Introduction—Interfaces in Adhesion and Adhesive Bonding

Abstract

The social background of research on adhesion and adhesive bonding is reviewed. Next, the interphase in the adhesive bond is defined, and the multi-scale and hierarchical structures contained in the interface are arranged based on their sizes. Finally, the analytical methodologies this book introduces to elucidate interfacial phenomena in adhesion and adhesive bonding are overviewed.

1 Social Background of Research on Adhesion and Adhesive Bonding

To realize an energy-saving and carbon–neutral society, the weight reduction of transportation structures such as automobiles and airplanes has become an issue to be addressed. In modern automobiles and aircraft, reducing weight, fuel consumption, and CO₂ emissions is crucial, and one way to achieve this is through the use of lightweight hybrid designs. To meet the rising demand for energy-efficient vehicles, manufacturers are turning to lightweight materials like high-strength steel, magnesium alloys, aluminum alloys, and polymer-based composites to make automotive body components. The utilization of lightweight materials has led to changes in the structural design of lightweight constructions.

Furthermore, researchers and engineers are developing and investigating new joining technologies that can be applied to both similar and dissimilar material combinations. A prevalent approach is to enhance the strength, weight, and durability of hybrid structures by combining traditional metals with polymeric composites. While composites are more structurally efficient than metals and are not prone to galvanic corrosion, metals have better damage tolerance and failure predictability than composites and are resistant to solvents and high temperatures that tend to degrade polymers. As a result, dissimilar joints between metals and composite materials are being developed to optimize the advantages of both materials.

In engineering applications, adhesive bonding is becoming an increasingly popular alternative to mechanical joints due to its numerous advantages over conventional mechanical fasteners. Adhesive bonding offers benefits such as lower structural weight, reduced fabrication cost, improved damage tolerance, design flexibility, and the ability to bond a wide range of materials, both similar and dissimilar. Additionally, adhesive bonding minimizes the thermal effects on the bonding substrates. The automotive industry has seen a significant increase in the application of adhesive bonding in recent years. This is primarily due to the potential for weight reduction, fuel savings, and reduced emissions in vehicles. A new manufacturing concept for car bodies that utilizes lightweight and high-strength materials placed in the appropriate locations is under research and development, as shown in Fig. 1 [1]. Adhesive bonding is considered a promising joining method for constructing multi-material car bodies because conventional welding joints are challenging to implement. However, there are still significant issues that need to be addressed before this technique can be fully trusted. A critical limitation of adhesive bonding is that the heat distortion temperatures of adhesives are often closer to the operating temperatures of the products than those of mechanical joints, which raises concerns about the adhesive bonding's mechanical resistance and durability. To ensure safety, adhesively bonded structures may need to include mechanical fasteners as an additional safety precaution. These practices result in heavier and more costly components, scarifying the advantages of adhesive bonding. More efficient use of lightweight materials and adhesives could be realized by developing reliable joint design and predictive methodologies for the strength and durability of adhesive bonding. To introduce adhesive bonding to such engineering applications that concern safety and reliability strongly, a deeper understanding of the adhesion and bonding mechanisms is necessary. Research on adhesion and its mechanisms is not only crucial as basic science, but it is also essential to connect it to social implementation. The fundamentals and applications of adhesive bonding are described in many books [2,3,4,5,6]. This book features comprehensive studies of interfacial phenomena in adhesion and adhesive bonding, focusing on a wide range of length-scale structures from the atomic level to a large-scale product fabricated using adhesive bonding with cutting-edge analytical techniques and evaluation test methods developed for the strength and durability of interfaces.

