| Change in Characteristics of (La1-xGdx)2Zr2O7 TBCs |
| Seongwon KIM |
| Korea Institute of Ceramic Engineering and Technology |
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Studies on thermal barrier coatings (TBCs) have been conducted over the past decades in order to increase the operating temperature of gas turbines for power generation and aircraft. One of the most commonly used materials for TBC application is yttria-stabilized zirconia (YSZ), which has low thermal conductivity and a high thermal expansion coefficient. Recently, rare-earth zirconate oxides with pyrochlore and/or fluorite have been investigated as candidate materials for future TBCs. Pyrochlore and fluorite have analogous cubic structures with a space group of Fd3(-)m for the former and Fm3(-)m for the latter. The low thermal conductivities of rare-earth zirconate oxides with these structures are attributed to the phonon scattering by point defects in the crystallographic structures.
In this study, (La1-xGdx)2Zr2O7 TBCs are fabricated by suspension plasma spray with a variety of suspension preparation conditions, such as conventional or high energy ball milling. Lanthanum/gadolinium zirconate oxides were prepared for this study using La2O3(High Purity Chemicals , 99.9% , 11¥ìm), Gd2O3(High Purity Chemicals , 99.9% , 11¥ìm), and ZrO2(Aldrich , 99%, 40nm) oxide powders. The suspension with composition of lanthanum or gadolinium zirconate was prepared by a planetary mill or ball mill. Mixed suspensions were deposited on the superalloy substrate by suspension plasma spray (Axial III, Northwest Mettech Corp., Canada). With deposited or heat-treated coatings, crystallographic phases of samples were identified by X-ray diffractometer (XRD, RIGAKU D/MAX-2500/PC, Japan) with Cu K¥á radiation(0.1506nm). The microstructures of polished samples were analyzed by scanning election microscope (SEM, JEOL JSM-6390, Japan).
Figure 1 XRD patterns of (La1-xGdx)2Zr2O7 coatings : (a) as-deposited and (b) after heat treatment at 1400¡É.
Figure 1 shows the XRD patterns of (La1-xGdx)2Zr2O7 coatings as deposited as well as after heat treatment. Among these XRD patterns, the existence of (331) and (511) peaks of superlattice confirms the presence of pyrochlore phases in this lanthanum zirconate coatings. The LZ or GZ composition turns out to have a pyrochlore or a fluorite phase, respectively. In addition, both as-deposited coatings have fully developed crystallographic phases with different crystallite sizes.
Phase formation, microstructures, and thermal conductivities are examined with the deposited coatings of (La1-xGdx)2Zr2O7 compositions. The change in structures and thermal conductivities after heat treatment is investigated as well.
References:
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Acknowledgements : This research was supported by a grant from the Fundamental R&D Program for Strategic Core Technology of Materials funded by the Ministry of Trade, Industry and Energy and by a grant from the Basic and Strategic R&D Program funded by the Korea Institute of Ceramic Engineering and Technology, Republic of Korea.
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