What Is The Coating
Coating is a solid continuous film obtained by one-time coating. It is a thin plastic layer coated on metal, fabric, plastic and other substrates for the purpose of protection, insulation and decoration. The coating can be gaseous, liquid or solid. The type and state of the coating are usually determined according to the substrate to be sprayed.
introduce
There are different names according to the type of coating used. For example, the coating of primer is called primer layer, and the coating of topcoat is called topcoat layer. The coating obtained from general coatings is thin, about 20 ~ 50 microns, while thick paste coatings can obtain a coating with a thickness of more than 1mm at a time. It is a thin plastic layer coated on metal, fabric, plastic and other substrates for protection, insulation, decoration and other purposes.
High temperature electrical insulation coating is outside the conductor made of copper, aluminum and other metals, or with insulating paint, plastic, rubber and other insulating coatings. However, insulating paints, plastics and rubber are afraid of high temperature. Generally, they will be concentrated and lose their insulating properties when they exceed 200 ℃. And many wires need to work at high temperature. What should we do? Yes, let the high-temperature electrical insulation coating help. This coating is actually a kind of ceramic coating. In addition to maintaining the electrical insulation performance at high temperature, it can also be closely "united" with the metal conductor to achieve "seamless". If you wrap the conductor seven times and eight times, they will not separate. This coating is very dense. Apply it, If two wires with large voltage difference touch together, breakdown will not occur.
High temperature electrical insulation coatings can be divided into many kinds according to their chemical composition. For example, boron nitride or alumina and copper fluoride coatings on the surface of graphite conductors still have good electrical insulation performance at 400 ℃. The enamel on the metal conductor can reach 700 ℃, the phosphate based inorganic binder coating can reach 1000 ℃, and the plasma sprayed alumina coating can still maintain good electrical insulation performance at 1300 ℃.
High temperature electrical insulation coating has been widely used in power, motor, electrical appliance, electronics, aviation, atomic energy, space technology and so on.
classification
According to the classification method of thermal spraying coating by f.n.longo in the United States, the coating can be divided into:
1. Wear resistant coating
It includes anti adhesion wear, surface fatigue wear coating and erosion resistant coating. In some cases, there are wear resistant coatings against low temperature (< 538 ℃) and high temperature (538 ~ 843 ℃).
2. Heat resistant and oxidation resistant coating
The coating includes coatings applied in high temperature process (including oxidation atmosphere, corrosive gas, erosion above 843 ℃ and thermal barrier) and molten metal process (including molten zinc, molten aluminum, molten iron and steel, molten copper).
3. Atmospheric and immersion corrosion resistant coatings
Atmospheric corrosion includes corrosion caused by industrial atmosphere, Salt atmosphere and field atmosphere; Immersion corrosion includes corrosion caused by drinking fresh water, non drinking fresh water, hot fresh water, salt water, chemistry and food processing.
4. Conductive and resistive coatings
The coating is used for conductivity, resistance and shielding.
5. Restore size coating
The coating is used for iron-based (machinable and grindable carbon steel and corrosion-resistant steel) and non-ferrous metal (nickel, cobalt, copper, aluminum, titanium and their alloys) products.
6. Gap control coating for mechanical components
The coating is grindable.
7. Chemical resistant coating
Chemical corrosion includes the corrosion of various acids, alkalis, salts, various inorganic substances and various organic chemical media.
Among the above coating functions, wear-resistant coating, heat-resistant anti-oxidation coating and chemical corrosion-resistant coating are closely related to the production of metallurgical industry.
application
Cemented carbide coating
In cutting, tool performance has a decisive impact on cutting efficiency, precision and surface quality. There is always a contradiction between the two key indexes of cemented carbide tool performance - hardness and strength. The material with high hardness has low strength, and improving the strength is often at the cost of reducing the hardness. In order to solve this contradiction in cemented carbide materials and better improve the cutting performance of cutting tools, a more effective method is to use various coating technologies to coat one or more layers of materials with high hardness and high wear resistance on cemented carbide matrix.
As a chemical and thermal barrier, the coating on the surface of cemented carbide tools reduces the crater wear of cemented carbide tools, which can significantly improve the machining efficiency, improve the machining accuracy, prolong the service life of tools and reduce the machining cost.
The characteristic of the coating is that the coating film is combined with the tool matrix to improve the wear resistance of the tool without reducing the toughness of the matrix, so as to reduce the friction factor between the tool and the workpiece and prolong the service life of the tool. In addition, because the thermal conductivity of the coating itself is much lower than that of the tool matrix and processing materials, it can effectively reduce the heat generated by friction, form a thermal barrier and change the heat loss path, so as to reduce the thermal impact and force impact between the tool and the workpiece, tool and cutting, and effectively improve the service performance of the tool.
