Thermal Barrier Coatings
Modern engines are starting to reach the limits of performance that metallurgy alone can provide, but new coatings are helping to increase heat tolerance.
The Office of Naval Research has invested in developing a fundamental understanding of thermal and environmental barrier coatings for more than 20 years. Gas turbine engine performance in terms of thrust, specific fuel consumption, and range is directly proportional to the temperature of engine combustion gases. The limitations of conventional nickel-base super-alloys had prevented further increases in these temperatures until the introduction of thermal barrier coatings (TBCs), the first of which were yttria-stabilized zirconia layers approximately 150 nanometers in thickness.
To improve and better predict coating lifetimes and performance, ONR established the first program to develop a scientific understanding of failure mechanisms in TBCs linking laboratory results to actual engine samples. The experimental results were translated into validated computational models and simulations, which elucidated understanding for a variety of mechanistic failure modes in TBCs. These results were provided in the form of maps characterizing the domains within which failure phenomena are most prevalent, and predicted the degradation within each domain, as well as established fundamental principles for the enhancement of performance within each.
A thermal barrier coating developed with the support of the Office of Naval Research is now a standard feature on the F135 engine, which powers the F-35 Lightning II aircraft.
(Photo courtesy of Pratt & Whitney)
A consequence of this understanding has been the development, in a subsequent applied research program, of an improved thermal barrier coating (Gd2Zr2O7) with half the thermal conductivity of the standard yttria-stabilized zirconia, and improved resistance to spallation (i.e, ejection of material after an impact) and attack by molten sand deposits. The coating is so successful, that within four years, it was transitioned, and is now baseline in most Pratt & Whitney engines including the F135 that powers all models of the F-35 Lightning II aircraft. Current research is exploring molten sand attack, which tends to contain variable ratios of calcium, magnesium, alumino-silicates and explore ways to mitigate this attack on TBCs and other propulsion materials. This research is critical to enable further capability and engine efficiency gains in naval aircraft.