陶瓷结合立方氮化硼磨削工具材料制备研究

陶瓷结合立方氮化硼(CBN)磨削工具是一类用于磨削加工的新型陶瓷/玻璃复合材料。本文作者从复合材料制备理论角度探讨了这类材料的制备原则,并进行了试验研究。研究结果表明:CBN磨具的烧成温度以低于800℃为宜;陶瓷结合剂与CBN磨料的热膨胀系数匹配性对磨具强度有较大影响,具有较小热膨胀系数和较高强度的C₁结合剂较适合CBN磨具制备。在一定温度范围内,适当提高烧结温度,有利于提高陶瓷结合剂桥相本身强度及结合剂与CBN磨料颗粒的结合强度。     

Fracture mechanisms of epoxy-based ternary composites filled with rigid-soft particles

Epoxy composites filled with different amounts of aggregate-free silica nanoparticles and phase-separated submicron rubber particles were fabricated to study the synergistic effect of multi-phase particles on mechanical properties of the composites. Compared with binary composites with single-phase particles, the ternary composites with both rigid and soft particles offer a good balance in stiffness, strength and fracture toughness, showing capacities in tailoring the mechanical properties of modified epoxy resins. It was observed that debonding of silica nanoparticles from matrix in the ternary composites was less pronounced than that in the binary composites. Moreover, the rubber particles became smaller and their shape tends to be irregular, affected by the presence of rigid silica nanoparticles. The toughening mechanisms in the epoxy composites were evaluated, and the enlarged plastic deformation around the crack tip, induced by the combination of rigid and soft particles, seems to be a dominant factor in enhancing fracture toughness of the ternary composites.     

NaBF4 as a hygrothermal durability enhancer for glass fibre reinforced polypropylene composites

The long term performance of composite materials is highly desired for their expanding application range. Tuning the interphase properties has been proven to be a practical way to enhance the performance of composites. In this study, short glass fibre (GF) reinforced polypropylenes (PPs) with improved hygrothermal durability were obtained by incorporating NaBF₄ into the sizing and thus the interphases of GF/PP composites. Detailed investigations were performed on the surface properties of sized GFs and the mechanical properties of virgin and aged composites. It was found that the retention in both ultimate tensile strength and Charpy impact toughness of aged composites monotonically increased with increasing NaBF₄ content. The improvement in hygrothermal durability was related to the enhanced fibre/matrix adhesion strength induced by the presence of NaBF₄ as indentified by fracture surface analysis using field-emission scanning electron microscopy and single fibre pull-out test.     

Effects of chemical modification of wood particles with glutaraldehyde and 1,3-dimethylol-4,5-dihydroxyethyleneurea on properties of the resulting polypropylene composites

Both glutaraldehyde (GA) and 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) can crosslink the cell wall polymers and dimensionally stabilize wood particles and the treated wood particles are thus expected to enhance the properties of the resulting wood particle/polypropylene composites. Compared to the composites filled with untreated particles, treatments of wood particles with both GA and DMDHEU showed a great reduction in water uptake and dimensional swelling of the resulting composites up to 39% and 46%, respectively. Both the flexural and tensile moduli increased due to wood particles treatments with GA and DMDHEU. Treatments of wood particles improved the tensile strength but moderately weakened the flexural strength and Charpy impact strength of the composites. Dynamic mechanical analysis and microscopy suggested an improved interfacial compatibility between wood particles and matrix due to GA and DMDHEU treatments. Chemical treatment resulted in smaller particle sizes and altered microscopic fracture appearance after composite production as compared to untreated particles. Morphological changes were attributed to embrittlement of wood particles, which may negatively influence the mechanical properties of the resulting composites.     

