Thermal spraying technology is actually to use a heat source to melt the coating material to the substrate material that needs to be processed. Due to the wide range of coating materials, fast deposition speed, high flexibility, easy automated processing, strong adaptability and other advantages, so The technology has been widely used.
Statistics show that more than 5,000 parts are required for thermal spraying of an advanced aeroengine abroad. Thermal barrier coating has become one of the key technologies for the manufacture of high-performance aeroengines. In addition, according to foreign According to the statistical report, thermal spray ceramic coatings account for two-thirds of the North American high-performance ceramic preparation market, which is basically consistent with domestic reports that the output value generated by thermal spray is about one-third of all output values of surface technology. , Indicating that thermal spraying technology has become one of the important surface engineering technologies.
The thermal energy is used to melt the sprayed material, and then it is atomized by the high-speed airflow, and the particles hit the surface of the substrate under the high-speed airflow. After condensation, a coating with a certain function is formed, that is, thermal spraying technology.
The technology of thermal spraying was born in different eras with different methods and uses:
1910: Wire flame spraying technology; wire flame spraying is widely used in the preparation of corrosion-resistant and wear-resistant coatings on the surfaces of mechanical parts, chemical containers and rollers. After making appropriate changes to the nozzle part of the spray gun, it can also be used to spray plastic powder. 1920s: Arc wire spraying technology, the characteristics are as follows:
(1) Spray special and aluminum coatings on steel components to provide long-term protection for the components.
(2) Spraying aluminum on steel parts can prevent high temperature oxidation.
(3) Spray stainless steel or other wear-resistant metals on steel parts for anti-corrosion protection.
(4) Coated with carbon steel, bronze and other materials on mechanical parts for repairing parts.
(5) Spray shielding coating on plastic products, etc.
(6) Use arc spray aluminum or spray casting to produce composite copper plate.
50s explosive spraying and plasma spraying technology: Plasma arc flame flow temperature is high, suitable for spraying high melting point materials. The coating density can reach 85-98%, the bonding strength can reach 35-70Mpa, and the spraying quality is much better than the flame spraying layer. Mainly used:
(1) Wear-resistant and wear-reducing coating;
(2) Corrosion-resistant coating;
(3) High temperature oxidation resistance, high temperature air flow erosion resistance, thermal barrier coating;
(4) Manufacture of metal and ceramic high-burning point composite materials.
Explosive spraying:
Inject the powder into the spray gun, introduce the oxygen-acetylene mixture gas at the same time, ignite the detonation gas to produce a high temperature of 3300 ℃, the powder is heated and sprayed onto the surface of the workpiece at a speed of more than 2 times the speed of sound, forming a high bonding strength and high of ~ 20mm diameter and ~ 8μm Density coating. The entire coating is formed by overlapping small round sheets.
(1) 0.25mn thick WC is sprayed on the damping table of aero-alloy iron alloy fan blades, and the service life is increased by 10 times:
(2) A layer of 0.12mm thick Cr3C2 is sprayed on the positioning snap ring of the combustion chamber, and the service life is increased by 7 times.
(3) At present, explosive spraying is mainly used to prepare high-quality wear-resistant coatings.
1960s: Self-fusible alloy powder flame, plasma spraying and spray welding technology
1980s: Supersonic flame spraying technology
Supersonic flame spraying
Fuel gas (hydrogen, propane, propylene or MAPP) and oxidant are introduced into the combustion chamber at a certain ratio for mixing and explosive combustion. The high-temperature gas enters the copper nozzle through the four inclined nozzles in the combustion head above the combustion chamber, and the powder The powder feed gas (Ar or N2) is fed into the high-temperature gas quantitatively along the carbide central sleeve in the combustion head. The high-temperature high-speed gas takes out the nozzle, and the high-speed nozzle forms a coating on the workpiece. The entire spray gun is cooled by circulating water.
90s: Laser cladding technology
The thermal spraying technology in China started in the 1950s. At that time, Wu Jianchun and Zhang Guanbao established the first domestic professional spraying plant in Shanghai, developed oxygen-oxygen block flame wire spraying and electrospraying devices, and developed metal spraying business externally. China's thermal spraying technology started early, and wire arc spraying was developed in the 1950s; some military departments began to study plasma spraying in the 1960s: spraying equipment and materials with more complete varieties and models appeared in the 1970s, but overall progress Slowly, only in recent years has it achieved rapid development.
The characteristics of thermal spraying technology: thermal spraying technology has developed from the preparation of decorative coatings to the preparation of various functional coatings, such as wear resistance, corrosion resistance, oxidation resistance, thermal insulation, electrical conductivity, insulation, friction reduction, lubrication , Anti-radiation and other coatings. Thermal spraying can be used for both repair and manufacturing. Because the performance of the coating material is better than that of the substrate, coating the surface of the part with it can greatly improve the quality of the product.
Choice of coating materials:
(1) The thermal spraying material preferably has a wider liquid phase region. The wider liquid phase region can keep the droplets of the coating material in the liquid phase for a longer period of time.
(2) Good thermal stability, no sublimation or decomposition in high temperature flame. If the material is easily decomposed at the thermal spraying temperature, it is not suitable for thermal spraying.
(3) The sprayed material should have good wettability with the substrate in the molten state to ensure a good bonding performance between the coating and the substrate.
(4) The thermal expansion coefficient is similar to that of the base material, so as to prevent the thermal expansion due to the difference of the expansion coefficient being too large.
Coating formation process:
(1) The sprayed material is heated to a molten state.
(2) The sprayed material is atomized into tiny droplets and hits the surface of the substrate at high speed. The larger the particle kinetic energy and the larger the impact deformation of the substrate, the better the coating combination.
(3) The melted high-speed particles deform after impacting the surface of the substrate and condense to form a coating.
Thermal spraying has become an important method for the preparation of wear-resistant coatings and is widely used in various national economic fields. The porous layer structure of the coating makes its wear-resistant service behavior significantly different from similar blocks. The most important feature is the load influences. Under low-stress service conditions, thermal spray coatings exhibit good wear resistance, but when the stress is above a certain level, accelerated wear and tear due to the accelerated expansion of cracks along the interface of the deposited particles, especially in erosion and cavitation , Fretting, fatigue, abrasive wear conditions, making the potential of the coating material can only play 10% ~ 30%. Therefore, to establish a basic study on the systematic relationship between coating organization, performance, service conditions, and service effectiveness, and to establish a database to guide the design and use of wear-resistant coatings, will be to fully exploit the potential of materials and improve the service of thermal spray coatings An important way to achieve efficiency and save resources and energy.
