Oxidation and hot corrosion behaviour of functionally graded yttria stabilized zirconia and lanthanum zirconate thermal barrier coatings

Abstract

In the present work, NiCoCrAlY + YSZ (Yttria stabilized Zirconia) and NiCoCrAlY + LZ (Lanthanum Zirconate) multilayer functionally graded thermal barrier coatings (FGTBC) were developed on Inconel 738 using the thermal plasma spray method. The gradual increase in the proportion of top coat material with a simultaneous gradual decrease in the proportion of NiCoCrAlY bond coat minimizes the thermal mismatch resulting in the improvement of the life of the coating. Developed coatings were tested for oxidation and hot corrosion study at 900 °C temperature. The hot corrosion of the coated specimen was conducted in an aggressive environment of 90 wt% Na2SO4 + 5 wt% NaCl +5 wt% V2O5 for 240 h with intermediate removal of specimen after 24 h. The specimens were evaluated by Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS) and X-ray diffraction (XRD) analysis after oxidation and hot corrosion. The study revealed the dominance of vanadium with the presence of YVO4 phase and LaVO4 phase after hot corrosion on the coating surface. The study near the crack and open surface after crack showed that the use of NiCoCrAlY bond coat in combination with YSZ and LZ, will form an oxidation product i.e. Al2O3 layer protecting the surface. The electrochemical study of the functionally graded YSZ and LZ after the hot corrosion test has revealed the better performance of LZ FGTBC over functionally graded YSZ FGTBC.