Development of Functionally Graded Coating Material for Metallic Thermal Protection System of Reusable Launch Vehicle

Functionally graded coating material (FGM) based on yttria-stabilized zirconia (YSZ) and Ni–Cr–Al–Y was designed and developed for metallic thermal protection system of reusable launch vehicle (RLV). Coating was made using premixed mechanically alloyed YSZ and Ni–Cr–Al–Y powders through plasma spray technique. Thermal stress analysis was carried out, which showed significant reduction in stress in FGM coating as compared to dual coating. The phase composition of coating was found to be close to the designed one. Porosity varied in the range of 8–18%. Average emissivity of three different time exposures of 30, 60 and 90 s was found to be 0.8. Solar absorptivity was found to be 0.55. Fatigue life of FGM coating evaluated along with Inconel and Ti6Al4V metallic substrate was compared with dual coating. FGM coating could be fatigue tested to relatively higher thermal cycles as compared to dual coating on the Inconel substrate. Heat flux measured at top surface was found to be close to simulated heat flux for windward side of RLV. Top surface temperature was similar for both type of metallic substrates and was matching with predicted temperature. However, substrate temperature was higher for Ti6Al4V as compared to Inconel alloy due to higher thermal diffusivity of Ti6Al4V.