The role of reactive elements in thermal barrier coatings

The thermal barrier coating (TBC) of jet engine turbines permits operation at what otherwise would be prohibitively high temperatures. Ideally, a jet engine should operate at very high temperatures to maximize fuel efficiency and power. Combustion gases are often held at temperatures above 1370° C, while engine metal superalloys have melting points that range between 1230° and 1315° C. As a result, engine turbine blades must be cooled by flowing air through holes drilled in the blade or be protected from the hot combustion gases by some form of thermal barrier. Cooling results in a rather minimal gain in allowable combustion gas inlet temperature; such a technique’s effectiveness is counterbalanced by a decrease in engine efficiency. Therefore, it is important to develop a ceramic coating to act as a thermal barrier that increases the operational lifetime of jet engines and permits higher operating temperatures, thereby increasing thrust and fuel efficiency.1 The TBC of choice, a thin yttria-stabilized-zirconia (YSZ) coating, reduces the temperature to which the underlying superalloy is exposed by hundreds of degrees Celsius. A TBC consists of three primary layers that cover the macroscopic engine superalloy: the YSZ thermal protective layer, the thermally grown oxide (TGO) formed through bond coat oxidation, and the metal alloy bond coat composed of Ni (Co) CrA1Y. Figure 1 shows a schematic representation of a TBC’s cross section. Our interface studies provide insight into atomic-level culprits in observed TBC failure in the form of spallation, a process where the TBC peels off the underlying substrate. Improving the bond coat performance might provide the most fruitful avenue for limiting TBC spallation. Accordingly, we explore the effect of chemical modifications to the bond coat alloy on ceramic/metal interface adhesion and discuss implications for macroscopic systems based on our atomic-level DFT calculations.