Adhesion and sliding response of a biologically inspired fibrillar surface: experimental observations

Inspired by the adhesion mechanisms of several animal species such as geckos, beetles and flies, several efforts in designing and fabricating surface engineering strategies have been made recently to mimic the adhesive and frictional behaviour of biological foot pads. An important feature of such biological adhesion systems is the ability to switch between strong attachment and easy detachment, which is crucial for animal locomotion. Recent investigations have suggested that such a 'switching' mechanism can be achieved by the elastic anisotropy of the attachment pad, which renders the magnitude of the detachment force to be direction dependent. This suggestion is supported by the observations that the fibres of the foot pads in geckos and insects are oriented at an angle to the base and that geckos curl their toes backwards (digital hyperextension) while detaching from a surface. One of the promising bio-inspired architectures developed recently is a film-terminated fibrillar PDMS surface; this structure was demonstrated to result in superior detachment force and energy dissipation compared with a bulk PDMS surface. In this investigation, the film-terminated fibrillar architecture is modified by tilting the fibres to make the surface vertically more compliant and elastically anisotropic. The directional detachment and the sliding resistance between the tilted fibrillar surfaces and a spherical glass lens are measured: both show significant directional anisotropy. It is argued that the anisotropy introduced by the tilted fibres and the deformation-induced change in the compliance of the fibre layer are responsible for the observed anisotropy in the detachment force.     

Biomimetic interfaces based on S-layer proteins,lipid membranes and functional biomolecules

Designing and utilization of biomimetic membrane systems generated by bottom-up processes is a rapidly growing scientific and engineering field. Elucidation of the supramolecular construction principle of archaeal cell envelopes composed of S-layer stabilized lipid membranes led to new strategies for generating highly stable functional lipid membranes at meso- and macroscopic scale. In this review, we provide a state-of-the-art survey of how S-layer proteins, lipids and polymers may be used as basic building blocks for the assembly of S-layer-supported lipid membranes. These biomimetic membrane systems are distinguished by a nanopatterned fluidity, enhanced stability and longevity and, thus, provide a dedicated reconstitution matrix for membrane-active peptides and transmembrane proteins. Exciting areas in the (lab-on-a-) biochip technology are combining composite S-layer membrane systems involving specific membrane functions with the silicon world. Thus, it might become possible to create artificial noses or tongues, where many receptor proteins have to be exposed and read out simultaneously. Moreover, S-layer-coated liposomes and emulsomes copying virus envelopes constitute promising nanoformulations for the production of novel targeting, delivery, encapsulation and imaging systems.     

Adhesion, friction and wear on the nanoscale of MWNT tips and SWNT and MWNT arrays

The nanotribological characterization of carbon nanotubes is fundamental for the exploration of new sliding applications. In this study, a comprehensive investigation of adhesion, friction and wear of a multiwalled nanotube (MWNT) tip, and SWNT (single-walled nanotube) and MWNT arrays has been carried out. A nonlinear response of the MWNT tip is observed when the tip is brought into and out of contact with various surfaces. A nonlinear response occurs due to the buckling of the nanotube and its subsequent sliding on the surface. In addition to the role of surface chemistry, it can also explain the relatively high value of the coefficient of friction obtained on different surfaces, as compared to that of Si and Si(3)N(4) tips. The adhesion and friction studies carried out on SWNT and MWNT arrays using Si tips show that SWNT arrays, compared to MWNT arrays, exhibit lower values, possibly due to lower van der Waals forces as a result of lower packing density and higher flexibility. The wear tests conducted with the MWNT tip and a Si tip on a gold film, at two normal loads, show less damage of the surface when the MWNT tip is used because of the MWNT acting as a compliant spring, absorbing part of the load. Wear tests conducted with a Si tip on SWNT and MWNT arrays show that the arrays do not wear. The tip wear and the friction force in the SWNT array are lower, because of lower adhesion and higher flexibility of the SWNTs, which causes less opposition to the motion of the tip.     

Sensitivity series and friction surface analysis of non-metallic friction materials

Nine non-metallic friction material formulations contained fibers, fillers and binder without strong abrasives were designed using Golden Section sequence combined with least-square method. Seven ingredients used without strong abrasives were selected based on the combinatorial approach. Wear (w) and coefficient of friction (μ) are expressed as a function of volume fraction of the ingredients selected as and . The formulations were optimized using the sensitivity series obtained from least-square method. An optimized formulation (S-10) was obtained with a total wear loss of 6.69 wt% and an average μ of 0.375. Friction surface of both brake pad and disc was observed using SEM, EDX and XRD. The non-metallic friction materials without abrasives exhibit unique friction performance and phenomena compared with the non-metallic friction materials with abrasives and semi-metallic brake linings. The temperature measured during friction is lower and the oxidation of the cast iron disc is not rigorous.     

Fracture of metal/ceramic interfaces

The present paper examines metal/ceramic interfaces. Energy release rates are calculated with the finite element method for different elastic–plastic material laws of the metal. The local strain field of the metal is measured during a four-point bending test with an optical method and compared with results from the simulations. The aim of the work is to understand the influence of interface strength and material properties on the energy release rate.     

Stress intensity factor analysis of friction sliding at discontinuity interfaces and junctions

A stress intensity factor (SIF) analysis for two-dimensional fractures with frictional contact (crack friction) is presented. This analysis is carried out using the symmetric-Galerkin boundary element method, and a modified quarter-point crack tip element. As in case of non-contact fracture, it is shown that highly accurate SIFs can be obtained, even with the simple Displacement Correlation SIF technique. Moreover, with the modified crack tip element, the mesh on the crack does not need to be excessively refined in order to achieve high accuracy. This meshing advantage is especially important in the context of the nonlinear frictional contact problem, as the computing time for the iterative process strongly depends on the number of elements used. Several numerical examples are presented and the SIF results are compared with available analytical or reference solutions. This research was supported in part by the University of South Alabama Research Council, and by the Applied Mathematical Sciences Research Program of the Office of Mathematical, Information, and Computational Sciences, U.S. Department of Energy under contract DE-AC05-00OR22725 with UT-Battelle, LLC.     

Interaction of dynamic mode-I cracks with inclined interfaces

In this paper, we report on an experimental study of the deflection/penetration behavior of dynamic mode-I cracks propagating at two different crack velocities (slower and faster) toward inclined weak interfaces of three dissimilar angles (α): 30°, 45° and 60°. A simple wedge-loading specimen configuration as proposed by Xu et al. [Xu LR, Huang YY, Rosakis AJ. Dynamic crack deflection and penetration at interface in homogenous materials: experimental studies and model predictions. J Mech Phys Solids 2003;51:461-86], made of brittle Homalite-100, is used. A modified Hopkinson bar setup is used to achieve well-controlled impact loading conditions. Dynamic photoelasticity in conjunction with high-speed photography is used to capture real-time isochromatics associated with deflected/penetrated cracks.     

Adhesion and friction properties of micro/nano-engineered superhydrophobic/hydrophobic surfaces

Hydrophobic micro/nano-engineered surfaces (MNESs) with good adhesion and frictional performances were fabricated by the combination of aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) and octadecyltrichlorosilane (OTS) coating. The AIC of a-Si technique was used to produce silicon micro/nano-textured surfaces, while an OTS self-assembled monolayer was used to lower the surface energies of the textured surfaces. The wetting properties of the MNESs were studied using a video-based contact angle measurement system. The adhesion and friction properties of the MNESs were investigated using a TriboIndenter. This study shows that the adhesion and frictional performances of all MNESs are significantly improved compared to untreated silicon substrate surfaces, and the adhesion and frictional performances of the OTS-modified textured surfaces strongly correlate to their surface wetting property, i.e., the larger the water contact angle, the better the adhesion and frictional performances of the OTS-modified textured surfaces.     

Strength statistics of adhesive contact between a fibrillar structure and a rough substrate

Equal distribution of load among fibrils in contact with a substrate is an important characteristic of fibrillar structures used by many small animals and insects for contact and adhesion. This is in contrast with continuum systems where stress concentration dominates interfacial failure. In this work, we study how adhesion strength of a fibrillar system depends on substrate roughness and variability of the fibril structure, which are modelled using probability distributions for fibril length and fibril attachment strength. Monte Carlo simulations are carried out to determine the adhesion strength statistics where fibril length follows normal or uniform distribution and attachment strength has a power-law form. Our results indicate that the strength distribution is Gaussian (normal) for both the uniform and the normal distributions for length. However, the fibrillar structure having normally distributed lengths has higher strength and lower toughness than one having uniformly distributed lengths. Our simulations also show that an increase in the compliance of the fibrils can compensate for both the substrate roughness and the attachment strength variation. We also show that, as the number of fibrils n increases, the load-carrying efficiency of each fibril goes down. For large n, this effect is found to be small. Furthermore, this effect is compensated by the fact that the standard deviation of the adhesive strength decreases as 1/ square root n.     

A combined molecular dynamics and Raman spectroscopy approach for designing ice-metal interfaces

Summary Understanding the structure and interatomic interactions of an ice-metal interface plays a fundamental role in the design of deicing coatings. This is demonstrated by a novel approach, combining vibrational results from laser Raman spectroscopy with molecular dynamics simulations to obtain insights into icing on solids which, in turn, lead to design criteria for minimizing adhesion. An atomistic model of ice-copper interaction is constructed based on electronic structure calculations and used to show that reasonable molecular geometry and binding energy at the interface can be obtained. Through molecular dynamics simulations we find that the ice layer adjacent to the copper surface is structurally more disordered than the layers further away, a result which is verified by the Raman spectra of vibrational frequencies. The primary adhesive bond is made by the adsorption of oxygen atoms at the lattice sites of the metal substrate. The information obtained by Raman spectroscopy and molecular dynamics is then exploited to arrive at specific recommendations for designing polymeric deicing coatings and materials.     

