Hardness refers to the ability of a material to resist scratching or pressing into its surface by other substances. In practical applications, due to different measurement methods, the measured hardness represents different material properties. For example, the scratch hardness used by crystal materials reflects the ability of the material to resist fracture damage, while the static indentation hardness used by metal materials represents the ability of the material to resist plastic deformation. The static indentation hardness is to press a hard object into the surface of the material to be tested under static pressure, and the hardness of the object to be tested is expressed by the load on the unit area of the surface pressed into the concave surface. Therefore, there is no uniform definition of hardness, various hardness units are different, and there is no fixed conversion relationship between them. According to the different test methods, the commonly used ones are Shore hardness, Brinell hardness, Barcell hardness, Rockwell hardness, Vickers hardness, microhardness, Mohs hardness, etc. Diamond is the hardest material in the world. Scientists divide the Mohs hardness of materials into 10 grades, and the first grade is called 1 degree. The hardness of diamond is 10 degrees, which is called the king of hardness!
There are two types of hardness scales, one is absolute hardness, and the other is relative hardness. For example, Mohs hardness is a kind of relative hardness. It does not indicate the degree of hardness, but only the order of hardness from small to large. The material in the order of the latter can scratch the surface of the front material. The commonly used hardness in mineralogy refers to Mohs hardness. Mohs hardness is a standard of mineral hardness. This method was named by Mohs' Friedrich, a German professor of mineralogy, in 1824. The method to determine this standard is to use a pyramid diamond drill needle to scratch the surface of the mineral to produce a scratch, and use the measured depth of the scratch to express the hardness. Generally, ten common minerals are used as the standard to distinguish the hard from the soft by scraping each other. Traditionally, the Mohs hardness is used in mineralogy or gemology.
Diamond has a stable tetrahedral structure, and each carbon atom forms a covalent bond with the atoms arranged on each corner of its neighboring tetrahedron, and the bond length between carbon atoms is short. This short covalent bond has strong deformation resistance, so diamond has great hardness. Since it was first discovered by humans more than 2700 years ago, diamond has been considered as the hardest material in nature.
Standard minerals and measured minerals with unknown hardness are used to describe each other. If the mineral can be scratched by apatite but not fluorite, the hardness of the mineral is 4-5. When there is no standard hardness mineral, it can also be measured with daily supplies. For example, the nail hardness is 2.5, the coin hardness is 3.5, the blade hardness is 5.5, and the glass hardness is 6. More than 6 should almost all belong to minerals such as gemstones. As a gem, it usually has high hardness. For example, the hardness of opal is 5.5~6.5, crystal is 6.5~7, and zinc spinel is 7.5~8. The hardness of emerald is 8.5, and that of sapphire and ruby is 9, second only to diamond.
Accurate determination of mineral hardness also requires microhardness tester or hardness tester. For example, the microhardness tester is actually a microscope with a loading device. Diamond is the hardest material on the earth at present. The following table shows the hardness comparison between diamond and some other hard materials. The hardness of diamond is about 8.5 times that of quartz, 4.4 times that of corundum, 3.7 times that of tungsten carbide, 3.1 times that of boron carbide and 1.56 times that of cubic boron nitride.