Relative hardness and absolute hardness are clearly stated in textbooks. The relative hardness of minerals is divided into 10 grades. A mineral that scratches the surface of a crystal when two minerals rub against each other is less hard, meaning that the harder crystal will scratch the less hard crystal. Minerals with relative hardnesses ranging from 1 to 10 are: 1 Talc, 2 Gypsum, 3 Calcite, 4 Fluorite, 5 Apatite, 6 Orthoclase, 7 Quartz, 8 Topaz, 9 Corundum, and 10 Diamond.
This set of relative hardness scales was pioneered by the mineralogist Friedrich Mohs (1773-1839), also known as Mohs, hence the name Mohs or Mohs Hardness. Mohs was born in Germany and moved to Austria in 1801 to engage in mineral identification work. Therefore, some books say that Mohs was an Austrian, while some Internet sources say that he was a German.
The Mohs scale of hardness is a relative hardness, so that the hardness of orthoclase, 6, is not equal to 3 times the hardness of gypsum, 2; similarly, the hardness of diamond, 10, is not equal to 2 times the hardness of apatite, 5. Similarly, the hardness of a diamond, 10, is not twice the hardness of apatite, 5. In other words, relative hardness is only a qualitative classification, not a quantitative scale.
Another hardness scale, called absolute hardness (Vickers indentation hardness), is a quantitative measurement made by striking the probe of a hardness tester against the smooth surface of a mineral crystal while holding the probe on the crystal surface for a few seconds. When the probe is removed, an indentation can be seen on the surface of the crystal. Under the same conditions of impact force and time, the indentation is smaller for harder crystals and larger for less hard crystals. From the size of the indentation, the size of the impact force, and the time of application, the absolute hardness of the tested crystal can be calculated. The unit is Kg/mm2, which is equivalent to the unit of pressure. This absolute hardness is also called indentation hardness because the hardness is obtained by indentation.
hardness(wiki)
In materials science, hardness refers to “the property of a solid material to resist permanent deformation”.
The three main ways of defining hardness include:
- Scratch hardness
- Indentation hardness
- Rebound hardness(dynamic hardness,or absolute hardness）
