Mullite matrix

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

T. N. Tiegs, P. F. Becher, and P. Angelini, Microstructures and Properties of SiC Whisker-Reinforced Mullite Composites, in Mullite and Mullite Matrix Composites, Ceramic Transactions, Vol. 6, eds. S. Somiya, R. F. Davis, and J. A. Pask, American Ceramic Society, Westerville, OH, 1990, pp. 463-472. 

A.T. Hunt, CCVD Processing of Lanthanum Phosphate and Beta-Alumina Fiber Coatings for Mullite Matrix Composites, Contract DM19561712, NSF SBIR Phase 1 Grant, 1996. 

FIGURE 12.7 (a) Scanning electron micrograph (SEM) of a mullite matrix CMC reinforced with submicronic zirconia (fiber diameter 12 pm), (b) TEM of a cobalt dispersion in an aluminosilicate matrix synthesized by the infiltration-H2 firing route. ... 

FIGURE 12.17 Spectral in-line scan (2 pm step) of a composite cross section. Spectra were recorded from one NLM202 fiber to another, through the aluminosilicate + (Zr02 Ge02) tailored interface and the mullite matrix. (Reprinted from Colomban, R, Raman microspectrometry and imaging of ceramic fibers in CMCs and MMCs, 103, 517, 2000. With permission from The American Ceramic Society.)... 

Colomban, P., Bruneton, E., Lagrange, J.L., Mouchon, E., Sol-gel mullite matrix-SiC and -mullite 2D woven fabrics composites with or without zirconia containing interphase. Elaboration and properties, J. Eur. Ceram. Soc., 16, 301, 1996. 

Wang Jing, Kou Hua-min, Liu Xue-jian,Pan Yu-bai, Guo Jing-kun. Reinforcement of mullite matrix with multi-walled carbon nanotubes. Ceramics International, 2007, 33 719-722. 

At the DLR Institute of Materials Research an all-oxide CMC (WHIPOX = wound highly porous oxide) consisting of alumina fibres Nextel 610 or mullite based fibre (Nextel 720, both 3M) and an alumina or mullite matrix, respectively, has been developed in recent years [1,2]. Mullite-based CMCs typically offer higher creep stability than alumina-based composites but display lower thermal conductivity. Lower thermal stability of alumina-based materials, however, can be accepted, since service temperature is significantly lower for these materials as a result of the better cooling efficiency. Therefore material development was focused on alumina... 

Domier of Germany developed oxide fiber-reinforced mullite matrix composite systems in the 1980s. The system was based on SMs Nextel 312 fibers and slurry impregnation of a mullite matrix. This composite demonstrated modest levels of strength (120 MPa [17.4 ksi]) and low failure strain (0.2 to 0.3 percent). Several hot gas exhaust nozzles for Dormer s turboprop airerafl were successfully deployed and continue to be used in regular service. 

DAVID WILSON is a research specialist with 3M and has more than 10 years of research and development experience in the synthesis of new ceramic fibers using sol-gel techniques and is the inventor of the Nextel 610 and Nextel 720 ceramic fibers. His experience includes the development of novel fiber precursor formulations, continuous fiber processing, and characterization of ceramic fibers at room and elevated temperatures. Current research activities include preceramic polymer-derived SiC fibers and MOCVD coating and composite fabrication for the Mullite Matrix Composite Consortium Program. Mr. Wilson holds four patents and is the author of several publications. 

The hydroxide suspensions were mixed with the SiC whiskers on the weight basis ofthe final fired composition ofthe mullite-zirconia or mullite matrix. Mixing was accomplished in a food blender for 12 m. The slurry-whisker mixtures were then cast into dishes and... 

Alternately, an alumina-mullite matrix was developed in an effort to improve long-term thermal stability of porous-matrix CMCs. This matrix utilizes a bimodal mixture of mullite ( 1 jLm) and alumina ( 0.2 p,m) powders [4], The mullite exhibits relatively slow sintering that inhibits shrinkage and maintains the overall porous structure, while the fine alumina sinters more readily, cementing the mullite particles and providing strength to the porous matrix structure [66, 67]. Zirconia has also been studied as the primary phase, with mullite used to stabilize the matrix [68]. 

The UCSB Nextel 720 reinforced material with its lower stiffness mullite matrix has a 45° tensile elastic modulus of only 23 MPa, or less than 40% of its fiber direction stiffness [142]. 

C. Kaya, E. G. Butler, A. Selcuk, A. R. Boccaccini, and M. H. Lewis, Mullite (Nextel 720) fibre-reinforced mullite matrix composites exhibiting favourable thermomechancial properties, J. Eur. Ceram. Soc., 22 2333-2342 (2002). 

Pnipcrty Nextel 610-1-AhOj-Mairix Nextel 720 AliOa-Matrix Nextel 720 + mullite Matrix... 

Various material properties (e. g. thermal diffiisivity, air permeability) have been determined for WHIPOX CMCs. Reliable data are important for potential applications such as thermal insulators, filters or burners. Thermal conductivity perpendicular to fiber orientation is about IW/mK. Closer inspection reveals lower conductivity if a mullite matrix is employed instead of alumina. Thermal conductivity in fiber direction, on the other hand, is about three times higher as perpendicular to the fiber direction, reflecting the non-isotropic structure of the composite (Figure 11). 

FIGURE 10. Load/deflection curves of WHIPOX CMCs with alumina plus mullite matrix. Damage-tolerant fracture behavior upon firing at 1500°C and 1600°C is indicated. 

Y. Hirata, S. Matsushita, Y. Ishihara, H. Katsuki, Colloidal processing and mechanical properties of whisker-reinforced mullite matrix composites, 7. Am. Ceram. Soc. 74 2438-2442(1991). 

H. Schneider, M. Schmiicker, J. Goring, B. Kanka, J. She, P. Mechnich, Porous alumino silicate fiber/mullite matrix composites Fabrication and properties, in Innovative Processing and Synthesis of Ceramics, Glasses, and Composites IV (N. P. Bansal and J. P. Singh, Editors), Ceram. Trans. 115,415-434 (2000), Am. Ceram. Soc., Westerville, OH. 

The microstructure of zirconia grains is observed by TEM and analyzed by CBED (Fig. 6). The large intergranular grain is transformed and the twin domains and a microcrack formed in the mullite matrix due to the different expansion coefficient after transformation are visible. In this case, the intragranular tetragonal zirconia particles show only a small concentric stress field around them. 

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