Thickness Distribution and Structure of External Ceramic Coatings on Gas Turbine Blades

Abstract

The rotor and nozzle blades are critical components of a gas turbine. Their materials and design determine the allowable gas temperature at the turbine inlet and directly influence the technical and economic performances of gas turbine engines. Improving gas turbine cycle parameters requires the development of fundamentally new blade protection systems and transition from oxidation-resistant multicomponent coatings to thermal barrier coatings. The structure and properties of an external zirconium dioxide ceramic coating deposited on a gas turbine blade airfoil using high-speed evaporation–condensation were studied to assess the potential for extending blade service life. X-ray diffraction analysis of yttria-stabilized zirconia showed that the content of the tetragonal and monoclinic phases was 30 wt.% and 50 wt.%, respectively. This indicates incomplete transition (stabilization) of the monoclinic phase into the tetragonal phase during powder synthesis. Hightemperature annealing of the ceramics promotes phase redistribution, which positively influences the powder structure by increasing the tetragonal phase content to 70 wt.%. The ceramic coating was deposited in a vacuum of 1–10^–2 Pa using electron-beam heating of the blades to 870–900°C. Optimal process parameters were established to enable the formation of an external ceramic layer on the blade airfoil with a thickness ranging from 80 to 120 μm. The deposited ceramic coating exhibits a columnar structure, with an average crystallite diameter of 2–3 μm and length approximately equal to the coating thickness. The microhardness of the ceramic coating ranges from 500 to 700 MPa. The findings demonstrate that the external thermal barrier ceramic coating applied to gas turbine blades by high-speed evaporation–condensation ensures a monoclinic (5–10%) to tetragonal (90–95%) phase ratio that corresponds to the most acceptable service properties.