Characterisation of interfacial imperfections of TiAl and 40Cr diffusion bonding by ultrasonic amplitude and phase

Abstract

A characteristics extraction algorithm is proposed to characterise the interfacial imperfections in TiAl and 40Cr diffusion bonding. The algorithm is based on analysing the variation of the ultrasonic amplitude and phase after interacting with the bonding interface. Ultrasonic measurements were performed by an ultrasonic imaging testing system, and broadband transducers with central frequency of the 10 and 20 MHz were employed. Metallographic analyses and shear tests were also performed on the joints. It was found that the amplitude of the reflection coefficient is almost a constant, and the phase of the reflection coefficient is the same for the perfectly bonded interface; for the kissing bond interface, the amplitude increases with the ultrasonic frequency, and the phase is the same at the low frequencies and opposite at the high frequencies; the amplitude does not vary with the frequency, and the phase is opposite for the unbonded interface.     

Atomistic simulations of interfacial sliding in amorphous carbon nanocomposites

Nanocomposites with amorphous carbon matrix reinforced by hard crystalline nanoparticles are fast developing as next generation super-tough and wear resistant coatings. The frictional wear and toughness properties of these composites are determined by the properties of the interface between crystalline- and amorphous-phases. In this paper we use molecular dynamics and statics simulations to study the interfacial energetics, internal stresses, sliding and friction behavior of diamond- and amorphous-carbon interfaces. It is found that the orientation of crystalline phase has minor effect on sliding behavior. Sliding behavior is affected by two mechanisms (1) bond breaking and reattaching at the interface and (2) deformation of amorphous carbon in the region surrounding the interface. It is found that the deformation away from the interface reduces the resistance to sliding. In structures with higher SP3 content, bond breaking at the interface dominates and there is much lesser deformation in the amorphous phase. The frictional resistance is significantly less if the interfacial bonding is primarily due to the Van Der Waal’s interactions.     

Bulk and interfacial effects in Co-Cr2O3 nanocomposites

The exchange bias field has been measured in a set of Co-Cr2O3 nanocomposites in order to distinguish between the bulk and interfacial contributions to H(ex). The studies were carried out on a set of samples prepared by the sol gel technique in which the Co concentration was varied between 30 and 80 wt%. The particle sizes in all samples were carefully controlled so as to enable a comparison of their magnetic properties. Using thermal activation measurements we are able to distinguish between contributions to H(ex) arising from the thermal stability of the antiferromagnetic particles (bulk behaviour) and that due to changing interface density with increasing Co concentration. We have interpreted our results in terms of the independent particle volume model.     

不连续界面相Al4C3对SiC/Al复合材料界面结合影响的第一性原理及实验

采用密度泛函理论的第一性原理及实验相结合的方法,探讨了不连续界面相Al₄C₃对SiC/Al复合材料界面结合的影响,并与无界面新相生成时进行对比。研究表明,当Al(111)表面吸附C原子时,在Bridge位置上吸附C原子最为稳定;随着C覆盖率的增加,C原子吸附能逐渐减小;当界面相呈不连续分布时,界面由原来的SiC/Al转变为(SiC+Al₄C₃)/Al,界面黏着功由原来的0.851 J/m2增加至1.231 J/m2,这主要由于当C原子在Al表面吸附时,C原子和Al原子间形成共价键和离子键,且与界面处的Si原子也形成共价键,从而促进界面结合。利用第一性原理计算的SiC/Al和(SiC+Al₄C₃)/Al体系黏着功与实验值较为接近,且变化规律相同,具有较高的参考价值。      

碳纳米管/聚合物复合材料界面结合性能的研究进展

碳纳米管(CNT)优异的力学性能使其成为复合材料优选的增强体。CNT/聚合物复合材料的力学性能主要受其界面结合性能的影响。综述了CNT/聚合物复合材料界面结合性能的研究方法和研究现状。对CNT/聚合物复合材料界面结合性能的研究,实验上采用微观表征技术、拉曼光谱分析技术和纳米力学拔出法,分子模拟方法则是通过对CNT施加位移或外力模拟CNT从聚合物基体中的抽拔过程。概述了聚合物的类型、晶态结构以及CNT的手性、功能化处理等因素对CNT/聚合物复合材料界面结合性能的影响,并展望了CNT/聚合物复合材料界面结合性能未来研究的重点方向。     

