Skip to page navigation menu Skip entire header
Brown University
Skip 13 subheader links

Degradation and Mitigation Mechanisms of Molten Silicate Deposits in Thermal and Environmental Barrier Coatings

Description

Abstract:
Calcia-magnesia-alumino-silicate (CMAS) deposits cause spallation and degradation of thermal barrier coatings (TBCs), which are used to insulate and protect metallic components in gas turbine engines. The reaction between CMAS and conventional TBCs, 7 wt.% Y2O3 partially stabilized ZrO2 (7YSZ), was investigated by isothermal studies at 1340 oC to establish the chemical and microstructural changes that occur. Additionally, a novel 'model' experiment was designed and conducted to evaluate the reaction behavior without the effects of a TBC microstructure, where 7YSZ powders were immersed in molten CMAS at 1300 oC. The study revealed how the reaction products depend on the solubility and transport of Y3+ in CMAS. Specifically, low Y-content in the CMAS, caused by a relatively large glass 'sink,' produces Y-depleted ZrO2 grains, which undergo the undesirable tetragonal to monoclinic transformation upon cooling. A thermomechanical model was presented that demonstrated the effect of the phase transformation on the strain field and, thus, delamination. The dissolution-reprecipitation process found in 7YSZ coatings is utilized in new CMAS-mitigating TBCs, which introduce high concentrations of solute into the melt upon dissolution to effectively alter the CMAS composition and nucleate crystalline phases that seal the attack front. 2ZrO2∙Y2O3(ss) air plasma sprayed (APS) TBCs were fabricated and found to significantly reduce CMAS-penetration by forming silicate-apatite crystals. 'Model'-experiments revealed that the transport of Y3+ is restricted in the melt and that this unique transport allowed for nucleation of apatite at regions of high flux. This is desirable for CMAS-mitigation, as it causes apatite formation at the CMAS/TBC interface. Three important reaction criteria were established to evaluate the TBC's CMAS-mitigation capability: the TBC must (i) react vigorously with the CMAS, (ii) alter the CMAS composition to nucleate a crystalline phase, and (iii) the phase must preferentially nucleate at the CMAS/TBC boundary. It was demonstrated that the optical basicity model can be used to quantify the reactivity between potential ceramic coatings and CMAS for future engine designs.
Notes:
Thesis (Ph.D. -- Brown University (2016)

Access Conditions

Rights
In Copyright
Restrictions on Use
Collection is open for research.

Citation

Krause, Amanda Rochelle, "Degradation and Mitigation Mechanisms of Molten Silicate Deposits in Thermal and Environmental Barrier Coatings" (2016). Materials Science Engineering Theses and Dissertations. Brown Digital Repository. Brown University Library. https://doi.org/10.7301/Z0PK0DKQ

Relations

Collection: