Hardness is the property of a material that enables it to resist
plastic deformation, usually by penetration. However, the term hardness
may also refer to resistance to bending, scratching, abrasion or
cutting.
Measurement of Hardness:
Hardness is not an intrinsic material property dictated by precise
definitions in terms of fundamental units of mass, length and time. A
hardness property value is the result of a defined measurement
procedure.
Hardness of materials has probably long been assessed by resistance to
scratching or cutting. An example would be material B scratches
material C, but not material A. Alternatively, material A scratches
material B slightly and scratches material C heavily. Relative hardness
of minerals can be assessed by reference to the Mohs Scale that ranks
the ability of materials to resist scratching by another material.
Similar methods of relative hardness assessment are still commonly used
today. An example is the file test where a file tempered to a desired
hardness is rubbed on the test material surface. If the file slides
without biting or marking the surface, the test material would be
considered harder than the file. If the file bites or marks the
surface, the test material would be considered softer than the file.
The above relative hardness tests are limited in practical use and do
not provide accurate numeric data or scales particularly for modern day
metals and materials. The usual method to achieve a hardness value is
to measure the depth or area of an indentation left by an indenter of a
specific shape, with a specific force applied for a specific time.
There are three principal standard test methods for expressing the
relationship between hardness and the size of the impression, these
being Brinell, Vickers, and Rockwell. For practical and calibration
reasons, each of these methods is divided into a range of scales,
defined by a combination of applied load and indenter geometry.
Hardness conversion between different methods and scales cannot be made
mathematically exact for a wide range of materials. Different loads,
different shape of indeters, homogeneity of specimen, cold working
properties and elastic properties all complicate the problem. All
tables and charts should be considered as giving approximate
equivalents, particularly when converting to a method or scale which is
not physically possible for the particular test material and thus
cannot be verified. An example would be converting HV/10 or HR-15N
value on a thin coating to the HRC equivalent.