Coexistence of interfacial stress and charge transfer in graphene oxide-based magnetic nanocomposites

In this paper, the existence of both compressive stress and charge transfer process in hydrothermally synthesized cobalt ferrite–graphene oxide (CoFe₂O₄/GO) nanocomposites has been established. Transmission electron microscopy results reveal the decoration of CoFe₂O₄ nanoparticles on GO sheets. Magnetic response of nanocomposites was confirmed from superconducting quantum interference device magnetometer measurement. Optical properties of these nanocomposites were investigated by Raman spectroscopy. The interfacial compressive stress involved in this system has been evaluated from observed blue shift of characteristic G peak of graphene oxide. Increase in the full-width half-maximum value as well as upshift in D and G peaks is clear indications of involvement of charge transfer process between GO sheets and dispersed magnetic nanoparticles. The effect of charge transfer process is quantified in terms of shifting of Fermi energy level of these nanocomposites. This is evaluated from variation in contact surface potential difference using scanning Kelvin probe microscopy. XRD spectra of CoFe₂O₄/GO confirm the polycrystalline nature of CoFe₂O₄ nanoparticles. Lattice strain estimated from XRD peaks is correlated with the observed Raman shift.     

Fracture toughness of Kevlar-epoxy composites with controlled interfacial bonding

The fracture toughness of Kevlar-epoxy resin composites with intermittent fibre bonding of a silicone vacuum fluid (SVF-200) and a polyurethane varnish (Estapol 7008) have been studied over the temperature range –60 to 40° C and strain rates 0.03 to 5000 min^–1. Whilst both coating materials give similar tensile strengths their effects on toughness are very different. As far as toughening is concerned Estapol 7008 is more effective than SVF-200. The toughening effect increases with increasing intermittent lengths of the Estapol-7008 coating, i.e. coating parameterC, increasing temperature and decreasing strain rate. At low strain rates and high temperatures, forC=1, the toughness increase is some 200 to 300% compared to the uncoated composites. Some initial work has also been conducted for hygrothermally aged uncoated and coated fibre composites. The SVF-200 coated composites do not show any toughness degradation compared to the dry control samples. However, both the uncoated and Estapol-7008 coated composites suffer some toughness loss. Even so, the toughness of the fully coated aged specimens is as good as the uncoated dry controls. A fracture analysis is presented which gives reasonable agreement between predicted fracture toughness values and experimental measurements. It is shown that fibre pull-out toughness and fibre fracture work are the main contributors to the total fracture toughness of these fibre composites; their relative significance being dependent on the type of coating material, the temperature and strain rate of testing.     

Dynamic recrystallization behavior and interfacial bonding mechanism of 14Cr ferrite steel during hot deformation bonding

In this study, hot compression bonding was first applied to join 14Cr ferrite steel at temperatures of 950–1200°C and strains of 0.11–0.51 under strain rates of 0.01–30 s~(-1).Subsequently, tensile tests were performed on the joints to evaluate the reliability of the joints formed.Detailed microstructural analyses suggest that two different competing dynamic recrystallization(DRX) mechanisms occur during the bonding process depending on the strain rate, and the joints obtained at different strain rate exhibits distinct healing effect.At a low strain rate(0.01 s~(-1)), continuous DRX occurs, as expected in high-stackingfault-energy materials, and is characterized by the progressive conversion of the sub-boundaries into larger-angle boundaries, which involves very limited grain boundaries migration.In addition, straininduced precipitation(SIP) is sufficient under this condition, further impeding the healing of bonding interface.Hence, the joints obtained at low strain rate fractured at the bonding interface easily.Whereas discontinuous DRX is activated at high strain rates(10 and 30 s~(-1)).Under this condition, the formation of sub-boundaries is severely suppressed, resulting in the piling-up of dislocations and hence the storage of a greater amount of stored energy for nucleation and subsequent nuclei growth via the long-distance grain boundaries migration.Meanwhile, the SIP process is sluggish, making the conditions much more favorable for grain boundaries migration which plays a key role in the healing of the original bonding interface.Thus, the joints can be successfully bonded when a high strain rate is applied, with the joints exhibiting tensile properties similar to that of the base material.     

