1 Introduction

Investigating the failure of adhesive interfaces is important to study the bonding mechanisms and evaluate the performance of adhesives and surface treatments. Failure behavior has been generally speculated by inspecting fracture surfaces by optical or scanning electron microscopy [1,2,3]. However, such classical fractography limits the capability to understand the complicated bonding mechanism and properties. The direct observation of the failure behavior under high-resolution electron microscopy is expected to provide information on complicated failure processes in adhesive interfaces. Recently, new equipment for performing in-situ experiments to apply tensile force to a joint specimen under high-resolution scanning transmission electron microscopy (STEM). We previously reported the real-time observation of the failure processes of the interfaces between a model epoxy/amine mixture and aluminum and the interface in a direct joint of engineering plastic and aluminum [4]. STEM allows us to perform high-spatial resolution analysis of interfaces through imaging and local elemental and chemical analysis using a focused electron probe with a diameter smaller than 1 nm diameter [5]. The advantages of STEM over conventional transmission electron microscopy (TEM) are that high-quality and high-contrast images can be acquired even with a thicker specimen up to 200 nm. This work attempts in-situ tensile testing in a STEM instrument to observe the real-time failure in commercial adhesives with phase-separated morphologies and inorganic fillers.

2 Experimental

Acid treatment of the Al surface was employed to ensure the bonding of Al to the adhesive. 3 mm thick Al6051 plates were preliminarily immersed in sodium hydroxide (NaOH) aqueous solution (ph12) at 60 °C for 30 or 45 s, followed by the treatment with 60 wt% nitric acid for 1 min. The acid treatment was employed with two immersion times (30 and 45 s) in the NaOH solution to control the bonding condition. Those are corresponded to “soft” and “hard” acid treatments, respectively. Then, those two plates were bonded with a commercial structural second-generation acrylic adhesive (SGA) [6,7,8], HARDLOC C355-20A/20B (DENKA Corp., Tokyo, Japan), or two-component epoxy adhesive [9, 10], DENATITE (NAGASE ChemTex Corp., Osaka, Japan). SGA adhesives are two-component room-temperature curing structural acrylic adhesives. After mixing and pasting the adhesives, they were left at approximately 24 °C (room temperature, RT) for 24 h and subsequently at 60 °C for two hours to cure the SGA adhesive. The epoxy adhesive was cured at 100 °C for 30 min. The non-bonded region for introducing a pre-crack into the interface was made by inserting 100 µm thick Kapton film into one end of the lamination.

The Al5052 and polyphenylene sulfide (PPS) joint specimens were provided by Taisei Plus Co. (Tokyo, Japan), Ltd., prepared by insert-injection molding of PPS (SGX-120, TOSO Corp., Japan) onto surface-modified Al5052 plate at the melt temperatures of 290–330 °C and the mold temperature of 120 °C. The Al surface was chemically treated by the method developed by Taisei Plas Co., Ltd., using a hydrazine-based aqueous solution [11], producing nano-sized pores with three-dimensional inter-connected structures within approximately 100 nm thick surface layers. The 2 ± 0.1 mm thick PPS/Al5052 joint laminate was prepared with the non-bonded region at one side of the joint laminate, which is a pre-crack part.

Figure 1 shows the specimen holder for in-situ tensile testing in a STEM equipped with a device manufactured by Mel-Build Corp. (Fukuoka, Japan). A small device for applying a tensile force to the specimen is built into the tip of the sample holder, as shown in Fig. 1a, b. The specimen is mounted on the isolated thin metal cartridge attached to the actuator built into the device. Pushing the cartridge by the actuator with 100 nm/sec can open the narrow slit with 20 µm width fabricated in the cartridge, applying tensile load into the specimen fixed in the slit. Figure 1c, d show an Al/adhesive/Al triple layer sectioned into about 100 nm thick, about 300 µm in size. The thin sections were cut with a diamond knife using an ultramicrotome, and the floating sections on the water in the trough of the diamond knife were collected onto the cartridge [5]. Then the sections were fixed on the slit as the desired position and direction, as shown in Fig. 1c, d. As shown in Fig. 1c, the section is fixed to apply tensile force to the interface. Here, the interface is arranged parallel to the slit, and both sides are fixed with adhesive. On the other hand, for applying a shear force to the interface, the section was positioned with the interface perpendicular to the slit, and the two corners, diagonally opposite each other, were fixed with adhesive, as depicted in Fig. 1d.

Fig. 1
figure 1

Nano-order tensile specimen holder for the in-STEM work. a The head part of the specimen holder. b Configuration of the tensile loading device built in the head part of the holder indicated in (a). Al/adhesive/Al triple layered thin specimens fixed on the slit part of the metal cartridge as the interface aligned parallel (c) and normal to the slit (d). Insets are illustrations depicting the fixation of the specimens with adhesive (red parts)

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A focused ion beam (FIB) [5] was used to prepare a thin test specimen for an inorganic filler-containing adhesive. A small section of the aluminum test specimen, along with epoxy adhesive, was fixed over a 20 µm wide slit in the metal plate of the specimen holder, and a thin window that included the Al/adhesive interface for electron beam transmission was created.

