What Is a Phosphate Coating?

Coating is a solid continuous film obtained by one-time coating application. It is a thin layer of plastic coated on a substrate such as metal, fabric, plastic, etc. for protection, insulation, decoration and other purposes. The coating can be gaseous, liquid, or solid. Usually, the type and state of the coating are determined according to the substrate to be sprayed.

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Coating is a solid continuous film obtained by one-time coating application. It is a plastic coated on metal, fabric, plastic and other substrates for protection, insulation, decoration and other purposes.

Different names depending on the type of coating used, such as

According to the classification of thermal spray coatings by FNLONGO in the United States, coatings can be divided into:

1.Abrasion resistant coating

Includes anti-adhesive wear, surface fatigue wear coatings, and erosion resistant coatings. In some cases, the coatings are resistant to low temperature (<538 ° C) wear and high temperature (538 to 843 ° C) wear.

2.Heat-resistant and oxidation-resistant coating

This coating includes high temperature processes (of which

Coated skin coating

It can protect aluminum alloy from wind and sand erosion during high-speed flight, corrosion from seawater and aviation fuel, and can improve aerodynamic performance. The coating should withstand instantaneous temperature changes around 200 ° C and strong sunlight. Aircraft are very bulky and have limited baking conditions. Self-drying coatings, such as acrylic or polyurethane coatings, must be used.

Coating engine coating

The entire engine, from the fan to the tail nozzle, is used without coating. Engine coatings are divided into anti-oxidation and anti-corrosion coatings, heat-resistant coatings, wear-resistant coatings and sealing coatings according to their uses.

Anti-oxidation and anti-corrosion coating: Because of the short operating time of early engines, the high-temperature alloy contains sufficient chromium and can resist oxidation itself, so no coating is applied. However, with the increase of engine life and temperature, and the reduction of chromium content in high-temperature nickel-based alloys to 50% of the original, it can no longer resist high-temperature oxidation and thermal corrosion, and requires coating protection. High temperature oxidation and thermal corrosion are the main causes of turbine blade damage, which can shorten the working life to 300 hours. After coating, the working life of high-temperature parts can be extended by 2 to 3 times. The compressor rotor and stator blades are protected with an aluminum phosphorous (chromate) coating. The combustion chamber can be used with both high-temperature enamel and aluminum-phosphorus (chromate) -containing coatings. Turbine rotors and stator blades often use aluminide diffusion coatings or diffusion barrier coatings with modified elements such as chromium, titanium, silicon, and yttrium. Afterburners use high temperature enamel or ceramic coatings. The service life of the developing metal-chromium-aluminum-yttrium coating is more than double that of the diffusion coating, and the service temperature is up to 1100 ° C. This coating is often used in combination with a zirconia-based thermal insulation coating, which can reduce the temperature by 50 to 100 ° C.

Wear-resistant coating: Another factor affecting engine life is high-temperature wear, including impact wear and micro-vibration wear. Explosive or plasma sprayed tungsten carbide-cobalt, chromium carbide-nickel chromium coatings are most effective. After coating, the wear life of parts can be extended by 7 to 100 times, and it has been widely used in engines of large transport aircraft.

Sealed coating: coated on the clearance part of the airflow passage of the engine. For every 0.13 mm increase in the radial clearance of the turbine, the unit fuel consumption of the engine increases by about 0.5%; conversely, a decrease of 0.25 mm increases the turbine efficiency by 1%. In addition, reducing the radial clearance of the compressor can also improve the anti-surge ability of the engine, thereby improving flight safety. Common seal coatings require moderate hardness, both strength and ease of scraping. Talc powder coatings and nickel-graphite coatings have been applied. The zirconia coating under development can withstand high temperatures of 1300 ° C.

Engine coating

Coating temperature control coating

The thermal environment of the spacecraft in space is very harsh, the temperature on the back side can reach -100 ° C, and the side on the sun side can reach + 120 ° C. In order to ensure the safety of the astronauts and the normal operation of the equipment, applying a temperature-control coating on the surface of the spacecraft can balance the heat exchange with the space and maintain the normal temperature in the cabin. Temperature-controlled coatings that have gained application include silicone zinc oxide, potassium silicate zirconia, and aluminum oxide coatings.

Coated Rocket Engine Coating

Liquid rocket engines generally use regenerative cooling and do not require coating protection, but in order to increase the temperature drop, alumina or zirconia thermal insulation coating is sprayed on the interior of the combustion chamber. Attitude control rocket engines mostly use refractory alloys such as niobium and molybdenum, and must be protected by an anti-oxidation coating to work. The attitude control rocket of the Apollo spacecraft's command and lunar module uses a small molybdenum alloy engine coated with a molybdenum disulfide coating.

Coating camouflage coating

Used to conceal military targets. The detection capability of modern reconnaissance instruments has been greatly improved. Camouflage coatings not only require the color and appearance to be coordinated with the background, but also have spectral reflection performance close to the background. Camouflage coatings are divided into anti-ultraviolet, anti-visible, anti-near-infrared, anti-mid-infrared, anti-radio waves, and anti-multispectral photographic camouflage coatings in development according to the applicable band. The aircraft can be protected by camouflage in monochrome, in order to make the outline more difficult to distinguish in the complex background area, often use deformed camouflage.

Coated textile coating

It is a kind of high molecular compound evenly coated on the surface of fabric. It forms one or more films on the surface of the fabric through adhesion, which can not only improve the appearance and style of the fabric, but also increase the function of the fabric. Special functions such as reflection.

