A Study On Properties Improvement Of TBC By Pretreatment

In this paper, NiCoCrAlY/YSZ nanometer thermal barrier coatings have beensuccessfully deposited on the substrate of GH3128superalloy by magnetron sputtering andelectrophoresis deposition composite technologies. With the help of XRD, SEM andTriboIndenter, several studies such as phases, microstructure appearance, microhardness,elastic modus of oxide films which forming in the process of preoxidation heat treatmentof the bond coats, at the temperature of500℃,700℃,900℃,1100℃in different oxygenpartial pressure were investigated. Besides, the effects of oxide films with variousperformances due to the preoxidation parameters on the high temperature oxidation weightgain and hot corrosion fatigue life of TBC were carried out. It is defined that the phasecomposition of the bond coat by magnetron sputtering was γ-Ni and the microsructure isdense and uniform. While the YSZ by eletrophoresis deposition was t-ZrO₂, with a littlemicro pores.The phases of oxide film were-Al₂O₃, Cr₂O₃, NiO,（Co,Ni）（Al,Cr）2O₄after the bondcoat preoxidized in the aerosphere environment at five different temperatures. Due to thetemperature, it was believed that microstructure morphologies on the surface of oxide filmwere 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 onthe subsurface of bond coat, while the loosened oxides formed at the end of heat treatmentwere found on the surface of bond coat. Owing to this, microhardness and elastic modus ofcross section oxide film increase firstly and then decrease with the indentation depthincreases. Therefore, during the cyclic oxidation tests, affected by the microstructure andmechanical performance of oxide film, the high temperature oxidation weight gain of TBCwith preoxidation heat treatment has been clearly declined, especially preoxidized at thetemperature of900℃. In addition, the hot corrosion fatigue life has also been prolonged.The phases of oxide film were-Al₂O₃, NiCr₂O₄after bond coat preoxidized in thelow vacuum environment at various temperatures. Affected by the oxygen concentration, the main phase of oxide fim was-Al₂O₃after heat treatment. It has been defined thatsurface appearances of oxide film also exist obvious differences. The uniform and densestructure was analyzed after preoxidation at the temperature of900℃, and microhardnesshas been maintained on the whole cross section. But microhardness of loosened oxide filmchanges greatly with the indentation depth increases.Compared to the preoxidation hear treatment of bond coat in the aerosphereenvironment, the high temperature oxidation weight gain can be reduced and hot corrosionfatigue life can be prolonged further when TBC preoxidized in the low vacuumenvironment, especially at the temperature of900℃. It is mainly depending on the uniformand dense microstructure of oxide film. It is believed that more-Al₂O₃with uniformmicrostructure was obtained after preoxidized in low vacuum environment. When the TBCwas used, for one thing, attributed to the presence of the initial TGO, the formation ofresidual stress, due to the thermal expansion coefficient mismatch of the new/old phases atthe interface of top/bond coat can be avoided. For another thing, it is advantageous to theformation of a dense and homogeneous oxide film through reducing the pressure of oxygenat the ceramic/bond coat interface.