Fabrication of pure SiC ceramic foams using SiO2 as a foaming agent via high-temperature recrystallization

A novel method was developed to produce the pure silicon carbide foams via the high-temperature recrystallization with the presence of a novel foaming agent-SiO₂. In this method, SiO₂ reacts with SiC to produce the gases in the silica liquid at high temperature, which leads to the formation of foams. The foams consist of the directional and interconnecting SiC crystals, and numerous intercommunicating pores that are located between them. The phase of foams was identified as 6H-SiC without the presence of SiO₂ since SiO₂ particles could react completely with SiC particles and vaporize from the sample at high temperature. The total porosity, weight loss and volume expansion rate can be increased with increasing SiO₂ contents while the three-point bending strength decreasing. The porosity of SiC foam with 25 wt.% SiO₂ as a foaming agent exhibits the maximum value while the three-point bending strength shows minimum value correspondingly. The sintered samples presented the porosities of 61-81%, the bending strength from 1.5 MPa to 4.8 MPa, and the volume expansion rate from 17.4% to 65%. This research can develop the theory for the preparation of SiC ceramics foams with controlled structure.     

用K2ZrF6溶液处理碳化硅涂层碳纤维制造碳/铝复合丝

在碳纤维表面用CVD法涂覆碳化硅涂层,接着用K₂ZrF₆溶液处理,干燥后通过700℃的熔融铝制得碳/铝复合丝。复合丝的强度约为复合准则强度的70%。当碳纤维的体积分数为54%时,复合丝的强度达到1200MPa。用扫描电镜观察复合丝的断口发现界面结合状态良好。用X射线衍射分析了碳化硅-K₂ZrF₆一铝在高温下的反应情况,表明K₂ZrF₆的良好的改善润湿的作用是由于它与铝和碳化硅之间发生了强烈的化学反应。     

Rapid sintering of silicon nitride foams decorated with one-dimensional nanostructures by intense thermal radiation

Silicon nitride foams were prepared by direct foaming and subsequent rapid sintering at 1600 °C. The intense thermal radiation generated under the pressureless spark plasma sintering condition facilitated necking of Si₃N₄ grains. The prepared foams possessed a porosity of ～80 vol% and a compressive strength of ～10 MPa, which required only ～30 min for the entire sintering processes. Rapid growth of one-dimensional SiC nanowires from the cell walls was also observed. Thermodynamic calculations indicated that the vapor–liquid–solid model is applicable to the formation of SiC nanowires under vacuum.     

A comparison of composite metal foam's properties and other comparable metal foams

New closed cell composite metal foams are processed using casting and powder metallurgy (PM) techniques. The foam is comprised of steel hollow spheres packed into a random loose arrangement, with the interstitial spaces between spheres occupied with a solid metallic matrix. The characterization of composite metal foams was carried out using monotonic compression, compression-compression fatigue, loading-unloading compression, micro-hardness and nano-hardness testing. The microstructure of the composite metal foams was studied using optical, scanning electron microscopy imaging and electron dispersive spectroscopy. The composite metal foams displayed superior (5-20 times higher) compressive strengths, reported as 105 MPa for cast foams and 127 MPa for PM foams, and much higher energy absorbing capability as compared to other metal foams being produced with similar materials through other technologies.     

Mesophase AR pitch derived carbon foam: Effect of temperature, pressure and pressure release time

For the carbon foam production, mesophase pitch pellets are heated up in a reactor in an aluminum mold to specified pressures and finally pressure released to obtain green carbon foam samples. The green foams were then stabilized and carbonized. The effects of various temperatures, pressures and pressure release times on production of carbons foams are investigated. The samples are subjected to SEM, mechanical testing, mercury porosimetry analysis and bulk density determination for characterization. For the processing temperatures of 553, 556, 566 and 573 K, the densities of the foams produced were 380, 390, 410 and 560 kg/m³ respectively. The compressive strengths of the respective samples were increased from 1.47, to 3.31 MPa for the lowest and highest temperatures. The processing pressures were 3.8, 5.8, 6.8 and 7.8 MPa. The bulk density and the compressive strength of the carbon foams produced were changed from 500 to 580 kg/m³, and 1.87 to 3.52 MPa for the lowest and highest pressures respectively. Pressure release times of 5 s, 80 s, 160 s and 600 s are used to produce different carbon foam samples. The densities and the comprehensive strengths measured for the highest and lowest pressure release times changed from 560 to 240 kg/m³ and 3.31 to 2.16 MPa respectively. The pore size distribution of all of the products changed between 0.052×10^-6m and 120×10^-6m. Increase in temperature and pressure increased the bulk density and compressive strength of the carbon foams. The mercury porosimetry results show % porosity increase with increasing temperature and pressure. On the other hand, increase in pressure release time decreased the bulk density, compressive strength of the carbon foam.     

