钼基体表面硅化物涂层的包埋渗法制备工艺和机理研究

目前,国内以NH4F为活化剂对钼基合金包埋渗硅,研究渗料组分对涂层沉积效率及作用机理的报道不多。为此,以NH4F为活化剂,研究并对比了不同活化剂含量下涂层的沉积效率,并通过热力学和动力学计算,分析了涂层沉积过程中活化剂的作用机理。结果表明:埋渗料配比按Si∶NH4F∶Al2O3=40∶10∶50于1 000℃下埋渗5 h,涂层厚度可达53μm,高于相同工艺下,以NH4Cl、NaF为活化剂的涂层沉积厚度。当埋渗硅含量充足时,包埋渗硅涂层具有2层结构,外层为MoSi2相,扩散层为Mo5Si3相。通过热力学分析知Mo5Si3较Mo3Si更为稳定,贫硅过渡区更易生成Mo5Si3相。包埋渗硅涂层的沉积受反应扩散过程控制。相同工艺条件下拟合的速率方程中,NH4F为活化剂所得速率常数值最大为18.5。该活化剂下埋渗的涂层,Si F2的平衡分压对涂层生长的气相扩散过程起控制作用,且Si F2的平衡分压均高于以NH4Cl、NaF为活化剂下Si X2(X=Cl,F)的平衡分压。     

界面涂层对气相渗硅Cf/SiC复合材料力学性能的影响

在1650℃气相渗硅（Vapor Silicon Infiltration—VSI）制备了3D碳纤维增强SiC基复合材料（Cf／SiC），其密度约为1．85g／cm^3．当C／SiC界面涂层存在时，气相渗硅cf／SiC强度为239．5MPa；而无界面涂层存在时，Cf／SiC弯曲强度大幅下降，约为67．4MPa．无界面涂层保护时，气相渗硅过程中纤维与硅蒸气发生反应，使得纤维硅化，造成材料性能下降．纤维表面沉积的C／SiC涂层，不仅保护纤维，避免被硅侵蚀，而且具有弱化界面、偏转裂纹等作用，复合材料的断裂功得到显著提高．将气相渗硅温度提高到1700℃后，有界面涂层存在情况下Cf／SiC复合材料密度显著提高，达到2．25g／cm^3，强度基本与1650℃时相当．     

低压沉积温度对MoSi2涂层微观结构与性能影响

以SiCl4和H2为原料,采用低压化学气相沉积(LPCVD)渗硅法在Mo基体表面原位反应制备了MoSi2涂层,研究了沉积温度对MoSi2涂层微观形貌、物相组成、沉积速率、涂层的硬度、涂层与基体结合强度的影响.研究结果表明:在1100～1200℃下制备的涂层结构致密,由单一MoSi2组成,沉积速率、涂层的硬度以及与基体的结合强度均表现为増加的趋势;当沉积温度高于1200℃,涂层出现开裂现象,由游离Si和MoSi2两相组成,涂层沉积速率、硬度和结合强度均出现下降的趋势.1100℃以下沉积的主要控制步骤为Si与Mo反应,而1100℃以上Si在涂层中的扩散对沉积过程起控制作用.     

溅射方式对氧化铌涂层微观结构与性能的影响

分别采用射频溅射和射频反应溅射方式在AZ31镁合金表面制备了氧化铌涂层,并利用扫描电子显微镜、X射线衍射仪、X射线光电子能谱仪、多功能材料表面性能试验仪和电化学工作站对比研究了两种涂层的微观形貌、物相组成、附着力和耐腐蚀性能。研究结果表明,两种涂层都呈非晶柱状结构,铌的价态为Nb5+,对AZ31镁合金的腐蚀保护率达93%以上;与射频溅射沉积的氧化铌涂层相比,射频反应溅射沉积的氧化铌涂层的表面致密性和耐蚀性能更强,附着力提高约7.4倍。     

