Interfacial Design for Joining Technologies: An Historical Perspective

This paper gives an historic perspective of the concept of “Interfacial Design” in joined (e.g. soldered, brazed, diffusion bonded) assemblies. During the course of history, the awareness grew that the interface in a material joint can be perceived at different length scales. With the continuing development of joining materials and technologies, it became evident that the performance of assemblies is critically dependent on the structure and composition of the multiple internal interfaces in the material joints. Resulting trends in the microstructural design of soldering, brazing, and other bonding materials by smart engineering of internal interfaces, as driven by increasingly complex technological requirements, are briefly addressed.     

Modelling the influence of reactive elements on the work of adhesion between a thermally grown oxide and a bond coat alloy

The durability of thermal barrier coating systems is primarily determined by the degree of adhesion between the thermally grown oxide (TGO) and the bond coat. Failure of the TBC is often the result of delamination at this interface. Adhesion can be improved by the addition of reactive elements (RE) to the bond coat alloy. REs include oxide forming elements such as Y, Zr and Hf. The so‐called reactive element effect has been attributed to a direct improvement of the bonding between the TGO and the bond coat. A macroscopic atom model has been developed to allow the work of adhesion between two compounds (e.g. an oxide and a metal compound) to be estimated. By calculating the work of adhesion across a number of different interfaces, the influence of reactive elements and impurities present in the substrate can be assessed. It has been found that the REs have a limited direct influence on the work of adhesion and can even result in a weaker interface. A large reduction in the work of adhesion is calculated when S and C are present at the interface. REs have a high affinity for both S and C. This indicates that the RE effect is primarily that of impurity scavenging, preventing diffusion of impurities to the interface. A number of experiments are reported, which demonstrate the RE effect and support the modelling results.     

Atomistic modeling of the interaction of glide dislocations with “weak” interfaces

Using atomistic modeling and anisotropic elastic theory, the interaction of glide dislocations with interfaces in a model Cu–Nb system was explored. The incoherent Cu–Nb interfaces have relatively low shear strength and are referred to as “weak” interfaces. This work shows that such interfaces are very strong traps for glide dislocations and, thus, effective barriers for slip transmission. The key aspects of the glide dislocation–interface interactions are as follows. (i) The weak interface is readily sheared under the stress field of an impinging glide dislocation. (ii) The sheared interface generates an attractive force on the glide dislocation, leading to the absorption of dislocation in the interface. (iii) Upon entering the interface, the glide dislocation core readily spreads into an intricate pattern within the interface. Consequently, the glide dislocations in both Cu and Nb crystals are energetically favored to enter the interface when they are located within 1.5 nm from the interface. In addition to the trapping of dislocations in weak interfaces, this paper also discusses geometric factors such as the crystallographic discontinuity of slip systems across the Cu/Nb interfaces, which contribute to the difficulty of dislocation transmission across an interface. The implications of these findings to the unusually high strengths experimentally measured in Cu/Nb nanolayered composites are discussed.     

The mechanisms of interfacial failure for lateral force-sensing microindentation test: finite element analysis

The interfacial bond strength for coatings and composites can be quantitatively determined using a newly developed lateral force-sensing microindentation method. In this study, a finite element analysis was made to investigate the interfacial failure mechanisms for Cu–ceramic and Al alloy–ceramic interfaces. The model is validated by comparing obtained results of the finite element analysis with analytical solutions. Two different interfacial failure mechanisms, depending on material properties and microindentation positions, are proposed. As demonstrated, interfacial debonding may result from shear stress or a coupling of tensile stress and shear stress at the interface, corresponding to material “pile-up” deformation or “sink-in” deformation. In addition, the high sensitivity of the lateral force response to interfacial debonding, associated with two different interfacial failure mechanisms, is also examined.     