Interfacial,fire retardancy,and thermal stability evaluation of graphite oxide (GO)-phenolic composites with different GO particle sizes

Mechanical and thermal properties of graphite oxide (GO)-phenolic composites were evaluated for different sizes of GO. Tensile tests on the composites with larger sizes of GO particles typically exhibited better mechanical properties. After ageing tests at 200 °C a decline in the mechanical properties of GO-phenolic composites was observed but this decline was less than that for neat phenolic resin. This was attributed to the GO absorbing thermal energy and thereby reducing damage to the molecular chain in the resin. The ageing tests, also suggested that the wettability of specimens improved with the addition of GO, which might be attributed to microvoid formation on specimen’s surface during the elapsed time at the elevated temperature. The chemical structures of neat phenolic resin was relatively easily broken-up by thermal damage, whereas GO-phenolic composites exhibited better thermal stability in both thermal analysis and flame retardant testing. The GO particles exhibited reinforcing effects that served to protect chemical bonding in the phenolic resin. It appears, therefore, that GO composites may be good candidates for us as heat and flame resisting materials, for various applications.     

Effect of surface modification of bamboo cellulose fibers on mechanical properties of cellulose/epoxy composites

Bamboo cellulose fibers were treated with NaOH aqueous solution and silane coupling agent, respectively, before they were applied into epoxy composites. The effect of surface modification on mechanical properties was evaluated by tensile and impact tests under controlled conditions. Compared with the untreated cellulose filled epoxy composites, the NaOH solution treatment increased the tensile strength by 34% and elongation at break by 31%. While silane coupling agent treatment produced 71% enhancement in tensile strength and 53% increase in elongation at break. The scanning electron microscopy (SEM) was used to observe the surface feature of the cellulose fibers and the tensile fractures as well as cryo-fractures of the composites. The Fourier transform infrared (FTIR) was employed to analyze the chemical structure of the cellulose fibers before and after modifications. The results indicated different mechanisms for the two modifications of cellulose. The NaOH solution partly dissolved the lignin and amorphous cellulose, which resulting in splitting the fibers into smaller size. This led to easier permeating into the gaps of the fibers for epoxy resin (EP) oligmer and forming effective interfacial adhesion. Based on the emergence of Si–O–C and Si–O–Si on the cellulose surface, it was concluded that the enhancement of mechanical properties after coupling agent modification could be ascribed to the formation of chemical bonds between the cellulose and the epoxy coupled with the coupling agent.     

Effect of surface etching on interphase and elastic properties of a biocomposite reinforced using glass–silica particles

Biopolymer based composites are designed using glass–silica reinforcement. Surface etching of spherical glass–silica particles is performed using chemical and physical treatments. In particular, treatment with hydrofluoric acid proved to be efficient to achieve acceptable anchoring effect. Experimental testing of thermomoulded composites confirms that samples with chemically modified microbeads have improved mechanical properties, irrespective of phase content. A quantitative evaluation of the improvement of the starch/glass–silica interphase properties is achieved using a finite element model. Generation of typical microstructures is used to simulate phase arrangement and interphase properties. Microstructures are meshed taking into account the interphase region. Finite element results indicate that for all samples, interphase Young’s modulus is lower than those of the intrinsic materials. The thickness weighted interface modulus increases for composites where the mechanical adhesion is improved using HF chemical treatment.     

Mechanical characterization of hierarchical carbon fiber/nanofiber composite laminates

Susceptibility to matrix driven failure is one of the major weaknesses of continuous-fiber composites. In this study, helical-ribbon carbon nanofibers (CNF) were dispersed in the matrix phase of a continuous carbon fiber-reinforced composite. Along with an unreinforced control, the resulting hierarchical composites were tested to failure in several modes of quasi-static testing designed to assess matrix-dominated mechanical properties and fracture characteristics. Results indicated CNF addition offered simultaneous increases in tensile stiffness, strength and toughness while also enhancing both compressive and flexural strengths. Short-beam strength testing resulted in no apparent improvement while the fracture energy required for the onset of mode I interlaminar delamination was enhanced by 35%. Extrinsic toughening mechanisms, e.g., intralaminar fiber bridging and trans-ply cracking, significantly affected steady-state crack propagation values. Scanning electron microscopy of delaminated fracture surfaces revealed improved primary fiber–matrix adhesion and indications of CNF-induced matrix toughening.     

