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Interfacial interaction and joining property of direct injection-molded polymer-metal hybrid structures: A molecular dynamics simulation study
Applied Surface Science 478 (2019) 680–689
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Interfacial interaction and joining property of direct injection-molded polymer-metal hybrid structures: A molecular dynamics simulation study Mingyong Zhoua,b, Xiang Xiongb, Dietmar Drummerc, Bingyan Jianga,
T
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a
State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha, Hunan 410083, PR China Powder Metallurgy Research Institute, Central South University, Changsha, Hunan 410083, PR China c Institute of Polymer Technology (LKT), Friedrich-Alexander-University Erlangen-Nürnberg, Am Weichselgarten 9, 91058 Erlangen-Tennenlohe, Germany b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer-metal hybrid structure Injection molding Molecular dynamics Interfacial joining
Polymer-metal hybrid structures have been increasingly used to replace metallic components. Direct injection joining is one of the most promising technologies in fabricating polymer-metal hybrid structure. In this study, molecular dynamics method was used to study the interfacial interaction and joining property. Interfacial models with different surface structures on aluminum (Al) substrate were constructed. The influences of surface structure, non-bonded interaction strength, and pull-out force during the separation were systematically explored. Simulation results show that an obvious difference in polymer structure along z-axis is observed during the joining process. Influenced by the interfacial interaction, the atomic density is at its largest near the interface. As the separation continues, growing voids are observed near the interface. Polymer chains close to the Al substrate are greatly stretched, while the chains far away from the interface are relatively steady. Nevertheless, the changes in radius of gyration during the separation are much higher regardless of the substrate structures. The contact area mainly contributes to the interaction energy, whereas the mechanical interlocking formed in the undercut area contributes to the maximum inner force and work of separation. Moreover, the interfacial interaction is also affected by the non-bonded interaction strength and the pull-out force.
1. Introduction In recent years, polymer-metal hybrid structures have been increasingly used to replace all-steel load-bearing structural components for weight saving and cost efficiency purpose in automobiles, digital electronics, and other industrial devices, such as instrument panels and front-end modules [1–4]. In 1996, the polymer-metal hybrid front end in the Audi A6 was the first publicly reported case into the automotive manufacturing practice. The metal sheet was used as the main bearing structure while the glass fiber reinforced polyamide was injection over molded to connect the metal sheet, forming a dual-layer structure with improved mechanical property [5]. In this method, the joining strength is the key aspect in direct injection molding polymer-metal hybrid structure. And the joining processes usually involve adhesive bonding [6] and mechanical joining method [7]. Injection molding technology has attracted wide attention in the polymer industry due to advantages in low cost, simple process, and large volume production. It is the commonly used method in fabricating polymer-metal hybrid structure [8–10]. The metallic substrate is assembled in the injection mold, and subsequently over-molded with
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polymer melt to achieve a strong joining. Direct injection joining, normally with surface treatment on the metallic substrate, is one of the most promising joining technologies for polymer-metal hybrid structure. Here, the surface treatment can be realized by sandblasting [11], laser processing [12,13], chemical etching [14], nano-molding technology [15,16], and etc. With these surface methods, the micro/nanopatterned texture is fabricated, thus enriching the contacting area of polymer melt on the metal surface. The joining strength at the interface can be then further improved. In particular, the nano-molding technology is utilized as a special surface treatment to form nano-pores on the aluminum alloy surface. The surface structure consisted of not a simple array of vertical nano-holes of approximately 20 nm but a threedimensionally foam network, which contributes to the formation of mechanical interlocking [15]. In direct injection joining technology, the joining of interface is mainly determined by the synergistic effects of non-bonded interaction in nanoscale, chemical bonding, and mechanical interlocking. However, the inner mechanism has not been sufficiently revealed, especially when the nano-molding technology is applied. In this case, the scale of surface structure on metal substrate is within tens of
Corresponding author. E-mail address: [email protected] (B. Jiang).
https://doi.org/10.1016/j.apsusc.2019.01.286 Received 20 August 2018; Received in revised form 29 January 2019; Accepted 30 January 2019 Available online 31 January 2019 0169-4332/ © 2019 Published by Elsevier B.V.
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computational capability restricted by MD theory. After that, the PP layer was heated up to the melt temperature of 503 K and relaxed with constant temperature for 10 ps, in order to reach a stable state for polymer melt in the actual condition. Al substrate was built with the same dimensions in both length and width as the PP layer. It is known that the surface roughness strengthens the joining property by either increasing the effective contact area or forming the mechanical interlocking [8,10,13,44]. In order to analyze the influence of surface treatment on the metal substrate in nano-molding technology, three different structures including undercut structure, vertical structure, and the smooth substrate were constructed on the Al substrate, as shown in Fig. 2. For undercut structure, the lateral “T” shape cavity was built to represent the micro/ nano interlock structure that was formed by micro-arc oxidation or other electrochemical treatment [10,44]. Dimensions of the inner cavity were 4 nm in y-axis and 1 nm in z-axis, while the size of the outer cavity was 2 nm in both directions. The vertical structure and the smooth substrate were constructed to compare the influence of increased contact area on metal surface. As described in the introduction section, the surface structure/roughness in direct injection-molded process is a few tens nanometers. Despite the limitation of dimension scale in MD simulation, this work can provide a qualitative estimation of interfacial characteristics and the separation behavior at the molecular level, which is impossible in a simulation based on continuum mechanics [29]. The Al layer was kept at a constant mold temperature of 353 K. The hybrid system for MD simulation was then built by assembling the PP layer and Al layer together, in which the Al layer was the substrate. Periodic boundary conditions in both x and y-axis were utilized for approximating a large system, while the non-periodic and shrinkwrapped boundary condition along z-axis was set so that polymer chains in PP layer can be shrunk or stretched during the whole process. All the atoms were encompassed in z-axis, thus the interaction between atoms on the top of PP layer and Al atoms at the bottom could be avoided. In order to get access to the actual processing condition, energy minimization and subsequently anneal treatment were done to optimize the molecule conformation in the polymer layer.