Coated carbide coating

In cutting processing, tool performance has a decisive influence on the efficiency, accuracy and surface quality of cutting processing. There are always contradictions between the two key indicators of the performance of cemented carbide toolshardness and strength. Materials with high hardness have low strength, and the increase in strength often comes at the cost of reduced hardness. In order to solve this contradiction existing in cemented carbide materials and better improve the cutting performance of tools, a more effective method is to use various coating technologies to apply one or more layers of cemented carbide on the cemented carbide substrate. Hard, high abrasion resistant material. The coating on the surface of the cemented carbide tool acts as a chemical barrier and a thermal barrier, which reduces the wear of the crater of the cemented carbide tool, which can significantly improve the machining efficiency, improve the machining accuracy, extend the service life of the tool, and reduce the machining cost.
The coating is characterized by the combination of the coating film and the tool substrate, which improves the wear resistance of the tool without reducing the toughness of the substrate, thereby reducing the friction factor between the tool and the workpiece and extending the tool's service life. In addition, the coating's own thermal conductivity is much lower than that of the tool base and the processing material, which can effectively reduce the heat generated by friction, form a thermal barrier, and change the way of heat dissipation, thereby reducing the Thermal shock and force shock effectively improve the performance of the tool.

The study of tool wear mechanism shows that the cutting edge temperature can reach 900 ° C during high-speed cutting. At this time, the tool wear is not only mechanical friction wear (wear behind the tool), but also bond wear, diffusion wear, and friction oxidation wear (tool blade wear And crater wear) and fatigue wear, these five kinds of wear directly affect the tool life. ^[1]

Coated tool coating

Tool coating technology can generally be divided into two categories: chemical vapor deposition (CVD) technology and physical vapor deposition (PVD) technology, which are reviewed separately below.
1. Development of CVD technology Since the 1960s, CVD technology has been widely used in the surface treatment of cemented carbide indexable tools. Due to the relatively easy preparation of the metal source required for CVD vapor deposition, TiN, TiC, TiCN, TiBN, TiB2, Al2O3 and other single-layer and multi-layer multilayer composite coatings can be deposited. The thickness can reach 7 ~ 9m. Therefore, by the mid-to-late 1980s, 85% of hard alloy tools in the United States had been treated with surface coating, of which CVD coating accounted for 99%; by the mid-1990s, CVD coating Carbide inserts still account for more than 80% of coated carbide tools.
Although the CVD coating has good abrasion resistance, the CVD process also has its inherent defects: first, the high processing temperature can easily cause the bending strength of the tool material to decrease; second, the film is in a tensile stress state, which easily leads to the use of the tool Microcracks sometimes occur. Third, the exhaust gas and waste liquid emitted by the CVD process will cause large environmental pollution, which is inconsistent with the current green manufacturing concept. Therefore, since the mid-1990s, the development and application of high-temperature CVD technology has been limited Constraints.
At the end of the 1980s, the low temperature chemical vapor deposition (PCVD) technology developed by Krupp.Widia reached a practical level. Its process temperature has been reduced to 450 to 650 ° C, which effectively inhibits the generation of phase. It can be used for thread cutting tools and milling cutters. , Mold TiN, TiCN, TiC and other coatings, but so far, the PCVD process is not widely used in the field of tool coating. In the mid-1990s, the emergence of new technologies for medium temperature chemical vapor deposition (MT-CVD) revolutionized 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 ° C to generate TiCN. MT-CVD technology can be used to obtain a dense fibrous crystalline coating, and the coating thickness can reach 8-10 m. This coating structure has extremely high wear resistance, thermal shock resistance and toughness, and can be deposited on the blade surface by high temperature chemical vapor deposition (HT-CVD) process, such as Al2O3, TiN, etc. Materials with low affinity and good self-lubricating properties. MT-CVD coated blades are suitable for use under high speed, high temperature, high load, dry cutting conditions, and their life can be doubled compared to ordinary coated blades. At present, CVD (including MT-CVD) technology is mainly used for surface coating of carbide turning tools. Coated tools are suitable for high-speed roughing and semi-finishing of medium and heavy cutting. -Al2O3 coating can also be achieved by CVD technology, which is currently difficult to achieve by PVD technology. Therefore, CVD coating technology still occupies a very important position in dry cutting processing.

Second, the development of PVD technology PVD technology appeared in the late 1970s, because its process temperature can be controlled below 500 , so it can be used as a final treatment process for the coating of high-speed steel tools. Since the cutting performance of high-speed steel tools can be greatly improved by using the PVD process, this technology has been rapidly promoted since the 1980s. By the end of the 1980s, the proportion of PVD coatings on high-speed steel complex tools in industrial developed countries had exceeded 60%. .
The successful application of PVD technology in the field of high-speed steel tools has attracted great attention from manufacturing countries around the world. While people are racing to develop high-performance, high-reliability coating equipment, they have also expanded their application fields, especially in hard alloy, The application of ceramic tools has been further studied. The results show that compared with the CVD process, the PVD process has a lower processing temperature and has no effect on the bending strength of the tool material below 600 ° C; the internal stress state of the film is compressive stress, which is more suitable for the carbide complex precision tool Coating; 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 cutting, the proportion of high-speed steel tools has gradually decreased, and the proportion of carbide tools and ceramic tools has increased. It has become an inevitable trend. Therefore, industrial developed countries have been working on carbide tools since the early 1990s. The research on PVD coating technology has made breakthrough progress to the mid-1990s. PVD coating technology has been widely used in hard alloy end mills, drills, step drills, oil hole drills, reamers, taps, indexable Coating of milling inserts, special tools, welding tools, etc. ^[1]