Effectiveness of lanthanum zirconate and Yttria stabilised zirconia freestanding APS thermal barrier coatings against natural CMAS attack at high temperatures

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.

KEYWORDS:

• Air plasma spray (APS)
• natural CMAS
• bond strength and ionicity
• lanthanum zirconate
• yttria stabilised zirconia
• freestanding coatings
• thermal diffusivity
• wetting and spreading

Acknowledgments

The authors are grateful to Dr B.R. Mishra, IREL-Chhatrapur, for providing LZ coating material and Mr Suresh Ch. Mahapatra, HAL-Koraput, for painstakingly collecting the natural CMAS powder from the actual engines. The authors thank Director, DMRL-Hyderabad, for his permission to pursue this research.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Defence Research and Development Organisation; Department of Atomic Energy, Government of India.