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 [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.