Fig. 1

An example of the multi-material car body structure

2 Interphase in Adhesive Bonding

Before discussing the mechanism of adhesion and adhesive bonding, it is necessary to clarify what an “interface” is in adhesion and adhesive bonding. “Interface” is an important issue not only in material science but also in various fields such as electronic devices, catalysts, colloid chemistry, and biological tissues, and interfacial interactions at the molecular and atomic levels are being studied in these fields. The interfaces that should be dealt with when discussing the adhesion mechanism refer to regions with structures and properties different from the bulk part of the adherend or adhesive. These are formed during the surface pretreatment of adherend or the bonding process in the adhesive layer. It is not simply a two-dimensional interface where dissimilar materials come into contact. It can be called an “interphase” because it is a three-dimensional area [7]. In metal/polymer joints, three terms have been frequently used: interface, interfacial region, and interphase. In material science, the interface refers to the boundary between two phases, namely the metal or oxide metal and the polymer. The interfacial region refers to the volume of material slightly below the metal's surface and extending into the polymer. In contrast, the interphase refers to the volume of the polymer that is adjacent to the substrate.

The bonding performance is determined by the two-dimensional interface between the adherend and adhesive and a three-dimensional region with different properties and structural characteristics that extend into the bulk materials. Therefore, the interphase or interfacial region, the area below the interface, plays a crucial role in adhesion phenomena and requires analysis. Researchers have studied the thickness of this region extensively, but it is still unknown. To understand the bonding mechanism, it is essential to identify the interfacial region, how it is formed during the bonding process, and how it can be damaged and degraded.

3 Testing of Adhesion and Adhesive Bonding

It is necessary to perform appropriate mechanical measurements and clarify the interfacial properties to elucidate the adhesion and adhesive bonding mechanism. Many ASTM and ISO standards have been written to evaluate the mechanical behavior of adhesive joints. New joining technologies have been developed especially for joining dissimilar materials, and their new applications are being investigated. Since these bonding technologies can achieve extremely high bonding strength, existing test methods do not cause the failure of the joint part and cannot correctly evaluate bonding characteristics. Therefore, several new test methods have been established as ISO standards in the last decade.

The test method for measuring the lap shear joint strength has been standardized as ISO4587 [8]. When a metal-plastic lap joint having high joint strength is tested by this standard, the relatively weak plastic part breaks due to the large bond area of the specimen. ISO 19095 specifies the test specimen geometries, as shown in Fig. 2 to quantify the metal-plastic joint performances regarding lap-shear strength, tensile strength, and peeling resistivity [9]. This standard also specifies the test method for sealing properties and environmental conditions for durability tests. Sealing property evaluation of metal/resin joints requires a test method different from that for joint strength. To evaluate the sealing performance of a metal-plastic joint, the detection of the leakage (leak rate) of helium gas is employed using the specimen shown in Fig. 2c, where a vacuum line is sealed with the specimen and the leaked helium is monitored by a mass spectrometer [10]. Using the test specimens and the test methods specified in the standard, the aluminum-PPS (polyphenylene sulfide) direct joints prepared by injection molding can be evaluated as described in Sect. 7 in chapter “Interfacial Phenomena in Adhesion and Adhesive Bonding Investigated by Electron Microscopy”. For the dissimilar adhesive bonding of carbon fiber-reinforced plastic and metal, the test method to determine the cross-tension strength, a standard test method for spot welding as defined by ISO 14272 [11], has been defined as ISO 24360 [12]. The detail of this test is described in Sect. 4 in chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)”.

Fig. 2

Test specimens of metal-plastic assemblies specified by ISO 19095: a lap joints of steel/PPS (left) and copper/PPS (right); b butt joints of aluminum/PPS (left) and steel/PP (right); c sealing test specimens of aluminum/PPS (left) and copper/PPS (right)

Commonly employed test methods are developed for the tensile, shear, bending, and peeling strength measurements of the specimens having a joint part. Even if there is an interfacial failure, it is difficult to determine the intrinsic adhesion forces acting across the interface using standard test methods. It is important to note that the strength measured for traditional joint designs and test methods depends on the intrinsic adhesion and the mechanical properties of the adhesive, substrates, and joint geometry. For instance, the stiffness, strength, and resistance to creep of a single-lap joint can vary depending on the substrates’ modulus, thickness, and overlap length. This is due to the complex stress distributions in the joint's geometry.

As described in chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)”, the mechanoluminescence (ML) successfully visualizes the strain distribution in lap joints, depending enormously on Young’s modulus of the adhesive and the adherend to be used (Figs. 33 and 45 in chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)”). Conventional adhesion tests, such as lap shear and peel adhesion tests, are often affected by inelastic deformations that occur in the adherend away from the interface, making them unsuitable for providing accurate interfacial characteristics. To characterize the adhesion properties of interfaces, the fracture resistance of the interfaces is estimated under tensile opening force in the appropriate double cantilever beam (DCB) geometry, as shown in Fig. 3 [13]. During the DCB test, a specimen is pulled apart at a constant velocity, and the locus of failure is examined to deepen the understanding of the bonding mechanism of surface treatments. Sections 8 in chapter “Interfacial Phenomena in Adhesion and Adhesive Bonding Investigated by Electron Microscopy” and 4 in chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)” provide details of the DCB test performed on dissimilar joints of carbon fiber-reinforced thermoplastic and aluminum.

Fig. 3

DCB test of a dissimilar adhesive joint of CFRTP and aluminum: a FEM analysis; b visualization of the crack propagation by ML

4 Multiscale and Hierarchical Structures in the Interphase and Interfacial Region in Adhesive Bonding

Adhesion and adhesive bonding mechanisms refer to the questions of how bond strength is achieved and how initial bond strength degrades under different environmental conditions. There are various ways in which bonding can occur. Mechanical interlocking into rough surface structures is one possibility to explain the bonding. Another potential factor is chemical bonding, which may enhance joint strength and occasionally even exceed the role of mechanical interlocking. The bonding mechanism has long been considered simply by mechanical adhesion (anchor effect), chemical bonding, or intermolecular forces based on surface energy. However, it is challenging to fully understand the adhesion mechanism based on such a simple concept. Gaining the trust of those considering the use of adhesive bonding in manufacturing their products requires deep insight into the phenomena that occur in the interphase of adhesive bonding.

The “interphase” involves several characteristics with different size features from atomic to macro scales, as shown in Fig. 4, where typical structures involved in the interphase are arranged in the order of their scales:

Fig. 4

Overview of multi-scale structures involved in the interphase in adhesive bonding: a chemical bond; b polymer chain entanglement; c lamellar morphology of a semicrystalline polymer; d porous structure in the oxide layer of metal adherent; e interfacial layer formed vis interdiffusion of polymers; f surface roughness created by surface treatment; g segregation or absence of a component in adhesive; h local deformation at a crack tip; i strain propagation and stress distribution in a structural body

(a) Chemical bonding

Van der Waals forces, ionic interaction, hydrogen bonding, and covalent bonding are the primal sources for adhesion, the smallest structure in the interphase. One practical approach to enhancing adhesion is incorporating chemically reactive moieties into the adherend. The formation of chemical bonds at interfaces is widely considered the primary factor in improving adhesion. Detecting these chemical bonds at adhesion interfaces has long been a significant objective of researchers studying the bonding mechanism and assessing bonding reliability. However, directly observing covalent bonding at interfaces is challenging. The chemical bonding at interfaces is investigated by sum-frequency generation spectroscopy (SFG) in chapter “Analysis of Molecular Surface/Interfacial Layer by Sum-Frequency Generation (SFG) Spectroscopy” and by electron energy loss spectroscopy (EELS) in Sect. 6 in chapter “Interfacial Phenomena in Adhesion and Adhesive Bonding Investigated by Electron Microscopy”.

(b) Interfacial segmental entanglement

Intermolecular chain entanglements across the interface control the adhesion between polymers. The chain coupling across an interface can provide physical links. The length of the entanglements that determines the adhesion strength is around 10 nm. The correlation between the entanglement of polymer chains at an interface and its resistance to crack propagation is investigated in Sect. 3 in chapter “Interfacial Phenomena in Adhesion and Adhesive Bonding Investigated by Electron Microscopy” by fractography studies with scanning electron microscopy (SEM).