nanometers. Due to the obvious particle characteristic and scale effect, the flow behavior of polymer melt is highly affected by those factors that is commonly ignored in conventional injection molding, like wall slip, surface interaction, intermolecular forces, and so on [17–20]. In particular, the interfacial interaction is more relevant and sensitive to the surface quality of the metal insert or metal substrate [21,22]. It has been reported that when the surface roughness of metal insert is down to 16 nm, the strength of interfacial interaction, such as van der Waals force, is drastically increased, which results in a strong adhesion between the polymer and metal substrate [23]. Although experimental researches have been undertaken to analyze the influences of processing materials and parameters, the development time would be substantially decreased if sufficiently robust simulation results allow the determination of the interfacial behaviors. Simulation methods based on continuum mechanics, however, fail to accurately predict the polymer behavior in nanoscale because of the drastic changes in material properties. Conventional analysis for interface needs data about the mechanical behavior of the interphase that is difficult to obtain from direct measurement. Recently, molecular dynamics (MD) simulation has attracted great attention for the analysis in nanometer scale. Atomic movements that are allowed to interact can be observed by this method, giving a view of the dynamic evolution [24–26]. Through MD modeling at atomistic scale, we can predict not only the flow behavior of single polymer in molding process like nanoimprint lithography [27–29] and injection molding [30–32], but also investigate the viscoelastic property [33,34] and the interfacial interaction of polymer composites [35–38] or polymer-mold insert hybrid [29,39,40]. However, there are few MD studies focused on the interfacial joining of polymer-metal hybrid structure, such as the bonding process between vinyl ester polymer and steel [41–43]. Besides that, MD study on the polymer-metal interface should focus on not only the joining process but also the separation process, since little is known regarding the joining mechanism at the polymer-metal interface in nanoscale. Nevertheless, it is theoretically feasible to reveal the interfacial interaction of injection-molded polymer-metal hybrid structure by MD method. The main purpose of this paper is to provide insights into the interfacial interaction and joining property of direct injection-molded polymer-metal hybrid structure by using MD method. In this study, interface model of polypropylene-aluminum (PP-Al) was constructed in accordance with the real experimental processing. Metal substrates with different surface structures were built to study the effect of surface treatment in direct injection joining technology. Both joining and separation process of the hybrid structure were investigated by analyzing the interaction energy, radius of gyration, inner force, and conformation change of polymer chains. Meanwhile, the influences of parameters such as non-bonded interaction strength and pull-out force were investigated comparatively. The output of this study will provide useful reference for future design in polymer-metal hybrid structure.
2.2. Force field The consistent valence force field (CVFF) was used to represent the intermolecular and non-bonded interaction between atoms in polymer layer. In this work, bond stretching potential, angular bending potential, torsion potential, and non-bonded interactions were selected [26,30,45,46]. Standard Lennard-Jones 12–6 potential and Coulombic pairwise interaction were adopted to describe the non-bonded interactions between the substrate and the polymer, as shown in Eqs. (1) and (2)
Utotal = Ubond + Uangle + Utorsion + Unon − bonded 2. Simulation model and methodology
= kb (r − r0 )2 + ka (θ − θ0 )2 + kt (1 + cos(nϕ − ϕ0)) + Unonbond (1)
2.1. Model constructing
Unon − bonded = UL − J + UCoulombic = 4ε ((σ / r )12 − (σ / r )6) + qi qj / r
Before assembling the injection-molded hybrid system, the polymer and metal layer were separately prepared. Amorphous polypropylene (PP) and aluminum (Al) were selected as the two layers in this study since they are commonly used in polymer-metal hybrid structure [1,8,11]. Atomistic model of the PP layer for MD simulation was constructed by Materials Studio, as shown in Fig. 1. The layer was in a rectangular cuboid with dimensions 7.7 nm × 7.7 nm in length and width. The other settings, such as the degree of polymerization, number of chains, initial density, and box size, are shown in Table 1. Compared to that with real material, the degree of polymerization in the simulation was relatively small. This is because the dimension scale in this simulation was also reduced to several nanometers due to the limited
(2)
where the kb, ka, kt, and ε were constants of the bond stretching potential, angular bending potential, torsion potential, and non-bonded interaction respectively. Parameters were derived from the lattice constant and adhesive energy of Al atom. The cut-off distance of both potentials in non-bonded interaction was 1.25 nm. Other values of the constants mentioned above were given in the supplementary material S1. It has been seen that the Lennard-Jones 12–6 potential and Coulombic pairwise interaction can effectively describe the polymer–metal interaction in nanoimprint lithography [31] and nano-injection molding process [29], or with calculated result in good agreement with experiment for interfacial property [47–49]. 681
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Fig. 1. Atomistic models of (a) a single PP chain and (b) the whole layer.
interaction. It is demonstrated in Fig. 4a that the undercut structure cavity is fully filled at 10 ps, forming a mechanical interlock at the interface. Since molecules are penetrating the nano-cavity, the total height of the hybrid structure is decreasing. Similar phenomenon is observed in the other two models with vertical structure and smooth substrate. For Al layer with smooth substrate, both two chains are confined and twisted because of the restrained boundary. It is demonstrated that the conformational change of the chain away from the interface is hardly influenced by the structure on Al surface. In this present work, the final configuration of PP-Al hybrid structure after the cooling process is used as the initial atomistic model for the separation simulation. In order to quantify the capacity of molecule to adopt a more compact conformation, the radius of gyration (Rg) in PP layer during MD simulation was calculated from Eq. (3):
2.3. Simulation procedure One representative simulation procedure for PP-Al hybrid structure by direct injection joining is shown in Fig. 3. In this study, the joining process and the separation process were mainly focused. All the Al atoms were positionally restrained by setting each component of force to zero during the whole simulation. In this case, these atoms were kept constrained and the metal layer was treated as a rigid body since the metal substrate has a much higher stiffness than that of common polymer. A negative force of 0.16 kcal/mol∙Ȧ along z-axis was applied to each atom in PP layer, so that molecules were able to be filled into the nano-cavity structure with excellent replication fidelity. At the same time, the PP layer was gradually cooled down to the mold temperature (353 K) in 10 ps in a constant particle number, volume, and temperature (NVT) ensemble. Once the joining process ended, the whole system was then cooled down to 303 K in another 10 ps. The next step was to study the joining strength under the separation process. In this step, an external force of 0.1 kcal/mol∙Ȧ (equal to 6.95 pN) along z-axis was applied to pull out the PP layer from the Al substrate. The interfacial interaction and joining property would be further studied by varying the pull-out force from 0.5 kcal/mol∙Ȧ to 0.01 kcal/mol∙Ȧ and by decreasing the constant value (ε) in non-bonded interaction from 100% to 10%. All the MD simulations mentioned above were performed by LAMMPS [50], an open source code for Large-scale Atomic/Molecular Massively Parallel Simulator in a computer cluster.
N
Rg 2 =
N
∑ mi ri2/ ∑ mi i=1
(3)
i=1
where mi is the atomic weight and ri is the distance of an atom to the mass centre of a single molecule. If Rg increases, it indicates the elongation of the whole layer, while its reduction illustrates the contraction of the polymer molecules. Regarding the relaxed PP layer without substrate, the radius of gyration is 2.63 nm, as shown at a simulation time of 0 ps in Fig. 5. It is observed that the gyration radius first increases during the joining process, mainly due to the unfilled space near the interface. Nevertheless, although the elongated molecule near the nano-cavity is observed during the filling process (seen in Fig. 4a and b), the radius of gyration decreases rapidly, as shown in Fig. 5. It is demonstrated that once the PP layer is close to the substrate surface, the majority of PP molecules begin to bend. The whole layers are compressed because of the boundary restriction from the rigid substrate. Due to the fact that there is no nano-cavity in terms of smooth substrate, PP molecule shows largest and quickest deformation, from 2.63 nm at initiation to 2.37 nm at 3.4 ps. Oscillating phenomenon in polymer layer is also observed with the progress of joining process, which reveals the elongation and spring-back of polymer chains. Besides, the cooling process in the following simulation can be beneficial to the balancing process, in order to reach a balanced state. Because the atomic density at the interface is highly correlated with
3. Results and discussion 3.1. Joining process of hybrid structure Configurations of PP molecules penetrating the Al substrate with three different structures are shown in Fig. 4. Two PP chains in particular, one close to the metal substrate and the other away from the interface, were marked to analyze the structural differences in PP layer along z-axis during the process. For undercut structure, the PP molecules are pushed towards the nano-cavity by the injection molding force. The chain near the substrate is stretched along the interface, while the other marked chain is gradually compressed due to the restricted boundary. Conformational changes can be clearly observed under combined influence of the external force and the interfacial Table 1 Main parameters of PP atomistic models. Material PP
Degree of polymerization
Number of chains
Total amount of atoms
Initial density (g/cm3)
150
32
43,264
0.85
682
Box size (nm) 7.7 × 7.7 × 6.7
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Fig. 2. Atomistic models of Al substrate with different structures: (a) undercut structure, (b) vertical structure, and (c) smooth substrate.
