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Thermal spray technology features and knowledge sharing of coating function classification

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Author : Pei Ze Technology
Update time : 2019-10-14 10:25:20
  Thermal spraying technology has a long history. The origins can be traced back to the beginning of this century. Earlier, people used only molten metal to compress to form a liquid stream, and then sprayed the surface of the object to be sprayed to form a membranous structure. The difficulty of the core of the technology lies in the spraying temperature, the droplets and the impact velocity on the surface of the sprayed object. Throughout the history of the entire thermal spray technology, the three major points are taken as the starting point.
  Temperature and speed depend on different heat sources and equipment structures. In a sense, the higher the temperature and the higher the speed, the better the formation of an excellent coating, which leads to the competition and coordination of the two factors of temperature and speed in the whole process of technology development. The selectivity of a wide range of spray materials is another advantage of thermal spray technology, which allows the work of different equipment to be “point iron into gold, wear helmet and armor”. It is these three elements, as well as many other controllable factors that make thermal spraying a truly unique technology with superposition effects, which can be designed to meet the wide variety of modified surfaces required for general mechanical maintenance. Until a wide range of applications in high-tech fields such as aerospace and bioengineering.
First, spraying materials
  Spray materials include powder, wire, tape and rod. Their composition is metal, alloy, ceramic, cermet and plastic. Powder materials play an important role in more than 100 types. Wire and strip are mostly metal or alloy (composite wire still contains ceramic or plastic); there are only a dozen bars, mostly oxide ceramics.
Second, the spraying method
  In order to provide different heat sources, it can be divided into combustion method and electrothermal method. The former includes combustion flame spraying, explosive spraying and high-speed flame spraying (HVOF); the latter includes arc spraying and plasma spraying (also divided into atmospheric pressure plasma spraying, low pressure plasma spraying and water stable plasma spraying). The spraying process has a significant effect on the coating temperature (in the strict sense, the temperature at which the droplets impact the surface of the substrate) and the velocity at which the droplets impact the surface of the substrate.
Third, the characteristics of thermal spraying technology
  1. The base material is not limited, it can be metal and non-metal, and can be sprayed on various base materials;
  2. Sprayable coating materials are extremely extensive, thermal spraying technology can be used to spray almost all solid engineering materials, such as cemented carbide, ceramics, metals, graphite, etc.;
  3. The temperature rise of the base material during spraying is small, and no stress or deformation occurs;
  4. The operation process is flexible and convenient, and is not restricted by the shape of the workpiece, and the construction is convenient;
  5. The coating thickness can be from 0.01 to a few millimeters;
  6. The coating properties are various and can form coatings with various special functions such as abrasion resistance, corrosion resistance, heat insulation, oxidation resistance, insulation, electrical conductivity and radiation protection;
   7. Advantages such as strong adaptability and good economic returns.
Fourth, the formation and evaluation of the coating
  After the spray material is sprayed through a spray apparatus having a form of heat source, its flight time is only a few thousandths of a second or less before reaching the surface of the substrate being sprayed. In such a short period of time, it is heated, melted or semi-melted to form fine, dispersed droplets that rush toward the surface of the substrate and are struck into flat, flat pieces that were previously covered by the latecomers. Soon, a cover layer made up of many flat ridges is formed, which is a sprayed layer. The higher the heat source temperature, the greater the droplet impact velocity and the denser the coating formed. Coating performance is related to many factors.
Fifth, the coating function
  When an engineering problem is raised, it is first necessary to clearly determine the working conditions of the part where the workpiece is to be sprayed (usually the working surface of the equipment or facility), and the coating function is to determine the main spraying process and materials. Based on the consideration, it is also necessary to consider whether it is economically allowed.
Coatings can be divided into functions according to their functions:
  1. Abrasion resistant coating. These include anti-adhesive wear, surface fatigue wear coatings and erosion resistant coatings. Some of these conditions are also resistant to low temperature (<538oC) wear and high temperature (538 ~ 843oC) wear coating.
  2. Heat resistant oxidation resistant coating. The coating includes high temperature processes (including oxidizing atmosphere, corrosive gases, erosion and thermal barriers above 843oC) and molten metal processes (including molten zinc, molten aluminum, molten iron and steel, molten copper). Coating.
  3. Resistance to atmospheric and immersion corrosion coatings. Atmospheric corrosion includes corrosion caused by industrial atmosphere, salty atmosphere, field atmosphere, etc. Impregnation corrosion includes corrosion caused by drinking fresh water, non-drinking fresh water, hot fresh water, salt water, chemical and food processing.
   4. Conductivity and resistance coating. This coating is used for conductance, resistance and shielding.
  5. Restore the size coating. This coating is used in iron-based (cuttable and grindable carbon steels and corrosion resistant steels) and non-ferrous metals (nickel, diamond, copper, aluminum, titanium and their alloys).
   6. Mechanical component clearance control coating. This coating can be ground.
  7. Chemical resistant coating. Chemical corrosion includes corrosion of various acids, bases, salts, various inorganic substances, and various organic chemical media.
  Among the above coating functions, closely related to metallurgical production are wear resistant coatings, heat resistant oxidation resistant coatings and chemical resistant coatings.