31 Introduction

As shown in the previous chapter, ceramics are finding use where temperatures exceed the capability of other materials, especially metals. Even so, they are

Handbook of Advanced Materials Edited by James K. Wessel ISBN 0-471-45475-3 Copyright © 2004 John Wiley & Sons, Inc.

not selected for many applications because of the brittleness of these monolithic ceramics. In the search for improved toughness, material scientists conceived the idea of reinforcing ceramics with continuous strands of high-temperature ceramic fiber, analogous to continuous fiberglass-reinforced plastics. Embedded continuous ceramic fibers reinforce the ceramic matrix by deflecting and bridging fractures.

These continuous fiber-reinforced ceramic composite (CFCC) materials offer the advantages of ceramics: resistance to heat, erosion, and corrosion—while adding toughness and thermal shock resistance. The result is a lightweight, hard, tough, high-temperature, thermal shock, erosion, and corrosion-resistant structural material. These materials are used where designers seek less downtime, reduced maintenance, lower operating costs, increased operating temperature, increased efficiency, lower emissions, and reduced life-cycle costs (see Table 3.1). Designers are evaluating and using them in applications in major industries.

Monolithic ceramics, although strong in tension, tend to fracture suddenly with total loss of strength. Conversely, when the yield strength of CFCC is exceeded, failure occurs "gracefully," with the material able to continue to bear load. This feature reduces the risk of catastrophic failure and encourages designers to use CFCC materials for this and other benefits (see Fig. 3.1).

All CFCC materials are composed of a ceramic fiber, a fiber-matrix interface coating and a ceramic matrix, arranged to form a continuously reinforced material. The fiber is converted to useful form by using conventional textile-forming techniques: single-fiber filaments can be grouped into a tow, woven into fabrics, cut, sewn, laminated, and tooled to form a net-shape preform for subsequent processing. Other forming processes include winding the coated fiber filaments onto a mandrel to form tubes, cylinders, and related shapes. This formed fiber shape, or preform, is infiltrated with a ceramic matrix by various techniques and converted to a ceramic by the application of heat and pressure.

The fibers provide toughness by arresting cracks, bridging cracks, and by a phenomenon known as fiber "pull-out."

For a crack to grow, energy must be expended. When the crack comes to a fiber, it must divert around that fiber. This consumes more energy than linear growth and the crack will stop. If the crack is propagated by sufficient energy to pass around the fiber, the fiber can bridge the crack and hold the composite together. Finally, if the forces are sufficient to fail the composite, the fiber must be pulled out of the composite. This pull-out requires additional energy and, as the fibers continue to carry the load, a noncatastrophic, load-bearing failure mode

TABLE 3.1 CFCC Characteristics

Characteristic

Resists corrosion Resists high temperatures Fiber reinforced Near-net-shape fabrication

Advantage

Survives hostile environments Use temperatures to 2200°F Survives cyclic loading

Lowers life-cycle cost

FIGURE 3.1 Continuous fiber reinforcement changes the shape of typical ceramic stressstrain curve. Yields occurs "gracefully." This slow yield eliminates sudden failure.

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