A comparison of the uniaxial tensile and pure bending strength of SiC filaments

The uniaxial and bend strengths of AVCO-SCS-6 SiC filaments were experimentally determined to be 3250 and 4320 M Pa, respectively, at room temperature. Using Weibull statistics, a relationship between the uniaxial and bend strength of the fibres was derived and was found to compare favourably with experimentally determined strength values. The elastic modulus of the filaments was also determined, using a gauge length extrapolation technique.     

PyC/SiC界面相对PIP法制备3D HTA C/SiC复合材料性能的影响

利用三维编织炭纤维预制件通过先驱体浸渍裂解法制备C／SiC复合材料。研究了热解碳（PyC）／SiC界面相对复合材料的微观结构和力学性能的影响。弯曲性能通过三点弯曲法测试，复合材料的断口和抛光面通过扫描电镜观察。结果表明：通过等温化学气相沉积法在纤维表面沉积PyC／SiC界面相以后，复合材料的三点抗弯强度从46MPa提高到247MPa。沉积界面的复合材料断口有明显的纤维拔出现象，纤维与基体之间的结合强度适当，起到了增韧作用；而未沉积界面相复合材料的断口光滑、平整，几乎没有纤维拔出，纤维在热解过程中受到严重的化学损伤，性能下降严重，材料表现为典型的脆性断裂。     

Stochastic honeycomb sandwich cores

This study presents a new type of cellular material that is simpler to make than conventional honeycombs and foams, while possessing many of the exceptional mechanical properties that make foams and honeycombs valuable as low density structural materials. The stochastic honeycombs had relative densities ranging from 7% to 13% with compressive strengths between 1 and 3 MPa, and compressive stiffnesses between 60 and 130 MPa. These values are comparable to those seen for commercial polypropylene honeycombs and significantly higher than those of commercial polypropylene foams. Rigid sandwich panels could be fabricated by reinforcing the stochastic honeycomb cores with external polypropylene face sheets. The panels could be assembled without adhesives, creating a rigid sandwich made entirely of a single material.     

UD-Cf/SiC陶瓷基复合材料的高温拉伸性能

采用两种形状(纺锤形、矩形)的拉伸试样对热压单向M40JB-C_f/SiC和T800-C_f/SiC复合材料进行了高温拉伸强度测试,得到了C_f/SiC复合材料的拉伸强度,并对纺锤形试样断裂应变的表达式进行修正,得出了复合材料的弹性模量。M40JB-C_f/SiC复合材料1300℃的拉伸强度及模量分别为374 MPa和134 GPa, 1450 ℃的拉伸强度及模量为338 MPa和116 GPa,T800-C_f/SiC复合材料1300 ℃拉伸强度和模量为392 MPa 和115 GPa。测试结果与试样的断裂方式密切相关,在有效部位断裂的测试结果大于在非有效区断裂的测试结果。M40JB-C_f/SiC复合材料的拉伸断裂应力-应变曲线表现出塑性变形的非线弹性破坏特征,而T800-C_f/SiC主要表现为线弹性特征。      

A study of the wetting,microstructure and bond strength in brazing SiC by Cu-X(X = Ti,V,Nb,Cr) alloys

In the active brazing of SiC by copper-based alloys, the effects of various active elements such as titanium, vanadium, niobium and chromium on the wetting, microstructure and bond strength are investigated. In wetting, Cu-Cr alloys have the lowest wetting angles on SiC of 10°–20° depending on the chromium content. SiC is decomposed on contact with alloy melts during brazing. Carbon and silicon released from this decomposition of SiC react with active elements to produce their carbides and suicides at the interface. The reacted layers have different microstructures depending on the brazing alloys, but Cu-Ti and Cu-Cr alloys show similar microstructure, as do Cu-V and Cu-Nb alloys. In the four-point bend tests, the brazed joints of Cu-5 at % Ti, Cu-5 at % V and Cu-5 at % Nb alloys have similar bend strengths of 86.9, 80.3 and 92.4 MPa, respectively. The brazed joints of Cu-2 at % Nb alloys show a high bend strength of 154 MPa, although the wetting angle is a little higher, at about 60°. Niobium is found as a new active element of copper-based alloys to braze SiC. Cu-Nb alloys are promising for substitution for Cu-Ti alloys.     

