TEST TYPES

Knoop Hardness Testing

Hardness is a characteristic of a material, not a fundamental physical property. It is defined as the resistance to indentation, and it is determined by measuring the permanent depth of the indentation.

More simply put, when using a fixed force (load) and a given indenter, the smaller the indentation, the harder the material. Indentation hardness value is obtained by measuring the depth or the area of the indentation using one of over 12 different test methods. Learn more about hardness testing basics here.

The Knoop hardness test method, also referred to as a microhardness test method, is mostly used for small parts, thin sections, or case depth work.

The Vickers method is based on an optical measurement system. The Microhardness test procedure, ASTM E-384, specifies a range of light loads using a diamond indenter to make an indentation which is measured and converted to a hardness value. It is very useful for testing on a wide type of materials as long as test samples are carefully prepared.

A pyramid shaped diamond is used for testing in the Knoop scale. This indenter differs from the pyramid indenter used on a Vickers test. The Knoop indenter is more elongated or rectangular in shape. The Knoop method is commonly used when indentations are closely spaced or very near the edge of the sample.

The width of the Knoop indentation can provide more resolution for measurement and the indentation is also less deep. Consequently, it can be used on very thin materials.

In a Knoop test, a predetermined test force is applied with a pyramid-shaped diamond indenter for a specified dwell time period. The indenter used on a Knoop test is pyramid-shaped but more elongated than the indenter used on a Vickers test. After this dwell period, the force is removed.

Unlike the Vickers test where the indentation length on the vertical and horizontal axes are measured and averaged, the Knoop method only uses the long axis. This measurement is then converted to a Knoop hardness number using a chart.

Since the test indentation is very small in a Knoop test, it is useful for a variety of applications: testing very thin materials like foils or measuring the surface of a part, small parts or small areas, measuring individual microstructures, or measuring the depth of case hardening by sectioning a part and making a series of indentations to describe a profile of the change in hardness.

Sectioning is usually necessary with a microhardness test in order to provide a small enough specimen that can fit into the tester. Additionally, the sample preparation will need to make the specimen’s surface smooth to permit a regular indentation shape and good measurement, and to ensure the sample can be held perpendicular to the indenter.

Often the prepared samples are mounted in a plastic medium to facilitate the preparation and testing. The indentations should be as large as possible to maximize the measurement resolution. (Error is magnified as indentation sizes decrease) The test procedure is subject to problems of operator influence on the test results.

For more information, see our guide Selecting a Newage Microhardness Tester or contact us.

Image:
The long side faces are set at 172 degree, 30 minute angle to one another. Short side faces are set at a 130 degree angle one another.

TEST TYPES

Knoop Hardness Testing

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