新型铌合金研究进展

高强铌合金具有比重小、强度高、韧性好、易焊接等优点,是制造高性能航空航天飞行器高温部件的重要材料,研究者通过碳化物强化、高温固溶淬火、大变形挤压、时效和热机械处理等方法研制出系列高强铌合金。航空航天高温结构件减重是研究新型铌合金的一个重要方向,选用密度为6～7．2g／cm^3的系列低密度铌合金,无涂层可在700℃以下工作,加涂层可在1200℃以下工作。铌硅复合材料有望成为在1350℃以上工作的航空发动机叶片材料,研究者通过前期研究筛选出多元Nb-Si-Ti-Al-Cr-X合金作为满足高温应用要求的新型铌合金的研究方向,揭示了铸态显微结构、热处理和热变形（热压、挤压、锻造）条件和机械性能,还研究了Al,Mo,B等合金元素对Nb-Si-Cr系合金抗氧化性能的影响。     

制备参数对氧化铌涂层晶相成分与结合性能的影响

为探讨制备参数对直流反应溅射氧化铌涂层晶相成分与结合性能的影响规律,采用不同的氧气流量、功率和沉积时间,通过直流反应溅射技术在AZ31镁合金表面制备了氧化铌涂层。利用X射线衍射仪和X射线光电子能谱仪对涂层进行表征,采用划痕仪测定了涂层的结合力。结果表明：当氧气流量为0.5~3.0 mL/min、功率为60~100 W、沉积时间为3~7 h时,制备参数对氧化铌涂层的晶相结构和化学价态没有影响,但是对涂层的结合性能影响显著。涂层结合力随氧气流量的增大而减小,随溅射功率和沉积时间的增加而先增大后减小。当溅射功率为80 W（W3试样）和沉积时间为300 min（T3试样）时,涂层的结合力分别为最大。     

铌硅化物基超高温合金表面包埋渗Al改性硅化物涂层的组织结构

分别采用两步包埋渗法和包埋共渗法在铌硅化物基合金表面制备了Al改性硅化物抗氧化涂层.两步包埋渗法是先在合金表面包埋渗Si制备(Nb,X)Si2(X表示Ti,Cr和Hf元素)涂层,然后再将渗Si试样于800～1000℃包埋渗Al.结果表明:当渗Al温度达到860℃时渗入(Nb,X)Si2层中的Al可形成(Nb,Ti)3Si5Al2相;当渗Al温度达到900℃时渗入的Al还可穿过(Nb,X)Si2层在渗Si层与基体之间形成(Nb,Ti)(Al,Si)3层.经1150℃/20h Si-Al包埋共渗后在合金表面形成的涂层具有多层复合结构:外层主要由(Nb,Ti)3Si5Al2和(Nb,Ti)(Al,Si)3组成;中间层上部为(Nb,X)Si2,下部为低硅化物(Nb,X)5Si3;内层由基体相(Nb,Ti)Al3和柱状晶(Cr,Al)2(Nb,Ti)组成.     

粉末粒度对喷涂二硅化钼涂层微观结构的影响

分别以2.5～15μm、15～30μm、30～54μm和54～74μm四种粒径的二硅化钼为热喷涂粉末,利用大气等离子喷涂技术制备了二硅化钼涂层,通过XRD和带能谱的SEM表征了涂层的微观组织结构。结果表明,喷涂过程中,喷涂粉末中部分四方相MoSi2(t)转变为六方相MoSi2(h),MoSi2中的Si含量出现了损失。随着喷涂粉末粒径的增大,MoSi2粒子在沉积过程中的氧化程度减弱,涂层中的Mo、Mo5Si3等富钼相和MoO3、MoO2等氧化产物的相对含量逐渐减少,而富硅相MoSi2逐渐增加。粉末粒度分别在30～54μm和54～74μm范围时,均可获得以MoSi2为主相的涂层。以粒径30～54μm的粉末制备的涂层组织较致密,内部出现了富钼相"网状"结构的组织。     

低压沉积温度对MoSi2涂层微观结构与性能影响

以SiCl₄和H₂为原料，采用低压化学气相沉积（LPCVD）渗硅法在Mo基体表面原位反应制备了MoSi₂涂层，研究了沉积温度对MoSi₂涂层微观形貌、物相组成、沉积速率、涂层的硬度、涂层与基体结合强度的影响. 研究结果表明：在1100~1200℃下制备的涂层结构致密，由单一MoSi₂组成，沉积速率、涂层的硬度以及与基体的结合强度均表现为増加的趋势；当沉积温度高于1200℃，涂层出现开裂现象，由游离Si和MoSi₂两相组成，涂层沉积速率、硬度和结合强度均出现下降的趋势. 1100℃以下沉积的主要控制步骤为Si与Mo反应，而1100℃以上Si在涂层中的扩散对沉积过程起控制作用.     

