Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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2nd Revised Pages

9.6 Brittle Fracture of Ceramics • 305

Frequency of fracture

Strength (MPa)

300 400

60 80

Strength (ksi)

100 120

500 600 700 800 900

0.000

0.002

0.004

0.006

0.008

Figure 9.12 The

frequency distribution of

observed fracture

strengths for a silicon

nitride material.

ceramic materials to include porcelain, portland cement, high-alumina ceramics, bar-

ium titanate, and silicon nitride.

There is usually considerable variation and scatter in the fracture strength for

many specimens of a specific brittle ceramic material. A distribution of fracture

strengths for a silicon nitride material is shown in Figure 9.12. This phenomenon

may be explained by the dependence of fracture strength on the probability of the

existence of a flaw that is capable of initiating a crack. This probability varies from

specimen to specimen of the same material and depends on fabrication technique

and any subsequent treatment. Specimen size or volume also influences fracture

strength; the larger the specimen, the greater is this flaw existence probability, and

the lower the fracture strength.

For compressive stresses, there is no stress amplification associated with any

existent flaws. For this reason, brittle ceramics display much higher strengths in com-

pression than in tension (on the order of a factor of 10), and they are generally

utilized when load conditions are compressive. Also, the fracture strength of a brittle

ceramic may be enhanced dramatically by imposing residual compressive stresses at

its surface. One way this may be accomplished is by thermal tempering (see Section

14.7).

Statistical theories have been developed that in conjunction with experimental

data are used to determine the risk of fracture for a given material; a discussion of

these is beyond the scope of the present treatment. However, due to the dispersion

in the measured fracture strengths of brittle ceramic materials, average values and

factors of safety as discussed in Sections 7.19 and 7.20 are not normally employed

for design purposes.

Fractography of Ceramics

It is sometimes necessary to acquire information regarding the cause of a ceramic

fracture so that measures may be taken to reduce the likelihood of future incidents. A