Classical atomistic simulations of surfaces and heterogeneous interfaces with the charge-optimized many body (COMB) potentials

Interest in atomic scale computational simulations of multi-phase systems has grown as our ability to simulate nanometer-sized systems has become commonplace. The recently developed charge optimized many body potential (COMB) potentials have significantly enhanced the atomic-scale simulation of heterogeneous material systems, including chemical reactions at surfaces and the physical properties of interfaces. The COMB formalism, which merges variable charge electrostatic interactions with a classical analytical potential, has the capacity to adaptively model metallic, covalent, ionic and van der Waals bonding in the same simulation cell and dynamically determine the charges according to the local environment. Presented here is the theoretical background and evolution of the COMB potential family. The parameterization of the potential is described for several metals, ceramics, a semiconductor, and hydrocarbons, with the intent that the final parameter sets are consistent among materials. The utility of this approach is illustrated with several examples that explore the structure, stability, and mechanical and thermal properties of metallic systems and metal-ceramic and semi-conductor oxide interfaces, including surfaces and/or interfaces of copper and cuprite, copper and silica, silicon and silica, silicon and hafnia, and copper and zinc oxide. The potential is also applied to the simulation of atomic scale processes such as early stage oxidation of copper surfaces, tensile test of polycrystalline zirconium, and hyper-thermal deposition of ethyl radicals on selected copper surfaces.     

摩擦材料中树脂对其性能的影响

阐述了硼桐油双改性酚醛树脂的改性原理,对以此为基体的编织型摩擦材料的磨损率和摩擦系数进行了测试和分析,并与其他几种酚醛树脂基摩擦材料进行了对比。结果表明,对酚醛树脂进行硼桐油双改性后应用于摩擦材料的效果理想。     

Mechanical and tribological properties of ceramic-matrix friction materials with steel fiber and mullite fiber

The purpose of the present work was to investigate and compare the mechanical and tribological behaviors of ceramic-matrix friction material (CMFM) with steel fiber (SF), mullite fiber (MF), and mixing SF and MF. The CMFM was prepared by hot-pressing sintering, and the tribological behaviors were determined using a constant speed friction tester. The worn surfaces and wear debris were observed by a scanning electron microscopy (SEM). Experiment results show that the combination of SF and MF can improve the mechanical properties that each single fiber does not have. The sever fade for the specimen reinforced by single MF during the whole friction testing can be attributed to the poor interface cohesive strength between MF and matrix. Mixing the SF and MF can improve the friction stability, and the friction coefficients for friction material with a mixture of the SF and MF increases with increasing MF content. For all specimens, increasing in the friction temperatures result in the increase of wear rates.     

Formation of friction layer of Ni3Al matrix composites with micro- and nano-structure during sliding friction under different loads

Research on friction layer is needed for mechanical systems and assemblies in order to control failure and wear of contacting materials. This study investigates the formation of friction layer of Ni₃Al matrix composites (NMCs) with multi-layer graphene (MLG) under different contact loads. The results show that, under 15 N condition, the friction layer with the ultrafine grain and nanocrystalline structure mainly consists of the thin debris re-embedded layer (DREL) and the matrix refinement layer (MRL). The build up of the DREL is a dynamic equilibrium recovery process. The severe plastic deformation induces the dislocations and twinning, as well as misorientation boundaries, resulting in the formation of the stress dissipative structure in MRL. The low friction coefficient (0.27–0.30) and wear rate (1.2–1.6 × 10⁻⁵ mm³N⁻¹ m⁻¹) of NMCs at 15 N load are attributed to the formation of friction layer with micro- and nano-structure during sliding friction.     

Reflection of ultrasonic waves from imperfect interfaces: A combined boundary element method and independent scattering model approach

A numerical technique for obtaining interface reflection coefficients for imperfect bonds between similar materials for a wide range of distributed defects is developed. A numerical boundary element method is utilized to find the far-field scattering amplitudes of a single defect for a normally incident plane wave. Then, the normal incidence reflection coefficient for a planar distribution of such defects is obtained from the independent scattering model. As a validation, the reflection coefficients are compared to the quasi-static model results where the latter are available. This establishes the basis for one application of the new model, the determination of spring constants which are not available. Other applications of the model, including studies of the response at frequencies beyond the quasi-static limit, the ratio of longitudinal to transverse wave reflectivities, and the effects of selected multiple scattering are discussed.     

