Page 92 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 2.6 Hardness Tl

light loads that are applied, the Knoop test is a
microhardness test; therefore, it is suitable for very
small or very thin specimens, and for brittle mate-
rials such as carbides, ceramics, and glass. 1 QT
This test is also used for measuring the hard- “ia” - 1
ness of the individual grains and components in a T <0
metal alloy. The size of the indentation is general-
ly in the range from 0.01 to 0.10 mm; consequent-
ly, surface preparation is very important. Because
the hardness number obtained depends on the ap-
plied load, Knoop test results should always cite
the load used.
Scleroscope and Leeb Test. The scleroscope
(from the Greek skleros, meaning “hard”) is an in-
strument in which a diamond-tipped indenter
(hammer) enclosed in a glass tube is dropped onto
the specimen from a certain height. The hardness is
related to the rebound of the indenter: the higher
the rebound, the harder the material. The impres-
sion made by a scleroscope is very small. Because
obtaining reliable results with a scleroscope is diffi- (C)
cult, a modern electronic version, called a Leeb, or
Equotip, test, has been developed (see Fig. 2.12d). FIGURE 2.l4 Indentation geometry in Brinell hardness testing:
In this test, a carbide hammer impacts the surface, (a) annealed metal; (b) work-hardened metal; (c) deformation of
and incident and rebound velocities are electroni- mild steel under a spherical indenter. Note that the depth of
cally measured. A Leeb number is then calculated the permanently deformed zone is about one order of magnitude
and usually converted to Rockwell or Vickers larger than the depth of indentation. For a hardness test to be
valid, this zone should be fully developed in the material.
hardness.
Source: Courtesy of M.C. Shaw and C.T. Yang.
Mohs Hardness. Developed in 1822 by F. Mohs, this test is based on the capability
of one material to scratch another. The Mohs hardness is based on a scale from 1 to
10, with 1 being the measure for talc and 10 that for diamond (the hardest substance
known). A material with a higher Mohs hardness number always scratches one with
a lower number. Soft metals have a Mohs hardness of 2 to 3, hardened steels about 6,
and aluminum oxide (used for cutting tools and as an abrasive in grinding wheels)
of 9. Although the Mohs scale is qualitative and is used mainly by mineralogists, it
correlates well with Knoop hardness.

Shore Test and Durometer. The hardness of materials such as rubbers, plastics,
and similar soft and elastic nonmetallic materials is generally measured by a Shore
test with an instrument called a durometer (from the Latin durus, meaning “hard”).
An indenter is pressed against the surface and then a constant load is rapidly
applied. The dept/J of penetration is measured after 1 second; the hardness is in-
versely related to the penetration. There are two different scales for this test. Type A
has a blunt indenter and a load of 1 kg; it is used for softer materials. Type D has a
sharper indenter and a load of 5 kg, and is used for harder materials. The hardness
numbers in these tests range from 0 to 100.

Hot Hardness. The hardness of materials at elevated temperatures (see Fig. 22.1) is
important in applications such as cutting tools in machining and dies in hot-working
and casting operations. Hardness tests can be performed at elevated temperatures
with conventional testers, with some modifications such as enclosing the specimen
and indenter in a small electric furnace.