Interfacial toughness measurements for thin films on substrates

There are more than 200 different methods for measuring adhesion, suggesting it to be material, geometry and even industry specific. This availability has exploded at least partly due to the arrival of dissimilar material interfaces and thin films and the ease with which microfabrication techniques apply to silicon technology. Having an eye toward those tests utilized for thin films, this paper reviews only a few of these techniques. The emphasis is on measuring thin film adhesion from the standpoint of fracture mechanics, when the film is mechanically or by other means removed from the substrate, and the amount of energy necessary for this process is calculated per unit area of the removed film. This tends to give values approaching the true work of adhesion at small thickness and greater values of the practical work of adhesion at larger thickness, all being in the 30–30,000 nm range. The resulting large range of toughnesses is shown to be dependent on the scale of plasticity achieved as controlled by film thickness, microstructure, chemistry and test temperature.While the tests reviewed largely address the measurement of elastic strain energy release rates, we also briefly address a few theoretical models which are specific to the resistance side of the delamination equation. The weight of the evidence suggests for ductile metallic films that the major extrinsic variables are film stress, extent of delamination, thickness and temperature while the major intrinsic ones are modulus, yield strength, the thermodynamic work of adhesion and one or more length scales. For some 25 film/substrate multilayers, with emphasis on Al, Au and Cu, the comparison of several theoretical models as to how the extrinsic and intrinsic variables intertwine is made.     

腐蚀介质对复合材料界面影响的模型研究

为阐明界面对复合材料耐腐蚀性能的影响,本文作者采用单丝模型研究了偶联剂处理对复合材料界面粘接能和耐腐蚀性能的作用。实验证明:单丝模型中单丝直径与界面粘接能无关;硅烷偶联剂处理液浓度对玻纤表面处理效果有很大影响:界面上化学吸附的及与玻纤表面起化学作用的偶联剂层对改善界面粘接起主要作用:对玻纤进行适当的表面处理能大大改善界面耐腐蚀性能。     

Effect of alloying element segregation on the work of adhesion of metallic coating on metallic substrate: Application to zinc coatings on steel substrates

A thermodynamic model based on the ‘Macroscopic Atom’ approach is proposed to assess the effect of alloying element segregation on the adhesion of metallic coating on metallic substrate. The interfaces that occur in hot-dip galvanized steels are considered, which include: Zn/Fe, Zn/Fe₂Al₅, Zn/FeZn₁₃, FeZn₁₃/Fe₂Al₅, and Fe₂Al₅/Fe. The effect of the alloying element on the work of adhesion of these interfaces is investigated, which includes Mg, Al, Si, P, Ti, V, Cr, Mn, Fe, Ni, Zn, Nb, Mo, Sn and Bi. Among these elements, Bi, Sn and Mg are predicted to decrease the work of adhesion of the Zn/Fe interface, whereas P, Nb, Mo, V, Ti and Ni tend to enhance this adhesion. The effect of element M (M = Al, Si, Cr, Mn) is positive when it exists in the zinc coating or negative when it occurs in the iron substrate. Among these interfaces, the Fe₂Al₅/Fe interface with a value of 3.8 J m⁻² is the strongest, whereas the Zn/FeZn₁₃ interface with of a value of 1.7 J m⁻² is the weakest. Delamination of the coating upon deformation is predicted to occur along the FeZn₁₃/Fe₂Al₅ and Zn/Fe₂Al₅ interfaces. This agrees with microscopic observations of hot dip galvanized steel after tensile testing.     

Meso-Scale Modeling the Orientation and Interface Stability of Cu/Nb-Layered Composites by Rolling

Metallic-based multilayered nanocomposites are recognized for their increased plastic flow resistance and indentation hardness, increased ductility, improved radiation damage resistance, improved electrical and magnetic properties, and enhanced fatigue failure resistance compared to conventional metallic materials. One of the ways in which these classes of materials are manufactured is through accumulated roll bonding where the material is produced by several rolling and heat-treatment steps during which the layer thickness is reduced through severe plastic deformation. A single rolling pass of the accumulated roll bonding process in which a Cu/Nb-layered composite with an initial average layer thickness of 24 μm subjected to a 50% height reduction is modeled. A single-crystal model based upon thermally activated dislocation motion is used. Nanohardness tests for both the Cu and Nb layers are used to help initialize the model for each of the two materials. Electron backscatter diffraction (EBSD) data of the heat-treated material is used to characterize the initial state of the composite and to produce 40 combined morphological and crystallographic numerical model realizations of the material. The results suggest very good agreement between the predicted and experimental textures for both the materials. Highly oriented microstructure develops during severe plastic rolling deformation of Cu/Nb nanocomposites. The deformation textures significantly deviate from those expected when rolling Cu or Nb alone, and the Cu/Nb interfaces do not correspond to those with the lowest possible formation energies. We study the interfacial stability of specific Cu/Nb bicrystal configurations under rolling conditions using a finite-element crystal plasticity model. Specifically, we examine how slip activity and lattice reorientation are affected by the kinematic constraint imposed by the interface. Our results show that for certain configurations the slip activity and lattice rotation of the individual crystallites display some sensitivity to the kinematic constraint, yet the overall stability of a given bicrystal can be predicted by the stability of the individual single-crystal orientations. Future work will account for the influence of the bimetal interface on the interface stability and development of enhanced properties.     

Influence of “Island-Like” Oxides in the Bond-Coat on the Stress and Failure Patterns of the Thermal-Barrier Coatings Fabricated by Atmospheric Plasma Spraying During Long-Term High Temperature Oxidation

Thermal-barrier coatings (TBCs) are very important ceramic-coating materials due to their excellent performance at high temperature. The inner zone of the bond-coat is often easily endured oxidized (internal oxidation) in the process of thermal spraying and the long-time exposure to the high temperature, and the “island-like” oxides can be formed. Especially, when the bond-coat was fabricated by atmospheric plasma spraying (APS), this trend is more evident. In this paper, the stress distribution around the thermally grown oxide (TGO) has been calculated by the finite element method when the “island-like” oxides have been considered. The simulation results indicate that the maximum tensile stress and compressive stress existed in the TGO, and the existence of the “island-like” oxides will further decrease the maximum tensile stress level in the TGO. While the “island-like” oxides in the bond-coat will decrease the effective thickness of the TGO at the metallic layer/ceramic layer interface due to the oxidation of the metallic elements in the bond-coat. The crack propagation equation has been established and the failure mechanism of the TBC due to the formation and growth of the TGO has also been discussed in detail. The lifetime of the TBCs which have experienced high temperature oxidation has been predicted and the theoretical results agreed well with the experimental data.     

Modeling and Simulation of a Donor Material Concept to Reduce Tool Wear in Friction Stir Welding of High-Strength Materials

This research proposes the “donor material” concept for reduction of tool's wear at the plunge phase by providing localized preheating at the plunge area using a softer material as a “donor.” This process generates heat in a relatively soft “donor” material, which is transferred to the much harder workpiece material by conduction. This research includes several numerical simulations of the donor material concept with different donor materials and plain carbon steel as the workpiece. A Significant decreases in both axial force and contact pressure were observed when a donor material was used in the plunge area. The decreases in both axial force and contact pressure are very likely to contribute to decreasing tool's wear.     

Structure–Property–Functionality of Bimetal Interfaces

Interfaces, such as grain boundaries, phase boundaries, and surfaces, are important in materials of any microstructural size scale, whether the microstructure is coarse-grained, ultrafine-grained, or nano-grained. In nanostructured materials, however, they dominate material response and as we have seen many times over, can lead to extraordinary and unusual properties that far exceed those of their coarse-grained counterparts. In this article, we focus on bimetal interfaces. To best elucidate interface structure?Cproperty?Cfunctionality relationships, we focus our studies on simple layered composites composed of an alternating stack of two metals with bimetal interfaces spaced less than 100?nm. We fabricate these nanocomposites by either a bottom?Cup method (physical vapor deposition) or a top?Cdown method (accumulative roll bonding) to produce two distinct interface types. Atomic-scale differences in interface structure are shown to result in profound effects on bulk-scale properties.     