Theoretical approach to the fracture of two-phase glass-crystal composites

This paper proposes a fracture theory for two-phase glass-crystal composites. It is hypothesized that the fracture mechanisms of such solids consist of the processes of crack nucleation and of crack propagation round the dispersed particles. At lower volume fractions of dispersed phase, macroscopic fracture will occur as a result of the growth of the micro-cracks originating in the vicinity of the pre-existing structural imperfections through a heterogeneous nucleation process; in this case, strength decreases with the proportion of the dispersed phase. At higher volume fractions where further crack propagation is prohibited by the hard crystalline particles, the process of crack propagation round the dispersed particles may be responsible for the macroscopic fracture of the composite; in this case, strength is an increasing function of the volume fraction. Expressions are formulated for mechanical strength of the glass-crystal composites, based upon the nucleation theory and Griffith's criterion. The published data on the strength of glass-alumina composites are used for the verification of the theory. The proposed theory explains well the strength behaviour of glass-alumina composites, and in particular, the dependence of the strength reduction on particle size at lower volume fractions.     

Strength analysis of particulate polymers

Impregnating polymeric matrix with stiff particles may significantly improve structural response of a composite material. Such improvements have to be weighed against the effects of the stress concentration at the particle–matrix interface that influence local strength and toughness. In the present paper we elaborate on the issue of local stresses and strength in particulate polymer matrix composites considering polyurethane matrix impregnated with alumina particles in numerical examples. The parametric analysis presented in the paper is concerned with the effects of the particle volume fraction and the particle-to-matrix stiffness ratio on the local stresses and initial damage. We also discuss the resilience of the impregnated polyurethane, i.e. the density of energy necessary to produce initial damage. The approach to the analysis of fracture in the composite with initial damage is discussed accounting for available experimental observations. Three scales of fracture corresponding to different phases of the development and propagation of the crack are identified, including microfracture at the particle–matrix interface and mesofracture limited to the matrix surrounding the particle. While these scales of fracture should be analyzed by numerical methods, macrofracture that occurs after the crack “emerged” from the representative unit cell where it originated can be considered using available analytical techniques. The methodology of the stress analysis of a particulate material consisting of an incompressible hyperelastic matrix and much stiffer elastic particles is also proposed in the paper.     

Effect of granulated sugar as pore former on the microstructure and mechanical properties of the vitrified bond cubic boron nitride grinding wheels

For vitrified bond cubic boron nitride (CBN) grinding wheel, introduced pores play a very important role for its mechanical properties and performance. In this paper, granulated sugar was used as pore former of the vitrified bond in CBN grinding wheel. The effects of content and particle size of the granulated sugar on the porosity and the flexural strength of the sintered vitrified bond CBN wheel samples have been investigated. It was found that the porosity of the vitrified bond CBN wheel is positively correlated with the content of the granulated sugar. The smaller and more irregular shaped pores are uniformly distributed in the bond when the content of granulated sugar is between 1 and 3 wt.%. Larger and more non-uniform pores and pore channels appear as the content of granulated sugar is increased from 5 to 7 wt.%. The flexural strength of the vitrified bond CBN wheel specimens decreases with an increase in pore former’s content and the porosity. With the increase of pore former’s particle size at the content of 3 wt.%, the flexural strength reaches to a peak value of 49 MPa with average particle size of granulated sugar is 250 μm. When the average size of granulated sugar is from 100 to 125 μm, the pores’ size is similar with the size of pore former and distributed homogeneously. The larger granulated sugar with the size from 160 to 500 μm can introduce different size of pores which could be smaller or larger than the size of pore former.     

Review of nanocarbon-engineered multifunctional cementitious composites

As structural materials, cementitious materials are quasi-brittle and susceptible to cracking, and have no functional properties. Nanotechnology is introduced into cementitious materials to address these issues. Nano materials, especially nano carbon materials (NCMs) were found to be able to improve/modify the mechanical property, durability and functional properties of cementitious materials due to their excellent intrinsic properties and composite effects. Here, this review focuses on the recent progress of fabrication, properties, and structural applications of high-performance and multifunctional cementitious composites with NCMs including carbon nanofibers, carbon nanotubes and nano graphite platelets. The improvement/modification mechanisms of these NCMs to composites are also discussed.     

