【作者】 王璐；

【导师】 付前刚；

【作者基本信息】 西北工业大学 ， 材料学， 2018， 博士

【摘要】 在高温含氧环境中,涂层技术是保护碳/碳（C/C）复合材料不被氧化的有效手段。Mo Si₂优异的高温抗氧化性能使其成为最佳涂层材料之一。然而Mo Si₂涂层在低温易脆性开裂、防护温区窄、现有的制备技术可设计性差,严重限制其应用范围。本文以拓宽Mo Si₂涂层的氧化防护温区为目的,采用高效能的超音速等离子喷涂技术（SAPS）作为涂层制备方法,系统研究了制备工艺、愈合相种类和梯度结构设计对Mo Si₂涂层微观结构和性能的影响。采用SEM、XRD、EDS、WDS、拉伸力学测试和有限元分析等手段对涂层的微观形貌、相组分和分布、力学性能和残余应力进行表征;考察了涂层在大气中的裂纹自愈合、抗氧化和抗热循环性能,并根据测试前后的微结构和组分演变,探究涂层的自愈合和抗氧化机理。主要研究内容和结果如下:分析了粉料粒径和后处理工艺对Zr B₂-Mo Si₂涂层致密度和界面结合强度的影响。结果显示粉料粒径越小,涂层片层的铺展程度和氧化程度越高,则致密度和拉伸结合强度越高。氩气热处理可降低Si O₂和B₂O₃的粘度并加快元素互扩散,使致密度和拉伸结合强度提高。研究了Zr B₂对Mo Si₂涂层在900～1500℃空气中抗氧化性能的影响,结果表明20～30%Zr B₂能显著提高其抗氧化性能,表面形成的Zr Si O₄颗粒钉扎Si O₂玻璃层能抑制氧气的扩散,并与涂层形成多相镶嵌结构,阻碍裂纹的扩展。该涂层体系在900℃的失效原因是界面热失配导致的脆性开裂以及B₂O₃含量过少无法有效愈合裂纹。在1500℃的失效原因是B₂O₃剧烈挥发以及具有较高热膨胀系数的Zr O₂发生相变体积膨胀使氧化防护层结构被破坏。引入纳米Si B₆愈合相显著提升了Mo Si₂涂层在中低温的抗氧化性能。15%Si B₆-Mo Si₂涂层在900和1200℃的有效氧化防护时长可达84h和120h,增重仅0.24%和0.93%。其优异的性能是因为纳米Si B₆减少了喷涂涂层的片层界面及其开裂,并氧化形成B₂O₃·Si O₂玻璃愈合裂纹。通过在Si B₆-Mo Si₂陶瓷中预制裂纹研究了高温下的裂纹愈合行为,结果表明增加氧化温度和Si B₆含量都能提高裂纹愈合速率。同时建立了Si B₆-Mo Si₂陶瓷氧化的理论计算模型,可初步估算裂纹的愈合时间和温度。引入Zr Si₂合金愈合相提高了Mo Si₂涂层的高温自愈合和抗氧化性能。Zr Si₂缓解了Mo Si₂涂层的界面应力集中现象,促使表面裂纹偏转,降低了大尺寸裂纹宽度,增加了微裂纹长度,有利于裂纹被及时愈合。在1500℃氧化10h后,20～60%Zr Si₂-Mo Si₂涂层表面的预制裂纹（宽度:2.6～9.6μm）被完全愈合,且Zr Si₂含量越高,裂纹愈合速率越快。当Zr Si₂含量为40%时,Mo Si₂涂层在1500℃的氧化防护性能最优,氧化42h后增重率为1.89%。其优异的自愈合和抗氧化性能有四方面因素:（1）裂纹宽度的减小有利于快速愈合;（2）Si源的增多提高玻璃相产量;（3）Zr Si₂的氧化体积膨胀率1.5倍于Mo Si₂,有利于减小裂纹空隙;（4）适量Zr O₂的形成,并与Si O₂形成Zr Si O₄稳定相,钉扎玻璃层,避免大量Zr O₂发生体积相变破坏氧化层结构。设计了梯度Si-Zr-Mo和自愈合多层B-Si-Zr-Mo涂层体系。采用Abaqus有限元和XRD对其残余应力进行分析,结果表明梯度结构设计明显缓解了涂层和基体的界面应力集中现象,并使最大应力值位置从界面转移至涂层表面。梯度和自愈合多层涂层在900℃可经历热循环30和40次以上。自愈合多层涂层更优异的抗热循环性能一方面是因为梯度结构对热应力的缓解作用,另一方面是该涂层在低温氧化形成B₂O₃·Si O₂流动相和Zr Si O₄颗粒使裂纹空隙减小,进而填充并愈合裂纹,阻止裂纹扩展导致的涂层失效。两类多层涂层在宽温域表现出优异的抗氧化性能,尤其是自愈合多层涂层,将900、1200和1500℃的抗氧化时长从10h、30h、22h提高到107h、125h、130h以上,增重仅0.41%、1.7%、1.1%。其优异的氧化防护效果有三方面的因素:（1）梯度结构对裂纹扩展的抑制;（2）Si B₆在低温生成流动性好的B₂O₃玻璃,促使Zr Si O₄颗粒和少量Si O₂愈合裂纹并形成颗粒钉扎玻璃保护层;（3）高温时,外层Zr Si₂可快速形成Zr Si O₄钉扎Si O₂致密防护层,抑制氧气向内扩散,避免生成B₂O₃挥发相使氧化层结构被破坏。更多还原