force, interfacial failure occurs at the substrate. The PP chains gradually separate from the substrate, leading to the formation of distinctive voids. These voids expand and grow in size as the stretching of polymer chain continues. Particularly, some atoms are constrained by the undercut structure at 10 ps. At around 15 ps, the majority of PP molecules are pulled out. These chains near the substrate are highly oriented and elongated along z-direction. As shown in Fig. 7c, the end-to-end distance of the marked chain near the substrate increases from 5.7 nm at initiation to 26.8 nm at 15 ps. However the chain away from the substrate shows little conformational change during the whole separation. In general, it costs about 19 ps for the complete separation of PP layer. Once the layer is completely pulled out from the Al substrate, nonbonded interaction at the interface no longer exists. Further studies on the radius of gyration will be carried out to have a better understanding of the configuration changes during the separation. Radius of gyration in PP layer during the separation was calculated, as shown in Fig. 8. It is demonstrated that the radius of gyration first increases and afterwards gradually appear to equilibrate. In other words, the chains are stretched away from the substrate until the polymer layer is totally removed from the Al substrate. Soon after the complete separation, the radius of gyration drops slightly due to the elastic recovery of the stretched chains. Therefore a distinct peak is observed around the complete separation moment. For substrate with undercut structure, it implies greater flexibility for these chains because there are more chains filled into the cavity. In addition, the mechanical interlock further enhances the joining at the interface. As a result, the largest change in gyration radius can be observed in this case. The gyration radius of the stretched chains reaches to 4.24 nm at 30 ps, increased by 71.6%. Meanwhile, a longer time is required to reach the equilibrium state when the substrate is constructed with undercut structure. Compared to the decrease of gyration radius in the joining process, the increase of gyration radius during the separation is much higher regardless of the substrate structure. Under combined effects of non-bonded interaction at the interface and the pull-out force applied to the PP layer, polymer chains are highly stretched, which makes a good agreement with the conformation change shown in Fig. 7.
Fig. 3. Representative simulation procedure for PP-Al hybrid structure.
the interfacial interaction and the corresponding interface strength, average density along z-direction was calculated with a slice thickness of 0.2 nm. Fig. 6 represents the density profile at the end of the joining process with different substrate structures. All three profiles reach a peak at a distance of 5.2 nm, which is exactly within the interface area between PP and Al layer. This is mainly because the nearby atoms in PP layer are adsorbed to the interface surface with the influence of nonbonded interactions. As the distance between Al surface and PP molecules decreases, the greater oscillation range in PP density can be observed. Polymer melt must overcome greater resistance to flow into the cavity, so the density in nano-cavity is relatively lower than that outside the cavity, especially in the undercut area. In terms of undercut structure and vertical structure, higher densities are observed near the interface inside the nano-cavity, at distances approximately 2.0 nm for undercut structure and 3.0 nm for vertical structure. 3.2. Separation process of hybrid structure
3.3. Influence of surface structure
The snapshots of PP-Al hybrid structure with undercut structure during the separation are demonstrated in Fig. 7. As the separation progresses, conformational changes in PP layer are clearly observed. Polymer chains especially those close to the substrate surface are highly stretched, while the conformations of chains far away from the interface are relatively steady. PP layer is gradually displaced along z-axis and away from the Al substrate. Fig. 7a shows the snapshot of PP-Al hybrid structure at the simulation time of 5 ps. By analyzing the marked chain that partly penetrated the nano-cavity, it is found that the outside part of this chain is first stretched while the rest part still remains inside the cavity. As the polymer layer displaces further under the pull-out
The snapshots of two marked chains and the whole system with vertical structure and smooth substrate at the separation time of 5 ps are shown in Fig. 9. Like the situation with undercut structure, the outside part of the marked chain is obviously stretched while the other end of this chain is adherent to both side interfaces of the vertical cavity. For Al substrate with smooth surface, the middle part of the marked chain is adherent to the substrate due to the non-bonded interaction. On the other hand, both ends of the chain are stretched under the axial loading along z-direction. For all Al substrates, the other 683
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(a) Undercut structure
(b) Vertical structure
(c) Smooth substrate Fig. 4. Snapshots of two specific chains and the whole PP layer penetrating the Al substrate at a simulation time of 2 ps, 4 ps, and 10 ps during the joining process.
The results described above indicated that the interfacial interaction in polymer-metal interface is critical to the joining property. The strength of interfacial interaction between polymer and metal substrate can be evaluated by their interaction energy that is formed during the joining process. The interaction energy was calculated from Eq. (4):
Einter ation = Etotal − (Epolymer + Emetal )
(4)
where Etotal is the total potential energy of polymer-metal hybrid structure, Epolymer is the potential energy of PP without Al substrate, and Emetal is the potential energy of Al substrate without PP layer. Fig. 10a shows the interaction energy at PP-Al interface during the joining process. The interaction energy is determined by the close contact between PP and Al atoms that are within the cut-off distance of the nonbonded interaction as shown in Eq. (2). The negative value of energy indicates that the polymer and metal substrate attracts each other. It can be seen that all the interaction energies increase since more atoms in PP layer are within the cut-off distance during the joining process. Abrupt change in their slopes is observed at around 4 ps. This is mainly because some atoms near the interface are within the distance of σ at that time. The interactions between PP and Al atoms turn from negative to positive. As a result, these atoms bounce off the substrate surface due to the Lennard-Jones potential, like the elastic balls. The change in radius of gyration at the same time also demonstrates that the PP chains are slightly stretched, as shown in Fig. 5. Because there are more contact areas at Al substrate with undercut structure, it shows a higher
Fig. 5. Radii of gyration during the joining process with different substrate structures.
marked chain shows little conformational change, which means the chains that are away from the substrate are hardly influenced by the interfacial interaction.
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Fig. 6. Densities along z-direction at the end of the joining process with different substrate structures.
