BACKGROUND - Design Perspective
What Can Go Wrong
Given all the positive implications associated with the use of advanced ceramics, the reader is cautioned that utilizing this material is not straight forward and does have its drawbacks. The history of the application of engineering ceramics is littered with examples where swapping out metal components for ceramic components have resulted in spectacular failures. As with metal systems, ceramic engineering design requires consideration of materials properties and operating environment. However, the design penalty associated with ceramic materials is that they typically exhibit rather high tensile strength with a relatively low fracture toughness (typically quantified by KIC). This inherent undesirable combination must be considered when designing components fabricated from ceramics. Lack of ductility (i.e., lack of fracture toughness) leads to low strain tolerance and large variations in observed fracture strength. When a load is applied, the absence of significant plastic deformation or micro-cracking causes large stress concentrations to occur at microscopic flaws. These flaws are unavoidably present as a result of fabrication or in-service environmental factors. To date non-destructive evaluation (NDE) inspection programs with sufficient resolution have not been successfully implemented into fabrication operations. The combination of high strength and low fracture toughness leads to relatively small critical defect sizes that can not be detected by current NDE methods. As a result, components with a distribution of defects (characterized by various crack sizes and orientations) are produced which leads to an observed scatter in component tensile strength. The distribution of defects leads to an apparent decrease in tensile strength as the size of the component increases. This is the so-called strength-size effect, i.e., with bigger components there is an increased chance that larger more deleterious flaws are present. Design methods for ceramic components must admit size dependence. This is accomplished through the use of system reliability concepts where the component is treated as a system, and the probability of failure of the system must be ascertained.
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