Scientists Explore Heat-resistant Coating to Improve Aircraft Gas Turbine Engines

TAGS:  Aerospace Coatings      Industrial Coatings    

Scientists investigate oxidative reactions in ytterbium silicide, a heat-resistant coating, to improve heat efficiency in aircraft gas turbine engines.

Ytterbium Silicide (Yb-Si) - Coating Material for High-temperature Sections

Ytterbium silicide (Yb-Si) is a promising coating material for the high-temperature sections of aircraft gas turbine engines. Although Yb-Si is heat-resistant and prevents formation of structurally harmful SiO₂ in the coating, its oxidation mechanisms are unclear.

In a recent study, scientists from Japan demonstrate how the Yb to Si ratio in the material, and the surrounding atmosphere, affect the oxidation processes in Yb-Si, opening doors to more energy efficient gas turbines.

Certain sections of aero gas-turbine engines, which are widely used in aircrafts, regularly reach temperatures above 1,200° C.

However, these materials require a heat-resistant coating layer to prevent the oxidation of SiC and subsequent evaporation of SiO₂, which is a process that leads to a decrease in the material volume and, therefore, structural defects such as large cracks or the topmost layer flaking off.

Unfortunately, existing coating layers cannot fully prevent this oxidization to SiO₂ because oxygen can permeate through microscopic cracks in these layers or by simple diffusion.

Yb-Si with High-melting Points

To address this issue, some scientists have focused on using ytterbium silicide (Yb-Si) as a coating material because Yb-Si can reach high melting points and their oxides are mainly Yb-silicates, which remain attached as an oxide layer and do not evaporate easily. However, not much is known about the fundamental phenomena that take place in these materials at high temperatures in either air or water vapor environments.

Through X-ray diffraction analysis, energy dispersive spectroscopy, and scanning electron microscopy, the scientists were able to accurately visualize and quantify the morphology and composition of the Yb-Si samples before and after the heat exposure tests.

One of the main findings was that the Yb to Si ratio was a major player in defining the oxidation behavior of the material; Yb5Si3 oxidized more than Yb3Si5 because of the preferential oxidation of Yb in silicide. Moreover, the amount of oxide decreased considerably in more water vapor-rich atmospheres.

Most importantly, the researchers explored the mechanisms by which ytterbium content can affect the formation of SiO₂. "After heat exposure of both silicides in steam, we found SiO₂ in Yb5Si3, whereas Si was actually still present in Yb3Si5," remarks Dr Inoue, who led the study.

"Our analysis indicate that SiO₂ growth is suppressed in Yb3Si5 because SiO₂ partakes in, and is the limiting factor of, reactions that form Yb-silicates," he adds. Though the exact intermediate reactions that lead to the formation of the various Yb-silicates are not completely understood yet, the team presented two highly possible reaction pathways. This will likely be clarified through future studies with even more detailed characterization techniques.

Overall, this study provides meaningful insight into what happens during the oxidation of Yb-Si, which will help in the development of protective coatings for aero gas-turbine engines. "If a coating that can withstand harsher environments can be realized, engine parts will become more heat resistant, which naturally leads to higher engine efficiency," remarks Dr Inoue.

Hopefully, further advances in coating technology will reduce aerial transportation costs and fuel consumption, making flying cheaper and less harmful to the environment.

Source: Tokyo University of Science