J. A Cera Soc.,86[0]1733-4002003) ournal Processing and Properties of a Porous Oxide Matrix Composite Reinforced with Continuous Oxide Fibers Magnus G. Holmquist'-and Fred F. Lange*, Materials Department, University of California, Santa Barbara, California 93106 Volvo Aero Corporation, 461 81 Trollhattan, Sweden A process to manufacture porous oxide matrix/polycrystalline nvironment, such as nitrogen oxides, carbon monoxide and oxide fiber composites was developed and evaluated. The unburned hydrocarbon nethod uses infiltration of fiber cloths with an aqueous slurry Most CFCCs that are commercially available are based on SiC of mullite/alumina powders to make prepregs. By careful fibers, with either oxide or non-oxide matrixes, and interphases anipulation of the interparticle pair potential in the slurry, a consisting of carbon, BN, SiC or combinations thereof. The consolidated slurry with a high particle density is produced interphases are designed to provide a crack-deflecting layer be with a sufficiently low viscosity to allow efficient infiltration of tween the matrix and fibers that prevents matrix cracks from the fiber tows. Vibration-assisted infiltration of stacked, cloth extending through the fibers, thus allowing crack bridging to occ prepregs in combination with a simple vacuum bag technique on matrix cracking enabling damage tolerance via notch insensi- produced composites with homogeneous microstructures. The method has the additional advantage of allowing complex tivity. SiC fiber based composites have attractive high- reep resistance, microstructural shapes to be made. Subsequent infiltration of the powder temperature properties such mixture with an alumina precursor was made to strengthen stability, high tensile strength, and high thermal conductivity. the matrix. The porous matrix, without fibers, possessed good phase will cause embrittlement of the composite after service at treatment at 1200C. Mechanical properties were evaluated in high temperature for long times. Embrittlement is most severe with flexural testing in a manner that precluded interlaminar shear cyclic loading beyond the proportional limit because oxygen that failure before failure via the tensile stresses. It was shown tha penetrates via the matrix cracks will react with the interphase and he composite produced by this method was comparable to the fibers. ,0 This effect is most pronounced for carbon coatings orous ox kide matrix composites manufactured by other pre but the introduction of bn coatings and boron additives has cesses using the same fibers(N610 and N720). The ratio of improved the situation in oxidizing environments, where BI notch strength to unnotch strength for a crack to width ratio of oxidation products (liquid boron oxide)help in healing matrix cracks. However, in wet environments the problem persists since 0.5 was 0.7-0.9, indicating moderate notch sensitivity. Inter- the boron oxidation products volatilize as boron hydroxide aminar shear strength, which is dominated by matrix strength, changed from 7 to 12 MPa for matrix porosity avoid degradation in oxidizing (especially wet)environments. ranging from 38% to 43%, respectively. The porous micro- tructural design strategies therefore usually require that the structure did not change after aging at 1200%C for 100 h. Heat stresses remain below the matrix cracking stress. End-user treatment at 1300C for 100 h reduced the strength for the experience indicates that stress excursions above the matrix N610 and N720 composites by 35% and 20%, respectively, and cracking stress is very difficult to avoid, and thus local ncreased their brittle nature embrittlement will be one of the dominant life-limiting phe nomena of non-oxide composites. These shortco promoted the development of environmentally stable all-oxide L. Introduction composites, i.e., materials where all constituents(fiber, inter C ONTINUOUS fiber ceramic composites(CFCCs) have attracted Two approaches have been used to develop damage-tolerant interest for a variety of high-temperature thermostructural all-oxide composites. The more traditional approach requires a applications in gas turbine engines, rocket engines, heat ex- crack-deflecting interface between the matrix and fibers. This can be hangers, and hot gas filters. The reason is that they offer achieved by adding an interphase which either forms a crack uperior refractoriness compared with conventional metal alloys deflecting interface with the fibers. -l5 has itself a low fracture combined with an inelastic deformation behavior rendering them toughness(e.g, "cleavable"oxides or a porous layer-9), or forms damage tolerant. Of particular interest is their use in combustor a gap between fiber and matrix(fugitive coating) king advantage of the FcCS ability to op The use of a porous matrix to isolate fibers from matrix cracks at high temperatures with reduced need for cooling air, it is a second, more recent approach for developing damage-tolerant possible to increase the efficiency and also control the combus- opposites. In this approach, the crack does not have a contin- tion process to minimize formation of species harmful to the uous front, but, instead, the matrix is held together by grai Matrix failure by the sequential failure pairs. Fibers are isolated from the stress singularity of a matrix E. Lara-Curzio--contributing editor crack because the matrix is not sufficiently continuous to support a crack. There are several examples of CFCCs which rely on a porous matrix for damage tolerance 3,21 24-27 The failure mech- anisms have been examined in some detail. 