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[July 2005]
Thermograph Maps Coating Failure
A novel technique based on infrared imaging is being developed at NPL (Teddington, Middx, England) to detect early signs of spallation. One example involves studying the integrity of a high temperature ceramic coating.
The intense heat inside gas turbine engines, used in aircraft and power stations, is sufficient to melt the blades of the engine so that in addition to air-cooling, a thermally insulating coating is also applied. The Thermal Barrier Coating is about 200 micron of zirconia, the application of which allows the engine to operate around 150C higher than without it, thus making the engine significantly more efficient and reducing both operating costs and perhaps even more importantly emission of greenhouse gases.
Unfortunately, over time, metal oxidation causes the insulation to fall off (spallation), exposing the bare blade to temperatures far exceeding the safety limits, leading to potentially catastrophic failure.
The 'thermograph' at NPL revealed the onset of failure before it actually happened. In the early stages of failure, the thermal coating begins to separate from the underlying material creating a small delamination. The presence of the flaw beneath the surface changes the local thermal conductivity leading to a small temperature change of about 0.5C.
The technique is almost instantaneous and holds promise for a portable non-destructive test that will be invaluable to maintenance engineers.
For more information on the thermographic technique, contact John Nunn at NPL at: john.nunn@npl.co.uk
ENDS