【Abstract】 In a high-temperature oxygenated environment,coating is an effective route to protect carbon/carbon（C/C）composites from oxidation.The excellent performance of Mo Si₂ at elevated temperature make it promising candidates for coating.However,at low temperature Mo Si₂ is prone to brittle cracking and could only protect the substrate in a narrow temperature range.Besides,its application is seriously limited due to the poor designability of the preparation technology.In this dissertation,with the aim of widening the oxygen protection temperature range of Mo Si₂ coating,supersonic plasma spray technology（SAPS）was employed to prepare the coatings,and the effects of preparation process,healing phases and gradient structure design on microstructure and property of Mo Si₂ coating were systematically investigated.SEM,XRD,EDS,WDS,tensile strength test and finite element analysis were employed to characterize the morphology,phase composition and distribution,mechanical property and residual stress of the coatings.The self-healing performance,oxidation and thermal cycle resistance and mechanism of the coatings were studied.Detailed research contents and main conclusions attained were listed below:The effects of particle diameter and post treatment process on the density and interfacial bonding strength of Zr B₂-Mo Si₂ coatings were investigated.Results showed that the smaller of the particle diameter,the higher degree of molten particle unfold and oxidation,and then the density and bonding strength became higher.Heat treatment in argon atmosphere could reduce the viscosity of Si O₂ and B₂O₃,and accelerate element mutual diffusion in the coatings,which were benefit for density and bonding strength of the coatings.The effect of Zr B₂ on the oxidation resistance of Mo Si₂-based coatings was investigated at the temperatures ranged from 900 to 1500°C.Results showed that 20～30%Zr B₂ could significantly improve the oxidation resistance,which was mainly attributed to Zr Si O₄ pinned Si O₂ glass layer to restrain oxygen permission and to form multi-phase mosaic microstructure with original coating to inhibit crack growth.The failure of the coatings at 900°C was due to interfacial thermal mismatch and brittle nature of ceramic coatings,which induced cracks difficult to be healed by insufficient B₂O₃.At 1500°C,the failure of the coatings was due to the destruction of protective oxide layer by vigorous evaporation of B₂O₃ and phase transition of Zr O₂ with higher CTE accompanied by volume change.Introducing nano-Si B₆ dramatically improved the oxidation resistance of Mo Si₂-based coatings at low and medium temperature regions.15%Si B₆-Mo Si₂ coating could effectively protect the substrate from oxidation for 84h and 120h at 900°C and 1200°C with mass gain of only 0.24%and 0.93%.The outstanding property of the coating was ascribed to nano-Si B₆,which reduced lamellar interfaces and splat cracking of as-sprayed coating.At the same time,B₂O₃·Si O₂,oxidation product of Si B₆,could flow to heal the cracks.The crack healing behavior of pre-cracked Si B₆-Mo Si₂ ceramics was studied at 900°C and 1200°C.Results showed that enhancing the oxidation temperature and the content of Si B₆ could speed up the crack healing rate.A theoretical calculation model of Si B₆-Mo Si₂ ceramics during oxidation was built to estimate crack healing time and temperature preliminarily.Introducing Zr Si₂ alloy could improve self-healing property and oxidation resistance of Mo Si₂ coating at high temperature.Zr Si₂ alloy possessed the abilities to relieve interfacial stress concentration,induce surface crack deflection,reduce the width of large size cracks and increase the length of micro-cracks,which were beneficial to heal cracks immediately.Following oxidation at 1500°C for 10h,the cracks prefabricated in the surface of 20～60%Zr Si₂-Mo Si₂ coatings were completely healed.The crack healing rate was positively related with the content of Zr Si₂.Among these coatings,40%Zr Si₂-Mo Si₂ presented the best oxidation resistance,which gained mass of 1.89%after oxidized at 1500°C for 42h.Its distinct protective performance was attributed to four aspects:（1）The reduction of crack width speeded up the healing rate;（2）The increase of Si source raised the production of glass phase;（3）The volume expansion of Zr Si₂ was 1.5 times higher than Mo Si₂ during oxidation,which was helpful for crack reduction;（4）Moderate amount of Zr O₂ formation could react with Si O₂ to produce the more stable phase-Zr Si O₄,which pinned glass layer to prevent cracking and avoid damaging protective layer by plentiful Zr O₂ accompanied with volume change during phase transition.Gradient Si-Zr-Mo and self-healing B-Si-Zr-Mo coatings were designed and prepared on Si C coated C/C composites.The residual stress in the coated composites was analyzed by Abaqus FEA and XRD.Results showed that interfacial stress concentration was remarkable relieved through gradient structural design,and the position of maximum stress transferred from the interface to the surface of the coatings.These two kinds of multilayer coatings could withstand 30 and 40 thermal cycles between 900°C and room temperature.On one hand,the superior thermal cycle resistance of self-healing coating was due to thermal stress relief by gradient structure design.On the other hand,the coating was oxidized at low temperature to form liquid B₂O₃·Si O₂ phase and Zr Si O₄ particles,which healed cracks and further restrained crack propagation to prevent coating failure.These two kinds of multilayer coatings exhibited outstanding oxidation resistance in wide temperature range,especially the self-healing B-Si-Zr-Mo coating,whose protection time increased from 10h,30h,22h to over 107h,125h,130h at 900,1200 and 1500°C with mass gains of only 0.41%,1.7%,1.1%,respectively.The superior protective performance was influenced by three factors:（1）The inhibition of crack growth through gradient structure design;（2）At low temperature,Si B₆ was oxidized to produce liquid B₂O₃,which facilitated Zr Si O₄ particles and Si O₂ to heal cracks and form protective layer;（3）At high temperature,the inward diffusion of oxygen and production of B₂O₃ volatile phase,which were fatal to the protective layer,were avoided by forming a dense Zr Si O₄ pinned Si O₂ protective layer through oxidation of Zr Si₂.更多还原