ournal 1. Am Ceram Soc, s1 [8]2077-86 (1998) Processing and Performance of an All-Oxide Ceramic Composite Carlos G. Levi, James Y. Yang, Brian J Dalgleish, Frank W. Zok, and Anthony G. Evans"T High Performance Composites Center, Departments of Materials and Mechanical E University of California, Santa Barbara, California 93160-5050 Continuous fiber ceramic composites(CFCCs) based on rous interlayers as crack deflection paths,0 and extends the oxides are of interest for high-temperature applications ow concept to utilize a porous matrix as a surrogate. The concept ing to their inherent oxidative stability. An enabling ele- has been successfully demonstrated, 14, I8 but only limited in- ment is a matrix with an optimum combination of tough formation is available in the open literature. The design and ness and strength, which may be achieved by incorporating stability of the matrix microstructure are arguably more critical a controlled amount of fine, well-distributed porosity aken to explore both a concept for a stable, porous oxide mas in the latter approach. The present investigation was und Implementation of this concept by vacuum infiltration of aqueous mullite-alumina slurries into two-dimensional wo- and the mechanical performance of the resulting comy fhe ven preforms of alumina fibers has been investigated The information is organized in the following manner Evaluation of these materials shows stress-strain chara derlying microstructural design concept for the matrix is teristics similar to other CFCCs, especially carbon-matrix elaborated, followed by an identification of the materials to be composites. Moreover, promising notch and creep proper- used. Subsequently, key elements of the processing science and ties have been found. Microstructural and processing issues technology are addressed. These include a specification of the relevant to the attainment of these behaviors are discussed manufacturing sequence, a sintering study that guides the choice of materials to be used for the matrix, as well as a . Introduction characterization of the composite microstructure. Thereafter, Ca ONTINUoUS fiber ceramic composites(CFCCs) with suit several essential thermomechanical properties are measured ably tailored interfaces can exhibit inelastic deformation and analyzed. Initially, the capacity of the composite to exhibit haracteristics, which enable the composites to retain streng inelastic deformation is determined in the0°/90°and±45° in the presence of holes and notches. They also render them orientations. Such results reveal that these composites exhibit amenable to design and life prediction strategies developed for damage tolerance comparable to other fiber-dominated CFCCs metals. This damage tolerance, coupled with their inherent re Moreover, the effects of thermal exposure on the 0/90 tensile fractoriness, has enabled CFCCs to emerge as candidates for rties are determined in order to characterize fiber de dation effects that might arise either during manufacture or in icular interest is their use in combustors wherein the ability service. Finally, some preliminary results are obtained concern- to operate at high temperatures with reduced need for cool air can yield substantial benefits in efficiency and also provide creep strength and notch performance, which relate to desigr control of deleterious emissions. such as No. .4 and lifing issues Most CFCC systems are based on SiC fibers, with either The general conclusion reached is that these materials have oxide or non-oxide matrices, and interphases consisting of vari- echanical characteristics comparable to those established for ous combinations of carbon, BN, and SiC. The interphas carbon-matrix materials(such as SiC/carbon and carbon/ tailored to enable interfacial debonding and crack bridging to arbon), with attendant implications for thermostructural per- occur upon matrix cracking accompanied by internal fric ormance. The key differences with the carbon-matrix materi- tion,However, these systems are susceptible to embrittle als are their superior oxidative stability and their differing nent by oxygen ingress through the matrix cracks, followed by creep response eaction with the interphase and the fibers. 6 -The kinetics ar particularly debilitating at intermediate temperatures(500 IL. Microstructural Design 900 C)and upon cyclic loading. 7,9 The embrittlement problem The microstructure must be designed to have a sufficientl y imposes major design limitations by requiring that the stresses low toughness to enable crack deflection through the matrix remain below the matrix cracking stress. This deficiency has while maintaining enough strength for adequate off-axis and stable oxide constituents 10-18 e Development of all-oxide composites has followed two dis- amount of fine, uniformly distribute a rporating a controlled ct microstructural design paths. The first is based weak interface concept, typical of most CFCCs. It uses eithe matrix performance dictates a stable and well-bonded particle fugitive layers or stable oxide interphases with suitably low network with substantial vold space, of order%, fracture toughness. 7 The second implicitly accepts the forma- comparable to the interparticle spacing. Fine matrix tion of strong interfaces. It builds on the experience with po- form. as well as the nominal strength of the matrix. However fine particles also reduce the stability of the matrix against densification during processing and service, promoting the B. N. Cox--contributing editor evolution of undesirable flaws under the constraint imposed by he fibers 22,23 Manuscript No. 191156. Received March 7 pproved October 6 Q Mullite emerges as an attractive matrix material, owing to its cellent creep resistance, low modulus, and, as noted beloy generally sluggish sintering kinetics below -1300oC. The sin- monitored by Dr.S. G. Fishman of the Office tering kinetics suggests adequate microstructural stability for applications in the gas-turbine engine, where initial target wall Now with the Division of Applied Sciences, Harvard University, Cambridge, MA. temperatures are in the range -1000o to -1200C. HoweverProcessing and Performance of an All-Oxide Ceramic Composite Carlos G. Levi,* James Y. Yang, Brian J. Dalgleish, Frank W. Zok,* and Anthony G. Evans*,† High Performance Composites Center, Departments of Materials and Mechanical Engineering, University of California, Santa Barbara, California 93160–5050 Continuous fiber ceramic composites (CFCCs) based