MAVERAL ELSEVIER Materials Science and Engineering A195(1995)145-150 Processing of damage-tolerant, oxidation-resistant ceramic matrix composites by a precursor infiltration and pyrolysis method FF. Lange W.C. Tu, A.G. Evans Materials Department, College of Engineering, University of California at Santa Barbara, Santa barbara, CA 93106 USA Abstract methonage-tolerant, continuous fiber ceramic matrix composites een produced by an inexpensive method. according to this particles are heat treated to form a porous framework with out shrinkage, which is then strengthened with an inorganic synthesized from a precursor. High particle packing densities can be achieved within the fiber preform provided that the particle-to-fiber diameter ratio is small. Filling the interstices with a increased the composite density and also limits the size of the crack-like voids within the matrix. In this review we descr mechanical properties of partially dense materials produced from powders to show that a porous matrix can be strong. We strate that the packing density of particles around fibers is highest when the particle-to-fiber diameter ratio is small. The kinetics and mechanical behavior of composite systems is summarized to demonstrate the requirements of damage-tolerant properties. An all oxide ceramic matrix composite produced by this method is discussed Keywords: Infiltration; Ceramics; Composites; Pyrolysis 1. Introduction made strong(above 200 MPa)by a cyclic precursor d. In additio Liquid precursor infiltration and pyrolysis can be the porous matrix itself can induce cracking mecha used for processing ceramics and their composites nisms that provide damage-tolerant behavior. this [1-7 The precursor is a liquid, comprising metal discovery implies that "weak "interfaces are not neces- organic molecules dissolved in an appropriate solvent. sarily a requirement of damage-tolerant, high strain to The excess solvent is removed by evaporation and the failure ceramic matrix composites reinforced with solid precursor molecules are decomposed( pyrolyzed) strong, ceramic fibers. For two different composite sys- to the desired inorganic with a heat treatment. a tems explored in this study, the fibers are well bonded powder compact can be infiltrated with a liquid pre- to the matrix and do not appear to be degraded by the cursor and pyrolyzed to synthesize an inorganic phase processing. In one of these systems, since both the within the porous ceramic[1-4]. A variety of unique matrix(mullite)and fibers(alumina)are oxides, high microstructures(graded, multiphase, partially porous temperature degradation will not occur in oxidizing to fully dense etc. )having unique thermomechanical environments properties can be fabricated. In addition, the pyrolyzed he processing method reviewed here involves precursor can be used both to increase the relative three steps: (i) the packing of powder around fibers by density and to strengthen the powder compact without pressure filtration, (i)a heat treatment to strengthen shrinkage l5」 he powder matrix without shrinkage and (iii)the The lack of powder shrinkage during strengthening additional strengthening of the n advantage for ceramic composites. Conventional trated solution precursor that pyrolyzes to a desired strengthening by densification is constrained by the inorganic material. The inorganic can have a different fibers and leads to the formation of crack-like voids 8. composition from either the fibers or the matrix Moreover, a powder matrix surrounding fibers can be Because the precursor does not completely fill the 221-5093/95/$95001995-Elsevier Science S.A. All rights reserve SSD0921-5093(94)06513-6MATERIALS SCIENCE & ENGINEERING A ELSEVIER Materials Science and Engineering A195 (1995) 145-150 Processing of damage-tolerant, oxidation-resistant ceramic matrix composites by a precursor infiltration and pyrolysis method F.F. Lange, W.C. Tu, A.G. Evans Materials Department, College of Engineering, University of California at Santa Barbara, Santa Barbara, CA 93106, USA Abstract Damage-tolerant, continuous fiber ceramic matrix composites have been produced by an inexpensive method. According to this method, the space between the fibers is filled with a powder. The powder particles are heat treated to form a porous framework with￾out shrinkage, which is then strengthened with an inorganic synthesized from a precursor. High particle packing densities can be achieved within the fiber preform provided that the particle-to-fiber diameter ratio is small. Filling the interstices with a powder increased the composite density and also limits the size of the crack-like voids within the matrix. In this review we describe the mechanical properties of partially dense materials produced from powders to show that a porous matrix can be strong. We demon￾strate that the packing density of particles around fibers is highest when the particle-to-fiber diameter ratio is small. The kinetics and mechanical behavior of composite systems is summarized to demonstrate the requirements of damage-tolerant properties. An all￾oxide ceramic matrix composite produced by this method is discussed. Keywords: Infiltration; Ceramics; Composites; Pyrolysis I. Introduction Liquid precursor infiltration and pyrolysis can be used for processing ceramics and their composites [1-7]. The precursor is a liquid, comprising metal organic molecules dissolved in an appropriate solvent. The excess solvent is removed by evaporation and the solid precursor molecules are decomposed (pyrolyzed) to the desired inorganic with a heat treatment. A powder compact can be infiltrated with a liquid pre￾cursor and pyrolyzed to synthesize an inorganic phase within the porous ceramic [1-4]. A variety of unique microstructures (graded, multiphase, partially porous to fully dense etc.) having unique thermomechanical properties can be fabricated. In addition, the pyrolyzed precursor can be used both to increase the relative density and to strengthen the powder compact without shrinkage [5]. The lack of powder shrinkage during strengthening is an advantage for ceramic composites. Conventional strengthening by densffication is constrained by the fibers and leads to the formation of crack-like voids [8]. Moreover, a powder matrix surrounding fibers can be made strong (above 200 MPa) by a cyclic precursor infiltration-pyrolysis processing method. In addition, the porous matrix itself can induce cracking mecha￾nisms that provide damage-tolerant behavior. This discovery implies that "weak" interfaces are not neces￾sarily a requirement of damage-tolerant, high strain to failure ceramic matrix composites reinforced with strong, ceramic fibers. For two different composite sys￾tems explored in this study, the fibers are well bonded to the matrix and do not appear to be degraded by the processing. In one of these systems, since both the matrix (mullite) and fibers (alumina) are oxides, high temperature degradation will not occur in oxidizing environments. The processing method reviewed here involves three steps: (i) the packing of powder around fibers by pressure filtration, (ii) a heat treatment to strengthen the powder matrix without shrinkage and (iii) the additional strengthening of the matrix with an infil￾trated solution precursor that pyrolyzes to a desired, inorganic material. The inorganic can have a different composition from either the fibers or the matrix. Because the precursor does not completely fill the 0921-5093/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSD10921-5093(94)06513-6