Brittle and Ductile Deformation Associated with Interfaces in Ceramic Fibers, Coatings, and Composites

Ceramic fiber-matrix composites require a fiber-matrix interface that is weak compared to the fiber. A functional composite might therefore be formed either by weakening the interface or strengthening the fiber. Fiber tensile strength is in turn affected by the processing used to form the interface and matrix. Typically strength is reduced, often by a large amount, but occasionally increased strength is observed. Degradation mechanisms are difficult to identify and probably complex. Evidence, consequences, and possible control methods for the following mechanisms will be discussed:

1) Stress corrosion cracking from surface active gases in the coating, processing, and application environment. Hermetic coatings or matrices may protect fibers from the application environment, but trap their own corrosive decomposition products at the fiber surface. A dispersed second phase that scavenges decomposition products may be beneficial. Increased surface roughness may be associated with corrosion.

2) Grain boundary segregation of coating components in the fiber.

3) Solid-state reactions between coating and fiber.

4) Flaw size - grain size correlation and fiber grain growth.

5) Coating flaws that function as fiber flaws.

6) Flaw healing in different chemical environments.

Tough ceramic composites also require displacement in shear along a fiber-matrix interface crack. A variety of deformation mechanisms may allow this displacement, such as microfracture, cleavage, cataclastic flow, twinning, dislocation plasticity, and diffusional creep and dynamic recrystallization associated with local frictional heating. Compositions, microstructures, and textures that promote such mechanisms are considered. Although observations are presented in the context of ceramic fiber-matrix composites, the phenomenon are general to structural and geologic materials. Broader implications will be discussed.