Hardness testing: Metals and plastics

The functional principle of hardness testing

The hardness testing is based on the resistance of a material to an indenter. We use various indenters, including spheres (Brinell), pyramids (Vickers) or cones (Rockwell) for testing hardness on metals, depending on the material and normative specifications. The hardest material, diamond, serves as the indenter for Vickers and Rockwell.

The Shore A, Micro Shore A, and IRHD M methods with their special indenters are used for plastic hardness testing in our laboratory. Apart from conditioning in the test climate, no further sample preparation is usually required.

For metals, it is normally necessary to prepare the test surfaces of the sample before testing. If testing is to take place in the core area, then the sample is cut. The separated part is usually embedded in plastic (i.e. caked with liquid plastic and solidified into a standard-sized "section"). This facilitates the subsequent grinding and polishing process, which is necessary during most tests for valid execution. Grinding and polishing are carried out fully automatically in our laboratory. This not only ensures high productivity, but also consistent quality.

The surface hardness test eliminates the separation process. Grinding and polishing of the sample surface are carried out manually. To avoid falsifying the test results, it is essential to act with great care to remove and polish only the coating, not the underlying surface area of the sample.

For the test, the indenters are applied on the prepared test sample with a defined force and time. In the Vickers method, for example, the diagonals of the resulting permanent indentation are then measured. This is done automatically on our test devices using a high-resolution camera. The hardness value of the respective scale is then calculated using a formula. With the Rockwell method, the depth of the indentation is recorded during testing instead of the diagonal measurement, and the hardness value is determined from this. The process, therefore, does not require a camera.

It is also possible to determine the surface hardening depth by means of hardness testing. In this case, a series of hardness indentations is made in a cross-section or longitudinal section of the sample, starting at the edge and running towards the inside of the component. A hardening depth can be determined based on the decrease in hardness and a defined hardness limit.