Breviary Technical Ceramics






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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