ABSTRACT
In this paper, NiCoCrAlY/YSZ nanometer thermal barrier coatings have been successfully deposited on the substrate of GH3128 superalloy by magnetron sputtering and electrophoresis deposition composite technologies. With the help of XRD, SEM and TriboIndenter, several studies such as phases, microstructure appearance, microhardness,elastic modus of oxide films which forming in the process of preoxidation heat treatment of the bond coats, at the temperature of 500℃, 700℃, 900℃, 1100℃ in different oxygen partial pressure were investigated. Besides, the effects of oxide films with various performances due to the preoxidation parameters on the high temperature oxidation weight gain and hot corrosion fatigue life of TBC were carried out. It is defined that the phase composition of the bond coat by magnetron sputtering was γ-Ni and the microsructure is dense and uniform. While the YSZ by eletrophoresis deposition was t- ZrO2, with a little micro pores.
The phases of oxide film were -Al2O3, Cr2O3, NiO, (Co,Ni)(Al,Cr)2O4 after the bond coat preoxidized in the aerosphere environment at five different temperatures. Due to the temperature, it was believed that microstructure morphologies on the surface of oxide film were different. And it also has been proved that oxide film distributes with clear-structure.The dense oxides formed at the beginning of preoxidation heat treatment were obtained on the subsurface of bond coat, while the loosened oxides formed at the end of heat treatment were found on the surface of bond coat. Owing to this, microhardness and elastic modus of cross section oxide film increase firstly and then decrease with the indentation depth increases. Therefore, during the cyclic oxidation tests, affected by the microstructure and mechanical performance of oxide film, the high temperature oxidation weight gain of TBC with preoxidation heat treatment has been clearly declined, especially preoxidized at the temperature of 900℃. In addition, the hot corrosion fatigue life has also been prolonged.
The phases of oxide film were -Al2O3, NiCr2O4 after bond coat preoxidized in the low vacuum environment at various temperatures. Affected by the oxygen concentration, the main phase of oxide fim was -Al2O3 after heat treatment. It has been defined that surface appearances of oxide film also exist obvious differences. The uniform and dense structure was analyzed after preoxidation at the temperature of 900℃, and microhardness has been maintained on the whole cross section. But microhardness of loosened oxide film changes greatly with the indentation depth increases.
Compared to the preoxidation hear treatment of bond coat in the aerosphere environment, the high temperature oxidation weight gain can be reduced and hot corrosion fatigue life can be prolonged further when TBC preoxidized in the low vacuum environment, especially at the temperature of 900℃. It is mainly depending on the uniform and dense microstructure of oxide film. It is believed that more -Al2O3 with uniform microstructure was obtained after preoxidized in low vacuum environment. When the TBC was used, for one thing, attributed to the presence of the initial TGO, the formation of residual stress, due to the thermal expansion coefficient mismatch of the new/old phases at the interface of top/bond coat can be avoided. For another thing, it is advantageous to the formation of a dense and homogeneous oxide film through reducing the pressure of oxygen at the ceramic/bond coat interface.
KEY WORDS：Magnetron sputtering, Electrophoresis deposition, TBC, Preoxidation heat treatment, Oxygen partial pressure
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