Microstructural Formation And Oxidation Behaviors Of Silicide Compound Coatings Prepared On Nb-Si Based Alloy

MoSi₂ could be employed as the protective coating for Nb-Si based ultrahigh temperature alloy due to its superior oxidation resistance.However,the interface stability between MoSi₂ coating and the substrate alloys is relatively poor at high temperatures.The inward diffusion of Si will cause degradation of MoSi₂,and then decrease its protection and service life at elevated temperatures.Besides,intense outward diffusion of alloying elements could result in formation of Kirkendall voids,which may weaken the mechanical properties of the substrate and derease the interfacial bonding strength.Therefore,it is essential to prepare a barrier layer to restrict the diffusion reaction at the interface between MoSi₂ and Nb-Si based ultrahigh temperature alloy.The purpose of this dissertation is to prepare silicide compound protective coatings with diffusion barrier on the Nb-Si based ultrahigh temperature alloy.The MoSi₂/ReSi₂/NbSi₂ compound coating was prepared by a two-step method involving electrodeposition of Mo/Re duplex film and then halide activated pack cementation（HAPC）,and the MoSi₂/WSi₂ and MoSi₂/WSi₂/NbSi₂ compound coatings were fabricated by magnetron sputtering（MS）Mo/W duplex film and then HAPC treatment.The microstructure,phase constituent,chemical composition and elemental distribution of these compound coatings were characterized by XRD,SEM,EDS and EPMA analysis methods.Static oxidation resistance and the protective mechanism of the compound coatings at different temperatures were investigated.In addition,the suppression mechanism and failure mechanism of barrier layers have also been studied.The main contents and conclusions of this work are as follows:（1）Both Ru and Re films have been electrodeposited on the Nb-Si based ultrahigh temperature alloy.The optimization experiments indicated that the growth rate of these films could be significantly improved by increase in both current density and main salt concentration.The vacuum heat treatment（1250℃/2 h）results indicated the better diffusion barrier performance of Re layer.Subsequently,the Mo layer was deposited on the Re-coated Nb-Si based ultrahigh temperature alloy from the water deficient aqueous electrolyte.Influences of experimental parameters on the chemical composition and phase constituent of Mo film were investigated.The deposits showed metallic appearance when the mole ratio of water to acetate was limited in the range of 1.6:1—2.3:1 and current density was kept in 200—400 m A·cm^-2.In these ranges,oxygen content of the Mo film and the amount of nodules on its surface increased with increase in current density and water content accordingly.Re was helpful to improve the deposition of Mo,and hence could decrease the oxygen content of the deposited Mo film.The typical Mo film was amorphous and about 10—15μm thick,consisting of dominant metallic Mo and a few Mo₂O₃ and MoO₃ oxides.（2）The MoSi₂/ReSi₂/NbSi₂ compound coating was prepared by combination of electrodeposition of Mo/Re duplex film and then Si-B-Y co-deposition technique.Owning to the molybdenum oxides in the Mo film,there were a few micro holes in the MoSi₂ layer.Pesting oxidation was probably suppressed by the oxide shell.Protection effect of this shell was relatively poor at 1250℃,and the MoSi₂ layer was nearly exhausted after 10 h’s exposure due to its porous structure and the exsistance of original oxides.The existence of ReSi₂ layer could improve the integrity of the MoSi₂ layer and the adherence between each layer in the compound coating.As a result,the MoSi₂/ReSi₂/NbSi₂ compound coating exhibited better oxidation resistance than the MoSi₂/NbSi₂ compound coating.After oxidation at 800℃for100 h,the mass gains per unit area were 0.48 and 2.15 mg·cm^-2 for these two compound coatings respectively.The mass gains per unit area were 2.61 and 2.95 mg·cm^-2 after oxidation at 1250℃for 10 h for these two compound coatings respectively.Due to the discontinuity and structural defects,the prepared ReSi₂ layer showed limited suppression ability to the outward diffusion of alloying elements,and new techniques should be adopted to prepare the compact compound coating with WSi₂ barrier layer.