22-24,28-3On loading has scpt h o as 5 0. Red ebv d Agust r 200ch appice d nagessbes-51 3095. the matrix experiences continuous microcracking during loading oundation for financial support. and appears to have completely disintegrated at the onset of fiber failure. Contrary to the conventional weak interface CFCCs where ation: Advanced Engineering, SAAB, 461 80 Trollhattan, Sweden the fibers slide out of the matrix, leaving distinct holes, when Califonia fibers fail in a porous matrix, they release a large volume ofProcessing and Properties of a Porous Oxide Matrix Composite Reinforced with Continuous Oxide Fibers Magnus G. Holmquist†,‡ and Fred F. Lange* ,§ Materials Department, University of California, Santa Barbara, California 93106 Volvo Aero Corporation, 461 81 Trollha¨ttan, Sweden A process to manufacture porous oxide matrix/polycrystalline oxide fiber composites was developed and evaluated. The method uses infiltration of fiber cloths with an aqueous slurry of mullite/alumina powders to make prepregs. By careful manipulation of the interparticle pair potential in the slurry, a consolidated slurry with a high particle density is produced with a sufficiently low viscosity to allow efficient infiltration of the fiber tows. Vibration-assisted infiltration of stacked, cloth prepregs in combination with a simple vacuum bag technique produced composites with homogeneous microstructures. The method has the additional advantage of allowing complex shapes to be made. Subsequent infiltration of the powder mixture with an alumina precursor was made to strengthen the matrix. The porous matrix, without fibers, possessed good thermal stability and showed linear shrinkage of 0.9% on heat treatment at 1200°C. Mechanical properties were evaluated in flexural testing in a manner that precluded interlaminar shear failure before failure via the tensile stresses. It was shown that the composite produced by this method was comparable to porous oxide matrix composites manufactured by other pro￾cesses using the same fibers (N610 and N720). The ratio of notch strength to unnotch strength for a crack to width ratio of 0.5 was 0.7–0.9, indicating moderate notch sensitivity. Inter￾laminar shear strength, which is dominated by matrix strength, changed from 7 to 12 MPa for matrix porosity ranging from 38% to 43%, respectively. The porous micro￾structure did not change after aging at 1200°C for 100 h. Heat treatment at 1300°C for 100 h reduced the strength for the N610 and N720 composites by 35% and 20%, respectively, and increased their brittle nature. I. Introduction CONTINUOUS fiber ceramic composites (CFCCs) have attracted interest for a variety of high-temperature thermostructural applications in gas turbine engines,1 rocket engines,2 heat ex￾changers,3 and hot gas filters.4 The reason is that they offer superior refractoriness compared with conventional metal alloys combined with an inelastic deformation behavior rendering them damage tolerant.5 Of particular interest is their use in combustor walls.6–8 By taking advantage of the CFCCs ability to operate at high temperatures with reduced need for cooling air, it is possible to increase the efficiency and also control the combus￾tion process to minimize formation of species harmful to the environment, such as nitrogen oxides, carbon monoxide, and unburned hydrocarbons.9 Most CFCCs that are commercially available are based on SiC fibers, with either oxide or non-oxide matrixes, and interphases consisting of carbon, BN, SiC or combinations thereof. The interphases are designed to provide a crack-deflecting layer be￾tween the matrix and fibers that prevents matrix cracks from extending through the fibers, thus allowing crack bridging to occur on matrix cracking enabling damage tolerance via notch insensi￾tivity.3,5 SiC fiber based composites have attractive high￾temperature properties such as creep resistance, microstructural stability, high tensile strength, and high thermal conductivity. However, the oxidation sensitivity of the crack-deflecting inter￾phase will cause embrittlement of the composite after service at high temperature for long times. Embrittlement is most severe with cyclic loading beyond the proportional limit because oxygen that penetrates via the matrix cracks will react with the interphase and the fibers.3,10 This effect is most pronounced for carbon coatings, but the introduction of BN coatings and boron additives has improved the situation in oxidizing environments, where BN oxidation products (liquid boron oxide) help in healing matrix cracks. However, in wet environments the problem persists since the boron oxidation products volatilize as boron hydroxides.3,11 To avoid degradation in oxidizing (especially wet) environments, structural design strategies therefore usually require that the stresses remain below the matrix cracking stress. End-user experience indicates that stress excursions above the matrix cracking stress is very difficult to avoid, and thus local embrittlement will be one of the dominant life-limiting phe￾nomena of non-oxide composites. These shortcomings have promoted the development of environmentally stable all-oxide composites, i.e., materials where all constituents (fiber, inter￾phase, and matrix) are oxides.3,12,13 Two approaches have been used to develop damage-tolerant all-oxide composites. The more traditional approach requires a crack-deflecting interface between the matrix and fibers. This can be achieved by adding an interphase which either forms a crack￾deflecting interface with the fibers,13–15 has itself a low fracture toughness (e.g., “cleavable” oxides16 or a porous layer17–19), or forms a gap between fiber and matrix (fugitive coating).18,20 The use of a porous matrix to isolate fibers from matrix cracks is a second, more recent approach for developing damage-tolerant composites.21 In this approach, the crack does not have a contin￾uous front, but, instead, the matrix is held together by grain pairs.22,23 Matrix failure occurs by the sequential failure of grain pairs. Fibers are isolated from the stress singularity of a matrix crack because the matrix is not sufficiently continuous to support a crack. There are several examples of CFCCs which rely on a porous matrix for damage tolerance.13,21,24–27 The failure mech￾anisms have been examined in some detail.22–24,28–31 On loading, the matrix experiences continuous microcracking during loading and appears to have completely disintegrated at the onset of fiber failure. Contrary to the conventional weak interface CFCCs where the fibers slide out of the matrix, leaving distinct holes, when fibers fail in a porous matrix, they release a large volume of E. Lara-Curzio—contributing editor Manuscript No. 187504. Received August 22, 2001; approved November 8, 2002. This research was supported by the Army Research Office, DAAG55-98-1-0455. M.G.H. thanks the Hans Werthe´n Foundation for financial support. *Member, American Ceramic Society. † Volvo Aero Corp. ‡ Current affiliation: Advanced Engineering, SAAB, 461 80 Trollha¨ttan, Sweden. § University of California. J. Am. Ceram. Soc., 86 [10] 1733–40 (2003) 1733 journal