Study On Design And Prolonging Life Mechanism Of Diffusion Barrier Layer In Thermal Protective Coating For Aeroengines

The hot end components of aeroengine are faced with wide temperature range service conditions such as medium high temperature and ultra-high temperature,which also puts forward strict requirements for the high temperature performance of component materials.Martensitic stainless steel and nickel base superalloy are typical materials for medium and high temperature components,while cobalt base superalloy is typical materials for ultra-high temperature components.A large number of studies have reported that thermal protective coating is one of the effective methods to improve the service life of hot end components.Among them,aluminum based alloy coating is the preferred protective material for martensitic stainless steel.However,in the process of high temperature service,the mutual diffusion between coating and substrate is serious,which accelerates the growth of brittle hard phases and the initiation of high stress,leading to premature spalling and failure of the coating.The superalloy coating is a protective material of nickel base superalloy,which attracts much attention.However,there is a competitive oxidation of coating elements during high-temperature service,which leads to the growth of brittle and hard compounds and the initiation of high stress,accelerates the formation of defects such as coating structure fracture and micro-pore,and intensifies the degradation and premature failure of coating protective performance;Thermal barrier coating is one of the widely used ultra-high temperature barrier materials,but there are a lot of pores and micro-cracks in the coating structure,which accelerate the diffusion of elements in the metal bonding layer and the growth of brittle oxides,induce high interface stress and lead to the damage of the coating structure.Therefore,it has become a common key scientific problem in the field of thermal protection coatings for aeroengines to effectively inhibit or mitigate the mutual diffusion between coating and substrate,the competitive growth of alloy elements,and the growth of thermally grown oxides for key protective coating materials in different temperature domains.In view of the urgent need for the coordinated control of diffusion and thermal stress at the interface of thermal protective coatings in a wide temperature range,this thesis innovatively carried out the design of a new diffusion barrier layer and the research on the mechanism of prolonging life,that is,by introducing a Cr barrier layer between the martensitic stainless steel and the Al Si protective coating,the interdiffusion between the coating and the substrate was suppressed.The YSZ/LMA double ceramic thermal barrier coating was coated with aluminum to repair the inherent defects of the structure.The in-situ reaction of aluminum oxide can inhibit the diffusion of oxygen generation and corrosion media;The element diffusion,competitive growth and brittle spinel formation of Co Cr Ni Al Y coating can be effectively controlled by precise aluminum doping design.The microstructure and high temperature performance of Cr/Al Si coating were systematically studied.The results showed that the single-layer Al Si coating had poor high temperature oxidation resistance and hot salt corrosion resistance.The diffusion of Al and Si atoms led to the formation of a large number of holes,and the interdiffusion of Fe and Al atoms led to the formation of Fe Al and Fe₂Al₅ phases,leading to the cracking and premature failure of the coating interface.The Cr/Al Si composite coating has improved high-temperature oxidation resistance and hot salt corrosion resistance.The Cr interlayer induces significant diffusion barrier and self-healing effects.The Cr dominated composite oxide layer effectively blocks the interdiffusion of Fe and Al atoms,inhibits the formation of Fe Al and Fe₂Al₅ brittle alloy compounds,and reduces the fracture sensitivity of the coating interface.The upward diffusion of Cr atoms effectively heals the hole defects formed by uneven diffusion of the coating,and improves the barrier ability,structural stability and protective effect of the coating.The high-temperature oxidation resistance and hot salt corrosion behavior of aluminized YSZ/LMA double ceramic thermal barrier coatings were studied.The results showed that the unplated Al coatings had poor high-temperature oxidation resistance and hot salt corrosion resistance.The high temperature drove the crystallization reaction of LMA coatings accompanied by significant volume shrinkage,which led to the initiation and propagation of a large number of micro-cracks,intensified the internal diffusion of oxygen and corrosion media,and accelerated the diffusion of metal elements in the bonding layer The rapid growth of TGO and the formation of brittle（Ni,Co）Cr₂O₄ spinel phase led to the crack failure of the coating interface.Al coating can effectively repair the micro-cracks generated by LMA crystallization reaction,while Al₂O₃ generated by in-situ reaction further inhibits the internal diffusion of oxygen and corrosion medium,effectively retards the diffusion of metal elements in the bonding layer,the rapid growth of TGO and the formation of brittle（Ni,Co）Cr₂O₄ spinel phase,and the aluminized YSZ/LMA coating has better oxidation resistance and hot salt corrosion resistance.The high temperature structural stability and oxidation resistance of Al doped Co Cr Ni Al Y bonding layer were studied.The research results show that the undoped Co Cr Ni Al Y coating in high temperature environment has serious element diffusion,competitive oxidation and Al consumption,leading to the formation of a large number of brittle（Ni,Co）Cr₂O₄ spinel phases,inducing a large number of holes and high stress initiation,causing serious fracture damage to the coating structure,and its high temperature protection effect is significantly degraded.However,Al doping leads to the formation of multi-level and large area of aluminum rich area in the coating structure.In the process of high temperature oxidation,the preferential oxidation of Al leads to the formation of a large area of continuous and dense aluminum oxide layer,which can effectively prevent the oxidation and local diffusion of metal elements.Al doped Co Cr Ni Al Y coating has excellent structural stability and high temperature oxidation resistance.Based on systematic experimental evaluation and micro-scale analysis and characterization,this thesis confirms that the interdiffusion problem between martensitic stainless steel and aluminum based coating interface can be significantly improved through the design of Cr diffusion barrier;The design of aluminum coating on the surface can significantly inhibit the internal diffusion of oxygen and corrosion medium,delay the diffusion of metal elements in the bonding layer and the rapid growth of TGO layer,and improve the structural stability and service life of thermal barrier coatings;The Al doping design can significantly inhibit or alleviate the rapid diffusion of coating elements,competitive growth and the formation of brittle hard metal compounds,giving the coating better structural stability and oxidation resistance.The research results have opened up a new theoretical vision for the design and control of diffusion barriers of high-temperature thermal protective coatings for aeroengines,and laid a theoretical and technical foundation for the performance control,service life extension and engineering application of high-temperature thermal protective coatings with wide temperature range.