February_AMP

ADVANCED MATERIALS & PROCESSES •

FEBRUARY 2014

s part of a research project funded by the National

Science Foundation and Rolls Royce Inc., scien-

tists developed a novel method of measuring

stress and damage in the protective ceramic coatings ap-

plied to turbine engine blades to prolong their life. Blades

are exposed to some of the most intense heat generated by

the engine, and their reliability is crucial to extending en-

gine performance. The thermal barrier coatings that pro-

vide insulation between the cooled blade and the hot gases

in the engine are highly stressed and age over time eventu-

ally resulting in failure.

Maintenance crews currently rely on visual blade in-

spections to judge wear and tear along with a routine main-

tenance schedule based on engine performance data

accumulated over time. If the protective ceramic coating

is severely damaged internally, cracked, or chipped, the tur-

bine blades can be damaged and even melt under the en-

gine’s intense temperatures. Amore quantitative approach

to determine the remaining life of the coating is desired so

that good blades can be returned to service while coatings

near the end of their life can be replaced.

A process that uses laser technology to measure stress

and damage in the ceramic coatings by exciting the bond-

ing material between the coating and metallic blades was

recently refined by UConn researchers. A low power laser

is focused through the coating and the energy from the

laser makes the bonding material fluoresce and that fluo-

rescence is stress-dependent. During the process, the wave-

length of light emitted by the fluorescence can be measured

to determine how strong the bond is, how much viable

coating is left, and whether the blade is damaged and prone

to failure.

Previous lab correlation work indicates that when the

coating is first made, it is very high stress because the bond

between the coating and metal is stronger. As the coating

cracks, the stress becomes less and less, and when it be-

comes too low, it simply flakes off. Determining this stress

is key to predicting remaining life. However, applying this

technique to actual hardware from an engine exposed new

challenges.

Turbine jet engines—because they draw in huge quan-

tities of outside air—are notorious for accumulating a lot of

dust and other contaminants during performance, which

makes testing the stress of engine blade coatings difficult.

The contaminant on the turbine blades is highly irregular

and in some places it is very thick and in others very thin.

Attempting to sand it off would damage the ceramic coat-

ing itself and using chemicals would eat through the coat-

ing as well.

To get around this, a technique was developed that uses

a pulsing laser to blast off the fine layer of contaminants,

without damaging the underlying ceramic coating or

blades. The laser is focused to a sufficient energy density

to ionize the contaminants, ripping them off the surface and

making them glow. Each laser pulse strips away about 1 µ

of material at a time. By monitoring the ionization process

with a spectrometer, the chemical composition of the re-

moved material can be determined and the point at which

all the contaminant is removed can be detected.

After the surface has been cleaned, the stress meas-

urement can be performed. The technique has been de-

veloped into an instrument that can automatically clean

an area on a blade and perform stress maps across the

surface of the blade. It is a unique application of the same

technology used by the Mars Rover to determine the

chemical composition of rocks, and by art experts to re-

store valuable paintings to their original condition if they

have been touched up or altered over time.

Development of an application for the technology to

make it more portable for use while engines are still in

service and attached to a plane is underway.

iTSSe

For more information:

Mike Renfro is UTC associate profes-

sor of engineering innovation and associate department head,

University of Connecticut, 860/486-5934,

renfro@engr.uconn. edu, www.uconn.edu

Method Measures Ceramic Coating

Stress and Damage

on Turbine Blades

Researchers are supplying the aerospace

industry with innovative technologies to

support the next generation of more

efficient and reliable aircraft.

Mike Renfro

Eric Jordan

University of Connecticut, Storrs

Fig. 1 —

An image showing the successful laser ablation of

contaminants from the thermal barrier coating on the surface

of a turbine engine blade. Courtesy of Michael Renfro/

Surface & Coatings Technology.