Ceramics

Ceramics

Applications

Ceramics are used in a variety of applications:

Compressive strength makes ceramics good structural materials (e.g., bricks in houses, stone blocks in the pyramids)

High voltage insulators and spark plugs are made from ceramics due to its electrical conductivity properties

Good thermal insulation has ceramic tiles used in ovens and as exterior tiles on the Shuttle orbiter

Some ceramics are transparent to radar and other electromagnetic waves and are used in radomes and transmitters

Hardness, abrasion resistance, imperviousness to high temperatures and extremely caustic conditions allow ceramics to be used in special applications where no other material can be used

Chemical inertness makes ceramics ideal for biomedical applications like orthopaedic prostheses and dental implants

Glass-ceramics, due to their high temperature capabilities, leads to uses in optical equipment and fiber insulation

Ceramics - Typical Properties

It is known that ceramic materials have the potential to achieve mechanical properties equal to or better than most metals. Their limitations have resulted from flaws or micro cracks which led to brittleness. Recent improvements in processing techniques, however, have maximized the density and minimized flaws so that new high performance ceramics are now being examined as direct replacements for many metal parts.

An important example of this is the ceramic turbocharger for use in automotive and gas turbine applications. Being subject to high temperature exhaust gases and high G forces, this application is very demanding on the materials used. The high reliability required also adds to the premium it can command, making it commercially attractive to develop new structural ceramics - such as zirconia ceramics, silicon nitride and silicon carbide - and new production processes. Ultimately, new ceramic manufacturing processes may lead to lower cost parts that are lighter and capable of withstanding very high temperatures. The potential of these new so-called fine ceramics has spawned a tremendous amount of research in the improvement of monolithic ceramics as well as the development of new ceramic-ceramic composite materials.

Ceramics - Testing Overview

The testing of ceramics has taken a different approach to that of conventional materials testing. The nature of ceramics makes it extremely difficult to perform a conventional tensile test. Without absolutely perfect alignment, the act of gripping a specimen applies bending that is often enough to result in failure. Many researchers in the 1960's and 1970's made attempts at direct tension testing but had limited success or used methods that were too costly and time consuming to be considered worthwhile. Most subsequent testing was conducted using specially shaped specimens (e.g., C-ring test) or gripless flexure testing. Recently, however, the need for engineering data is growing in conjuction with the development of structural ceramics.

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Ceramics Overview Typical Properties Testing Overview

The requirements of ceramics testing are quite rigorous. Typical environmental requirements include temperatures of 1600C in air, and 2200C in a vacuum; requirements in inert gas depend upon its properties. Both three- and four-point bend testing are widely accepted, but specimen sizes vary widely from lab to lab and from country to country. Testing systems must be capable of precise slow-speed control (0.0001 in/min) and are often required to hold loads for extended periods of time for creep analysis. The need for tension testing, fracture mechanics, bend testing, even reverse-stress fatigue testing is growing and can only be met by carefully designed systems that can overcome the challenges facing ceramics. As an example, Instron 8562 Systems are used in many research institutes throughout the world, including Massachusetts Institute of Technology and Institut fur Werkstoffkunde, University of Karlsruhe, to study behavior of ceramics, composites and cermets.

To speed the ceramics development process, Instron has designed a series of automated ceramics test systems that can perform creep and three- and four-point bend tests. These tests are typically conducted within a furnace capable of 1650C in a vacuum with multiple test stations so that once the system is set up, all twelve specimens are tested automatically. Precise temperature control is essential, as is the load and strain measurement accuracy.