(c) Lamellar structures of semicrystalline polymers

Semicrystalline polymers such as polyethylene (PE), polypropylene (PP), and polyamide (PA) form lamellar structures by folding polymer chains. The lamellae's orientation and the interfacial region's crystallinity influence the adhesion properties. The thicknesses of the lamellae are 10–20 nm and the lamellar structures below the surface with a depth of about 100 nm are affected by surface pretreatment. The effect of the lamellar structures on adhesion properties is described in Sect. 4 in chapter “Interfacial Phenomena in Adhesion and Adhesive Bonding Investigated by Electron Microscopy”.

(d) Metal oxide porous structure

Natural aluminum surfaces are covered with a thin oxide and/or hydroxide open porous layer with pore sizes of about 10 nm. Chemical or laser treatments can also artificially create porous surfaces on aluminum surfaces. Adhesive molecules infilter these narrow pores to enhance bond strength. Sections 6–8 in chapter “Interfacial Phenomena in Adhesion and Adhesive Bonding Investigated by Electron Microscopy” describe the effect of the aluminum oxide surface structures on bonding.

(e) The interfacial layer formed via the interdiffusion of polymers

When similar or dissimilar polymers are welded at high temperatures, interdiffusion occurs, creating a coexistent interfacial layer with a concentration gradient between the two polymers. Layer thicknesses range from a few to 10 nm for immiscible polymer pairs determined according to the interaction parameter (χ parameter). In contrast, thermodynamically miscible pairs exhibit quite a fast diffusion, forming the layer with a few hundred nm thicknesses. The interfacial layers are visualized and characterized by energy-filtering transmission electron microscopy (EFTEM), as described in Sects. 1 and 2 in chapter “Interfacial Phenomena in Adhesion and Adhesive Bonding Investigated by Electron Microscopy”.

(f) Surface roughness created by surface pretreatments

Atmospheric plasma treatment of the polymer surface creates a rough surface in the order of several hundred nm together with polar functional groups. The topological features of the surface roughness of isotactic PP (iPP) are described in Sect. 4 in chapter “Interfacial Phenomena in Adhesion and Adhesive Bonding Investigated by Electron Microscopy”.

(g) Interfacial segregation of the components in the adhesive

Commercial adhesives intentionally contain several organic and inorganic components to meet the specifications of a particular application. These components may be unevenly distributed in the adhesive, with different distributions of components between the interfacial region and the bulk well-separated from the adherend surface. The regions that form at these interfaces can be several microns in size and affect bonding properties. The segregation of a component in an adhesive is investigated in the bonding of aluminum to iPP in Sect. 5 in chapter “Interfacial Phenomena in Adhesion and Adhesive Bonding Investigated by Electron Microscopy”.

(h) Plastic deformation ahead of the crack

When a crack opening load is applied to a laminated specimen, plastic deformation occurs before the crack tip during crack propagation along the interface. This region absorbs the energy that causes an interfacial fracture, and the plastic deformation region's expansion enhances interfacial toughness. Mechanoluminescence (ML) visualization of plastic deformation at the crack tip under specific test conditions in various specimen geometries is useful for stress–strain analysis of joint structures, as presented in Sect. 4 in chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)”.

(i) Strain distribution in adhesive joints in structural bodies

It is possible to visualize the strain propagation of the structure due to adhesion when impact strength is applied to the car body by combining ML and high-speed video recording. This technique helps predict possible points of failure in large-scale adhesive-bonded structures, as described in Sect. 3 in chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)”.

5 Visualization and Analysis of Interphases in Adhesion and Adhesive Bonding

The structures mentioned above with variable length scales are investigated mainly by following three analytical techniques:

• 1. Multi-dimensional visualization of interphases by electron microscopy (EFTEM, STEM, EELS, tomography, and in-situ tensile TEM, etc.);

• 2. Mechanoluminescence (ML) visualization of strain and crack propagation at interfaces;

• 3. Atomic characterization of buried interfaces by sum-frequency generation spectroscopy (SFG).

