Coating technology --- tool coating

- Dec 18, 2018-

Coating technology --- tool coating


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Through chemical or physical methods on the surface of the tool to form a certain film (that is, coating process), so that the cutting tool to obtain excellent comprehensive cutting performance, so as to meet the requirements of high-speed cutting; Since the advent of hard coated tools in the early 1970s, chemical vapor deposition (CVD) technology and physical vapor deposition (PVD) technology have been developed successively, which has opened a new chapter in the history of tool performance improvement. Compared with uncoated tools, coated tools have significant advantages: it can greatly improve the life of cutting tools; Effectively improve the cutting efficiency; Improve machining precision and surface quality of machined workpiece obviously; Effectively reduce the consumption of tool materials, reduce the cost of processing; Reduce the use of coolant, reduce the cost, conducive to environmental protection.


Characteristics of tool coating:


1. The application of coating technology can greatly improve the surface hardness of the tool without reducing the strength of the tool, and the hardness that can be achieved now is close to 100GPa;

2. With the rapid development of coating technology, the chemical stability and high temperature oxidation resistance of the film become more prominent, which makes high-speed cutting possible.

3. Lubrication film has good solid phase lubrication performance, can effectively improve the quality of processing, but also suitable for dry cutting;

4. As the final process of tool manufacturing, coating technology has little impact on tool accuracy and can be used for repeated coating process.


Commonly used coatings:


1, titanium nitride coating: titanium nitride (TiN) is a general PVD coating, which can improve the hardness of the tool and has a high oxidation temperature. The coating can be used for high speed steel cutting tools or forming tools to obtain good machining results.

2, chromium nitride coating: CrN coating has good adhesion resistance, making it the preferred coating in the processing of chip tumor. With this almost invisible coating, the machining properties of HSS or carbide tools and forming tools will be greatly improved.

3, diamond coating: CVD diamond coatings for non-ferrous metal material processing cutting tool provides the best performance, the processing of graphite, metal matrix composites (MMC), high silicon aluminum alloy, and many other high abrasion material, the ideal coating (note: pure diamond coated tools cannot be used for processing steel, because processing steel will produce a large number of cutting heat, and causing a chemical reaction, ruined the adhesive layer between the coating and cutting tool).

4. Titanium nitride coating: the carbon element added to the titanium nitride coating (TiCN) can improve the hardness of the tool and obtain better surface lubricity, making it an ideal coating for high-speed steel tools.

5, aluminum nitride/titanium nitride/aluminum nitride coating (TiAlN/AlTiN) : aluminum oxide layer formed in the TiAlN/AlTiN coating can effectively improve the tool's high temperature processing life. The coating can be used for carbide cutting tools mainly used for dry or semi-dry cutting. AlTiN coatings offer higher surface hardness than TiAlN coatings, depending on the ratio of aluminum to titanium contained in the coatings, making it another viable coating option for high speed machining.

6. Diamond-like coating (DLC) :

DLC has many excellent features: high hardness -60GPa or Hv6000 or above; Low friction coefficient Excellent film density; Good chemical stability and optical properties. DLC coating applied to the tool shows much more special performance than other hard coatings. DLC coated tools mainly include: graphite cutting, cutting various non-ferrous metals (such as aluminum alloy, copper alloy, etc.), non-metallic hard materials (such as acrylic, glass fiber, PCB materials) cutting.


Because DLC has very low friction coefficient (0.05~0.2) and self-lubrication, the tool coated with DLC is the best choice for dry cutting. Compared with the tool without coating, the cutting force of DLC coating tool can be reduced by 6% under the condition of dry cutting. . Compared with other coated tools (such as TiN, TiAlN), the cutting force of DLC coated tools can be reduced by 23% under dry cutting conditions; Similarly, DLC's low friction coefficient can greatly reduce the cutting force of metal cutting (aluminum alloy cutting), thus greatly reducing energy consumption. Under the cutting conditions with coolant, the cutting force ratio of DLC coated tools without coated tools and other coated tools decreased by 18%, and the roughness of the processed metal surface decreased by Ra0.04~R0.23um) to Ra0.38.


Coating technology and tool coating knowledge:


1. Titanium nitride carbide (TiCN) coating has higher hardness than titanium nitride (TiN) coating. Due to the increased carbon content, the hardness of TiCN coating was increased by 33%, and the hardness range was about hv3000-4000 (depending on the manufacturer).


