Doctor of Philosophy (PhD)
Thermal barrier coatings (TBCs) are used gas turbine engines. The current material of choice (YSZ) degrades when it contacts with impurities arise from low quality fuels such vanadium and sulfur. YSZ cannot be used in temperature higher than 900˚C. Higher efficiency and performance of gas turbine engines will require a new generation of thermal barrier coatings (TBCs). In current work, hot corrosion behavior of new candidates including Gd2Zr2O7, ZrO2 stabilized with Ta2O5, zirconia stabilized with both Ta2O5 and Y2O3 and zirconia stabilized with CeO2 and TiO2 is investigated. For YSZ case, the reaction between NaVO3 and Y2O3 produces YVO4 and leads to the transformation of tetragonal ZrO2 to monoclinic ZrO2. Comparing to YSZ, under a temperature of 1050 °C, Gd2Zr2O7 is found to be more stable, both thermally and chemically, than YSZ, and exhibits a better hot corrosion resistance. To examine the effect of stabilizing zirconia with tantalum oxide, different compositions of ZrO2-Ta2O5 samples in the presence of molten mixture of Na2SO4 + V2O5 at 1100˚C were tested. Hot corrosion results show that orthorhombic zirconium-tantalum oxide is more stable, both thermally and chemically in Na2SO4+V2O5 media at 1100˚C, and shows a better hot corrosion resistance than the tetragonal phase. When zirconia stabilized with yttria and tantalum oxide (TaYSZ sample), minor amounts of NaTaO3, TaVO5 and Ta9VO25 are formed as the hot corrosion products with only traceable amounts of YVO4. Due to the synergic effect of doping of zirconia with both Y2O3 and Ta2O5, the TaYSZ sample has a much better hot corrosion resistance than YSZ. In zirconia stabilized with CeO2 coating, the formation of CeVO4 crystals possibly postponed the formation of YVO4 and transformation of tetragonal zirconia to monoclinic. On surface of zirconia stabilized with TiO2 coating, TiVO4 crystals are significantly smaller (about 5µm in length) than the large plate shaped YVO4 and CeVO4 found on the YSZ and CSZ samples. TiSZ coating was found to have a better hot corrosion resistance at a temperature of 1050˚C than both YSZ and CSZ coatings.
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Habibi, Mohammad Hamed, "Hot corrosion behaviour of new candidates for thermal barrier coatings application in turbine simulated environments" (2014). LSU Doctoral Dissertations. 301.