Original Date: 03/08/1999
Revision Date: 01/18/2007
Information : Functional Gradient Thermal Barrier Ceramic Coatings
By using an electron beam-physical vapor deposition system and the flexibility it offers, the Applied Research Laboratory at the Pennsylvania State University (ARL Penn State) developed a process for producing functional, gradient thermal barrier coatings (TBCs). Prior to electron beam-physical vapor disposition (EB-PVD), TBCs were applied by using a flame spray process. However, this process did not provide a metallurgical bond to the substrate, and was inconvenient at best for turbine blades where cooling passages became filled with spray material. By contrast, EB-PVD allows for a uniform deposition of multiple materials in varying concentrations to form a gradient of properties, thereby optimizing substrate protection and thermal coefficient match.
Figure 3-3 depicts the EB-PVD system. The system contains six 45-kilowatt electron beam guns. Typically, two guns are used to heat the target component, while the other four are used to selectively evaporate the three ingots of coating material. The ability to vary the material choice, target position and motion, and the gun energy provides a wide spectrum of possible coating configurations. The following is an evolution of TBC configurations:
Lamellar TBC Produced by Plasma Flame Spray Poor grain structure, voids, and low adhesion to the substrate. EB-PVD Columnar structure for high radial strength.
EB-PVD with Graded Coating Improved substrate thermal match and adhesion.
TBC with Alloying Introduction of secondary elements (tantalum) to engineer voids and lower thermal conductivity.
Multilayer Columnar TBC Discontinuities introduced by removing and reintroducing target from chamber, but provided even lower thermal conductivity.
Inherent advantages of EB-PVD for TBCs include high deposition rate (14 kilograms per hour); potential for multilayered or gradient coatings in metals, ceramics, and composites; flexibility allowed for engineered material configurations; and ability to accommodate large-size components (1 meter x 1 meter x 1 meter). These features also provide superior adhesion; thermal matching between substrate and graded coatings; and lower thermal conductivity. As a result, TBCs improve component life and engine performance; increase operational temperature from 1300°C to 1500°C; create a reduction in required active cooling; and reduce specific fuel consumption of approximately 1%.
Figure 3-3. Electron Beam-Physical Vapor Deposition System
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