Improved interfacial mechanical properties of Al2O3-13wt%TiO2 plasma-sprayed coatings derived from nanocrystalline powders

The interfacial toughness of two types of Al₂O₃-13wt%TiO₂ plasma-sprayed ceramic coatings on steel substrates—“conventional” and “nano”—has been measured using the Rockwell indentation method. The interfacial toughness of the “conventional” coating and the “nano” coating is found to be 22 and 45 J.m⁻², respectively. The “conventional” coating, which was prepared using a fused feedstock powder available commercially, has a microstructure consisting primarily of fully-molten (FM) and solidified “splats”. The feedstock powder for the “nano” coating comprised reconstituted agglomerates of nanocrystalline Al₂O₃ and TiO₂ powders. The microstructure of the “nano” coating, as characterized using scanning and transmission electron microscopy techniques, consists of regions of FM “splats” interspersed with partially-molten (PM) rounded microstructural features. The substructure in these PM features (20–50 μm diameter) consists of α-Al₂O₃ grains (0.5–1 μm) surrounded by a TiO₂-rich amorphous phase. The FM/steel interfaces in both the “conventional” and the “nano” coatings are found to be cracked (before mechanical testing), whereas the PM/steel interfaces in the “nano” coating are found to be adherent. It is believed that the unique bimodal microstructure, together with the presence of the TiO₂-rich amorphous phase at the PM/steel interface, is responsible for the significantly improved interfacial toughness of the “nano” coating. The key differences in the failure modes in the two types coatings are also discussed, with reference to a simple model.     

Effect of limited matrix–reinforcement interfacial reaction on enhancing the mechanical properties of aluminium–silicon carbide composites

An unconventional approach to strengthening Al/SiC composites through controlled matrix–reinforcement interfacial reactions was studied. Composites with two distinct interfacial microstructures were prepared by varying the contact time between the SiC particles and molten aluminium during processing. The formation of a thin Al₄C₃ reaction layer along the particle–matrix interface was found to increase the composite yield strength, ultimate tensile strength, work-hardening rate and work-to-fracture, and change the fracture pattern from one involving interfacial decohesion to one where particle breakage was dominant. These changes were attributed to a stronger interface bond, which is thought to result from the tendency for the Al₄C₃ reaction layer to form semicoherent interfaces and orientation relationships with the aluminium matrix and SiC particles and for it to be mechanically “keyed-in” to both these phases. The stronger interface bond also enhanced the levels of plastic constraint which, when coupled with the greater work hardening, promoted local matrix failure, thereby reducing the composite ductility.     

Phase transformation yield surface determination for some shape memory alloys

Like in the “plasticity” theory, the prediction of phase transformation yield surfaces constitutes an essential issue in the modelling of polycrystalline shape memory alloys thermomechanical behaviour. Usually for “micro–macro” integration, the nature of the interface between austenite and twinned or untwinned martensite under stress free state and the choice of correspondence variants (CV) or habit plane variant (HPV) are predominant toward the explicit shape of the phase transformation surface. If the predictions for Cu–Al–Be, Cu–Al–Zn (interface between austenite and one single variant of martensite for cubic to monoclinic phase transformation) and Cu–Al–Ni (interface between austenite and twinned martensite for cubic to orthorhombic phase transformation) are fairly good; the prediction is not efficient in the important case of Ti–Ni (interface between austenite and twinned martensite with stress free state for cubic to monoclinic phase transformation). The usual hypothesis consisting in neglecting the effect of stress on the interface geometrical configuration must be revised.     

Stress Generation and Adhesion in Growing Oxide Scales

The experimental evidence for stress generation in growing oxide scales is briefly summarised, and the origin of these stresses is discussed. The limited experimental data related to oxide adhesion is discussed, and it is concluded that the adhesion between metal and oxide probably involves chemical interaction, and that the effects of impurities collecting at the interface are not easy to predict. It is emphasised however that the problem of adhesion at the metall oxide interface is complicated by the dynamic nature of the situation, so that it is necessary to postulate a growth mechanism which will allow contact between the oxide and the metal to be maintained. Finally, the problem of plastic flow in the oxide is considered, and it is suggested that some of the evidence which has been adduced to support the idea of large plastic flow is less than entirely convincing. Some of the phenomena may possibly be interpreted in terms of “stress-directed growth”.     

