Doctor of Philosophy (PhD)
Thermal barrier coatings (TBCs) are used in gas turbine engines to achieve a higher working temperature and thus lead to better efficiency. Yttria-Stabilized-Zirconia (YSZ), a material with low thermal conductivity, is commonly used as the top coat layer to provide the thermal barrier effect. In this dissertation the thermo-physical properties of a variety of TBCs samples made out of different fabrication techniques were investigated and compared. The first set of samples was fabricated using a pressing machine device to fabricate 0.5 inch diameter disk shaped YSZ-Al2O3 samples. The YSZ-Al2O3 powder mixture was made of 0, 1, 2, 3, 4 and 5 wt% Al2O3 /YSZ powder ratio. The second set of samples was fabricated by Atmospheric Plasma Spray process for two different microstructure configurations, standard (STD) and vertically cracked (VC), and two different thicknesses, 400 and 700 micron respectively. A laser flash system was used to measure the thermal properties of the coatings. Experiments were performed over the temperature range from 100 to 800 C. The porosity of the YSZ samples was measured using a mercury porosimetry analyzer, POREMASTER 33 system. A Scanning Electron Microscope (SEM) was used to study the microstructure of the samples. An analytical model is proposed to estimate the effective thermal conductivity of the TBCs. Results showed that the change of thermo-physical properties is directly linked to the microstructure of the samples, demonstrated by the porosity measurements and SEM images. The addition of alumina was effective to suppress sintering behavior of YSZ ceramic. The YSZ-Al2O3 composite samples reported lower thermal conductivity values compared to pure YSZ, this due to the increase of porosity of the samples. For the TBCs, the thermal diffusivity and thermal conductivity increased for the VC-TBC samples in comparison to the STD-TBC samples over the temperature range tested. The analytical predictions were compared to the experimental data.
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Bohorquez de Silva, Monica, "Study of microstructure effect on the thermal properties of Yttria-stabilized-Zirconia thermal barrier coatings made by atmospheric plasma spray and pressing machine" (2010). LSU Doctoral Dissertations. 855.