Low pressure plasma treatment of CFRP substrates for adhesive bonding: an investigation of joint durability under severe temperature-moisture conditioning
Introduction
The wide diffusion of composite materials in many manufacturing fields such as automotive, racing and aerospace has led to progressive research for joining methods which allow one to maintain quality and reliability of the components over time, reducing their deterioration even when severe environmental conditions are involved during their use. Adhesive bonding is well known as one of the best assembly techniques for ensuring high mechanical strength to joints made of composite substrates, because of its advantages in better stress distribution, absorbing energy during impact and enhancing fatigue-life of the components. Besides, the adhesive layer yields a continuous bond between the two substrates, and this is essential especially when carbon fiber reinforced polymer (CFRP) is employed, as use of adhesive avoids damage to the fibers and, thus, preserves all mechanical properties of this material. However, selection of a high-performance substrate material and an adhesive having optimal properties might not be sufficient to fulfill the desired quality criteria required for the adhesive joint. Indeed, the success of the adhesive bonding process is commonly related to the pre-bonding operations (namely, degreasing and treatment) carried out on the faying surfaces in order to obtain essential characteristics such as good wettability, surface activation, increased chemical interactions between adherend and resin, and proper roughness extent for mechanical interlocking.
The most common treatments employed to prepare CFRP substrates for adhesive bonding are mainly based on the purely mechanical effect provided by roughness increasing and morphological modification obtained either during composite manufacturing (peel-ply) or before the application of the adhesive on the substrate (abrasion and grit blasting). Abrasive techniques are of course easier to perform, but, as Wingfield [1] argues, they require more care in avoiding defects or delamination which may strongly affect the joint strength. Furthermore, even the risk of contamination brought about by these kinds of treatments has to be taken into account. This is obvious for abrasion and grit-blasting, but real for peel-ply as well. Indeed, as a state-of-art review of Kanerva et al. [2] highlights, despite allowing achievement of a more uniform surface, peel-ply often represents an inefficient preparation owing to the residues released after removal of the ply.
Hence, in recent years, valid alternatives have been sought and non-standard physical processes have garnered an increasingly widespread interest. Most of these are based on laser processes, which can be employed with different laser sources. As an example, Palmieri et al. [3] describe performance evaluations of peel-ply and laser ablation treatments, the latter performed by using a Nd:YAG laser-source. Interesting results are also shown by Oliveira et al. [4], who employed a Yb:KYW chirped pulse-regenerative amplification laser system, and analyzed its morphological effect by varying process parameters and direction of the carbon fibers. Use of laser, however, requires in-depth experience of the process and knowledge of the proper wavelength to adopt in order not to damage the substrate, as highlighted by Reitz et al. [5] with a comparison between UV- and IR-laser pretreatments for CFRP/aluminum adhesive joints.
As reported by Ebnesajjad and Landrock [6], among the physical treatments suitable for CFRP materials, even plasma processes have been increasingly attracting attention, thanks to advantages that include contaminant removal, surface activation, and ease in the process management and automatization. Plasma treatments of polymer substrates generally result in morphological modification, oxidation and, thus, enhancement of surface wettability, as observed by Williams et al. [7] treating CFRP laminates with an atmospheric pressure He/O2-plasma. As Baldan [8] explains, the more these effects are concurrent, the more the interactions between adhesive and adherends are stronger and more stable (in terms of both type and number of such interactions), and, consequently, the more the adhesive joint shows high resistance. In this regard, Iqbal et al. [9] recognized that plasma treatment is particularly suitable for treatment of high-performance polymer and composite materials, like polyetheretherketone (PEEK) and fiber-reinforced polyphenylene sulfide (PPS), respectively.