Fig. 8. Radii of gyration in PP layer during the separation with different substrate structures.
interaction energy at the end of filling, which is −10.904 × 106 cal/ mol. The joining strength in this case is higher than those at the other two interfaces. It indicates that the undercut structure can effectively enhance the interfacial interaction. Fig. 10b shows the interaction energy during the separation of PP-Al hybrid structure. The interaction energy goes to zero at the end of separation. The interfacial stress in PP layer during the separation is another indicator to evaluate the joining property. It can be calculated by dividing the inner force by the cross-sectional area at the interface [51]. In this present work, precise estimation of the cross section area can hardly be achieved. Therefore, we only analyze the inner force along zaxis versus simulation time during the separation. It was calculated by summing up the force in each atom in PP layer, then minus the applied
pull-out force. The inner force curves based on B-Spline modeling are shown in Fig. 11. Maximum inner force is observed at the early stage of the separation, at 1.2 ps, 1.3 ps and 2.0 ps for undercut structure, vertical structure, and smooth surface respectively. After that, the inner force is gradually reduced. A higher force is generated in PP-Al hybrid structure with undercut structure substrate, which means the interfacial strength is relatively stronger. Therefore, in the following section, we will mainly focus on the Al substrate with undercut structure by studying the influences of non-bonded interaction strength and pull-out force on the interfacial interaction and joining strength. For all substrate structures, the contact area at the interface, the interaction energy at the end of joining process, the maximum inner force and the work for a total separation of PP layer from the Al
Fig. 7. Snapshots of two specific chains and the whole system with undercut structure during the separation, at a simulation time of (a) 5 ps, (b) 10 ps, and (c) 15 ps. 685
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Fig. 11. Inner forces in PP layer during the separation with different substrate structures. Fig. 9. Snapshots of two specific chains and the whole system with (a) vertical structure and (b) smooth substrate at the separation time of 5 ps.
Fig. 12. Velocities of the PP layer during the separation with different substrate structures.
substrate were calculated. In particular, the work of separation was calculated by integrating the average inner force over the corresponding velocity at each moment. The velocity mentioned above is defined as the average velocity for the whole atoms in PP layer, by recording the speed for each atom in every 200 steps. The velocity grows gradually and reaches a steady state eventually, as shown in Fig. 12. The atoms in substrate with undercut structure show the lowest velocity due to the highest interfacial interaction. As shown in Table 2, all the interaction energy, maximum inner force, and the work of separation increase with the increasing contact area, but not simply proportional to the contact area. More non-bonded interactions are formed during the joining process, and therefore the joining strength will get stronger. By comparing the interfacial joining property between vertical structure and smooth substrate, both interaction energy and work of separation are largely increased. However, the maximum inner force doesn't increase that much, only with a percentage of 7.56%. By comparing the results in vertical structure and undercut structure, the contact area increases by 51.28%, while the interaction energy, maximum inner force, and the work of separation increase by 33.17%, 42.24%, and 42.89% respectively. A relatively higher force and more separation work are required compared to the increase of interaction energy. The result implies that the joining strength of PP-Al hybrid
Fig. 10. Interaction energies at PP-Al interface during (a) the joining process and (b) the separation with different substrate structures.
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Table 2 Comparisons between contact area, interaction energy, maximum inner force, and work of separation with different structures on Al substrate. Substrate structure
Contact area (nm2)
Interaction energy (cal/mol)
Maximum inner force (nN)
Work of separation (J)
59.3 90.1 136.3
−5.576 × 10 −8.189 × 106 −10.904 × 106
65.08 70.58 100.39
2.76 × 10−16 4.24 × 10−16 6.06 × 10−16
Smooth substrate Vertical structure Undercut structure
6
structure is further enhanced by the mechanical interlocking that was formed in the undercut area. 3.4. Influence of non-bonded interaction strength These results described above indicate that non-bonded interaction plays an important role in forming the polymer-metal interface. In direct injection joining technology, substrates such as aluminum, aluminum alloy, magnesium, steel, and etc. are most commonly used metallic materials. However, the combinations of different metal substrate and polymer materials definitely result in different interaction strengths. It is known from experiments that the joining strength of PPAl hybrid structure is relatively higher, compared to most other materials. In order to investigate the influence of non-bonded interaction strength, the energy magnitude in non-bonded potential (ε in Eq. (2)) between PP and Al atoms was reduced to varying degrees, from the initial value of 100% to 10%. Fig. 10 shows the inner force in PP layer under varying interaction strengths which are 100%, 40%, 20%, and 10% respectively. It is demonstrated that inner force first increases dramatically, reaching the highest value within 2.5 ps. With further removal of polymer layers, the inner force gradually reduces, indicating fewer polymer atoms are available to interact with Al atoms. After an oscillating downward phase, the inner force drops to zero or a relatively steady value, corresponding to the end of separation for the initial layer. With the increase of energy magnitude, the inner force during the separation process is obviously higher and more time is required to realize a complete separation, as shown in Fig. 13.
Fig. 14. Interaction energies at PP-Al interface with different pull-out forces during the separation.
Fig. 14. The greater pull-out force is, the less time for the total separation is required. It means that the failure of hybrid structure happens more quickly when a larger external load is applied. When the force applied on PP layer is higher than 0.01 kcal/mol∙Ȧ , the final interaction energy drops to 0 kcal/mol. It means that the distance between atom in PP layer and Al atom is beyond the cutoff distance. All the chains in PP layer are separated from the Al substrate. On the other hand, when the pull-out force is equal to or less than 0.01 kcal/mol∙Ȧ , molecules near the substrate stay adhered to the substrate due to the non-bonded interaction between PP and Al atoms. Particularly, the interaction energy at a simulation time of 45 ps is −6.756 × 106 cal/ mol when the applied force is 0.01 kcal/mol∙Ȧ . Compared to the initial interaction energy that is formed in the filling process, the decrease percentage of interaction energy is approximately 38.3%. By observing the snapshot at 45 ps, we can find that the PP layer is separated into two parts, which means cohesive failure occurs in the polymer bulk. Molecules that formed interlock mechanism in the joining process are mostly stuck on the substrate, while those molecules situated far away from the substrate are pulled out. It is demonstrated that the interfacial joining strength in this condition is higher than the stretching strength of the initial PP layer. However, interfacial failure occurs at the substrate in terms of undercut structure and smooth substrate. All the chains in PP layer are pulled out from Al substrate at the separation time of 45 ps.
3.5. Influence of pull-out force It is known that the polymer-metal hybrid structure is mainly used as the load-bearing automobile structures. Therefore, the failure behaviors under different loading conditions need to be analyzed. To explore the influence of pull-out force on the interfacial interaction, PP layer were removed away from Al substrate with different pull-out forces. The interaction energy during the separation process can be shown in
4. Conclusions In this study, molecular dynamics was used to study the interfacial interaction and joining property of direct injection-molded polymermetal hybrid structure. Interfacial models of PP-Al systems with different surface structures were constructed. The influences of surface structure, non-bonded interaction strength, and pull-out force were investigated comparatively. The simulation results illustrate that an obvious difference in polymer structure along z-axis is observed during the joining process. The radius of gyration first increases and afterwards decreases rapidly because of the compression and restriction from the
Fig. 13. Inner forces in PP layer during the separation with different interaction strengths. 687
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rigid substrate. Influenced by the interfacial interaction, the atomic density reaches a peak at the interface between PP and Al layer. As the separation progresses, polymer chains especially those close to the substrate surface are greatly stretched, while the conformations of chains far away from the interface are relatively steady. The change in radius of gyration during the separation is much higher regardless of the substrate structures. The contact area mainly contributes to the interaction energy, whereas the mechanical interlocking formed in the undercut area contributes to the maximum inner force and work of separation. Inner force first increases dramatically, reaching the highest value within 2 ps. As the polymer layers displace further, the chains separate from the Al substrate, leading to the formation of growing voids near the interface. The interfacial interaction is also affected by the non-bonded interaction strength and the pull-out force. Normally interfacial failure occurs at the substrate, except under conditions when the pull-out force is down to 0.01 kcal/mol∙Ȧ . In this case, cohesive failure occurs in the polymer bulk.
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Full length article
Interfacial interaction and joining property of direct injection-molded polymer-metal hybrid structures: A molecular dynamics simulation study Mingyong Zhoua,b, Xiang Xiongb, Dietmar Drummerc, Bingyan Jianga,
T
⁎
a
State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha, Hunan 410083, PR China Powder Metallurgy Research Institute, Central South University, Changsha, Hunan 410083, PR China c Institute of Polymer Technology (LKT), Friedrich-Alexander-University Erlangen-Nürnberg, Am Weichselgarten 9, 91058 Erlangen-Tennenlohe, Germany b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer-metal hybrid structure Injection molding Molecular dynamics Interfacial joining
Polymer-metal hybrid structures have been increasingly used to replace metallic components. Direct injection joining is one of the most promising technologies in fabricating polymer-metal hybrid structure. In this study, molecular dynamics method was used to study the interfacial interaction and joining property. Interfacial models with different surface structures on aluminum (Al) substrate were constructed. The influences of surface structure, non-bonded interaction strength, and pull-out force during the separation were systematically explored. Simulation results show that an obvious difference in polymer structure along z-axis is observed during the joining process. Influenced by the interfacial interaction, the atomic density is at its largest near the interface. As the separation continues, growing voids are observed near the interface. Polymer chains close to the Al substrate are greatly stretched, while the chains far away from the interface are relatively steady. Nevertheless, the changes in radius of gyration during the separation are much higher regardless of the substrate structures. The contact area mainly contributes to the interaction energy, whereas the mechanical interlocking formed in the undercut area contributes to the maximum inner force and work of separation. Moreover, the interfacial interaction is also affected by the non-bonded interaction strength and the pull-out force.
1. Introduction In recent years, polymer-metal hybrid structures have been increasingly used to replace all-steel load-bearing structural components for weight saving and cost efficiency purpose in automobiles, digital electronics, and other industrial devices, such as instrument panels and front-end modules [1–4]. In 1996, the polymer-metal hybrid front end in the Audi A6 was the first publicly reported case into the automotive manufacturing practice. The metal sheet was used as the main bearing structure while the glass fiber reinforced polyamide was injection over molded to connect the metal sheet, forming a dual-layer structure with improved mechanical property [5]. In this method, the joining strength is the key aspect in direct injection molding polymer-metal hybrid structure. And the joining processes usually involve adhesive bonding [6] and mechanical joining method [7]. Injection molding technology has attracted wide attention in the polymer industry due to advantages in low cost, simple process, and large volume production. It is the commonly used method in fabricating polymer-metal hybrid structure [8–10]. The metallic substrate is assembled in the injection mold, and subsequently over-molded with
⁎
polymer melt to achieve a strong joining. Direct injection joining, normally with surface treatment on the metallic substrate, is one of the most promising joining technologies for polymer-metal hybrid structure. Here, the surface treatment can be realized by sandblasting [11], laser processing [12,13], chemical etching [14], nano-molding technology [15,16], and etc. With these surface methods, the micro/nanopatterned texture is fabricated, thus enriching the contacting area of polymer melt on the metal surface. The joining strength at the interface can be then further improved. In particular, the nano-molding technology is utilized as a special surface treatment to form nano-pores on the aluminum alloy surface. The surface structure consisted of not a simple array of vertical nano-holes of approximately 20 nm but a threedimensionally foam network, which contributes to the formation of mechanical interlocking [15]. In direct injection joining technology, the joining of interface is mainly determined by the synergistic effects of non-bonded interaction in nanoscale, chemical bonding, and mechanical interlocking. However, the inner mechanism has not been sufficiently revealed, especially when the nano-molding technology is applied. In this case, the scale of surface structure on metal substrate is within tens of
Corresponding author. E-mail address: [email protected] (B. Jiang).
https://doi.org/10.1016/j.apsusc.2019.01.286 Received 20 August 2018; Received in revised form 29 January 2019; Accepted 30 January 2019 Available online 31 January 2019 0169-4332/ © 2019 Published by Elsevier B.V.
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computational capability restricted by MD theory. After that, the PP layer was heated up to the melt temperature of 503 K and relaxed with constant temperature for 10 ps, in order to reach a stable state for polymer melt in the actual condition. Al substrate was built with the same dimensions in both length and width as the PP layer. It is known that the surface roughness strengthens the joining property by either increasing the effective contact area or forming the mechanical interlocking [8,10,13,44]. In order to analyze the influence of surface treatment on the metal substrate in nano-molding technology, three different structures including undercut structure, vertical structure, and the smooth substrate were constructed on the Al substrate, as shown in Fig. 2. For undercut structure, the lateral “T” shape cavity was built to represent the micro/ nano interlock structure that was formed by micro-arc oxidation or other electrochemical treatment [10,44]. Dimensions of the inner cavity were 4 nm in y-axis and 1 nm in z-axis, while the size of the outer cavity was 2 nm in both directions. The vertical structure and the smooth substrate were constructed to compare the influence of increased contact area on metal surface. As described in the introduction section, the surface structure/roughness in direct injection-molded process is a few tens nanometers. Despite the limitation of dimension scale in MD simulation, this work can provide a qualitative estimation of interfacial characteristics and the separation behavior at the molecular level, which is impossible in a simulation based on continuum mechanics [29]. The Al layer was kept at a constant mold temperature of 353 K. The hybrid system for MD simulation was then built by assembling the PP layer and Al layer together, in which the Al layer was the substrate. Periodic boundary conditions in both x and y-axis were utilized for approximating a large system, while the non-periodic and shrinkwrapped boundary condition along z-axis was set so that polymer chains in PP layer can be shrunk or stretched during the whole process. All the atoms were encompassed in z-axis, thus the interaction between atoms on the top of PP layer and Al atoms at the bottom could be avoided. In order to get access to the actual processing condition, energy minimization and subsequently anneal treatment were done to optimize the molecule conformation in the polymer layer.
nanometers. Due to the obvious particle characteristic and scale effect, the flow behavior of polymer melt is highly affected by those factors that is commonly ignored in conventional injection molding, like wall slip, surface interaction, intermolecular forces, and so on [17–20]. In particular, the interfacial interaction is more relevant and sensitive to the surface quality of the metal insert or metal substrate [21,22]. It has been reported that when the surface roughness of metal insert is down to 16 nm, the strength of interfacial interaction, such as van der Waals force, is drastically increased, which results in a strong adhesion between the polymer and metal substrate [23]. Although experimental researches have been undertaken to analyze the influences of processing materials and parameters, the development time would be substantially decreased if sufficiently robust simulation results allow the determination of the interfacial behaviors. Simulation methods based on continuum mechanics, however, fail to accurately predict the polymer behavior in nanoscale because of the drastic changes in material properties. Conventional analysis for interface needs data about the mechanical behavior of the interphase that is difficult to obtain from direct measurement. Recently, molecular dynamics (MD) simulation has attracted great attention for the analysis in nanometer scale. Atomic movements that are allowed to interact can be observed by this method, giving a view of the dynamic evolution [24–26]. Through MD modeling at atomistic scale, we can predict not only the flow behavior of single polymer in molding process like nanoimprint lithography [27–29] and injection molding [30–32], but also investigate the viscoelastic property [33,34] and the interfacial interaction of polymer composites [35–38] or polymer-mold insert hybrid [29,39,40]. However, there are few MD studies focused on the interfacial joining of polymer-metal hybrid structure, such as the bonding process between vinyl ester polymer and steel [41–43]. Besides that, MD study on the polymer-metal interface should focus on not only the joining process but also the separation process, since little is known regarding the joining mechanism at the polymer-metal interface in nanoscale. Nevertheless, it is theoretically feasible to reveal the interfacial interaction of injection-molded polymer-metal hybrid structure by MD method. The main purpose of this paper is to provide insights into the interfacial interaction and joining property of direct injection-molded polymer-metal hybrid structure by using MD method. In this study, interface model of polypropylene-aluminum (PP-Al) was constructed in accordance with the real experimental processing. Metal substrates with different surface structures were built to study the effect of surface treatment in direct injection joining technology. Both joining and separation process of the hybrid structure were investigated by analyzing the interaction energy, radius of gyration, inner force, and conformation change of polymer chains. Meanwhile, the influences of parameters such as non-bonded interaction strength and pull-out force were investigated comparatively. The output of this study will provide useful reference for future design in polymer-metal hybrid structure.
2.2. Force field The consistent valence force field (CVFF) was used to represent the intermolecular and non-bonded interaction between atoms in polymer layer. In this work, bond stretching potential, angular bending potential, torsion potential, and non-bonded interactions were selected [26,30,45,46]. Standard Lennard-Jones 12–6 potential and Coulombic pairwise interaction were adopted to describe the non-bonded interactions between the substrate and the polymer, as shown in Eqs. (1) and (2)
Utotal = Ubond + Uangle + Utorsion + Unon − bonded 2. Simulation model and methodology
= kb (r − r0 )2 + ka (θ − θ0 )2 + kt (1 + cos(nϕ − ϕ0)) + Unonbond (1)
2.1. Model constructing
Unon − bonded = UL − J + UCoulombic = 4ε ((σ / r )12 − (σ / r )6) + qi qj / r
Before assembling the injection-molded hybrid system, the polymer and metal layer were separately prepared. Amorphous polypropylene (PP) and aluminum (Al) were selected as the two layers in this study since they are commonly used in polymer-metal hybrid structure [1,8,11]. Atomistic model of the PP layer for MD simulation was constructed by Materials Studio, as shown in Fig. 1. The layer was in a rectangular cuboid with dimensions 7.7 nm × 7.7 nm in length and width. The other settings, such as the degree of polymerization, number of chains, initial density, and box size, are shown in Table 1. Compared to that with real material, the degree of polymerization in the simulation was relatively small. This is because the dimension scale in this simulation was also reduced to several nanometers due to the limited
(2)
where the kb, ka, kt, and ε were constants of the bond stretching potential, angular bending potential, torsion potential, and non-bonded interaction respectively. Parameters were derived from the lattice constant and adhesive energy of Al atom. The cut-off distance of both potentials in non-bonded interaction was 1.25 nm. Other values of the constants mentioned above were given in the supplementary material S1. It has been seen that the Lennard-Jones 12–6 potential and Coulombic pairwise interaction can effectively describe the polymer–metal interaction in nanoimprint lithography [31] and nano-injection molding process [29], or with calculated result in good agreement with experiment for interfacial property [47–49]. 681
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Fig. 1. Atomistic models of (a) a single PP chain and (b) the whole layer.
interaction. It is demonstrated in Fig. 4a that the undercut structure cavity is fully filled at 10 ps, forming a mechanical interlock at the interface. Since molecules are penetrating the nano-cavity, the total height of the hybrid structure is decreasing. Similar phenomenon is observed in the other two models with vertical structure and smooth substrate. For Al layer with smooth substrate, both two chains are confined and twisted because of the restrained boundary. It is demonstrated that the conformational change of the chain away from the interface is hardly influenced by the structure on Al surface. In this present work, the final configuration of PP-Al hybrid structure after the cooling process is used as the initial atomistic model for the separation simulation. In order to quantify the capacity of molecule to adopt a more compact conformation, the radius of gyration (Rg) in PP layer during MD simulation was calculated from Eq. (3):
2.3. Simulation procedure One representative simulation procedure for PP-Al hybrid structure by direct injection joining is shown in Fig. 3. In this study, the joining process and the separation process were mainly focused. All the Al atoms were positionally restrained by setting each component of force to zero during the whole simulation. In this case, these atoms were kept constrained and the metal layer was treated as a rigid body since the metal substrate has a much higher stiffness than that of common polymer. A negative force of 0.16 kcal/mol∙Ȧ along z-axis was applied to each atom in PP layer, so that molecules were able to be filled into the nano-cavity structure with excellent replication fidelity. At the same time, the PP layer was gradually cooled down to the mold temperature (353 K) in 10 ps in a constant particle number, volume, and temperature (NVT) ensemble. Once the joining process ended, the whole system was then cooled down to 303 K in another 10 ps. The next step was to study the joining strength under the separation process. In this step, an external force of 0.1 kcal/mol∙Ȧ (equal to 6.95 pN) along z-axis was applied to pull out the PP layer from the Al substrate. The interfacial interaction and joining property would be further studied by varying the pull-out force from 0.5 kcal/mol∙Ȧ to 0.01 kcal/mol∙Ȧ and by decreasing the constant value (ε) in non-bonded interaction from 100% to 10%. All the MD simulations mentioned above were performed by LAMMPS [50], an open source code for Large-scale Atomic/Molecular Massively Parallel Simulator in a computer cluster.
N
Rg 2 =
N
∑ mi ri2/ ∑ mi i=1
(3)
i=1
where mi is the atomic weight and ri is the distance of an atom to the mass centre of a single molecule. If Rg increases, it indicates the elongation of the whole layer, while its reduction illustrates the contraction of the polymer molecules. Regarding the relaxed PP layer without substrate, the radius of gyration is 2.63 nm, as shown at a simulation time of 0 ps in Fig. 5. It is observed that the gyration radius first increases during the joining process, mainly due to the unfilled space near the interface. Nevertheless, although the elongated molecule near the nano-cavity is observed during the filling process (seen in Fig. 4a and b), the radius of gyration decreases rapidly, as shown in Fig. 5. It is demonstrated that once the PP layer is close to the substrate surface, the majority of PP molecules begin to bend. The whole layers are compressed because of the boundary restriction from the rigid substrate. Due to the fact that there is no nano-cavity in terms of smooth substrate, PP molecule shows largest and quickest deformation, from 2.63 nm at initiation to 2.37 nm at 3.4 ps. Oscillating phenomenon in polymer layer is also observed with the progress of joining process, which reveals the elongation and spring-back of polymer chains. Besides, the cooling process in the following simulation can be beneficial to the balancing process, in order to reach a balanced state. Because the atomic density at the interface is highly correlated with
3. Results and discussion 3.1. Joining process of hybrid structure Configurations of PP molecules penetrating the Al substrate with three different structures are shown in Fig. 4. Two PP chains in particular, one close to the metal substrate and the other away from the interface, were marked to analyze the structural differences in PP layer along z-axis during the process. For undercut structure, the PP molecules are pushed towards the nano-cavity by the injection molding force. The chain near the substrate is stretched along the interface, while the other marked chain is gradually compressed due to the restricted boundary. Conformational changes can be clearly observed under combined influence of the external force and the interfacial Table 1 Main parameters of PP atomistic models. Material PP
Degree of polymerization
Number of chains
Total amount of atoms
Initial density (g/cm3)
150
32
43,264
0.85
682
Box size (nm) 7.7 × 7.7 × 6.7
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Fig. 2. Atomistic models of Al substrate with different structures: (a) undercut structure, (b) vertical structure, and (c) smooth substrate.
