GTBL042-07 GTBL042-Callister-v2 August 6, 2007 12:43
228 • Chapter 7 / Mechanical PropertiesRockwell hardness60 70 80 90 100 HRB25020015010050015001000500(^00100200300400500)
Brinell hardness number
20 30 40 50 HRC
Steels
Brass Cast iron (nodular)
Tensile strength (10
3
psi)
Tensile strength (MPa)
Figure 7.31 Relationships
among hardness and tensile
strength for steel, brass, and
cast iron. [Data taken from
Metals Handbook: Properties
and Selection: Irons and
Steels, Vol. 1, 9th edition, B.
Bardes (Editor), American
Society for Metals, 1978,
pp. 36 and 461; andMetals
Handbook: Properties and
Selection: Nonferrous Alloys
and Pure Metals, Vol. 2, 9th
edition, H. Baker (Managing
Editor), American Society
for Metals, 1979, p. 327.]
Concept Check 7.6
Of those metals listed in Table 7.3, which is the hardest? Why?
[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]
7.17 HARDNESS OF CERAMIC MATERIALS
One beneficial mechanical property of ceramics is their hardness, which is often
utilized when an abrasive or grinding action is required; in fact, the hardest known
materials are ceramics. A listing of a number of different ceramic materials according
to Knoop hardness is contained in Table 7.7. Only ceramics having Knoop hardnesses
of about 1000 or greater are utilized for their abrasive characteristics (Section 13.8).
7.18 TEAR STRENGTH AND HARDNESS
OF POLYMERS
Mechanical properties that are sometimes influential in the suitability of a polymer
for some particular application include tear resistance and hardness. The ability to
resist tearing is an important property of some plastics, especially those used for thin
films in packaging.Tear strength,the mechanical parameter that is measured, is the
energy required to tear apart a cut specimen of a standard geometry. The magnitude
of tensile and tear strengths are related.