Aircraft manufacturers and their government and industrial customers have designed new aircraft engines that can operate at extremely high temperatures, meaning the engines can generate more energy while burning less fuel.
Haydn Wadley, Edgar Starke Professor of Materials science and engineering at the University of Virginia’s School of Engineering and Applied Science, and Jeroen Deijkers, a postdoctoral research associate in Wadley’s group, have found a way to greatly extend the useful life of these engine materials.
A jet engine draws in large amounts of air that powers an aircraft’s propulsion system when it is compressed and mixed with hydrocarbon fuel and burned in a burner. The hotter the burner, the more efficient the engine.
Aircraft engines now burn at or above 1,500 degrees Celsius, much higher than the melting temperatures typically found in engine parts made of nickel and cobalt alloys. Research has turned to ceramics that can withstand these temperatures, but they have to contend with the chemical reactions created by water vapor and unburned oxygen in extreme combustion environments.
Silicon carbide is the ceramic of choice. Engine parts made of silicon carbide, however, can only last a few thousand hours of flight time. At such high temperatures, carbon reacts with oxygen to form carbon monoxide (gas), while silicon forms silicon dioxide (solid), but silicon dioxide reacts with water vapor to form gaseous silicon hydroxide. In other words, the engine part gradually turns to gas and disappears from the liner.
To protect the ceramic parts, engine manufacturers apply two coats, called environmental barrier coating systems, to the silicon carbide. The outer layer is designed to slow the diffusion of oxygen and water vapor to silicon carbide during flight, while the inner adhesive coating made of silicon protects the surface of silicon carbide by reacting with oxygen to form a thin layer of silicon dioxide. But the design still has challenges.
The life of an engine component usually depends on the time it takes for the thickness of the silica layer to reach a critical point at which stresses caused by expansion and contraction during repeated heating and cooling cause the coating to peel off.
Scientists and engineers have two basic strategies to delay the separation of coatings and extend the life of expensive engine components. They can make the outer coating very thick to slow the rate at which oxygen reaches the adhesive coating, but this adds weight and cost. Or, they can produce a different type of protective oxide that doesn’t “fall off.”
Their solution uses an outer layer of ytterbium disilicate, a rare earth element that has the thermal expansion properties of silicon and silicon carbide and is slow to deliver oxygen and water vapor to the silicon layer. They first deposited a silicon-bonded coating, then placed a thin layer of hafnium oxide between the silicon and ytterbium disilicate outer layers.