In order to achieve a greater thrust-to-weight ratio for aero engines, the inlet temperature in front of the engine turbine must be increased. Therefore, the requirements for the high temperature resistance of hot-end components such as aero engine combustion chambers and turbine blades must be increased accordingly. Thermal barrier coating on the surface of the base alloy is one of the effective ways to improve its high temperature resistance. The team of Professor Feng Jing from the School of Materials Science and Engineering of Kunming University of Science and Technology is currently studying a new type of ceramic thermal barrier coating material, which is expected to make China's thermal barrier coating technology produce a leapfrog and lead development internationally.
In what ways are thermal barrier coatings important for aero engines? What is the level of domestic research and application of thermal barrier coatings?
Feng Jing: The important technology of aero engine is two-piece one-plate and thermal barrier coating. Thermal barrier coating is one of the four key core technologies. The efficiency of aero engines depends on the temperature. The higher the temperature, the higher the efficiency. However, to increase the operating temperature of the engine, we must consider whether the material is resistant. The current combustion temperature of the engine gas can reach 1500 ~ 1600 ° C. Around 1100 ° C. In the future, the requirements for aero engines will become higher and higher, and the operating temperature may reach 1800 ° C, 2000 ° C or even higher. One of the problems that faced was how to ensure that the material can still operate normally at such a high temperature. At present, the most commonly used material for engines is nickel-based ultra-high-temperature alloys, and the highest temperature in service is about 1100 ° C, and this index is actually difficult to complete, so it is necessary to use a thermal barrier coating to meet its requirements.
The engine blade is mainly cooled by air cooling, but we also hope that it can withstand more heat, then a ceramic thermal barrier coating is required on the surface of the nickel-based superalloy.
The advantage of ceramics is that its melting point and strength are higher than the matrix material, and the thermal conductivity is lower. The low thermal conductivity will cause a temperature gradient between the gas and the substrate. The higher the temperature limit that can be tolerated.
Traditional thermal barrier coating materials generally use zirconia-based ceramics, which can reduce the temperature of the material by 50 ~ 150 ° C under different conditions and thicknesses. The comprehensive performance of zirconia-based ceramics is very good, and it is widely used in civil aviation passenger aircraft and military aircraft. It is very important for the development of aviation aircraft and is an important thermal barrier coating material on aviation engines.
Compared with the international advanced level, the research and application of our domestic thermal barrier coatings still have a big gap. Take the United States as an example. In the 1950s and 1960s, the US Department of Defense and NASA and others began to study the thermal barrier coatings. In the 1960s and 1970s, the types of materials and production processes were basically fixed. In the 1970s and 1980s, Start promoting applications. In China, the application of thermal barrier coatings was mainly after 2000. At that time, we introduced some related equipment from Russia, Ukraine, etc. The institutions that studied thermal barrier coatings earlier were mainly aviation companies and aviation companies under the Chinese aviation industry. Research institutes, and some universities including Beijing University of Aeronautics and Astronautics, have made great progress after more than 10 years of hard work. Now we can apply zirconia-based ceramic materials to blades, turbines, and other key components.
However, coating is a very complicated project. Although it has already begun to be applied, but our experience is insufficient. As a result, the quality and life of China ’s aviation engines are not as good as those of international first-class engine products, and our technology is not mature enough. GE of the United States, Rolls-Royce of the United Kingdom, Mitsubishi of Japan, and Siemens of Germany are the international leaders in the field of coating research. They have basically completed the development of materials 30 years ago, and have accumulated a large number of experience. At present, some coating materials in our country have just been successfully developed and put into use. To reach a mature level, it takes time and experience in product application and continuous improvement.
Compared with other coating materials, what are the advantages of the new high-temperature ferrite phase-change toughened ceramic materials you are studying? What is the current application in aerospace and other fields?
Feng Jing: The advantages are very obvious. I just mentioned that the most widely used zirconia-based materials in the world. A few companies have begun to gradually apply rare earth zirconate materials, but there is a very serious problem with zirconate. The problem is that it has poor fracture toughness, can not work for a long time, and has a relatively short life. In comparison, zirconia-based ceramics are very good high-temperature materials.
