Scientists study the oxidation reaction of ytterbium silicide (a heat-resistant coating) to improve the thermal efficiency of aircraft gas turbine engines
Image: The oxidation process of ytterbium silicide largely depends on the amount of air in the environment. Scanning electron microscope images and X-ray diffraction peaks prove this. See more
Image source: Ryo Inoue, Tokyo University of Science
Certain parts of aviation gas turbine engines that are widely used in aircraft often reach temperatures above 1,200 °C. There is no doubt that any material used in such a harsh environment must be durable and competent for the task. Ceramic matrix composites made of silicon carbide (SiC) have recently attracted interest as promising candidates for gas turbine engines. However, these materials require a heat-resistant coating to prevent the oxidation of SiC and subsequent evaporation of SiO2, which is a process that leads to a reduction in the volume of the material, so large cracks or peeling of structural defects such as the top layer will occur.
Unfortunately, existing coatings cannot completely prevent oxidation to SiO2 because oxygen can penetrate through microscopic cracks in these layers or through simple diffusion.
To solve this problem, some scientists have focused on using Ytterbium silicide (Yb-Si) as a coating material, because Yb-Si can reach a high melting point, and their oxides are mainly Yb-silicates, which remain as oxide layers. Adheres and will not evaporate easily. However, little is known about the basic phenomena that occur in these materials at high temperatures in air or water vapor environments.
In a recent study published in the journal "Intermetallic Compounds," Ryo Inoue, Junior Associate Professor of Tokyo University of Science, Assistant Professors Yutaro Arai and Professor Yasuo Ogo, and Senior Researcher Takuya Aoki of the Japan Aerospace Exploration Agency (JAXA) The team of scientists included set up to understand the oxidation mechanism of Yb-Si. They conducted various experiments to gain insight into the oxidation behavior (and degradation) of different Yb-Si coatings in three atmospheres at high temperatures: air, water vapor, and a mixture of the two.
Through X-ray diffraction analysis, energy dispersive spectroscopy, and scanning electron microscopy, scientists can accurately visualize and quantify the morphology and composition of Yb-Si samples before and after the heat exposure test. One of the main findings is that the ratio of Yb to Si is the main factor determining the oxidation behavior of the material; due to the preferential oxidation of Yb in the silicide, the degree of oxidation of Yb5Si3 is higher than that of Yb3Si5. In addition, in a more water vapor-rich atmosphere, the amount of oxides is significantly reduced.
Most importantly, the researchers explored the mechanism by which ytterbium content affects the formation of SiO2. "After thermally exposing the two silicides to steam, we found SiO2 in Yb5Si3, but in fact Si is still present in Yb3Si5," commented Dr. Inoue, who led the research. "Our analysis shows that the growth of SiO2 in Yb3Si5 is inhibited because SiO2 participates in the reaction to form Yb-silicate and is the limiting factor in the reaction to form Yb-silicate," he added. Although the exact intermediate reactions leading to the formation of various Yb silicates are not fully understood, the team proposed two very possible reaction pathways. This may be clarified through future research and more detailed characterization techniques. Overall, this research provides meaningful insights into what happens during the Yb-Si oxidation process, which will help develop protective coatings for aviation gas turbine engines. "If a coating that can withstand harsher environments can be achieved, engine components will become more heat-resistant, which will naturally lead to higher engine efficiency," commented Dr. Inoue. It is hoped that further advancements in coating technology will reduce air transportation costs and fuel consumption, make flying cheaper and less harmful to the environment. ### About Tokyo University of Science Tokyo University of Science (TUS) is a well-known and respected university and the largest private research university in science in Japan. It has four campuses in central Tokyo and its suburbs and Hokkaido. Founded in 1881, the university has continuously contributed to the development of science in Japan by instilling a love of science in researchers, technicians, and educators. With the mission of "creating science and technology for the harmonious development of nature, mankind and society", TusHoldings has carried out extensive research from basic science to applied science. TUS adopts multidisciplinary research methods and conducts in-depth research in some of the most important fields today. TUS is an elite system in which leaders in the field of science are recognized and cultivated. It is the only private university in Japan that produces Nobel Prize winners, and the only private university in Asia that produces Nobel Prize winners in the field of natural sciences. Website: https://www.tus.ac.jp/en/mediarelations/ About Tokyo University of Science Junior Associate Professor Dr. Ryo Inoue Inoue received his Ph.D. from Tokyo University in 2014 and worked there as a project researcher for one year. Joined Tokyo University of Science in 2015 as an assistant professor in the Department of Materials Science and Technology. He now leads the Inoue Laboratory as a junior associate professor in the Department of Mechanical Engineering, where he develops and researches composite materials for automobiles, airplanes, and research. He has published more than 30 peer-reviewed articles and is a member of the Ceramic Society of Japan and the Society of Mechanical Engineers of Japan. Journal of Intermetallic Compounds DOI 10.1016/j.intermet.2020.106992
Most importantly, the researchers explored the mechanism by which ytterbium content affects the formation of SiO2. "After thermally exposing the two silicides to steam, we found SiO2 in Yb5Si3, but in fact Si is still present in Yb3Si5," commented Dr. Inoue, who led the research. "Our analysis shows that the growth of SiO2 in Yb3Si5 is inhibited because SiO2 participates in the reaction to form Yb-silicate and is the limiting factor in the reaction to form Yb-silicate," he added. Although the exact intermediate reactions leading to the formation of various Yb silicates are not fully understood, the team proposed two very possible reaction pathways. This may be clarified through future research and more detailed characterization techniques.
Overall, this research provides meaningful insights into what happens during the Yb-Si oxidation process, which will help develop protective coatings for aviation gas turbine engines. "If a coating that can withstand harsher environments can be achieved, engine components will become more heat-resistant, which will naturally lead to higher engine efficiency," commented Dr. Inoue.
It is hoped that further advancements in coating technology will reduce air transportation costs and fuel consumption, make flying cheaper and less harmful to the environment.
About Tokyo University of Science
Tokyo University of Science (TUS) is a well-known and respected university and Japan's largest private research university for science. It has four campuses in central Tokyo and its suburbs and Hokkaido. Founded in 1881, the university has continuously contributed to the development of science in Japan by instilling a love of science in researchers, technicians, and educators.
With the mission of "creating science and technology for the harmonious development of nature, mankind and society", TusHoldings has carried out extensive research from basic science to applied science. TUS adopts multidisciplinary research methods and conducts in-depth research in some of the most important fields today. TUS is an elite system in which leaders in the field of science are recognized and cultivated. It is the only private university in Japan that produces Nobel Prize winners, and the only private university in Asia that produces Nobel Prize winners in the field of natural sciences.
Website: https://www.tus.ac.jp/en/mediarelations/
About Ryo Inoue, Junior Associate Professor, Tokyo University of Science
Dr. Ryo Inoue received his Ph.D. from Tokyo University in 2014, where he worked as a project researcher for one year. Joined Tokyo University of Science in 2015 as an assistant professor in the Department of Materials Science and Technology. He now leads the Inoue Laboratory as a junior associate professor in the Department of Mechanical Engineering, where he develops and researches composite materials for automobiles, airplanes, and research. He has published more than 30 peer-reviewed articles and is a member of the Ceramic Society of Japan and the Society of Mechanical Engineers of Japan.
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Copyright © 2021 American Association for the Advancement of Science (AAAS)