Microstructure Tailored Functionally Graded Alumina/Lanthanum Hexaaluminate Ceramics for Application as Thermal Barrier Coatings

✍ Scribed by Zahra Negahdari; Monika Willert-Porada

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 455 KB
• Volume: 11
• Category: Article
• ISSN: 1438-1656
• DOI: 10.1002/adem.200900147

✦ Synopsis

Lanthanum hexaaluminate (LHA) is a promising competitor to Y-ZrO 2 -based thermal barrier coatings. A layered functionally graded alumina/lanthanum hexaaluminate with a gradient in composition was developed to control the microstructure of composite ceramics and adjusting the porosity upon, to arrive at a material exhibiting low thermal conductivity at high temperature combined with structural reliability. Investigation on thermal and mechanical properties of composite layers showed that alumina-rich composite layers provide excellent mechanical properties whereas LHA-rich composite layers present lower thermal conductivity.

A thermal barrier coating (TBC) ''system'' as an advanced applied to metallic surfaces operating at elevated temperature is generally composed of a porous, insulating ceramic oxide top layer, to-date yttrium oxide stabilized zirconia (YSZ), a thermally grown aluminum-rich oxide (TGO) layer, and an underlying aluminide bond layer which is used to form the TGO layer. [1] Despite the fact that so many efforts have been done for the development of TBC, still some requirements are not fulfilled: lower thermal conductivity, increased temperature capability, improved lifetime as well as higher durability to foreign object damage, and resistance to chemical reactions with gas impurities, [2] or with deposits like calcium-magnesium alumino-silicate (CMAS). [3] In addition, the coefficient of thermal expansion (CTE) mismatch stress between different layers and the underlying superalloy substrate should be minimized for increased spallation resistance. [1] Enhancing the engine performance via increasing the operating temperature reveals a need for materials with lower thermal conductivity and sinter-ability. A major disadvantage of YSZ limited operation temperature for long time applications, is caused by considerable aging including densification at temperature above 1100 8C which increases thermal conductivity and Young's modulus, and therefore, decreases resistance to CTE mismatch stress. In addition to the deterioration of thermal and mechanical properties, the oxygen conductivity of YSZ increases rapidly at elevated temperature. [4] Lanthanum hexaaluminate, LaAl 11 O 18 (LHA) with a magnetoplumbite structure and plate-like grain is a promising material for replacing YSZ, not only because of low thermal conductivity and Young's modulus but also due to lower sinter-ability, superior structural and thermo-chemical stability up to 1400 8C, stable pore structure, lower oxygen diffusivity. [1,5] In addition phase compatibility with Al 2 O 3 , which has the lowest oxygen diffusivity among the common oxides, is preserved up to 18008C. [6] An inherent low thermal conductivity of a dense ceramic top layer, which LHA ceramics have been proposed for, is based on the defect structure of this oxide. [5] The defective structure is also responsible for low fracture toughness. [6] Thus, further lowering of the thermal conductivity of the ceramic top layer could only be achieved with LHA composite ceramics with a high LHA content and with additional defects, like e.g., porosity.

In previous work, development of multiphase ceramic material with special architecture of the pore structure, i.e.,