Effect of heat treatment on the interfacial element diffusion and hardness of FeCoNiCrAl high-entropy alloy coatings

FeCoNiCrAl high-entropy alloy (HEA) coatings have here been prepared on Q235 steel by using the plasma spraying technology. The effects of heat treatment (300 °C, 400 °C, and 500 °C) on the coating phase, surface and cross section morphologies, surface Brinell hardness (HRB), cross section Vickers hardness (HV), and element diffusion at the coating interface, were then investigated. The distribution of pores, cracks, and unmelted particles on the surface of the coating was analyzed by scanning electron microscopy (SEM). The results showed that the BCC crystalline structure of the HEA coating did not change significantly after heat treatment at 300 °C, 400 °C, and 500 °C. In addition, the bonding between the coating and the matrix interface became significantly improved, and the cracks at the coating interface were eliminated. New oxides that filled the pores and cracks inside and at the interface of the coating were also observed as the temperature and holding time increased. Furthermore, the surface hardness of the Q235 steel was reduced as a result of the spray coating. However, this surface hardness was increased to HRB 85 (or even more) after heat treatment. Compared with the other samples, the coating sample that was heat-treated at 500 °C showed the best strength.