As gas turbine engines are driven to operate at higher temperatures to maximize efficiency, components become susceptible to attack by deposits of calcium magnesium alumino-silicate (CMAS) ingested with the intake air. Of particular interest to this work is the degradation of thermal barrier coatings (TBCs) by CMAS. Molten CMAS is known to interact with TBCs both thermochemically, by dissolving the ceramic and reprecipitating it as a new or modified phase, and thermomechanically, by infiltrating the porosity and degrading the strain tolerance.
The thermochemical degradation of TBCs was investigated using primarily differential scanning calorimetry (DSC) by comparing the endotherms and exotherms recorded for pure, model silicates to those observed for silicates mixed with various TBC materials including YSZ and GZO. The five ternary silicates studied (CaO-AlO1.5-SiO2) began melting over a relatively narrow range (∼1125-1145°C). Introducing magnesium to the ternary results in higher melting temperatures and only minor changes to the crystallization behavior. Iron decreases the melting temperature, and markedly improves the crystallization kinetics of pure silicate systems, especially absent magnesium.
Modification of the crystallization behavior of pure silicates has been proposed in the literature as a mitigation strategy for CMAS. This work utilizes DSC to look for characteristic changes as described above to probe potentially effective TBCs. The addition of YSZ to a quaternary CMAS results in little change to the melting or crystallization in the DSC, despite the dissolution of YSZ into the silicate. In stark contrast, GZO with CMAS generates a significant crystallization exotherm that appears in the DSC immediately after the silicate melts. As the fraction of GZO is increased, the melting endotherm begins to shrink due to the thermal overlap of the melting and crystallization processes. This signifies a rapid reaction, and a potentially useful TBC material for CMAS mitigation. Several additional TBC materials are tested and discussed including Y2Zr2O7, La2Zr 2O7, La2Ce2O7, and GdAlO 3. Stemming from these results, isothermal exposures of silicates on GZO TBCs demonstrated the influence of CMAS loading and exposure time on both penetration and the recession front depth, as well as the importance of silicate viscosity on the competition between infiltration time and reaction kinetics.
Furthermore, the present study illuminates several factors relevant to the mechanical degradation of both yttria stablizaed zirconia (YSZ) and gadolinium zirconate (GZO) by molten CMAS through the use of a laser gradient test (LGT) designed following the mechanical model of Evans and Hutchinson. Number of cycles, CMAS loading, CTE mismatch, and most of all, for the LGT, coating toughness influenced the location and extent of cracking within the TBC exposed to CMAS. Both the chemical and mechanical considerations and experimental protocol developed in this investigation lay the groundwork for assessing the CMAS resistance of next generation TBCs.
|Advisor:||Levi, Carlos G.|
|Commitee:||Bennett, Ted D., McMeeking, Robert M., Seshadri, Ram|
|School:||University of California, Santa Barbara|
|School Location:||United States -- California|
|Source:||DAI-B 74/08(E), Dissertation Abstracts International|
|Keywords:||Cmas attack, Silicates, Tbc, Thermal barrier coatings, Yttria stabilized zirconia|
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