In this scenario, even Low-Pressure Plasma (LPP) plays a main role. Its effectiveness as pre-bonding treatment for CFRP substrates was investigated in a preliminary study of the same authors [10], in which 36 different LPP-treatment conditions (by varying process gas, power input and exposure time) were employed on an adhesive system similar to that considered in the following (i.e. same adhesive, resin matrix and fiber-type). That experimental work showed an overall increase in joint performance after this treatment, when compared to the most conventional pre-bonding preparations of CFRP substrates (abrasion and peel-ply), especially when air and oxygen were adopted as process gases. The improvement was related to aspects involving both morphological and chemical-state modifications, resulting in the following main findings:
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LPP treatment, in general, provided the creation of crater-shaped roughness profiles which incremented the actual contact area between adhesive and adherend; it was observed that shear strength enhancement followed a trend which seemed to be in accordance with the increase in roughness extent;
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wetting analysis on the plasma-treated surfaces highlighted a substantial increase of the polar component of the surface free energy, resulting in a raise in surface wettability (i.e. reduction of the apparent contact angle values); such behavior is believed to be related to the oxidizing effect of plasma, as determined by XPS assessment:
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especially air-plasma and oxygen-plasma entailed a remarkable increase of the oxygen/carbon ratio and a deep stimulation of polar species (e.g. C–O, CO, O–C–O, O–CO): this means creation of more active-sites and improved adhesive-adherend interactions.
However, as Sargent [11] reports, “Designers are not usually concerned with the as-manufactured strength of bonded joints, but with the lowest level to which the strength will fall during the life [of the component] due to adverse effects of the ambient environment”. Thus, to establish success of LPP for CFRP substrates to be adhesively bonded, it was necessary to verify whether and how much its qualities had been maintained even under severe ambient conditions. Budhe et al. [12] highlight that the most common environmental threats are indeed related to the effect of both temperature and moisture absorption; when they act simultaneously, hygro-thermal aging occurs, seriously affecting strength and durability of the composite joint. Indeed, most researchers (see Reis et al. [13] and Zafar et al. [14]) report an overall joint strength reduction during aging, the rate of which depends on exposure time and environment as well as the adhesive system undergoing conditioning. Hence, this investigation was focused on verifying both short- and long-term effectiveness of cold-plasma surface treatment for CFRP adhesive-joining. A preliminary testing campaign was performed under standard laboratory conditions to evaluate the shear strength of SLJ, comparing the effectiveness of eighteen LPP treatments to a traditional mechanical abrasion of the adherends. From this, four sets of LPP-treatment conditions were selected and then subjected to an accelerated aging process. The same conditioning was also adopted for abraded joints, which were employed as control references. To assess the durability of the CFRP-epoxy adhesive system under accelerated aging conditions, tensile shear strength (TSS) testing and wedge cleavage testing (WT) were performed in parallel.
The experimental results showed that low-pressure plasma treatment of the CFRP substrates resulted in an increased short-term quality of the adhesive joint as well as in an enhancement of its durability even under severe aging conditions.
Section snippets
Materials
The CFRP material used as substrate for tensile shear strength (TSS) evaluation was manufactured arranging 7 layers of 2/2-twill carbon-plies with a 0°-orientation, pre-impregnated with epoxy resin by hand lay-up technique. A medium curing process was performed using a vacuum bag in an autoclave for 2h at 135 °C and pressure of 6 bar. CFRP panels having thickness 1.50 ± 0.02 mm and Young's modulus equal to 70 ± 5 GPa were obtained. To perform wedge testing, for the reasons discussed in
SLJ strength under standard laboratory conditions
A preliminary trial campaign was carried out by testing differently plasma-treated TSS-joints in order to determine, for both air and oxygen, how shear strength varied with the set process parameters and, consequently, which sets of joints to select for aging. In this regard, Fig. 4 displays the results of shear strength obtained testing the SLJ under standard laboratory conditions (23 °C, 50% RH); the number present at the base of each column represents the average value of shear strength
Conclusion
An evaluation of the aging behavior of adhesively bonded joints manufactured using a toughened epoxy adhesive applied on CFRP substrates was carried out. Low-pressure plasma treatment was employed, adopting air and pure oxygen as process gases, and varying working parameters such as power and exposure time in order to determine those treatment conditions to undergo a severe accelerated aging cycle. Thus, a preliminary testing campaign was performed under standard laboratory conditions to
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