Development of the new materials for hard coatings of cutting tools

- Jan 08, 2019-

Development of the new materials for hard coatings of cutting tools

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1. Development of multiple and composite hard coating materials


PVD technology because of its processing temperature controlled under 500 , can be used as the final treatment process, very suitable for the coating of high speed steel tools, greatly improve the cutting performance of high speed steel cutter, so the technology from the 1980 s rapid promotion;By the end of 1980s, the PVD coating ratio of HSS complex cutting tools in industrial developed countries exceeded 60%.By the beginning of the 21st century, the PVD coating ratio of HSS cutters in industrial developed countries reached 50% ~ 70%, and the PVD coating ratio of HSS complicated cutters reached up to 90%.The successful application of PVD technology in the field of high-speed steel tools has attracted great attention from the manufacturing industry all over the world. While competing to develop high performance and high reliability coating equipment, the application field of PVD technology has been expanded (especially in the application of cemented carbide and ceramic tools).The results show that compared with the CVD process, PVD process temperature is low, under 600 had no effect on the bending strength of cutting tool material.The internal stress state of the film is compressive stress, which is more suitable for the coating of cemented carbide precision and complex cutting tools.No adverse impact on the environment, in line with the development direction of green manufacturing.In the late 1990s, PVD technology has been widely applied to the coating treatment of carbide end milling, drill bit, oil hole drilling, reamer, tap, indexable milling cutter piece and special-shaped cutter, etc.


The German Eurasia consulting company has analyzed the tool coating market in China in 1999.The market size of tool coating in China in 1999 was 75.12 million yuan.According to the statistics of China machinery industry yearbook (scope is a member of the tool association) high-speed steel tool products in China accounted for the largest proportion, its output value accounted for 85.9%, sales accounted for 89.8%, and the same year coating tool accounted for only 3% ~ 4%.The company forecasts that the Chinese tool coating market will grow by more than 20 percent annually, and the PVD tool coating market will grow even more, and the size of the Chinese tool coating market is expected to reach 247.69 million yuan by 2005.Because China's tool processing market is very large, so the development of tool coating technology is of great significance.Although the current domestic tool coating technology and the international advanced level compared to the large gap, the ordinary TIALN coating technology is not mature, but the huge application market and the country's increasingly urgent requirements for innovation, green manufacturing technology, has promoted the innovative development of domestic tool coating technology.


Cutting tool surface of the hard film on the material requirements: 1, high hardness, good wear resistance; 2. Stable chemical properties, no chemical reaction with workpiece materials; 3, heat and oxidation resistance, low friction coefficient, and matrix adhesion firm. It is difficult for a single coating material to meet all the above technical requirements. The development of coating materials, has been from the original single TiN coating, TiC coating, has gone through the TiC Al, 03-tin composite coating and TiCN, TiAlN and other multi-composite coating development stage, now the latest development of TiN/NBN, TiN/CN and other multi-composite film materials, the performance of the tool coating has been greatly improved.


TiN is the most mature and widely used hard coating material.At present, the use rate of TiN coated HSS cutters in industrial developed countries has accounted for 50% ~ 70% of HSS cutters, and the use rate of some non-regrindable complex cutters has exceeded 90%.Due to the high technical requirements of modern metal cutting tools, TIN coating gradually failed to adapt.The oxidation resistance of TiN coating is poorer, when use temperature is 500 ;The film was obviously oxidized and ablated, and its hardness could not meet the need.TiC has higher microhardness, so the wear resistance of this material is better.At the same time, it adheres firmly to the substrate. In the preparation of multi-layer wear-resistant coating, TiC is often used as the underlying film in contact with the substrate. It is a very common coating material in the coating tool.The development of TiCN and TiAlN further improved the performance of coated cutters.TiCN can reduce the internal stress of the coating, improve the toughness of the coating, increase the thickness of the coating, prevent the diffusion of cracks, and reduce the blade breakage.TiCN is set as the main wear layer of the coated tool, which can significantly improve the tool life.TiAlN has good chemical stability and anti-oxidation abrasion. When processing high-alloy steel, stainless steel, titanium alloy and nickel alloy, the service life of tin-coated cutter can be increased by 3~4 times.If there is a high concentration of Al in the TiAlN coating, a very thin amorphous Al2O3 will be generated on the surface of the coating during cutting, forming a hard inert protective film. The coated tool can be more effectively used in high-speed cutting.Oxygen-doped titanium nitride TiCNO has very high microhardness and chemical stability, which can produce a composite coating equivalent to TIC+AL2O3.Some transition metal nitrides, carbides, borides and their complex compounds, some of which have high hardness, these materials can be developed and applied to coating tools, will make a new breakthrough in the performance of coating tools.


2. Application of low pressure gas phase synthesis diamond film


Among the hard film materials mentioned above, there are three kinds of diamond film, cubic boron nitride CBN and carbon nitride whose microhardness HV can exceed 50GPa.These few ultra-high hardness film materials have opened up a very important use for the development of coated cutting tool hard film, because rare and expensive natural diamond is far from meeting the needs of modern industry.In the mid-1950s, the United States general motors company artificially synthesized diamond, and obtained granular and powder diamond.Because granular diamond is difficult to be machined, it is difficult to apply it to tool surface.Polycrystalline diamond blades (PCD) commonly used in the mechanical industry also have limited performance due to their simple geometry, no chip breaker and reasonable geometric parameters.In the early 1970s, diamond thin films were synthesized by low-pressure chemical vapor deposition (LCVD). After more than 20 years of technological breakthroughs, the technology of low-pressure vapor deposition for diamond finally made a major breakthrough, and diamond research has become a hot topic worldwide.


