Adhesive Bond Used on Columbia Space Shuttle Known to be Less Heat Resistant, Says UB Materials Engineer

BUFFALO, N.Y. -- The adhesive-bonding method used to secure heat-resistant ceramic tiles to the body of the Space Shuttle Columbia is known for its inability to withstand high temperatures, and should not be used exclusively in the construction of new space shuttles, according to a materials engineer at the University at Buffalo.

"Ceramic tiles are brittle, so using both the tiles and the adhesive bond creates a situation that is prone to malfunction and could spell trouble," says Deborah D. Chung, Niagara Mohawk Professor of Materials Research in the UB School of Engineering and Applied Sciences and director of UB's Composite Materials Research Laboratory.

In the case of the Columbia, Chung speculates that cracked or missing tiles would have exposed adhesive silicone bonds to very high temperatures. This would cause the degradation of the bonds and the loss of more tiles -- exposing the shuttle body to extremely high temperatures upon re-entry in the Earth's atmosphere.

"Very often in scientific research we tend not to emphasize issues that are considered mundane or less dazzling technologically, like a bond," Chung says. "But so often it's the seemingly mundane things that kill the project. In the case of the American Airlines Airbus crash in Queens (in 2001) a simple fastener was the culprit; in the Challenger shuttle case it was the O-ring. I think there's a lesson here to learn."

According to Chung, there are other -- more heat-resistant -- ways to connect tiles to the shuttle body. Brazing, which is like soldering except that it involves higher temperatures, creates joints able to withstand temperatures as high as 1000 degrees Fahrenheit, depending on the brazing material used. In contrast, adhesives joints can only withstand much lower temperatures -- around 250 degrees Fahrenheit.

Brazing, however, is a more expensive and involved process than adhesion. This could be a reason that it wasn't used for the Columbia, Chung says.

"But given the tragic outcome of the Columbia mission, having a more involved bonding process should not be out of the question," she adds.

Chung also suggests that NASA consider using a much higher proportion of carbon-carbon composites, in place of aluminum, to construct the shuttle's airframe. Carbon-carbon composite materials, which were in their infancy when the Columbia was built about 20 years ago, are now well-developed. They are much more heat resistant than aluminum and are considered to be the best material available today for high-temperature, lightweight structures, Chung says.

Ideally, according to Chung, future shuttle bodies should be designed as a single "monolithic piece," with gradient change in composition and in function across the thickness of the body. Thermal insulation function could be at one end of the gradient and structural function could be at the other end of the gradient, for example.

"This would replace the need to bond together the different layers of the shuttle's body, and lessen the risk of break up," Chung says.

Chung's lab conducts research on composite materials for aerospace, automotive, construction and electronic applications.