In common micro-indention measurements (ASTM E368), a diamond probe of specific shape (Knoop or Vickers) is pressed into the sample specimen using a controlled force (9.8 x 10-3to 9.8N.) Once the pre-set test force is applied, the probe is typically held for 10 to 15 seconds and retracted. The dimensions of the resulting indent are then measured using an optical microscope. The Vickers or Knoop hardness of the material is calculated from the applied force and the dimensions of the indent that persists in the material after the probe is retracted. The Vickers hardness number (VH) is calculated from the force applied divided by the surface area of the permanent impression made by the indenter. The Knoop harness number is obtained by diving the applied force by the projected area of the permanent impression made by the indenter. 1
This powerful technique is an important characterization tool used in many industries to quantify surface hardness variations due to a wide range of processes such as flame hardening, shot hardening, carburization and nitriding. However, many types of materials are not amendable to analysis using ASTM E384. For example: