Current Research Interests
Thermal barrier coatings (TBCs) are used extensively in both commercial and military gas turbine engines to increase component life and engine performance. TBCs are really a system of coatings. A TBC system consists of a bond coat, a thermally grown oxide (TGO), and a thermally insulating ceramic top coat[1,2]. In most applications, the bond coat is either a MCrAlY (where M=Ni or NiCo) or a Pt modified aluminide coating. A dense bond coat is required to provide protection to the superalloy substrate from oxidation and hot corrosion attack and to form an adherent, slow growing TGO on its surface. The TGO is formed by oxidation of the aluminum that is contained in the bond coat. The current (first generation) TBC layer is composed of 7wt % yttria stabilized zirconia (7YSZ) with a typical thickness of 100-300 µm. Yttria stabilized zirconia is used as the insulating layer due to its low conductivity (2.6W/mK for fully dense material), relatively high coefficient of thermal expansion, and good high temperature stability. The EB-PVD process used to apply the TBC for turbine airfoils produces a columnar microstructure with several levels of porosity. The porosity between the columns is critical to providing strain tolerance (via a very low in-plane modulus), as it would otherwise spall on thermal cycling due to thermal expansion mismatch with the superalloy substrate.

Use of these multilayer systems in advanced gas turbine engines is anticipated. However, this will require improved TBC durability and an increasing resistance to high temperature and long time exposures in corrosive environments. New materials having improved high temperature properties must therefore be developed to allow for their use in the higher temperature, corrosive environments where performance benefits are greatest. Current top coat compositions are limited by a lack of phase and thermal stability at elevated temperatures and localized coating spallations caused by erosion/impact or CMAS damage[3]. The bond coats require improved oxidation resistance at increased temperatures and a higher creep strength[4].

Our research at UVA focuses on the use of a novel directed vapor deposition processing approach for depositing compositional and morphologically controlled top coat and bond coat layers intended for use in next generation TBC systems.

Thermally Stable Top Coats
Zirconia-7wt%Yttria (7YSZ) is current used as TBC topcoat material because it possesses a suite of desirable properties, such as a high melting point, low thermal conductivity, chemical inertness with the TGO and a high thermal expansion coefficient. However, this top coat composition is limited by a lack of phase and thermal stability at elevated temperatures. Next generation TBCs will result in increased temperature exposures of the top coat and may therefore require new materials. Our recent work in this area consists of studying the properties of rare earth based zirconate materials (La, Gd, Sm and Yb etc.), which are promising candidate materials due to their high melting point, low thermal conductivity, high temperature phase stability and good sintering resistance. Specifically we are interested in the effect of zirconate compositions (for three, four and five component systems) on the phase stability, thermal conductivity and thermalchemical stability with alumina. To achieve this we are creating coatings having laterally graded compositions so that the properties of a range of coating compositions can be assessed in parallel. Characterization techniques include SEM, XRD and direct thermal conductivity measurement based on the use of an IR camera.

A La₂Zr₂O₇ coating is shown in the figure below. The coating was reactively deposited on the 1” square alumina substrate from two different metal source rods (La and Zr). The coating was deposited at 1050^oC. A helium-5.0 vol.% O₂ carrier gas jet was introduced into the chamber at 20 slm resulting in a deposition chamber pressure of 26 Pa.

a) top surface	b) cross section
Lanthanum Zirconate coating deposited by DVD. In (a) surface facets of the columnar microstructure are shown. In (b) a cross-sectional view is given revealing intercolumnar porosity.