Evaluation of AlCoCrFeNiTi-high entropy alloy (HEA) as top coat material in thermal barrier coating (TBC) system and investigation of its high temperature oxidation behavior

To obtain compatible properties of high-temperature performance and mechanical strength properties, AlCoCrFeNiTi high-entropy alloy (HEA) was designed as a new candidate material for metal-based thermal barrier coating (TBC) systems. The aim of this study is to investigate potential applications of AlCoCrFeNiTi-HEA as a coating material for TBC systems and to determine its behavior under high temperature conditions. CoNiCrAlY bond coatings were produced on the Inconel-718 substrate surface using high-velocity oxygen fuel (HVOF) technique. AlCoCrFeNiTi-HEAs were produced on CoNiCrAlY bond coatings using atmospheric plasma spray (APS) technique and a typical TBC system structure was obtained. The produced AlCoCrFeNiTi-HEA TBC system was exposed to oxidation at temperatures of 1000 °C and 1100 °C for time periods of 5 h, 25 h, 50 h and 100 h in order to determine the oxidation resistance under isothermal conditions and to investigate formation and growth behavior of oxide structures formed at the coating interface. As a result of oxidation tests, the growth behavior of the thermally grown oxide (TGO) layer formed between the coating interfaces and the microstructural changes occurring in the coating system were investigated depending on temperature and time processes. In the TBC system with Ti-containing HEA content, a transformation from body-centered cubic (BCC) structure to rhombohedral crystal lattice structure occurred as a result of increasing temperature. Many spinel compound forms were formed at the coating interface. It was observed that the coating system with AlCoCrFeNiTi-HEA content maintained its structural integrity without any damage such as microstructural and mechanical spalling and cracking under conditions of high temperature and different time periods.