Synchronous effects of multiscale reinforced and toughened CFRP composites by MWNTs-EP/PSF hybrid nanofibers with preferred orientation

MWNTs-EP/PSF (polysulfone) hybrid nanofibers with preferred orientation were directly electrospun onto carbon fiber/epoxy prepregs and interlaminar synchronously reinforced and toughened CFRP composites were successfully fabricated. With MWNTs-EP loading increasing, the oriented nanofibers were obtained accompanying with enhanced alignment of inner MWNTs-EP. Flexural properties and interlaminar shear strength of composites were improved with increasing MWNTs-EP loadings, whereas fracture toughness attained maximum at 10 wt% MWNTs-EP loading and then decreased. Based on these results, multiscale schematic modeling and mechanism schematic of hybrid nanofibers reinforced and toughened composites were suggested. Due to the preferred orientation of nanofibers, MWNTs-EP was inclined to align vertically to carbon fiber direction along the in-plane of interface layer. The proposed network structures, containing four correlative phases of MWNTs-EP/PSF sphere/carbon fiber/epoxy matrix, contributed to simultaneous improvement of strength and toughness of composites, which was realized by crack pinning, crack deflection, crack bridging and effective load transfer.     

The effect of alkaline treatment on mechanical properties of kenaf fibers and their epoxy composites

In this work, kenaf fibers were pre-treated in a NaOH solution (6% in weight) at room temperature for two different periods (48 and 144 h). The chemical treatment of kenaf fibers for 48 h allowed to clean their surface removing each impurity whereas 144 h of immersion time had detrimental effect on the fibers surface and, consequently, on their mechanical properties.Untreated and NaOH treated kenaf fibers (i.e. for 48 h) were also used as reinforcing agent of epoxy resin composites. The effect of the stacking sequence (i.e. using unidirectional long fibers or randomly oriented short fibers) and the chemical treatment on the static mechanical properties was evaluated showing that the composites exhibit higher moduli in comparison to the neat resin. As regards the strength properties, only the composites reinforced with unidirectional layers show higher strength than the neat resin. Moreover, the alkali treatment increased the mechanical properties of the composites, due to the improvement of fiber–matrix compatibility.The dynamic mechanical analysis showed that the storage and the loss moduli are mainly influenced by the alkali treatment above the glass transition temperature. Moreover, the alkali treatment led to a notable reduction of tan δ peaks in addition to significant shifts of tan δ peaks to higher temperatures whereas the stacking sequence did not influence the trends of storage modulus, loss modulus and damping of the composites.     

Flax fibre and its composites – A review

In recent years, the use of flax fibres as reinforcement in composites has gained popularity due to an increasing requirement for developing sustainable materials. Flax fibres are cost-effective and offer specific mechanical properties comparable to those of glass fibres. Composites made of flax fibres with thermoplastic, thermoset, and biodegradable matrices have exhibited good mechanical properties. This review presents a summary of recent developments of flax fibre and its composites. Firstly, the fibre structure, mechanical properties, cost, the effect of various parameters (i.e. relative humidity, various physical/chemical treatments, gauge length, fibre diameter, fibre location in a stem, oleaginous, mechanical defects such as kink bands) on tensile properties of flax fibre have been reviewed. Secondly, the effect of fibre configuration (i.e. in forms of fabric, mat, yarn, roving and monofilament), manufacturing processes, fibre volume, and fibre/matrix interface parameters on the mechanical properties of flax fibre reinforced composites have been reviewed. Next, the studies of life cycle assessment and durability investigation of flax fibre reinforced composites have been reviewed.     

莫来石纤维增强SiO2气凝胶复合材料的制备及性能研究

以正硅酸乙酯为硅源,采用溶胶-凝胶及超临界干燥技术制备了掺杂莫来石纤维的SiO2气凝胶复合材料,并对材料的热学性能和力学性能进行了测试,结果表明:SiO2气凝胶复合材料的热导率与其密度、温度和纤维添加量有关;添加莫来石纤维可以明显提高SiO2气凝胶的弹性模量和机械强度,改善材料的力学性能;莫来石纤维添加量控制在3%左右可以使SiO2气凝胶材料保持较低的热导率和较高的机械强度.     

