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The perfect combination of thermal spray technology and ceramic coating

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Author : Pei Ze Technology
Update time : 2020-08-03 14:25:28

     Thermal spraying technology is an important part of surface engineering in the field of materials science. It is a surface strengthening and surface modification technology. By spraying a layer of coating on the surface of the metal substrate, the metal has wear resistance, corrosion resistance and high temperature resistance. Oxidation, electrical insulation, heat insulation, radiation protection, anti-wear and sealing properties. Thermal spraying technology is mainly used for the pre-protection of high-temperature, wear-resistant and corrosion-resistant components, the preparation of functional coatings, and the repair of failed components.

     In the thermal spraying process, flame spraying, arc spraying, plasma spraying, explosive spraying and supersonic spraying technology are widely used. Flame spraying is realized by a flame spray gun. The spray gun introduces acetylene, oxygen or compressed air through a gas valve. After the acetylene and oxygen are mixed, a combustion flame is generated at the nozzle outlet. The introduced powder or rod coating material is heated and melted in the flame , Under the action of the flame flow, the mist-like droplets are sprayed onto the surface of the substrate to form a coating. The two wire-shaped materials used in arc spraying are automatically introduced by the wire feed wheel. When a large current passes between the two wire-shaped materials, an arc will be generated. The wire-shaped materials will melt rapidly under the high temperature of the arc and be compressed The air acts as small droplets and is sprayed onto the surface of the substrate to form a coating. Plasma spraying is suitable for powder coating materials. The plasma spray gun converts electrical energy into heat energy to generate a high-temperature and high-speed plasma flame. The plasma flame temperature can be as high as 50,000 ℃, which can melt all spray materials. Explosive spraying uses the energy provided by the ignition and explosion of a mixture of combustible gas and oxygen to spray powder onto the surface of the substrate to form a coating. The supersonic flame spraying method has a high particle impact speed, which improves the bonding strength, hardness, compactness and wear resistance of the coating.
     Most ceramic materials have ionic bond or covalent bond structure, high bond energy, strong interatomic bonding force, and low surface free energy, which endows the ceramic material with high melting point, high stiffness, high chemical stability, high insulation and thermal expansion. Small coefficient, low friction coefficient and other characteristics; but compared with metal materials, its plastic deformation ability is poor, and it is sensitive to stress concentration and cracks. Obviously, using ceramics as mechanical structural materials has worse reliability than metal materials, difficult machining, and high cost. However, the use of thermal spray technology to prepare ceramic coatings on the metal substrate can organically combine the characteristics of the metal material with the characteristics of the ceramic material to obtain a composite material structure. Due to the exceptionally superior comprehensive performance of this composite material structure, thermal spraying technology has rapidly expanded its application from high-end fields to civil industrial fields such as energy, transportation, metallurgy, textile, petrochemical, and machinery.
     The characteristics of ceramic coating technology Compared with the overall structure of ceramic materials, ceramic coating technology has the following characteristics:
1. It can organically combine the strength, toughness, easy processing, etc. of metal materials with the high temperature resistance, wear resistance and corrosion resistance of ceramic materials.
2. Reasonable selection of coating materials and suitable spraying process can obtain various functional surface strengthening coatings.
3. Not limited by the substrate: the substrate material used for thermal spraying can be inorganic materials such as metal, ceramics, cement, refractory materials, stone, gypsum, etc., or organic materials such as plastic, rubber, wood, and paper.
4. It is not restricted by the size of the workpiece and the construction site.
5. The coating deposition rate is fast, the thickness is controllable, and the process is simple. 6. The ceramic coating has good workability, and spraying can be carried out after the coating is damaged.
  The application fields of thermal spray ceramic coating technology are very wide, mainly including:
1. Thermal barrier coating. The key components of aero-engines are superalloy turbine blades and turbine disks. These heated components are exposed to harsh environments such as high temperature, oxidation and high-speed air erosion. For gas turbine components that can withstand temperatures up to 1100°C, which has exceeded the temperature limit (1075°C) used by nickel-based superalloys, the effective solution is to apply a high-melting-point ceramic coating with good thermal insulation, called thermal barrier coating.Thermal barrier coatings are mainly used for heating parts of aviation, naval and land gas turbines, as well as civil internal combustion engines, turbocharged turbines, and oxygen lances for metallurgical industries.
2. High-temperature adhesive and wear-resistant coating: heat treatment furnace rolls, support rolls, sintering furnace rolls and other high-temperature rolls are mostly operated at a high temperature of 800 ℃-1200 ℃. Thermal spraying technology is used to spray special ceramic or cermet coating on the surface of the high temperature furnace roll. The layer has good high temperature resistance, oxidation resistance, anti-adhesion, nodules prevention and self-cleaning purification functions, which can significantly increase the service life of furnace rolls and produce steel with excellent surface smoothness.
3. Wear-resistant and corrosion-resistant coating: The plunger of the high-pressure reciprocating metering pump used in the chemical plant uses sprayed Al2O3-TiO2 composite coating instead of the traditional chrome plating process, and its service life is increased by 6 times. Under low-stress sliding wear and abrasion conditions, almost all original chrome-plated products can be replaced with thermal spray ceramic coatings.
4. Functional coating: Plasma sprayed supersonic sprayed superconducting ceramic coatings have been successfully applied. Superconducting ceramic coatings have shown good application prospects in magnetic shielding, microwave components, various types of transmitters, and quantum electronic devices. Spraying a 30μm BaTiO3 coating on a 0.1mm iron sheet, its dielectric constant exceeds 6000, and has been widely used in ceramic chip capacitors. When the thickness of the Al2O3 coating formed by plasma spraying is less than 1mm, it can withstand a voltage of more than 2500°C at a high temperature of 1300°C, which meets the requirements for high-temperature electrical insulation. Spraying bioactive ceramics such as 50-75μm hydroxyapatite on the titanium alloy substrate provides good chemical compatibility. Therefore, it can be used as an ideal artificial bone material.
     There are a lot of corrosion and abrasion problems in oilfield drilling and oil production engineering. It can be expected that thermal spraying technology and ceramic coating have a wide range of application prospects.