Studies On Design,Preparation And Oxidation Behaviors Of Mcraly Coatings With Diffusion Barriers

MCrAlY coatings are widely used in hot sections of aero engines and gas turbines at present.However,severe interdiffusion between coating and substrate would occur during high temperature service,and this would weaken oxidation resistance of coating.In order to retard the interdiffusion between coating and substrate,and improve the oxidation resistance of the coating,MCrAlY coatings with Re-based diffusion barrier,Ni-W diffusion barrier,Ni-W/Ni-Re diffusion barrier,double Re-based diffusion barrier and Al₂O₃ diffusion barrier were prepared by electroplating,arc ion plating（AIP）,preoxidation and vacuum annealing.The microstructures as well as evolution processes of diffusion barriers and MCrAlY coatings were studied by SEM/EDS,XRD,EPMA and TEM.The main conclusions are shown as follows:The NiCrAlYSi coatings with and without Re-based diffusion barriers were prepared on K438 and K465 respectively by electroplating Ni-Re,AIP NiCrAlYSi coating and vacuum annealing,and the coatings were all composed of γ/γ’ and a few β-NiAl phases.The oxidation results showed that the coatings with Re-based diffusion barriers both possessed better oxidation resistance and the Re-based diffusion barrier suppressed the interdiffusion between coating and substrate apparently.During oxidation,Re diffusing into the coating from diffusion barrier reduced the CTE difference between coating and oxide scale,therefore reducing the thermal stress of the oxide scale and improving the spallation resistance of oxide scale for coating.Meanwhile,the W in substrate would diffuse into Re-based diffusion barrier and was beneficial for improving the stability of diffusion barrier.In superalloy/NiCrAlYSi coating system,when the Cr content in superalloy was high,the Cr content in coating would keep in a high level,therefore promoting forming a dense Al₂O₃ scale.When the W content in superalloy was high,the W atoms would diffuse into coating and weaken the adhesion of oxide scale.The NiCoCrAlYHfZr,Ni-W/NiCoCrAlYHfZr and Ni-W/Ni-Re/NiCoCrAlYHfZr coatings were fabricated on a second generation single superalloy using electroplating Ni-W,electroplating Ni-Re,AIP NiCoCrAlYHfZr coating and vacuum annealing.The three coatings were composed of γ/γ’ and β-（Ni,Co）Al phases.In the process of oxidation,Ni-W and Ni-W/Ni-Re diffusion barriers both lowered oxidation rates of coatings.And Ni-W/Ni-Re diffusion barrier was more stable than Ni-W diffusion barrier.This was because that the solid solubility of W in γ/γ’ phase was higher than that of Re in γ/γ’ phase,and W and Re were solidly dissolved together as oxidation went on,which would improve the stability of diffusion barrier.Besides,the Ni-W layer in Ni-W/Ni-Re diffusion barrier could serve as a buffer layer,reducing the inward diffusion of Re into substrate and delaying its degradation rate.Moreover,the oxidation rate of coating decreased after W diffusing into NiCoCrAlYHfZr coating from Ni-W diffusion barrier,while the oxidation rate of coating increased after Re diffusing into coating from Ni-W/Ni-Re diffusion barrier.The NiCoCrAlYHfZr coating,NiCoCrAlYHfZr coating with single Re-based diffusion barrier and NiCoCrAlYHfZr coating with double Re-based diffusion barrier were prepared on a fourth generation single crystal superalloy by electroplating Ni-Re,electroplating pure Ni,AIP NiCoCrAlYHfZr and vacuum annealing.The three coatings consisted of γ/γ’ and β-（Ni,Co）Al phases.According to the results of oxidation,the formation of secondary reaction zone（SRZ）and TCP phases were suppressed by double Re-based diffusion barrier significantly.The NiCoCrAlYHfZr coating with double Re-based diffusion barrier possessed the lowest internal oxidation.During oxidation,Re diffused into coating and promoted the uniform distribution of Hf.The NiCoCrAlYHfZr and Al₂O₃/NiCoCrAlYHfZr coatings were prepared by AIP a buffer layer of NiCoCrAlYHfZr,pre-oxidation,AIP NiCoCrAlYHfZr coating and vacuum annealing.The two coatings were both composed of γ/γ’ and β-（Ni,Co）Al phases.The oxidation results showed that the Al₂O₃ diffusion barrier improved oxidation resistance of coating obviously and restrained the formation of SRZ and TCP phases totally.Meanwhile,the Al₂O₃ diffusion barrier significantly inhibited the inward diffusion of Ni,Co and Cr from coating as well as the outward diffusion of refractory elements W,Ta,Re and Ru from substrate.In this way,the Cr content was high and the content of refractory elements was low in Al₂O₃/NiCoCrAlYHfZr coating.A continuous and dense Al₂O₃ scale was formed under the influence of "third-element effect" of Cr,and the adhesion of oxide scale was improved by reducing the content of refractory elements,therefore the oxidation resistance of Al₂O₃/NiCoCrAlYHfZr coating was increased.