An investigation on high entropy alloy for bond coat application in thermal barrier coating system

Abstract

The paper aims to investigate standalone FeCoCrNi2Al, FeCoCrNiAl0.3, FeCoNiAlTi0.4 and NiCoCrAlSi high entropy alloys (HEA) as a possible bond coat material for thermal barrier coating (TBC) system. For this high entropy alloys prepared by mechanical activated synthesis (MAS) were densified by spark plasma sintering (SPS). MAS HEAs were investigated for phase formation, melting temperature and coefficient of thermal expansion by X-ray diffractometer (XRD), Differential scanning calorimetry (DSC) and Dilatometer respectively. Isothermal oxidation of sintered samples was carried out at 1050 °C for a period of 5, 25, 50, 100, 200 and 300 h in the air. The formation and growth of thermally grown oxides (TGO) were investigated by Raman spectroscopy, X-Ray Diffraction, and Scanning electron microscopy (SEM). The oxidation study shows that FeCoCrNi2Al and FeCoNiAlTi0.4 HEA follow parabolic rate weight gain due to the formation of TGO enriched in Al2O3. Discontinuous weight changes due to the formation of CoAl2O4, NiCrFeO4, and Cr2O3 phases were observed in FeCoCrNiAl0.3 HEA. TGO enriched in Al2O3 and NiAl2O4 were observed in NiCoCrAlSi HEA, whereas FeCoNiAlTi0.4 HEA shows the formation of TGO enriched in Al2O3, NiAl2O4, Ti2O3, and Al2O5Ti. Increase in coefficient of thermal expansion (CTE) with increasing temperature is observed for FeCoCrNi2Al and FeCoNiAlTi0.4 HEA. FeCoCrNi2Al HEA showing average CTE of 15.16 ± 0.25 × 10−6/K, good mechanical properties, and containing α-alumina TGO layer, makes it a potential candidate for a bond coat material.