Microstructures and tribological behaviors of Cf/C-SiC composites tailored by Cu and Fe-Si matrices

Ternary Cu-Fe-Si alloy were applied to modify tribological behavior of carbon fiber/carbon-silicon carbide (C_f/C-SiC) composites by reactive melt infiltration. Microstructures, physical properties and tribological properties on a full-scale train brake test rig of the modified composites were studied. Results indicate that both Cu and Fe-Si alloy as matrices lead to significantly enhanced thermal conductivity and compressive strength for C_f/C-SiC composites. Moreover, the average friction coefficient of the modified composites is between 0.25 and 0.55, which is higher than that of copper metal matrix composites. In addition, the average volume wear rate of the modified composites is only 0.168 cm³/MJ. The C_f/C-SiC composites modified by Cu and Fe-Si alloy with improved physical properties and tribological properties meet the technical requirement and show high application potential in express train brake systems.     

Effects of cross-linked networks on dielectric properties of epoxy resins based on molecular dynamics

In this paper, three epoxy resin systems commonly used in power equipment are prepared to obtain cross-linked networks with structural differences. The relationship between microscopic structures and dielectric properties of epoxy resins is investigated and discussed. Experimental results show that the polarization and conductance losses are inhibited in the anhydride-cured systems with methyl groups. The molecular dynamics (MD) simulation shows that rigid anhydride structures (such as methyl groups) play a major role in reducing the local segment mobility and increasing the free volume at cross-linking points. The decrease in local segment mobility has been confirmed by the decrease of mean square displacement (MSD) at the cross-linking points, which is consistent with the change of measured polarization and conductance loss. In the glassy state, the dielectric properties of different anhydride-cured systems can be reflected by local MSD at the cross-linking points. At high temperatures near the glass transition temperature, both the free volume and network mobility increase significantly, which reflects the increased dielectric relaxation strength and conductance loss. The understanding of the structure–property relationships could provide a theoretical foundation for epoxy modification in a controlled manner for power equipment applications.     

Molecular dynamics simulation of crosslinking process and mechanical properties of epoxy under the accelerator

To simulate the crosslinking process of epoxy resin under the accelerator action, the crosslinking system of bisphenol-A diglycidyl ether (DGEBA) as a monomer, methyl tetrahydro-phthalic anhydride (MTHPA) as a thermal curing agent and 2,4,6-tris (dimethylaminomethyl) phenol (DMP-30) as a thermal curing accelerator has been studied using molecular dynamics (MD) simulation. An algorithm that can construct the high-crosslinked system with different crosslinking density is completed based on the Perl language, and the subsequent properties are simulated. The results of molecular dynamics simulation show that modulus have an increasing trend, and glass transition temperature (Tg) raises from 325 K to 480 K when crosslinking density is from 0% to 95.5%. Conversely, cohesive energy density lessens from 620 J/cm³ to 170 J/cm³, solubility and Poisson's ratio decrease, and the tensile strength firstly increases and then diminishes. The friction coefficient decreases firstly and then increases, meanwhile, the temperature suddenly adds by 20 K and the relative concentration distribution (RCD) grows by 1.58 times at the contact surface. This study predicts for the crosslinking process and micro mechanical properties in the DGEBA/MTHPA/DMP-30 system.     

新型无石棉有机物摩擦材料的制备及其性能研究

以热压固化方法制备了一种新型无石棉有机物（NAO）摩擦材料,研究了其力学性能和摩擦磨损性能。结果表明,NAO的冲击强度可达4.25 kJ/m2,硬度为68 HRM,力学性能优异;材料具有适中的摩擦系数,摩擦系数随温度升高而稳步升高,无热衰退现象,且高温和低温的摩擦系数曲线均较平稳;材料具有极低的磨损率;主要摩擦性能均优于国家标准所规定的技术要求。     

Molecular dynamics simulation in single crystal 3C-SiC under nanoindentation: Formation of prismatic loops

