Effectiveness of lanthanum zirconate and Yttria stabilised zirconia freestanding APS thermal barrier coatings against natural CMAS attack at high temperatures
R. K. Satpathy, I. Sophia Rani, Zafir Alam, Laxmidhar Besra
Abstract
CMAS-resistant thermal barrier coating is essential for durability of turbine components in advanced aero-engines. They are mostly evaluated using synthetic model CMAS powders. However, the degradation by natural CMAS can be higher considering their multiple constituents, nanostructure and low fusion temperature. The study compares the resistance of APS LZ and 8-YSZ freestanding disks to natural CMAS up to 1450°C and 100 h. The readily formed sealing layer of apatite, stable at 1450°C for 100 h, restricts further damage to LZ. But YSZ undergoes extensive destabilisation from metastable tetragonal (t´) to monoclinic and cubic phases after 1 and 10 h at 1450°C and 1250°C, respectively. Wetting, spread of CMAS and thermal diffusivities of the coatings are assessed. A 4–10 fold increase in thermal diffusivities is noticed. The paper attempts to bring some insight on CMAS reactivity with coating constituents based on fundamental considerations of ionicity, basicity, bond strength and diffusivities of species.