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John O | August 2017

New optical method finds weak spots in jet engine thermal coatings


researchers, led by a team from heriot-watt university in edinburgh, scotland in partnership with roll-royce, have demonstrated for the first time an optical analysis method that reveals weak spots in ceramic thermal barrier coatings that are used to protect jet engine turbines from high temperatures and wear.

 


researchers used a tensile machine to pull a metal specimen with
a ceramic thermal barrier coating sprayed on its surface.
(peter j. schemmel, heriot-watt university)      

 

according to a report from the optical society, this method could be used to predict the lifetime of coatings on airplanes as well as lead to new thermal barriers that could be more efficient and reduce pollution.

 

the article noted, “the lifetime of a thermal barrier coating used on airplane turbine blades can range widely from as little as 1,000 hours up to 10,000 hours at full turbine 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.”

 

the researchers determined that a piece of metal coated with ceramic thermal barrier being pulled in a certain way changes the refractive index, which measures how fast light travels through a material. if researchers can correlate the strain distribution with a coating’s lifetime, then it would be possible to see which coatings will fail first.

 

the key to this new method was using gigahertz (ghz) illumination because the wavelengths travel through opaque materials.

 

“the researchers tested their technique with pieces of metal sprayed with the same ceramic coatings used on rolls royce turbine blades,” the report indicated. “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 coating and bounced off the metal beneath.”

 

it continued, “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 blade.”

 

researchers believe that there could be improvements to the method by using terahertz (thz) illumination to enhance the spatial resolution. they are also using strain measurements of coatings undergoing accelerated aging to determine what the measurements will be at the point of failure.

 

the research was recently published in optics express. the abstract read:

 

“we report the first observation of stress induced birefringence in air plasma sprayed (aps) thermal barrier coatings (tbcs) using a reflection based polariscope and ghz illumination. strain optic coefficients of (−0.0133 ± 0.0102) × 10−9 and (−0.0190 ± 0.0043) × 10−9 were measured for yttria-stabilized zirconia (ysz) aps coatings of the same thickness deposited on substrates of 3 mm and 1 mm mild steel.

 

“the reflection measurement approach was validated by additional measurements of the stress optic coefficient of bulk yttria-partially stabilized zirconia (ytzp) ceramic that were in agreement with previously reported transmission measurements.

 

“the ultimate application of this technique is the prediction of remaining life in tbcs.”

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