PCBN tool (Polycrystalline Cubic Boron Nitride tools) is made by sintering CBN powder and the bond under ultra-high pressure and high temperature. It is the most suitable tool material for high speed cutting iron metal.
Introduction of PCBN tool:
CBN(Cubic boron nitride material) is a kind of new inorganic material, which is first synthesized by GE company in 1957. The synthetic hardness of CBN is second only to synthetic diamond. It features high hardness, thermal stability, and chemical inertness, as well as good infrared transmittance.
Monocrystalline cubic boron nitride grains are very small. They are difficult to obtain high-quality large grains of monocrystalline cubic boron nitride due to limitations of high-pressure cavity and synthesis technology.
Polycrystalline cubic boron nitride can be divided into solid PCBN insert and PCBN compact. The solid PCBN is sintered by numerous small CBN grains under high temperature and high pressure. The PCBN compact is sintered by CBN layer and tungsten carbide matrix under high temperature and high pressure. The polycrystalline structure makes PCBN isotropic, overcoming the defects of the cleavage plane of monocrystalline. At the same time, PCBN has impact toughness similar to the matrix of tungsten carbide.
These advantages make PCBN widely used, especially as tool material, and it plays a great role in the cutting process. It is widely used in the fields of automotive, machine tool, aerospace, petroleum geology, instrumentation, furniture manufacturing, and etc. PCBN tool can be sued for turning instead of grinding, and its life is increased by several tens of hundreds of times compared to the life of tungsten carbide tool.
2.Synthesis technology of PCBN
At present, PCBN compacts with bonded are widely used. Although the solid PCBN overcomes the problems of the small size of CBN monocrystalline and anisotropy, there has some problem in the connection and welding as a tool. Because solid PCBN is not infiltrated and bonded by usual solder easily. It is difficult to directly weld. PCBN compacts have advantages in welding.
As tool material, the quality of PCBN directly affects the cutting efficiency and service life of PCBN tool. Pay attention to the following problems in the synthesis process in order to obtain high-quality PCBN.
PCBN is sintered by CBN grains and a certain proportion of bond under high temperature and high pressure. The bond plays an important role in the process of synthesizing PCBN.
Add the proper amount of bond can lower the sintering temperature and pressure and improve the performance of the solid.
Pay attention to the following points when choosing the bond.
- The coefficient of linear expansion is as close as possible to that of CBN to reduce the temperature difference stress
- The bone should have a strong chemical affinity with nitrogen or boron to improve the bond strength.
- When grinding hard metal materials (such as wear-resistant cast iron and hardened steel), the content of bond has a great effect on the cutting effect. Therefore, when manufacturing PCBN with high content bond, the bond should have High hardness and toughness.
There are many materials used as PCBN bond, which can be classified into 3 categories.
- Metal bond, it is composed of metals or alloys( such as aluminum, titanium, cobalt, nickel, etc.) (2) vitrified bond, it is mainly nitrides, carbides borides, etc. (3) metal and vitrified bond, it is composed of vitrified and metal or metal alloys in a certain ratio.
PCBN with metal bonded usually has high CBN content and good toughness, and its thermal conductivity is better than of PCBN with the vitrified bond. But the metal usually softens at the temperature of 700℃-800℃. The effect of softening metal at high temperature causes the wear resistance of PCBN to decrease. There is a big difference between the coefficient of linear expansion of metal and that of CBN. It causes CBN structure changed at high temperature, resulting in a decrease in red hardness.
The melting point of ceramic is relatively high, and there is no softening effect at high temperature. PCBN with vitrified bond has high temperature wear resistance and strong chemical wear resistance. But the ceramic has a poor thermal conductivity at high temperature, and it is easy to concentrate the cutting temperature on the material to be processed and soften it. Therefore, it is suitable for cutting hardened steel.
Metal and vitrified bond have the advantages of both pure metal bond and pure vitrified bond. When synthesizing PCBN, choose a suitable bond according to the different materials of the object to be processed.
2.2 CBN content
CBN content affects the hardness of PCBN. The higher the CBN content, the higher the hardness of PCBN is. Usually, the hardness of PCBN is 3000 HV-5000 HV. The relationship between CBN content and PCBN hardness is shown in Fig.1. CBN content is usually 40%-95%. When CBN content is over 95%, the sintering performance is deteriorated, and the wear resistance is greatly lowered.
2.3 CBN grain size
The finer CBN grain, the better the wear resistance of PCBN, and the higher the compressive strength is. But the fine-grained CBN is not easily sintered. The larger the CBN grain size, the stronger the mechanical wear resistance of PCBN is. And the damage resistance is weaker, the worse the sharpness of the cutting tool.
It is shown in the tests that mixing different grain size CBN in a certain ratio, improves the PCBN quality. In raw materials, add a small amount of CBN grains to increase the overall performance of the tool.