Formation of heterogeneous interfaces in the TiAl/Ti2AlNb layered composite and its effect on the fracture toughness

TiAl/Ti2AlNb intermetallic-intermetallic laminated(IIL)composites featuring brittle/ductile heterogeneous interfaces were fabricated through vacuum hot-pack rolling.The microstructures and the phase transfor-mation behaviors of the interfaces of the IIL composites before and after annealing at 900℃/6 h were in-vestigated.The heterogeneous interfaces are composed of four distinct regions,individually Ⅰ(βo+γ+α2),Ⅱ(βo/B2+ω)(brittle part),Ⅲ(O lath),and Ⅳ(equiaxed O)(ductile part)regions from TiAl to Ti2AlNb side.Notably,after annealing,an equiaxed O band approximately 50 μm wide was observed in region Ⅳ of the interface.In addition,a significant microhardness variation was observed between regions Ⅱ and Ⅳ of the interface,where region Ⅱ exhibited higher hardness compared to the TiAl alloy,and region Ⅳ displayed lower hardness than the Ti2AlNb alloy.The enhanced fracture toughness of the IIL composites,three times that of the TiAl base alloy,is attributed to the formation of the brittle/ductile heterogeneous interfaces and the layered design incorporating the Ti2AlNb alloy.The corresponding toughening mech-anism was further discussed.The brittle Ⅱ region plays a role in increasing crack branching,while the ductile Ⅳ region inhibits the propagation of microcracks and prevents the formation of main cracks.This work highlights the crucial role of the brittle/ductile heterogeneous interface in the toughening of lam-inated composites.Furthermore,the discovery of the O band provides novel insights into the design of TiAl/Ti2AlNb heterostructures.     

Abnormal grain growth via the migration of planar growth interfaces

Nanocrystalline nickel electrodeposits undergo a sequence of morphologically distinct grain growth stages during annealing. The nanostructure initially undergoes a rapid sequence of abnormal grain growth, followed by a much slower normal grain growth stage. Out of this uniformly growing structure comes a second stage of abnormal grain growth which not only accelerates the overall growth rate, but the transformation also occurs by the migration of planar reaction fronts. These planar growth interfaces are composed of many individual grain boundary segments, migrating together at essentially the same velocity. Grain shape was studied from intergranular fracture surfaces; it was found that the abnormally growing grains were cuboidal in shape and were present either as individually growing grains or as cuboid clusters. Electron backscatter diffraction showed numerous twin-related cuboidal grain clusters having complex compound planes.     

Mechanical properties and microstructure of friction stir welded tailor-made blanks

This paper studies the microstructural features and mechanical properties of friction stir welds with dissimilar alloys and different thicknesses. The welds are produced in five different thickness/material combinations from 2024-T3 and 7075-T6 sheets with different thicknesses. A parametric study is conducted to optimize the welding parameters such that the different configurations can be compared. The paper is divided into two chapters: microstructural features and mechanical properties. In the first chapter, a study of the chemical composition and microstructure of the welds shows that a narrow chemical mixing zone is present in the dissimilar-alloy welds and that the stirring zone embodies the union rings and exhibits heterogeneous texture for most configurations. Study of the hardness, tensile properties and fracture surfaces in the second chapter shows that an asymmetric softened region, which is harder at the advancing side and extends more into the retreating side, is formed in the stirring zone and that the mechanical properties decrease as the thickness ratio increases. The fracture was partially ductile and partially brittle for all configurations.     

Direct joining of oxygen-free copper and carbon-fiber-reinforced plastic by friction lap joining

Oxygen-free copper(Cu) was successfully joined to carbon-fiber-reinforced thermoplastic(CFRTP,polyamide 6 with 20 wt% carbon fiber addition) by friction lap joining(FLJ) at joining speeds of 200–1600 mm/min with a constant rotation rate of 1500 rpm and a nominal plunge depth of 0.9 mm.It is the first time to report the joining of CFRTP to Cu by FLJ. As the joining speed increased, the tensile shear force(TSF) of joints increased first, and decreased thereafter. The maximum TSF could reach 2.3 kN(15 mm in width). Hydrogen bonding formed between the amide group of CFRTP and the thin Cu_2O layer on the Cu surface, which mainly contributed to the joint bonding. The influence factors of the TSF of the joints at different joining speeds were discussed. The TSF was mainly affected by the joining area, the degradation of the plastic matrix and the number and the size of bubbles. As the joining speed increased,the influence factors varied as follows: the joining area increased first and then decreased; the degradation of the plastic matrix and the number and the size of bubbles decreased. The maximum TSF was the comprehensive result of the relatively large joining area, small degradation of the plastic matrix and small number and sizes of bubbles.