Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 72

small ones in hard materials. There are several different eponymic tests
that produce scales of relative hardness. All the scales are arranged so
that a higher number corresponds to a harder material.
The most common is the Rockwell test, which is based on the mea-
surement of the depth of penetration of the indentor under a fixed load.
Different indentors and different loads produce a variety of scales; the
Rockwell number reported ( R C , R M , etc.) is calculated from the recipro-
cal of the penetration depth. The less common Vickers and Brinell scales
express hardness as the mean stress at the point of the indentor (load/
area or the impression) and use units of either kgf/mm 2 or pascals.
The relationship between the different engineering hardness scales
is shown in Figure 3.13. An older, geologically oriented scale, the Mohs
scale, based on scratch testing, is included for comparison. Orthopaedic
alloys are among the harder metals, whereas polymers as a group tend to
be fairly soft. Ceramics may be hard or soft depending on their structure
and composition, as shown by the Mohs scale. Relative hardness may be
important when several materials are used in combination. For instance,
softer materials tend to be transferred to harder ones during wear pro-
cesses. This may be controlled to some degree by altering the hardness
of one or the other material.

10,000

Diamond 10

5000

2000 Corundum

or
Sapphire
To paz
Quartz
Orthoclase
Apatite

Alumina
Nitrided steels

9
8
7
6
5

1000

80
60
40

Cutting tools

500

110
100

Orthopaedic alloys
Easily machined tools
To oth enamel

20

200

Fluorite
Calcite

0 4

3

80
6 40

Rockwell
C

100

Brasses and
aluminum alloys

140
120
100

0

50

Rockwell
B

80
60
40
20

Compact bone
Most plastics

Gypsum 2

20

130
120
100

10

Rockwell
M

UHMW
Polyethylene

80
40

5
Brinell
hardness
(kgf/mm 2 )

Talc 1

Rockwell
R

Materials
properties

MOHS
hardness

FIGUre 3.13 hardness of materials.