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.
In the Knoop hardness test, the size of the indentation that the test specimen leaves in the material is analysed in order to determine the hardness value.
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°.

What is the Knoop test procedure?

As part of the Knoop hardness test, the indentation surface left behind by the indenter is precisely measured in order to calculate the hardness value.

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.

Specimen requirements for the Knoop method

What Knoop methods and applications are there and what are they used for?

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 hardness test is suitable for all materials - from very soft to extremely hard materials - and therefore covers a broad hardness spectrum.
A single indenter is used for all Knoop test methods, which simplifies handling.
Due to the longer longitudinal diagonal of the Knoop indentation with the same test force, the evaluation is particularly precise - in some cases even more precise than with the Vickers method.
The test causes only minimal damage to the sample surface and is therefore considered virtually non-destructive - ideal for sensitive materials such as glass or ceramics.
Thanks to its shape, the method is ideal for very thin layers, small or elongated samples and brittle materials where other methods reach their limits.

The Knoop method has the following disadvantages:

The surface of the sample must be carefully prepared - without grinding or polishing, an exact evaluation of the optical impression is hardly possible.
The entire testing process takes considerably longer than with the Rockwell method, for example - including sample preparation, 30 to 60 seconds per measurement is realistic.
As high-quality optics are required to analyse the indentation, Knoop hardness testers are generally more expensive than comparable Rockwell systems.
In practice, the method has rarely been used in Europe to date - it has mainly established itself in specialised applications.