force, interfacial failure occurs at the substrate. The PP chains gradually separate from the substrate, leading to the formation of distinctive voids. These voids expand and grow in size as the stretching of polymer chain continues. Particularly, some atoms are constrained by the undercut structure at 10 ps. At around 15 ps, the majority of PP molecules are pulled out. These chains near the substrate are highly oriented and elongated along z-direction. As shown in Fig. 7c, the end-to-end distance of the marked chain near the substrate increases from 5.7 nm at initiation to 26.8 nm at 15 ps. However the chain away from the substrate shows little conformational change during the whole separation. In general, it costs about 19 ps for the complete separation of PP layer. Once the layer is completely pulled out from the Al substrate, nonbonded interaction at the interface no longer exists. Further studies on the radius of gyration will be carried out to have a better understanding of the configuration changes during the separation. Radius of gyration in PP layer during the separation was calculated, as shown in Fig. 8. It is demonstrated that the radius of gyration first increases and afterwards gradually appear to equilibrate. In other words, the chains are stretched away from the substrate until the polymer layer is totally removed from the Al substrate. Soon after the complete separation, the radius of gyration drops slightly due to the elastic recovery of the stretched chains. Therefore a distinct peak is observed around the complete separation moment. For substrate with undercut structure, it implies greater flexibility for these chains because there are more chains filled into the cavity. In addition, the mechanical interlock further enhances the joining at the interface. As a result, the largest change in gyration radius can be observed in this case. The gyration radius of the stretched chains reaches to 4.24 nm at 30 ps, increased by 71.6%. Meanwhile, a longer time is required to reach the equilibrium state when the substrate is constructed with undercut structure. Compared to the decrease of gyration radius in the joining process, the increase of gyration radius during the separation is much higher regardless of the substrate structure. Under combined effects of non-bonded interaction at the interface and the pull-out force applied to the PP layer, polymer chains are highly stretched, which makes a good agreement with the conformation change shown in Fig. 7.
Fig. 3. Representative simulation procedure for PP-Al hybrid structure.
the interfacial interaction and the corresponding interface strength, average density along z-direction was calculated with a slice thickness of 0.2 nm. Fig. 6 represents the density profile at the end of the joining process with different substrate structures. All three profiles reach a peak at a distance of 5.2 nm, which is exactly within the interface area between PP and Al layer. This is mainly because the nearby atoms in PP layer are adsorbed to the interface surface with the influence of nonbonded interactions. As the distance between Al surface and PP molecules decreases, the greater oscillation range in PP density can be observed. Polymer melt must overcome greater resistance to flow into the cavity, so the density in nano-cavity is relatively lower than that outside the cavity, especially in the undercut area. In terms of undercut structure and vertical structure, higher densities are observed near the interface inside the nano-cavity, at distances approximately 2.0 nm for undercut structure and 3.0 nm for vertical structure. 3.2. Separation process of hybrid structure
3.3. Influence of surface structure
The snapshots of PP-Al hybrid structure with undercut structure during the separation are demonstrated in Fig. 7. As the separation progresses, conformational changes in PP layer are clearly observed. Polymer chains especially those close to the substrate surface are highly stretched, while the conformations of chains far away from the interface are relatively steady. PP layer is gradually displaced along z-axis and away from the Al substrate. Fig. 7a shows the snapshot of PP-Al hybrid structure at the simulation time of 5 ps. By analyzing the marked chain that partly penetrated the nano-cavity, it is found that the outside part of this chain is first stretched while the rest part still remains inside the cavity. As the polymer layer displaces further under the pull-out
The snapshots of two marked chains and the whole system with vertical structure and smooth substrate at the separation time of 5 ps are shown in Fig. 9. Like the situation with undercut structure, the outside part of the marked chain is obviously stretched while the other end of this chain is adherent to both side interfaces of the vertical cavity. For Al substrate with smooth surface, the middle part of the marked chain is adherent to the substrate due to the non-bonded interaction. On the other hand, both ends of the chain are stretched under the axial loading along z-direction. For all Al substrates, the other 683
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(a) Undercut structure
(b) Vertical structure
(c) Smooth substrate Fig. 4. Snapshots of two specific chains and the whole PP layer penetrating the Al substrate at a simulation time of 2 ps, 4 ps, and 10 ps during the joining process.
The results described above indicated that the interfacial interaction in polymer-metal interface is critical to the joining property. The strength of interfacial interaction between polymer and metal substrate can be evaluated by their interaction energy that is formed during the joining process. The interaction energy was calculated from Eq. (4):
Einter ation = Etotal − (Epolymer + Emetal )
(4)
where Etotal is the total potential energy of polymer-metal hybrid structure, Epolymer is the potential energy of PP without Al substrate, and Emetal is the potential energy of Al substrate without PP layer. Fig. 10a shows the interaction energy at PP-Al interface during the joining process. The interaction energy is determined by the close contact between PP and Al atoms that are within the cut-off distance of the nonbonded interaction as shown in Eq. (2). The negative value of energy indicates that the polymer and metal substrate attracts each other. It can be seen that all the interaction energies increase since more atoms in PP layer are within the cut-off distance during the joining process. Abrupt change in their slopes is observed at around 4 ps. This is mainly because some atoms near the interface are within the distance of σ at that time. The interactions between PP and Al atoms turn from negative to positive. As a result, these atoms bounce off the substrate surface due to the Lennard-Jones potential, like the elastic balls. The change in radius of gyration at the same time also demonstrates that the PP chains are slightly stretched, as shown in Fig. 5. Because there are more contact areas at Al substrate with undercut structure, it shows a higher
Fig. 5. Radii of gyration during the joining process with different substrate structures.
marked chain shows little conformational change, which means the chains that are away from the substrate are hardly influenced by the interfacial interaction.
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Fig. 6. Densities along z-direction at the end of the joining process with different substrate structures.
