反应烧结碳化硅陶瓷材料的研究

本文介绍了不同工艺参数对反应烧结碳化硅材料(RBSC)的显微结构和力学性能的影响,给出了这种材料的一些实验结果总结如下:1.素坯密度对 RBSC 材料的显微结构和力学性能产生很大的影响,所以控制适当的素坯密度是非常重要的;2.RB-24的 RBSC 试样性能:密度3.05～3.09g/cm~3;游离硅15vol%,室温抗弯强度640MPa;室温时 K_(1c)为4.3,硬度 HR_A=92.5;韦伯尔模数为14;3.RBSC 材料的耐腐蚀性能和耐磨性能优于硬质合金和高纯 Al_2O_(?)。     

粉体表面包碳对3Y-TZP陶瓷烧结性能及微观结构的影响

采用对纳米氧化锆陶瓷粉体表面包碳,研究了在不同烧结工艺下,碳含量(0%～7.0%(wt))的变化对纳米氧化锆陶瓷烧结性能及微观结构的影响.分析结果表明:包裹少量碳能明显提高烧结活性,增大烧结坯密度.在本实验条件下,碳含量为1.5wt%的纳米氧化锆陶瓷在1250℃低温氧化气氛中烧结,可得到相对密度约为96%、晶粒尺寸约为85m的陶瓷体,此后随着碳含量的增加,致密度减小;采用真空烧结,由于碳没有被氧化,包裹层的存在阻止了晶界的扩散,延缓了陶瓷体烧结的致密化过程,烧结性能较差;同时碳的加入有效地抑制了晶粒的长大.     

热等静压制备亚微米细晶氧化铝陶瓷的研究

采用商业氧化铝粉体为原料,MgO为烧结助剂,采用干压结合冷等静压成型素坯,再经适当温度预烧得到具有一定密度的预烧体,对预烧体的晶粒生长与致密化过程进行了研究.素坯在1275℃/160MPa下热等静压烧结2h后得到了平均晶粒尺寸为650～850nm的氧化铝陶瓷.通过万能材料试验机、显微硬度仪测试抛光样品力学性能,得到其三点抗弯强度为(620±30) MPa,硬度HV10为(19.7±0.4)GPa,断裂韧性约2.52MPa·m1/2.     

碳包纳米氧化锆粉体的烧结行为与力学性能

采用粉体表面包碳技术和两步烧结方法制备出具有良好力学性能的细晶3Y-TZP材料,研究了在无压烧结和两步烧结条件下,碳含量对碳包3Y-TZP材料烧结行为及力学性能的影响.结果表明:对于包裹少量碳的3Y-TZP,与未包碳的试样相比,采用两步烧结不但能提高材料的密度还能细化晶粒;结构致密和ZrO2相变增韧使材料具有较高的维氏硬度和断裂韧性.在碳含量为1.5%时,3Y-TZP材料的维氏硬度和断裂韧性达最大值,碳含量进一步提高使材料中的气孔和缺陷增多,导致材料的性能下降.     

橡胶等静压成型纳米ZrO2(3Y)粉素坯

对橡胶等静压成型(Rubber isostatic pressing,RIP)制备纳米Y-TZP陶瓷作了初步研究.研究结果表明,通过RIP成型,可以获得相对密度较高、体积较大的ZrO₂(3Y)素坯,并在较低温度下无压烧结得到纳米Y-TZP陶瓷.在1100℃下烧结2h所得的Y-TZP陶瓷的相对密度可达97％,晶粒仅为70nm左右.相对密度较高、平均孔径小是RIP成型素坯烧结温度低的主要原因.     

超高压成型制备Y-TZP纳米陶瓷

研究了用超高压成型制备Ｙ－ＴＺＰ纳米陶瓷的新方法．通过采用新的成型方法,在５０００吨六面顶压机上实现了高达３ＧＰａ的超高压成型,获得相对密度达６０％的３ｍｏｌ％Ｙ_２Ｏ_３－ＺｒＯ_２陶瓷素坯,比在４５０ＭＰａ下冷等静压成型所得素坯的密度高出１３％．这种超高压成型所得素坯具有极佳的烧结性能,可在１０５０～１１００℃下经无压烧结致密化．研究表明,这种素坯烧结性能好的主要原因是素坯的相对密度比较高,从而大大增加了物质的迁移通道．由于烧结温度极低,有利于制备ＺｒＯ_２晶粒尺寸＜１００ｎｍ的纳米陶瓷。在１０５０℃／５ｈ的条件下,可烧结得到相对密度达 ９９％以上的 Ｙ－ＴＺＰ纳米陶瓷,平均晶粒仅为 ８０ｎｍ．     

碳含量对高合金铁基材料组织与性能的影响

研究了高合金铁基材料中碳含量对材料显微组织与性能的影响.采用光学显微镜、扫描电镜和能谱分析表明:碳含量对材料烧结密度和性能影响显著.热处理后,材料硬度上升,但强度和韧性下降.烧结时,碳含量增加,使液相量增加,晶界上的液相膜增厚,晶粒球化;碳化物大部分在晶界上析出,形成半连续网络组织.热处理后,由于部分碳化物的溶解,一些连续的网络组织断裂.实验得出,1250±5.°C烧结,碳含量为1.4%时,烧结致密化程度最高,相对密度达到96%,各项性能也达到最高.本实验通过合理控制碳含量,获得了高性能的铁基粉末冶金材料.     

原料粉末碳、氧含量对无粘结相硬质合金性能的影响

采用放电等离子(spark plasma sintering,简称SPS)烧结制备出了无粘结相硬质合金材料,并结合XRD、SEM、金相显微镜等分析测试手段,研究了原料粉末中碳、氧含量对无粘结相硬质合金的微观组织和性能的影响。结果表明,原料粉末中游离碳含量过高会造成烧结体晶粒的显著长大,氧含量较高会降低烧结体的致密度,从而导致烧结体的性能变差;采用纯度较高的原始粉末时,维氏硬度达到2566kg.f/mm2,断裂韧性为6.2MPa/m1/2。另外,在500℃对原料粉末进行氢气预处理可以明显降低氧含量,在1700℃下可制备出相对密度达98.8%,维氏硬度为2731kg.f/mm2,断裂韧性为6.16MPa/m1/2的无粘结相硬质合金材料。     

两段式无压烧结制备纳米二氧化锆(3Y)材料

研究了两段式无压烧结制备纳米二氧化锆(ZrO_2)(3Y)陶瓷材料过程中晶粒生长与致密化的协同控制。将粒度为30nm的商品纳米ZrO_2(3Y)粉体进行冷等压成型制成相对密度为49%的素坯,然后将坯体加热至1 250℃以获得94%的相对密度,后降温至1 050℃保温20 h,制得相对密度大于99%晶粒尺寸为100 nm左右的ZrO_2(3Y)。研究表明,在烧结过程中利用晶粒生长与气孔收缩的活化能的差异以协调晶粒生长与致密化的关系,找出晶界迁移与晶界扩散、晶格扩散协同作用区域,在该区域晶粒生长受到抑制,而致密化得以维持,从而实现晶粒生长的抑制和坯体的完全致密化是两段式无压烧结的关键所在。     

质子交换膜燃料电池(PEMFC)双极板制备过程的研究

以中间相碳微球(MCMB)为原料,采用凝胶注模成型工艺,制得高质量的素坯.将素坯置于真空气氛中进行1600℃高温烧结制得符合PEMFC严格要求的双极板.简述了浆烧结过程温度的控制,通过测试素坯和烧结体的物理性能,研究了体积密度和抗弯强度及线收缩率与固相含量间的关系.     

炭布叠层针刺预制体性能分析

通过X-Y向拉伸强度、Z向剥离强力、NOL环整体拉伸强度表征预制体性能,研究了炭布叠层针刺预制体的结构特点.结果表明:X-Y向拉伸强度反映了针刺对连续纤维的损伤程度,其随针刺密度升高而降低.网胎面密度对Z向预制体剥离强力的影响规律性不明显,3 K炭布针刺预制体剥离强力高于6K和12K炭布针刺预制体,斜纹炭布针刺预制体剥离强力高于缎纹炭布预制体.NOL整体拉伸环破坏有完全断裂、褶皱式不完全断裂、层间剥离三种模式;3 K缎纹炭布针刺预制体NOL环拉伸强度最低,只有3 MPa,呈现整体拉伸完全断裂破坏模式;12 K缎纹炭布针刺预制体呈现层间破坏模式;6 K缎纹炭布针刺预制体的破坏方式为褶皱式不完全断裂模式,整体力学性能较好.相同工艺预制体环向拉伸强度远大于X-Y向拉伸强度.     