So why do we have to develop new ceramic materials and think that zirconia-based materials will eventually be replaced? This is because a zirconia-based material undergoes a phase transition between 1100 and 1200 ° C. Once the material undergoes a phase change, its crystal structure and properties will all change. When used below 1200 ° C, zirconia does have a comparison Good properties, but as the temperature exceeds 1200 ° C and continues to increase, the life of zirconia-based materials will decrease exponentially. In the future, the operating temperature of aircraft engines may reach 2000 ° C, so we must make thermal barrier coatings of 1400 ~ 1500 ° C. Under these conditions, zirconia-based materials can no longer be used.
The highest operating temperature of the new rare earth tantalate high-temperature ferroelastic phase change ceramic material we developed can reach 1600 ° C, or even 1800 ° C. It is a very stable ceramic, which has three advantages compared to zirconia-based materials:
The first is low thermal conductivity, which is half lower than that of zirconia-based materials, which means that when zirconia is reduced by 100 ° C, it can be lowered by 200 ° C, which will produce a large temperature gradient. For the protection of engine blades And other parts work well.
The second is the toughness of ferroelastic transformation. The toughening of zirconia-based materials at high temperatures is due to its iron elasticity, which is a major feature that is superior to other ceramic materials. Other ceramic materials will become brittle at high temperatures. Because it does not have this iron elasticity. We searched for new materials along this line of thought, and found rare earth tantalate ceramics based on the crystal structure of zirconia. After research, we found that rare earth tantalate also has this kind of ferroelastic phase transition, which will form ferroelectric domains at high temperatures During loading and releasing stress, it will not deform immediately like a rubber band, thus acting as a stress buffer, which greatly improves the high-temperature fracture toughness of the material. In other words, it will not become brittle so easily at high temperatures, which greatly increases the life of the material.
The third is that the two have different low thermal conductivity mechanisms. Oxygen vacancy defects in zirconia materials cause phonon scattering, which reduces the process of phonon heat transfer, which is the essence of its low thermal conductivity mechanism. The low thermal conductivity mechanism of rare earth tantalate is an anharmonic effect caused by the relatively large mass of the tantalum atom itself. The low thermal conductivity material formed by oxygen vacancies is a conductor of oxygen ions. Therefore, zirconia material can be used as an electrode of a fuel cell. It is a good conductor of oxygen ions at high temperatures. Oxygen ions can enter and leave the zirconia material freely. The lower layer of the ground layer oxidizes the alloy layer, resulting in a rapid growth of an oxide layer on the surface of the alloy layer. Because the thermal expansion coefficient of this oxide does not match the thermal barrier coating and alloy layer, it is particularly prone to failure. Therefore, the failure of the engine blade coating is not that the thermal barrier coating itself is damaged, but that this oxide causes the coating to Too much stress during thermal cycling, resulting in shedding. The rare earth tantalate material is an insulator of oxygen ions, and the growth rate of the thermal oxide in the alloy layer is more than 1000 times lower than that of the zirconia material.
Rare earth raw materials
In addition, the rare earth tantalate material is softer than zirconia and can withstand more stress. Therefore, the thermal stress at high temperature is much lower than that of zirconia, which makes its life much longer than that of zirconia in thermal cycling. Under the working conditions, rare earth tantalate with the same stress state can also prepare thicker thermal barrier coatings, which can achieve a larger temperature gradient.
The new rare earth tantalate ceramic thermal barrier coating material currently has only one research team around the world, which is my team. Before us, no domestic team paid attention to rare earth tantalate. From material discovery to basic property determination to final application, the entire material system was established by my team. We have published dozens of research papers in this field. Dozens of invention patents have also been applied for. In the field of new rare earth tantalate ceramic coatings, our country has independent intellectual property rights. At present, there are no reports of the application of this material in the world, and it has just begun to be applied in China. It is mainly used for high temperature insulation on space equipment and some equipment. In trials, in the field of aviation, we are also actively working with Beijing University of Aeronautics and Astronautics and research institutes affiliated to the aviation industry. If this excellent material is successfully applied to the field of aerospace, I think that our country's thermal barrier coating Layer technology will lead to leapfrog and lead development in the world.