新型铌合金研究进展

高强铌合金具有比重小、强度高、韧性好、易焊接等优点,是制造高性能航空航天飞行器高温部件的重要材料,研究者通过碳化物强化、高温固溶淬火、大变形挤压、时效和热机械处理等方法研制出系列高强铌合金。航空航天高温结构件减重是研究新型铌合金的一个重要方向,选用密度为6~7.2 g/cm3的系列低密度铌合金,无涂层可在700℃以下工作,加涂层可在1 200℃以下工作。铌硅复合材料有望成为在1 350℃以上工作的航空发动机叶片材料,研究者通过前期研究筛选出多元Nb-Si-Ti-Al-Cr-X合金作为满足高温应用要求的新型铌合金的研究方向,揭示了铸态显微结构、热处理和热变形(热压、挤压、锻造)条件和机械性能,还研究了Al,Mo,B等合金元素对Nb-Si-Cr系合金抗氧化性能的影响。     

Deposition of silicon modified aluminide coatings on nickel base superalloys by pack cementation process

Abstract

The equilibrium partial pressures of vapour species generated in halide activated pack powder mixtures at high temperatures were calculated for a series of compositions using thermochemical analysis tools. The results obtained were applied to identify suitable activators and pack compositions for codepositing Al and Si to form diffusion coatings on nickel base superalloys by the pack cementation process. The calculation results suggested that compositions of the packpowder mixtures activated by CrCl₃.6H₂O may be adjusted to create deposition conditions favourable for codepositing Al and Si, but, those activated byAlF₃or AlCl₃ may only deposit Al.A series of coating deposition experiments were also carried out at 1000 ° C and 1100 ° C and the results obtained confirmed that, with adequate control of pack compositions and deposition conditions, codeposition of Al and Si can be achieved with CrCl₃.6H₂O activated pack powder mixtures. A mixture of elemental Al and Si powders may be used as a depositing source instead of using Al-Si master alloy powders as conventionally recommended. The coatings could be formed either through the inward diffusions of Al and Si or through the outward diffusion of Ni together with other substrate elements such as Cr and Co, depending on the deposition temperature used. Prolonged deposition at 1100 ° C ledtothe formationofa coatingwith amultilayeredstructure consistingofanouter nickelsilicide layerand a middle Simodified NiAl layer followed by a diffusion zone. The pack compositions and deposition conditions may be adjusted to control the microstructure of the coatings formed by the codeposition process.     

NbSi基原位复合材料表面包埋渗制备SiCrY共渗涂层

通过热力学计算分析了分别使用NH₄Cl和CrCl₃·6H₂O两种不同激活剂时, 各个共渗元素的卤化物蒸汽压分压的变化, 得到了实现Nb-Si基原位复合材料上Si-Cr-Y三元包埋共渗的最佳条件。采用包埋共渗法在Nb-Si基原位复合材料表面制备了Si-Cr-Y共渗涂层, 研究了涂层的组织形貌、 成分及其相组成。结果表明: 使用NH₄Cl做激活剂, 通过调整包埋渗料的成分, 可以在适当的温度下实现Si-Cr-Y的三元共渗。当渗料成分为12Cr-6Si-0.75Y₂O₃-5NH₄Cl-76.25Al₂O₃(质量分数)时, 在1350℃可以实现Si-Cr-Y三元共渗。制备的涂层具有多层结构, 分为外层、 内层和明显的互扩散层。互扩散层的存在, 表明涂层的形成是一个连续生长过程, 伴随着Cr、 Si、 Y元素向基体内的扩散。涂层的主要成分由Cr₂(Nb,Ti)、 (Nb,Ti)₅Si₃和HfSi₂组成, Y元素的添加起到了细化涂层的作用。     

Y_2O_3对铌硅化物基超高温合金硅化物渗层组织的影响

采用包埋共渗工艺在铌硅化物基超高温合金表面制备了Si-Y2O3共渗层,共渗温度为1050℃,共渗时间为10h。利用SEM,EDS和XRD等方法分析了渗剂中Y2O3添加量对渗层结构、组织形貌及其成分分布的影响,并与相同包埋渗温度和时间下单独渗Si渗层的组织进行了对比。结果表明:在渗剂中添加不同含量Y2O3后的渗层具有相似的结构,均具有明显分层的结构,由外至内依次为(Nb,X)Si2(X表示Ti,Hf和Cr)层,(Nb,X)5Si3过渡层和富Al扩散区。与单独渗Si渗层相比,渗剂中添加Y2O3没有改变渗层表层的相组成,但抑制了渗层中孔洞的产生,使相同包埋渗温度和时间处理后Si-Y2O3共渗层的组织较单独渗Si渗层的更为致密。EDS能谱分析结果表明,Y在渗层中的分布是不均匀的,在靠近过渡层与基体界面处的Y含量较高,并由内向外逐渐递减。随渗剂中Y2O3含量增加,渗层中的平均Y含量出现先增加后降低的规律。当渗剂中Y2O3的加入量为1%~2%(质量分数)时,Y2O3具有明显的催渗作用。     

Co-deposition of aluminide and silicide coatings on γ-TiAl by pack cementation process

Thermochemical analyses were carried out for a series of pack powder mixtures for deposition of aluminide and for co-deposition of aluminide and silicide coatings on -TiAl by the pack cementation process. Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures for depositing adherent and coherent aluminide and silicide coatings. Pack powder mixtures activated by 2 wt% AlCl₃ was used to aluminise -TiAl at 1000°C. With proper control of pack compositions and coating conditions, an aluminide coating of TiAl₃ with a coherent structure free from microcracking was deposited on the substrate surface via inward diffusion of aluminium. The results of thermochemical calculations indicated that co-deposition of Al and Si is possible with CrCl₃ · 6H₂O and AlCl₃ activated pack powders containing elemental Al and Si as depositing sources. Experimental results obtained at 1100°C revealed that CrCl₃ · 6H₂O is not suitable for use as an activator for co-depositing aluminide and silicide coatings on -TiAl. It caused a significant degree of degradation instead of coating deposition to the substrate. However, adherent coatings with excellent structural integrity consisting of an outer TiSi₄ layer and an inner TiAl₃ layer were successfully co-deposited at 1100°C and 1000°C using pack powder mixtures activated by AlCl₃. IT is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and silicon. Discussion is presented on the issues that need to be considered to ensure the deposition of aluminide and silicide coatings with coherent structure free from microcracking on -TiAl by the pack cementation process.     

Multi-layer CVD-SiC/MoSi2–CrSi2–Si/B-modified SiC oxidation protective coating for carbon/carbon composites

To further improve the oxidation resistance of coating for carbon/carbon (C/C) composites, a multi-layer CVD-SiC/MoSi₂–CrSi₂–Si/B-modified SiC coating was prepared on the surface of C/C composites by pack cementation and chemical vapour deposition method, respectively. The microstructures, oxidation and thermal shock resistance of the coating were studied. The influence of B content in pack powder on the microstructure and oxidation resistance of B-modified SiC coating was also investigated. The results show that the B-modified SiC coating prepared with 10 wt.% B exhibited the best oxidation protection ability for C/C composites at 1173 K. The multi-layer coatings could protect the C/C composites at 1173 K for 30 h and 1873 K for 200 h, and endure 30 thermal cycles between 1873 K and room temperatures. The oxidation resistance and thermal shock resistance is mainly attributed to their dense structure and self-sealing property.     

W-Mo-Si/SiC Oxidation Protective Coating for Carbon/Carbon Composites

A W-Mo-Si/SiC double-layer oxidation protective coating for carbon/carbon （C/C） composites was prepared by a two-step pack cementation technique. XRD （X-ray diffraction） and SEM （scanning electron microscopy）results show that the coating obtained by the first step pack cementation was a thin inner buffer layer of SiC with some cracks and pores, and a new phase of （WxMo1-x）Si2 appeared after the second step pack cementation. Oxidation test shows that, after oxidation in air at 1773 K for 175 h and thermal cycling between 1773 K and room temperature for 18 times, the weight loss of the W-Mo-Si/SiC coated C/C composites was only 2.06%. The oxidation protective failure of the W-Mo-Si/SiC coating was attributed to the formation of some penetrable cracks in the coating.     