High-temperature strengths of aluminium composite reinforced with continuous SiC fibre

Aluminium alloys were reinforced unidirectionally with 30, 35, 40 and 50 vol %SiC fibres by a liquid-pressing method. The SiC fibres for reinforcement were produced from a polycarbosilane and were yarns consisting of 500 continuous filaments of length 300 m and diameter 13m, having a tensile strength of 2000 MPa. High-temperature tensile and bending strengths of the composites were examined in air in the temperature range from room temperature to 500° C. The strengths were not influenced by temperature up to 400° C, but were decreased at 500° C. The decrease is considered to be caused by the reduction in transfer efficiency of the stress accompanying the decrease in adhesion between fibres and matrix.     

基于聚焦粒子束技术的碳纤维单丝断裂韧性实验研究

以商用碳纤维T300和T800为研究对象, 采用聚焦粒子束(FIB)技术精确刻蚀了碳纤维单丝的缺陷, 分析了碳纤维单丝的断裂性能。通过单丝拉伸试验获得碳纤维拉伸强度, 并利用扫描电镜观察试件的断裂截面, 基于镜像方法和Griffith断裂理论获得拉伸强度与镜像半径之间的关系, 进而对碳纤维单丝的断裂韧性 K_IC进行了估算。结果表明: 采用FIB刻蚀缺陷的方法估算得到的T300碳纤维单丝的 K_IC 为1.32 MPa·m1/2, 与采用试剂溶解方法得到的数据(1.25 MPa·m1/2)相比较, 两者相差小于10%。      

SiC 纤维的强度测试和评价

测定了 SiC(W 芯)纤维的抗张强度,实验表明:对用 CVD 法制备的 SiC(W 芯)纤维的抗张强度测试值随试样标距的增大而降低,其室温抗张强度呈正态分布状态。当纤维拉伸试样标距为50mm 和25mm 时,SiC(W 芯)纤维室温抗张强度分别为3584.2±403.7MPa 和3669.9±348.1MPa,其 Weibull 模数分别为9.8和11.9。     

Mechanical characterization of dense calcium phosphate bioceramics with interconnected porosity

Porous hydroxyapatite/tricalcium phosphate (HA/TCP) bioceramics were fabricated by a novel technique of vacuum impregnation of reticulated polymeric foams with ceramic slip. The samples had approximately 5–10% interconnected porosity and controlled pore sizes appropriate to allow bone ingrowth, combined with good mechanical properties. A range of polyurethane foams with 20, 30 and 45 pores per inch (ppi) were used as templates to produce samples for testing. The foams were inpregnated with solid loadings in the range of 60–140 wt%. The results indicated that the average apparent density of the HA/TCP samples was 2.48 g/cm³, the four-point bending strength averaged 16.98 MPa, the work of fracture averaged 15.46 J/m² and the average compressive strength was 105.56 MPa. A range of mechanical properties resulted from the various combinations of different grades of PU foam and the solid loading of slips. The results indicated that it is possible to manufacture open pore HA/TCP bioceramics, with compressive strengths comparable to human bone, which could be of significant clinical interest.     

The properties of carbon fibre/SiC composites fabricated through impregnation and pyrolysis of polycarbosilane

Unidirectional carbon fibre reinforced SiC composites were prepared from four types of carbon fibres, PAN-based HSCF, pitch-based HMCF, CF50 and CF70, through nine cycles or twelve cycles of impregnation of polycarbosilane and subsequent pyrolysis at 1200°C. The polycarbosilane-derived matrix was found to be -SiC with a crystallite size of 1.95 nm. The mechanical properties of the composites were evaluated by four-point bending tests. The fracture behavior of each composite was investigated based on load-displacement curves and scanning electron microscope (SEM) observation of fracture surfaces of the specimens after tests. It was found that CF50/SiC and CF70/SiC exhibited high strength and non-brittle fracture mode with multiple matrix cracking and extensive fibre pullout, whereas HSCF/SiC and HMCF/SiC exhibited low strength and brittle fracture mode with almost no fibre pullout. The differences in the fracture modes of these carbon fibre/SiC composites were thought to be due to differences in interfacial bonding between carbon fibres and matrix. Values of flexural strengths of CF70/SiC and CF50/SiC were 967 MPa and 624 MPa, respectively, which were approximately 75% and 38% of the predicted values. The relatively lower strength of CF50/SiC, compared with CF70/SiC, was mainly attributed to the shear failure of CF50/SiC during bending tests.     