Effect of filler geometry on interfacial friction damping in polymer nanocomposites

Single-walled carbon nanotube polycarbonate and C60 polycarbonate nanocomposites were fabricated using a solution mixing method. The composite loss modulus was characterized by application of dynamic (sinusoidal) load to the nanocomposite and the pure polymer samples. For a loading of 1 weight %, the single-walled nanotube fillers generated more than a 250% increase in loss modulus compared to the baseline (pure) polycarbonate. Even though the surface area to volume ratio and surface chemistry of C60 is similar to that for nanotubes, we report no significant increase in the energy dissipation for the 1% weight C60 nanoparticle composite compared to the pure polymer. We explain these observations by comparing qualitatively, the active sliding area (considering both normal and shear stresses) for a representative volume element of the nanotube and the nanoparticle composites. These results highlight the important role played by the filler geometry in controlling energy dissipation in nanocomposite materials.     

Effect of alkali treatment on interfacial bonding in abaca fiber-reinforced composites

Abaca fibers demonstrate enormous potential as reinforcing agents in composite materials. In this study, abaca fibers were immersed in 5, 10 or 15 wt.% NaOH solutions for 2 h, and the effects of the alkali treatments on the mechanical characteristics and interfacial adhesion of the fibers in a model abaca fiber/epoxy composite system systematically evaluated. After 5 wt.% NaOH treatment, abaca fibers showed increased crystallinity, tensile strength and Young’s modulus compared to untreated fibers, and also improved interfacial shear strength with an epoxy. Stronger alkali treatments negatively impacted fiber stiffness and suitability for composite applications. Results suggest that mild alkali treatments (e.g. 5 wt.% NaOH for 2 h) are highly beneficial for the manufacture of abaca fiber-reinforced polymer composites.     

Pressure-assisted direct bonding of copper to silicon nitride for high thermal conductivity and strong interfacial bonding strength

To achieve superior thermal and mechanical properties of copper-bonded (Cu-bonded) Si₃N₄ substrate, a pressure-assisted direct bonded Cu (DBC) technique was applied to bond Cu foil with Si₃N₄ plate. The effects of oxide layer (SiO₂) thickness of Si₃N₄ plate on the microstructure, thermal and mechanical properties of the Si₃N₄-DBC samples were investigated. The successful bonding of Cu foil to Si₃N₄ plate was confirmed by the presence of the interfacial products of Cu₂MgSiO₄ and CuYO₂. Additionally, it was demonstrated that a thin SiO₂ layer can result in a discontinuous distribution of interfacial products while a thick one can lead to the formation of pores in SiO₂ layer. Notably, the sample prepared by Si₃N₄ plate with 5-μm-thickness SiO₂ layer and Cu foil with 5.9-μm-thickness oxide layer (Cu₂O) exhibited the optimally comprehensive properties with thermal conductivity of 92 W·m^?1·K^?1 and shearing strength of 102 MPa, which demonstrates significant promise for application in power electronic modules.

Graphical abstract

     

Computational and experimental study of interfacial bonding of single-walled nanotube reinforced composites

In the development of nanotube reinforced polymer composites, one of the fundamental issues that scientists and engineers are confronting is the nanotube/polymer interfacial bonding, which will determine load transfer capability from the polymer matrix to the nanotube. In this paper, the interfacial bonding of single-walled nanotube (SWNT) reinforced epoxy composites was investigated using a combination of computational and experimental methods. The interfacial bonding was predicted using molecular dynamics (MD) simulations based on a cured epoxy resin model, which was constructed by incorporating three-dimensional cross-links formed during curing reaction. Based on the pullout simulations, the interfacial shear strength between the nanotube and the cured epoxy resin was calculated to be up to 75 MPa, indicating that there could be an effective stress transfer from the epoxy resin to the nanotube. In the experiments, single-walled nanotube reinforced epoxy composites were fabricated, characterized and analyzed. The uniform dispersion and good interfacial bonding of the nanotubes in the epoxy resin resulted in a 250–300% increase in storage modulus with the addition of 20–30 wt% nanotubes. These experimental results provided evidence of stress transfer in agreement with the simulation results.     

