EQ Coating: A New Concept for SRZ-Free Coating Systems

Abstract

Ni-base single crystal (SC) superalloys with high concentrations of refractory elements prone to generate a diffusion layer called Secondary Reaction Zone (SRZ) beneath their bond coating during long time exposure at high temperatures. SRZ causes a reduction of the load bearing cross section and it is detrimental to the creep properties of thin-wall turbine airfoils. In this study, a new bond coat system, “EQ coating” which is thermodynamically stable and suppresses SRZ has been proposed. Diffusion couples of coating materials and substrate alloys were made and were heat treated at 1100°C for 300h and 1000h. Cyclic oxidation examinations were carried out at 1100°C in air and the oxidation properties of EQ coating materials were discussed. High velocity frame sprayed (HVOF) EQ coatings designed for 2 nd generation Ni-base superalloy were deposited on 4 th generation Ni-base superalloys. The stability of microstructure at the interface and creep property of the coating system were investigated. Less than 50µm-thick of the interdiffusion zone and no SRZ were observed in EQ coating system after 300h heat treatment at 1100°C, in contrast to beyond 90µm-thick of diffusion zone and 140µm-thick of SRZ in the conventional CoNiCrAlY coating system. Creep strength of Al-diffusion and CoNiCrAlY coated 4 th generation superalloys showed decrease in thin creep specimens, but EQ coated 4 th generation superalloy showed equivalent creep strength for bare material.