Production of Metallic Foams From Ceramic Foam Precursors

A process has been developed for obtaining closed cell metallic foams using a ceramic foam precursor. In this approach, the major constituent of the ceramic foam precursor is iron oxide (Fe₂O₃), which is mixed with various foaming/setting additives. The mixture sets rapidly at room temperature to stabilize the foam generated by hydrogen release. The oxide foam is then reduced in a non‐flammable hydrogen/inert gas mixture to obtain a metallic foam with a cell diameter of 0.5–2 mm. Iron foams with a relative density of 0.23 were tested in compression and yielded an average compressive strength of ～ 34 MPa. The compressive stress‐strain curves obtained were typical of cellular metals. The normalized strengths of the metal foams obtained in the present study compare very favorably with that of steel foams produced by other techniques.     

The thermal expansion and mechanical properties of high reinforcement content SiCp/Al composites fabricated by squeeze casting technology

With mixing different sized SiC particles, high reinforcement content SiCp/Al composites (V_p=50, 60 and 70%) for electronic packaging applications were fabricated by squeeze casting technology. The composites were free of porosity and SiC particles distributed uniformly in the composite. The mean linear coefficients of thermal expansion (20–100 °C) of SiCp/Al composites ranged from 8.3 to 10.8×10⁻⁶/°C and decreased with an increase in volume fraction of SiC content. The experimental coefficients of thermal expansion agreed well with predicted values based on Kerner's model. The Brinell hardness increased from 188.6 to 258.0, and the modulus increased from 148 to 204 GPa for the corresponding composites. The bending strengths were larger than 370 MPa, but no obvious trend between bending strength and SiC content was observed.     

Ultrasonic Torsion Welding of Sheet Metals to Cellular Metallic Materials

One of the most important requirements for finding new applications for cellular metals is to integrate them in complex technical structures. The metal foams have to be joined to each other, or to sheet materials, by suitable joining techniques. The main topics of this paper are the ultrasonic torsion welding of cellular metallic materials to sheet metals and the investigation of the mechanical properties of the joints. The basic materials of foams and sheet metals were different aluminum and iron alloys. Depending on the materials used, weldings with tensile shear strengths of up to 25 MPa were realized. Using aluminum foam sandwich (AFS) and sheet metals, successful weldings were performed before and after the foaming process. Furthermore, it was possible to perform a successful foaming process with the unfoamed AFS/sheet metal joints. Microscopic investigations showed that the ultrasonic welding technique allows the joining of the metal foams with sheet metals without significant deformation of the joining partners. The temperatures during the welding process in the interface were below the melting point of the foams and the sheet metals.     

Fabrication of ferrous metallic foams by reduction of ceramic foam precursors

A process has been developed for obtaining closed cell metallic foams using a ceramic foam precursor. In the present study, the major constituent of the ceramic foam precursor was iron oxide (Fe₂O₃), which was mixed with various foaming/setting additives. The mixture set rapidly at room temperature, stabilizing the foam generated by hydrogen release. The oxide foam was then reduced by annealing at 1240^∘C in a non-flammable hydrogen/inert gas mixture to obtain a metallic foam with a relative density of 0.23 ± 0.017, and an average cell diameter of 1.32 ± 0.32 mm. The iron foams were tested in compression and yielded an average compressive strength of 29 ± 7 MPa. The compressive stress-strain curves obtained were typical of cellular metals. The normalized strengths of the metal foams obtained in the present study compare favorably with those of steel foams produced by other techniques.     

A novel material for lightweight concrete production

This paper presents the results of an experimental study on the effects of using recycled waste expanded polystyrene foam (EPS), as a potential aggregate in lightweight concrete. In this study, thermally modified waste EPS foams have been used as aggregate. Modified waste expanded polystyrene aggregates (MEPS) were obtained by heat treatment method by keeping waste EPS foams in a hot air oven at 130 °C for 15 min. Effects of MEPS aggregate on several properties of concrete were investigated. For this purpose, six series of concrete samples were prepared. MEPS aggregate was used as a replacement of natural aggregate, at the levels of 0%, 25%, 50%, 75%, and 100% by volume. The density of MEPS is much less than that of natural aggregate; MEPS concrete becomes a lightweight concrete with a density of about 900–1700 kg/m³. The 28-d compressive strengths of MEPS concrete range from 12.58 MPa to 23.34 MPa, which satisfies the strength requirement of semi-structural lightweight concrete.