Thermal properties of La 2O 3-YSZ powder fabricated by heterogeneous precipitation

Abstract

This study focused on whether La 2O 3 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. La 2O 3 was coated on YSZ powder by HP using YSZ as a base material, La(NO 3) 3·6H 2O as an La source, and NH 4HCO 3 as a precipitant. The phase of the as-precipitated La on YSZ was La 2(CO 3) 3·8H 2O, which was changed to La 2(CO 3) 3·8H 2O + La 2O 3CO 3 at 200 °C and La 2O 3CO 3 + La 2O 3 at 500 °C in air after 2 h. La 2O 3 was finally obtained by heat treatment of as-precipitated La 2(CO 3) 3·8H 2O at 800 °C in air for 2 h. In order to compare several properties of La 2O 3 mixed with YSZ (LYSZ) powder by HP, LYSZ powder was prepared by ball-milling (BM). It was found that La 2O 3 was more uniformly distributed in the as-prepared HP-LYSZ powder than in the BM-LYSZ powder. This different distribution of La 2O 3 affected the thermal properties of the two LYSZ powders at high temperature. After sintering at 1300 °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 μm. 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 °C. Therefore, HP-LYSZ powder could be a useful material for TBC applications.