GTBL042-07 GTBL042-Callister-v2 August 6, 2007 12:43
7.12 Influence of Porosity on the Mechanical Properties of Ceramics • 21340302010(^00)
250
200
150
100
50
0 0.0004 0.0008 0.0012
Strain
Glass
Aluminum oxide
Stress (10
3 psi)
Stress (MPa)
Figure 7.19 Typical stress–strain
behavior to fracture for aluminum
oxide and glass.
7.11 ELASTIC BEHAVIOR
The elastic stress–strain behavior for ceramic materials using these flexure tests is
similar to the tensile test results for metals: a linear relationship exists between stress
and strain. Figure 7.19 compares the stress–strain behavior to fracture for aluminum
oxide and glass. Again, the slope in the elastic region is the modulus of elasticity;
also the moduli of elasticity for ceramic materials are slightly higher than for metals
(Table 7.1 and Table B.2, Appendix B). From Figure 7.19 note that neither glass nor
aluminum oxide experiences plastic deformation prior to fracture.
7.12 INFLUENCE OF POROSITY ON THE
MECHANICAL PROPERTIES OF CERAMICS
For some ceramic fabrication techniques (Sections 14.8 and 14.9), the precursor ma-
terial is in the form of a powder. Subsequent to compaction or forming of these
powder particles into the desired shape, pores or void spaces will exist between the
powder particles. During the ensuing heat treatment, much of this porosity will be
eliminated; however, it is often the case that this pore elimination process is incom-
plete and some residual porosity will remain (Figure 14.27). Any residual porosity
will have a deleterious influence on both the elastic properties and strength. For ex-
ample, it has been observed for some ceramic materials that the magnitude of the
modulus of elasticityEdecreases with volume fraction porosityPaccording to
E=E 0 (1− 1. 9 P+ 0. 9 P^2 ) (7.21)
Dependence of
modulus of elasticity
on volume fraction
porositywhereE 0 is the modulus of elasticity of the nonporous material. The influence of
volume fraction porosity on the modulus of elasticity for aluminum oxide is shown
in Figure 7.20; the curve represented in the figure is according to Equation 7.21.
Porosity is deleterious to the flexural strength for two reasons: (1) pores reduce
the cross-sectional area across which a load is applied, and (2) they also act as stress
concentrators—for an isolated spherical pore, an applied tensile stress is amplified by