Properties mullite

Most structural PMCs consist of a relatively soft matrix, such as a thermosetting plastic of polyester, phenolic, or epoxy, sometimes referred to as resin-matrix composites. Some typical polymers used as matrices in PMCs are listed in Table 1.28. The list of metals used in MMCs is much shorter. Aluminum, magnesium, titanium, and iron- and nickel-based alloys are the most common (see Table 1.29). These metals are typically utilized due to their combination of low density and good mechanical properties. Matrix materials for CMCs generally fall into fonr categories glass ceramics like lithium aluminosilicate oxide ceramics like aluminnm oxide (alnmina) and mullite nitride ceramics such as silicon nitride and carbide ceramics such as silicon carbide. 

Table 5.1 Mechanical properties of graded mullite/alumina...

Table 5.2 Physical and mechanical properties of graded mullite-ZTA...

Marple, B.R. Green, D.J. (1991) Mullite/alumina particulate composites by infiltration processing III, Mechanical properties. J. Am. Ceram. Soc. 74, 2453-2459. 

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. 

Whereas the ambient mechanical properties have been studied extensively, the high temperature properties of mullite and mullite/zirconia matrix composites have been examined by only a few studies.18 30,32 34... 

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. 

T. Sugama, N.R. Carciello, T.M. Nayberg, and L.E. Brothers, Mullite microsphere-filled lightweight calcium phosphate cement slurries for geothermal wells setting and properties, Cem. Cone. Res., 25 [6] (1995) 1305-1310. 

MuUite is another important crystalline ceramic that is an oxide It is a solid solution of alumina and silica in the compositional range 71-75 wt% alumina. Mullite is represented by the formula, 3 Al203.2Si02. It has excellent strength and creep resistance as well as low thermal expansion and conductivity. For more detailed information on the structure and properties of mullite the reader is referred to Schneider et al. (1994). Table 6.2 provides a summary of the properties of mullite. 

Determine the temperature required to induce cracking in a mullite green body in the form of a plate with a void fraction of 0.4 that has a strength of only 200 KPa caused by interparticle van der Waals interactions. The following properties of muUite are needed ay = 5 x lO rC, E = 69 GPa [28, pp. 593, 770, 777], v = 0.25. 

FIGURE 12.12 Examples of FGM CMCs (left) the combination of NLM Nicalon SiC fiber reinforced zirconia (in white) and mullite (in black) matrices (right) a Nasicon matrix reinforced with mullite (Nextel) and SiC (NLMTM) fibers (see the sketch). (Reprinted from Colomban, R, Process for fabricating a ceramic matrix composite incorporating woven fibers and materials with different compositions and properties in the same composite, Mater. TechnoL, 10, 89, 1995. With permission.)... 

Colomban, R, and Mazerolles, L., Si02-Al203 phase diagram and mullite nonstoichiometry of sol-gel prepared monoliths influence on mechanical properties, J. Mater. Sci. Lett., 2, 1077, 1990. 

Mouchon, E., and Colomban, R, Microwave absorbent preparation, mechanical properties and RF/microwave conductivity of SiC (and/or mullite) fibres reinforced NASICON matrix composites, J. Mater. ScL, 31, 323, 1996. 

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. 

Different applications generally require different filter properties (temperature resistance, pressure drop, filtration efficiency, etc.), and therefore a number of different types of filters, based on either ceramic (SiC, mullite, cordierite, etc.) or metallic (stainless steel, Hastelloy, Fecralloy, etc.) materials, are currently produced and commercialized. Table 1 summarizes such filter types, their producers, and their major application fields. 

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