Callister - An introduction - 8th edition

Summary • 491
Table 12.6
Vickers (and Knoop) Hardnesses for Eight Ceramic Materials
Vickers Hardness Knoop Hardness
Material (GPa) (GPa) Comments
Diamond (carbon) 130 103 Single crystal, (100) face
Boron carbide (B 4 C) 44.2 — Polycrystalline, sintered
Aluminum oxide (Al 2 O 3 ) 26.5 — Polycrystalline, sintered, 99.7% pure
Silicon carbide (SiC) 25.4 19.8 Polycrystalline, reaction bonded, sintered
Tungsten carbide (WC) 22.1 — Fused
Silicon nitride (Si 3 N 4 ) 16.0 17.2 Polycrystalline, hot pressed
Zirconia (ZrO 2 ) (partially 11.7 — Polycrystalline, 9 mol% Y 2 O 3
stabilized)
Soda-lime glass 6.1 —
Possibly the most desirable mechanical characteristic of ceramics is their hardness;
the hardest known materials belong to this group. A listing of a number of different
ceramic materials according to Vickers hardness is contained in Table 12.6. 4
These materials are often utilized when an abrasive or grinding action is required
(Section 13.6).
Creep
Often ceramic materials experience creep deformation as a result of exposure
to stresses (usually compressive) at elevated temperatures. In general, the time–
deformation creep behavior of ceramics is similar to that of metals (Section 8.12);
however, creep occurs at higher temperatures in ceramics. High-temperature compressive
creep tests are conducted on ceramic materials to ascertain creep deformation
as a function of temperature and stress level.
SUMMARY
Crystal Structures
• Interatomic bonding in ceramics ranges from purely ionic to totally covalent.
• For predominantly ionic bonding:
Metallic cations are positively charged, whereas nonmetallic ions have negative
charges.
Crystal structure is determined by (1) the charge magnitude on each ion and
(2) the radius of each type of ion.
• Many of the simpler crystal structures are described in terms of unit cells:
Rock salt (Figure 12.2)
Cesium chloride (Figure 12.3)
Zinc blende (Figure 12.4)
Fluorite (Figure 12.5)
Perovskite (Figure 12.6)
• Some crystal structures may be generated from the stacking of close-packed
planes of anions; cations fill interstitial tetrahedral and/or octahedral positions
that exist between adjacent planes.
4
In the past the units for Vickers hardness were kg/mm 2 ; in Table 12.6 we use the SI units
of GPa.