SAND BLASTED: Thermal-barrier aircraft coatings that must withstand extreme and varying temperatures are expected to last thousands of takeoffs and landings, and, in industrial gas-turbine engines, must endure up to 30,000 hours of operation. Image: Courtesy of U.S. Department of Defense
Hard to believe, but a grain of sand—or rather millions of them traveling at high velocity—can have a devastating effect on aircraft and industrial gas-turbine engines. The granules eat into the zirconium dioxide ceramic thermal-barrier coatings that insulate and protect engine components from extremely high temperatures. In an effort to protect these coatings and ensure that turbine engines continue to operate properly, a team of Ohio State University engineers is testing a new formulation of zirconium dioxide, more commonly known as zirconia.
Zirconia, which has gained notoriety outside the scientific community (because its chemical cousin is the cubic zirconia of fake diamond fame), has for decades been used to make ceramic coatings for turbines because it is highly insulating. This gives jet engines the ability to run at high temperatures—thousands of degrees Fahrenheit. The downside: at these higher temperatures, sand and other particles drawn into the turbines stick to the ceramic, turn to molten glass, and erode the protective coating, which is about 250 microns (0.01 inch) thick, says Nitin Padture, a professor of materials science and engineering at The Ohio State University.
After the engine cools, the glass solidifies into an inflexible glaze on the ceramic surface. When the engine heats up again and the metal blades expand, the ceramic coating cannot expand because the glaze has locked it in place. The ceramic thereby breaks off, shortening the life of the engine blades.
The solution, Padture and his team report in a recent issue of Acta Materialia, is a new coating method that changes the composition of the zirconia so that the molten glass cannot eat through and damage the turbines. "Our innovation is a new way of making the coating by adding aluminum and titanium to the zirconia crystals," says Padture, who has been studying thermal-barrier coatings for more than a decade. The aluminum and titanium turns molten glass into a stable crystal that poses no danger to the underlying ceramic surface.
"The next step is to see if we can actually deposit this on turbine blades, which have a complicated shape," he says. The University of Connecticut, where Padture began his research, has applied for a patent on his method for strengthening zirconia, and he plans to work with Inframat Corporation, a Farmington, Conn., nanotech company, to further develop the technology. If their tweaked version of zirconia flies, the airline industry and military will be able to design engines that run hotter, and as such, burn fuel more efficiently and create less pollution.