We investigate the formation mechanism of prismatic loops in a 3C-SiC single crystal subjected to nanoindentations on its (111) and (110) surface with a fictitious spherical indenter using molecular dynamics simulation. On the (111) surface, the simulated plastic deformation proceeds with the nucleation and expansion of shear loops, which react with each other to form prismatic loops eventually. Further analysis reveal that the prismatic loops form from one single shear loop or two independent shear loops. For the former, the two screw components of one shear loop undergo cross-slip and form a prismatic loop by the "lasso"-like mechanism. For the latter, two independent shear loops intersect and evolve into a prismatic loop by an extended "lasso"-like mechanism, which is also manifested in the nanoindentation on 3C-SiC (110). These results could provide a further understanding for plastic deformation in 3C-SiC nanocrystal materials.     

Infection‐resistant hyperbranched epoxy nanocomposite as a scaffold for skin tissue regeneration

Implantable devices and scaffolds are recently the focal points of biomedical research to address various human ailments. Infections associated with such appliances face severe consequences. In this report, hyperbranched epoxy/clay ? silver nanocomposites are shown to be an infection‐resistant tough implantable scaffold material. ‘Green’ silver nanoparticle embedded nanocomposites with varying percentages of nanoparticles were prepared by an ex situ technique. The thermosetting nanocomposites demonstrated an improvement of mechanical properties (tensile strength from 38 to 60 MPa) and adhesive strength from 768 to 2819 MPa compared with the pristine polymeric system. The nanocomposite surface inhibited the growth of antibiotic‐resistant microbes such as Staphylococcus aureus, Escherichia coli and Candida albicans which are mostly responsible for surgical infections. The material is highly compatible with primary liver and cardiac cell lines of wistar rat. In vivo implantation of the material fostered wound healing in wistar rat. Hematological and histopathological parameters of the tested animals confirmed the compatibility of the scaffold with the in vivo system. Thus, the study forwards the nanocomposite as a potential infection‐resistant implantable scaffold for skin tissue regeneration. © 2014 Society of Chemical Industry     

Material properties of the cross-linked epoxy resin compound predicted by molecular dynamics simulation

Molecular dynamics (MD) simulations were conducted to estimate the material properties of the cross-linked epoxy resin compound. A periodic amorphous structure of the cross-linked epoxy resin compound was constructed and it was simulated by continuous accumulation of structure configurations at various temperatures. Based on the simulation results, glass transition temperature (T_g), linear thermal expansion coefficients and Young's modulus of the cross-linked epoxy resin compound were predicted. The predicted values of these material properties are in good agreement with the experimental values in the literature.     

环氧树脂／纳米蒙脱土胶粘剂耐蚀及耐磨性研究

通过正交试验研究了环氧树脂、固化剂、增韧剂、有机化纳米蒙脱土、偶联剂对环氧胶粘剂拉伸剪切强度的影响,并对所得配方的胶粘剂进行腐蚀和冲蚀磨损试验,由此推荐环氧树脂／纳米蒙脱土胶粘剂各组分的最佳加入量。     

Mechanical reinforcement and wear resistance of aligned carbon nanotube/epoxy nanocomposites from nanoscale investigation

The anisotropic mechanical reinforcement compression and scratch resistance of epoxy nanocomposites filled with well-aligned carbon nanotubes (ACNTs) sliding in different orientations were investigated by nanoindentation techniques. It was found the addition of ACNTs to epoxy very effectively improved the microscopic hardness, elastic modulus, and nanoscratch resistance of pure epoxy, and the nanocomposites in antiparallel orientation of ACNTs showed the highest enhancement. However, macroscopic compression test showed the normal orientation of ACNTs enhanced the compression strength the most. Based on nanoscale observations, new ACNTs related reinforcing and scratch resistance mechanisms were further proposed and discussed. This study will enhance the understanding of the anisotropic reinforcement effect of ACNTs with new characteristics, and will be helpful for the design and application of ACNT nanocomposite materials into precision instruments and related anisotropic nanomaterials.     

