Synthesis and Development of Sm2SrAl2O7 Based Air Plasma Sprayed Ceramic Thermal Barrier Coatings: Oxidation, Hot Corrosion and High Temperature Erosion Study

Abstract

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.