Grain boundary engineering and the role of the interfacial plane

Abstract

Grain boundary engineering (GBE) involves the use of microstructural design to improve bulk material properties and enhance resistance to intergranular degradation. More specifically, the patented GBE procedure involves the design and control of fcc metallic microstructures using thermomechanical treatments and grain boundary characterisation based on the coincidence site lattice model. The phenomenon of multiple twinning is used to create a ‘twin limited’ microstructure, i.e. a microstructure composed entirely of special grain boundaries and triple junctions that is highly resistant to intergranular degradation. However, the theory behind GBE is not fully developed and therefore further study of the interfacial geometry, including the grain boundary plane and its role in GBE, is required to improve understanding of multiple twinning with the ultimate aim of improving the bulk and intergranular properties of metallic materials. An introduction to GBE is presented, including a number of cases where grain boundary design has improved the properties of fcc alloys for industrial applications. The theoretical characterisation of grain boundaries, including interfacial structure and geometry, is reviewed, highlighting the problems associated with microstructural characterisation based on limited knowledge of the grain boundary geometry. The importance of the grain boundary network is discussed: the grain boundary and triple junction character distributions are known to have a significant influence on bulk properties. Finally, the role of the interfacial plane is considered. It is concluded that although GBE has produced significant results, its theoretical basis and the ultimate creation of twin limited microstructures require further development.     

Overview of transient liquid phase and partial transient liquid phase bonding

Transient liquid phase (TLP) bonding is a relatively new bonding process that joins materials using an interlayer. On heating, the interlayer melts and the interlayer element (or a constituent of an alloy interlayer) diffuses into the substrate materials, causing isothermal solidification. The result of this process is a bond that has a higher melting point than the bonding temperature. This bonding process has found many applications, most notably the joining and repair of Ni-based superalloy components. This article reviews important aspects of TLP bonding, such as kinetics of the process, experimental details (bonding time, interlayer thickness and format, and optimal bonding temperature), and advantages and disadvantages of the process. A wide range of materials that TLP bonding has been applied to is also presented. Partial transient liquid phase (PTLP) bonding is a variant of TLP bonding that is typically used to join ceramics. PTLP bonding requires an interlayer composed of multiple layers; the most common bond setup consists of a thick refractory core sandwiched by thin, lower-melting layers on each side. This article explains how the experimental details and bonding kinetics of PTLP bonding differ from TLP bonding. Also, a range of materials that have been joined by PTLP bonding is presented.     

盐溶液干湿循环对CFRP-混凝土界面粘结性能的影响

为了考察盐溶液干湿循环条件对碳纤维增强复合材料（Carbon fiber reinforced polymer,CFRP）-混凝土界面粘结性能的影响,本文采用5%的NaCl溶液来模拟海水环境,经过不同次数的干湿循环后,利用单面剪切试验对48个试件界面性能的变化情况进行了研究,分析了上述环境对界面破坏形式、界面承载力及界面剪应力等参数的影响。结果表明：在盐溶液干湿循环作用下,界面粘结性能发生显著地退化,具体表现为,随着干湿循环次数的增加,界面承载力会不断降低,且下降程度与混凝土强度和CFRP的粘贴尺寸有关,界面剪应力在不同环境下的分布具有相似性,即荷载的不断增加会使剪应力逐渐由加载端向自由端传递,但在传递过程中,有效传递区域的长度不会发生改变。     

Copper-containing nanocomposites: Synthesis and phase composition

Composites comprising copper nanoparticles dispersed in a low-density polyethylene matrix were obtained by thermal decomposition of copper diacetate. The powdered nanocomposite was studied by X-ray diffraction. The phase composition of nanoparticles depends on the metal content in the matrix. The average size of nanoparticles determined from the characteristic broadening of diffraction peaks falls within 10–25 nm.     

Photoelastic study of the durability of interfacial bonding of carbon fibre-epoxy resin composites

The physical techniques of polarizing microscopy, including the quantitative measurements of small optical retardations, have been used to investigate elastic fields adjacent to short carbon fibres in epoxy resin composites. The elastic fields associated with shear stress distribution along the fibre-matrix interface have been employed to monitor the initiation of interface debonding during hot (100 °C) water uptake. By examining the development of stress birefringence during resin swelling in the resin adjacent to individual fibres, the differences in the durability of interfacial bonding and the fibre failure modes for differently coated fibres have been obtained. The results show that the state of self-stress in model composites, comprising a single carbon fibre in a film of epoxy resin, can, by immersion in hot distilled water, be enhanced to such an extent that the axial tension in the fibre can be sufficient to initiate fibre fracture. The results also show that, for fibres that have been given certain proprietary surface treatments, the fibre fractures by different failure modes.