1 Introduction

Multi-material hybrid structures exhibit various useful properties, including both low density and high performance. In particular, metal-polymer hybrids are expected to replace metals in industrial applications because they can reduce weight compared with pure metals and so lower costs, such as by providing improved fuel efficiency in the aerospace and automotive sectors [1,2,3,4,5]. Thermoplastics are also advantageous because they can be joined without the use of adhesives [6, 7] or items such as screws and rivets [8, 9]. To date, these materials have been joined via thermal press fitting using ultrasonic [10,11,12], induction [13], laser [14,15,16,17] or solid friction [18] heat sources. The metals-polymers direct bonding is known to result primarily from hydrogen bonds between oxides formed on metal surfaces and polar functional groups (amino groups, hydroxyl groups, carboxyl groups, etc.) present on polymers. Thus, the addition of functional groups to the surface of polymer using some surface treatment methods is required to obtain high strength, high quality direct metal-polymer bonding. At present, these surface treatment methods include chemical etching using acids and bases [19] and exposure to ultraviolet radiation [20], coronas [21] or plasmas [22]. Among these, plasma treatments are considered to be superior in that these techniques can modify only the polymer surface [22].

The present study performed surface treatments employing an atmospheric pressure plasma jet excited by radio frequency (RF) power. The oxygen radical density in such plasmas is two orders of magnitude higher than that in conventional high-voltage low-frequency atmospheric pressure plasma jets [23, 24]. On this basis, the process demonstrated herein was expected to efficiently add polar functional groups on polymer surfaces. In a prior work, stainless (SUS304) steel and polycarbonate (PC) were joined by joining technology by a combination of atmospheric pressure RF plasma jet and thermal press fitting [25].

In engineering plastics including PC and PEEK, it is difficult to modify the polymer surfaces using a low-frequency pulse plasma source because the irradiated surface is not heated to the temperature required for the surface reaction of radicals generated by low-frequency pulse plasma irradiation [25]. The atmospheric pressure RF-excited atmospheric plasma jet used in this study is more efficient than the common low-frequency high-voltage pulsed atmospheric plasma, since the plasma jet not only produces high-density plasmas but also promotes surface reactions by heating through plasma irradiation, thus allowing the surface treatment of the polymer and the metal without preheating the surface with other heating sources [25]. The results of this previous research demonstrated the feasibility of bonding metal-polymer dissimilar materials using this technology.

Modern automobile bodies often incorporate aluminum (Al) alloys or hybrids of Al alloys with other materials to achieve both weight reduction and rigidity [1, 26,27,28,29]. Thus, the development of technologies for the joining of Al alloys to materials such as engineering plastics is desirable. In the work reported herein, direct joining of an Al alloy (A5052) to the engineering plastic (poly ether ether ketone: PEEK) was performed using the plasma-assisted process and the influence of irradiating an atmospheric pressure plasma on the bonding strength of A5052-PEEK joints was investigated.

2 Experimental procedures

An atmospheric pressure RF plasma jet using Ar gas was employed for plasma-assisted direct joining. The plasma source consisted of a quartz tube wound with a wide (15 mm) metal strip serving as the electrode, to which high-frequency power was applied, and a narrow (5 mm) strip acting as a ground electrode (Fig. 1(a)). The narrow electrode (grounding electrode) was located at the quartz tube end along the path of gas flow, while the wide electrode to which RF power is applied was situated 5 mm from the top edge of the narrow electrode. A quartz tube (outer diameter: 6 mm and inner diameter: 4 mm) was used and high-frequency (60 MHz) sine wave power was applied to the power electrode. The pure Ar was feed to the plasma source at 3slm in gas flow rate [23, 24].

