The first widely accepted indentation type hardness test was proposed and developed by J.A. Brinell in 1900. The Brinell hardness test is similar to the Rockwell test where a simple indentation is created in the work piece by applying a force between 500 and 3000 kg using a standardized five to ten mm diameter hardened steel or tungsten carbide ball. An adequate dwell time, normally 30 seconds, is required to ensure that material plastic flow has ceased before the load is released.
After unloading the material, the average indentation diameter is measured using a low power microscope. The measured hardness is calculated from the applied load, indenter diameter, and indentation diameter. Accurate Brinell measurements require a smooth and clean test surface.
Brinell tests are commonly used on softer materials such as aluminum and copper alloys and can be correlated to Rockwell and Vickers test values readily. Also, the larger indentations associated with the Brinell scale provide a more representative measure of the test material since more material is used per indentation as compared to the Rockwell scale for example.
Micro-Hardness tests are used when extremely precise, pinpoint hardness measurements are needed. Micro-hardness tests are based on the penetration depth of a diamond pyramid indenter under a light 10 to 3000 gram load. The procedure involves deforming the test piece under a light microscope and measuring both diagonal lengths. Either a Knoop or Vickers indenter can be used. The Vickers indenter has a square, diamond pyramid shape; the Knoop indenter has an elongated diamond pyramid profile. In both cases, material hardness can be measured in very localized regions of a test sample. For example, individual microscopic crystals that comprise a metal can be individually selected and tested using this technique. Paints, adhesives, and plastics are also viable candidate materials for such a test.
Of course a precise measurement requires a precise test sample. Test materials destined for micro-hardness testing must be flat, parallel, and metallographically polished which unfortunately adds time and expense. Vickers and Knoop testing are the most expensive of the hardness scales to complete.
The following and last article of this series will provide several case studies to illustrate how material hardness testing contributes to engineering investigations.