Page 223 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

202 Chapter 8 Ceramics, Graphite, Diamond, and Nanomaterials: Structure, General Properties. and Applications

8.3.1 Mechanical Properties
The mechanical properties of several engineering ceramics are presented in Table 8.2.
Note that their strength in tension (transverse rupture strength, Section 2.5) is
approximately one order of magnitude lower than their compressive strength. The
reason for this is their sensitivity to cracks, impurities, and porosity. Such defects lead
to the initiation and propagation of cracks under tensile stresses and significantly
reduce the tensile strength of the ceramic. Thus, reproducibility and reliability (accept-
able performance over a specified period) is an important aspect in the service life of
ceramic components.
The tensile strength of polycrystalline ceramic increases with decreasing grain
size and porosity. This relationship is represented approximately by the expression

Urs = UTS,,e5"P, (8.1)

Where P is the volume fraction of pores in the solid (thus, if the porosity is 15%,
then P = 0.15 ), UTSO is the tensile strength at zero porosity, and the exponent 71
ranges between 4 and 7. The modulus of elasticity of ceramics is related to its poros-
ity by the expression

E ~ E,(1 - i.9P + 0.9P2), (8.2)

where E0 is the elastic modulus at zero porosity.
Unlike most metals and thermoplastics, ceramics generally lack impact tough-
ness and thermal-shock resistance because of their inherent lack of ductility; once
initiated, a crack propagates rapidly. In addition to undergoing fatigue failure under
cyclic loading, ceramics (particularly glasses) exhibit a phenomenon called static
fatigue. When subjected to a static tensile load over time, these materials suddenly
may fail. This phenomenon occurs in environments Where Water vapor is present.
Static fatigue, which does not occur in a vacuum or in dry air, has been attributed to
a mechanism similar to the stress-corrosion cracking of metals.
Ceramic components that are to be subjected to tensile stresses may be pre-
stressed in much the same Way that concrete is prestressed. Prestressing the shaped

TABLE 8.2
Properties of Various Ceramics at Room Temperature
Transverse
rupture Compressive Elastic
strength strength modulus Hardness Poisson’s Density
Material Symbol (MPa) (MPa) (GPa) (HK) ratio, 1/ (kg/m3)
Aluminum oxide A1203 140-240 1000-2900 310-410 2000-3000 0.26 4000-4500
-
Cubic boron cBN 725 7000 850 4000-5000 3480
nitride
-
-
Diamond - 1400 7000 830-1000 7000-8000 - 3500
Silica, fused SiO2 1300 70 550 0.25
Silicon carbide SiC 100-750 700-3500 240-480 2100-3000 0.14 3 100
-
Silicon nitride Si3N4 480-600 300-310 2000-2500 0.24 3300
-
Titanium carbide TiC 1400-1900 3100-3850 310-410 1800-3200 - 5500-5800
Tungsten carbide WC 1030-2600 4100-5900 520-700 1 800-2400 10,000-15,000
Partially stabilized PSZ 620 - 200 1100 0.30 5800
zirconia
Note: These properties vary Widely depending on the condition of the material.