Effects of initial oxidation on β phase depletion and oxidation of CoNiCrAlY bond coatings fabricated by warm spray and HVOF processes

Abstract

Thermal barrier coatings (TBC) have been applied extensively onto the high-temperature components in turbine engines to prolong their service life in extremely harsh environments. TBCs are typically composed of a ceramic top coating for thermal insulation and a metallic bond coating (BC) for oxidation resistance and providing adhesion to the top coating. MCrAlY, where M stands for Co, Ni or an alloy of these elements, is a widely used material for BC and usually produced by low pressure plasma spraying (LPPS) in industry. Recently high velocity oxy-fuel (HVOF) spraying is attracting significant attention as a more economical alternative procedure to LPPS. In terms of the quality of sprayed coatings, however, LPPS is still superior in terms of oxygen pick-up during coating preparation, which should affect the performance as a bond coating in service. In this study, a modified HVOF process, so called 2-stage HVOF or warm spray (WS) was applied to deposit a CoNiCrAlY alloy. Comparisons between BCs fabricated by HVOF and WS were made in terms of microstructure, surface morphology, and cyclic and isothermal oxidation behaviors in air at 1423K up to 100cycles and 100h respectively. The results showed that rougher and less oxidized BCs were deposited by the WS process, which exhibited slower kinetics of β-phase depletion during oxidation. A simple Al diffusion model revealed that apparently a small difference in the initial oxidation between the two spraying processes had significant influence on the β-depletion phenomena, which may influence the life time as a bond coating.