Cubic boron nitride (CBN) was synthesized manually by General Electric (GE) in the 1950s under high temperature and pressure. Its hardness is only second to that of diamond and much higher than that of other materials. Therefore, CBN and diamond are collectively called superhard materials. Cubic boron nitride is synthesized by hexagonal boron nitride and catalyst under high temperature and pressure. It is another new high-tech product after the advent of synthetic diamond. It has high hardness, thermal stability and chemical inertia, as well as good infrared transmission and wide band gap. Its hardness is second only to diamond, but its thermal stability is much higher than diamond, and it has greater chemical stability to iron-based metal elements. Cubic boron nitride grinding tool has excellent grinding performance. It can not only be competent for grinding difficult materials, improve productivity, but also effectively improve the grinding quality of workpiece. The use of cubic boron nitride is a great contribution to metal processing, leading to revolutionary changes in grinding, which is the second leap in grinding technology.
There are two kinds of cubic boron nitride: single crystal and polycrystalline sinter. The single crystal is made of hexagonal boron nitride and catalyst in the range of pressure 3000-8000MPa and temperature 800-1900 ~C. Typical catalysts are selected from alkali metals, alkaline earth metals, tin, lead, antimony and their nitrides. The crystal forms of cubic boron nitride include tetrahedral truncated cone, octahedron, distorted crystal and twin crystal. Industrial cubic boron nitride is black, amber and metal-plated, and its particle size is usually less than 1 mm. It is superior to diamond in thermal stability and chemical inertia to iron group metals. Abrasives made of it are suitable for processing hard and tough materials, such as high speed steel, tool steel, die steel, bearing steel, nickel and cobalt base alloy, chilled cast iron, etc. When cubic boron nitride grinding tool is used to grind steel, high grinding ratio and surface quality can be obtained.
1. Manufacturing abrasives
2. Making polycrystalline composite sheet as tool material
CBN characteristics of cubic boron nitride:
The hardness of CBN is second only to that of diamond, but the thermal stability of CBN is much higher than that of diamond. CBN is chemically inert to iron-based metal elements. Therefore, CBN processing ferrous metal materials have unique features, providing new processing tools for hard and tough hard-to-process steel. Material suitable for diamond processing and material suitable for CBN processing can complement each other. In recent years, CBN has developed rapidly, and the annual increment rate of CBN production reaches 15%-20%, which is more than double that of diamond. At present, CBN, together with diamond, corundum and silicon carbide, is known as the four major types of abrasives in the world. China has developed CBN500 high-grade products equivalent to GE company. At the same time, varieties of surface plating metal have been developed. CBN powder suitable for grinding and polishing ferrous and non-ferrous metals has been selected, and polycrystalline composite turning tool has been developed. CBN is currently mainly used in processing tool steel (including high-speed steel, die steel), bearing steel, stainless steel, nickel-based alloy and chilled cast iron. In these processing areas, CBN replaces corundum and achieves good technical and economic results. Especially when processing cemented carbide steel above HRC50, because CBN is sharper than corundum and has more stable performance than diamond, it shows a series of advantages.
The advantages of CBN are as follows:
1. CBN grinding wheel has high durability. Grinding ratio can reach 3500-5500, 100 times higher than corundum wheel and 3-5 times higher than diamond wheel.
2. CBN grinding wheel has less grinding force, less grinding heat, less stress, thin stress layer, high surface roughness and accuracy than corundum grinding wheel. It does not produce burns, cracks and softening after annealing. The durability of the processed workpiece can be increased by 30%~50%.
3. The abrasive tool has good shape retention and less times of dressing and replacement, which shortens the total processing time, and is conducive to the realization of automatic on-line processing, ensuring the accuracy of the workpiece being processed.
4. CBN polycrystalline composite cutting tool has “red hardness” and can be used in high-speed dry cutting. The high temperature produced by high speed cutting softens the machinable material and improves its machinability. When processing nickel-based alloy, the life of CBN turning tool is hundreds times longer than that of cemented carbide turning tool.