Fake diamonds take the heat

[hari]

A technology that could enable manufacturers to use new kinds of heat-resistant materials in engine blades, so that engines will be able to run hotter and more efficiently has been developed.

Ohio State University engineers are developing the technology to coat jet engine turbine blades with zirconium dioxide — commonly called zirconia, the stuff of synthetic diamonds — to combat high-temperature corrosion.
Outer coating

The zirconia chemically converts sand and other corrosive particles that build up on the blade into a new, protective outer coating. In effect, the surface of the engine blade constantly renews itself.

"In the desert, sand is sucked into the engines during takeoffs and landings, and then you have dust storms," said Nitin Padture, professor of materials science and engineering at Ohio State, who had military aircraft in mind when he started the project. "But even commercial aircraft and power turbines encounter small bits of sand or other particles, and those particles damage turbine blades."

Jet engines operate at thousands of degrees Fahrenheit, and blades in the most advanced engines are coated with a thin layer of temperature-resistant, thermally-insulating ceramic to protect the metal blades.

The coating — referred to as a thermal-barrier coating — is designed like an accordion to expand and contract with the metal.

The problem: When sand hits the hot engine blade it melts — and becomes glass.

"Molten glass is one of the nastiest substances around. It will dissolve anything," Padture said. The hot glass chews into the ceramic coating. But the real damage happens after the engine cools, and the glass solidifies into an inflexible glaze on top of the ceramic.

When the engine heats up again and the metal blades expand, the ceramic coating can't expand, because the glaze has locked it in place. The ceramic breaks off, shortening the life of the engine blades. The key, Padture said, is that the coating contains aluminium and titanium atoms hidden inside zirconia crystals.
Stable crystal

When the glass consumes the zirconia, it also consumes the aluminium and titanium. Once the glass accumulates enough of these elements, it changes from a molten material into a stable crystal, and it stops eating the ceramic.

"The glass literally becomes a new ceramic coating on top of the old one. Then, when new glass comes in, the same thing will happen again. It's like it's constantly renewing the coating on the surface of the turbine," Padture said,

according to an Ohio State University press release.