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A significant portion of the college’s annual research activity is conducted in collaboration with the Department of Energy’s Ames Laboratory and the research centers of the university’s Institute for Physical Research and Technology (IPRT), four of which are directed by engineering faculty. Other partnerships cut across campus boundaries to involve the College of Business, the College of Education, and others.

Here’s a look at how some of our partnerships have helped us leverage resources to accomplish greater research output.

As high-temperature mechanical systems, such as advanced turbine engines, are pushed towards higher performance standards, they are also subjected to increasingly severe operating environments. Thermal barrier coatings (TBCs) coupled with advanced cooling schemes allow the current families of nickel-based superalloys to meet the materials needs of today’s turbine engine systems. However, as materials researchers anticipate future improvements in gas turbine engines, such as higher operating efficiencies, longer operating lifetimes, and reduced emissions, they are looking to develop new or improved structural materials with inherently higher temperature performance capabilities that can be successfully incorporated into the next generation of TBC systems.

In two separate three-year Office of Naval Research (ONR) projects totaling over $700,000 in funding, MSE Associate Professor Brian Gleeson and Ames Lab Scientist and MSE Adjunct Assistant Professor Dan Sordelet are collaborating to investigate methods to fabricate or modify existing coating systems that can significantly enhance the performance level of TBC systems. Both projects started in 2000.

“A lack of reliability, more than any other design factor, is limiting the extensive commercial use of TBC systems for gas turbines,” explained Gleeson. “Commercial advanced TBC systems are typically two-layered, consisting of a ceramic topcoat and an underlying metallic bond coat. The topcoat, which is usually applied either by air plasma spraying (APS) or electron beam-physical vapor deposition (EB-PVD), is most often yttria-stabilized zirconia (YSZ). The properties of YSZ are such that it has a low thermal conductivity, high oxygen permeability, and a relatively high coefficient of thermal expansion.”

The YSZ topcoat is also made “strain tolerant” by depositing a structure that contains numerous pores and/or pathways. The high oxygen permeability of the YSZ topcoat provides the metallic bond-coat resistance to oxidation attacks. Thus the bond coat, which is rich in aluminum, forms a protective, thermally grown oxide (TGO) scale of a-Al₂O₃.

“It has been generally found that spallation and/or cracking of the thickening TGO scale is the ultimate failure mechanism of commercial TBCs, particularly EB-PVD TBCs,” said Gleeson. “Improving the adhesion and integrity of the interfacial TGO scale is critical to the development of more reliable TBCs.” This aspect comprises the central focus of the first ONR project.

“The durability and reliability of TBC systems is critically linked to the oxidation behavior of the bond coat together with minor elements that, with time, diffuse into the coating from the substrate during service,” said Gleeson. “Ideally, within the TBC system, the bond coat should oxidize to form a slow-growing, non-porous, and adherent TGO.”

To arrive at an optimum bond-coat oxidation behavior, said Gleeson, necessarily means gaining a fundamental understanding of the influences of alloy/bond-coat composition and microstructure and the effect of surface condition on TGO formation and growth. Gleeson and Sordelet are experimenting with different bond-coat compositions and structures and characterizing their oxidation performance during isothermal and thermal cycling tests. The interdiffusion behavior between the coating and the alloy substrate is another aspect that is also being investigated.

In their second ONR project, Gleeson and Sordelet are developing a simple, economical method to improve the salt-induced hot corrosion resistance of TBC systems. When sulfur from the combustion gas combines with sodium in the air to form sodium sulfate, it leaves a corrosive liquid deposit on the top coat of the TBC system. “Since the ceramic top coat contains inherent cracks necessary for strain tolerance, the liquid salt has a tendency to seep through to the bond coat, resulting in rapid degradation of the metal parts,” said Gleeson.

It becomes necessary, then, to alter the top-coat chemistry of the system to enable it to resist salt-induced attacks. An APS coating system, available through the Ames Lab Plasma Spray Facility that is operated by Sordelet, is utilized to deposit coatings necessary for this investigation. The APS system is sufficiently versatile in that a range of top-coat modifications is possible.

Both projects involve interactions with researchers at Rolls-Royce Allison, Howmet, and the NASA Glenn Research Center. Commercial producers are always on the lookout for improved products, said Gleeson, but depend on research facilities like the MSE department and Ames Laboratory to conduct detailed study and experimentation.

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