on oxides are of interest for high-temperature applications ow￾ing to their inherent oxidative stability. An enabling ele￾ment is a matrix with an optimum combination of tough￾ness and strength, which may be achieved by incorporating a controlled amount of fine, well-distributed porosity. Implementation of this concept by vacuum infiltration of aqueous mullite–alumina slurries into two-dimensional wo￾ven preforms of alumina fibers has been investigated. Evaluation of these materials shows stress–strain charac￾teristics similar to other CFCCs, especially carbon-matrix composites. Moreover, promising notch and creep proper￾ties have been found. Microstructural and processing issues relevant to the attainment of these behaviors are discussed. I. Introduction CONTINUOUS fiber ceramic composites (CFCCs) with suit￾ably tailored interfaces can exhibit inelastic deformation characteristics, which enable the composites to retain strength in the presence of holes and notches.1 They also render them amenable to design and life prediction strategies developed for metals. This damage tolerance, coupled with their inherent re￾fractoriness, has enabled CFCCs to emerge as candidates for many high-temperature thermostructural applications.2 Of par￾ticular interest is their use in combustors,3 wherein the ability to operate at high temperatures with reduced need for cooling air can yield substantial benefits in efficiency and also provide control of deleterious emissions, such as NOx. 4 Most CFCC systems are based on SiC fibers, with either oxide or non-oxide matrices, and interphases consisting of vari￾ous combinations of carbon, BN, and SiC. The interphases are tailored to enable interfacial debonding and crack bridging to occur upon matrix cracking accompanied by internal fric￾tion.1,5 However, these systems are susceptible to embrittle￾ment by oxygen ingress through the matrix cracks, followed by reaction with the interphase and the fibers.6–9 The kinetics are particularly debilitating at intermediate temperatures (500°– 900°C) and upon cyclic loading.7,9 The embrittlement problem imposes major design limitations by requiring that the stresses remain below the matrix cracking stress. This deficiency has motivated the search for CFCCs based on environmentally stable oxide constituents.10–18 Development of all-oxide composites has followed two dis￾tinct microstructural design paths. The first is based on the weak interface concept, typical of most CFCCs. It uses either fugitive layers12 or stable oxide interphases with suitably low fracture toughness.17 The second implicitly accepts the forma￾tion of strong interfaces. It builds on the experience with po￾rous interlayers as crack deflection paths19,20 and extends the concept to utilize a porous matrix as a surrogate. The concept has been successfully demonstrated,11,14,18 but only limited in￾formation is available in the open literature. The design and stability of the matrix microstructure are arguably more critical in the latter approach. The present investigation was under￾taken to explore both a concept for a stable, porous oxide matrix and the mechanical performance of the resulting composites. The information is organized in the following manner. The underlying microstructural design concept for the matrix is elaborated, followed by an identification of the materials to be used. Subsequently, key elements of the processing science and technology are addressed. These include a specification of the manufacturing sequence, a sintering study that guides the choice of materials to be used for the matrix, as well as a characterization of the composite microstructure. Thereafter, several essential thermomechanical properties are measured and analyzed. Initially, the capacity of the composite to exhibit inelastic deformation is determined in the 0°/90° and ±45° orientations. Such results reveal that these composites exhibit damage tolerance comparable to other fiber-dominated CFCCs. Moreover, the effects of thermal exposure on the 0°/90° tensile properties are determined in order to characterize fiber degra￾dation effects that might arise either during manufacture or in service. Finally, some preliminary results are obtained concern￾ing the interlaminar shear properties as well as the in-plane creep strength and notch performance, which relate to design and lifing issues. The general conclusion reached is that these materials have mechanical characteristics comparable to those established for carbon-matrix materials (such as SiC/carbon and carbon/ carbon), with attendant implications for thermostructural per￾formance. The key differences with the carbon-matrix materi￾als are their superior oxidative stability and their differing creep response. II. Microstructural Design The microstructure must be designed to have a sufficiently low toughness to enable crack deflection through the matrix while maintaining enough strength for adequate off-axis and interlaminar properties.18 These seemingly contradictory re￾quirements are achievable by incorporating a controlled amount of fine, uniformly distributed porosity.18,21 Acceptable matrix performance dictates a stable and well-bonded particle network with substantial void space, of order ∼30%, on a scale comparable to the interparticle spacing. Fine matrix particles enhance packing density and uniformity within the fiber pre￾form, as well as the nominal strength of the matrix. However, fine particles also reduce the stability of the matrix against densification during processing and service, promoting the evolution of undesirable flaws under the constraint imposed by the fibers.22,23 Mullite emerges as an attractive matrix material, owing to its excellent creep resistance, low modulus, and, as noted below, generally sluggish sintering kinetics below ∼1300°C. The sin￾tering kinetics suggests adequate microstructural stability for applications in the gas-turbine engine, where initial target wall temperatures are in the range ∼1000° to ∼1200°C. However, B. N. Cox—contributing editor Manuscript No. 191156. Received March 7, 1997; approved October 6, 1997. Supported by the Defense Advanced Research Projects Agency under University Research Initiative Grant N00014-92-J-1808. Supervised by Dr. W. Coblenz and monitored by Dr. S. G. Fishman of the Office of Naval Research. *Member, American Ceramic Society. † Now with the Division of Applied Sciences, Harvard University, Cambridge, MA. J. Am. Ceram. Soc., 81 [8] 2077–86 (1998) Journal 2077