“This coating has greater potential than we thought; we need to develop it and put it in an actual engine, to move it further along the path toward commercialization.”Jeroen Deijkers, University of Virginia
As a way to reduce commercial aviation's greenhouse gas emissions, airplane manufacturers and their customers in government and industry have invested in the design of new aircraft engines that function at extremely high temperatures, allowing them to generate more energy while burning less fuel. However, these very high temperatures can be a problem for the materials used to make the engine.
Haydn Wadley, professor of materials science and engineering at the University of Virginia (UVA) School of Engineering and Applied Science, and Jeroen Deijkers, a postdoctoral research associate in Wadley’s group, have now found a way to greatly extend the life of the materials used in these jet engines. They report their advance in a paper in Acta Materialia.
“A jet engine gulps huge quantities of air, which, when compressed and mixed with hydrocarbon fuel and burned in a combustor, powers the plane’s propulsion system. The hotter the combustor, the more efficient the engine,” Wadley explained.
Combustion in airplane engines now reaches or exceeds 1500°C, well above the melting temperatures of engine parts typically made of nickel and cobalt alloys. Research has thus turned to ceramics that can withstand these temperatures, but the ceramics must also contend with chemical reactions from the water vapor and unburnt oxygen in the extreme combustion environment.
Silicon carbide is the ceramic of choice. However, engine parts made of silicon carbide would last only a few thousand hours of flight time. At such high temperatures, the carbon reacts with oxygen to form carbon monoxide (a gas), while the silicon forms silica (a solid), which then reacts with water vapor to form a gaseous silicon hydroxide. In other words, the engine part progressively turns into gas and disappears out the tail pipe.
To protect the ceramic parts, engine manufacturers apply a two-layer coating, called an environmental barrier coating system, to the silicon carbide. The outer layer is designed to slow the spread of oxygen and water vapor toward the silicon carbide during flight, while an inner bond coat made of silicon protects the silicon carbide’s surface by reacting with the oxygen to form a thin layer of silica. But there are still challenges to this design.
“The life of the engine component is often dictated by the time it takes for the silica layer thickness to reach a critical point where the stress caused by expansion and contraction during repeated heating and cooling causes the coating to pop off,” Wadley said.
Scientists and engineers have two basic strategies to delay the coating’s separation and extend the life of expensive engine components. They can make the outer coating layer very thick to slow down the arrival of oxygen at the bond coat, but that adds weight and cost. Or they can create a different kind of protective oxide, one that does not 'pop off'. Deijkers and Wadley decided to pursue the second strategy.
Their solution employs an outer layer of ytterbium disilicate, a rare earth element that shares the thermal expansion characteristics of silicon and silicon carbide and is slow to transport oxygen and water vapor toward the silicon layer. They first deposit the silicon bond coat on the silicon carbide and then place a thin layer of hafnium oxide, or hafnia, between the silicon and the ytterbium disilicate outer layer.
Their experimental studies show that as the silica forms on the silicon, it immediately reacts with the hafnia to form a silicon-hafnium oxide, or hafnon. The hafnon’s thermal expansion and contraction is the same as the rest of the coating and so will never cause the coating to pop off or crack. Wadley calls it adding a little “hafnia fairy dust”.
“When we deposit a very thin layer of hafnia on top of silicon, followed by a layer of ytterbium disilicate, the oxygen that passes through the ytterbium disilicate creates a chemical reaction with the underlying materials to form the hafnon,” Deijkers said.
Deijkers’ access to unique equipment in Wadley’s lab, specifically a directed vapor deposition system, enabled this breakthrough in environmental barrier coatings. The ability to deposit a film of ytterbium disilicate that is thinner than the diameter of a human hair is key to their success.
The directed vapor deposition process uses a powerful 10-kilowatt focused electron beam to melt the hafnia in a low-pressure chamber. A supersonic jet of gas transports the resulting vapor to the silicon-coated silicon carbide where it condenses, creating a thin film. Deijkers and Wadley then use a plasma spray method to deposit the final ytterbium disilicate layer, and the coated component is ready for testing.
Deijkers successfully defended his dissertation in October 2020, combining his interests in aircraft and high-temperature materials for his PhD research, and following his father’s path into materials science and engineering. “My dad used to work on dredging ships. Seeing the pump house glowing orange-white in the furnace, that’s how I caught the engineering bug,” Deijkers said.
Deijkers, who is from the Netherlands, combined these early memories with his interest in serving in the Dutch Air Force, earning a bachelor’s and master’s degree in aerospace engineering from Delft University of Technology.
When Deijkers began applying to PhD programs in the US, his master’s thesis on thermal barrier coatings captured Wadley’s attention. Deijkers’ arrival was well timed. Group member Brad Richards, who earned his PhD in materials science and engineering from UVA in 2015, had developed the silicon-ytterbium disilicate coating system for ceramics that was subsequently found to be very similar to that being used by the makers of aircraft engines.
Deijkers’ dissertation improves Richards’ coating system, deepening understanding of the surface chemistry involved and increasing the coating system’s viability for commercial adoption.
“One set of questions driving my research focused on how long it takes for the hafnon to form through the oxidation process,” Deijkers said. “I wanted to understand how this process really works, and whether we could actually put it to use.
“This coating has greater potential than we thought; we need to develop it and put it in an actual engine, to move it further along the path toward commercialization.”
Today’s methods are rooted in deposition techniques developed in the 1970s. “Compared to the state-of-the-art in industry, our research makes a major improvement,” Deijkers said. “My rough estimate, if industrial manufacturers were able to implement these newer processing techniques, they could extend the engine parts’ lifetime by as much as 200 times. But there are a lot of hurdles to jump through to get that level of performance.”
Wadley’s research group made these advancements with the support of the US Office of Naval Research, which awarded Wadley’s team two successive grants over a period of six years.
“The problems we have to solve are multi-disciplinary and multi-institutional,” Wadley said. “We need to fuse together knowledge from mechanics, chemistry and materials science in order to make progress. Beyond the immediate need to reduce CO2 emitted by propulsion technology, our research supports the global shift from carbon-containing hydrocarbons to hydrogen fuels and the eventual electrification of air travel platforms.”
Whereas Deijkers hopes to attract private industry to the team’s coatings system and deposition process, his career ambition is to pursue scientific discoveries at a national laboratory or in academia.
“The nation has an urgent need for talent in this arena,” Wadley said. “We are in desperate need for bright, creative people who want to be trained to solve these kinds of problems for society going forward.”
This story is adapted from material from the University of Virginia School of Engineering and Applied Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.