As the name implies, superhard materials are materials with extremely high hardness. Generally speaking, diamond has the highest hardness, with a Mohs hardness of 10, and cBN is slightly lower than diamond, so superhard materials usually refer to diamond and cBN, or composite materials made of these two materials as the main components. The hardness of the four types of hard materials used as "industrial teeth", corundum, SiC, cemented carbide, and high-speed steel, is much lower than the hardness of diamond and cBN, so superhard materials are also called the hardest and sharpest "industrial teeth" or "king of materials". Structure and properties of superhard materials 1. Structure and properties of diamond Like other carbon materials, the main chemical element composition of diamond is carbon. Whether it is natural diamond or artificial diamond, no matter what kind of diamond, it will contain more or less impurities. Diamond generally contains nitrogen impurities. According to the difference in the nitrogen content in the diamond crystal, diamond can be divided into two types (type I diamond and type II diamond).
Classification of diamond
Diamond hybrid orbital electron cloud distribution, atomic structure and unit cell structure
In recent years, carbon materials are a type of material with a very hot research topic. The 21st century is also known as the "Carbon Age". Carbon materials are widely used in various fields due to their outstanding performance, especially in the application of national strategic emerging industries. Graphene and carbon nanomaterials, carbon fibers and their composite materials, diamonds, carbon-based films and traditional carbon materials (carbon black, porous carbon, graphite, special graphite, etc.) have broad application prospects in lithium batteries, capacitors, energy storage, photovoltaics, semiconductors, optoelectronic displays, 5G communications, sensors, general aviation, future transportation, high-end equipment and other fields.
2. Structure and properties of cBN
Cubic boron nitride (cBN) is the second largest variety of superhard materials. The chemical structure of boron nitride is BN, which is composed of two elements: boron and nitrogen. Boron nitride has four different crystal structures, mainly hexagonal boron nitride (hBN), cubic boron nitride (cBN), rhombohedral boron nitride (rBN) and dense hexagonal boron nitride (wBN). In hBN and rBN, nitrogen atoms and boron atoms are hybridized in SP2 mode, while in cBN and wBN, nitrogen atoms and boron atoms are hybridized in SP3 mode.
Several different brands of cBN single crystals of a certain company (from the company's website) The hardness of cBN is slightly lower than that of diamond, and the color is diverse. The color of the crystal is related to the type and amount of impurities contained. cBN has unique optoelectronic properties. Synthesizing large-sized, high-quality cBN single crystals is an inevitable choice for obtaining functional device applications. However, the large-sized cBN crystals are far less successful than diamonds. This may be because the conditions for synthesizing large-grained cBN are more stringent, and its application has not found a suitable field. Application of superhard materials 1. Main types of superhard material products Superhard materials and their products and tools have been widely used in industry. They not only solve the problem that traditional tools cannot process or are difficult to process, but also significantly improve the efficiency of traditional processing and significantly reduce consumption and waste emissions. Several superhard material products and tools (a. Blades; b. Grinding wheels; c. Saw blades; d. Drill bits) The main types of superhard material products and tools include sawing tools, abrasives (including bonded abrasives, coated abrasives and loose abrasives), cutting tools, drilling tools, dressing tools, wire drawing dies, other tools and different functional components. 2. Natural diamonds and artificial diamonds (1) Natural diamonds Diamonds obtained from natural diamonds are called natural diamonds. Natural diamonds are bright, gorgeous, rare and precious, and are regarded as treasured by people. There is an advertising slogan "Diamonds are forever, and one diamond will last forever" that is known to everyone in my country. In addition to being used as jewelry, natural diamonds are more often used in industry. The industrial use of diamonds has long been based on their extremely high hardness. Diamond knives for cutting glass remind us of the use of diamonds to make drill bits for geological exploration and oil and coal mining. Diamonds undergo varying degrees of oxidation reactions with oxygen at high temperatures, especially with iron, and are not suitable for ferrous metal processing. (2) Artificial diamonds Artificial diamonds are often referred to as laboratory-grown diamonds or cultured diamonds in the industry. Large particles or large single crystals of artificial diamonds have been used to process synthetic diamond jewelry. Artificial diamonds are also real diamonds, with the same composition and structure as natural diamonds. The two can only be distinguished by very special methods, such as under a cathode luminometer, the growth texture of the former is geometric, while the latter is ring-shaped. Since 2006, jewelry institutions such as GIA have begun to provide identification services and issue certificates, mainly to prevent merchants from selling lower-priced synthetic diamonds as natural diamonds. In the past, researchers in the diamond industry have been looking for ways to efficiently synthesize 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 single crystal diamond by ultra-high pressure and high temperature method under static catalyst conditions has matured. my country has made rapid progress in the development and application of large single crystal artificial diamond synthesized by ultra-high pressure and high temperature method, as well as the technology of synthesizing artificial diamond by microwave plasma CVD method. Synthetic diamond jewelry has been sold on the market and has begun to take shape. 3. Main applications of cBN Compared with diamond, cBN has unique advantages, such as high thermal stability and chemical inertness with iron family elements. There are currently two main uses of cBN: one is to manufacture abrasive tools, and the other is to make polycrystalline cubic boron nitride as a tool material. Therefore, cBN has unique features in processing ferrous metal materials and provides new processing tools for hard and tough iron-based materials that are difficult to process. The main applications of cBN are as follows: (1) Used as abrasive tool material. It can be used for processing both iron-based materials and non-ferrous metal materials. (2) Used as a tool material. The material used for cutting tools is generally polycrystalline cubic boron nitride. PcBN is a micro powder prepared from cBN single crystal. It is made by adding titanium carbide, cobalt and other adhesives, and then sintering it under high pressure and high temperature using a six-sided top press. It is particularly effective in processing iron metals and their alloys, and is particularly suitable for high-speed cutting and dry cutting. It can also replace grinding with turning and milling, greatly improving production efficiency.
(3) Used as functional materials. High thermal conductivity cBN can be used in optoelectronic functional devices.Superhard materials are a small variety among many different materials, but they are an irreplaceable material and may gradually replace some other materials.
