New optical method pinpoints weak spots in jet engine thermal coatings

August 9, 2017, Optical Society of America
Researchers used a tensile machine to pull a metal specimen with a ceramic thermal barrier coating sprayed on its surface. With a polariscope, they could measure changes in refractive index resulting from this applied strain. Some of the components of the GHz polariscope are seen on either side of the tensile machine. Credit: Peter J. Schemmel, Heriot-Watt University

Researchers have demonstrated, for the first time, that an optical analysis method can reveal weak areas in ceramic thermal barrier coatings that protect jet engine turbines from high temperatures and wear. The technique could be used to predict how long coatings would last on an airplane and might eventually lead to new thermal barrier coatings, making engines more efficient and cutting both the cost and pollution of air travel.

The lifetime of a thermal barrier coating used on airplane can range widely from as little as 1,000 hours up to 10,000 hours at full thrust, even when the coating is applied in the exact same way. Because the lifetime is unpredictable and failure during flight could be catastrophic, turbine blades are scheduled for replacement based on the shortest estimated lifetime.

"Our strain-measurement can analyze the coatings immediately after manufacturing and work to identify the turbine blades that would last the longest in the airplane," said leader of the research team, Andrew J. Moore, of Heriot-Watt University, UK. "Ultimately, we want to develop an imaging device that would show the strain distribution in the coating of an entire turbine blade, information that would be used to decide if that turbine blade would go into service."

In The Optical Society journal Optics Express, the researchers demonstrated that changes in , a measure of how fast light travels through a material, could be observed when a piece of metal coated with a ceramic thermal barrier coating was pulled in a controlled manner. Moore's research team is collaborating with Rolls-Royce, a leading manufacturer of jet engines.

"If we can correlate how the strain distribution is related to the coating's lifetime, then we could determine which coatings will fail first and shouldn't be put into an aircraft and which ones will last much longer," said Moore. "This would increase the time between services significantly, which would bring huge savings."

The new technique could also be used to predict the lifetimes of coatings developed to be more reliable or tolerate higher temperatures, which allows engines to run more efficiently. It might also find use in automotive and nuclear power applications where ceramics are also used as thermal barriers.

Seeing through opaque materials

Using gigahertz (GHz) illumination was key to the new technique because these wavelengths can travel though some opaque materials, such as ceramics, allowing analysis from within the material. Visible wavelengths, on the other hand, can only be used for surface analysis of .

The researchers tested their technique with pieces of metal sprayed with the same ceramic coatings used on Rolls Royce turbine blades. They put the pieces into a tensile machine that applied strain by slowly pulling the metal. Researchers then applied GHz illumination (280-380 GHz) during the process, which traveled through the ceramic and bounced off the metal beneath. The reflected light was then measured using a polariscope to determine how the refractive index of the ceramic changed with the applied strain. Although the team's current optical setup only acquires point-based measurements, the researchers say the technique could be used with an imaging setup to analyze an entire .

"With the GHz illumination we were able to see changes in the refractive index with applied strain," said Moore. "This shows that our approach could be applied for quality assurance in the future."

The researchers recently started experimenting with using higher frequency illumination in the terahertz (THz) range, which could improve the technique's spatial resolution. In collaboration with Cranfield University, UK, they are also using their technique to make strain measurements of ceramic-coated metal samples that undergo accelerated aging. "We will be looking to see when the coatings fail and then correlating that with GHz and THz measurements we took prior to the aging process," said Moore. "This is a step toward using our technique to identify which coatings fail first."

Explore further: New engine coatings promise operational efficiencies and longer life

More information: Peter Schemmel et al, Measurement of direct strain optic coefficient of YSZ thermal barrier coatings at GHz frequencies, Optics Express (2017). DOI: 10.1364/OE.25.019968

Related Stories

Fake diamonds help jet engines take the heat

March 17, 2008

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

Faster, better, safer jet engines

July 17, 2013

From Igor Sikorsky's first American helicopter to Pratt & Whitney's latest engine for the nation's new Joint Strike Fighter jet, Connecticut has a proud history of aviation science innovation.

Recommended for you

On the rebound

January 22, 2018

Our bodies have a remarkable ability to heal from broken ankles or dislocated wrists. Now, a new study has shown that some nanoparticles can also "self-heal" after experiencing intense strain, once that strain is removed.

Nanoparticle gel controls twisted light with magnetism

January 22, 2018

"Help me, Obi Wan Kenobi. You're my only hope." For many of those around at the release of Star Wars in 1977, that scene was a first introduction to holograms—a real technology that had been around for roughly 15 years.

Information engine operates with nearly perfect efficiency

January 19, 2018

Physicists have experimentally demonstrated an information engine—a device that converts information into work—with an efficiency that exceeds the conventional second law of thermodynamics. Instead, the engine's efficiency ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

swordsman
not rated yet Aug 10, 2017
Excellent concept provides a measure of the physical relaxation of molecules utilizing electrooptic methodology. A+.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.