【作者】 陈俊华；

【导师】 耿浩然；

【作者基本信息】 山东大学 ， 材料加工工程， 2009， 博士

【摘要】 20世纪80年代以来,C_f/C复合材料以其密度低、高比强度、耐磨、耐烧蚀等特点已广泛应用于航天航空、化工、医用等领域,但是C_f/C复合材料制造成本昂贵、高温抗氧化性差和复杂成型预制体制作难度大等因素制约了其多领域应用。本文利用快速化学液相沉积（Rapid Chemical Liquid Deposition,简称RCLD法）专利技术,快速制备出密度达1.7g/cm³以上的C_f/C复合材料,分别采用浓硝酸、电化学对预制体碳纤维表面进行改性;利用SEM分析和三点弯曲强度测试,对比了两种改性方法对C_f/C复合材料组织及性能的影响,获得了试验条件中较理想的碳纤维“温和”处理工艺条件:电化学处理,1.2V,5分钟。研究了不同密度的预制体和不同的沉积时间对C_f/C复合材料密度及结构的影响。结果表明:基体碳围绕纤维呈同心圆层状排列,层数的多少和厚度与预制体的密度有关,碳纤维之间依靠基体碳连接。C_f/C复合材料的密度与沉积时间有关,对于密度为0.12和0.58g/cm³的预制体,沉积时间达到4h之前,材料密度随时间增长而增大的速度较快,4h～5h之间,材料密度增大很小,5h～6h之间,材料的密度基本保持不变;对于密度为0.66g/cm³的预制体,在3h～6h过程中,材料的密度一直呈增大趋势。C_f/C复合材料的密度与预制体密度有关,在相同的沉积时间下,材料的密度随预制体密度的增加而变小。测试了C_f/C复合材料的电阻率和肖式硬度,电阻率介于87.5Ω·μm～119.5Ω·μm,平均值为103.5Ω·μm;当预制体密度为0.12g/cm³、0.58g/cm³、0.66g/cm³时,C_f/C材料的肖式硬度分别为78HSD、90HSD、96HSD。使用万能材料试验机测量了C_f/C复合材料的抗弯强度和压缩强度,最大值分别为122.51MPa和102.57MPa。通过分析研究了C_f/C复合材料抗弯强度的变化规律以及C_f/C复合材料的弯曲应力—应变曲线特征。SEM观察C_f/C复合材料断面特征,分析了C_f/C复合材料弯曲断裂机制。结果显示,C_f/C复合材料抗弯强度随预制体密度的增加而增大;界面结合能力强的材料呈现脆性断裂特征,界面结合适中的材料出现假塑性效应。利用二次沉积法,在C_f/C复合材料外层沉积含C和S化物的复合层,通过正交实验设计对二次沉积工艺参数进行优化,根据材料的各项性能指标,确定了优化参数:硅油作为二次沉积原料中的硅源,硅油质量百分含量为20%,控制器单位电压升起时间为60分钟。密度最大达1.94g/cm³;显气孔率最大为17.07%。研究发现:材料密度越大,其显气孔率越小。利用XRD、SEM及EDS等分析手段研究了C_f/C-Si系化合物复合材料物相形态和分布,结果表明:Si化物和碳共沉积到C_f/C材料外表面和表层孔隙中。测量了C_f/C-Si化物复合材料抗弯强度和压缩强度,C_f/C-Si化物复合材料抗弯强度最大为119.54MPa,相比C_f/C复合材料大约降低5%～10%;抗压强度最大为80.02MPa,相比C_f/C复合材料大约降低10%～15%。利用热失重、DSC及氧化动力学等分析手段,研究了C_f/C-Si化物复合材料在空气中的氧化行为。研究发现,C_f/C-Si化物复合材料相比C_f/C复合材料其抗氧化性能提高4～5倍。利用热失重法、DSC分析及氧化动力学理论,研究了C_f/C-Si化物复合材料在空气中的氧化行为。结果表明,Si化物显著地占据了C_f/C复合材料表层的反应活化点,有效地阻碍了氧的扩散氧化作用,使C_f/C复合材料的氧化起始温度达到684.0℃。研究了氧化效应对C_f/C复合材料的组织结构和宏观尺度的影响。各温度点下的等温氧化失重曲线均服从线性规律,随着温度的升高,氧化失重率呈增加趋势。结合SEM,分析了预制体密度对C_f/C复合材料氧化微观形貌的影响,研究了预制体密度对C_f/C复合材料失重率的影响。研究表明:热解碳更易于被氧化,随着预制体密度的增加,C_f/C复合材料的氧化失重率减小,抗氧化性增强。采用真空钎焊法对C_f/C复合材料的钎焊进行了研究。分析了C_f/C复合材料/Cu-Ti/C_f/C复合材料的真空钎焊接头的界面组织结构、室温力学性能,以及不同的工艺参数等对界面组织和力学性能的影响,建立了C_f/C复合材料钎焊过程的物理模型。以Cu为基体,设计了不同配比的Cu-Ti钎料。对不同配比的钎料,采用座滴试样的比接触面积法,测试了钎料对C_f/C复合材料的润湿性能。试验结果表明,随着Ti含量的增加,钎料对C_f/C复合材料的润湿性能显著提高。当Ti含量在15%时,钎料的润湿性最好,铺展面积最大。熔化钎料中的活性元素Ti向接触面偏聚,反应生成TiC,这样会降低液态金属与C_f/C复合材料的界面能,促进润湿。当Ti含量过少时,C与Ti在界面反应生成的化合物太少而且不均匀,使得钎料的润湿性变差。而Ti含量过高时,生成大量β-TiCu₄化合物,润湿性变差。利用Cu-Ti钎料钎焊时,接头区域的组织为C_f/C、TiC、Cu-Ti共晶组织（β-TiCu₄+α-Cu）,依据所发现的界面结构,提出了界面结构的形成机理和界面形成过程模型。试验结果表明,当钎焊工艺参数变化时,C_fC复合材料/Cu-Ti/C_f复合材料C接头界面生成物的种类并未改变,TiC层的厚度和Cu-Ti共晶组织（β-TiCu₄+α-Cu）所占界的比例会发生一定的变化。随着钎焊温度的提高或保温时间的延长,TiC的厚度逐渐增大;而钎缝中的Cu-Ti共晶组织逐渐减少。当钎焊温度为1050℃、保温时间为30min时,接头可以获得最佳的剪切强度21MPa。更多还原

【Abstract】 Since 1980s,the C_f/C composite has more and more applications in many industries such as aerospace,chemistry,medicine,etc,due to its low density,high specific strength,excellent wear and ablation resistance.However,the high cost,low oxidation resistance at high temperature and complexity in preformed parts’ fabrication of this kind of material restrict its further application.In this work,rapid fabrication of C_f/C composites were successfully achieved by using rapid chemical liquid deposition technology,and C_f/C composites with a