Mechanical Properties of LaTi 2 Al 9 O 19 and Thermal Cycling Behaviors of Plasma-Sprayed LaTi 2 Al 9 O 19 /YSZ Thermal
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X.Y. Xie, H.B. Guo, and S.K. Gong (Submitted January 29, 2010; in revised form June 22, 2010) Recently, extensive efforts have been made to develop new thermal barrier coating (TBC) materials which can operate at temperatures above 1523 K over a long term. In this article, LaTi2Al9O19 (LTA) was synthesized by solid-state reaction at 1773 K, and the mechanical properties of the LTA bulk were evaluated. The microhardness is about 14 GPa, comparable to that of YSZ bulk, whereas the YoungÕs modulus is about 44 GPa, lower than the value of YSZ. However, the fracture toughness of 0.8-1 MPa m1/2 is much lower than that of bulk YSZ. A double-ceramic-layer LTA/YSZ TBC structure was proposed and the TBC sprayed by plasma spraying. Thermal cycling tests of the TBC specimens were performed at 1373 K with a dwell time of 10 min. The LTA remained good stability with ZrO2 and Al2O3. However, the single layer LTA TBC was cracked at the LTA/bond coat interface after about 300 cycles, due to its poor thermal shock resistance, while the YSZ TBC yielded a lifetime of about 1000 cycles. The LTA/YSZ TBC remained intact even after 3000 cycles, exhibiting a promising potential as new TBC materials.
Keywords
LaTi2Al9O19, mechanical property, plasma spray, thermal barrier coatings (TBCs), thermal cycling
1. Introduction The performance and efficiency of gas turbine engines are directly related to the operating temperature. Thermally sprayed thermal barrier coatings (TBCs) have been used successfully in gas turbine components to protect the metal parts from hot burner gases, leading to further increase in operating temperature and decrease of the amount of cooling air (Ref 1-3). Nowadays, state-of-the-art 7-8 wt.% yttria-stabilized zirconia (YSZ) is widely used as a standard TBC topcoat material (Ref 4-6). However, the most critical issue for YSZ is the limited operation temperature (95% 1.1 (Ref 12)
1.3-1.5 (Ref 16)
Journal of Thermal Spray Technology
Volume 19(6) December 2010—1181
Peer Reviewed
Thermal cycling tests were performed in an air furnace by heating the specimens to 1373 K with a holding time of 10 min at the temperature, followed by compressed air cooling for 90 s. The lifetime is defined as the number of cycles at which spallation of more than 20% surface area of TBC specimens occurred. Thermal-cycled TBC specimens were impregnated with epoxy, and then sectioned, ground, and polished. The microstructure and composition of the coatings were investigated by scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS, INCA Oxford, Britain), respectively.
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proposed, and it proved to be effective in improving the thermal cycling lifetime (Ref 17, 27, 28). This suggests that the DCL structure could be adopted in this study, in which LTA with high temperature capability is used as the top layer while YSZ with good mechanical properties forms the bottom layer.
3.2 Chemical Stability of LTA As thermal barrier materials, LTA must maintain good chemical stability with other parts such as th
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