颗粒整形对碳化硅陶瓷密封材料成型及烧结性能的影响

采用球磨对SiC粉体颗粒进行整形,并借助反应烧结制备SiC陶瓷密封材料,考察了颗粒整形对反应烧结SiC陶瓷成型、烧结性能、显微结构和力学性能的影响规律。结果表明,整形后的SiC颗粒的球形度高,粒径分布更为均匀;整形SiC粉体的振实密度和素坯密度明显提高,烧结体的显微结构更加均匀,主晶相为6H-SiC和Si,分布均匀,残炭很少;颗粒整形明显改善SiC陶瓷的成型性能及力学性能,当压力为15MPa时,整形后的SiC素坯密度为2.08g/cm~3,烧结体密度为3.06g/cm~3,抗弯强度和断裂韧性分别达到456MPa和3.87MPa·m1/2。     

Brazing of reaction-bonded silicon carbide and Inconel 600 with an iron-based alloy

The objective of the present work was to join reaction-bonded silicon carbide to Inconel 600 (IN600, a nickel-based superalloy) for use in high temperature applications by brazing with an Fe-20wt% alloy. This joining method resulted in the molten filler metal reacting with the IN600 to form a Ni-Fe-Si solution, which in turn formed a liquid with the free silicon phase of the RBSC. This liquid reacted vigorously with the SiC component of the RBSC to form low melting point phases in both starting materials and chromium carbides at the metal-ceramic interface. By using solution thermodynamics, it was shown that a Ni-Fe-Si liquid with equimolar nickel and iron contents and silicon content of less than 30 at% Si will decompose -SiC at the experimental brazing temperatures; it was also shown that these predictions agree with the experimentally observed microstructures and line composition profiles.     

Herstellung faserverstärkter,reaktionsgebundener Siliziumcarbidkeramiken unter Verwendung intermetallischer Siliziumlegierungen

Manufacturing of reaction bonded, carbon-fiber reinforced SiC-composites using intermetallic silicon alloys An advanced manufacturing technology for reaction bonded, carbon-fiber reinforced SiC-composites is shown using intermetallic silicon alloys as reactant. By infiltration of pure silicon in porous preforms, the carbon fibers are converted to siliconcarbide. A toughening effect is only possible, if the C-fiber strands are preserved during the melt infiltration. A screening in the FeSi-system was performed to control the reactivity of silicon with the free carbon, contained as powder as well as fibers in the preform. The chemical reactivity of FeSi alloys with 10–15 wt% Fe provide both, fiber preservation and optimized microstructural properties. The results of tribological tests show long term stable friction behaviour with asymptotic μ-values corresponding to a contact pattern, where the Si(Fe)SiC matrix is bearing and the friction is controlled by the carbon (C-fiber)/SiC-ceramic ratio in the surface. The low specific weight and the low wear of the CMC material open up the possibilities to use it for car lifetime brake disks.     

Effects of sintering temperature of whisker preform on the microstructures and tensile properties of Bi(OH)3-coated Al18B4O33 whisker-reinforced aluminum matrix composites

A alumina borate whisker with Bi(OH)₃ coating was prepared by a chemical method. The coated whiskers were sintered at various temperatures. The coated whisker-reinforced pure aluminum matrix composite was fabricated by squeeze casting method. The microstructures of the coated whiskers and coated composites with the different sintering temperature of whisker preform were studied, and the tensile properties of the coated composites at room temperature were also investigated. It can be found that the microstructures of coatings on the whisker surfaces and at the interface in the coated composites are strongly dependent on the sintering temperature of whisker preform. The ultimate tensile strength and elongation to fracture of the coated composites increased with the increasing of sintering temperature of the whisker preform.     

