碳纳米管增强铝基复合材料的力学和物理性能

采用磁力搅拌与放电等离子烧结技术制备了碳纳米管(CNT)增强铝基复合材料.对试样进行了扫描电镜和透射电镜表征,测试了试样的力学性能、摩擦性能、电学性能和热学性能.当碳纳米管在试样中的质量分数为1%时,可在铝基体中均匀分布且CNT/Al界面结合良好,此时试样的抗拉强度和硬度较纯A1分别提高了29.4%和15.8%.在获得最佳力学性能强化和最佳减磨效果的同时.试样电导率较纯Al仅降低8.0%.碳纳米管可提高基体的热导率.但强化效果不明显.      

Improvement of Wear Resistance in Alumina Matrix Composites Reinforced with Carbon Nanotubes

Alumina matrix composites reinforced with carbon nanotubes (CNTs) fabricated by CNT purification, mixing, compaction, and sintering processes, and the effects of the CNT addition on wear resistance were investigated in relation to the relative density, hardness, and fracture toughness. Wear resistance and fracture toughness were measured by the dry sliding wear test method and the indentation fracture test method, respectively. Zero to ~3 vol pct of CNTs were homogeneously distributed in the composites, although some pores existed. The wear resistance and fracture toughness increased with an increasing CNT fraction, but the composite specimen containing 3.0 vol pct of CNTs hardly showed an increase over the specimen containing 2.25 vol pct of CNTs. Observations of worn surfaces revealed that the wear mechanism involved both the abrasive and delamination wear modes in the specimens containing 0 to ~0.75 vol pct of CNTs, whereas the surface was worn largely in an abrasive wear mode in the specimens containing 1.5 to ~3.0 vol pct of CNTs. This was because CNTs helped to change the delamination wear mode to the abrasive wear mode by preventing crack initiation and propagation at alumina grains. The fracture toughness increase provided beneficial effects in the resistance to crack initiation and propagation, the reduction in delamination wear on the worn surface, and the consequent improvement in wear resistance. Because the effect of the porosity increase due to the CNT addition unfavorably affected the improvement of wear resistance and fracture toughness in the specimen containing 3.0 vol pct of CNTs, the appropriate level of CNT fraction was 1.5 to ~2.25 vol pct.     

Three-dimensional modelling of thermal residual stresses and mechanical behavior of cast SiC/A1 particulate composites

Thermal residual stresses developed during casting of SiC/aluminum particulate-reinforced composites were investigated as a function of cooling rate and volume fraction of particles using thermo-elastoplastic finite element analysis. The phase change of the matrix during solidification and the temperature-dependent material properties as the composite is cooled from the liquidus temperature to room temperature were taken into account in the model. Further, the effect of thermal residual stresses on the mechanical behavior of the composites was also studied. Based on the study, it was found that the matrix undergoes significant plastic deformation during cool down and has higher residual stress distribution as the cooling rate increases. The model which does not include the solidification of the matrix tends to overestimate the residual stresses in the matrix and underestimate the tensile modulus of elasticity of the composites. In addition, the presence of thermally induced residual stresses tends to decrease the apparent modulus of elasticity and increase the yield strength of the composites compared to those without residual stresses.     

机械球磨与放电等离子体烧结制备碳纳米管/铜复合材料

采用机械球磨和放电等离子体烧结(SPS)工艺制备了碳纳米管(CNTs)/铜复合材料.利用SEM和TEM对材料组织和形貌进行了表征,研究了球磨时间、CNTs含量、SPS烧结压力对复合材料组织和性能的影响.结果表明:质量分数为1％的CNTs可在铜基体中获得良好分散;CNTs与铜基体界面结合良好,有利于应力在基体与CNTs之...     

苎麻纤维增强聚乳酸复合材料性能研究

通过密炼?注塑成型工艺制备了不同苎麻纤维含量的聚乳酸基复合材料,研究了纤维含量对复合材料性能的影响规律,并揭示了纤维增强机理。研究表明,苎麻纤维的添加提高了复合材料的耐热性能,尤其是当纤维质量分数为40%时,复合材料的热变形温度提高了10.5%。此外,苎麻纤维均匀地分散在基体中,由于纤维与聚乳酸的界面强度较弱,断面上有大量的纤维拔出和纤维孔洞;差示扫描量热仪测试表明高含量的纤维限制了聚乳酸分子链的运动,促进复合材料形成更加致密完善的晶核;同时,流变行为也表明苎麻纤维含量的增加有助于提高复合材料的黏弹响应和复合黏度;最后,苎麻纤维的加入提高了复合材料的拉伸和弯曲强度,且随纤维含量的增加而增大。与聚乳酸相比,当纤维质量分数为40%时复合材料的拉伸和弯曲强度分别提高了30%和21.9%。      

