The hardness of a material is the quality of being able to withstand localized permanent or plastic deformation, penetration, scratching or bending. If hardness increases, so does the resistance to wear, on the one hand. However, hard materials on the other hand are hard to change in shape. Therefore, hardness is an important property of metals.
What is the definition of hardness?
Hardness is defined as the ability of a material to withstand a plastic deformation, usually caused by indentation or abrasion. Unlike elastic deformations, plastic deformations are permanent. This means that the material does not return to its original state after no force is applied any more. Further, hardness can be described as the ability of a material to withstand deformation by other actions, such as:
Hardness has a direct influence on other properties of the material. For this reason, it is an important factor in the evaluation of suitability and performance. However, due to the correlation between the material properties, hardness should always be observed in conjunction with other characteristics. Therefore, hardness depends on toughness, ductility, plasticity and strength among others.
Types of hardness
Since a material behaves differently under varying loads, hardness is classified into several types. The most important are considered to be scratch, rebound, and indentation hardness. To measure them, specific scales and tools are used. However, there are conversion tables to establish comparability.
Scratch hardness defines the ability to resist scratches on the surface of a material. Typically, the tests are carried out with the help of a stylus which is scratched over the surface under a specific load. Normally, ceramics and other brittle materials are subjected to such a test because they do not exhibit plastic deformation.
Materials are exposed to permanent loads during storage and construction. Indentation hardness describes the ability to withstand the resulting compression or deformation. Since this is the most common form of loading, indentation hardness is often referred to as only hardness.
Rebound or dynamic hardness is more related to elasticity than hardness. Typically, it is measured with the help of a diamond-tipped hammer which is dropped on the surface of a material from a specified height. The bounce of the hammer determines the rebound hardness. There is negative correlation here.
Since the types of hardness are determined with different test methods, several units of measurement exist. Therefore, comparability is difficult. However, it can be achieved through conversion tables where it is possible.
These are the most common used units for hardness measurement:
- Rockwell hardness number (HR)
- Brinell hardness number (HB)
- Vickers hardness number (HV)
To measure the various types of hardness, different test methods are used. Typically, a hardness test is applied by pressing an intender into the surface of a material. Which test should be performed depends on many factors, e.g. the type of hardness you want to measure, the type of material, the size of the workpiece, etc.
Rockwell hardness test
This method is mainly used for metals. It is a fast procedure which is often used in production control. It is the standard test for the measurement of indentation hardness. An important part of the method is the scale which is determined by the used material. During the process, an indenter is pushed with the testing load into the surface of the material. The Rockwell method measures the permanent depth of indentation produced by a force from an indenter.
The formula used for this is:
N,s = scale factors which depend on the scale
d = depth of the permanent penetration in mm
Brinell hardness test
The Brinell method utilizes a ball of steel or tungsten as the indenter. The ball is pressed into the surface of the metal with a predefined force for a specific time. The resulting dent is measured with the help of a microscope. With the help of the following formula, the Brinell hardness number can be calculated:
F = Force in kp
D = indenter diameter in mm
d = indentation diameter in mm
Vickers hardness test
Vickers hardness test is also known as microhardness test. The procedure always applies the same diamond indenter for all materials. Therefore, the calculation is easier compared to the other tests.
The approach is similar to the other methods. A pyramid-shaped diamond indenter is pushed into the workpiece with a predetermined force. With the help of the resulting indentation and the following formula, the Vickers hardness number can be calculated:
F = Force in kp
d = indentation diagonal in mm