As the name suggests, superhard materials are materials with extremely high hardness. Generally speaking, the hardness of diamond is the highest, the Mohs hardness is 10, and the hardness of CBN is slightly lower than that of diamond. Therefore, superhard materials usually refer to diamond and CBN, or composites made of these two materials as the main components.
The hardness of corundum, SiC, cemented carbide and high-speed steel, the four kinds of hard materials used as “industrial teeth”, is far lower than that of diamond and CBN. Therefore, superhard materials are also known as the hardest and sharpest “industrial teeth” or “king of materials”.
Like other carbon materials, the main chemical element composition of diamond is carbon. No matter it is natural diamond or man-made diamond, but no matter what kind of diamond will contain more or less impurities. Diamond generally contains nitrogen impurities. According to the difference of nitrogen content in diamond crystal, diamond can be divided into two types (type I diamond and type II diamond).
In recent years, carbon materials are a kind of very hot materials. The 21st century is also known as the “carbon age”. Carbon materials are widely used in various fields because of their excellent properties, especially in national strategic emerging industries, such as graphene and carbon nano materials, carbon fiber and its composites, diamond Carbon based films and traditional carbon materials (carbon black, porous carbon, graphite, special graphite, etc.) have broad application prospects in lithium battery, capacitor, energy storage, photovoltaic, semiconductor, photoelectric display, 5g communication, sensor, general aviation, future transportation, high-end equipment and other fields
Cubic boron nitride (CBN) is the second largest variety of superhard materials. The chemical structure formula of boron nitride is BN, which is composed of boron and nitrogen. Boron nitride has four different crystal structures, mainly hexagonal boron nitride (hBN), cubic boron nitride (CBN), rhombic boron nitride (RBN) and dense hexagonal boron nitride (WBN). The nitrogen and boron atoms in hBN and RBN are hybridized in SP2, while the nitrogen and boron atoms in CBN and WBN are hybridized in SP3.
The hardness of CBN is slightly lower than that of diamond, and the crystal color is related to the type and quantity of impurities. CBN has unique photoelectric characteristics. The synthesis of large-size and high-quality CBN single crystal is an inevitable choice for the application of functional devices. However, the large-size of CBN crystal is far less successful than that of diamond. This may be because the conditions for the synthesis of large particle CBN are more stringent and its application has not found a suitable field.
Application of superhard materials
1. Superhard material products
Superhard materials and their products and tools have been widely used in industry. They not only solve the problems that cannot be processed or difficult to process with traditional tools, but also significantly improve the traditional processing efficiency and significantly reduce consumption and waste discharge.
Several superhard material products and tools (A. blade; B. grinding wheel; C. saw blade; D. drill bit)
The main varieties of superhard material products and tools include sawing tools, abrasives (including consolidated abrasives, coated abrasives and loose Abrasives), cutting tools, drilling tools, finishing tools, wire drawing dies, other tools and different functional parts.
2. Natural and man-made diamonds
(1) Natural diamond
Diamonds extracted from natural diamonds are called natural diamonds. Natural diamonds are bright, gorgeous, rare and precious. They are regarded as treasures. There is an advertisement that says “a diamond will last forever, an eternal”, which is well known to Chinese women and children.
In addition to being used as jewelry, natural diamond is more used in industry. The industrial use of diamond has long been to use its special high hardness. The diamond knife for cutting glass reminds us that diamond is used to prepare drill bits for geological exploration and oil and coal mining. Diamond will oxidize with oxygen in varying degrees at high temperature, especially with good affinity with iron, so it is not suitable for ferrous metal processing.
(2) Artificial diamond
Artificial diamond, also known as laboratory synthetic diamond or cultivated diamond in the industry. Artificial diamonds with large particles or single crystals have been used to process synthetic diamond jewelry. Artificial diamonds are also genuine diamonds, with the same composition and structure as natural diamonds. The two can only be distinguished in a very special way. For example, under the cathodoluminescence instrument, the growth texture of the former is geometric, and the latter is annular. Since 2006, GIA and other jewelry institutions have started to provide identification services and issue certificates, mainly to prevent merchants from selling synthetic diamonds with lower prices as natural diamonds.In the past, researchers in the diamond industry have been looking for efficient synthetic methods of artificial diamonds, but there has been no breakthrough. The two main obstacles come from cost and production factors. After decades of research, the technology of synthesizing large diamond single crystals by ultra-high pressure and high temperature method under the condition of static catalyst has been mature. China has made rapid development in the technical development and application of synthetic diamond large single crystal synthesized by ultra-high pressure and high temperature method and synthetic diamond synthesized by microwave plasma CVD method. Synthetic diamond jewelry has been sold in the market and has begun to take shape.
3. Main applications of CBN
CBN has unique advantages compared with diamond, such as high thermal stability and chemical inertness with iron group elements.
At present, CBN is mainly used in two aspects: one is to manufacture abrasive tools, and the other is to make polycrystalline cubic boron nitride as tool material. Therefore, CBN is unique in processing ferrous materials, which provides a new processing tool for hard and tough difficult to process iron-based materials.
Used as abrasive material. It can be used not only for the processing of iron-based materials, but also for the processing of non-ferrous metal materials.
Used as tool material. Polycrystalline cubic boron nitride is generally used for cutting tools. PCBN is a micro powder prepared from CBN single crystal. It is prepared by adding titanium carbide, cobalt and other adhesives and then sintering at high pressure and high temperature with a six sided top press. It is especially effective for the processing of ferrous metals and their alloys, especially suitable for high-speed cutting and dry cutting. It realizes turning instead of grinding and milling instead of grinding, and greatly improves the production efficiency.
Used as functional material. High thermal conductivity CBN can be applied to photoelectric functional devices.
Superhard material is a small variety of many different materials, but it is an irreplaceable material and may gradually replace some other materials.