1 tnt Cera So,861017340(2003 urna Processing and Properties of a Porous Oxide Matrix Composite Reinforced with Continuous Oxide Fibers lagnus G. Holmquist.and Fred F, Lange* s Materials Department, University of California, Santa Barbara, California 93106 Volvo Aero Corporation, 461 81 Trollhattan, Sweden a process to manufacture porous oxide matrix/polycrystalline environment, such as nitrogen oxides, carbon monoxide. and xide fiber composites was developed and evaluated. The burned hydrocarbons. method 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 manipulation 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 produce 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 occur prepregs in combination with a simple vacuum bag technique on matrix cracking enabling damage tolerance via notch insensi produced composites with homogeneous microstructures. The tivity. .5 SiC fiber based composites have attractive high method has the additional advantage of allowing complex temperature prope y sensiti, and high thermal conductivity shapes to be made. Subsequent infiltration of the powder es such as creep resistance, microstructura tability, high ten mixture with an alumina precursor was made to strengthen However, the oxidation sensitivity of the crack-deflecting inter phase will cause embrittlement of the composite after service at thermal stability and showed linear shrinkage of 0.9% on heat high temperature for long times. Embrittlement is most severe with treatment at 1200 C Mechanical properties were evaluated in lexural 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 that penetrates via the matrix cracks will react with the interphase and the composite produced by this method was comparable to the fibers. This effect is most pronounced for carbon coatings, porous oxide matrix composites manufactured by other pr but the introduction of BN coatings and boron additives has cesses using the same fibers(N610 and N720). The ratio of oxidation products (liquid boron oxide) help in healing matrix improved the situation in oxidizing environments, where BN notch strength to unnotch strength for a crack to width ratio of 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 hydroxides. . To laminar shear strength, which is dominated by matrix avoid degradation in oxidizing (especially wet) environments. strength, changed from 7 to 12 MPa for matrix porosity structural 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 1300.C 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 increased their brittle nature embrittlement will be one of the dominant life-limiting phe nomena of non-oxide composites. These shortcomings have romoted the development of environmentally stable all-oxide I. Introduction composites, i.e., materials where all constituents(fiber, inter hase, and matrix)are oxides. 3.12, 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 changers, and hot filters. The reason is that they offer achieved by adding an interphase which either forms a crack combined with an inelastic deformation behavior rendering them toughness (e. g, "cleavable"oxides 6 or a porous layer7-19),or forms a gap between fiber and matrix(fugitive coating) walls. By taking advantage of the CFCCs ability to operate The use of a porous matrix to isolate fibers from matrix cracks at high temperatures with reduced need for cooling air, it is is a second, more recent approach for developing damage-tolerant the efficiency an nd also control the coml tion process to minimize formation of species harmful to the composites. In this approach, the crack does not have a contin- uous front, but, instead, the matrix is held together by grain pairs. ,23 Matrix failure occurs by the sequential failure of grain bers are isolated from the stress singula E, Lara-Curzio-comntributing 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 mecl anisms have been examined in some detail. 22-24.28-3 On loading Manuscript No. 1875004 Received August 22, 2001: approved November 8. 2002. ice, DAAG55-98-1-0 the matrix experiences continuous microcracking during loading M. G.H. thanks the Hans Werthen Foundation for financial support and appears to have completely disintegrated at the onset of fiber failure. Contrary to the conventional weak interface CFCCs where irent affiliation: Advanced Engineering. SAAB, 461 80 Trollhattan, Sweden. the fibers slide out of the matrix. leaving distinct holes, when niversity of California. fibers fail in a porous matrix. they release a large volume of 1733