Fig. 1
figure 1

Diagrams showing (a) the apparatus used to generate an atmospheric pressure RF plasma jet, (b) the dimensions of the specimen and (c) tensile-shear configuration for A5052 and PEEK direct joining with thermal press fitting via pre-plasma treatment

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The test pieces used in this work comprised sheets of Al alloy (A5052) or PEEK (Mitsubishi chemical advanced materials, Ketron 1000), both having 500 mm × 15 mm × 5 mm in dimensions. Figure 1(b) and (c) shows the dimensions of the specimen and tensile-shear configuration for A5052 and PEEK direct joining with thermal press fitting via pre-plasma treatment. Figure 2 shows the procedure for metal-polymer direct joining based on atmospheric pressure RF plasma jet irradiation. In this process, the plasma jet was applied to the surfaces of one or both materials as a pre-treatment. After exposure to plasma, A5052 was heated by a heater until the temperature of A5052 reached 320°C, close to the melting point of PEEK, and then thermal press fitting was performed. The test pieces of metal and polymer were joined such that the two materials overlapped by 10 mm.

Fig. 2
figure 2

Procedure for metal-polymer direct joining based on atmospheric pressure RF plasma jet irradiation

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In order to evaluate bond strength, specimens were also bonded using an epoxy adhesive and the tensile shear stress for these samples was compared with that for specimens jointed using the thermal press fitting. Tensile tests were demonstrated employing a tensile tester (Autograph AGS-X, Shimadzu Corp.). In this measurement, a shear force acted on the bond interface at a crosshead speed of 1.66 × 10−3 mm/s and a tensile shear stress was evaluated by measuring the maximum load at failure of specimens bonded under various conditions the maximum load at failure for specimens bonded under various conditions.

To investigate the physical properties of surfaces after irradiation of plasma, the surface morphology of A5052 was assessed using scanning electron microscopy (SEM, Hitachi SU-70) together with energy dispersive X-ray spectroscopy (EDX, INCA PentaFETx3, Oxford Instruments) and atomic force microscopy (AFM, KEYENCE VN-8000). To evaluate the chemical properties of surfaces after plasma irradiation, the chemical bonding state of a surface of plasma-exposed polymer was analyzed using X ray photoelectron spectroscopy (XPS, AXIS165, Shimadzu).

3 Results and discussion

The plasma irradiation effect of the A5052 and PEEK surfaces on the bond was assessed by applying plasma treatments to only the A5052 or the PEEK or to both. Figure 3 summarizes the tensile shear strength for the various A5052-PEEK samples bonded using a combination of the thermal press fitting method and the plasma-assisted joining. The results of a sample of untreated A5052 and PEEK bonded by thermal press fitting and adhesive are also shown in Fig. 3 for comparison. The tensile shear strength of specimens of untreated A5052 and PEEK bonded by thermal press fitting and adhesive was 10.2 and 8.6 MPa. In bonding with epoxy adhesives, it is known that the bonding is achieved by hydrogen bonding or ionic bonding between functional groups such as OH and NH in the epoxy resin and oxygen functional groups present on the metal/polymer surface [30, 31]. Therefore, it is considered that adhesive and t thermal press fitting result in almost the same bond strength because adhesion and thermal bonding are similar bonding mechanisms. The average bond strength for the samples for which only the A5052 was exposed to the plasma was 0.6 MPa. In contrast, exposing only the PEEK to the plasma produced a significant increase in bond strength to 15.5 MPa, and the bond strength was greater than that for the unirradiated samples by a factor of 1.5. On the other hand, the bond strength of the sample plasma-treated with both A5052 and PEEK was 12.1 MPa, which was slightly lower than that of the sample irradiated only with PEEK. Figure 4 presents photographic images of the fracture surfaces of specimens bonded with and without plasma irradiation after tensile testing. The plasma irradiation on the A5052 side caused greater delamination, and cohesive breakdown was observed only in a small area. On the other hand, the sample without plasma irradiation on the A5052 side showed cohesive breakdown of PEEK in a larger area than in the sample with plasma irradiation on A5052 side, although delamination of the interface was also pronounced. These results indicate that the plasma treatment of PEEK is effective in improving bond strength in the direct bonding of A5052-PEEK. In contrast, specimens in which the metal had been irradiated exhibited decreased bond strength.

