The phase equilibria between thermal barrier coating (TBC) materials and calcium–magnesium–aluminosilicate melts (CMAS, representing deposits formed when siliceous debris is ingested into modern turbine engines), were investigated at 1300 °C. The primary goal was to understand the influence of the deposit and thermal barrier oxide compositions on the melt solubility limits and reaction product constitution. Model deposit compositions with SiO2 to CaO ratios from 1.4 to 4.7 and with or without MgO were reacted with yttria- and gadolinia-zirconia thermal barrier oxides ranging from pure ZrO2 to the pure yttria and gadolinia. The reactions formed various crystalline silicate phases; rare earth–calcium–silicate apatite and zircon (ZrSiO4) were observed most frequently. Following the reactions, the residual melts were depleted in SiO2 and generally enriched in CaO, MgO, and Al2O3. The implications of the anticipated changes in the melt viscosity and the cation partitioning between the melt and various solid solution phases on the efficacy of degradation-mitigating crystallization reactions are discussed.
Bibliographical noteFunding Information:
This investigation was supported by the Office of Navy Research under STTR Phase II N00014-12-M-0340 (with Questek Innovations), and ONR grant N00014-16-1-2702 , both monitored by Dr. David Shifler. The work made use of the MRL Shared Experimental Facilities supported by the MRSEC Program of the NSF under Award No. DMR 1121053 ; a member of the NSF-funded Materials Research Facilities Network ( www.mrfn.org ). The funding agencies were not involved in the study design, the collection, analysis and interpretation of the data, or the decision to publish. The authors are grateful to Drs. G.G.E Seward, E. Zaleski, and R.W. Jackson (UCSB) for helpful discussions and assistance EPMA measurements, preparing the CMAS powders, and performing FactSage calculations, respectively.
- Phase equilibria
- Rare earth
- Solid solution
- Thermal barrier coatings (TBC)