were powder compact [5. Thus partly filling of the void partially dried in the mold, removed and then fully phase within a fiber preform decreases the size of the dried at 60C. The excess powder layer, on top of the crack-like voids during precursor pyrolysis to the size fiber-powder composite, was removed to prevent of the particle interstices, instead of the much larger matrix cracking on further processing interstices between fibers The Si3n4 matrix composite compacts were subse- quently heat treated at 1250C for 10 h under flowing n2 to strengthen partially the Si3N4 matrix without 5.“ Ceramic wood shrinkage by forming necks between the touching Si3N4 particles, by evaporation-condensation. To When two intrinsically brittle materials are com- strengthen this powder matrix fully, infiltration, pyroly- bined, damage tolerance can be achieved whenever sis and heat treatment procedures were implemented cracks can be induced to deflect along planes parallel using polysilazane [5]. This precursor pyrolyzes to an to the loading direction[36]. Most damage-tolerant amorphous"Si, N4". Up to three cycles were typically ceramic matrix composites(CMCs) have implemented employed with heat treatment at 1200'C for 4 h after this requirement by using a thin interphase between the each pyrolysis. A similar procedure was used to pro- fiber and the matrix. The interphases used in most duce a mullite matrix; a mixed alkoxide was used to commercial products consist of either C or BN. These strengthen the mullite powder matrix after gelation, interphases oxidize and cause embrittlement 37, 38]. drying and heat treatment permits the creation of low cost, damage-tolerance and fractured sections o big by was used on polished Here, a new concept is developed and exploited that nning electron CMCs, inherently resistant to oxidation embrittlement, features. Fig 3 illustrates a typical polished sect 9 nlight the microstr without the requirement for a matrix-fiber interphase. the Al2O3-Si3 N4 composite showing that the cube-like The ensuing composites have preformance character- Si, Ni4 particles are bonded together with amor istics similar to those demonstrated by various natural phous"Si3N4"and the Al,O3 fibers are bonded to the materials, particularly wood 39 matrix with the same amorphous"Si3N4". The dimen c The materials described here use high strength sion of the pores seen in Fig 3 are limited in size to the ramic fibers in a porous ceramic matrix. The space between particles materi als are selected to satisfy the two basic criteria Flexural testing demonstrates the fracture mode and needed to achieve damage-tolerant behavior, accord- damage tolerance of these materials. Fig 4 shows the ing to the scheme elaborated elsewhere [7].(i)The load-displacement behavior of both the Al,O-Si3N fibers have a larger thermal expansion coefficient than the matrix. In consequence, the fiber bundles are esidual tension, whereas the matrix regions are in residual compression. (ii)The matrix consists of a fine- scale, porous framework having a relatively low resis tance k to crack extension but good tensile strength The latter criterion can be satisfied by using the CMC processing method described above, i.e. packing powders around fibers by the pressure filtration of particles within a dispersed slurry(Section 3),and strengthening the powder matrix by the cyclic infiltra- tion of a ceramic precursor (Section 4). The behavior is exemplified by Al,O3 fibers in porous matrices of either Si, N, or mullite [7 In order to create these composites, dispersed aqueous Si3n4 and mullite slurries with a particle size up to I um were first prepared Slip casting was used to pack the particles around fiber bundles. The as lum received AL2O, fibers were cut into 35 mm lengths. Fig. 3. Polished section of an AlO-SigN, composite showing Fiber bundles were dip coated into the slurry and that the cubic-like Si N particles are bonded together with stacked in a Teflon mold in contact with plaster of amorphous, precursor derived "Si, N, "and the AL,O, fibers are Paris. The slurry was poured into the mold to cover the bonded to the matrix with the same amorphous"Si, A148 F.F. Lange et al. / Materials Science and Engineering A195 (1995) 145-150 The size distribution of the crack-like voids pro￾duced within a pyrolyzed precursor is proportional to the size distribution of the voids within the initial powder compact [5]. Thus partly filling of the void phase within a fiber preform decreases the size of the crack-like voids during precursor pyrolysis to the size of the particle interstices, instead of the much larger interstices between fibers. 