Hot Extrusion of A356 Aluminum Metal Matrix Composite with Carbon Nanotube/Al2O3 Hybrid Reinforcement

Over the years, the attention of material scientists and engineers has shifted from conventional composite materials to nanocomposite materials for the development of light weight and high-performance devices. Since the discovery of carbon nanotubes (CNTs), many researchers have tried to fabricate metal matrix composites (MMCs) with CNT reinforcements. However, CNTs exhibit low dispersibility in metal melts owing to their poor wettability and large surface-to-volume ratio. The use of an array of short fibers or hybrid reinforcements in a preform could overcome this problem and enhance the dispersion of CNTs in the matrix. In this study, multi-walled CNT/Al₂O₃ preform-based aluminum hybrid composites were fabricated using the infiltration method. Then, the composites were extruded to evaluate changes in its mechanical properties. In addition, the dispersion of reinforcements was investigated using a hardness test. The required extrusion pressure of hybrid MMCs increased as the Al₂O₃/CNT fraction increased. The deformation resistance of hybrid material was over two times that of the original A356 aluminum alloy material due to strengthening by the Al₂O₃/CNTs reinforcements. In addition, an unusual trend was detected; primary transition was induced by the hybrid reinforcements, as can be observed in the pressure–displacement curve. Increasing temperature of the material can help increase formability. In particular, temperatures under 623 K (350 °C) and over-incorporating reinforcements (Al₂O₃ 20 pct, CNTs 3 pct) are not recommended owing to a significant increase in the brittleness of the hybrid material.     

Consolidation of Carbon Nanotube Reinforced Aluminum Matrix Composites by High-Pressure Torsion

Al-3 vol pct carbon nanotube (CNT) composites are fabricated by consolidation through high-pressure torsion (HPT) at room temperature. The densification behavior, microstructural evolution, and mechanical properties of Al/CNT composites are studied. The results show that density and microstructural homogeneity increase with increasing number of revolutions under a high pressure of 6 GPa. Substantial grain refinement is achieved after 10 turns of HPT with an average grain thickness of ~38 nm perpendicular to the compression axis of HPT. The Al/CNT composite shows a considerable increase in hardness and strength compared to the Al matrix. The strengthening mechanisms of the Al/CNT composite are found to be (i) grain refinement of Al matrix and (ii) Orowan looping. Raman spectroscopy and high-resolution transmission electron microscopy reveal that the structure of most of CNTs is changed during processing through mechanical milling and HPT.     

Carbon fiber reinforced root canal posts. Mechanical and cytotoxic properties

The aim of this study was to compare the mechanical properties of a prefabricated root canal post made of carbon fiber reinforced composites (CFRC) with metal posts and to assess the cytotoxic effects elicited. Flexural modulus and ultimate flexural strength was determined by 3 point loading after CRFC posts had been stored either dry or in water. The bending test was carried out with and without preceding thermocycling of the CFRC posts. The cytotoxicity was evaluated by an agar overlay method after dry and wet storage. The values of flexural modulus and ultimate flexural strength were for dry stored CFRC post 82 +/- 6 GPa and 1154 +/- 65 MPa respectively. The flexural values decreased significantly after water storage and after thermocycling. No cytotoxic effects were observed adjacent to any CFRC post. Although fiber reinforced composites may have the potential to replace metals in many clinical situations, additional research is needed to ensure a satisfying life-span.     

纤维类型和涂层对纤维增强MoSi2复合材料弯曲性能的影响

以短切碳纤维(Cf)和碳化硅纤维(SiCf)为增强相,并用化学气相渗透法对部分纤维进行炭涂层处理,采用热压法制备了4种纤维增强MoSi2基复合材料(SiCf-MoSi2、SiCf/C-MoSi2、Cf-MoSi2和Cf/C-MoSi2),研究了纤维类型及表面炭涂层对MoSi2基复合材料弯曲性能的影响.结果表明纤维的加入明显提高了MoSi2的抗弯强度,加入5%SiCf和5%Cf的复合材料的强度比纯MoSi2分别提高了9.0%和22.8%,Cf增强作用明显优于SiCf;纤维类型相同时,具有炭涂层的纤维增强效果更显著,5%Cf/C-MoSi2复合材料的强度最高,达到了364.7MPa,比纯MoSi2的强度提高了30%;扫描电镜分析表明,无炭涂层的SiCf与MoSi2基体间存在着明显的裂缝,炭涂层改变了纤维与基体的界面结合;有涂层纤维的断裂机制为首先脱粘然后拔出.     

