| Gas-turbine engines are used for aircraft/marine propulsion and power generation.The increase of gas temperatures is beneficial to increase the energy efficiency and reduce carbon emission.However,as gas temperatures are increased,there are new challenges to temperature capability of alloy components.Ni-based alloy and Ti-based light-weight alloy are the most used alloy in gas-turbine engines.With the improvement of mechanical performance of TiAl alloy,light-weight alloy has been applied in the hottest sections(turbines)of gas-turbine engines.One way to increase temperature capability of Ni-based alloy components is the application of thermal barrier coating system(TBCs)which is composed of ceramic top coat and metallic bond coat.Metallic bond coat can decrease thermal expansion coefficient mismatch between top coat and alloy substrate and increase high-temperature oxidation resistance and durability by forming protective Al2O3 scale(also thermal growth oxide,TGO).Compared with other Al2O3 scale forming coatings,Pt-modified aluminide metallic coatings(simply Pt-Al coatings)have better high-temperature oxidation resistance and durability by forming compact TGO with better adhesion.They protect blades up to 25,000 h thermal cycles at 1000 ?.However,because of complicated preparation technology,effects of electroplating Pt,aluminizing parameters and substrate chemical composition on phases composition and structure of Pt-Al coatings lack systematic research.Meanwhile,as multi-component coatings with multi-layer structure,its high-temperature oxidation behavior,componential/microstructural degradation and molten salts induced hot corrosion need further research.It contributes to obtain high-performance Pt-Al coatings and further promote its durability.In terms of high-temperature oxidation protection for Ti-based light-weight alloy,effective coatings are still not available.The outward diffusion of Ti from substrate blocks the popularity of light-weight alloy in the hottest sections of gas-turbine engines.Effects of Pt layer thickness,A1 content and substrate chemical composition on phases composition and structure of Pt-Al coatings were investigated.Pt-Al coatings with different Pt content and phases composition were fabricated on pure Ni,Ni-based and Ti-based alloy.After thermal cycling and hot corrosion experiments,it was found that Pt content,aluminizing parameters and substrate materials have an important influence on phases composition and structure of Pt-Al coatings.During thermal cycling,the key Pt element was depleted significantly,the surface was roughening gradually,the adhesion of TGO decreased and the hot corrosion behaviors induced by different deposited salts had great difference.Meanwhile,the coatings with double-layer structure can prevent the outward diffusion of Ti from Ti alloy substrate.The chemical composition,phases composition and morphologies of the samples were analyzed by XRD,SEM,EDS,XPS and EPMA.The rumpling magnitude of TGO was determined by combining surface roughness obtained by non-contact 3D optical profiler and cross-sectional morphologies.By analyzing the diffusion behaviors of Pt during thermal cycling,the effect of Pt/Hf content on oxidation kinetics,surface roughness and TGO adhesion,different hot corrosion mechanisms and high-temperature stability of Pt-Al coatings on Ti alloy,the following conclusions were obtained:1)With the increase of Pt content,the phases composition of Pt-Al coatings on Ni-based alloys changed:??PtAl2+??PtAl2.Comtrolling aluminizing parameters can optimize coating thickness,voids scale,phases composition and distribution.The coatings fabricated on Ti alloy had double-layer structure.The thickness and diffusion extent of electroplated Pt layer plays a crucial role in phases composition of each layer.2)The optimized Pt content is 8.19 at.%.its coating had lowest oxidation rate constant and surface roughness.The depletion of Pt led to PtAl2??*and Kirkendall effect.Further 0.46 wt.%Hf addition can decrease oxidation weight gain and TGO rumpling extent.3)While Na2SO4 resulted in basic fluxing of the initial oxide scale,Na2SO4+V2O5 mixed molten salts resulted in acidic fluxing of the initial oxide scale.V2O5 accelerated Pt precipitation from coatings and boosted the oxidation and sulfuration process of hot corrosion.4)The double-layer structure coating whose outer layer is(Pt)TiAl3+(Ti)PtAl2 and inner layer is(Ti)PtAl2 had a better thermal stability on Ti alloy substrate.It can decrease the oxidation depletion of surface Ti and prevent the outward diffusion of Ti from substrate. |
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