2.4 Sintering technology
The main parameters in the PCBN sintering process are pressure, temperature and sintering time. Sintering is controlled in the thermal stable zone of CBN. The temperature is usually marked by the heating power during the sintering process. The optimum powder value of sintering and the power value of reverse conversion of cubic boron nitride to hexagonal boron nitride are very similar, so the range of sintering temperature is very narrow. Usually, it is 1400 ° C – 1600 ° C. The pressure is about 5.0-7.0GPa and the holding time is about
2-30 minutes. In addition, the uneven temperature distribution in the high-pressure cavity affects the PCBN quality and performance. Therefore, measures should be taken to make the temperature distribution in the cavity as uniform as possible.
2.5Cleanliness and graininess of CBN grain
The surface cleanliness of CBN grain directly affects the sintering quality of the PCBN. Therefore, strictly treat the CBN grains before sintering, to remove impurities such as moisture and oxides on the surface of the grains. The method used is mainly heating under vacuum or hydrogen or ammonia etc. reducing gas for 1 to 2 hours. Otherwise, too many impurities affect the bonding between CBN-CBN grains and CBN and the bond, making the strength of the PCBN cutter reduced, and the wear resistance is lowered.
2.6 Grinding quality
In addition to raw material quality and reliable specification of the blank synthesis technology, the grinding technology and precision of PCBN cutter directly determine the microscopic performance when cutting. Therefore, the peripheral grinding machine is used, and the cutter cost is improved a lot, but the product quality and durability will be improved.
The ideal way is to use a diamond grinding wheel. After test comparison, vitrified bond diamond grinding wheel should be first chosen, and resin bond diamond grinding wheel with high heat resistance coefficient should be secondly chosen.
2.7 Inspection methods
Inspection methods: In addition to dimensional accuracy and surface quality, there will be microscopic defects under electron microscopy in the production of CBN tools. It will defect product. These microscopic defects are invisible to the naked eyes, which cause chipping.
3 PCBN performance features
PCBN not only features the excellent quality of CBN, but also the PCBN compact has the impact toughness of tungsten carbide. The crystal grains of PCBN are disorderly arranged, isotropic, no cleavage surface. It is not like monocrystalline CBN which has a difference in the strength and wear resistance.
3.1 Higher hardness, wear resistance and impact resistance
PCBN hardness is second only to that of the diamond, which is much higher than the hardness of ceramic and tungsten carbide. It has a high wear resistance. When machining hardened steel, the durability is 10-50 times that of tungsten carbide. The impact resistance of PCBN tool is much higher than that of the ceramic tool.
3.2 High thermal stability
PCBN has good thermal stability at 1200℃, and its hardness is higher than that of tungsten carbide and ceramic material at 800℃. Fig.2 shows the hardness of PCBN at high temperature. PCBN tool does not oxidize below 1000 °C, so the relative temperature of tooltip does not adversely affect it. Instead, it can accelerate the cutting tungsten carbide.
3.3 Excellent chemical stability
PCBN has excellent chemical stability and does not react chemically with iron group metals at 1200 ° C – 1300 ° C. It is not corroded in acid and is eroded in alkali at around 300 ° C. It is bonding and diffusion to various materials is much smaller than that of tungsten carbide. Therefore, the PCBN tool is suitable for steel materials
3.4 Good thermal conductivity
The thermal conductivity of PCBN is lower than that of a diamond, which is 20 times that of tungsten carbide. As the cutting temperature increases, the thermal conductivity of PCBN increases, while the thermal conductivity of alumina decreases. ( shown as Fig.3) Therefore, the heat of PCBN tooltip can be quickly transmitted, which is good for the improvement of machining accuracy and mechanical wear resistance.
3.5 Low coefficient of friction
The coefficient of friction of CBN and different materials is between 0.1 and 0.3, which is much smaller than the friction coefficient (0.4-0.6) of tungsten carbide. It decreases with the increase of friction speed and normal force.
4 Applications of PCBN tool
PCBN is mainly used for processing tool materials of ferrous metals.
PCBN tool is suitable for all kinds of difficult-to-machine materials, including various hardened steels( carbon steel, alloy steel, bearing steel, mold steel, high-speed steel and high hardness hardened steel parts), various spray(welding) materials, wear resistant cast iron materials, high-temperature alloys, can also be used in the processing of titanium alloys, pure nickel, pure tungsten, and other materials. As shown in Fig.5
PCBN with high CBN content features high thermal conductivity and toughness. It is usually used as tool materials for rough machining hardened steel and pearlitic cast iron.
PCBN with low CBN content features low thermal conductivity and high compressive strength and red hardness. It is usually used as tool materials for machining hardened steel.
PCBN compact has good toughness, high wear resistance, and chemical stability to iron group elements. It is tool material for processing hard ferrous materials such as nickel-chromium chilled cast iron, high-chromium iron, and infiltrated steel.
Solid PCBN insert can not only be used for turning to the machine but also milling workpiece is better. It not only improves the processing quality of the product but also has high production efficiency and reduces the unit manufacturing cost.
With the continuous development of industrialization, PCBN tool will be more and more widely used. Dry cutting, hard machining, and high-speed cutting have high efficiency, low energy consumption, low pollution, low cost, which is the mainstream of cutting in the future.