Fig. 3
figure 3

Tensile shear strength for samples jointed by the thermal press fitting after plasma irradiation or the thermal press fitting with untreated A5052 and PEEK

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Fig. 4
figure 4

Photographic images of fracture surfaces after tensile testing of specimens bonded with and without plasma irradiation

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To investigate the cause of the decrease in bond strength due to plasma irradiation of A5052, the physical and chemical analysis of the A5052 surface before and after plasma irradiation was performed. The physical and chemical states of material surfaces are known to play important roles in the direct joining of dissimilar materials. The surface morphology of A5052 surfaces before and after irradiation of the plasma was observed using AFM. Figure 5 plots the surface roughness, Ra, as estimated from AFM images as a function of exposure time. With increasing plasma irradiation time, the Ra of the A5052 surface increased slightly, from 208 nm at 0 min (that is, the pristine surface) to 247 nm after 5 min. Considering the physical aspects of joining dissimilar materials, it is generally known that an increase in surface roughness Ra increases the bond strength due to the mechanical interlocking (anchor effect). However, when A5052 is irradiated with plasma, bonding strength is dramatically improved with plasma irradiation compared to PEEK without plasma irradiation. On the other hand, there is only a slight increase in surface roughness due to plasma irradiation. Therefore, these data suggest that the influence of physical effects including anchor effect on the difference in bond strength with and without plasma irradiation of PEEK is small. In a previous study, it was confirmed that plasma irradiation adds oxygen functional groups (C=O and O-C=O) to the PEEK surface, and that the oxygen functional groups increase with increasing plasma irradiation time. [32] In this experiment, there was no significant increase in the surface roughness of A5052 due to plasma irradiation, and the bond strength increased only due to plasma irradiation on the PEEK side. These results indicate that the addition of functional group on PEEK surface, not the anchoring effect on the A5052 side, contributed to the increase in bond strength. These data therefore confirm that the increased bond strength of the samples after plasma exposure was primarily a result of chemical effects rather than physical effects.

Fig. 5
figure 5

Variation in surface roughness, Ra, for A5052 with plasma irradiation time

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Variation in the chemical composition of the surface of A5052 after irradiation of the plasma was also investigated by SEM-EDX. Figure 6 provides SEM images and SEM-EDX elemental maps of Al (blue), Mg (purple), O (red), Cr (yellow) and Fe (green) acquired from the surface of A5052 before and after 5 min of plasma exposure. These images indicate that minimal physical change was induced by irradiation. However, the O maps show some slight oxidation while the Mg maps indicate a significant increase in the amount of Mg across the entire metal surface after plasma treatment. Figure 7 summarizes the Mg/Al and O/Al ratios estimated from SEM-EDX elemental maps as functions of the plasma irradiation time. These results demonstrate that the plasma treatment increased the concentrations of both O and Mg on the A5052 surface. The increase in the relative atomic concentration of O is due to oxidation on A5052 by reactive oxygen species generated by the plasma jet.

Fig. 6
figure 6

SEM images and SEM-EDX elemental maps of Al (blue), Mg (purple), O (red), Cr (yellow) and Fe (green) on the surface of (a) untreated A5052 and (b) A5052 exposed to plasma for 5 min

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Fig. 7
figure 7

Variation of the ratios of O and Mg to Al on an A5052 surface (as estimated from SEM-EDX elemental maps) with plasma irradiation time

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The chemical bonding state of the surface of A5052 before and after irradiation of the plasma was examined using XPS. Figure 8 presents Al 2p, Mg 2p and O 1s XPS spectra of treated and untreated specimens. The Al 2p spectra could be deconvoluted into peaks related to Al (metal) at 71.8 eV and Al2O3 at 74.5 eV [33]. The Mg 2p spectra were deconvoluted into peaks related to Mg (metal), magnesium oxide (MgO) and hydroxides/oxides at 49.6, 50.8 and 51.6 eV, respectively [34, 35]. The O 1s spectra were deconvoluted into peaks associated with oxides and hydroxides at 531.3 and 532.8 eV [36]. The Al 2p spectrum of the pristine A5052 exhibited peaks related to AlOx and metallic Al while the Mg 2p spectrum showed only a low-intensity peak. The O 1s spectrum contained peaks attributed to a natural oxide film. Following plasma irradiation, the data indicated an increase in the amount of MgO on the A5052 surface. These results demonstrate that the alloy was initially covered with an AlOx coating without Mg but that a MgO layer was formed as Mg diffused to the surface as a consequence of the heat input from the plasma [37,38,39]. The presence of MgO as the major phase can be explained by the rapid growth rate of MgO. It is possible that in the early stages of oxidation, Mg has diffused rapidly through local paths of easy diffusion in the alloy surface forming MgO. Surface inhomogenities (rolling defects), dislocations, and grain boundaries could serve as short-circuit paths for diffusion of Mg. Usually, MgO does not form a continuous layer [40, 41].

