Investigation of CMAS Resistance of Sacrificial Suspension Sprayed Alumina Topcoats on EB-PVD 7YSZ Layers

Abstract

Molten calcium-magnesium-aluminum-silicate (CMAS) mineral particles cause significant degradation of thermal barrier coatings (TBCs) in aero-engines. One approach to protect the TBC coating against the CMAS attack is the application of a sacrificial coating on top of the TBC coating. In this work sacrificial Al 2 O 3 coatings were deposited on top of EB-PVD 7YSZ layers using suspension thermal spraying starting from an aqueous Al 2 O 3 suspension. Spray parameters were varied in order to produce sacrificial topcoats with two different microstructures and porosities levels. The coating systems were tested under CMAS attack by performing shortand long-time infiltration tests at 1250 °C. It was found out that the porosity and morphology of Al 2 O 3 coatings strongly influenced the CMAS infiltration kinetics and the formation of various phases. CMAS mitigation depended on the interaction between the coating morphology which rules the driving force for infiltration, as well as on the reaction speed between alumina and the CMAS deposit.

Experimental and Simulation Analysis of the Evolution of Residual Stress Due to Expansion via CMAS Infiltration in Thermal Barrier Coatings

“…Note that the depth R(T h ) of the CMAS-rich layer follows the parabolic law by Wu et al [25]. This study [24] also indicated that the infiltration behavior is initially faster and becomes slower as the holding time still operates.…”

“…The solid dots with bars and lines indicate the experimental data and the curve-fitting line. Based on previous studies [24], the depth ( ) h R T of the CMAS-rich layer can be defined by the parabolic law as follows:…”

Calcia–magnesia–alumina–silicate (CMAS) attack mechanisms and roadmap towards Sandphobic thermal and environmental barrier coatings

“…The use of sacrificial coatings have also been explored, utilising Al 2 O 3 based top coats deposited by suspension plasma spray or suspension high velocity oxy-fuel spraying, on top of existing EB-PVD 7YSZ coatings [134]. The approach is interesting in that the Al 2 O 3 coating yields spinel layers that appear protective.…”

Erosion behavior of CMAS/VA infiltrated EB-PVD Gd2Zr2O7 TBCs: Special emphasis on the effect of mechanical properties of the reaction products

“…GZO, Al2O3, 65YZ, etc. (19,(24)(25)(26)(27)(28)(29)(30).The principle of sacrificial layers is to form quickly a protective reaction layer during CMAS infiltration which consists of new crystalline phases with higher melting points. This layer inhibits further CMAS infiltration into the TBC.…”

“…This layer inhibits further CMAS infiltration into the TBC. These high-temperature stable phases, such as apatite, fluorite, garnet, zirconium, and especially spinel (in case of an Al2O3 sacrificial layer (31)), block the infiltration paths of the CMAS and serve as a barrier against further CMAS infiltration. Therefore, the further infiltration of CMAS into the underlying unreacted coating is inhibited.…”