"If you want diesel or aircraft engines to operate at higher power levels, you have to raise the temperature at which they operate," says Klod Kokini, professor of mechanical engineering at Purdue. "The metals being used to make engine parts do not have the ability to sustain higher temperatures, but ceramic materials do."
Engines and gas-powered turbines generate tremendous heat. If the temperature gets too high, parts such as pistons or turbine blades can crack or warp, bringing the machine to a grinding halt.
Kokini and his graduate students are experimenting with ceramic coatings, which act as a heat shield, that cover only the portion of a metal part exposed to the greatest heat.
"By putting a coating over the part to protect the metal underneath, the temperature can be increased to some extent, so the engine runs at higher power and lasts longer," Kokini says.
The concept of using ceramic coatings in engines is not new, Kokini says. It first was developed in the 1960s by the aerospace industry and later was used by diesel engine manufacturers. However, as the materials engineers began testing new designs, they found that the ceramic coatings would fall off when exposed to high temperatures.
Kokini's research focuses on determining why and how different types of ceramic coatings crack and fail.
"Most people who have worked on this have been materials scientists concentrating on the microstructure of the ceramics," Kokini says. "We take a more mechanical engineering approach to the problem, emphasizing that the whole system needs to be studied and understood. It's more than just applying a coating -- it's understanding how the coating interacts with the metal and the environment of the engine at high temperatures."
Ceramics and metals expand and contract in different ways when they are heated and cooled, Kokini says, resulting in stresses along the edges and the interface where ceramic meets metal. Although ceramics are much more resistant to high temperatures than metals, they are very brittle.
"If the engine is not designed properly using the right combination of materials, pieces of ceramic may indeed crack and fall off in a process called spalling," he explains. "The advantage to using ceramic coatings is, even if a piece falls off, it's not as catastrophic as if the whole part falls apart."
Kokini and his graduate students for six years have been subjecting ceramic-coated metal supplied by engine manufacturers to conditions that simulate the environments inside engines.
In the lab, Kokini and Brian Choules, a doctoral student from West Chicago, Ill., use a high-powered carbon dioxide laser to simulate the conditions inside an aircraft or gas turbine engine. The laser emits an invisible beam of light that can heat a sample of ceramic-coated metal to about 3000 F in less than two seconds. The 1500-watt laser, which also is used for manufacturing research, was purchased with funds from the university and the National Science Foundation.
In addition to constantly monitoring the temperature of the coating with an instrument called an infrared pyrometer, Choules also uses a telescopic microscope to magnify the sample 100 to 150 times. The whole experiment is recorded on videotape, and the images are stored digitally on a computer for closer analysis.
To simulate the cooler conditions inside a diesel engine, Kokini and another doctoral student, Yoshimi R. Takeuchi, from Pittsburgh, work at Purdue's Herrick Laboratories, where they use infrared lamps to heat samples to 1400 F to 1600 F, which takes 30 to 50 seconds. Depending on the experiment, the light generated by the lamps can be so intense that the researchers must wear protective goggles to shield their eyes.
The industries Kokini works with include aluminum companies and diesel engine and aircraft manufacturers. In a recent project, a large diesel engine company was testing experimental coatings of a ceramic called zirconia on pistons and cylinder heads. Purdue researchers worked closely with the company to understand the fracture mechanisms of the coatings in engine and laboratory tests.
As a result of the work, the researchers developed a coating called mullite for diesel engines, and the life of the coating increased from about 40 hours of operation to more than 1,000 hours.
One of the goals of Kokini's research is to develop models of various types of systems that eventually could be used by engine designers to predict where failures may occur.
"Ultimately, we'd like to isolate each type of failure and the conditions that cause it and develop a failure mechanism map," Kokini says. "A designer could look at this map and say 'I expect this engine to be operating at these temperatures, so I can expect this type of failure. Therefore I need to design a coating that will prevent that type of failure.'"
Praxair Surface Technologies, a supplier of coatings to industry, also is very supportive of Kokini's work and is interested in applying his coatings research. Praxair's coatings research and development department in Indianapolis has supplied samples of standard zirconia coatings and others for continued study.
"Ceramic coatings currently are not used in areas where the highest temperatures occur," Kokini says. "But when manufacturers are ready to build the next generation of engines, we hope our work will help make the use of these coatings a reality."
Source: Klod Kokini, (765) 494-5727; e-mail, email@example.com
Writer: Amanda Siegfried, (765) 494-4709; e-mail, firstname.lastname@example.org
Purdue News Service: (765) 494-2096; e-mail, email@example.com
NOTE TO JOURNALISTS: A color photo of Klod Kokini and a graduate student testing ceramic-coated samples in the lab is available from Purdue News Service. It is called Kokini/Ceramics or download here.
Purdue mechanical engineering Professor Klod Kokini and doctoral student Yoshimi Takeuchi use infrared lamps to test the performance of ceramic-coated metal under temperature conditions that simulate the inside of a diesel engine. The researchers are studying how ceramic coatings may be used on engine parts to protect them from extreme heat, increasing their durability. (Purdue News Service Photo by David Umberger)
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