The model accurately predicts how well certain coatings will work before they are created, providing a powerful design tool that promises to save time and money, said Klod Kokini, a professor of mechanical engineering and an assistant dean of the Schools of Engineering at Purdue.
Findings will be presented Tuesday (6/25) during the 14th U.S. National Congress of Theoretical and Applied Mechanics in Blacksburg, Va. A research paper also appeared earlier this year in the journal Mechanics of Time-Dependent Materials.
Thermal-barrier coatings are used primarily for turbine engines in jet aircraft, but researchers are trying to develop coatings for diesel engines in cars and trucks. The coatings are a mixture of ceramic and metal that are applied to engine parts in a process known as plasma spray technology. Powdered metal and ceramic are heated with a torch and then sprayed onto metal parts.
"It's like spray painting, except that it occurs at very high temperatures," Kokini said. "The coatings extend an engine's lifetime, and they enable you to run engines at higher temperatures, which improves performance."
The coatings are "graded," meaning they are applied in layers that have varying concentrations of metal and ceramic. The outermost layer contains mostly heat-resistant ceramic, and underlying layers contain increasingly higher concentrations of metal. Over time, as parts are subjected to the high operating temperatures inside an engine, the coating deforms. Then, after the engine is turned off and begins to cool down, cracks form in the coating, eventually causing pieces to break off.
Some coatings resist cracking better than others. But the only way for engineers to rate the performance of different coating mixtures and various layer combinations has been to actually create and test them in a laboratory.
"We know the behavior of the metal alone, and we know the behavior of the ceramic alone," Kokini said. "But what happens when we mix them in say, 20 percent metal and 80 percent ceramic, or 50 percent ceramic and 50 percent metal?
"We now have a comprehensive computational model that allows a designer to say, 'I'm mixing this and this together, what kind of properties can I expect?' It's a very powerful tool. No one had ever developed a methodology to actually do these predictions before."
The model, which has been shown to be more than 90 percent accurate, promises to save time and money by ruling out mixtures that will not work well. Kokini developed the simulation with doctoral student Sudarshan Rangaraj.
Their findings will be detailed in a paper to be presented during the conference at Virginia Polytechnic Institute and State University. The conference is sponsored by the United States National Committee on Theoretical and Applied Mechanics, the National Academy of Sciences and the National Academy of Engineering.
In a related matter, Kokini's research team also has uncovered important details about how cracks affect a coating's durability.
Certain types of cracks cause pieces of the coating to break loose from the metal and fall off, exposing portions of a metal part to extremely hot, potentially damaging temperatures.
But certain, so-called "segmented cracks" are actually beneficial because they help to prevent the coating from falling off. The bad cracks are ones that form parallel to the metal and are located deep beneath the coating's surface, at the point where the coating meets the metal. However, beneficial cracks form on a coating's surface and are perpendicular to the metal. These surface cracks increase the durability of a coating by delaying the formation of bad, parallel cracks, Kokini said.
Manufacturers deliberately create coatings that contain the beneficial perpendicular cracks.
"But no one has ever understood and explained how having these segmented cracks helps, in terms of what they do to the coating and why they prolong the life of the coating," Kokini said.
Purdue researchers, including doctoral student Bin Zhou, have used a high-power laser to heat coatings and study the behavior of cracks and durability of coatings, depending on how many segmented cracks exist and how deep the cracks are.
In findings detailed during a meeting of the American Ceramic Society in April, Purdue researchers showed that the length of beneficial cracks is critical to how effectively they slow the formation of harmful cracks. The engineers have found that segmented cracks half as thick as the coating are more effective than more shallow cracks that extend to only 15 percent of a coating's thickness.
Future research will focus on determining more precisely how cracks of varying depths improve the performance of thermal-barrier coatings. Findings will be presented during an upcoming American Ceramic Society conference in January.
The research has been funded by the National Science Foundation and the Purdue Research Foundation, in collaboration with Caterpillar Inc. and Praxair Surface Technologies Inc.
Time-Dependent Behavior of Ceramic (Zirconia)-Metal (NiCoCrAIY) Particulate Composites
Sudarshan Rangaraj and Klod Kokini
School of Mechanical Engineering, Purdue University
The estimation of time-dependent properties for ceramic-metal mixtures is important for analyzing the effects of thermal loading on compositionally graded thermal barrier coatings. A mean field micromechanics approach is developed to compute effective time-dependent properties of ceramic (zirconia)-metal (NiCoCrAIY) particulate composite materials. These properties are then used to predict the transient thermal stresses in these composites when they are subjected to thermal shock. These thermal stresses include the effects of time-dependent material response.
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