By Susan Dieterle
The extreme stickiness of a new glue for ceramic composite materials is nothing compared
to the stick-to-itiveness of its inventors.
The group of Ames Laboratory researchers went back to the drawing board repeatedly while trying to develop a glue that would hold together parts made of a rugged class of ceramic materials known as silicon carbide composites.
Undeterred by the setbacks, they now have an easy-to-use glue that produces strong joints without the expensive, high-temperature curing process associated with other types of ceramic glues. They hope that this simple, effective joining method will prompt industry to make greater use of ceramic composites in high-temperature settings.
The composites have properties that, in some applications, make them superior to steel and superalloys because they can withstand higher temperatures, do not melt and are less susceptible to corrosion. That makes them ideal candidates for such products as industrial furnace fans and turbines in hot, corrosive environments.
"If we had turbine engines that allowed fuel to burn hotter and industrial furnaces that could operate at uniform, constant temperatures, we would increase their efficiency immensely," says Iver Anderson, director of the Lab's Metallurgy and Ceramics Program. "Ceramic composites could make that possible and would result in a substantial reduction in energy costs for many industries."
Silicon carbide itself is a brittle material that lacks the strength needed for load-bearing applications. So some industries have turned to a composite in which silicon carbide fibers are woven together like a mat and then encased in a silicon carbide matrix. Just as steel rebar strengthens concrete, the fibers strengthen the matrix material. Silicon carbide composites are considered the most developed of the various types of ceramic composite materials and are closest to large-scale commercialization.
But there has always been a drawback associated with their use -- the lack of a satisfactory way to join parts made of the material. "If you want to make a fan, you have to join blades to a hub and a hub to a shaft," Anderson says. "And nobody had a convenient, robust method for doing that."
The problem has been difficult to solve because, although silicon carbide itself can withstand temperatures of up to 2000 C (3600 F), the fibers in the composite material begin to degrade at 1200 C (2200 F). Traditional ceramic-joining methods don't work well because they involve curing the joints at temperatures of at least 1600 C (2900 F), usually in furnaces with inert atmospheres.
Three years ago, the Department of Energy asked its scientists to come up with possible solutions to the joining problem. Ames Lab Director Tom Barton, whose background is in materials chemistry, participated in phone conferences regarding the issue and then paired up Anderson and materials chemist Sina Maghsoodi to attend a Salt Lake City workshop where it would be discussed.
During a long layover in Denver on the way back, the two scientists came up with a promising concept. What if they used Maghsoodi's silicon-bearing preceramic polymers along with Anderson's aluminum-silicon metal powder? The idea intrigued both the chemist and the metallurgist.
Preceramic polymers have several unique properties, Maghsoodi says. They are easily processed into a variety of shapes and forms and bind easily with metal alloy powders. This would give the glue adhesive properties in its "green state" while allowing it to convert to nanocrystalline ceramics at higher temperatures to form a strong joint, he says.
Anderson notes that the aluminum-silicon powder also had several purposes. "Aluminum readily forms a eutectic alloy with silicon; that is, a relatively ductile alloy that has a melting point lower than either aluminum or silicon alone," Anderson says. In addition to lowering the melting point of silicon, the aluminum in the powder would bond with the oxygen that enters the joint when the polymer was heated in air. The result, they believed, would be a joint filled with small islands of aluminum oxide, making it as strong or stronger than the parts themselves.
But getting their concept to work was simpler in theory than in actual practice.
First, a student couldn't make the long-chained polymers Maghsoodi wanted. The scientist ended up using intermediate-length polymers, called oligomers. Fortunately, the honeylike consistency of the oligomers proved preferable to the stiffness of long-chained polymers.
Then, the scientists determined that the alloy powder had too much aluminum and too little silicon, so they had to change the composition of the metal alloy powder.
"It wasn't working very well at first," Maghsoodi remembers with a rueful laugh.
The two scientists, who continued gathering input and direction from Barton, added another member to the team -- Mohammad Nosrati, a graduate student in ceramics. Nosrati began experiments to work out the kinks in the glue. He added a pinch of boron to the mixture, strengthening the bonds between the glue's components.
When they came up with a compound that worked well at room temperatures, they turned to ceramist Ozer Unal to measure the strength of the glue at high temperatures. The results were disappointing. "The joints had only 10 percent of their room-temperature strength at 1200 C," Maghsoodi says. "So, we had to go back to the drawing board and find a new formulation."
More adjustments followed. In the end, the group had a sticky, grayish paste that could be applied to the joint area with a syringe. From there, the joint can be heated and cured in a regular air atmosphere without clamping pressure. The glue makes a strong joint that can withstand pressures of up to 14,500 pounds per square inch, the equivalent of 100 megapascals.
The source for heating the joints can be something as simple and portable as a propane torch, as Nosrati discovered one evening. "Mohammad was working in a lab on campus late one night and needed to use a furnace that heats to 1500 C, but it wasn't working," Maghsoodi recounts. "He checked it and saw that the heating element was broken."
So, Nosrati smeared some of the glue on the broken ends of the heating element, heated it with a propane torch and then placed it back in the furnace. "It worked," Maghsoodi says, noting that he shows the heating element to interested companies to demonstrate how easily the glue can be used.
The glue, which already has two patents with a third in the works, is attracting attention from companies throughout the world. One of those is Amercom, A Synterials Company, a Virginia-based enterprise that conducts research and development work on ceramic-matrix and metal-matrix materials.
Bill Bustamante, Amercom's vice president of business development, is interested in the glue largely because it is air-stable and can be easily used in the field. "That's especially attractive for some of the military applications and for the aerospace industry," he says.
Some of the ceramic composite materials are used in the hot sections of gas turbine engines, Bustamante says. Because the engines are big enough to walk around in, the ceramic parts can be damaged through a worker's carelessness. "The possibility of using this glue to patch or repair a damaged component is very, very attractive," he says.
Maghsoodi has also been approached by companies wanting to know if the glue can be adapted for use with materials other than silicon carbide composites. "We're working with them on small projects to try to help them solve their problems," he says. "So what started as a project for DOE has now increased 100 times through involvement with companies trying to solve their own problems."
The level of interest in the glue makes the scientists glad that, despite the initial setbacks, they stuck with the work.
For more information:
Sina Maghsoodi, (515) 294-1110, ijadi@ameslab.gov
Iver Anderson, (515) 294-4446, andersoni@ameslab.gov
Current research funded by:
DOE Office of Basic Energy Sciences
Last revision: 12/17/99 sd
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