六辊冷轧机的弯辊力组合板形控制策略

针对现有六辊轧机对使用两组弯辊力进行四次板形控制的理论不足,提出了弯辊力组合板形控制策略.利用Marc有限元仿真计算软件建立辊系-轧件耦合模型,分析工作辊弯辊力与中间辊弯辊力板形调控特性的差别.在此基础上通过理论推导,建立了弯辊力组合板形控制策略的两种实现方式——在线闭环控制模型与基于弯辊力组合系数的设定参数在线调节方法.现场应用结果表明,弯辊力组合板形控制策略能够充分利用工作辊弯辊力与中间辊弯辊力板形调控特性的差别进行配合调节,对长期困扰生产的四次板形缺陷实施快速精确的控制.      

Microstructure and Mechanical Properties of CNTs/A356 Nanocomposites Fabricated by High-Intensity Ultrasonic Processing

Carbon nanotube (CNT)-reinforced A356 alloy nanocomposites were successfully fabricated by introducing a method of CNT predispersion and high-intensity ultrasonic treatment. The scanning electron microscope and energy-dispersive spectrometer results showed that high-intensity ultrasonic treatment was able to disperse the CNTs into the melt. When the ultrasonic power was less than 2.1 kW, the microhardness and tensile properties (ultimate tensile strength (UTS), yield strength (YS), and elongation) of the nanocomposites improved as the ultrasonic power increased. Further, the microhardness, UTS, and YS improved as the CNT content increased while elongation decreased. The microhardness, UTS, and YS of the 0.8 wt pct CNTs/A356 nanocomposites fabricated by high-intensity ultrasonic processing at an ultrasonic power of 2.1 kW were increased, respectively, by 27.8, 17, and 29.2 pct compared to the A356 alloy without CNT addition, and the ductility remained. The fracture analysis confirmed that CNTs were homogeneously distributed in the matrix, and strong interfacial bonding formed between CNTs and the matrix. Also, transmission electron microscope results confirmed that CNTs were stale embedded in the matrix and the formation of brittle Al₄C₃ was suppressed.

     

Micromechanical Modeling of Filament Wound Cement-Based Composites

A theoretical model to predict the response of laminated cement-based composites is developed. The micromechanical model simulates the mechanical response of a multilayer cement-based composite laminate under uniaxial, biaxial, and flexural loading modes. Tsai-Wu Criterion is used for each lamina and the stacking sequence is utilized to obtain the overall stiffness matrix. The effect of distributed cracking on the stiffness degradation of the cross ply layers under tensile loading is measured using a scalar damage parameter that is empirically related to the apparent strain. The model is calibrated by predicting the load versus deformation response of unidirectional, cross ply, and angle ply laminates under tensile and flexural loading. Results are then compared to the experimental results cross ply and angle composites with various stacking sequences.     

Tensile properties of short fiber-reinforced SiC/Al composites: Part II. Finite-element analysis

The tensile properties of aluminum matrix composites containing SiC whiskers or particulate were investigated analytically and compared to experimental results. Two finite-element models were constructed and used for elastoplastic analysis. In both models, the SiC fibers are represented as longitudinally aligned cylinders in a three-dimensional array. The cylinder ends are transversely aligned in one model and staggered in the other. Using the models, the sensitivity of the predicted composite properties to the deformation characteristics of the matrix alloy was examined, and the general behavior of the models was validated. It was determined that both models are necessary to predict the overall composite stress-strain response accurately. The analytic results accurately predict: the observed composite stress-strain behavior; the experimentally observed increase in Young’s modulus and the work-hardening rate with increasing fiber volume content and aspect ratio; and the decrease and subsequent increase in proportional limit as the SiC volume fraction is increased. The models also predict that the transverse material properties should be insensitive to fiber aspect ratio. In addition, the model predicts the location of initial yielding and the propagation of the plastic region. These results offer insights into the deformation mechanisms of short fiber-reinforced composites.     

ACF/CNT复合材料的制备及其对Cr(Ⅵ)的吸附性能

以硝酸镍为催化剂前驱体,C2H2为碳源,H2为还原气,N2为载气,采用化学气相沉积法(CVD)在活性炭纤维(ACFs)毡体的纤维表面催化生长碳纳米管(CNTs),制备ACF/CNT复合材料。经测定,所制复合材料比表面积可达62.56 m2/g;扫描电镜分析表明,CNTs在ACFs表面分布均匀而致密,经过表面修饰可以作为1种良好的吸附材料。选择低浓度的六价铬(Cr(Ⅵ))溶液进行吸附研究,考察振荡时间、溶液pH值以及溶液的初始浓度等因素对吸附行为的影响。实验结果表明,初始Cr(Ⅵ)浓度为1 mg/L,在25℃时,随着振荡时间的增长溶液中Cr(Ⅵ)的脱除率逐渐增加,在150 min时达到最大值49.48%。溶液中Cr(Ⅵ)的脱除率随着pH的减小而增大,当pH值为2.0时脱除率达91.50%,对Cr(Ⅵ)的吸附量随着溶液初始浓度的增加而增大,但是当初始浓度到达5.0 mg/L时,脱除效率到达最大值后开始降低。并对ACF/CNT复合材料的吸附机制进行了探讨。     

