CMAS/Volcanic ash resistant coatings for TBC and EBC applications based on novel Y-Si-Fe oxide system

Kurzfassung

Molten CMAS (CaO-MgO-Al2O3-SiO2) and volcanic ash (VA) infiltrate thermal and environmental barrier coatings (T/EBCs) leading to chemical degradation of EBCs and the loss of the strain-tolerant microstructure and finally spallation of TBCs leaving the bare underlying components exposed to extreme heat environment in aero engines engines. This CMAS attack can be of thermo-chemical nature, leading to chemical reactions between the TBC and EBC materials with molten CMAS at higher temperatures that result in phase crystallisation dependent on the coating material and CMAS chemistry. In this work, a novel YSiFe oxide coating produced by reactive magnetron sputtering was investigated as top layer on TBC/EBC’s reacting with and arresting CMAS/VA before it can infiltrate into underlying coatings. The coating infiltration resistance and reaction kinetics related to the FeO content in the coating were studied and evaluated on the basis of formed reaction products and coating recession influenced by several factors such as microstructure, coating and CMAS composition. Firstly, a post heat treatment of the YSiFe oxide coating was developed to stabilise crystalline single phase yttrium monosilicate (YMS), an yttria rich and FeO containing phase. The best coating heat-treatment and phase composition in terms of coating adhesion, microstructure and CMAS infiltration behaviour was achieved with the heat-treatment, that included fast heating of the coating followed by heating isothermally for 1 h at 1250 °C and air quenching to RT. Multiple phases consisting of the single phase X2-YMS (54 %), Y2O3 (22 %), and YFeO3 (24 %) were achieved in this heat-treatment. The FeO in the YSiFe oxide coatings reinforced the single phase X2-YMS compared to iron-free YSi oxide coatings which led to mixed YMS phases. Secondly, the infiltration behaviour of Colima volcanic ash and UCSB CMAS was studied as well as the influence of FeO in the YSiFe oxide coatings on the crystallisation reaction products with CMAS. The results showed that the studied YSiFe oxide coatings induced the crystallisation of the CMAS melt by forming oxyapatite, YDS and garnet phases, dependent on the CMAS composition. The Fe diffusion from the YSiFe oxide coating into the glass caused garnet formation which consumed further CMAS glass and prevented further infiltration, compared to iron-free YSi oxide coatings which did not induce the garnet formation. This YSiFe oxide coating has been proven to be a very good CMAS resistant coating and is therefore a promising candidate for CMAS resistant multilayer EBCs and failed to achieve reasonable results when applied on 7YSZ, that could be further improved for TBC compatibility.