A new strategy for high-value reutilization of recycled carbon fiber: Preparation and friction performance of recycled carbon fiber felt-based C/C-SiC brake pads

To achieve the high-value reutilization of recycled carbon fiber (rCF), a new strategy of preparing rCF-based C/C-SiC brake pads is proposed in this work. The results show that the rCF possesses crystal structure and tensile strength comparable with those of virgin CF (vCF) exception of pyrolytic char adhering to the surface of rCF after pyrolysis. The rCF was converted into C/C composites through impregnation-pyrolysis. Pyrolytic char was found to have no evident negative effect on the densification rates of the rCF C/C composites. By reactive melt infiltration, the rCF C/C-SiC composites were fabricated based upon the rCF C/C composites. The achieved rCF C/C-SiC composites do not differ markedly from the vCF group control in terms of microstructure and bending strength. Furthermore, the thermal diffusion coefficients of the rCF C/C-SiC composites are very close to those of vCF C/C-SiC composites in the temperature range 25°C-300 °C. The coefficient of friction values of the rCF C/C-SiC composites are as stable as those of vCF control group, both being maintained at approximately 0.4 during friction test, whether at 25 °C or 300 °C. The wear rate of the rCF C/C-SiC composites is 3.8 μm·min⁻¹, nearly indistinguishable from that of the vCF C/C-SiC composites, i.e., 4.5 μm·min⁻¹, further suggesting that the two materials resemble each other closely. The rCF C/C-SiC composites exhibit great potential for use as alternative brake pads to serve auto braking systems. This work opens up a new path for high-value reuse of rCF.     

Interdiffusion of thermoplastics and epoxy resin precursors: investigations using experimental and molecular dynamics methods

Toughening of epoxy thermosets with thermoplastics is an important avenue towards improved properties of high‐performance systems. The main focus of the work reported was to study the complex issue of interdiffusion between thermoplastics and epoxy resin precursors, using a combination of experimental and numerical simulation methods, in order to understand the key parameters driving the process. Diameter changes of thermoplastic filaments in contact with epoxide and diamine precursors have been followed and an original approach using carbon nanotubes as tracers of the filament swelling front has been developed. This method shows that the improved mobility of low‐molecular‐weight resin monomers as well as the greater mobility of phenoxy of lower glass transition temperature as compared to poly(ether sulfone) control dissolution by favouring the penetration of precursors into the thermoplastic and lead to both swelling of the outer part of the filaments and reduction of the inner unaffected core. Simulation by molecular dynamics supports the idea that diffusion of polymer macromolecules remains limited compared to the dominant effect of precursor diffusion. Indeed, low diffusion coefficients have been predicted for thermoplastic oligomers compared to resin monomers. For the latter, the calculated values correlate well with the experimental data measured during interdiffusion. Copyright © 2012 Society of Chemical Industry     

Key technologies for laser-assisted precision grinding of 3D C/C-SiC composites

Due to their exceptional and distinctive qualities, 3D C/C-SiC composites are widely utilized in producing high-end equipment and the aerospace national defense industries. However, the hard and pseudo plastic nature of the material and its anisotropies make it challenging to process. To improve the processing quality of 3D C/C-SiC composites, laser-assisted precision grinding technology is introduced in this paper, which innovatively controls the depth of the thermally induced damage layer by adjusting the laser process parameters to reduce the hard brittleness of the material, and then the surface is created by precision grinding with a grinding wheel on this basis. Experiments on laser-induced damage, laser-assisted grinding, and diamond scratching were carried out to investigate the effect of laser parameters on material damage and the effect of laser-assisted grinding processes, with an emphasis on revealing the mechanism of material removal. The results show that laser irradiation causes complex reactions such as sublimation, decomposition, and oxidation of 3D C/C-SiC composites, resulting in SiO2 and Si and recondensed SiC, causing surface/subsurface damage. A maximum reduction in normal grinding force, tangential grinding force, specific grinding energy, and surface roughness of 35.6%, 43.6%, 43.58%, and 24.22%, respectively, compared to conventional grinding processes with laser-assisted grinding. After laser irradiation, the degree of brittle fracture in the precision grinding of workpieces is significantly reduced due to the degradation of matrix and fiber damage caused by laser irradiation, which reduces the hard and pseudo plastic properties of the material. The removal mechanism shows a trend of ductile domain removal in the grinding of thermally damaged layers, which reduces the grinding force and improves the surface quality.     