炭纤维针刺预制体增强C/SiC复合材料的制备与性能研究

以炭纤维复合网胎针刺织物为预制体, 采用“化学气相渗透法+先驱体浸渍裂解法”(CVI+PIP)混合工艺, 制备了C/SiC陶瓷复合材料; 研究了针刺预制体的致密化效率以及复合材料的微观结构和力学性能, 并与目前常用的三维编织C/SiC复合材料和预氧丝针刺织物增强C/SiC复合材料进行了对比. 结果表明, 针刺预制体的致密化效率明显高于三维编织预制体, 在相同致密工艺条件下, 炭纤维针刺织物增强复合材料和预氧丝针刺织物增强复合材料的密度分 别达到2.08和2.02g/cm³, 而三维编织预制体增强复合材料的密度仅为1.81g/cm³. 炭纤维针刺复合材料的力学性能高于预氧丝针刺复合材料, 弯曲强度和剪切强度分别达到237和26MPa.     

Crack-healing in reaction-bonded silicon carbide

Crack healing in reaction-bonded silicon carbide (RBSC) was investigated by introducing cracks with the depth of 2.0–2.3 mm into flexure specimens and subsequently heat-treating the specimens at temperatures up to 1300 °C. Results showed that the cracks were healed by being filled with amorphous silica produced by the oxidation of silicon and silicon carbide. As the cracks were healed, the strength of cracked specimens was recovered to as much as 60% of the strength of as-machined specimens.     

Fabrication and characterization of some novel reaction-bonded silicon carbide materials

Attempts have been made to produce modified reaction-bonded silicon carbide (RBSC) ceramics by incorporating a dispersion of other phases into the initial powder mix. ZrC, TiC, TaC and B₄C were chosen as additives together with TiB₂ as a phase likely to produce microcrack toughening in the final compact. During fabrication an important factor appears to be the possible reactions of the added phase with liquid silicon during the infiltration stage of the process. Thus, while all the carbides react with liquid silicon to form refractory silicides and new silicon carbide, this only significantly affected the reaction-bonding process if the dissolution/reaction kinetics were so fast as to disrupt the formation of the new silicon carbide framework which grows epitaxially to bond the existing silicon carbide particles together. As with conventional RBSC, the initial SiC grits play no part in any reaction except to act as nucleation sites for the new SiC. The microstructures of the various new materials have been characterized by reflected light microscopy, scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction. This has led to an appraisal of the high-temperature reactions observed to have occurred and the unreliability of the high-temperature thermochemical data used to predict their occurrence. The mechanical properties of the new materials have been investigated by indentation testing (hardness and fracture toughness), including temperature-variant tests. Results are presented and the possibility for improving the properties of RBSC are discussed.     

A wax-mold route to fabricate polymer-derived SiC scaffold

A wax-mold route was developed to fabricate phenolic resin-derived SiC scaffold. Firstly, a wax-mold with the desired structure of the scaffold was obtained. Then resin mixtures were poured into the mold. After curing and pyrolyzing, porous carbon preform was obtained. The wax-mold was removed through being melted during curing, and the pattern material could be recycled. Finally, the SiC scaffold was fabricated by infiltrating liquid silicon into the preform. The dimension shrinkage of SiC scaffold was between 1.3 and 3 % before/after infiltration, which was affected by the infiltrating temperature and the starting components in resin mixtures, and there is no distortion. When the preform with apparent porosity of 45.5% was employed, the flexural strength and the density of SiC scaffold were 445 MPa and 3.07 g/cm³, respectively.     

High-temperature thermal conductivity of biomorphic SiC/Si ceramics

Thermal conductivity of biomorphic SiC/Si, a silicon carbide + silicon containing two phase material, was evaluated using the laser steady-state heat flux method. These materials were processed via silicon melt infiltration of wood-derived carbon scaffolds. In this approach, heat flux was measured through the thickness when one side of the specimen was heated with a 10.6-µm CO₂ laser. A thin mullite layer was applied to the heated surface to ensure absorption and minimize reflection losses, as well as to ensure a consistent emissivity to facilitate radiative loss corrections. The influence of the mullite layer was accounted for in the thermal conductivity calculations. The effect of microstructure and composition (inherited from the wood carbonaceous performs) on measured conductivity was evaluated. To establish a baseline for comparison, a dense, commercially available sintered SiC ceramic was also evaluated. It was observed that at a given temperature, thermal conductivity falls between that of single-crystal silicon and fine-grained polycrystalline SiC and can be rationalized in terms of the SiC volume fraction in biomorphic SiC/Si material.