In-situ STEM experiment was performed using TECNAI Osiris (FEI company, USA) STEM instrument with an accelerating voltage of 200 kV.

3 Results and discussions

3.1 Failure of the adhesive interfaces between SGA and Al in crack opening mode

Figure 2 is a STEM image in annular dark field (ADF) mode, where the bottom dark part is Al and the upper part is the adhesive. It shows that the adhesive has the phase-separated structure containing the two phases with bright domains dispersed in the continuous dark matrix. There are two types of domains that are dispersed, with significantly different sizes. Many small domains are dispersed in the narrow gap between the large spherical domains. Moreover, it was found that the bright phase preferentially covers the entire Al surface. It is indicated that the acrylic monomers in the adhesive are separated into two phases during the polymerization process [12], and the minor component tends to form the matrix phase. This adhesive exhibits phase-inversion during the increase in molecular weights of the components. At the same time, the major component prefers to lay on the Al surface.

Fig. 2
figure 2

shows a STEM-ADF micrograph revealing the phase-separated structure within the SGA adhesive layer and the selective interfacial segregation of one component onto the Al surface

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Figure S1 presents the configuration of the STEM instrument used in this work. The electron beam is focused on a spot and scanned across the specimen area for investigation while the transmitted electrons are collected. The elastically scattered electrons are collected in the three annular dark field (ADF) detectors located below the specimen, which can collect the elastically scattered electrons according to the scattering angles. The bright field (BF) detector, located at the lowest position in the array of detectors, collects the un-scattered and scattered electrons with low scattering angles. Those four imaging detectors allow us to acquire images with different contrasts simultaneously, and the desired contrast of images can be obtained. Here, the images taken with the three detectors are shown. A comparison of the three images indicates that the DF1 detector gives the image with better overall contrast.

Figure 3 shows the STEM images in high-angle annular dark field (HAADF) mode captured in the in-situ tensile experiment of the specimen with a pre-crack at the interface. When tensile force was applied in the lateral direction of the specimen, the pre-crack widened, and the crack tip became round-shaped before the crack propagated. It was confirmed that numerous branched fine cracks were generated from the crack tip on that round face (Fig. 3a). The tensile load continued to be applied to the specimen in the lateral direction. Then, the crack started propagating into the adhesive layer, producing the branched fine cracks in the wide region ahead of the crack (Fig. 3b). The crack, thus, escaped from the interface between Al and the adhesive, preventing the interfacial failure of the adhesive interface. Figure 3c is a magnified image of the fine cracks, showing that the fine cracks are running through the gap between the spherical dispersed domains. Figure 3d shows that microvoids and small fibrils are included in the fine cracks around the crack tip, which are elongated in the direction perpendicular to the crack growth direction. These features observed in the fine cracks indicate crazing occurs upon the failure of the adhesive bond when the tensile force is applied to the interfacial pre-crack. The fine fibrils elongate and break, causing the microvoids to grow and coalesce, then cracks forming. The real-time observation of the failure of adhesive bonding suggests that the adhesive forms the phase separation with continuous soft phase and the hard dispersed domains. Figure S2 shows the crazes developed around the crack tip captured using HAADF and DF2 detectors. A comparison of the two images suggests that the HAADF image provides better contrast, enabling clear differentiation between the voids and the deformed area surrounding them.

Fig. 3
figure 3

STEM HAADF images show the crack propagation during in-situ tensile testing ahead of the pre-crack prepared at the SGA/Al adhesive interface. a Depicts fine cracks observed after the pre-crack opening. b shows significant crack growth and crazing during the crack opening. c Presents a magnified view of the area highlighted in green in (b), revealing crazing development within the narrow gap between spherical domains. d Displays a high-magnification image at the crack tip, marked by the red square in (b)

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3.2 Failure of the adhesive interfaces between SGA and Al under shear force

The failure behavior of the Al/adhesive/Al triple layer under shear force was examined through in-situ tensile testing with STEM observation. Figure 4 shows the crazing in the adhesive layer of the Al plates under two different acid treatment conditions. Figure 4a presents the development of crazes and interface delamination with the Al treated in a short immersion time with sodium hydroxide (soft acid treatment), while Fig. 4b illustrates the failure process by the long immersion time (hard acid treatment). With the soft acid treatment, the failure of the interface of the upper Al layer occurred at the beginning of the shear force loading (left panel of Fig. 4a), and crazing started to develop in the middle part of the adhesive. Meanwhile, the voids were produced after the growth of the crazing (middle panel of Fig. 4a), and then the adhesive layer was entirely raptured with the delamination of the adhesive from the Al substrate (right panel of Fig. 4a). The bonding by the hard acid treatment, the interfacial delamination could be avoided in the earlier stage of the shear loading, as shown in the left panel of Fig. 4b. The crazing was observed to be produced simultaneously in the entire part of the adhesive layer, and then the adhesive layer was raptured entirely in the adhesive after the crazing grew into cracks.

