Abstract:The influence of the certain specific vacuum pre-oxidation process on the phase transformation of thermally-grown oxides (TGO) was studied. The CoCrAlY high temperature corrosion resistance coatings were produced onto the nickel-based superalloy substrate by high velocity oxygen fuel (HVOF). It suggests that the TGO usually consists of a great number of chromium oxides, cobalt oxides and spinel oxides besides alumina during the initial period of the high temperature oxidation if the specimens are not subjected to the appropriate vacuum pre-oxidation process. Furthermore, the amount of alumina is strongly dependent on the partial pressure of oxygen; while the CoCr₂O₄ spinel oxides are usually formed under the conditions of higher partial pressure of oxygen during the initial period and the lower partial pressure of oxygen during the subsequent period of the isothermal oxidation. After the appropriate vacuum pre-oxidation process, the TGO is mainly composed of alumina that contains lower Y element, while alumina that contains higher Y element sporadically distributes, and the spinel oxides cannot be found.After a longer period of the isothermal oxidation, a small amount of porous CoCr₂O₄ and the chrome oxide sporadically distribute near the continuous alumina. Additionally, after the appropriate vacuum pre-oxidation process, the TGO growth rate is relatively slow.

Key words: vacuum pre-oxidation process; thermally-grown oxides (TGO); high velocity oxygen fuel (HVOF); spinel oxides