“Hotter engines are more efficient,” said Elizabeth J. Opila, professor and chair of the Department of Materials Science and Engineering at UVA and a lead researcher on the project.
Turbine engines are known for aircraft propulsion, but stationary turbines have many industrial uses, including power generation. They burn fuel to rotate turbine blades, converting mechanical energy to electricity.
“You get more work output per heat input at higher temperatures,” Opila said. “The potential benefits drive interest in coatings that act as a barrier against the reactive gases produced by combustion at these high temperatures that can damage turbine blades.”
Efficiency translates to less fuel consumption and reduced emissions and operating costs — which helps account for why the U.S. Department of Energy’s ARPA-E ULTIMATE program funded the team’s work. They published their findings in the October print issue of Scripta Materialia.
Two primary material systems are used in the hot section of turbine engines today:
- Coated nickel-based superalloys can tolerate up to about 2,200°F — well short of the DOE’s goal of nearly 3,300°F.
- Ceramic composites use several coating layers to protect against degradation from oxidation, a chemical reaction that occurs with exposure to air and moisture. However, these systems are limited by the melting temperature of one layer, silicon, which melts at 2,577°F.
The UVA-led team focused on another material option called refractory metal alloys. Refractory metals were studied extensively in the 1960s. While durable and heat-resistant, they were abandoned due to poor oxidation resistance.
To protect the alloy, the researchers experimented with rare earth oxides — chemical compounds that naturally possess strong protective properties — to come up with one do-it-all coating.
“By combining multiple rare earth oxides, tailoring properties to better protect the underlying substrate can be achieved with just a single layer,” said Kristyn Ardrey, a Ph.D. alumna of Opila’s lab and first author of the paper. “This allowed us to achieve better performance without complex multi-layer coatings.”
Opila’s lab created and tested new combinations of rare earth elements, such as yttrium, erbium and ytterbium. To predict the best combinations and improve performance, they worked with UVA associate professors Bi-Cheng Zhou and Prasanna Balachandran, whose labs specialize in computer simulations and machine learning, a form of artificial intelligence.
The team applied the coatings to alloys using two standard manufacturing methods. One technique heats the material to a molten state before spraying on the surface. The other is applied as a liquid mixture that dries and hardens. The researchers tested and compared how well each method performed under extreme heat and reactive conditions, such as exposure to high-temperature steam.
They also partnered with UVA Professor Patrick Hopkins' ExSiTE Lab, which specializes in using lasers to measure heat resistance and material strength.
“This was a collaborative effort,” Opila said. “Using machine learning and computational methods allowed us to explore a huge range of possible material combinations, and Patrick’s lab was key to understanding the physical characteristics of the materials we developed.”