Fig. 4
figure 4

STEM-HAADF images were obtained during in-situ tensile testing under shear loading to observe the development of crazes in the SGA adhesive layer of the Al/adhesive/Al triple-layered specimen under different acid treatment conditions for aluminum. a represents soft acid treatment, while (b) represents hard acid treatment

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The findings suggest that the crazes developed in the soft component, acting as the matrix phase as the narrow gap between the dispersed domains, effectively resists crack propagation, thereby enhancing interface toughness. Furthermore, the selective segregation of the hard component at the Al/adhesive interface can contribute to the toughness of the interface by directing crazes towards the adhesive layer rather than toward the Al interface. Figure 5a, b show STEM-HAADF images capturing the moment of adhesive failure under tensile and shear loading, respectively. The gaps between neighboring domains are bridged by fine filaments of the matrix phase called fibrils before the rupture of the adhesive (Fig. 5a), while in the shear mode, the matrix phase is significantly deformed.

Fig. 5
figure 5

STEM-HAADF images showing the crack tip before and immediately after the rupture of the adhesive in tensile (a) and shear (b) mode

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The bonding performance of the SGA adhesive used in this study has been well investigated, and the mode I fracture energy of the bonding to steel was reported to be 1.7–2.0 kJ/m2 [6,7,8]. Such high toughness could be achieved due to its phase-separated structure, which promotes crazing and effectively absorbs energy to cause failure, and also due to the selective segregation of the hard component on the Al surface.

3.3 Failure behavior of the interfaces between an elastomer-containing thermoplastic and Al bonded directly via injection molding

To understand deeply the role of the phase-separated structure of the adhesive on interfacial toughness, the failure behavior of the interface in the direct bonding of thermoplastic and Al via injection molding was investigated. The details of this bonding technology are shown in the literature [11,12,13]. As shown in the left panel of Fig. 6a, the polyphenylene sulfide (PPS) contains elastomer as the dispersed domains. When applying the tensile force to open the pre-crack, cavities were produced in the elastomer domains close to the interface. The cavities could promote local plastic deformation around the domains, and then crazes were produced to connect the cavities (middle panel of Fig. 6a). Finally, the crazes coalesced, separating the PPS from the Al with a small amount of elongated PPS remaining on the Al surface (right panel in Fig. 6a). As compared to the SGA adhesive, crazing occurred in the limited region around the crack in the PPS/Al direct joint, indicating that the phase separation structure obtained in the SGA adhesive is effective in promoting the crazing into the wide region of the adhesive.

Fig. 6
figure 6

STEM HAADF images captured during in-situ tensile testing, showing the failure progression ahead of the pre-crack at the PPS/Al direct joint interface under tensile loading (a), and failure under shear loading (b). The arrows in (a) denote the direction of crack propagation, while in (b), they indicate the direction of shear loading

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Figure 6b depicts the STEM-HAADF images captured under shear loading. In the PPS/Al double-layered specimen, crazing was initiated in the middle part of the PPS layer (left in Fig. 6b) and propagated towards the Al surface (middle in Fig. 6b). It was found that the shear loading induced extremely widespread crazing in the PPS layer as compared with the tensile loading (right panel in Fig. 6b).

3.4 Preparation of the test specimen for in-situ tensile testing of an adhesive highly filled with inorganics

Many structural adhesives contain a large amount of dispersed inorganic filler. When such samples are cut with an ultramicrotome, the filler tends to fall out. Therefore, to create specimens for the in-situ tensile testing from adhesives containing filler, the use of FIB is necessary. As shown in Fig. 7a, the pre-cut test specimen was bridged across the slit, ensuring that the interface is positioned at the center of the slit. Both ends were fixed by beam-induced tungsten deposition, then the central portion containing the interface was milled using an ion beam to create a window thin enough for electron beam transmission. When tensile stress was applied to the miniature test specimen, the adhesive side was elongated while deformation of the Al side was negligible. Consequently, compressive force acted perpendicular to the tensile stress in the adhesive layer, leading to the formation of necking. Figure 7b suggests that the crack initially originates from the filler in the necking part, and it proceeds to grow towards the adjacent filler. Subsequently, the crack propagates from one filler to another until it eventually reaches the adhesive interface.

Fig. 7
figure 7

a SEM micrograph showing an Al and adhesive joint specimen fixed over the slit of a metal cartridge for the in-situ tensile testing in STEM. b The growth of cracks near the adhesive interface under tensile force observed during STEM observation

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4 Conclusion

We expanded the in-situ observation of adhesive interface failure using STEM to commercial adhesives. The generation and propagation of cracks near the interface, attributed to the adhesive's phase separation, contribute to the toughness of the adhesive interface. The direction of stress acting on the interface and the strength of the interface influence the initiation and growth of cracks throughout the adhesive layer.