Development of Al2o3 - Sm2sral2o7 Ceramic Composite Thermal Barrier Coatings for High Temperature Applications

Abstract

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.