The brittle nature of ceramics can be overcome as they sustain heavy loads, leading to more resilient dental implants, aircraft engine blade coatings, and other structures, according to a team of researchers led by Purdue University.
Though inherently strong, most ceramics tend to fracture suddenly when just slightly strained under a load unless exposed to high temperatures. Structural ceramic components also require high temperatures to form in the first place through a lengthy process called sintering, where a powdered material coalesces into a solid mass.
Applying an electric field to the formation of yttria-stabilized zirconia (YSZ) makes the material almost as plastic, or easily reshaped, as metal at room temperature. Engineers also could see cracks sooner since they start to slowly form at a moderate temperature as opposed to higher temperatures, giving them time to rescue a structure.
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, allows metals to resist fractures and change shape easily.
“These dislocations will move under compression or tension, such that the material doesn’t fail,” said Jaehun Cho, a graduate research assistant in materials engineering with Purdue University Materials Engineering.
Ceramics normally don’t 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.
“Smaller grains, such as nanocrystalline grains, may slide as the ceramic material deforms, helping it to deform better,” said Haiyan Wang, PhD, MS, Purdue’s Basil S. Turner Professor of Engineering.
Preexisting dislocations and small grain sizes enabled a flash-sintered YSZ sample thinner than human hair to grow increasingly plastic between room temperature and 600°C when compressed, with cracks starting to slowly spread at 400°C as opposed to conventionally sintered YSZ that requires 800°C and higher to plastically deform.
Improved plasticity means more stability during operation at relatively low temperatures. The sample also could withstand almost as much compression as some metals do before cracks started to appear.
“Metals can be compressed to 10% or 20% strain, no problem, but ceramics often fracture into pieces if you compress them to less than 2% to 3% strain,” said Xinghan Zhang, PhD, MS, professor of materials engineering at Purdue. “We show that flash-sintered ceramics can be compressed to 7% to 10% without catastrophic fracture.”
Even when the sample did begin to crack, the cracks formed very slowly and did not result in complete collapse, as would typically happen with conventional ceramics. The next steps would be using these principles to design even more resilient ceramic materials.
“In the past, when we applied a high load at lower temperatures, a large number of ceramics would fail catastrophically without warning,” said Zhang. “Now we can see the cracks coming, but the material stays together. This is predictable failure and much safer for the usage of ceramics.”
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, making them more dense and easier to deform. None have yet tested the ability of flash-sintered ceramics to change shape at room temperature or increasingly higher temperatures.
The study, “High Temperature Deformability of Ductile Flash Sintered Ceramics Via In-Situ Compression,” was published by Nature Communications.
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