Explosive spraying is to inject the powder into the spray gun and introduce oxygen acetylene mixture to ignite the ignition gas to produce 3300 ℃ high temperature. The powder is heated and sprayed on the surface of the workpiece at more than twice the speed of sound to form a diameter of ~ 20 mm and a diameter of ~ 8 mm μ M thick coating with high bonding strength and high density. The whole coating is formed by overlapping small circular flakes.
(1) When 0.25mn thick WC is sprayed on the damping table of ferroalloy fan blade of aeroengine, the service life is increased by 10 times
(2) A layer of Cr3C2 with thickness of 0.12mm is sprayed on the locating clasp of 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 coating.
Supersonic flame spraying (HVOF) is to introduce fuel gas (hydrogen, propane, propylene or MAPP) and combustion aids into the combustion chamber in a certain proportion for mixing and explosive combustion. The high temperature gas enters the copper nozzle through four inclined nozzles in the upper combustion head of the combustion chamber, and the powder is quantitatively sent into the high temperature gas along the carbide central sleeve in the combustion head by the powder feeding gas (AR or N2), The nozzle is brought out by high temperature and high speed gas, the coating is formed on the workpiece by high speed nozzle, and the whole spray gun is cooled by circulating water.
Characteristics of thermal spraying technology: the application of thermal spraying technology has developed from the preparation of decorative coatings to the preparation of various functional coatings, such as wear-resistant, anti-corrosion, oxidation-resistant, heat insulation, electrical conductivity, insulation, friction reduction, lubrication, radiation protection and so on. Thermal spraying can be used for both repair and manufacture. Because the performance of the coating material is better than that of the substrate, the product quality can be greatly improved by coating the surface of the parts with the coating material.
Selection of coating materials:
(1) It is better for the thermal spraying material to have a wide liquid region, which can keep the droplet of the coating material in liquid phase for a long time.
(2) Good thermal stability, no sublimation and no decomposition in high temperature flame. If the material is easy to decompose at the temperature of thermal spraying, it is not suitable for thermal spraying.
(3) The spraying material should have good wettability with the substrate in the molten state, so as to ensure a good bonding performance between the coating and the substrate.
(4) It has the similar coefficient of thermal expansion with the base material, so as to prevent the larger thermal stress due to the large difference of the coefficient of thermal expansion.
Coating formation process:
(1) The spray material is heated to a molten state.
(2) When the spray material is atomized into tiny droplets and impinges on the surface of the substrate at high speed, the greater the kinetic energy of the particles impinging on the substrate and the greater the impact deformation, the better the coating bonding.
(3) The molten 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 coating, which is widely used in various fields of national economy. The porous layered structure of the coating makes its wear-resistant service behavior significantly different from that of the same kind of block. The most important feature is the influence of load. Under low stress service conditions, the thermal spraying coating shows good wear resistance, but when the stress is higher than a certain level, due to the accelerated propagation of cracks along the interface of deposited particles, the wear is accelerated, especially under the conditions of erosion, cavitation, fretting, fatigue and abrasive wear, the potential of the coating material can only play 10% - 30%. Therefore, it is an important way to make full use of the material potential, improve the service efficiency of thermal spraying coating and save resources and energy to establish the basic research on the systematic relationship between the coating structure, performance, service conditions and service efficiency, and to establish the database to guide the design and use of wear-resistant coating.
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