Fig. 8. Radii of gyration in PP layer during the separation with different substrate structures.
interaction energy at the end of filling, which is −10.904 × 106 cal/ mol. The joining strength in this case is higher than those at the other two interfaces. It indicates that the undercut structure can effectively enhance the interfacial interaction. Fig. 10b shows the interaction energy during the separation of PP-Al hybrid structure. The interaction energy goes to zero at the end of separation. The interfacial stress in PP layer during the separation is another indicator to evaluate the joining property. It can be calculated by dividing the inner force by the cross-sectional area at the interface [51]. In this present work, precise estimation of the cross section area can hardly be achieved. Therefore, we only analyze the inner force along zaxis versus simulation time during the separation. It was calculated by summing up the force in each atom in PP layer, then minus the applied
pull-out force. The inner force curves based on B-Spline modeling are shown in Fig. 11. Maximum inner force is observed at the early stage of the separation, at 1.2 ps, 1.3 ps and 2.0 ps for undercut structure, vertical structure, and smooth surface respectively. After that, the inner force is gradually reduced. A higher force is generated in PP-Al hybrid structure with undercut structure substrate, which means the interfacial strength is relatively stronger. Therefore, in the following section, we will mainly focus on the Al substrate with undercut structure by studying the influences of non-bonded interaction strength and pull-out force on the interfacial interaction and joining strength. For all substrate structures, the contact area at the interface, the interaction energy at the end of joining process, the maximum inner force and the work for a total separation of PP layer from the Al
Fig. 7. Snapshots of two specific chains and the whole system with undercut structure during the separation, at a simulation time of (a) 5 ps, (b) 10 ps, and (c) 15 ps. 685
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Fig. 11. Inner forces in PP layer during the separation with different substrate structures. Fig. 9. Snapshots of two specific chains and the whole system with (a) vertical structure and (b) smooth substrate at the separation time of 5 ps.
Fig. 12. Velocities of the PP layer during the separation with different substrate structures.
substrate were calculated. In particular, the work of separation was calculated by integrating the average inner force over the corresponding velocity at each moment. The velocity mentioned above is defined as the average velocity for the whole atoms in PP layer, by recording the speed for each atom in every 200 steps. The velocity grows gradually and reaches a steady state eventually, as shown in Fig. 12. The atoms in substrate with undercut structure show the lowest velocity due to the highest interfacial interaction. As shown in Table 2, all the interaction energy, maximum inner force, and the work of separation increase with the increasing contact area, but not simply proportional to the contact area. More non-bonded interactions are formed during the joining process, and therefore the joining strength will get stronger. By comparing the interfacial joining property between vertical structure and smooth substrate, both interaction energy and work of separation are largely increased. However, the maximum inner force doesn't increase that much, only with a percentage of 7.56%. By comparing the results in vertical structure and undercut structure, the contact area increases by 51.28%, while the interaction energy, maximum inner force, and the work of separation increase by 33.17%, 42.24%, and 42.89% respectively. A relatively higher force and more separation work are required compared to the increase of interaction energy. The result implies that the joining strength of PP-Al hybrid
Fig. 10. Interaction energies at PP-Al interface during (a) the joining process and (b) the separation with different substrate structures.
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Table 2 Comparisons between contact area, interaction energy, maximum inner force, and work of separation with different structures on Al substrate. Substrate structure
Contact area (nm2)
Interaction energy (cal/mol)
Maximum inner force (nN)
Work of separation (J)
59.3 90.1 136.3
−5.576 × 10 −8.189 × 106 −10.904 × 106
65.08 70.58 100.39
2.76 × 10−16 4.24 × 10−16 6.06 × 10−16
Smooth substrate Vertical structure Undercut structure
6
structure is further enhanced by the mechanical interlocking that was formed in the undercut area. 3.4. Influence of non-bonded interaction strength These results described above indicate that non-bonded interaction plays an important role in forming the polymer-metal interface. In direct injection joining technology, substrates such as aluminum, aluminum alloy, magnesium, steel, and etc. are most commonly used metallic materials. However, the combinations of different metal substrate and polymer materials definitely result in different interaction strengths. It is known from experiments that the joining strength of PPAl hybrid structure is relatively higher, compared to most other materials. In order to investigate the influence of non-bonded interaction strength, the energy magnitude in non-bonded potential (ε in Eq. (2)) between PP and Al atoms was reduced to varying degrees, from the initial value of 100% to 10%. Fig. 10 shows the inner force in PP layer under varying interaction strengths which are 100%, 40%, 20%, and 10% respectively. It is demonstrated that inner force first increases dramatically, reaching the highest value within 2.5 ps. With further removal of polymer layers, the inner force gradually reduces, indicating fewer polymer atoms are available to interact with Al atoms. After an oscillating downward phase, the inner force drops to zero or a relatively steady value, corresponding to the end of separation for the initial layer. With the increase of energy magnitude, the inner force during the separation process is obviously higher and more time is required to realize a complete separation, as shown in Fig. 13.
Fig. 14. Interaction energies at PP-Al interface with different pull-out forces during the separation.
Fig. 14. The greater pull-out force is, the less time for the total separation is required. It means that the failure of hybrid structure happens more quickly when a larger external load is applied. When the force applied on PP layer is higher than 0.01 kcal/mol∙Ȧ , the final interaction energy drops to 0 kcal/mol. It means that the distance between atom in PP layer and Al atom is beyond the cutoff distance. All the chains in PP layer are separated from the Al substrate. On the other hand, when the pull-out force is equal to or less than 0.01 kcal/mol∙Ȧ , molecules near the substrate stay adhered to the substrate due to the non-bonded interaction between PP and Al atoms. Particularly, the interaction energy at a simulation time of 45 ps is −6.756 × 106 cal/ mol when the applied force is 0.01 kcal/mol∙Ȧ . Compared to the initial interaction energy that is formed in the filling process, the decrease percentage of interaction energy is approximately 38.3%. By observing the snapshot at 45 ps, we can find that the PP layer is separated into two parts, which means cohesive failure occurs in the polymer bulk. Molecules that formed interlock mechanism in the joining process are mostly stuck on the substrate, while those molecules situated far away from the substrate are pulled out. It is demonstrated that the interfacial joining strength in this condition is higher than the stretching strength of the initial PP layer. However, interfacial failure occurs at the substrate in terms of undercut structure and smooth substrate. All the chains in PP layer are pulled out from Al substrate at the separation time of 45 ps.
3.5. Influence of pull-out force It is known that the polymer-metal hybrid structure is mainly used as the load-bearing automobile structures. Therefore, the failure behaviors under different loading conditions need to be analyzed. To explore the influence of pull-out force on the interfacial interaction, PP layer were removed away from Al substrate with different pull-out forces. The interaction energy during the separation process can be shown in
4. Conclusions In this study, molecular dynamics was used to study the interfacial interaction and joining property of direct injection-molded polymermetal hybrid structure. Interfacial models of PP-Al systems with different surface structures were constructed. The influences of surface structure, non-bonded interaction strength, and pull-out force were investigated comparatively. The simulation results illustrate that an obvious difference in polymer structure along z-axis is observed during the joining process. The radius of gyration first increases and afterwards decreases rapidly because of the compression and restriction from the
Fig. 13. Inner forces in PP layer during the separation with different interaction strengths. 687
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rigid substrate. Influenced by the interfacial interaction, the atomic density reaches a peak at the interface between PP and Al layer. As the separation progresses, polymer chains especially those close to the substrate surface are greatly stretched, while the conformations of chains far away from the interface are relatively steady. The change in radius of gyration during the separation is much higher regardless of the substrate structures. The contact area mainly contributes to the interaction energy, whereas the mechanical interlocking formed in the undercut area contributes to the maximum inner force and work of separation. Inner force first increases dramatically, reaching the highest value within 2 ps. As the polymer layers displace further, the chains separate from the Al substrate, leading to the formation of growing voids near the interface. The interfacial interaction is also affected by the non-bonded interaction strength and the pull-out force. Normally interfacial failure occurs at the substrate, except under conditions when the pull-out force is down to 0.01 kcal/mol∙Ȧ . In this case, cohesive failure occurs in the polymer bulk.
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