2. Thesis and Dissertations

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Subject: search.filters.subject.Thermal barrier coatings

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    Development of Al2o3 - Sm2sral2o7 Ceramic Composite Thermal Barrier Coatings for High Temperature Applications
    (National Institute Of Technology Karnataka Surathkal, 2023) J, Fredy James; Arya, Shashi Bhushan
    Composite thermal barrier coatings with top coat composition 70 wt. % Al2O3 - 30 wt. % Sm2SrAl2O7 was prepared through atmospheric plasma spraying. The rare earth material Sm2SrAl2O7 was synthesized in the laboratory through molten salt technique at 1100 °C. Commercially available NiCrAlY powder was used as bond coat. The as-coated sample is characterized by XRD, SEM, EBSD and Raman spectroscopy. The top coat exhibited three phases Sm2SrAl2O7, ϒ-Al2O3, and α-Al2O3. The high temperature oxidation, hot corrosion and erosion tests were performed to examine the integrity of the coatings in critical conditions. Further, the surface of the coatings were modified with a laser beam to reduce the roughness and to seal the open porosities. The isothermal oxidation tests at 1100 °C for 150 h showed a parabolic weight gain and a nearly uniform Al2O3 layer formation at interface. The laser-treated samples showed a comparatively lower parabolic rate constant (kp = 2.2 mg2cm-4s-1) than the as- coated samples (kp = 2.5 mg2cm-4s-1). Dissociation of top coat was observed from the XRD analysis of oxidized surface. The electrochemical impedance spectroscopy of the laser-treated samples revealed a higher impedance than the as-coated samples due to eliminated porosity. The hot corrosion resistance of the samples at 700 °C and 900 °C were analyzed in aviation and marine conditions employing the salt combinations 50 %wt. Na2SO4 + 50 %wt. V2O5 and 90 %wt. Na2SO4 + 5 %wt. V2O5 + 5 %wt. NaCl, respectively. The samples manifested a lower resistance in marine conditions than aviation conditions, while the laser-treated samples showed a better hot corrosion resistance than the as-coated samples. The solid particle erosion tests using alumina erodent was carried out on the composite coatings at 200 °C and 800 °C. A mixed mode of failure with brittle and ductile material removal mechanisms was observed in the eroded areas. The weight monitoring calculated a higher average erosion value for the as-coated samples exposed to 800 °C at a 90° impact angle. Overall, the laser-treated samples showed a better performance at high temperatures due to the better mechanical properties and sealing of the porosities generated due to the coating process.
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    Synthesis and Development of Sm2SrAl2O7 Based Air Plasma Sprayed Ceramic Thermal Barrier Coatings: Oxidation, Hot Corrosion and High Temperature Erosion Study
    (National Institute of Technology Karnataka, Surathkal, 2018) T, Baskaran; Arya, Shashi Bhushan
    Samarium Stronium Aluminate (SSA) ceramic oxide was synthesized by molten salt method and coated on a bond coat (NiCrAlY)/Inconel 718 superalloy by air plasma spray (APS) process. The pre-oxidation, oxidation, hot corrosion and high temperature erosion tests were conducted at elevated temperatures. The pre-oxidation study was carried out to examine the role of thermally grown oxide (TGO) thickness on the oxidation kinetics of thermal barrier coatings (TBCs). TGO thicknesses were controlled at different pre-oxidation times of 10, 20 and 30 h at 1050 ºC in the argon atmosphere, the highest TGO thickness being found for 30 h pre-oxidized samples. Among these three pre-oxidation times, 20 h pre-oxidized SSA TBCs showed 65% higher oxidation resistance as compared to conventional Yttria stabilized zirconia (YSZ) TBCs (20 h) after oxidation at 1100 ºC in air mainly due to the presence of highly enriched α-Al2O3 at the interface. Electrochemical impedance spectroscopy was carried out and it was noticed that the highest charge transfer resistance and lowest capacitance about 0.48 × 106 Ohm cm2 and 1.1 nF cm-2 respectively were observed for 20 h of pre-oxidation. The impedance response was reduced significantly after oxidation at 1100 ºC for 10 h pre-oxidized specimen as compared to 20 and 30 h due to the compositional change of pure α-Al2O3 based TGO into more conductive NiCr2O4. The lowest diffusion coefficient, DCr3+ in the NiO lattice which reduced the formation of metal ion vacancies at the TGO-top coat interface caused to exhibit higher TGO resistance for 20 h pre-oxidized specimens over 10 and 30 h after oxidation at 1100 ºC. The hot corrosion resistance of SSA TBCs showed approximately 8-39% lower than conventional YSZ in hot corrosion environments of (i) 50 wt.% Na2SO4 + 50 wt.% V2O5 and (ii) 90 wt.% Na2SO4 + 5 wt.% V2O5 + 5 wt.% NaCl at 700 and 900˚C. The lower hot corrosion life of SSA TBCs was mainly due to the basicity of SrO. It is much higher than Sm2O3, Y2O3 and Al2O3, which indicates SrO has higher tendency followed by Sm2O3, Y2O3 and Al2O3 to react with molten salt at 700 and 900 ˚C. The high temperature erosion test was conducted in air jet erosion tester at an impingement angle of 30 and 90˚ for different testing temperatures of 200, 500, and 800 ˚C. The test was performed on pre-oxidized SSA and YSZ TBCs at 1050 ˚C for 10 h. The SSA TBCs exhibited 50% lower erosion resistance than conventional YSZ TBCs. The observed lower resistance is mainly due to the formation of mud-cracks during pre-oxidation treatment.