Thermal properties of La2O3-YSZ powder fabricated by heterogeneous precipitation

Abstract

This study focused on whether La2O3 was uniformly coated by heterogeneous precipitation (HP) on the thermal barrier coating (TBC) material, yttria stabilized zirconia (YSZ), in order to decrease densification and enhance thermal stability at operating temperature. La2O3 was coated on YSZ powder by HP using YSZ as a base material, La(NO3)(3)center dot 6H(2)O as an La source, and NH4HCO3 as a precipitant. The phase of the as-precipitated La on YSZ was La-2(CO3)(3)center dot 8H(2)O, which was changed to La-2(CO3)(3)center dot 8H(2)O + La2O3CO3 at 200 degrees C and La2O3CO3 + La2O3 at 500 degrees C in air after 2 h. La2O3 was finally obtained by heat treatment of as-precipitated La-2(CO3)(3)center dot 8H(2)O at 800 degrees C in air for 2 h. In order to compare several properties of La2O3 mixed with YSZ (LYSZ) powder by HP, LYSZ powder was prepared by ball-milling (BM). It was found that La2O3 was more uniformly distributed in the as-prepared HP-LYSZ powder than in the BM-LYSZ powder. This different distribution of La2O3 affected the thermal properties of the two LYSZ powders at high temperature. After sintering at 1300 degrees C for 12 h, the HP-LYSZ powder showed an approximately 5% lower relative density than the BM-LYSZ powder, caused by the different distribution of pores with diameters below 1 mu m. In addition, the HP-LYSZ had better thermal stability than the BM-LYSZ during annealing, because BM-LYSZ had a greater ratio of the monoclinic phase after lengthy annealing at 1400 degrees C. Therefore, HP-LYSZ powder could be a useful material for TBC applications.