5. "Ceramic wood" When two intrinsically brittle materials are com￾bined, damage tolerance can be achieved whenever cracks can be induced to deflect along planes parallel to the loading direction [36]. Most damage-tolerant ceramic matrix composites (CMCs) have implemented this requirement by using a thin interphase between the fiber and the matrix. The interphases used in most commercial products consist of either C or BN. These interphases oxidize and cause embrittlement [37,38]. Here, a new concept is developed and exploited that permits the creation of low cost, damage-tolerance CMCs, inherently resistant to oxidation embrittlement, without the requirement for a matrix-fiber interphase. The ensuing composites have preformance character￾istics similar to those demonstrated by various natural materials, particularly wood [39]. The materials described here use high strength ceramic fibers in a porous ceramic matrix. The materials are selected to satisfy the two basic criteria needed to achieve damage-tolerant behavior, accord￾ing to the scheme elaborated elsewhere [7]. (i) The fibers have a larger thermal expansion coefficient than the matrix. In consequence, the fiber bundles are in residual tension, whereas the matrix regions are in residual compression. (ii) The matrix consists of a fine￾scale, porous framework having a relatively low resis￾tance K c to crack extension but good tensile strength. The latter criterion can be satisfied by using the CMC processing method described above, i.e. packing powders around fibers by the pressure filtration of particles within a dispersed slurry (Section 3), and strengthening the powder matrix by the cyclic infiltra￾tion of a ceramic precursor (Section 4). The behavior is exemplified by AI203 fibers in porous matrices of either Si3N 4 or mullite [7]. In order to create these composites, dispersed aqueous Si3N 4 and mullite slurries with a particle size up to 1 ktm were first prepared. Slip casting was used to pack the particles around fiber bundles. The as￾received AIeO 3 fibers were cut into 35 mm lengths. Fiber bundles were dip coated into the slurry and stacked in a Teflon mold in contact with plaster of Paris. The slurry was poured into the mold to cover the fiber preform and packed by filtration under the capil￾lary pressure provided by the plaster of Paris. After complete powder consolidation, the bodies were partially dried in the mold, removed and then fully dried at 60 °C. The excess powder layer, on top of the fiber-powder composite, was removed to prevent matrix cracking on further processing. The Si3N 4 matrix composite compacts were subse￾quently heat treated at 1250 °C for 10 h under flowing N2 to strengthen partially the Si3N 4 matrix without shrinkage by forming necks between the touching Si3N 4 particles, by evaporation-condensation. To strengthen this powder matrix fully, infiltration, pyroly￾sis and heat treatment procedures were implemented using polysilazane [5]. This precursor pyrolyzes to an amorphous "Si3N4". Up to three cycles were typically employed with heat treatment at 1200 °C for 4 h after each pyrolysis. A similar procedure was used to pro￾duce a mullite matrix; a mixed alkoxide was used to strengthen the mullite powder matrix after gelation, drying and heat treatment. Scanning electron microscopy was used on polished and fractured sections to highlight the microstructural features. Fig. 3 illustrates a typical polished section of the A1203-Si3N 4 composite showing that the cube-like Si3Ni 4 particles are bonded together with amor￾phous "Si3N4" and the AI203 fibers are bonded to the matrix with the same amorphous "Si3N4". The dimen￾sion of the pores seen in Fig. 3 are limited in size to the space between particles. Flexural testing demonstrates the fracture mode and damage tolerance of these materials. Fig. 4 shows the load-displacement behavior of both the AI203-Si3N 4 Fig. 3. Polished section of an AI203-Si3N n composite showing that the cubic-like Si3N 4 particles are bonded together with amorphous, precursor derived "Si3N4" and the A1203 fibers are bonded to the matrix with the same amorphous "Si3N4