Why are Thermal Barrier Coatings Important?
TBC's help protect gas turbine blades from the increasingly harsh operating environment of the military and commercial aircraft engine. Turbine engines are being pushed beyond the use temperature of available metallic systems, generally exceeding 2600 F in engines. The TBC's allow a 300 F improvement in the temperature capability of the engine parts. TBC's have the potential to significantly improve engine performance (through higher engine operating temperatures) and reduce life cycle costs (by reducing the frequency of engine overhauls). However, durability and reliability issues limit the benefits that can be derived from TBC's.
Thermal Barrier Coatings are typically ceramic composites based on zirconia, alumina, and titanium -- relatively thick, brittle coatings attached to metal substrates. Stresses occur at the interface between the ceramic and the bond coat. The adhesion quality of the TBC to the substrate is considered to be one of the limiting factors for use of these materials.
Thermal barrier coatings often need to retain their integrity when subject to thermal cycling through large temperature ranges, despite significant thermal expansion misfit with the substrate. This is achieved by deliberately incorporating either aligned porosity or microcrack arrays. These methods for incorporating compliance also result in low strength and toughness. Yet, the systems have high durability (often remarkably so). Eventually, the coating spalls away and exposes the substrate.
One engineering problem is that the time to failure exhibits large variability. Understanding and reducing this variability would constitute a major benefit to the expanded and more confident implementation of coatings. A thorough understanding of the mechanisms that cause TBC failure is also key to increasing, as well as predicting, TBC durability.
ORNL researchers have significantly improved the accuracy of computed crack stress intensity factors by incorporating the ORNL-developed "modified quarter-point" element (MQP) into boundary integral fracture codes. This improvement in accuracy makes it feasible to reliably model real problems, particularly those where cracks follow curved paths. This new method is currently being applied to failure in thermal barrier coatings (TBC's). details