The research on tool wear mechanism shows that the maximum temperature of the tool edge can reach 900 ℃ in high-speed cutting. At this time, tool wear is not only mechanical friction wear (tool back wear), but also bonding wear, diffusion wear, friction oxidation wear (tool edge wear and crescent pit wear) and fatigue wear. These five kinds of wear directly affect the service life of the tool.
Tool coating
Tool coating technology can generally be divided into chemical vapor deposition (CVD) technology and physical vapor deposition (PVD) technology, which are reviewed as follows.
1、 Development of CVD technology
Since the 1960s, CVD technology has been widely used in the surface treatment of cemented carbide indexable tools. Because the metal source required for CVD process vapor deposition is relatively easy to prepare, the deposition of single-layer and multi-layer composite coatings such as tin, tic, TiCN, tibn, TiB2 and Al2O3 can be realized. The bonding strength between the coating and the substrate is high, and the film thickness can reach 7 ~ 9 μ m. Therefore, by the middle and late 1980s, 85% of cemented carbide tools in the United States had been treated with surface coating, of which CVD coating accounted for 99%; By the mid-1990s, CVD coated cemented carbide blades still accounted for more than 80% of coated cemented carbide tools. Although CVD coating has good wear resistance, CVD process also has its inherent defects: first, the process treatment temperature is high, which is easy to reduce the bending strength of tool materials; Second, the film is in a state of tensile stress, which is easy to cause microcracks when the tool is used; Third, the exhaust gas and waste liquid discharged by CVD process will cause great environmental pollution, which conflicts with the green manufacturing concept strongly advocated at present. Therefore, since the mid-1990s, the development and application of high-temperature CVD technology have been restricted to a certain extent.
In the late 1980s, Krupp The low temperature chemical vapor deposition (PCVD) technology developed by widia has reached the practical level, and its process temperature has been reduced to 450 ~ 650 ℃, which effectively inhibits η Phase can be used for tin, TiCN and tic coatings of thread cutters, milling cutters and molds, but so far, PCVD process is not widely used in the field of tool coating.
In the mid-1990s, the new technology of medium temperature chemical vapor deposition (mt-cvd) revolutionized the CVD technology. Mt-cvd technology is a new process that uses C / N-containing organic acetonitrile (CH3CN) as the main reaction gas to decompose and chemically react with TiCl4, H2 and N2 at 700 ~ 900 ℃. The coating with dense fibrous crystalline morphology can be obtained by mt-cvd technology, and the coating thickness can reach 8 ~ 10 μ m。 This coating structure has high wear resistance, thermal shock resistance and toughness, and can deposit Al2O3, tin and other materials with good high-temperature oxidation resistance, low affinity with processed materials and good self-lubricating performance on the blade surface through high-temperature chemical vapor deposition (ht-cvd).
Mt-cvd coated blade is suitable for high speed, high temperature, large load and dry cutting, and its service life can be about twice as long as that of ordinary coated blade. At present, CVD (including mt-cvd) technology is mainly used for the surface coating of cemented carbide turning tools. Coated tools are suitable for high-speed rough machining and semi finishing of medium and heavy cutting. It can also be realized by CVD technology α- Al2O3 coating, which is difficult to realize by PVD technology at present, so CVD coating technology still plays a very important role in dry cutting.
2、 Development of PVD Technology
PVD technology appeared in the late 1970s. Because its process treatment temperature can be controlled below 500 ℃, it can be used as the final treatment process for the coating of high-speed steel tools. Because the cutting performance of high-speed steel tools can be greatly improved by using PVD process, this technology has been rapidly popularized since the 1980s. By the end of the 1980s, the proportion of PVD coating of complex high-speed steel tools in industrial developed countries has exceeded 60%.
The successful application of PVD technology in the field of high-speed steel cutting tools has attracted great attention in the manufacturing industry all over the world. While competing to develop high-performance and high reliability coating equipment, people have also conducted more in-depth research on the expansion of its application field, especially in cemented carbide and ceramic cutting tools. The results show that compared with CVD process, PVD process has lower treatment temperature and has no effect on the bending strength of tool material below 600 ℃; The internal stress state of the film is compressive stress, which is more suitable for the coating of cemented carbide precision and complex tools; PVD process has no adverse impact on the environment and is in line with the development direction of modern green manufacturing.
With the advent of the era of high-speed machining, the application proportion of high-speed steel tools has gradually decreased, and the application proportion of cemented carbide tools and ceramic tools has increased, which has become an inevitable trend. Therefore, industrial developed countries have been committed to the research on PVD coating technology of cemented carbide tools since the early 1990s, and have made breakthrough progress by the mid-1990s, PVD coating technology has been widely used in the coating treatment of cemented carbide end milling cutter, drill bit, step drill, oil hole drill, reamer, tap, indexable milling insert, special-shaped cutter, welding cutter and so on.
