Abstract

MCrAlY powder where M stands for Nickel (Ni) or Cobalt (Co) is extensively used in the aerospace industry as a bond coat for its corrosion and oxidation resistant characteristics. MCrAlY can sustain prolonged exposure to high temperatures such as in gas turbines. The aluminum present predominantly in the β-phase of the alloy forms a protective oxide called aluminum oxide or alumina. This latter acts as a barrier for further oxidation provided that there is no chemical breakdown resulting in the formation of non-protective oxides.
Numerical analyses have been made in place to predict the degree of oxidation of the feedstock during their flight in thermal spray processes. The most prominent is the Mott and Cabrera theory developed in 1944. The theory stipulates that the film oxide can be associated with three growth processes/thicknesses namely, very thin films, thin films, and thick films. The in-flight oxidation of particles is proven to be in the order of milliseconds, hence only very thin films are formed. In this range of thickness, the oxidation is driven by the presence of a strong electric field.
High velocity air-fuel (HVAF) has revolutionized the very exclusive group of solid-state deposition. The process comes in with the promise of reducing the particle temperature while increasing their velocity with the stated goal of diminishing the oxidation and enhancing the adhesion particle-substrate. The combination of HVAF and MCrAlY delivers qualitative coatings with low oxide content and a longer lifespan. The numerical modeling of the in-flight oxidation of the particles is validated by examining the oxide layer thickness using a Transmission Electron Microscope (TEM).