Study of Corrosion by CMAS deposition on Topcoat of Thermal Barrier Coating

Abstract

Thermal barrier coating on gas turbine engine is applied to reduce the effective temperature in the super-alloy part of engine, and to enable us to use higher working temperature during operation. The coatings also prevent corrosion and degradation caused by molten calcium-magnesium-aluminium-silicon oxides(CMAS) from debris ingested into the exposed part of the engine. Though different top coat material such as Gd2Zr2O7 Pyrochlore and 8YSZ have been studied here with respect to their corrosion behavior with CMAS at high temperature of 1300 oC for different time duration.
8mole% YSZ and Gd2Zr2O7 powders were prepared using co-precipitation method and nano-grain 8YSZ and pyrochlore pellet was obtained after sintering the pellets at 1300 oC for 4 hours. The apparent porosity of 8YSZ pellet was 16% which is lower than that of pyrochlore pellets after sintering at 1300 oC for 4 hours. Phase pure non-transformable tetragonal zirconia was present in 8YSZ pellet, whereas Gd2Zr2O7 pellet showed phase pure single phase of pyrochlore. Precursor solution of CMAS was deposited on 8YSZ and pyrochlore surface and heat treated at 1300 oC for 14,18 and 32 hours to study CMAS corrosion on the respective pellet surface. It was observed that after heat treatment with CMAS for 14,18 and 32 hours, t’ phase of 8 YSZ pellet was entirely converted to tetragonal and monoclinic zirconia phase with some amount of yttrium silicate as shown by the x-ray diffraction pattern of the pellets in contact with CMAS. On the other hand for similar duration of corrosion with CMAS, most of the pyrochlore phase was converted into cubic zirconia, Ca2Gd8(SiO4)6O2, Gd2Si2O7 and Ca6Zr19O44, though little bit of pyrochlore still existed in the Gd2Zr2O7 pyrochlore pellet even after heat treatment at 32 hours. The non-CMAS contact surface of both 8YSZ and pyrochlore pellet showed negligible or no-phase change which confirms the fact CMAS reaction with 8YSZ and Gd2Zr2O7 was mainly responsible for phase transformation at the CMAS contact surface. From EDAX profile it is evident that pyrochlore was more resistant to CMAS corrosion, despite having higher porosity as compared to 8YSZ pellets.The study shows Gd2Zr2O7 pyrochlore may serve as a better top coat material to resist CMAS corrosion at the high operating temperature of gas turbine engine.