Effect of the epoxy molecular weight on the properties of a cyanate ester/epoxy resin system

Bisphenol A dicyanate (BADCy) was modified by diglycidyl ether of bisphenol A epoxy resins with different molecular weights [E20 (weight‐average molecular weight = 1000) and E51 (weight‐average molecular weight = 400)] to investigate the effects of the epoxy molecular weight on the properties of the modified systems. The reactions were monitored with differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy, and the results showed that more pentacyclic oxazolidinone rings were formed in BADCy/E51 than in BADCy/E20 with the same epoxy resin weight content. DSC showed that BADCy/E20 had a lower curing temperature than BADCy/E51 because of the higher concentration of hydroxyl groups (? OH) in E20. Thermal, moisture absorption, and mechanical testing showed that E51‐modified BADCy performed better because of its lower molecular weight. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1744–1750, 2006     

A facile synthesis of a novel optoelectric material: a nanocomposite of SWCNT/ZnO nanostructures embedded in sulfonated polyaniline

Functionalized single-walled carbon nanotubes (f-SWCNTs) hybridized with freshly prepared zinc oxide (ZnO) nanocrystals have been found to be good luminescent material with tuned emission properties. A three-phase nanocomposite of sulfonated polyaniline embedded with such SWCNT/ZnO nanostructures has been prepared by a simple solution mixing chemical process and characterized by using high-resolution transmission electron microscopy, X-ray diffractometry, Raman spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The study of UV-visible absorption and photoluminescence spectroscopies reveal that the ternary polymer nanocomposite is a luminescent material with enhanced emission intensity. Also an increase in DC conductivity indicates that the nanocomposite is also a good conductive material, satisfying Mott’s variable range hopping model for a two-dimensional conduction. Such a three-phase nanocomposite may find extensive application in dye-sensitized solar cells, sensors, and supercapacitors.     

Analysis of tensile modulus of PP/nanoclay/CaCO3 ternary nanocomposite using composite theories

The tensile modulus of PP/nanoclay/CaCO₃ hybrid ternary nanocomposite was analyzed using composite models. Rule of mixtures, inverse rule of mixtures, modified rule of mixtures (MROM), Guth, Paul, Counto, Hirsch, Halpin–Tsai, Takayanagi, and Kerner–Nielsen models were developed for three‐phase system containing two nanofillers. Among the studied models, inverse rule of mixtures, Hirsch, Halpin–Tsai, and Kerner–Nielsen models calculated the tensile modulus of PP/nanoclay/CaCO₃ ternary nanocomposite successfully compared with others. Furthermore, the Kerner–Nielsen model was simplified to predict the tensile modulus by volume fractions of nanofillers. Also, Takayanagi model was modified for the current ternary system. The developed Takayanagi model can predict the tensile modulus using Young's modulus and volume fractions of matrix and nanofillers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Nonisothermal crystallization and melting behavior of PP/nanoclay/CaCO3 ternary nanocomposite

Nonisothermal crystallization and melting behavior of PP/nanoclay/CaCO₃ ternary nanocomposite were investigated using different melt flow index (MFI) of PP, nanoclay and CaCO₃ contents. The rate of crystallization was also studied using relative crystallinity as a function of temperature and time. The results show that the increase of MFI of PP and CaCO₃ content in the prepared ternary nanocomposite shift the crystallization curve of PP to the higher temperature. However, increasing the content of nanoclay from 2 wt % to 6 wt % decreases the crystallization temperature possibly due to the restriction of molecular chain mobility. Further analysis of nonisothermal crystallization was carried out based on Avrami equation which the crystallization kinetic of prepared nanocomposite was evaluated. Except the significant variation in the heat of melting, the influence of these parameters on the melting behavior was much less than the crystallization process. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

