Hardness testing is a method by which a small indentation is made into a materials to determine its resistance to deformation. For certain materials, this can be a reflection of other properties while for most, it helps provide insight into the state of a material without unnecessarily destroying it.
Type of Hardness Testing
There are number of methods to test hardness. Below is a summary of the ones offered by BEAR. Hardness is a relative value--while some can be compared, it is not universally straightforward to convert one type of hardness value to another.
Rockwell Hardness (HRB/HRC)
This method requires little to no sample preparation and is used on metallic specimens. An indentor with a sepcified load is pushed into the sample and the final hardness value is dependent on the depth of penetration into the sample. HRB uses a carbide steel ball as an indenter while HRC uses a diamond cone indenter. HRC is typically for hardened steels or heat treated metals, while HRB is more appropriate for softer metallics. For Rockwell measurements, the hardness value can be linearly related to its tensile strength. While the classic Rockwell setup is not portable, BEAR also has a portable Hardness Testing set that can evaluate in-service, or onsite materials.
Measuring microhardness requires a polished sample surface as this methods utilizes measurements of the final indent to determine hardness. The indent is also much smaller than the Rockwell method, and is consequently much better for small or thin samples over the Rockwell method. For both Vickers and Knoop methods, the indenter is a diamond pyramid, but the Vickers test has a square base while Knoop uses one with a more diamond shape. The Knoop test also makes a shallower indent than the Vickers, making it better suited for brittle or thin samples.
Shore A Tester
The Shore scales of hardness are typically used in much softer materials such as polymers. The general principle is the same--it measures the resistance of a material to an indenter. This method is portable, but samples cannot be too thin.