Preparation And Performance Of Abradable Seal Coating On Titanium Alloy

As one of most important coatings used in turbo-machinery, abradable seal coating plays an important role in gas path closure system. It helps to reduce the distance between the turbine and the blade and can improve the efficient while working.The powders of abradable seal coating are multiphase material, and abradable seal coating can be prepared by thermal spraying method. The characteristics of the abradable seal coating prepared by thermal spraying method are uneven on the surface and polyporous throughout the coatings, mixed with unmelted metal particles and oxidates in the coatings. The pores will become the channels where the corrosion medium existed in the environment can infiltrate the internal of the coatings to accelerate the corrosion of the coatings to reduce the process safety in mechanical use.In this thesis, the air plasma spraying(APS) method was used to prepare bond coating(Ni Al) and top coating(Al BN) on TC4 titanium alloy substrate. The electrochemical noise test method, coupled with electrochemical test method(Tafel curves, EIS and OCP), SEM method, metallurgical test method and XRD method were used to study the galvinc corrosion behavior of the two galvanic couples(titanium alloysubstrate/Ni Al coating, Ni Al coating/Al BN coating).From the results, it can be shown that the galvanic corrosion behaviors between titanium alloysubstrate/ Ni Al coating and Ni Al coating/Al BN coating were partly similar and different from each other. The galvanic corrosion levels of the two galvanic couples are A levels, which mean that the couple could be used by contacting. The corrosion category of the anodes is pitting corrosion. For Ni Al coating, the porosity of Ni Al coating is about 10 percent. There exists galvanic interaction between Ni and Al on account of different potential existed on the Ni element and Al element. The corrosion products of the Ni-Al coatings show compact and accumulate on the surface of the Ni-Al coatings to block up the pores existed in the coatings, which helped to improve the corrosion resistance.For Al BN coating, the porosity of Al BN coating is about 20 percent. As an electrical insulator, the BN materials have no galvanic action when used as an anode. The corrosion behavior of the Al BN coating was similar to aluminum. But the surface complexity and porosity are different from the Al metal. The results show that corrosion products of Al BN coatings are loose and have poor adhesion, which can not accumulate on the surface of the coating to prevent penetration of corrosive ions. The corrosion products of the Al BN coatings can not accumulate on the coating surface and make the corrosion resistance of the coating be worse. Comparing with the Ni Al coating, the noise resistance(Rn), polarization resistance(Rp) and the charge transfer resistance(Rt) are consentaneous.The corrosion resistance of Ni Al coating increased slightly, and the Al BN coating reduced greatly after salt spray corrosion test. Al BN coating has a poor corrosion resistance itself, especially in the environment which contains a lot of corrosive ions.