Electric Field Boosts Plasticity of Ceramic Coatings

Applying an electric field to ceramics gives them metal-like characteristics needed for sustaining heavy loads. Photo by Jaehun Cho, Purdue University.

Researchers with Purdue University (West Lafayette, Indiana, USA) have observed a way that historically brittle ceramic coatings can overcome heavy loads. The researchers say most ceramics fracture when strained unless exposed to high temperatures. Ceramic components also require high temperatures to initially form via sintering, in which a powdered material coalesces into a solid mass. However, ceramic coatings used on industrial assets like metal engine blades must protect cores from many operational temperatures.

This study demonstrates that applying an electric field to yttria-stabilized zirconia (YSZ)—a typical thermal barrier ceramic—makes the material almost as plastic as metal at room temperature. Cracks can be seen sooner as well, since they begin forming at more moderate temperatures.

Historically, the researchers note, YSZ coatings have fractured when an engine heats up and cools down. “In the past, when we applied a high load at lower temperatures, a large number of ceramics would fail catastrophically without warning,” says Xinghang Zhang, professor of materials engineering. “Now we can see the cracks coming, but the material stays together.”

Recent studies show that applying an electric field, or “flash,” accelerates the sintering process, and at lower furnace temperatures. Flash-sintered ceramics have less porosity, which makes them denser and more likely to deform rather than break. Prior studies, however, did not test whether these ceramics could change shape at room temperature or higher. During in situ testing, a flash-sintered YSZ sample thinner than human hair grew increasingly plastic between room temperature and 600 °C when compressed, with cracks slowly spreading at 400 °C. In contrast, conventionally sintered YSZ requires 800 °C and higher to deform.

“Metals can be compressed to 10 or 20 percent strain, no problem, but ceramics often fracture into pieces if you compress them to less than 2 to 3 percent strain,” Zhang says. “We show that flash-sintered ceramics can be compressed to 7 to 10 percent without catastrophic fracture.”

The Purdue-led research is supported by the U.S. Office of Naval Research (Arlington, Virginia, USA), in collaboration with the University of California, Davis (Davis, California, USA) and Rutgers University (New Brunswick, New Jersey, USA). 

Source: Purdue University, www.purdue.edu.