Ceramic Matrix Composites

A composite material is a material that has a chemically and/or physically distinct phases distributed within a continuous phase. The composite generally has characteristics better than or different from those of either component. The matrix phase is the continuous phase, while the distributed phase, commonly called the reinforcement phase, can be in the form of particles, whiskers or short fibers, continuous fibers or sheet. Figure 1.1 shows the types of composites based on the form of reinforcement. Oftentimes it is convenient to classify different types of composites as per the matrix material characteristics, viz., polymer matrix composites (PMCs), metal matrix composites (MMCs), and ceramic matrix composites (CMCs). The reinforcement in any matrix can be polymeric, metallic, or ceramic. Polymeric matrix composites containing reinforcement fibers such as carbon, glass, or, aramid are quite commonly used as engineering materials. Metals containing ceramic particles, whiskers, or fibers (short or long) are also gaining in importance. The ceramic matrix composites are the newest entrants in the composites field.

Ceramic reinforcements can be produced in the form of continuous fiber, short fiber, whisker, or particle. Continuous ceramic fibers are very attractive for reinforcing ceramic materials. They combine rather high strength and elastic modulus with high temperature capability and a general freedom from environmental attack, making them attractive as reinforcements in high temperature structural materials. Continuous fibers are, however, more expensive than particulate reinforcements. It is convenient to divide the ceramic reinforcements into oxide and nonoxide categories. Table 3.1 lists some important ceramic reinforcement materials available in different forms.

In this chapter we describe some of the important processing techniques for fabricating ceramic matrix composites. Among the items that one should take into account for choosing reinforcement and matrix materials are:

The interface region has a great deal of importance in determining the ultimate properties of a given composite. There are two main reasons for this importance of the interface region:

In this chapter, we examine the elastic and physical properties of ceramic matrix composites. In particular, we give a micromechanical description of the different elastic constants, thermal expansion coefficients, thermal conductivity, density, etc. in terms of the same constants or of the individual components and the geometric arrangement of the components.

In this chapter, we discuss various aspects of mechanical behavior of ceramic matrix composites: mechanics of load transfer from the matrix to the fiber, behavior of CMCs under monotonic and cyclic (fatigue) loading, and creep conditions.

In this chapter, we discuss in some detail the important subject of thermal stresses in composites. This is of importance in all composites, PMCs, MMCs, and CMCs. The analytical expressions obtained in Sec. 8.2 and 8.3 have, of course, general validity for a variety of composites. We shall then apply the results obtained in these sections to CMCs and derive some important guidelines to obtain enhanced fracture toughness in these materials. Finally, we dwell on the subject of thermal shock and thermal fatigue in CMCs.

In this chapter, we discuss the role of interface in CMCs with a special emphasis on mechanics of the fiber/matrix interface as related to that somewhat elusive property called toughness. We discussed the importance and general features of interface in composites in Chapter 5. Now we look at the subject of tailoring this interface in CMCs with a view to obtain an enhanced fracture toughness in these materials. We give some examples of the interface structure in some CMCs, briefly review the topic of toughness in different types of CMCs and then we analyze in some detail the toughening mechanisms in fiber reinforced ceramic matrix composites because of their immense promise.

Laminated fibrous composites are made by bonding together two or more laminae. The individual unidirectional laminae or plies are oriented in such a manner that the resulting structural component has the desired mechanical and/or physical characteristics in different directions. In this way, one can exploit the inherent anisotropy of fibrous composites to design a composite material having a desired set of characteristics such as elastic constants, thermal expansion coefficients, etc. This has been employed quite extensively in polymer matrix composites to design PMCs having highly tailored elastic, thermoelastic, and strength characteristics, not so much in metal matrix and ceramic matrix composites. Techniques such as tape-casting and hot pressing of laminae can be used to produce laminated CMCs (Prewo and Brennan, 1982; Bhatt, 1991; Bhatt and Phillips, 1990; Amateau and Messing, 1990; Velamakanni and Lange, 1991; Gladysz et al., 1999). In this chapter, we provide the reader the very basic mathematical tools to analyze such laminated composites. For greater details on the mechanics of laminated composites, the references listed under Suggested Reading should be consulted.

In this chapter we describe some of the commercial applications of ceramic matrix composites, emphasizing the salient requirements in each application and how they are met by specific CMCs. Let us recall that the main attributes of CMCs are: high strength and modulus, low density, capability of being used at high temperatures, and a greater toughness than that of monolithic ceramics. In order to drive home some of these advantages of CMCs, we present some examples. Some of the novel processing techniques described in Chapter 4 have been used very profitably to make CMCs in useful forms. Figure 11.1a shows ceramic matrix composite turbine blades made by sol-gel vacuum impregnation of fibrous preforms while Fig. 11.1b shows a filament wound tube made of CMC (Hyde, 1989). The characteristic of toughness or damage tolerance is depicted by hammering a nail through a CMC (Fig. 11.1c) while the ability to withstand high temperatures is shown by an oxyacetylene flame impinging on the wall of a tube made of CMC (Hyde, 1989).