DEVELOPMENT OF NOVEL CMAS/VA RESISTANT TBCS: Influence of chemical composition and the microstructure.

Kurzfassung

The CMAS associated degradation of 7YSZ TBC layers is one of the serious problems in the aero engines that operate in dusty environments. CMAS infiltrates into TBC at high temperatures and stiffens the TBC which ultimately loses its strain tolerance and gets delaminated. Due to the serious effects that CMAS attack has on reducing the service life of 7YSZ TBCs, scientific and engineering research community is challenged to better understand the attack mechanisms and design methods for CMAS infiltration resistance. The most common infiltration mitigation strategies focus on using a reactive material that upon interaction with molten CMAS glass induces its partial or full crystallization sealing the porous features of the coating stopping any further infiltration. However, chemical composition of CMAS/VA and the TBC microstructure are highly influential parameters in case of CMAS/TBC interaction. Variation in the global CMAS/VA composition means difference in their melting points, viscosities, acidic/basic nature. Any developed novel TBC material will react differently to different CMAS/VA compositions and form numerous reaction products which might bring a completely new effect in the end. Novel TBCs such as gadolinium zirconate and 65YZ (65wt.% Y2O3 rest ZrO2) coatings were applied by means of EB-PVD method and their interactions with various CMAS and natural volcanic ash was studied in detail. It was found out that the reaction products were heavily dependent upon the CMAS chemistry and the presence of minor oxides in the CMAS should not be neglected. In addition, the TBC microstructure and the porosity have played a vital role in hampering the CMAS infiltration. Two different EB-PVD 7YSZ microstructures namely 'feathery' and ‘normal’ were tested against CMAS infiltration and found out that by tailoring the microstructure the CMAS infiltration kinetics can be hampered. It was also shown that the contact angle of molten CMAS would differ on these coating microstructures. Simultaneously, these results are well supported by microstructural simulations using ABAQUS where effects of feather arm lengths, inter-columnar gap width and feather inclinations on the infiltration kinetics are derived.