Transmission electron microscopy (TEM) conventionally presents two-dimensional (2D) projections of the internal structures of samples with a high spatial resolution. Here, we introduce additional 2D structural information using scanning transmission electron microscopy (STEM). As shown in Fig. 5, in STEM, the electron beam is focused on a spot on a specimen surface and is scanned across the specimen area to be investigated. In contrast, the transmitted electrons are collected in the annular detectors aligned “on-axis” below the specimen. A unique specimen holder performing in-situ tensile joint specimen testing allows real-time failure observation. As a result, it is possible to add dynamic (t) information to 2D structural information (x, y). The STEM instrument also can perform tomography for 3D visualization of interfaces. The combination of energy-dispersive X-ray spectrometry (EDX) and the tomography function allows us to construct 3D elemental distributions, which add elemental information (E) to 3D structural information (x, y, z). Electron energy loss spectroscopy (EELS) provides information on the electronic and bonding environment of the excited atom. Therefore, it can add chemical information (C) of an element of interest (E) to the 2D structural information.

Fig. 5

Multi-dimensional interface analyses by STEM: a STEM-HAADF images of in-situ observation of the crack propagation of Al5052/epoxy adhesive interface under tensile loading; b 3D elemental maps of the laser-irradiated Zn-coated steel surface created by STEM-EDX tomography; Fe (red), Zn (green) and Fe/Zn co-existing (yellow) region; c EELS spectra of aluminum compounds metallic Al, Al(OH)₃ (gibbsite), AlO(OH) (boehmite) and γ-alumina

Chapter “Electron Microscopy for Visualization of Interfaces in Adhesion and Adhesive Bonding” provides an overview of electron microscopy techniques used to investigate interphases in polymers and polymer/metal hybrid systems. First, how the instruments of energy-filtering transmission electron microscopy (EFTEM), STEM, and SEM work and are operated is briefly described. The principles of EELS and EDX are described. Next, the specimen preparation techniques such as ultramicrotomy, heavy metal staining, focused ion beam (FIB) fabrications, and replica method, which are essential for these electron microscopy tasks, are introduced. This chapter also reviews advanced electron microscopy techniques, such as STEM-EDX-tomography, and chemical phase mapping using electron energy-loss near-edge structure (ELNES) and in-situ tensile TEM. Numerous examples of the application of these techniques to various surfaces and interfaces present in polymer alloys and composites, crystalline polymers, adhesive bonds, and metal substrate surfaces are presented.

Interfacial phenomena in adhesion and adhesive bonding are investigated in chapter “Interfacial Phenomena in Adhesion and Adhesive Bonding Investigated by Electron Microscopy”. Polymer–polymer interfaces formed via interdiffusion are visualized and characterized by EFTEM. Fractographic studies using high-resolution SEM investigate entanglements at the polymer–polymer interfaces, and the adhesion mechanism is discussed about the interfacial entanglements. The effect of surface treatments of polymers for adhesion improvement is studied in terms of the surface roughness and the chemical functionality of the adherend. We then describe the role of chemical interactions between polymers and metals on bonding by the analysis of fracture surfaces by the STEM-replica technique. Bonding mechanisms of adhesive bonding and recently developed direct bonding of metal and plastic are also investigated by STEM-EELS/ELNES and STEM-tomography. Finally, special attention is made to the toughness and durability of adhesive joints between metal and carbon fiber-reinforced plastics (CFRP) and discuss the durability of the adhesive bonding.

In chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)”, we introduce recent progress in direct visualization of mechanical behavior in the failure process of adhesive bonding by mechanoluminescence (ML). In bonding and joining, obtaining the necessary “force” in the required period is extremely important. However, we cannot see mechanical behavior. We need to predict the information appropriately based on our accumulated experience and knowledge of experts, reflecting it in the design and simulation. Do we have full confidence in our past knowledge, simulations, and designs to be correct? Are there any assumptions included in the knowledge? Is there any information that we need to be made aware of? Questions always remain. To address this issue, we have utilized ML sensing technology as our originally developed (Fig. 6), which can visualize dynamic strain distribution generated at the adhesive bonding area and its interface. ML is a fascinating and promising visual sensing technology. However, many readers should not be familiar with ML sensing.