density up to 1.7 g/cm³ were fabricated.The surfaces of C_f/C composites were modified with electrochemical method by using concentrated nitric acid.The effects of two different modification methods on the microstructure and properties were compared by using SEM and three point bending test.The ideal surface modification procedure is as follows:electrochemical method,1.2 V,5 minutes.Effect of density of fibrous body and deposition time on the density and microstructure of C_f/C composite were investigated.The results show that the C matrix surrounds the C fibers in multiple circular layers,with the number of layers related to thickness and density of fibrous body.The density of C_f/C composite depends on its deposition time.With fibrous body with densities of 0.12 and 0.58 g/cm³,the density of C_f/C composite increased rapidly with deposition time at beginning.After 4 hours deposition,the density of C_f/C composite increased very slowly.And after 5 hours deposition,the density of C_f/C composite did not change any more.With fibrous body with a density of 0.66 g/cm³,the density of C_f/C composite kept increasing during the initial 6 hours of deposition.The density of C_f/C composite relate to the density of fibrous body.With a constant deposition time,the density of C_f/C composite decreased with increasing the density of fibrous body.The resistivity and scleroscope hardness of the resultant C_f/C composite were measured.The resistivities fall between 87.5Ω·μm and 119.5Ω·μm,with an average value of 103.5Ω·μm.With three kinds of fibrous body with different densities（0.12 g/cm³,0.58 g/cm³ and 0.66g/cm³）,the hardnesses of C_f/C composites are 78,90 and 96 HSD,respectively。The bending strength and compressive strength were measured by using universal testing machine.The maximum bending strength and compressive strength are 122.51 MPa and 102.57 MPa respectively.Various factors affecting the strength of C_f/C and the characteristic of stress-strain curve were analyzed.The fracture appearances of C_f/C composites were investigated by using SEM,and the fracture mechanism for C_f/C was analyzed.It is indicated that the bending strengths of C_f/C composites increased with the increasing of density of fibrous body.The fracture appearances of C_f/C composites with high interface bonding strength exhibit a typical brittle characteristic,and those of C_f/C composites with a modest interface bonding strength exhibit pseudo-plastic characteristics.A layer containing C and Si series chemical compound were deposited on C_f/C composite through secondary deposition method.The deposition parameters were optimized by Orthogonal design,the optimal parameters were obtained.Silicone oil with a Si fraction of 20%was used as source of silicon.The rise time for 1 volt is 60 minutes.The maximum density is 1.94 g/cm³,and the maximum porosity is 17.07%. The morphology and distribution of various phases were analyzed with XRD,SEM and EDS.The results show that C and Si