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DESIGNING,  EVALUATING and TESTING 

of Components Made from Engineering Ceramics 

Although ceramics have very desirable characteristics such as their ability to operate at high temperatures, good wear resistance, has high temperature strength and good corrosion resistance, their inherent brittleness requires careful and judicious use in particular applications.  

Metals have well-developed materials-property relationships and well-characterized metal applications data and use profiles as well as many years of experience of using metal components in various applications.

In sharp contrast, engineering ceramics do not have such well-developed materials-property relationships and do not have well-characterized ceramic applications data and use profiles. Substantial experience of use of ceramic components in particular applications is lacking. This means that the knowledge and understanding of the use of ceramic components in specific applications are limited.

Thus a customer must be prudent and cautious in the use of ceramic components in particular applications. See also reference 7 ( - M. M. Schwartz “Handbook of Structural Ceramics” McGraw-Hill Publishers, 1992).

UHM strongly recommends and urges our customers to first carry out a critical and thorough evaluation of UHM’s CeTZP ceramic components in operational service in any of the customer’s applications and/or in any of the customer’s products  - IMPORTANT - please click here to see the section on UHM's Product Liability Disclaimer.

Testing

Tensile property tests are routinely carried out with metals. In contrast, ceramic manufacturers generally tend to concentrate upon evaluating the following mechanical properties: bend strength, compression strength, Vickers hardness, fracture toughness and wear resistance. This is primarily because the measurement of tensile properties of ceramics is fraught with experimental difficulties in obtaining reliable values.

For monolithic engineering ceramics such as CeTZP, hardness will correlate with wear resistance and toughness will determine the strengths, strength variability, i.e. Weibull modulus, and general ease of handling/fitting/usage.

As with all monolithic homogeneous materials, the elastic moduli are not very dependent upon microstructure, so that the tensile modulus of the CeTZP will be around 200 GPa.

The few bend test fatigue results that have been carried out on room temperature fatigue of CeTZP would support the findings of others: that fatigue is very limited, with the fatigue limit being at least 90% (if not greater) of the monotonic bend strength.

As UHM’s CeTZP, like many engineering ceramics, has virtually no tensile ductility (although it is reasonably tough), the main usages envisaged are those which take advantage of its good hardness (and thus wear resistance), high compressive strength and very low thermal conductivity.

It is very important to note that great care would be needed to utilize this material in a safety critical component with a high tensile stress. There are three possible approaches that would be used in an attempt to guarantee safety under these conditions and they would be:

(a) a probabilistic analysis based upon converting bend strength data to the component stress state and volume using a Weibull approach, typified by the analyses of Stanley for example,

(b) using a LEFM analysis given the material's toughness and the minimum flaw size that could be detected by NDE given the component geometry etc.,

or

(c) proof testing of the component before being placed in service.

For use in safety critical applications with high tensile stresses, it is recommended that approach (c) be considered for UHM’s CeTZP components (or for any other ceramic). 

Approach (a) has been found to be exceedingly expensive as it involves tensile tests of quite large sized specimens.

For further details on design considerations, testing methods, reliability and failure analyses and other factors for ceramics, it would be advisable to consult texts such as "Handbook of Structural Ceramics" by M.M.Schwartz (reference 7) and "Engineered Materials Handbook" by S.J.Schneider (reference 5).

 

Useful Quotes

It might be worth noting the following:  In his Handbook on Structural Ceramics and his book on Ceramic Joining, Mel Schwartz of the Sikorsky Aircraft Division of United Technologies Corporation states that: 

"Educating and making engineers aware of the increasing body of knowledge about these materials is the first step in viewing ceramics as legitimate materials for use in demanding applications" (from page 1.2 in reference 7)

and

"The greatest misconception regarding ceramics is confusing brittleness with lack of strength. Consider, for example, a metal turbine rotor and a ceramic turbine rotor. Drop each from a desktop onto a floor. On impact, the ceramic turbine blades chip or break. The metal turbine blades, on the other hand, merely bend [............in the case of the two turbine rotors falling off the desk, neither one is usable]. Yet this example does not show that ceramics are inferior to metal. The same advanced ceramic gas turbine rotor can be spun at 80 to 100,000 rpm (8373 to 10,466 rad/sec) at a temperature of 13700C (25000F). Metals would fail miserably under those conditions" (from page 55 in reference 4).

 

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