A study on hygrothermal degradation and recovery of an epoxy adhesive using molecular dynamics simulation

For application of epoxy adhesive to joining similar or dissimilar materials in vehicle bodies, its hygrothermal degradation (HTD) caused by severe environmental conditions of service will always be an issue until the relevant mechanism is clearly addressed and the remedy is found. This study provides experimental observations of an epoxy adhesive in terms of HTD and recoverability of the mechanical performance, and in the meantime, molecular dynamics (MD) simulations are performed to analyse the underlying mechanism. Comparing experimental results of the adhesive among states of the initial, HTD and dried manifests that the glass transition temperature (T_g) and the uniaxial tensile properties both reduced after HTD but partially recovered when dried. In the MD simulations, both of the dominant HTD factors, plasticization and hydrolysis, are accounted for via characterizations of water inclusion and bond scission. The simulation results reveal that both of the HTD factors reduce T_g, while only hydrolysis weakens the tensile properties. A quantitative comparison between the influences of plasticization and hydrolysis implies that hydrolysis is reversible for this specific epoxy adhesive.     

阳离子聚合物/膨润土纳米复合吸附材料的性能及对红色染料的脱色

利用阳离子聚合物-聚环氧氯丙烷二甲胺为插层剂,采用 “有机高聚物溶液直接插层复合法”制备了一系列阳离子聚合物聚环氧氯丙烷二甲胺/膨润土纳米复合吸附材料,经特性分析表明,经聚环氧氯丙烷二甲胺插层复合后的膨润土比表面积显著增加,表面电性由负值变为正值,颗粒的聚集程度增加,有较好的吸附和沉降性能.在此基础上,研究了还原大红R、分散大红S-R、活性艳红K-2BP等红色染料在复合膨润土上的吸附行为.结果表明,聚环氧氯丙烷二甲胺/膨润土对三种染料的吸附脱色能力明显大于钠基膨润土,平衡吸附量q_e与平衡浓度C_e之间的关系均符合Langmuir和Freundlich等温吸附模型,吸附动力学行为遵循Langmuir方程所述规律.     

Preparation of epoxy resin/CaCO3 nanocomposites and performance of resultant powder coatings

Epoxy resin/CaCO₃ nanocomposites were prepared by the methods of extruding, solution blending, and in situ and inclusion polymerization, respectively. The contents of nanoparticles in the nanocomposites were varied from 5 wt % to 15 wt %. Powder coatings with different content of nanoparticles were made from the nanocomposites. The results showed that the cupping property and impact resistance decreased with the increase of coating film thickness. The dispersion of nanoparticles in epoxy matrix affected the impact resistance and cupping property of the obtained coating films greatly. The coating films made from the nanocomposite prepared by in situ and inclusion polymerization showed that the best impact resistance and the maximum cupping property was achieved when nano‐CaCO₃ content was 5 wt %. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2656–2660, 2006     

Viscoelasticity of epoxy resin/silica hybrid material prepared via sol–gel process: Considered in terms of morphology

We investigated the relationship between the morphology and viscoelasticity of epoxy/silica hybrid materials manufactured via two different processes: simultaneous formation of epoxy and silica phases and sequential formation of silica phase in the prepared epoxy phase. The glass transition phenomena of the hybrid materials mostly depended on their silica structure. The particular structure did not affect T_g much, while the silica chain structure greatly raised T_g of the hybrid samples. The storage modulus E′ depended on the volume fraction of the silica phase ζ, rather than the silica structure. In the glassy state, E′ of the hybrid samples slightly decreased when compared with the neat epoxy samples. Lack of chemical reaction between the silica and the epoxy phases could be attributed to this decrease at which the silica structure could have worked as a flaw. In the rubbery state, E′ greatly increased with increasing silica content ζ regardless of the silica structure, and this behavior well agreed with that predicted by the Davies model, because the physical interaction worked very well in the rubbery region. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008