Fig. 8
figure 8

Al 2p, Mg 2p and O 1s XPS spectra of an A5052 surface (a) without and (b) with plasma irradiation

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This MgO is thought to have decreased the bond strength. Other work has shown that the adhesion and ageing of paints [42, 43] and adhesives [44] on metals having a MgO surface layer are frequently poor because of the friability and susceptibility to hydrolysis of the oxide [39]. Typically, MgO formed on Al-Mg alloys such as A5052 via diffusion of Mg will show low cohesive strength and is easily chemically fed to form oxides and thus to generate weakly bonded layers [45]. The present results suggest that the migration of Mg to the surface as a consequence of heating by the plasma with subsequent oxidation by radicals induced the formation of MgO. This, in turn, lowered the bond strength between the A5052 and PEEK.

The effect of exposing the PEEK side of A5052-PEEK specimens to the plasma was also investigated. Figure 9 plots the tensile shear strength of A5052-PEEK joints as a function of the time span over which the PEEK was treated with the plasma, with no treatment applied to the A5052. With increasing irradiation time, the bond strength evidently underwent a moderate increase from 10.2 MPa at 0 min to 15.5 MPa after 5 min and then was almost constant.

Fig. 9
figure 9

Variation of the tensile shear strength of A5052-PEEK bonded samples following plasma treatment only of the PEEK side with plasma irradiation time

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In a previous study, the effect of plasma irradiation on the chemical state of PEEK surfaces was assessed using XPS [32]. These prior XPS analyses indicated the O-C=O bond formation due to irradiation of the plasma, as well as C-O and C=O bonds that were originally present in the PEEK. The amount of each of these groups on the PEEK surface was found to increase as the plasma irradiation time was increased. It has been determined that O=C-O groups on the polymer increase the bond strength between metals and polymers following direct bonding [46,47,48]. Hence, these results suggest that oxidation of the PEEK by radicals in the atmospheric pressure RF plasma jet generated oxygen-containing surface functional groups that increased the bond strength.

4 Conclusions

Direct bonding of an Al alloy to PEEK via non-thermal atmospheric pressure plasma-assisted joining technology has been demonstrated. The effect of plasma irradiation on the bond strength following thermal press fitting was investigated. The tensile shear strength of A5052-PEEK joined by thermal press fitting with plasma-assisted joining was established by comparison with specimens made using conventional thermal press fitting and adhesive bonding. The tensile shear stress for samples bonded after irradiation of the plasma of only the PEEK was as high as 15.5 MPa, 50% higher than that of the unirradiated sample. This improved bond strength can be attributed to the addition of oxygen-based functional groups on the surface of PEEK by radicals generated by plasma jet. In contrast, plasma treatment of the A5052 side led to a decrease in bond strength as a consequence of the generation of MgO, which formed on Al-Mg alloys such as A5052 via diffusion of Mg that will show low cohesive strength and is easily chemically fed to form oxides and thus to generate weakly bonded layers. This reduction in bond strength is attributed to the forming magnesium oxide, which forms in the early stages of participation due to plasma irradiation. The effect of exposing the PEEK side of A5052-PEEK specimens to the plasma was also investigated. With increasing irradiation time, the bond strength evidently underwent a moderate increase from 10.2 MPa at 0 min to 15.5 MPa after 5 min and then was almost constant. These results suggest that oxidation of the PEEK by radicals in the atmospheric pressure RF plasma jet generated oxygen-containing surface functional groups that increased the bond strength.