In recent years, oxide-oxide ceramic matrix composites (CMCs) have attracted ever-increasing attentions in high-temperature aerospace areas due to their low density and the capability of maintaining excellent mechanical strength and fracture toughness up to 1100 ℃ in air [1–4]. Although, compared to conventional non-oxide CMC, such as SiC-based materials, oxide-oxide CMC have inferior mechanical performance, their inherent oxidation resistance gives more advantage on the long-term applications in the oxidative atmosphere [1].
Generally speaking, three aspects contribute to mechanical performance of CMC. The first one is reinforcement fibers, which play a crucial role in CMC with respect to reinforcement and flaw tolerance. For oxide-oxide CMC, common fiber species used in the research and industrial areas are continuous Nextel™ 610 and Nextel™ 720 fibers [2, 4–13]. Nextel™ 610 is a high-purity alumina fiber (> 99% Al2O3) [14]. Nextel™ 720 is an alumina-mullite fiber (85 wt% Al2O3 and 15 wt% SiO2) with an alumina-mullite volume fraction ratio of 57:43 [15]. Their fiber tow contains 400 filaments with an average filament diameter of 12 µm. Nextel™ 610 has higher tensile strength (3.2 GPa) than Nextel™ 720 (2.1 GPa) at low and moderately high temperatures (up to approximately 1000 ℃) [16], but the latter possesses better creep resistance up to 1200 ℃ [9, 17]. Above 1200 ℃, both of Nextel™ 610 and Nextel™ 720 fibers are subjected to severe degradation of mechanical performance due to the rapid growth of crystalline grains.
The second one is ceramic matrix, which is commonly comprised of alumina, alumina-silica, or alumina-mullite [9], because that they can be partially sintered below 1200 ℃ and have excellent mechanical performance. However, it is not easy to uniformly fill these components into the ceramic fibric and make them stack closely due to the impediment of ceramic fibers. Also, these components are difficult to sinter and achieve good mechanical strength without the aid of pressure below 1200 ℃. Many researches have been done to solve these problems. Wang and Cheng et al. used diphasic Al2O3-SiO2 sols as the precursor of aluminosilicate ceramic matrix, which had a good processing ability and a high sintering activity [18, 19]. However, the mechanical performance and high-temperature stability need to be further improved. Jiang and Liu et al. used α-Al2O3 with a grain size of 200 nm as the matrix and Nextel™ 610 as the reinforcement fiber to fabricate CMC, whose flexural strength kept stable at 25-1050 ℃, while dramatically decreased at 1100–1200 ℃ [5]. Except for the growth of crystallites in fibers, we think that the shrinkage from the matrix may also be responsible for this result, since nanometer alumina powders had good sintering ability and could be steadily sintered above 1000 ℃. To resolve the contradiction between sintering activity and high-temperature stability, Levi and Evan et al. proposed an interesting route, in which large (~ 1 µm) mullite particles and small alumina particles (~ 200 nm) were used together. Large particles were packed between and within tows to form a touching, non-shrinking network, and small particles fitted within the void spaces of this network to form bridges between the larger mullite particles, as well as between the mullite particles and the fibers [17]. However, it was essential to use vacuum infiltration technique to fill the fiber preform with these particles. For large or thick CMC materials, defect might occur easily. Another attractive route for the matrix preparation was reaction-bonding aluminum oxide (RBAO) [20–22], in which metallic aluminum particles was added. The metallic aluminum particles will be oxidized and expanded during heat treatment in air to form alumina, obviating the shrinkage of the matrix in the presence of a rigid fiber network. However, special care should be taken to avoid the moisture during storage and processing, because fine metallic aluminum particles were highly active toward water.
In addition to the fiber and matrix, the interface between them also plays a vital role on the mechanical performance of CMC. Fugitive carbon coating [19], oxide rare-earth orthophosphates coating (REPO4) [23–25], and porous matrix [26] are common strategies to overcome brittle fracture of CMC. In this study, in order to simplify the material system, no coating was engineered between fibers and matrix, and a porous matrix was designed to provide weak interface that was responsible to toughen CMC materials through debonding, crack deflection, and pulling out mechanism. We aim to develop an aqueous ceramic slurry, which could be used to prepare the matrix of high-temperature CMC by the prepreg process, and adapt to the fabrication of complex shape CMC components. Microstructure evolution and functions of different alumina components in aqueous slurry will be clarified. The preceramic polymer will be used to strengthen CMC, and its influence on mechanical performance of CMC will be systematically investigated. This study will shed light on the design and realization of the matrix of CMC materials.