10.5.2 Hardness
Only the Vickers and Knoop procedures are
suitable for measuring the hardness of ceramics. The other
hardness measurement procedures are either unsuitable for
hard and brittle materials because, for instance, they use
a very hard indenter and a small test force, or their testing
technology has not yet been sufficiently developed. Assessing
the impression made by the indenter in the usual way that
this is done for metallic materials to determine the Vickers
or Knoop hardness reaches the limits of its applicability
with ceramic materials; a range of problems related to the
procedure and to the materials arise.
For this reason, procedures with modified methods such as
the
- UCI procedure (Ultrasonic Contact Impedance) or the
- universal hardness measurement technique are used.
The UCI procedure is based in the change
of the resonant frequency of a test probe, with the Vickers
indenter mounted on its tip, during the penetration process.
The changing frequency is a measure of the surface area of
the impression, and therefore an indicator of the hardness
of the material.
The change in the resonant frequency of the oscillating bar
is, however, also a function of the modulus of elasticity
of the indenter and of the sample, and these must be determined
through other procedures. This gives rise to material-specific
calibration functions.
Conversely, the modulus of elasticity of the material can
be found using the UCI procedure if its hardness is known.
The universal hardness measurement is based
on a determination of the functional relationship between
the indenter force and the penetration depth. In this way
a hardness that depends on the test force is determined, taking
the elastic and plastic deformation into account, and a hardness
figure that is independent of the test force, satisfying the
conditions for crack-free impressions.
It is important that the same method of measurement is used
for materials that are to be compared.
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