May 3 2002
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In the history of engineering ceramics applications, there have been many examples where imitated designs based on a metal’s performance have resulted in considerable failures of ceramic components.
In ceramic engineering design, it is essential to study the material properties, operating environment, and the fabrication process used. Factors like dimensional tolerances, reliability, risk of premature failure, cost, and component geometry are limited in ceramic designs.
A precondition of any ceramic engineering design is an unambiguous and prioritized specification of the application needs. So, what function is the component supposed to perform and what are the conditions it will experience? For most of the engineering applications, the following considerations will be involved:
- The load component will have to endure, more particularly, the stress distribution that forms
- The operating temperature including transients
- The chemistry associated with the operating environment
- The potential for impact and abrasion
- Other external environmental factors like high magnetic or electrical fields and ionizing radiation
The application conditions are compared with the suitable properties of the material to conclude the first stage of ceramic material selection. This apparently easy task is often made hard by the lack of property data, particularly data measured under the appropriate conditions.
The usual sources of property data for ceramics are reference works that are typically limited in scope and specify classes of materials, or suppliers’ data sheets that provide typical values or ranges of properties achieved by specific materials. With the lack of applicable data, designers must be prepared to deal with the need to measure the materials’ properties by themselves.
Prediction of Performance
In the subsequent step of the material selection process, ceramic performance in the application is predicted. This will take into consideration the dependence of numerous properties on component-specific features like shape and size, as well as the interfaces with neighboring components.
Finite element and other mathematical modeling methods are very crucial in estimating the behavior of ceramic components, mostly in intricate stress conditions or where transient mechanical and thermal conditions are present.
In numerous cases, well-documented performance predictions are available that are based on material properties. In other cases, the easiest and most economical approach is to develop the component and evaluate its performance directly in the application.
Fabrication and Manufacturing Issues
The fabrication technique chosen for the component is based on its shape and size, as well as the economics of production, which itself is mainly controlled by the manufacturing rate.
The fabrication technique also establishes the type and population of process-related defects, which limit the properties and reliability that can be achieved. For instance, identical parts that are created by slip casting, pressing, and firing will—regardless of having the same density—display dissimilar microscopic defects relating to shape and size (for example, small cracks in the pressed item and bubbles in the slip cast item).
These will result in differences in strength and toughness which could make the component produced by one technique unsuitable for the application.
The dependability of the measured properties is one of the most essential features impacted by the fabrication technique. The statistical scatter of test results formed by a batch of ceramic test specimens can be applied to predict the probability of component failure under specified operating environments.
Consequently, components can be refined (over-engineered) to realize an acceptable failure rate. What constitutes a tolerable failure rate will depend on the significance of the application, specifically in applications where threats to life are considered the most challenging.
After choosing the fabrication technique, the designer must assess the property data and establish that it is still relevant, and that any mathematical modeling that has been carried out stays valid.