Cubic Boron Nitride (CBN)
Cubic Boron Nitride (CBN) was first synthesized by General Electric (GE) under high temperature and high pressure in the 1950s. Its hardness is second only to diamond and far higher than other materials. Therefore, it and diamond are collectively called superhard material. Superhard material are widely used in sawing tools, grinding tools, drilling tools and cutting tools. Diamond is prone to oxidation at high temperatures, especially due to its good affinity with iron based elements, making it unsuitable for processing ferrous metals.
Its crystal structure is similar to diamond, with a slightly lower hardness than diamond, and is commonly used as an abrasive and tool material. In 1957, R.H. Wintov of the United States first developed cubic boron nitride. But so far, no natural cubic boron nitride has been discovered.
Cubic boron nitride is synthesized by hexagonal boron nitride and a catalyst under high temperature and pressure. It is another new high-tech product that has emerged after the emergence of artificial diamonds. It has high hardness, thermal stability and chemical inertness, as well as good red appearance and wide band gap and other excellent properties. Its hardness is second only to diamond, but its thermal stability is far higher than diamond, and it has greater chemical stability to iron series metal elements. The grinding performance of cubic boron nitride grinding tools is very excellent, not only capable of processing difficult to grind materials, improving productivity, but also effectively improving the grinding quality of workpieces. The use of cubic boron nitride is a significant contribution to metal processing, leading to revolutionary changes in grinding and marking the second leap in grinding technology.
There are two types of cubic boron nitride: single crystal and polycrystalline sintered bodies. Single crystal is prepared by using hexagonal boron nitride and catalyst at a pressure of 3000-8000MPa and a temperature range of 800-1900 ℃. Typical catalyst materials are selected from alkali metals, alkali earth metals, tin, lead, antimony, and their nitrides. The crystal forms of cubic boron nitride include tetrahedral truncated cones, octahedra, distorted crystals, and twin crystals. The cubic boron nitride produced in industrial production is available in black, amber, and metal plated on the surface, with particle sizes typically below 1 millimeter. It has superior thermal stability to diamond and chemical inertness to ferrous metals. The grinding tools used for manufacturing are suitable for processing materials that are both hard and tough, such as high-speed steel, tool steel, mold steel, bearing steel, nickel and cobalt based alloys, and chilled cast iron. When using cubic boron nitride grinding tools to grind steel, high grinding ratios and machined surface quality can mostly be achieved.
Cubic boron nitride has high hardness and thermal stability, and its microhardness is second only to synthetic diamond; Its thermal stability is superior to that of synthetic diamond, and it can still maintain sufficiently high mechanical properties and hardness at high temperatures, with good red hardness; With stable structure, high oxidation resistance and good chemical stability, it is especially good compared with diamond and does not react with iron group elements at temperatures up to 1100~1300 ℃, so it is particularly suitable for processing ferrous materials; The thermal conductivity is smaller than that of diamond, but higher than that of hard alloy, with good thermal conductivity; High bending strength; As a grinding tool material, it has a long service life and good wear resistance. However, single crystal cubic boron nitride has a small grain size, anisotropy, and a cleavage surface that is prone to splitting. It is highly brittle and prone to cleavage damage.
