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With the development of modern cutting technology, the metal removal rate continues to increase. In 1900, it took about 100 minutes to cut a standard steel bar (500mm × 100mm) with a carbon tool steel tool. After using a carbide tool in the 1950s, the machining time was shortened to a few minutes. The use of coated carbide tools began in 1969, initially using coated blades to cut standard steel bars in 1.5 minutes, while the GC1025 and GC015 coated blades further reduced processing time to 1.25 minutes and 1 minute. By analyzing the machining process, the most economical way to increase the cutting speed and reduce the cutting cost is to apply the new tool material. ?
Tool surface coating is an effective means to improve tool life and reduce cutting costs. The tool coating not only can improve the surface hardness of the tool, enhance its wear resistance, but also reduce the friction coefficient of the tool surface, increase the lubrication ability, increase the cutting speed, reduce the number of tool changes, and improve the precision and surface quality of the machined parts. Increase productivity. At present, in industrialized countries, coated tools account for more than 80% of all tools used, while China's ratio is less than 20%. Therefore, it is of great significance to promote the application of coated tools and develop new coated tool materials.
1.CBN performance characteristics
CBN (CubicBoronNitride) is another superhard inorganic material that appears after synthetic diamond. It has many excellent physical and chemical properties similar to diamond, such as ultra-high hardness (after diamond). High wear resistance, low coefficient of friction, low coefficient of thermal expansion, etc., while also having some properties superior to diamond (see Table 1 for comparison of cBN and diamond properties). Unlike diamonds, which are not suitable for processing steel materials, cBN has extremely stable chemical properties for iron-based metals, so it can be widely used for precision machining and grinding of steel products. In addition to its excellent wear resistance, cBN is also extremely excellent in heat resistance. It can also cut heat-resistant steel, hardened steel, titanium alloy, etc. at a relatively high cutting temperature, and can cut high-hardness chill rolls. Carburized and quenched materials and difficult-to-machine materials such as SiAl alloys that are very abrasive to tool wear.
Performance comparison between CBN and diamond
Performance - CBN - Diamond
Crystal structure - sphalerite - diamond
Lattice constant (A°) - 3.615 - 3.567
Density (g·cm-3) - 3.48 - 3.515
Microhardness (MPa) -46042～84337->88260
Modulus of elasticity (GPa) - 700 - 1000
Thermal expansion coefficient (×10 -6 power / ° C) - 4.7 - 3.1
Chemical stability - does not react with Fe-based metals, does not oxidize in the atmosphere to 1000 ° C - reacts with Fe-based metals, oxidizes at 600 ° C or higher in the atmosphere
Thermal stability - in vacuum, the phase change from cBN to hBN occurs at a temperature of 1550 ° C - in vacuum, the temperature exceeds 1300 ° C, and the diamond begins to transform into graphite.
CBN is not naturally occurring and needs to be artificially synthesized. The traditional method of synthesizing cBN is high temperature and high pressure. This method is demanding and costly in terms of synthesis conditions, and can only be obtained as a cBN fine powder, and can only be used as an abrasive and polycrystalline cBN tool. Despite this, the processing properties of this expensive tool are greatly reduced due to the use of adhesives. Therefore, there is an urgent need to develop a lower cost, higher quality cBN preparation technology.
2. Preparation method of CBN coating
CBN has three isomers, hBN, cBN and wBN. hBN (hexagonal boron nitride) has a structure very similar to graphite, also known as "white graphite", and the texture is also very soft; and the structure and properties of cBN are similar to diamond. In 1987, Inagawa et al. successfully prepared CBN films with higher purity. Subsequently, many researchers succeeded in preparing CBN by physical vapor deposition (PVD) and chemical vapor deposition (CVD), which led to the research of cubic nitrogen in the world. The craze of boron film. Among the various preparation methods, the most typical methods are plasma enhanced pulsed laser deposition and hot wire assisted radio frequency plasma chemical vapor deposition.
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