Around 1900, Martens proposed the following definition for hardness testing: “Hardness is the resistance of a body to the indentation by another (harder) body.” This simple but precise definition has taken its place in technical circles, and is just as valid today as it was then. Technical hardness is a mechanical characteristic used to describe a material or the state of a material.
Hardness cannot be measured directly, but is derived from primary measured variables (for example test load, indentation depth, indentation area). Depending on the test method, the hardness value is determined from one of the following:
- Test load and one of the geometric values characterizing the hardness indentation (e.g. indentation depth)
- Solely via a length characterizing the indentation
- Through a different material response (e.g. scratch resistance)
A clear hardness value can only be characterized by the following criteria: definition of the hardness value, geometry and material of the indenter, size of test load and duration and type of test load application, and the condition and surface quality of the specimen.
Primary considerations when selecting a hardness test method include:
- Cost/effort involved in specimen preparation
- Availability of machines and instruments
- Requirements of standards
- Specimen material and hardness
- Shape, dimensions and weight
- Accessibility of specimen
- Nature of test (e.g. series or traverse test)
- Permissible measurement uncertainty
For metals, methods involving static force application are usually employed. An indenter (ball, cone or pyramid) made of carbide or diamond is pressed vertically into the surface of a specimen supported on a firm base. The test load is applied smoothly and without impact, with defined application and dwell times. In many test methods the indentation is measured following load removal. The length measurement values (indentation depth, diagonals, diameter) and test load are used to calculate the hardness value.
In practice, the Rockwell, Brinel and Vickers and Knoop hardness testing methods are used, with the modern instrumented indentation method (Martens hardness) gaining steadily in significance. This method is seeing increasing use, particularly in research and development and industry, as it provides additional parameters for determining material properties as well as the hardness value. Moreover, this method can be employed with any material.
Methods featuring dynamic force application often require mobile devices, particularly when large components are being tested.