series chemical compound were deposited together into the surface pore of C_f/C composite.The bending strength and compressive strength of C_f/C-Si series chemical compound composites were measured.The maximum bending strength is 119.54 MPa,lower by 5%～10%compared with that of C_f/C composite.The maximum compressive strength is 80.02 MPa,higher by 10%～15%.The oxidation behaviors of C_f/C-Si series chemical compound composites were investigated with thermogravimetry,DSC and oxidation kinetics.Compared with C_f/C composite,the oxidation resistance of C_f/C-Si series chemical compound composite increases by 4-5 times.This is attributed to that the Si series chemical compound surface layer can effectively prevent the composite from oxidizing.The initial oxidation temperature of C_f/C-Si series chemical compound composites rised up to 684.0℃.The effect of oxidation on the structure and maco-dimension.The isothermal oxidation weight loss curve is linear curve.With increasing temperature,the weight loss rates due to oxidation increase.By means of SEM,the effect of the density of fibrous body on micro-morphology of oxidized C_f/C composite and weight loss rate were investigated.The results show that pyrolytic carbon is more liable to oxidation. With increasing the density of fibrous body,the weight loss rates of C_f/C decrease.Systematic studies were conducted on vacuum brazing of C_f/C composite. Microstructures and properties of C_f/C/Cu-Ti/ C_f/C brazing joints under various brazing parameters were investigated.Cu-based and Al-based brazes with different content of Ti were designed.The wet abilities of the brazes were measured by specific contact area method.The results show that the wet abilities of both Cu-based and Al-based brazes on C_f/C composites were improved with increasing content of Ti.In case of 15%of Ti,both Cu-based and Al-based brazes on C_f/C composites exhibit optimal wet abilities.Ti in molten braze diffuse towards interface between C_f/C and braze,and react with C to form TiC,reducing the interfacial energy and improve the wet ability.With low Ti content,only few isolated TiC particles form on the interface and thus the wet abilities are not acceptable.However,with too high Ti content,a lot ofβ-TiCu₄ chemical compounds were found,the wet abilities are not acceptable.The microstructure of joint brazed with Cu-Ti consists of C_f/C,TiC,Cu-Ti eutecticβ-TiCu₄+α-Cu.Based on the structure of joint,a physical model for brazing process of C_f/C composite and a mechanism for interface structure formation were developed.The constituents in the brazing joint do not vary with the brazing process parameter, only the fraction of each constituent（C_f/C,TiC,Cu-Ti eutectic andβ-TiCu₄+α-Cu） and the thickness of TiC change.With increasing of brazing temperature and soak time, the thickness of TiC increase,and the amount of Cu-Ti eutectic decease.With a brazing temperature of 1050℃for 30min,joints with a shear strength of 21MPa were obtained.更多还原