The Research Of Inter-diffusion Behavior And Diffusion Resistance Of The Interface Between Substrate And Bond Coating In Thermal Barrier Coatings

During high-temperature service,the Thermal Barrier Coating of an aircraft engine turbine blade is inevitably subjected to the long-term action of high temperature,which can cause a severe interdiffusion of the elements at the interface between the substrate and the bonding layer.The interdiffusion phenomenon can change the composition and structure of the bonding layer and the substrate,subsequently resulting in the bonding layer failure and the mechanical properties reduction of the substrate.Therefore,in the present paper,the interface between the Ni-based alloy Rene N5 substrate and the NiCrAlY bonding layer was considered as the research object,in order for the interfacial diffusion behavior and the formation mechanism of the reaction zoneto be analyzed,whereas the rules of interfacial mass transfer and structuralevolution were investigated.The ?-Al₂O₃ diffusion barrier layer that was in-situ formed during the YSZ active diffusion barrier layer service,was utilized for the control on the diffusion behaviors of both the substrate and the bonding layerto be achieved.Through the research on the structural evolution behavior of the YSZ active diffusion barrier layer,the formation,degradation and failure mechanisms during the high-temperature long-time service of the active diffusion barrier layer with a "sandwich" structure wereobserved.Subsequently,the multi-layer composite bonded structure was proposed,which contained the YSZ active diffusion barrier layer that had alamellar toughening effect,whereas the new bonding layer with a function of diffusion resistance and long serving life was prepared.The obtained main research results were as follows:1.In the 1000 oC high-temperature circumstance,both Aland Cr in the NiCrAlY bonding layer diffused to the N5 substrate,whereas the Ni in the N5 substrate diffuses into the NiCrAlY bonding layer.Therefore,the interdiffusion reaction zone and the secondary reaction zone containing the TCP harmful phase were formed at the interface between the substrate and the bonding layer.The interdiffusion reaction zone formation was mainly determined by the Al and Cr diffusion to the substrate,in order for the corresponding organizational structure to be mainly composed of the ?-NiAl and ?-Cr phases.The formation of the secondary reaction zone,however,was determined by the Ni diffusion to the NiCrAlY bonding layer,in order for the corresponding organizational structure to be mainly composed of the ?'-Ni3 Al phase.The TCP harmful phase in the secondary reaction zone were mainly composed of refractory metals.2.In the high-temperature circumstance,the YSZ active diffusion barrier layer hadan interfacial reaction with Al in the N5 substrate and the NiCrAlY bonding layer,subsequently forming an ?-Al₂O₃ diffusion barrier layer with a "sandwich" structure,which could effectively control the element diffusion occurring atthe interface between the substrate and the bonding layer.The active diffusion barrier layer formation was mainly affected by the ambient temperature.When the temperature was 800 oC,the ?-Al₂O₃ diffusion barrier layer was formed only at the interface between the YSZ precursor layer and the NiCrAl Y bonding layer.When the temperature increased to 900 oC and beyond,the ?-Al₂O₃ diffusion barrier layer with a "sandwich" structure was formed on both sides of the YSZ precursor layer.3.The YSZ active diffusion barrier layer had an excellent resistance against diffusion.Even subsequently to a 250 h cyclic oxidation at 1000 oC,such a barrier layer could effectively resist the element interdiffusion behavior between the N5 substrate and the NiCrAlY bonding layer,in order for the interdiffusion the reaction,the second reaction zones and the TCP harmful phase not to beformed.When the oxidation duration exceeded 250 h,a fracture occurred at the interface between the ?-Al₂O₃ diffusion barrier layer and the NiCrAlY bonding layer,resulting in the coatingfailure.4.As indicated by the analysis result of the ABAQUS finite element simulation,during the thermal cycling,the N5/YSZ/NiCrAlY sample formed apparentstress concentration zone sencircling the interface between the ?-Al₂O₃ diffusion barrier layer and the NiCrAlY bonding layer,resulting in an easier occurrence of cracks at the interface.In addition,the cracksexpandedquickly and subsequentlyproduced fracture due to the ?-Al₂O₃ layer brittleness.5.The multi-layer composite structure could increase the fracture toughness of the YSZ active diffusion barrier layer and form a long-life new bonding layer with diffusion resistance.Based on the researches on the structural evolution behavior of multi-layer composite bonding layer,it was discovered that the multi-layer composite structure could increase the fracture toughness of the YSZ active diffusion barrier layer,without affecting the corresponding diffusion resistance.Following 500 h cyclic oxidation in the 1000°C high-temperature circumstance,the coating still demonstrated good stability and excellent resistance against the diffusion behavior.Additionally,through the ABAQUS finite element simulation result analysis,it was discovered that the toughening mechan-ism of the multi-layer composite structure mainly resultedfrom both crack deflection toughening and residual stress toughening.