Cubic boron nitride (CBN), the second superhard material synthesized using a method similar to diamond manufacturing, is second only to diamond in terms of hardness and thermal conductivity. It has excellent thermal stability and can be heated to 10,000C in the atmosphere. No oxidation occurs. CBN has extremely stable chemical properties for ferrous metals and can be widely used in the processing of steel products.

⑴ Types of cubic boron nitride cutting tools

Cubic boron nitride (CBN) is a substance that does not exist in nature. It is divided into single crystal and polycrystalline, namely CBN single crystal and polycrystalline cubic boron nitride (Polycrystalline cubic bornnitride, PCBN for short). CBN is one of the allotropes of boron nitride (BN) and has a structure similar to diamond.

PCBN (polycrystalline cubic boron nitride) is a polycrystalline material in which fine CBN materials are sintered together through binding phases (TiC, TiN, Al, Ti, etc.) under high temperature and pressure. It is currently the second-hardest artificially synthesized material. Diamond tool material, together with diamond, is collectively called superhard tool material. PCBN is mainly used to make knives or other tools.

PCBN cutting tools can be divided into solid PCBN blades and PCBN composite blades compositely sintered with carbide.

PCBN composite blades are made by sintering a layer of PCBN with a thickness of 0.5 to 1.0mm on a cemented carbide with good strength and toughness. Its performance combines good toughness with high hardness and wear resistance. It It solves the problems of low bending strength and difficult welding of CBN blades.

⑵ Main properties and characteristics of cubic boron nitride

Although the hardness of cubic boron nitride is slightly lower than diamond, it is much higher than other high-hardness materials. The outstanding advantage of CBN is that its thermal stability is much higher than that of diamond, reaching temperatures above 1200°C (diamond is 700-800°C). Another outstanding advantage is that it is chemically inert and does not react with iron at 1200-1300°C. reaction. The main performance characteristics of cubic boron nitride are as follows.

① High hardness and wear resistance: CBN crystal structure is similar to diamond, and has similar hardness and strength to diamond. PCBN is particularly suitable for processing high-hardness materials that could only be ground before, and can obtain better surface quality of the workpiece.

② High thermal stability: The heat resistance of CBN can reach 1400~1500℃, which is almost 1 times higher than the heat resistance of diamond (700~800℃). PCBN tools can cut high-temperature alloys and hardened steel at high speeds 3 to 5 times higher than carbide tools.

③ Excellent chemical stability: It has no chemical interaction with iron-based materials up to 1200-1300°C, and will not wear as sharply as diamond. At this time, it can still maintain the hardness of cemented carbide; PCBN tools are suitable for cutting quenched steel Parts and chilled cast iron, can be widely used in high-speed cutting of cast iron.

④ Has good thermal conductivity: Although the thermal conductivity of CBN cannot keep up with diamond, among various tool materials, the thermal conductivity of PCBN is second only to diamond, and much higher than high-speed steel and cemented carbide.

⑤ Has a lower friction coefficient: A low friction coefficient can lead to a reduction in cutting force during cutting, a reduction in cutting temperature, and an improvement in the quality of the machined surface.

⑶ Application of cubic boron nitride cutting tools

Cubic boron nitride is suitable for finishing various difficult-to-cut materials such as quenched steel, hard cast iron, high-temperature alloys, cemented carbide, and surface spray materials. The processing accuracy can reach IT5 (the hole is IT6), and the surface roughness value can be as small as Ra1.25~0.20μm.

Cubic boron nitride tool material has poor toughness and bending strength. Therefore, cubic boron nitride turning tools are not suitable for rough machining at low speeds and high impact loads; at the same time, they are not suitable for cutting materials with high plasticity (such as aluminum alloys, copper alloys, nickel-based alloys, steels with high plasticity, etc.), because cutting these Serious built-up edges will occur when working with metal, deteriorating the machined surface.