Callister - An introduction - 8th edition

490 • Chapter 12 / Structures and Properties of Ceramics
40
250
Flexural strength (MPa)
200
150
100
50
30
20
10
Flexural strength (10 3 psi)
0
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Volume fraction porosity
Figure 12.36 The influence of porosity on the flexural strength for aluminum oxide at
room temperature. (From R. L. Coble and W. D. Kingery, “Effect of Porosity on Physical
Properties of Sintered Alumina,” J. Am. Ceram. Soc., 39, 11, Nov. 1956, p. 382. Reprinted
by permission of the American Ceramic Society.)
Dependence of
flexural strength on
volume fraction
porosity
10 vol% porosity will often decrease the flexural strength by 50% from the measured
value for the nonporous material. The degree of the influence of pore volume
on flexural strength is demonstrated in Figure 12.36, again for aluminum oxide. Experimentally
it has been shown that the flexural strength decreases exponentially
with volume fraction porosity (P) as
s fs s 0 exp 1nP2
In this expression 0 and n are experimental constants.
(12.10)
Hardness
Accurate hardness measurements are difficult to conduct inasmuch as ceramic materials
are brittle and highly susceptible to cracking when indenters are forced into
their surfaces; extensive crack formation leads to inaccurate readings. Spherical
indenters (as with Rockwell and Brinell tests) are normally not used for ceramic
materials because they produce severe cracking. Rather, hardnesses of this class of
materials are measured using Vickers and Knoop techniques that employ indenters
having pyramidal shapes (Section 6.10, Table 6.5). 3 Vickers is widely used for measuring
hardnesses of ceramics; however, for very brittle ceramic materials, Knoop is
often preferred. Furthermore, for both techniques, hardness decreases with increasing
load (or indentation size), but ultimately reaches a constant hardness
plateau that is independent of load; the value of hardness at this plateau varies from
ceramic to ceramic. An ideal hardness test would be designed so as to use a sufficiently
large load that lies near this plateau, yet of magnitude that does not introduce
excessive cracking.
3 ASTM Standard C1326, “Standard Test Method for Knoop Indentation Hardness of
Advanced Ceramics,” and Standard C1327, “Standard Test Method for Vickers Indentation
Hardness of Advanced Ceramics.”