2. CVD diamond coating: the application of CVD diamond coating with surface hardness up to Hv9000 has been relatively mature. Compared with PVD coated tools, the service life of CVD diamond coated tools has been improved by 10-20 times. The high hardness of diamond coated tools makes the cutting speed 2 -- 3 times higher than that of uncoated tools, making CVD diamond oxidation temperature refers to the temperature value at the beginning of the decomposition of the coating. The higher the oxidation temperature value is, the more favorable it is for cutting at high temperature. Although TiAlN coating may have lower hardness at room temperature than TiCN coating, it has proved to be much more effective than TiCN coating in high-temperature processing. The reason why TiAlN coating can still maintain its hardness at high temperature is that it can form a layer of aluminum oxide between the tool and the chip, which can transfer heat from the tool to the workpiece or chip. Compared with HSS cutters, carbide cutters usually have a higher cutting speed, which makes TiAlN the preferred coating for carbide cutters. PVDTiAlN coated stone coated cutters are usually used for PVDTiAlN coated stone coated cutters in carbide drill bits and end-milling cutters.


3, the tool surface of the hard film on the material has the following requirements: (1) high hardness, good wear resistance; (2) chemical properties are stable, do not have a chemical reaction with the workpiece material; Heat resistance and oxidation resistance, friction coefficient is low, and the matrix is firmly attached. 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 -- A12O3 a TiN composite coating and TiCN, TiAlN and other multivariate composite coating development stage, now the latest development of TiN/NbN, TiN/CN, and other multivariate composite film materials, the performance of tool coating has been greatly improved.


4, in the process of coating tool manufacturing, generally according to the hardness of the coating, wear resistance, high temperature oxidation resistance, lubrication and adhesion resistance to a few aspects to choose, including coating oxidation and cutting temperature is the most directly related to the technical conditions. Oxidation temperature refers to the coating began to decompose the temperature value, the higher the oxidation temperature value, the more favorable the cutting process in high temperature conditions. Although TiAlN coating may have lower hardness at room temperature than TiCN coating, it has proved to be much more effective than TiCN coating in high-temperature processing. TiAlN coating can maintain its hardness at high temperatures because it can form a layer of alumina between the tool and the chip, which can transfer heat from the tool to the workpiece or chip. Carbides usually cut at higher speeds than HSS cutters, which makes TiAlN the preferred coating for carbides. PVDTiAlN coatings are commonly used for carbides bit and end-milling cutters.


5. From the perspective of application technology: in addition to cutting temperature, cutting depth, cutting speed and coolant may have an impact on the application effect of tool coating.


Progress of common coating materials and superhard coating technology:


TiN is the most mature and widely used hard coating material. At present, the industrial developed countries TiN coating high speed steel tool utilization rate has accounted for high speed steel tool 50% to 70%, some non-regrinding complex tool utilization rate has exceeded 90%. Due to the high technical requirements of modern metal cutting tools TiN coating is increasingly unable to adapt. Poor oxidation resistance of TiN coating, the use of the temperature of 500 , the obvious oxidation and the ablation membrane layer, and it also can't satisfy the need. TiC has higher microhardness, so the wear resistance of this material is better. At the same time, it is firmly attached 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.


TiCN and TiAlN development, and the coating tool performance to a higher level. 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 cutting edge breakage. TiCN is set as the main wear resistant layer of the coated tool, which can significantly improve the tool life. TiAlN chemical stability is good, anti-oxidation wear, processing high alloy steel, stainless steel, qin alloy, nickel alloy, TiN coating tool life increased by 3-4 times. If there is a high concentration of Al in TiAlN coating, a very thin non-grade A12O3 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 for high-speed cutting. Oxygen-doped titanium carbide TiCNO has very high microhardness and chemical stability, which can produce the function equivalent to TiC 10 A12O3 composite coating.


Among the hard film materials mentioned above, there are three kinds of microhardness HV that can exceed 50GPa: diamond film, cubic boron nitride CBN, and carbon nitride.


Many of the depositing of diamond film temperature for 600 900 , so the technology used in deposition diamond film on the surface of cemented carbide cutting tools. The commercialization of diamond carbide tool is a great achievement of coating technology in recent years.


CBN is second only to diamond in hardness and thermal conductivity, excellent thermal stability, in the atmosphere, it doesn't happen heated to 1000 oxide. CBN has extremely stable chemical properties for ferrogroup metals. Unlike diamond, which is not suitable for processing steel, CBN can be widely used for finishing and grinding 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 relatively high cutting speed. It can also be used to cut hard rollers with high hardness, carbonized quenching materials and si-al alloy which is very severe to tool wear. CVD and PVD methods are mainly used to synthesize CBN thin films by low pressure gas phase. CVD includes chemical transport PCVD, hot wire assisted heating PCVD, ECR - CVD, etc. PVD has reactive ion beam plating, reactive electroless plating, laser electroless ion beam assisted deposition and so on. 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.


Carbon nitride may have a hardness equal to or greater than that of diamond. The success of carbon nitride synthesis is a remarkable example of molecular engineering. As a super hard material, carbon nitride is expected to have many other valuable physical and chemical properties.