Interface growth morphologies in pulsed laser deposited, room temperature grown multilayer hard coatings

The mechanical behaviour - hardness, elasticity, and adhesion - of multilayer coatings is strongly influenced by the type of the formed interfaces between the different layers. In industrially applied tribological coatings the interface region is predominantly not a perfect sudden change of the chemical composition of the adjacent crystal planes, but a transition zone of a thickness, which is strongly dependent on the energetic conditions during deposition. Multilayer coatings grown by high-energetic deposition techniques always struggle with high atomic mixing of both adjacent coating materials due to high energetic ion implantation.One of these high-energetic deposition techniques is the Pulsed Laser Deposition (PLD), characterized by pulsed and within one pulse alternating high- and low-energetic particle fractions, hitting successively the substrate surface. Such deposition conditions were shown to be highly advantageous for low temperature deposition by the densification of the growth structures due to activated diffusion and re-sputtering, but increases the difficulty in depositing multilayer structures.The current paper addresses these specific growth conditions based on Ti/TiN and Cr/CrN multilayer coatings. High resolution transmission electron microscopy results show that the atomic mixing at the interface is not highly critical for the deposition of multilayer coatings and that extremely dense growth structures are forming even in the interface regions.     

Designing architectured materials

Architectured or “hybrid” materials are combinations of two or more materials or of materials and space, configured in such a way as to have attributes not offered by any one material alone. This paper describes the rationale for creating architectured materials and criteria for deciding which combinations and configurations show the most promise.     

抗辐照损伤金属基纳米结构材料界面设计及其响应行为的研究进展

在高能粒子辐照条件下,金属基结构材料内部会出现不同类型的缺陷,这些辐照诱导缺陷的大规模聚集会造成损伤,降低材料的结构稳定性,从而严重影响结构材料的力学和物理性能。通过材料设计的手段引入界面充当缺陷陷阱,可通过对辐照诱导缺陷的分离、吸收和湮灭,有效减轻材料的辐照损伤。纳米结构材料由于含有高密度界面,其辐照损伤行为的研究于近20年快速发展,且界面能被证实是影响界面调控辐照损伤的重要因素。本文聚焦金属基纳米结构材料,围绕界面设计,详细阐述了低能和高能界面设计下,不同结构类型的界面对辐照损伤的影响及界面响应行为的研究进展,为进一步实现界面结构优化,平衡界面能、界面结构稳定性及良好辐照抗性之间的关系提供理论基础和科学依据。最后,基于前述界面设计的思想,总结了近年来发展的碳/金属界面设计及抗辐照损伤的研究进展,展望了未来先进抗辐照金属基纳米结构材料的设计和发展。     

Atomic Mixing in Metals Under Shear Deformation

The fundamental processes of shear-induced chemical mixing in heterogeneous Cu-based alloy systems have been studied by molecular dynamics computer simulations. These simulations reveal that two very disparate mechanisms operate depending on whether or not the two phases are coherent. For the coherent systems, mixing occurs as dislocations transfer across phase boundaries. The mixing in these systems is “superdiffusive,” and for spherical precipitates, the rate of mixing increases quadratically with precipitate radius. In systems that have incoherent phases, the mixing occurs by a local shuffling of atoms at the interface, and for them, the mixing is diffusive, with the mixing rates of spherical precipitates scaling linearly with particle radius. The morphologies of the interfaces for the two situations are also different. Coherent precipitates form rough interfaces that are relatively sharp, whereas the interfaces of incoherent precipitates are smooth but diffuse. These simulations also show that for incoherent precipitates, shear-induced mixing can be very different at different crystallographic interfacial planes as well as for different strain directions.     

The structural analysis of various hydro-graphene species

The marginal members of the carbon allotropes containing a significant amount of hydrogen atoms are of interest because they show a variety of interesting properties lacking pure carbon materials. They consist of many fractions of highly condensed aromatic rings and are characterized by the atomic ratio of hydrogen-to-carbon ([H]/[C]). A finite part of a graphite sheet is often called “graphene”. The graphenes terminated by hydrogen atoms like highly condensed aromatic rings may be called “hydro-graphene”. In this paper, we carried out the structural analysis of six kinds of “hydro-graphene” materials prepared by the pyrolysis of three different raw materials at 550 and 650 °C. The TEM results are consistent with the XRD analysis and clearly indicate the size of the crystallite structure and unique interlayer distance in each hydro-graphene material. These methods may be effective for the design of various kinds of hydro-graphenes and preferable applications of these materials.