Aviation Manufacturing Network: In addition to the ceramic coating materials mentioned above, what other results has your team achieved in the field of high temperature materials?
Feng Jing: In the field of high-temperature thermal barrier coatings, the previous world has achieved 2 layers and 3 layers. Through our improvement, we can achieve 8 layers or 10 layers. Our multi-level gradient functional thermal barrier coating materials It not only has the function of high-efficiency heat insulation, but also has the functions of anti-corrosion, anti-wear and long life. Other research includes welding of high-temperature ceramic materials and development of high-temperature alloy materials.
One of our more distinctive studies is the application of rare and precious metal materials in the high-temperature field. For the problem of poor adhesion between high-temperature ceramic coatings and nickel-based ultra-high temperature alloys on engine parts, a kind of adhesion between Bonding layer alloy, the commonly used bonding layer alloy in the world is NiCoCrAlY, but its oxidation speed is very fast after 1100 ℃, it is difficult to use at higher temperatures. We have added Pt to this material, or added a precious metal bonding layer alone, so as to create a higher temperature environment for the use of the bonding layer, this material has begun to be applied. This research is very important in the field of high temperature materials, and it is also a new breakthrough in the world of adhesive layer materials.
The combination of thermal barrier coating materials and 3D printing technology, such as 3D printing technology is very suitable in the process of making the bonding layer alloy. This year, the genetic engineering of rare and precious metal materials in Yunnan Province was started. One of the important tasks is thermal barrier The adhesive layer of the coating is made of rare metal materials. The main method is to prepare a layer of rare metal materials on the surface of the base material by 3D printing / laser forming, so that it can achieve high temperature resistance and oxidation resistance. A major function of genetic engineering of rare and precious metal materials in Yunnan is to serve the aerospace industry in China and develop new alloy materials.
Now the alloy composition is getting more and more complicated. Even with 8- and 10-yuan alloys, people can no longer complete such a huge amount of experimental work. However, through genetic engineering of materials, we will easily find and develop new ones. The bonding layer alloy material can achieve the overall promotion effect of the thermal barrier coating on the aero engine technology progress.
Aviation Manufacturing Network: A major science and technology project for genetic engineering of rare and precious metal materials in Yunnan province was launched in March 2018. As the chief scientist of computing and database, what is the main content of your work? What is the significance of this project for materials research?
Feng Jing: I am mainly responsible for the calculation of rare and precious metal materials and the construction of the database. We are mainly focused on the exploration and development of ultra-high-temperature alloys for aircraft engines. Through the method of material genetic engineering, we can explore 8-element and 10-element alloys that humans could not solve before. Through high-throughput calculations and machine learning, as well as the use of the "elements" of the database, I believe that in the future, firstly, we can obtain high-temperature bonding layer alloys of rare and precious metal materials, and secondly, we are likely to achieve greater success in the field of ultra-high-temperature alloys. Breakthrough, such ultra-high-temperature alloys are expected to achieve direct application in the high temperature environment of 1500 ~ 2000 ℃.
The importance of genetic engineering of rare and precious metal materials is also reflected in the fact that, on the one hand, the workload of our material scientists is greatly reduced. With the help of this high-performance computing tool, hundreds of thousands or even millions of material components and alloy materials can be designed, and Performance prediction; on the other hand, this project can not only greatly speed up the research and development of materials, but also save a lot of research and development costs in the process. Our current goal is to reduce the research and development time by half, that is, the research and development time is reduced by half and the research and development costs are reduced by half. To reduce human and material resources by half. Judging from the project implementation process, I think that the future reduction may be far more than this number, because it can not only design rare metal materials, but also other high temperature materials.