Diamond and graphite are allotrope, diamond crystal is cubic crystal system, and graphite is hexagonal crystal system.Due to the different bonding modes between atoms, their performance varies greatly.Thermodynamically, graphite is more stable than diamond.The growth of diamond in low pressure gas phase, in the phase diagram of carbon, is carried out in the region where graphite is stable and diamond is metastable.However, because the chemical potentials of the two phases are so close, both phases can be formed.The key technology of low pressure gas phase synthesis of diamond is inhibiting graphite phase and promoting diamond phase growth.The common synthesis method is the hot wire method.Plasma enhanced chemical vapor deposition (PECVD), including microwave PCVD, electron cyclotron resonance ecr-pcvd, dc and rf PCVD methods;Dc and high frequency arc discharge plasma method.The energy input in the reaction process (such as rf power, microwave power, etc.), the activation state and optimal ratio of the reaction gas, and the nucleation mode in the deposition process, etc., play a decisive role in the formation of diamond film.The crystal and lattice constants of substrate materials have great influence on the nucleation growth of diamond film. When diamond phase and graphite phase are nucleated at the same time on the substrate, the graphite phase will grow rapidly.If there is a high concentration of atomic hydrogen, it will corrode the growing graphite phase and remove the graphite phase. Although it can corrode the diamond phase, it is much slower, so as to inhibit the growth of graphite phase.Many requirements for the depositing of diamond film temperature 600 ~ 900 ℃, so the technology used to deposition diamond film on the surface of cemented carbide cutting tools.


3. CBN thin film technology is to be breakthrough yet


Compared with synthetic diamond film, synthetic CBN film is a late research field.CBN has three isomers;CBN cubic sphalerite structure;H-bn hexagonal graphite structure;W-bn hexagonal wurtzite structure.The properties of the three isomers are very different. H-bn has a very similar structure to graphite and a very soft texture.In w-bn and CBN, B and N atoms have to form a quad coordination structure with each other. They are both superhard materials.The CBN obtained by high temperature and high pressure method is granular crystal with the highest microhardness up to 84.3GPa. The highest microhardness of CBN film is 61.8GPa, and its comprehensive performance is no less than that of diamond film.CBN is second only to diamond in hardness and thermal conductivity and has excellent thermal stability.In the atmosphere, it doesn't happen heated to 1000 ℃ oxide.CBN has extremely stable chemical properties for iron group metals, which is different from diamond which is not suitable for processing steel. It can be widely used for finishing and grinding of steel products.In addition to excellent wear resistance, CBN coating can be used to process heat resistant steel, titanium alloy and hardened steel at a fairly high cutting speed, and can be used to cut hard rollers with high hardness, carbon-mixed quenching materials and si-al alloy with very severe tool wear.CVD and PVD methods are the main methods to synthesize CBN films.CVD includes chemical transport PCVD, hot wire assisted heating PCVD, ecr-cvd, etc.PVD includes reactive ion beam plating, reactive evaporation and laser assisted ion beam deposition.


The synthesis technology of CBN still has a lot of work to do in basic research and application technology, including reaction mechanism and film forming process, plasma diagnosis and mass spectrometry analysis, determination of the best process conditions, development of high-efficiency equipment, etc.


4. Carbon nitride that may exceed the hardness of diamond


In the late 1980s, American scientists I.I.U and co-hen designed p-c3n4, a new compound similar to p-si3n4. The bulk modulus, energy band and character constants of p-c3n4 were calculated by using solid state physics and quantum chemistry theory, and it was found that the bulk modulus of carbon nitride reached the numerical range of diamond.Since the hardness of the material is proportional to the volume modulus, the hardness of C3N4 may reach the hardness of diamond, which has attracted the attention of scientists all over the world.In 1994, I.I.U published the new research result E53. He adopted the ab initio calculation method of variable lattice model molecular dynamics (vs-md), extended the theoretical research of low-energy C3N and solid, and pointed out that C and N may have three structures: P phase of hexagonal crystal system, sphalerite structure of cubic crystal system and graphite-like structure of triangular crystal system.In 1996, Jeter and Hemley in the United States still used first-principles ab initio calculations, but changed the process.The conjugate gradient method is used to minimize the electron degrees of freedom under the initial conditions.The periodic function is used to expand the electron wave into two plane waves.Extended standard conservation and strength conservation (ENHC) were used.C3N4 with five structures is obtained, which are n-phase, p-phase, cubophase blende structure, cubophase silicozinc E structure and graphitic phase.In addition to the graphite-like phase, the other four are all superhard materials.The volume modulus of E - c c 3 n 4 in cubic - phase silicozinc ore is higher than that of diamond.Therefore, carbon nitride may have the hardness of diamond.


The success of synthesizing carbon nitride is a remarkable example of molecular engineering.Carbon nitride, as a superhard material, is expected to have many other valuable physical and chemical properties.The main synthesis methods of carbon nitride are dc and rf reactive sputtering.Laser evaporation and ion beam assisted deposition method, ECR-CVD method, double ion beam deposition method, etc.The carbon nitride film obtained by electron beam evaporative ion beam assisted deposition at okayama university in Japan reached the highest microhardness of carbon nitride at present -- 63.8GPa.China's tsinghua university also obtained 60.8GPa of high hardness carbon nitride, and wuhan university synthetic carbon nitride hardness reached 50.0GPa9, and deposited on the HSS braid drill, and obtained very good drilling performance.The key technology to prepare carbon nitride superhard coating is to avoid the precipitation of graphite phase.