（3）The Mo/W duplex film was prepared on the substrate via MS technique.The results suggested that the film appearance and its bonding to the substrate could be significantly affected by the sputtering power and working pressure.Besides,the sputtered film is metallic and amorphous.By controlling the deposition time and deposition temperature in HAPC process,two kinds of compound coatings of MoSi₂/WSi₂ and MoSi₂/WSi₂/NbSi₂ could be obtained respectively.As the coating formed via inward diffusion of[Si]in HAPC mainly,growth process of these coatings followed the parabolic law.The WSi₂ layer could retard the inward diffusion of[Si].Therefore,there were fewer[Si]atoms to diffuse into the substrate,and growth of the beneath NbSi₂ could be restricted consequently.The growth rate constant of NbSi₂ layer in the MoSi₂/WSi₂/NbSi₂ system is about 8.31×10²μm²·h^-1,nearly half of that in the MoSi₂/NbSi₂ system(1.87×10³μm²·h^-1).（4）WSi₂ layer could suppress the outward diffusion of alloying elements effectively at both 800 and 1250℃,and the MoSi₂/WSi₂ compound coating showed excellent oxidation resistance at this two temperatures.Growth of the scales followed the parabolic law and the rate constants were 0.34 and 0.93μm²·h^-1,respectively.The outward diffusion and pre-oxidation of Al was helpful to restrict the pesting oxidation at 800℃.At 1250℃,the WSi₂layer could suppress the outward diffusion of alloying elements effectively.As a result,there were no Ti O₂ and Nb₂O₅ formed and the scale was obviously thinner.The MoSi₂/WSi₂compound coating exhibited much better oxidation resistance than the MoSi₂/NbSi₂compound coating.However,the WSi₂ layer near the substrate could degrade into porous W₅Si₃ in situ,which may decrease its suppression ability accordingly.At 1350℃,the degradation process was accelerated,and the porous W₅Si₃ layer could not suppress the outward diffusion of alloying elements.Hence,a number of Kirkendall voids formed in the substrate and there were Al₂SiO₅,YNbO₄ and AlNbO₄ complex oxides formed in the scale.After oxidation for 40 h,the scale was estimated to be about 20μm thick and its growth rate constant was 12.89μm²·h^-1.The invalidation test at 1350℃indicated that both exhaustion of the lateral NbSi₂ coating of the specimen and interfacial oxidation were responsible for the failure of the coating system.（5）At 1400℃,degradation of MoSi₂ and WSi₂ layers could be suppressed by the presence of NbSi₂ layer,and MoSi₂/WSi₂/NbSi₂ compound coating showed excellent oxidation resistance at 1400℃.In this compound system,WSi₂ and NbSi₂ layers acted as diffusion barrier and buffering layers respectively.Growth of the scale followed the parabolic law and the growth rate constant was 1.48μm²·h^-1.After oxidation at 1400℃for 100 h,the scale was about 12.5μm thick.The interdiffusion between WSi₂ and MoSi₂ could weaken the barrier effect of WSi₂ and the grain boundary was the preferred diffusion path for the outward diffusion of W and alloying elements.By doping with W and other alloying elements,the hardness and elastic modulus of MoSi₂ layer could be improved evidently.Preferential oxidation of active elements like Ti,Cr and Al at the scale/MoSi₂ interface could be helpful to restrict the oxidation of Nb,and then the outward diffusion of Nb could be decreased consequently.The parabolic growth rate constant of inter-diffusion zone（IDZ）layer was about 31.92μm²·h^-1,indicating the inward diffusion of Si led to the fast degradation of NbSi₂layer mainly.Whereas,preferential oxidation of Hf,Al and Si and the resulted accumulation of their oxides in the vertical cracks located in NbSi₂ layer could lead the expansion and growth of these cracks,and then result in cracking of MoSi₂/WSi₂ complex coating upon oxidation at 1250℃.Intense oxidation of NbSi₂ at the interface could cause rupture and exfoliation of MoSi₂/WSi₂ complex coating,and then failure of the coating system.