Effects of pack composition on the formation of aluminide coatings on alloy steels at 650°C

This is a detailed study aimed to understand the effects of pack composition on the formation and growth of aluminide coatings on alloy steels by pack aluminisation at 650°C, a temperature below the melting point of Al (660°C), using pack powders consisting of Al as depositing source, a halide salt as an activator and Al₂O₃ as inert filler. The packs activated by AlCl₃, NH₄Cl, AlF₃ and NH₄F were used to investigate the effects of the type of halide salt on the coating formation and growth process and subsequently to identify the most suitable activator for pack aluminising alloy steels at 650°C. The effects of pack Al content on the rate of coating growth were then studied by varying the pack Al content from 1.4 wt% to 10 wt% whilst fixing the pack activator content at 2 wt%. It was observed that among the halide salts studied, AlCl₃ is the only suitable activator for pack aluminising alloy steels at 650°C and the rate of coating growth increases with the pack Al content. The equilibrium partial pressures of vapour species generated at the deposition temperature in packs activated by different types of halide salts were calculated and the results were discussed in relation to the observed deposition tendency of packs activated by different types of activators. A vapour phase transportation model was applied to elucidate the relationship between the rate of coating growth and the pack Al content. It was also demonstrated that by combining the low temperature pack aluminising parameters identified in this study with electroless or electro Ni plating, coherent nickel aluminide coatings free of microcracking can be produced on alloy steels at 650°C.     

钼网表面MoSi2涂层的制备及其1500℃高温氧化性能

钼网被广泛用作高温催化剂载体,但关于其高温防护涂层的制备及失效机制却鲜有报道.用包埋渗硅方法在钼网表面制备了MoSi2高温抗氧化涂层,并在静态大气环境中开展了1500℃恒温氧化试验.利用X射线衍射仪(XRD)、扫描电镜(SEM)、能谱分析(EDS)等对氧化前后涂层的微观形貌和组织结构进行了表征.结果表明:1500℃氧化2h,MoSi2涂层中的Si元素会发生选择性氧化,在涂层表面形成一层连续的、具有“自愈合”功能的熔融态SiO2保护膜,阻挡氧向基体一侧的扩散,展现出了良好的高温抗氧化性能;高温条件下,涂层中的Si元素会和钼基体发生界面扩散反应生成抗氧化性能差的Mo5Si3,同时MoSi2不断地和氧发生反应生成Mo5Si3和SiO2,当涂层中的MoSi2完全转化为Mo5Si3,涂层将快速氧化失效.     

包埋法制备SiC涂层内残留Si的高温抗氧化作用机制

采用包埋法在C/C基体上制备了SiC涂层, 借助X射线衍射仪(XRD)和扫描电镜(SEM)对涂层的相组成及微观形貌进行了观察和分析, 研究了涂层在1500℃静态空气中的氧化行为, 并进一步阐述了涂层的抗氧化机制。结果显示: 包埋法制备的涂层由α-SiC、 β-SiC及游离Si组成, 经XRD半定量分析得到不同工艺制备的涂层中游离Si含量不同; 游离Si含量越高涂层越致密; 氧化性能显示涂层中适量的游离Si有利于涂层的抗氧化, 当涂层中游离Si质量分数为1.3%和2.9%时其抗氧化性能均较好, 在1500℃静态空气中氧化7 h失重率分别为0.19%和0.16%。     

Preparation of a tantalum-based MoSi2-Mo coating resistant to ultra-high-temperature thermal shock by a new two-step process

To develop an ultra-high-temperature resistant coating for a reusable thermal protection system,the preparation of a tantalum-based MoSi2-Mo coating by a new two-step process of multi-arc ion plating and halide activated pack cementation is presented.The coating has a dense structure and is well compatible with the tantalum substrate,which can be thermally shocked from room temperature to 1750℃ for 360 cycles without failure.The mechanism of the coating's excellent resistance to high-temperature thermal shocks is that a strong-binding gradient interface and a dense SiO2 oxide scale with good oxygen resistance are formed by the high-temperature self-diffusion of Si.