Initial phase hot corrosion mechanism of gas tunnel type plasma sprayed thermal barrier coatings

The hot corrosion resistance of the top layer in TBC is one of the main constructive factors which determines the lifetime of the coatings under critical operating environments. In the present study, 8 wt% yttria stabilized zirconia (8YSZ), lanthanum zirconate (La2Zr2O7) and equal weight percentage of its composite (50%8YSZ + 50% La2Zr2O7) coatings were prepared by using gas tunnel type plasma spray torch at optimum spraying conditions. The hot corrosion performances of the above thermal barrier coatings were examined against 40 wt%V2O5–60 wt%Na2SO4 corrosive ash at 1173 K for 5 h in open air atmosphere. After hot-corrosion testing, the coating surface was studied using a scanning electron microscope to observe the microstructure and X-ray diffraction techniques were used to identify the phase compositions. The results showed that LaVO4 and YVO4 are the main hot corrosion products along with the ZrO2 phase transformation from tetragonal to monoclinic phases in La2Zr2O7 and 8YSZ coatings respectively. The microstructure and phase formation mechanism of the hot corrosion products varied with each coating and among these, composition of 50%8YSZ + 50%La2Zr2O7 coating exhibited least degradation against V2O5–Na2SO4 corrosive environment compared to the other coatings.

► Free standing TBC specimens were prepared by gas tunnel type plasma spraying with thickness of around 300–400 μm.
► 50%8YSZ + 50%La2Zr2O7 composite coating shows superior hot corrosion resistance.
► Corrosive crystals structure and phase transformation was well controlled in this coating.