The basic principle of thermal spraying is to melt or semi-melt the coating material with special heat source or high temperature, high pressure and high speed gas at a certain temperature, or accelerate the solid particles to several times the speed of sound, spray at a high speed and firmly bond on the surface of the part matrix, forming a continuous coating with set microstructure properties.
Thermal spraying technology has been widely applied in the field of aviation manufacturing technology because of its wide source of materials, stable preparation process, strong designability of coating composition and structure, controllable coating quality, various functions, and protective coatings can be prepared and automated production. Thousands of key parts of aero-engine and aircraft, such as compressor blade tenon, casing, sealing labyrinth seal, combustion chamber, turbine blade, guide blade, journal, bearing seat, sealing ring, nozzle and so on, need to be coated by thermal spraying. The application of coating greatly improves the reliability and service life of aero-engine.
High-temperature abrasion seal coating
As one of the most important coatings for engine components, the high-temperature abrasion seal coating is used to regulate the clearance between the high-pressure turbine rotor components and the casing, which is crucial to maintaining the efficiency of the engine. Once the thickness of the high-temperature abrasion seal coating exceeds 1.5mm, the robot automatic plasma spraying technology must be adopted. Computer closed-loop control of spraying parameters and on-line monitoring of coating thickness is beneficial to coating microstructure and thickness uniformity, and stable coating metallurgical quality.
Thermal barrier coating
The thermal barrier coating is widely used in aero-engines and ground gas turbines to protect high-temperature components of engines, such as combustion chambers, turbine blades, flame nozzles, etc. It can greatly improve the service life of components and the engine efficiency, reduce component temperature and increase the gas temperature.
The preparation methods of thermal barrier coatings include plasma spraying and electron beam physical vapor deposition (PVD). Thermal shock life and thermal conductivity are two key technical indexes of thermal barrier coatings. Without excellent thermal shock resistance, thermal barrier coating cannot be successfully applied to aero-engine with high-reliability requirements. The key to the successful application of thermal spraying coating in commercial aircraft engines is to achieve the coating life of thousands and tens of thousands of hours.
Scientists in the United States developed gadolinium oxide, ytterbium oxide, yttrium oxide ternary rare earth compound stable zirconia, the working temperature of which can be 1500 ℃, and has set up commercialization.
Development trend of thermal spraying technology
After more than 100 years of development, thermal spraying technology is becoming increasingly mature, and its applications cover aerospace, industrial gas turbine, automobile, electric power, fuel cell and solar energy, medical and health care, paper and printing and many other fields.
In order to achieve a significant breakthrough in high thrust-to-weight ratio and high efficiency of aero-engine, the gas temperature in the engine must be increased, which will inevitably result in a substantial increase in the surface temperature of hot end components of high-pressure turbines.
Carbide nitride ceramics such as silicon carbide and silicon nitride are most likely to replace nickel-based superalloys as high-temperature structural materials for engines operating at higher temperatures. The important factor that restricts its application is the insufficient stability of material structure in the engine high-temperature gas environment. Carbide and nitride ceramics can react with water vapor to produce volatile products, which seriously degrade the structure and properties of ceramic materials. The composite technology of vapor deposition and plasma spraying is used to prepare the environmental barrier coating on the ceramic surface, which can effectively prevent the contact between the high-temperature gas atmosphere and the ceramic substrate and improve the structural stability of the ceramic substrate.
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