Researchers at Purdue University have found a way to overcome the brittle nature of ceramics as they sustain heavy loads, potentially leading to more resilient structures such as aircraft engine blade coatings and dental implants.
While inherently strong, most ceramics tend to fracture suddenly when 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, in which a powdered material coalesces into a solid mass. These issues are particularly problematic for ceramic coatings of metal engine blades, which are intended to protect metal cores from a range of operational temperatures.
Now, in a paper published in Nature Communications, the researchers demonstrate for the first time 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 as easily reshaped, as metal at room temperature. Engineers could also see cracks sooner since they start to form more slowly at moderate temperatures than 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," explained Xinghang Zhang, professor of materials engineering at Purdue University. "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 allows the process to take place at much lower furnace temperatures. Flash-sintered ceramics also have very little porosity, which makes them denser and therefore easier to deform. But no one had tested the ability of flash-sintered ceramics to change shape at room temperature or higher.
"YSZ is a very typical thermal barrier coating – it basically protects a metal core from heat," said Haiyan Wang, a professor of engineering at Purdue University. "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 change shape easily 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. "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 at Purdue University.
Ceramics normally don't form dislocations unless deformed at very high temperatures. Flash-sintering can, however, introduce these dislocations and create 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," Wang said.
Pre-existing 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. Cracks started to spread slowly at 400°C, whereas conventionally sintered YSZ required temperatures of 800°C and higher to plastically deform.
Improved plasticity means more stability during operation at relatively low temperatures. The sample could also withstand almost as much compression strain 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 2–3% strain," Zhang said. "We show that flash-sintered ceramics can be compressed to 7–10% 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 will be to use these principles to design even more resilient ceramic materials.
To perform the in-situ experiments on a micron-sized ceramic sample, the researchers employed an an in-situ nanomechanical testing tool inside a high-resolution scanning electron microscope equipped with a focused ion beam tool at Purdue's Life Science Microscopy Center. They also used an FEI Talos 200X electron microscope facility in Purdue's Materials Engineering facility. Purdue is expecting an even higher-resolution aberration-corrected microscope that the researchers will soon use for future nanomaterials research.
This story is adapted from material from Purdue University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
