Superhard materials are those with hardness values of 10 or close to 10 on the Mohs scale, primarily including diamond and cubic boron nitride (cBN). Essentially, diamond is an allotrope of carbon,cubic boron nitride is a compound of boron and nitrogen. The differences in material properties also lead to distinctions in their applications.
Diamond is an allotrope of carbon and a classic atomic crystal. In its crystal structure, each carbon atom is connected to four other carbon atoms through covalent bonds in the form of sp3 hybrid orbitals, forming a tetrahedral structure. These tetrahedral structures extend into a very robust network in three dimensions. Thanks to this unique structure, diamond is the hardest substance found in nature.
Cubic boron nitride is synthesized from boron and nitrogen under high temperature and pressure, and it is the second hardest material in the world. Its structure is very similar to diamond, being a cubic crystal system based on a cubic lattice. In cBN's crystal structure, boron and nitrogen atoms alternate and are bonded via BN covalent bonds to form tetrahedral structures. However, compared to diamond, cBN has a slightly longer bond length of 1.57Å, which results in a slightly lower hardness.
The thermal stability of the abrasive grains determines the speed at which the grinding wheel can operate. Higher speeds lead to higher grinding temperatures. If the abrasive's thermal stability is poor, the grinding wheel will wear out quickly.
Cubic boron nitride, while having a similar structure to diamond, is a typical thermally degradable abrasive. The carbon bonds on its surface are unsaturated, and under high-temperature conditions (above 720°C), these unsaturated surface carbon atoms easily combine with oxygen atoms to form carbon oxides that escape from the crystal. This results in gradual crystal peeling and disintegration, leading to diminished cutting force. Its operational temperature is typically controlled below 800°C, making it more suitable for medium-speed grinding.
Conversely,cubic boron nitride crystals have surfaces covered by boron and nitrogen atoms. The electronic structure of boron's valence shell is 1S22S22P1, providing three bonding electrons and resulting in a surface where boron's valence bonds are saturated with no dangling bonds. Consequently, cBN maintains relative thermal stability at higher temperatures, capable of withstanding cutting temperatures above 1200°C. This allows cBN to exhibit self-sharpening characteristics in high-speed and ultra-high-speed grinding and polishing, maintaining sharpness.
Diamond has a strong affinity with transition metals such as iron, cobalt, nickel, chromium, and vanadium. At high temperatures, iron atoms easily interact with the carbon atoms in diamond, converting it into graphite, leading to reduced abrasive strength and cutting force. This requires frequent dressing, reducing processing efficiency and resulting in sticking debris and poor machining quality. Therefore, diamond cannot be used for grinding iron-based alloy materials.
In contrast,cubic boron nitride has good chemical stability with iron, steel, and in oxidative environments. It does not react with iron-based metals below 1150°C. In oxidative environments, cBN can also form a boron oxide layer, further enhancing its chemical stability. It is particularly suitable for processing various hardened steels and cold-hardened steel, which are difficult to machine with iron-based materials.
As superhard materials, diamond andcubic boron nitride have irreplaceable roles in grinding and polishing applications. Due to their complementary properties in material processing, they significantly expand the application range of abrasive tools for nearly all materials, including those with high hardness, high brittleness, and high toughness. Diamond, with its extreme hardness, is particularly advantageous for processing hard and brittle materials such as carbide, ceramics, and glass. Meanwhile,cubic boron nitride, with its excellent thermal and chemical stability, compensates for diamond's disadvantages in high-speed grinding and processing iron-based materials.
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