Alternating-Composition Layered Ceramic Barrier Coatings
- Thursday, 26 June 2008
These coatings are expected to be more durable, relative to prior thermal/environmental barrier coatings.
Ceramic thermal and environmental barrier coatings (T/EBCs) that contain multiple layers of alternating chemical composition have been developed as improved means of protecting underlying components of gas-turbine and other heat engines against both corrosive combustion gases and high temperatures. A coating of this type (see figure) is configured using the following layers:
- An outer, or top oxide layer that has a relatively high coefficient of thermal expansion (CTE) and serves primarily to thermally protect the underlying coating layers and the low-CTE ceramic substrate structural material (the component that is ultimately meant to be protected) from damage due to exposure at the high temperatures to be experienced in the application;
- An inner, or bottom silicon-containing/ silicate layer, which is in contact with the substrate, has a low CTE and serves primarily to keep environmental gases away from the substrate; and
- Multiple intermediate layers of alternating chemical composition (and, hence, alternating CTE).
The intermediate alternating-composition coating layers are chemically compatible with themselves as well as with the inner and outer coating layers. These intermediate coating layers act as an energy-dissipating interlayer: they dissipate strain energy associated with the CTE mismatch between the inner and outer coating layers, thereby reducing stresses and helping to increase (relative to prior ceramic T/EBCs) coating resistance to cracking and delamination from the substrate surface.
Typically, there are between four and ten alternating-composition intermediate layers, comprising higher-CTE oxide layers interspersed with lower-CTE silicate layers, each layer having a thickness between 5 and 50 μm. The compositions of the oxide and silicate alternating layers can be the same as those of the outer and inner layers, respectively. Alternatively, different oxide and silicate compositions can be chosen to increase tolerance of strain, resistance to cracking, and/or protection against chemical attack by gases in any intended application.
During thermal cycling, the alternating layers become regions of alternating tension and compression. This stress and strain configuration facilitates microsegmentation in the oxide layers while maintaining effective compression in the silicate layers. As a consequence, the thermal expansion of the energy-dissipating interlayer is reduced, stresses are reduced, and tolerance of strain is greatly enhanced. Cracking that starts in the outer oxide layer of the coating is arrested within the alternating layers because of the compressive stress in the silicate alternating layers and the tendency toward deflection and/or bifurcation of cracks at the interfaces between the alternating layers. Moreover, during cooling, the compression in the silicate alternating layers helps to ensure the integrity of the overall coating system in its role as an environmental barrier by helping to prevent penetration of combustion gases to the surface of the substrate.
The thickness of the alternating oxide and silicate layers are quite dependent on the intended engine application. A thicker-silicate-layer/thinner-oxide-layer structure could increase the strain tolerance of the coating and protect the substrate (or engine component in application) from the hot gases in the engine environment; however, on the other hand, a slightly thinner silicate- layer next to a slightly thicker oxide-layer structure could increase the coating’s resistance to stress and penetration of any damaging gas- constituents through cracks, potentially reacting with the substrate.
This work was done by Robert A. Miller of Glenn Research Center and Dongming Zhu of the U. S. Army Research Laboratory.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17536-1.
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