碳纳米管增强Cu基和Al基复合材料界面改性

在碳纳米管(carbon nanotubes,CNTs)增强Cu基和Al基复合材料的制备中,界面改性是提高复合材料性能的重要方法。金属基体和碳纳米管间的有效界面结合直接影响了复合材料中界面的载荷传递、导电以及导热性能,从而影响复合材料性能。本文综述了近几年碳纳米管增强Cu基和Al基复合材料界面改性的工艺方法,讨论了界面改性工艺对碳纳米管增强Cu基和Al基复合材料界面结构和性能的影响。     

颗粒增强钛基复合材料的研究进展

系统地介绍了颗粒增强钛基复合材料的最新研究进展和发展趋势,重点论述了该类复合材料组分的选择与改善匹配性的措施,并对颗粒增强钛基复合材料的制备技术和力学性能进行了评述。     

Damage due to fracture of brittle reinforcements in a ductile matrix

A micromechanical model is developed for brittle particle reinforced metal matrix composites sustaining damage. A composite with uniformly distributed damage is modelled by a three-phase damage cell consisting of a cracked particle in a cylindrical matrix cell embedded in an undamaged composite cylinder. The fraction of broken particles to all particles is taken as the ratio of the broken-particle/matrix cell volume to the whole damage cell volume. Systematic analysis is carried out for aligned spherical and cylindrical particles in an elastic-perfectly plastic matrix subject to tensile loading normal to the plane of particle cracks. The influence of damage evolution paths on the composite stress-strain behavior is investigated. Results are given for the effects of damaged particle percentage, total particle volume fraction and particle shape on the overall composite limit flow behavior. Significant reduction in composite limit flow stress may occur if most of the particles are broken. For composite with spherical reinforcement, the reduction is found to be linearly dependent on the percentage of damaged particles.     

Crack propagation directions in unfilled resins

Posterior composite restorative materials undergo accelerated wear in the occlusal contact area, primarily through a fatigue mechanism. To facilitate the timely development of new and improved materials, a predictive wear model is desirable. The objective of this study was to develop a finite element model enabling investigators to predict crack propagation directions in resins used as the matrix material in composites, and to verify these predictions by observing cracks formed during the pin-on-disc wear of a 60:40 BISGMA:TEGDMA resin and an EBPADMA resin. Laser confocal scanning microscopy was used to measure crack locations. Finite element studies were done by means of ABAQUS software, modeling a cylinder sliding on a material with pre-existing surface-breaking cracks. Variables included modulus, cylinder/material friction coefficient, crack face friction, and yield behavior. Experimental results were surprising, since most crack directions were opposite previously published observations. The majority of surface cracks, though initially orthogonal to the surface, changed direction to run 20 to 30 degrees from the horizontal in the direction of indenter movement. Finite element modeling established the importance of subsurface shear stresses, since calculations provided evidence that cracks propagate in the direction of maximum K(II)(theta), in the same direction as the motion of the indenter, and at an angle of approximately 20 degrees. These findings provide the foundation for a predictive model of sliding wear in unfilled glassy resins.     

Influence of particle size and volume percent of flaky mo particles on the mechanical properties of AI2O3/Mo composites

The influence of particle size and volume percent of Mo particles on flake-forming behavior of Mo powders during a ball milling process and three-point flexural strength and fracture toughness of A1₂O₃ composites reinforced with flaky Mo particles have been investigated. The flake-forming behavior of Mo powders mixed with A1₂O₃ powders becomes prominent with increasing Mo particle size, while remaining almost independent of Mo volume percent. The microstructure of the composites reinforced with flaky Mo particles is anisotropic, depending on the arrangement of these Mo particles in the A1₂O₃ matrix. The microdispersion of flaky Mo particles contributes remarkably to an increase in both flexural strength and fracture toughness. The flexural strength increases with decreasing Mo particle size, while the fracture toughness increases with increasing Mo particle size, which corresponds to an increase of the flake-forming tendency of these particles. Furthermore, the flexural strength and fracture toughness can be simultaneously improved by increasing the volume fraction of flaky Mo particles. The microstructural observations indicate that the improvement in strength may be attributed to a grain-refining effect due to inhibition of grain growth of the matrix by the presence of Mo particles. In addition, the improvement in fracture toughness may be due to plastic deformation of Mo particles at a crack tip, which is accelerated more by the flaky rather than the small spherical shape.