What is the hardness of a solid sample?
The hardness of a solid sample is what characterizes its resistance to mechanical penetration.
How is hardness measured?
To quantify the hardness of the samples, penetration tests are carried out in the laboratory. These are tests close to compression tests. With the difference that the compressive force is not applied to the whole sample, but very locally.
These tests consist of placing a solid of specific shape (point, ball, etc.) on a flat and horizontal face of the sample and applying a vertical force for a certain time. Then, several parameters can be used to quantify the hardness: dimension of the imprint formed when the point is withdrawn, height of penetration, etc. Some tests require several tests to be performed with different forces.
What standards are used?
There are many standards that define the conditions for measuring the hardness of solid samples. These standards define as many hardness scales that allow the samples to be compared with each other.
There are a number of general hardness scales and standards (Vickers, Brinnel, etc). Others, on the other hand, are specifically used on certain types of samples (Shore for polymers, Barcol for composites, etc.). Each standard defines the measurement conditions, the geometry and the material of the tips used (sphere, pyramid, etc.), etc.