In high speed rotating machinery (compressor and gas turbine) form the ideal sealing state between the blade and the shell, in order to obtain the largest hydrodynamic pressure, the abradable coating technology to improve the efficiency, reduce energy consumption and prolong the service life of the machine. In the manufacture of aero engine, the gap between the rotor and the casing can be reduced successfully by the wear coating. The gap between the turbine and compressor control loop, the result is not ideal: friction caused by turbine compressor components or other damage, such as bearing or gear; after the friction surface of abradable coatings are very smooth, and without coating material transfer to the turbine, if the residual surface is not smooth, will have a negative impact on the air the guide, thus affecting the efficiency of the machine, if the abradable material transferred to the turbine, may cause an imbalance, will also affect the performance of compressor. Until now, the jet engine gas path seal problem, has developed a series of abradable coating materials, the abradable coating can not only seal parts to reduce the gap surface for air, and can be used for labyrinth seal to divert cooling air, reduce the loss of compressed air engine, and keep the pressure balance of the rotor shaft.
Except in the early once by flame spraying aluminum coating for abradable coatings, the abradable coatings are composed mostly of two kinds of materials are metal and non metal body filler, filler is weakened overall metal body, can increase wear resistance coating. Mixed powder or agglomerate powder composed of two kinds of materials, which can be used as a coating powder for wear coating.
Adopted or powder flame spraying technology of plasma sprayed abradable coatings in the compressor casing is sprayed on the soft, at the end of the compressor blade spraying hard wear-resistant cobalt coated tungsten carbide coating, the control can be formed between the ideal sealing gap, advanced manufacturing technology is the rapid development of the aerospace sector in 1970s that is one of the important applications of modern thermal spray technology.
1 abradable coatings selection
In practical application, the selection of abradable coatings based on two main points, one is environmental temperature; two is the hardness of the coating requirements. The hardness, the highest temperature and the spraying technology used for wear coating.
Technology and performance of wear coating spraying
2 wear coating preparation
Preparation method mainly includes coating wear flame spraying and plasma spraying two, determination method in practical application the choice of method should be based on the supplier recommended or through the test of powder.
Can wear properties of coatings is not only closely related to the powder composition, but with larger spraying parameters, the spraying process is confirmed, it should be strictly controlled, it is very important to ensure the quality of the coating. The hardness is one of the important parameters that affect the performance of the coating, and it must be strictly controlled.
For hardness test of the specimen can wear coating need special preparation. First of all, the thickness of the coating requirements must meet certain requirements, if the coating is less than 2 mm, the hardness will lead to inaccurate measurements; secondly, the measurement of surface must be flat, it is best to use 60# sandpaper lightly dry silicon carbide grinding, the grinding pressure is too heavy will lead to inaccurate measurement of hardness, surface roughness measurement should be controlled in Ra6-9; third. The hardness of the surface hardness measurement to the standard gauge, 12.7 mm ball and 15 kg load, the hardness readings for HR15Y.
In compressor turbine and for a gear ring can be found in the study of wear coating, five kinds of abradable coatings did not appear due to friction and make the machine load the phenomenon of excessive use. However, with the flame spraying of Ni coated graphite (85-15) abradable coatings will make the turbine impeller edge damage, local discoloration phenomenon, serious damage of turbine; by plasma spraying of AlSi/40wt% polyester, AlSi/50wt% polyester and polyester AlSi-BN/ abradable coating, coating material transfer phenomenon occurs, the coating material is transferred to the turbine wheel rim. And the formation of scar; and by plasma spraying AlSi-BN abradable coatings, when the coating hardness more than 60HR15Y, also occur abradable coating material transfer to the turbine impeller edge phenomenon, and when the hardness is lower than 60HR15Y, the transfer does not occur, and the friction after the outer ring surface of coating is smooth and flawless, conducive to the flow guide compressed air, it will not affect the dynamic balancing machine, can fully meet the compressor turbine impeller and the outer ring between Clearance control requirements.
In preparation of abradable coatings should pay special attention to, every time before spraying parts should be spray trial, and hardness test to test piece, if the hardness of the coating to meet the requirements, the coating can wear performance can meet, then, can the parts in spray. If the coating hardness does not meet the need, the coating process should be properly adjusted. The flame spraying is one of the methods is to maintain and adjust the hardness of gas flow rate, spray distance and moving speed gun under the same conditions change the powder flow rate, reduce the flow rate of powder will increase the hardness of the coating, on the other hand, will reduce the hardness of the coating, successive change powder flow rate and test the hardness of the coating hardness, until qualified; two is to maintain the powder method flow rate, spray distance and moving gun at constant velocity and combustion adjustment of oxygen gas flow rate, while reducing oxygen and combustion gas flow will lead to the hardness of the coating decreased, on the other hand, will increase the hardness of the coating, successive change and test the hardness of the coating hardness, until the qualified.
In the preparation of abradable coatings but also pay attention to strictly control the temperature of the substrate, it is very important to obtain the hardness of the coating is correct, official spraying should be preheated to a temperature of 90-120 DEG C, and in the spraying process to control the substrate temperature does not exceed 180 degrees, the matrix coating hardness increase will lead to overheating.
In addition, the coating can wear coating, it is best to control the spraying rate at about 0.1 mm per pass.
3 wear coating processing
For abradable coatings is concerned, it is a composite material composed of metal or alloy coated core material, the core material is soft, lightweight, non metal particles are fragile and scraping, such as graphite, diatomite, bentonite, six boron nitride and ZrO2, properties. Ground pressure and heat will change the coating processing result, abradable coatings do not use grinding method, and recommend the use of the method of turning. In the turning process, to strictly control the processing parameters, it is as important as controlling the spraying parameters.
Abradable coatings commonly used in turning, rotating speed and slow feed rate, small slow cutting speed. For example, the Metco 6P Metco 310NS flame spraying aluminum coated graphite abradable coatings, recommended the use of tungsten carbide tip for dry machining lathe, lathe line speed control in 2m/s, feed control in 0.06mm/pr, the amount of feed control in 0.25mm/. When turning to ensure that the particles will not be blown out of the coating, and before the use of the surface must be thoroughly cleaned.
