BY Rahul Nair, Fischer Technology, Inc.; co-authors- Matt Taylor, Fischer Technology, Inc., Bernd Binder, Helmut Fischer, GmbH.
Indentation Testing is the technique of using a harder material commonly referred to as an indenter to deform a softer material. The calculated hardness (H) is the applied force (F) divided by the corresponding area of contact (A); H = F/A. One of the first modern forms of this technique was implemented by Johan August Brinell in 1900 . A very heavy load, up to 30,000 N, is applied through a 10mm diameter hard ball onto the test material. The hardness of the material is calculated by measuring the diameter of the residual imprint.
As materials increased in hardness over the years new techniques had to be developed to measure this property. Patented in 1914 the Rockwell Test employs smaller indenters; a diamond cone or a 1/16 inch diameter steel ball . A lower fixed load in the range of 600 N to 1,500 N is applied, the penetration depth measured and the corresponding area of contact calculated.
While the aforementioned techniques are used to measure hardness of metals and ceramics, Durometers were developed to measure the hardness of soft polymeric materials. Developed in the 1920s, ‘Shore’ hardness of material is characterized through this technique using Durometers with different spring constants and a conical or spherical shaped indenter per ASTM D 2240 and ISO 868.
Surface treatments of soft steels like case hardening, carburizing and carbonitriding require the surface mechanical properties to be measured, not the bulk. In order to limit the stress field from an indent to the treated surface, lower loads have to be applied through smaller indenters. The Vickers and Knoop hardness were developed in 1921 and 1939 respectively to meet this need. Indenters used in these techniques are diamond pyramids where the four sides meet at a point. Low loads of up to 5N are applied through these indenters and the area of the residual imprint is optically measured per ISO 6507-1, 2, ISO 4545-1, 2 or ASTM E384.