ATMOSPHERIC PLASMA SPRAYING OF SINGLE PHASE LANTHANUM ZIRCONATE THERMAL BARRIER COATINGS WITH OPTIMIZED POROSITY

摘要

The shortcomings at elevated operation temperatures of the standard material yttria stabilized zirconia (YSZ) for thermal barrier coatings (TBCs) have initiated many research activities seeking for alternatives. One candidate is the pyrochlore lanthanum zirconate La_2Zr_2O_7 (LZ) which is phase stable to its melting point. At the same time, it shows a lower thermal conductivity and a lower sintering tendency when compared to YSZ. Because of its low thermal expansion coefficient and poor toughness it is applied in combination with YSZ in double layer TBC systems. It is current state of knowledge that LZ is prone to lanthanum depletion if processed by plasma spraying. The process conditions have to be selected carefully to avoid this. Furthermore, the amount and morphology of the coating porosity is essential for a good thermo-mechanical performance. In this work, the development and testing of LZ/YSZ double layer TBC systems is described. Initially, suitable basic parameters (torch, plasma gas composition and power) were tested with respect to coating stoichiometry. Then, microstructures were optimized by adjusting feed rates, spray distances, and selecting a more appropriate feedstock. Powder particles and coatings were characterized by digital image analysis.