Study On Structure Control,Oxidation Resistance And Thermal Shock Of MoSi₂ Coating On Nb Alloy

Nb alloys are important structural materials used at high temperatures in the aerospace field for their moderate density,high melting point,and excellent high-temperature performance.However,due to the poor high-temperature oxidation resistance,Nb alloys need to be protected by the coating.MoSi₂ is considered as a promising material for high-temperature oxidation resistant coating of Nb alloys due to its excellent oxidation resistance and its thermal expansion coefficient(8.0～9.0×10^-6/K)similar to that of niobium alloys(7.6～9.1×10^-6/K).However,there are a few problems in the application of MoSi₂ coatings:（1）the intrinsic brittleness of MoSi₂ causes the cracking of the coating;（2）the diffusion of interface elements at high temperature leads to the degradation of the coating;（3）the mismatch between the CTE of the oxidation layer and the coating results in the peeling of the oxidation layer.Regarding the above problems,in this paper,dense and crack-free MoSi₂ coatings were prepared by hot press sintering（HPS）,the glass phase（SGP）content was optimized in the coating,the intrinsic brittleness of MoSi₂ was improved by adding SiC_w,Ta diffusion barrier layer was used to retard the diffusion of interface elements,Cr was added to further delay the inward diffusion of Si and improve the anti-spalling performance of the oxidation layer,and finally,MoSi₂-SiC_w-Cr/Ta coatings were successfully obtained.XRD,XPS,SEM,EDS and TEM were employed to analyze thephase composition,morphology,and chemical composition of the coating,explore the toughening mechanism of SiC_w on MoSi₂ and the growth behavior of the diffusion layer,clarify the mechanism of Ta layer delaying the inward diffusion of Si,and investigate the relationship between the microstructure of the coating and the anti-spalling performance and interface diffusion behavior of the coating after Cr addition.The main findings are as below:（1）It was found that as the sintering temperature increased from 1250℃to 1550℃,the density of the coating increased from 90.86%to 96.78%,the oxidation weight gain dropped from 0.50 mg/cm² to 0.35 mg/cm²,the thickness of the diffusion layer increased to 11.97μm after static oxidation for 150 h at 1250℃.But the relative density of the coating increased slightly after the HPS exceeded 2 h.Therefore,the optimal preparation method was found:sintering at 1550℃for 2 h.In such condition,the obtained coating could effectively protect the substrate for more than 100 h at 1550℃and showed excellent oxidation resistance.（2）To investigate the effect of the oxygen content on the performance of the coating,MoSi₂ coatings with different SGP content were prepared by adding different bentonite content.It was found that the growth of the diffusion layer and the oxidation layer conformed to the parabolic growth law and the formula to estimate the thickness of the diffusion layer and the oxidation layer of the coatings was L=12.34+145960×exp（（-17297）/T）t^1/2 and L=0.19+16447×exp（（-16436）/T）t^1/2,respectively,and the estimation error was less than 6%.It was also found that MoSi₂/SGP interface in the diffusion layer and Si O₂/oxide interface and oxidation layer of MoSi₂ coatings with low SGP content were less,resulting in a higher diffusion activation energy and a smaller diffusion coefficient.The results showed that MoSi₂ with low SGP content had better high-temperature oxidation resistance and longer life.（3）To improve the fracture toughness and the thermal shock resistance of MoSi₂ coatings,MoSi₂-SiC_w composite coatings were prepared.The Si O₂ layer formed on the surface of SiC_wprotected it from degeneration and improved the fracture toughness of the composite coating to7.32 MPa·m^1/2 and the thermal shock times of the coating could reach 157 times at room temperature～1250℃,which is about three times of the unmodified coating.The coating failure was caused by the stress concentration at the coating edge.Considering the influence of incubation time on the thickness growth behavior,together with theoretical derivation and unknown parameters that were obtained by design experiments,the formula of the thickness of the diffusion layer in cyclic oxidation was obtained as follows::=11.84+266590 exp（（-18585）/T）（t-0.04nexp（（4633）/T））^1/2,and the estimation error was validated to be less than 2%.This could provide a theoretical basis for the estimation of degradation rate and life of the coating.（4）To retard the diffusion of interface elements,Ta and Ta Si₂ diffusion barriers were prepared between the Nb alloy and MoSi₂ coating.It was found that Ta layer had the best diffusion barrier effect.The growth rate of the diffusion layer with Ta was 1.93μm²/h,lower than 4.68μm²/h of that without the diffusion barrier layer and 7.36μm²/h of Ta Si₂at 1250℃.The reason for the poor diffusion barrier effect of Ta Si₂ was that the porous layer produced during the preparation process accelerated the diffusion of Si,while the reason for the excellent diffusion barrier effect of Ta layer was that the diffusion barrier energy of Si in Ta was 0.257eV,higher than 0.023 eV in Nb.Moreover,MoSi₂/Mo₅Si₃/Ta₅Si₃/Ta₂Si/Ta multilayer structure formed during sintering greatly reduced the concentration gradient and diffusion driving force,and inhibited the diffusion of Si.（5）To further delay the diffusion of interface elements and improve the anti-spalling performance of the oxidation layer,MoSi₂-SiC_w-Cr and MoSi₂-SiC_w-Cr/Ta coatings were prepared.Since the diffusion energy barrier of Cr in Ta was 0.641 eV,higher than 0.368 eV in Nb,Cr diffused more slowly in Ta layer,occupied Si points,reduced the vacancy of Si diffusion,further slowing down the diffusion of Si in Ta layer.The growth rate constant of the diffusion layer of the coating with Ta was 0.50μm²/h at 1250℃,nearly 89%less than that of the MoSi₂coating.The number of the thermal shock failure of MoSi₂-SiC_w-Cr and MoSi₂-SiC_w-Cr/Ta coatings decreased to 141 and 126,respectively,which indicates that the addition of Cr and the preparation of Ta layer had a certain negative impact on the thermal shock resistance of composite coatings.Since Cr₂O₃ formed after Cr was oxidized can adjust the CTE of the oxidation layer and the overflow of Cr O₃ at high temperatures can adjust the thickness of the oxidation layer,the addition of Cr was beneficial to the anti-spalling performance of the oxidation layer.Therefore,MoSi₂-SiC_w-Cr/Ta obtained the best comprehensive effect.