Fig. 6

Brief introduction of chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)”; direct visualization of mechanical behavior in failure process of adhesive bonding by mechanoluminescence (ML)

Therefore, firstly basic ML technologies are introduced in terms of materials, sensors, and sensing technologies in Sects. 1 and 2 in chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)”. Then, for considering the effective application of ML sensing that take advantage of technological features, (Sect. 3 in chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)”) structural health monitoring (SHM)/conditioning-based monitoring (CBM) and (Sect. 4 in chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)”) innovation in design and prediction are discussed from the viewpoint of visualizing mechanical behavior, deterioration, and failure process as the killer application of ML sensing. Furthermore, using internationally standardized adhesion strength tests, visualizing the mechanical behavior of adhesive joints, fracture initiation points, and fracture processes will be introduced based on time-series information of ML images.

The purpose of this chapter “Direct Visualization of Mechanical Behavior During Adhesive Bonding Failure Using Mechanoluminescence (ML)” is to show the invisible mechanical behavior of adhesive joints, which are becoming increasingly crucial in multi-material lightweight design, and to provide an opportunity to gain confidence in conventional experience and inspiration for completely different designs and predictions.

In chapter “Analysis of Molecular Surface/Interfacial Layer by Sum-Frequency Generation (SFG) Spectroscopy”, we introduce recent progress in analyses of the various kinds of interfaces using sum-frequency generation (SFG) vibrational spectroscopy. Surfaces of materials are important sites for a wide variety of physical properties, such as hydrophilicity, hydrophobicity, friction, adhesion, biocompatibility, and catalysis, and sometimes exhibit properties “different” from those of the bulk and are very complicated. Wolfgang Pauli, the Nobel Prize winner in physics, was annoyed by the complexity of surfaces and said, “God made solids, but surfaces were made by the Devil”. The orientation of molecules at the surface is also important in considering the probability of the chemical reaction occurring. For example, the SN-2 reaction, a significant chemical reaction, proceeds when negatively charged functional groups attack positively charged sites, such as the carbon atoms of the carbonyl groups. Therefore, one can easily imagine that if the carbonyl carbon sites are not facing a direction favorable to the reaction on the surface, the reaction will not proceed easily.

SFG spectroscopy is a novel spectroscopic technique that uses laser light at two different wavelengths to “specifically” obtain the information from molecules at surfaces and interfaces. Since this spectroscopic technique uses lights, it can obtain information not only from the surfaces, but also from the molecules at the interfaces, as far as the lights can penetrate and reach the interface. This makes it possible, for example, to study the chemical reactions of molecules at the interface of electrodes in liquids or the orientation of molecules at the interface of adhesives and the curing process, which have been difficult to investigate directly.

Figure 7 presents the example of the SFG spectra obtained at the aluminum oxide interface with an adhesive containing a small amount of aminopropyltrimethoxysilane. Immediately after the application (black line), an SFG peak can be observed at 2840 cm⁻¹. This peak originates from the methoxy group of the silane coupling agents, indicating that the coupling agents are concentrated at the interface immediately after the application. As time proceeds, the intensity of this peak gradually decreases, and instead, new peaks appear at 2850 and 2880 cm⁻¹. These peaks can be attributed to the methylene (CH₂) and methyl (CH₃) groups in the polymer components of the adhesive. The peaks derived from the adhesive components are not visible immediately after the application, which is thought to be due to the molecules of the adhesive polymer being in random orientation at the interface immediately after the application. The methoxy groups of the silane coupling agent readily react with a small amount of water present at the interface and are decomposed. As the adhesive cures, its main component, the macromolecule, is thought to become ordered at the interface. Thus, SFG spectroscopy allows us to understand the static chemical structures and molecular orientations at the “buried interfaces” and the dynamics of the molecular behaviors at the interfaces.

Fig. 7

SSP polarized SFG spectra following the time evolution of the silyl-terminated polyether adhesives and aluminum oxide interface after application