Thermal properties of oxides with magnetoplumbite structure for advanced thermal barrier coatings

Abstract

Oxides having magnetoplumbite structure are promising candidate materials for applications as high temperature thermal barrier coatings because of their high thermal stability, high thermal expansion, and low thermal conductivity. In this study, powders of LaMgAl 11O 19, GdMgAl 11O 19, SmMgAl 11O 19, and Gd 0.7Yb 0.3MgAl 11O 19 magnetoplumbite oxides were synthesized by citric acid sol–gel method and hot-pressed into disk specimens. The thermal expansion coefficients (CTE) of these oxide materials were measured from room temperature to 1500 °C. The average CTE value was found to be ∼ 9.6 × 10 − 6 /C. Thermal conductivity of these magnetoplumbite-based oxide materials was also evaluated using steady-state laser heat flux test method. The effects of doping on thermal properties were also examined. Thermal conductivity of the doped Gd 0.7Yb 0.3MgAl 11O 19 composition was found to be lower than that of the undoped GdMgAl 11O 19. In contrast, thermal expansion coefficient was found to be independent of the oxide composition and appears to be controlled by the magnetoplumbite crystal structure. Preliminary results of thermal conductivity testing at 1600 °C for LaMgAl 11O 19 and LaMnAl 11O 19 magnetoplumbite oxide coatings plasma-sprayed on NiCrAlY/Rene N5 superalloy substrates are also presented. The plasma-sprayed coatings did not sinter even at temperatures as high as 1600 °C.