I. Types and Development of Ceramic Tools
Types and development of ceramic tools: the most obvious development line of ceramic tools is the increase of blade toughness in turn: alumina ceramic tools-composite alumina ceramic tools-silicon nitride ceramic tools-cubic boron nitride tools.
In the field of metal cutting, alumina ceramic tools and silicon nitride ceramic tools are collectively called ceramic tools; In the science of inorganic nonmetallic materials, cubic boron nitride belongs to ceramic materials, and the advent of cubic boron nitride tools is the revolution of ceramic tools. Henan Institute of Materials, as one of the earliest research institutes in China to study polycrystalline cubic boron nitride tools, recently introduced pure boron nitride sintered ceramic tools, which have significantly improved toughness and wear resistance.
Second, the performance of ceramic tool and its application in metal cutting.
Compared with cemented carbide blades, ceramic tools can withstand the high temperature of 2000℃, while cemented carbide becomes soft at 800℃. Therefore, the ceramic tool has better chemical stability at high temperature and can be cut at high speed, but its disadvantage is that the strength and toughness of alumina ceramic tool are very low and it is easy to break. Due to the high temperature resistance of ceramic tools, it is more conducive to high temperature and high speed cutting. Because of the low thermal conductivity of ceramics, the high temperature is only at the tip of the tool, and the heat generated by high-speed cutting is taken away with the chips, so most researchers believe that the advantages of ceramic tools can be truly reflected if the linear speed of alumina ceramic tools is higher than that of cemented carbide cutting 10 times.
In order to reduce the sensitivity of ceramic tools to crushing, zirconia or a mixture of titanium carbide and titanium nitride is added in an attempt to improve their toughness and impact resistance. Although these additives are added, the toughness of ceramic tools is much lower than that of cemented carbide blades.
Another way to improve the toughness of alumina ceramic tools is to add crystal structure or silicon carbide whiskers to the materials. These special whiskers have an average diameter of only 1 nm and a length of 20 microns, which greatly increases the toughness, strength and thermal shock resistance of ceramics. Limited by its impact toughness, it has been used in the field of finishing.
Like alumina ceramic tools, silicon nitride ceramic tools have higher thermal hardness than cemented carbide blades. Its high temperature resistance and mechanical impact resistance are also good. Compared with alumina ceramic cutter, it has the disadvantage of insufficient chemical stability when processing steel. However, gray cast iron can be machined with a silicon nitride ceramic cutter at a speed of 1450 ft/min or higher.
Processing materials suitable for ceramic tools: ceramic tools can not be used to process aluminum, but are especially suitable for gray cast iron, ductile iron, hardened steel and some non-quenched steels and heat-resistant alloys. However, for these materials, the successful application of ceramic tools requires the appearance and micro-quality assurance of the blade and the best cutting parameters.
Third, the performance and application of cubic boron nitride tool
The hardness of cubic boron nitride tool is much higher than that of ceramic tool. Cubic boron nitride tool is called material together with diamond because of its high hardness, and is often used to process materials with hardness higher than RC48. It has excellent high temperature hardness-up to 2000℃. Although it is more brittle than cemented carbide cutter, its impact strength and crushing resistance are obviously improved than those of ceramic cutter. At present, cubic boron nitride (CBN) tool is the tool material with the best wear resistance in the field of ferrous metal processing. Practice has proved that the service life of CBN tools is generally several to dozens times that of cemented carbide blades and ceramic tools. For example, when using BN-S30 cubic boron nitride cutter to process brake disc made of gray iron, the cutting linear speed is 600M/min, which is 2 times higher than that of cemented carbide blade and 6- 12 times longer than that of rough machining. Moreover, with the development of research, CBN tools can adapt to the processing of various high-hardness complex materials, and can process tungsten carbide with hardness above HRC70, which is the first case in China.
In addition, some special cubic boron nitride tools (such as BN-S 10, BN-S20 brand) can bear the chip load of rough machining, the impact of intermittent cutting and the wear and cutting heat of finish machining, and can meet the requirements of rough machining hardened steel and other difficult-to-machine materials with cubic boron nitride tools. BN-K 1 brand in HLCBN series is widely used in metal processing of high hardness and wear-resistant castings such as mining machinery and metallurgical rollers because of its excellent impact toughness.
Pure boron nitride sintered blade: The latest research achievement is the blade made of pure boron nitride material, and the CBN content reaches 100%. The boron nitride ceramic tool formed by pure sintering has a high hardness and thermal conductivity ratio, and when cutting high-hardness ferrous metal materials, there will be no common hot cracking and fragmentation at the blade; Compared with CBN sintered bodies containing other binders, the service life of boron nitride ceramic tools can be extended by more than 6 times.
Cubic boron nitride cutter is suitable for processing materials: gray cast iron, various wear-resistant cast iron, cast steel, high-hardness materials (such as hardened cold working tool steel, high-speed steel, bearing steel, powder metallurgy steel, high-strength steel, high-manganese steel, high-chromium cast iron, white cast iron, austenitic iron, cemented carbide, etc. ).
Fourth, the application differences between ceramic tools and cubic boron nitride tools
Although cubic boron nitride belongs to the category of ceramics in materials science, the toughness and hardness of ceramic tools are not as good as those of cubic boron nitride tools because of the different properties of CBN, Sn, Al2O3 and s in. Therefore, according to the difference of hardness and toughness, ceramic tools are a better choice when cutting hardened steel workpieces with hardness below 50HRC and small cutting depth and feed. Cubic boron nitride tool is more suitable for the case that the hardness of workpiece is higher than 50HRC. In addition, it should be noted that ceramic tools and traditional embedded boron nitride blades are rarely used for rough machining, and only some brands of boron nitride blades such as BN-S20 are qualified for rough machining and intermittent cutting.