Tailoring of dual-interface in high tenacity PP composites – Toughening with positive hybrid effect

The study proves the feasibility of manufacturing injection moulded polypropylene composites reinforced with short rayon cellulose fibres of two selectively tailored fibre–matrix interfaces. The originally developed method relies on selective chemical grafting of two different polymer waxes onto the surface of cellulose fibres in order to obtain two different strengths of fibre–matrix interfaces in one composite. This selective tailoring of a dual-interface is meant to improve the notched impact strength without deteriorating of its flexural strength. Compatibilised fibres have a strong interphase, which conditions the transfer of strain from the matrix to fibres during deformation. Fibres tailored for a weak interface more efficiently hinder the crack propagation at crash. A 32% improvement of composite notched impact strength was achieved with merely a 5% deterioration of its flexural strength. Its specific properties are on the level or better than those of polypropylene counterpart reinforced with the same content of glass fibres.     

Deformation and failure of PP composites reinforced with lignocellulosic fibers: Effect of inherent strength of the particles

PP/wood composites were prepared from two lignocellulosic fibers with different particle size and aspect ratio in order to determine the effect of these factors on the deformation and failure mechanism as well as on the properties of the composites. Wood content was changed from 0 to 80 wt%. Maleinated polypropylene (MAPP) was added to improve interfacial adhesion. The MAPP/wood ratio was kept constant at 0.1. Mechanical properties were determined by tensile testing. Micromechanical deformation processes were followed by acoustic emission (AE) and volume strain (VOLS) measurements, and by the study of fracture surfaces. The results proved that micromechanical deformations change drastically both with decreasing particle size and changing interfacial adhesion. Less debonding, fiber pull out and fiber fracture occur in composites containing small particles. Hardly any change was observed in the mechanical properties of the composites with decreasing particle size, in spite of the drastic modification of the deformation mechanism. The apparently slight influence of particle size on composite strength results from the smaller aspect ratio of the small particles, which indicates that orientation and orientation distribution must have a strong effect on reinforcement. Further improvement in composite strength is possible only through the optimization of particle size, aspect ratio and the inherent strength of wood.     

Enhanced tensile properties of aluminium matrix composites reinforced with graphene encapsulated SiC nanoparticles

Due to a high propensity of nano-particles to agglomerate, making aluminium matrix composites with a uniform dispersion of the nano-particles using liquid routes is an exceptionally difficult task. In this study, an innovative approach was utilised to prevent agglomeration of nano-particle by encapsulating SiC nano-particles using graphene sheets during ball milling. Subsequently, the milled mixture was incorporated into A356 molten alloy using non-contact ultrasonic vibration method. Two different shapes for graphene sheets were characterised using HRTEM, including onion-like shells encapsulating SiC particles and disk-shaped graphene nanosheets. This resulted in 45% and 84% improvement in yield strength and tensile ductility, respectively. The former was ascribed to the Orowan strengthening mechanism, while the latter is due primarily to the fiber pull-out mechanism, brought about by the alteration of the solidification mechanism from particle pushing to particle engulfment during solidification as a consequence of high thermal conductive graphene sheets encapsulating SiC particles.     

Modification of surface functionality of multi-walled carbon nanotubes on fracture toughness of basalt fiber-reinforced composites

In this work, we studied the influence of surface functionality of multi-walled carbon nanotubes (MWCNTs) on the mechanical properties of basalt fiber-reinforced composites. Acid and base values of the MWCNTs were determined by Boehm's titration technique. The surface properties of the MWCNTs were determined FT-IR, and XPS. The mechanical properties of the composites were assessed by measuring the interlaminar shear stress, fracture toughness, fracture energy, and impact strength. The chemical treatments led to a change of the surface characteristics of the MWCNTs and of the mechanical interfacial properties of MWCNTs/basalt fibers/epoxy composites. Especially the acid-treated MWCNTs/basalt fibers/epoxy composites had improved mechanical properties compared to the base-treated and non-treated MWCNTs/basalt fibers/epoxy composites. These results can probably be attributed to the improved interfacial bonding strength resulting from the improved dispersion and interfacial adhesion between the epoxy resin and the MWCNTs.