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Knoop hardness test

Classification, process, differentiation and practical recommendations.

Categorisation of the Knoop method

The Knoop method is a static hardness testing method, which was developed as an alternative to the Vickers method and is used in particular for the testing of thin layers and brittle materials. It can be characterised as follows:

  • It is one of the standardised procedures (ISO 4545, ASTM E92, ASTM E384).
  • The Knoop method has a test load range of 1 gf to 2 kgf according to ISO and ASTM, which means that this method can be used for hardness testing in the micro and low-load ranges.
  • It is an optical method. This means that the size of indentation left by the indenter is measured to determine the hardness value of a test specimen.
  • Indenter shape and material: The indenter consists of a pyramid-shaped diamond with a rhomboid base, with a longitudinal edge angle of 172.5° and a transverse edge angle of 130°.

Knoop test procedure

Knoop test procedure

In the Knoop hardness test, an optical method, the size of indentation left by the indenter is measured.

The larger the indent left by the indenter at a defined test force in the surface of a workpiece (specimen), the softer the tested material.

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Specimen requirements for the Knoop method

Knoop methods and applications

How to read and represent a Knoop hardness value?

Comparison between Knoop and Vickers methods

Advantages and disadvantages of the Knoop method

The Knoop method has the following advantages:

  • The Knoop method can be used with any and all materials and test specimens, from soft to hard, as the procedure covers the entire hardness range.
  • There is only one type of indenter, which can be used for all Knoop methods.
  • Evaluation is more precise than the Vickers method, because the Knoop measurement diagonal (longitudinal diagonal) is longer for any given indentation depth.
  • The test is non-destructive, and there is only very minor damage to the specimen surface (less than that with Vickers, because both the indentation depth and the risk of crack formation at the indent edge in glass and ceramics is lower than with Vickers).
    It is particularly suitable for testing small, longish components and very thin layers as well as brittle materials (glass and ceramics) for which no other method is appropriate.

The Knoop method has the following disadvantages:

  • The surface quality of the specimen must be good, because the indent is measured optically. This means that the test location should be prepared, otherwise precise evaluation is difficult.
  • The process is rather slow (compared with the Rockwell method). The test cycle takes somewhere between 30 and 60 seconds, not including the time taken to prepare the specimen.
  • Due to the need to conduct optical indent evaluation, Knoop hardness testers must be equipped with an optical system, which makes them more expensive to purchase than Rockwell testers.
  • Use of this method is not very common in Europe.
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