Study On Preparation And Oxidation Resistance Of Reactive Element Doped Nial Bond Coat

Thermal barrier coatings?TBCs?are widely used in land-based gas turbines and aero-engines to reduce the surface temperature of hot-section components and protect them from high temperature gas corrosion and oxidation,thus improve the operation efficiency and enhance the service life.After long time service under high temperature?isothermal or cycled?,TBCs would spall off and leave superalloy components indirectly exposed to high temperature gas,resulting in terrible accidents.The TBC durability is mostly dependent on the oxidation resistance of the bond coat,which is directly determined by its chemistry.Therefore,improving the oxidation resistance of the bond coat by optimizing its chemistry is a most important approach to enhance the TBC durability.As the inlet temperature of gas turbines continues to rise,the conventional MCrAlY?M:Ni or Co?material can not meet its demand.NiAl,with high melting point,low density,low cost and good oxidation resistance,is considered as the most promising replacement for MCrAlY.However,the thermally grown oxide?TGO?formed on NiAl spalls readily during thermal cycling,due to its weak adhesion to the underlying alloy.To improve the scale adhesion,a thirdly additional element is essential,particularly reactive elements?REs?.Atmospheric plasma spray?APS?,as a versatile and economic method,is widely used to deposit metallic bond coats,such as MCrAlY.Compared with MCrAlY,fabrication of NiAl bond coat by APS is relatively difficult and is rarely reported.Therefore,optimizing the APS parameters to deposit NiAl bond coat and improving its oxidation resistance is of great importance.This work attempts to develop an economic and high oxidation performance bond coat,thus enhances the application temperature and lifetime of TBCs.On the one hand,in order to improve the oxidation resistance of NiAl,reactive element?e.g.,Hf?with optimized concentration is doped into NiAl.On the other hand,APS technique is used to fabricate NiAl-RE bond coat.The oxidation resistance of NiAl-RE bond coat with and without ceramic top coat is tested under high temperature exposure,and compared with NiCoCrAlY sample.Experimental results demonstrate that the oxidation resistance of NiAl-RE bond coat is significantly higher than the NiCoCrAlY sample,and the TBC lifetime with NiAl-RE bond coat is substantially enhanced.However,due to the low strength of NiAl material under high temperature,the surface of NiAl-RE bond coat has a rapid roughening rate,which hinders further enhancement of TBC lifetime.Therefore,Al₂O₃dispersion strengthened NiAl is prepared and HfO₂ is added to improve its oxidation resistance.The main research content and conclusions are described as follows:?1?NiAl alloys with and without RE addition were prepared by arc-melting,and the influences of Lu,Hf and Cr addition on the oxidation resistance of NiAl were investigated.It demonstrated:?a?both Lu and Hf decreased the oxidation rate of NiAl,while the oxidation rate of NiAl-Lu was almost four times that of NiAl-Hf;?b?segregation of S and C occurred not only at the alloy/TGO interface,but also within the TGO from the interface to some depth.Both Lu and Hf significantly improved the interfacial adherence by suppressing this phenomenon;?c?the beneficial effect of Lu in enhancing the TGO adherence can be easily overcompensated by Cr.?2?NiAl bond coats with and without Hf addition were deposited on two types of superalloy substrates?i.e.,DZ125L,no Hf,and DZ125,0.51at.%Hf?using APS.Oxidation test demonstrated that the NiAl-Hf bond coat showed significantly improved oxidation resistance compared with the NiCoCrAlY bond coat.In addition,it was found that Hf addition in the bond coat was more effective than in the superalloy substrate for improving the bond coat oxidation resistance.?3?Four TBC systems with different types of bond coats?NiAl,NiAl-Lu,NiAl-Hf and NiCoCrAlY?were fabricated by APS,and the influences of bond coat chemistry on TBC lifetime and failure mode were investigated.Compared with NiCoCrAlY TBC,the lifetime of NiAl TBC was slightly shorter,while the NiAl-Lu and NiAl-Hf samples showed significantly enhanced lifetimes.It was proposed that the enhanced lifetime is a result of combination of decreasing of the oxidation rate and retention of the TGO/bond coat interfacial toughness,thus changing the preferred cracking positions in TBC.?4?Oxide dispersion strengthened?ODS?NiAl bulks were obtained by mechanical alloying and spark plasma sintering,and the effect of Al₂O₃ and HfO₂ addition on surface roughening and TGO spallation were investigated.Compared with arc-melted NiAl alloy,the ODS NiAl showed a rapid surface roughening rate,and the addition of Al₂O₃ accelerated this phenomenon.The ODS NiAl showed poor oxidation resistance and had limited improvement by 0.5 wt.%HfO₂ addition.However,with high content addition of Al₂O₃?e.g.20 vol.%?,its oxidation resistance was substantially improved,which was mainly attributed to the decrease in thermal mismatch stress.