While inherently strong, most ceramics used in industrial applications tend to fracture suddenly when just slightly strained under a load unless exposed to high temperatures. Structural ceramic components also require high temperatures during manufacture: Sintering, in which a powdered material coalesces into a solid mass, requires high processing temperatures.
Researchers at Purdue University, West LaFayette, Ind., have identified a way that the brittle nature of ceramics can be overcome as they sustain heavy loads. The research is expected to lead to more resilient structures such as aircraft engine-blade coatings.
The Purdue researchers found that applying an electric field to the formation of yttria-stabilized zirconia (YSZ), a typical thermal barrier ceramic, makes the material almost as plastic (or easily reshaped) as metal at room temperature. Engineers could also see cracks sooner because they start to slowly form at a moderate temperature as opposed to higher temperatures, giving them time to rescue a structure.
“In the past, when we applied a high load at lower temperatures, a large number of ceramics would fail catastrophically without warning,” said Xinghang Zhang, professor of materials engineering. “Now we can see the cracks coming, but the material stays together; this is predictable failure and much safer for the usage of ceramics.”
Recent studies have shown that applying an electric field, or flash, significantly accelerates the sintering process that forms YSZ and other ceramics, and at much lower furnace temperatures than conventional sintering. Flash-sintered ceramics also have very little porosity, note the researchers, which makes them denser and therefore easier to deform. None have yet tested the ability of flash-sintered ceramics to change shape at room temperature or increasingly higher temperatures.
“YSZ is a very typical thermal barrier coating — it basically protects a metal core from heat,” said Haiyan Wang, Purdue’s Basil S. Turner Professor of Engineering. “But it tends to suffer from a lot of fractures when an engine heats up and cools down due to residual stresses.”
What allows metals to be fracture-resistant and easy to change shape is the presence of “defects,” or dislocations – extra planes of atoms that shuffle during deformation to make a material simply deform rather than break under a load. Ceramics normally do not form dislocations unless deformed at very high temperatures. Flash-sintering them, however, introduces these dislocations and creates a smaller grain size in the resulting material.
The research was published in Nature Communications.