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Effect of surface pre-treatment on surface characteristics and adhesive bond strength of aluminium alloy
International Journal of Adhesion & Adhesives 70 (2016) 265–270
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Effect of surface pre-treatment on surface characteristics and adhesive bond strength of aluminium alloy K. Leena a,n, K.K. Athira c,1, S. Bhuvaneswari b, S. Suraj a, V. Lakshmana Rao a a
Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala 695022, India Analytical and Spectroscopy Division, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala 695022, India c National Institute of Technology, Calicut, Kerala 673601, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 13 September 2015 Accepted 17 July 2016 Available online 22 July 2016
In this work aluminium alloy surfaces have been subjected to three different methods of surface pretreatments such as solvent degreasing, FPL (Forest Products Laboratory) etching and priming using an epoxy based primer. The treated surfaces were evaluated for surface energy, contact angle, surface topography, surface roughness and adhesive strength characteristics. The influence of surface pre-treatments on the variation of polar, dispersive and total surface energy of the surfaces is addressed. A wettability test was performed on the surfaces using an epoxy adhesive in order to assess the influence of the pre-treatment techniques on substrate/adhesive interaction. Theoretical work of adhesion values for the various pre-treated surfaces were calculated using the contact angle data and further tested experimentally by adhesive bond strength evaluation by tensile testing of a single lap aluminium-epoxyaluminium assembly. The method of surface pre-treatment showed a profound effect on the surface topography and roughness by AFM. This study reveals that a combination of high surface energy and high surface roughness of the substrate along with good wettability of the adhesive contributed to the highest joint strength for the aluminium alloy through the FPL etching pre-treatment. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Atomic Force Microscopy C Wettability D Surface energy Surface roughness
1. Introduction Adhesively bonded aluminium joints have wide spread applications in aerospace, automotive and general engineering sectors because of their high strength-to-weight ratio, excellent corrosion resistance and improved manufacturability compared to those made by traditional welding techniques [1–4]. The durability of the adhesive bond strength and the long service life under demanding conditions necessitates the pre-treatment of the surface before adhesive bonding. Aluminium surfaces are usually covered with a weakly bound naturally formed surface oxide layer and adsorbed contamination, which needs to be removed to establish a durable bond between the metal and the adhesive. Various mechanical, chemical and electrochemical surface pre-treatment methods have been reported for aluminium substrates like liquid or vapour degreasing, abrading, gritn
Corresponding author. Tel.: þ 91 4712564298. E-mail addresses: [email protected] (K. Leena), [email protected] (K.K. Athira), [email protected] (S. Bhuvaneswari), [email protected] (S. Suraj), [email protected] (V.L. Rao). 1 Present address: Four dimensional X-ray microscopy lab, Department of Mechanical Engineering, IIT Bombay, Powai, Mumbai-400076, India http://dx.doi.org/10.1016/j.ijadhadh.2016.07.012 0143-7496/& 2016 Elsevier Ltd. All rights reserved.
blasting, acid / alkaline etching, anodising etc. to name a few [5–12]. A full review on the surface pre-treatments for aluminium alloys has been reported elsewhere [13]. Alkaline etching removes the unstable aluminium oxide/hydroxide film and cleans the stubborn oils and greases off the bonding surfaces. Vapour degreasing consists of removing oils and other organic contaminants from the roughened surface using suitable solvents. Boiling water can act as a surface treating agent for aluminium alloy which results in durable adhesive strength with epoxy adhesive [14]. A combination of different pre-treatment techniques such as grit-blasting, acetone degreasing, alkali etching and phosphoric acid anodising, provides a better adhesive bonding property for aluminium alloys [15]. The most exploited chemical pre-treatment for aluminium adherents is based on chromic-sulphuric acid etching which generates a suitable oxide layer on the substrate surface and can produce strong and durable adhesive bonds [16]. The surface pre-treatment method adopted for the adherent would greatly influence the contact angle. An aluminium alloy surface after alkaline etching, dipping in warm water followed by treating with silane solution showed a decreased contact angle for water and a polyurethane adhesive on the surface [17]. A higher
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surface roughness of the adherent favoured the spreading of the adhesive drop on its surface. Studies conducted elsewhere have shown that surface roughness and chemical heterogeneities greatly influence the contact angle values of the adhesive formed on the adherent surface [18–21]. One of the factors deciding the durability of an adhesive joint is the extent of penetration of the adhesive into the pores of the surface film formed after the pre-treatment [22]. The penetration of the adhesive depends on many factors such as pore dimensions, contact angle between the adhesive and the substrate, viscosity of the adhesive and the viscosity–time characteristics at the temperature of applications [23]. In the case of adhesively bonded aluminium, there have been comparatively few studies on the influence of surface roughness on joint strength. It has been reported that a commonly used chromate pre-treatment improves the lap shear strength of bonded joints with optimal joint strength corresponding to a surface morphology consisting of etch pits of 1–5 mm in diameter [24]. Similar structures have been reported, where the etched and anodised aluminium surfaces were scanned using transmission electron microscopy (TEM) [25,26]. These workers have attributed much of the increase in joint strength of pre-treated aluminium to fine scale oxide structures. The present study is focused on the effectiveness of various surface pre-treatments for producing strong adhesive bonds on aluminium interfaces which is evaluated using the single lap shear test with an epoxy adhesive. The influence of surface energy, contact angle, surface topography and surface roughness on the experimental shear strength properties of aluminium substrates was explored.
2.2.1.3. Method b followed by primer application (method c). Aluminium substrates were primed immediately after method b. Substrates were dipped in primer BR127, kept in a vertical position for removal of excess primer at room temperature for 30 min. and further cured at 120 °C for 30 min. in a hot air oven. This surface was designated as Al-SDFPLP. 2.2.2. Substrate surface characterisation The substrate surface was analysed for surface energy, contact angle, surface topography, roughness and adhesive strength properties. The contact angles of the reference liquids and the dynamic contact angle of an epoxy adhesive on variously pretreated aluminium substrates were analysed using a sessile drop technique using a video based optical contact angle measuring equipment OCA20 (M/s. Data Physics, Germany). The surface energy of the substrates, which is calculated as the sum of the polar and dispersive components were measured by the Owens Wendt Rabel Kaelble (OWRK) method [28,29]. The theoretical work of adhesion of the epoxy adhesive on the variously pre-treated substrates was determined using the Dupre-Young equation. Single lap shear strength of the bonded joints was determined according to ASTM D1002-72. The testing was carried out with a universal testing machine (UTM) Instron Model 5569, where, five samples were tested in each case. The surface topography of the various pre-treated aluminium substrates was measured using a 300R atomic force microscope (M/s. WILec, Alpha, Germany) in a non-contact mode at a scanning speed of 1 μm/s. for an analysing area of 100 μ 100 μ. All the images had 256 data points with a scan rate of 1.0 line/s. Three individual scan areas were considered and the roughness values were averaged to calculate the surface roughness using WITec project plus software.
2. Materials and methods 2.1. Materials
3. Results and discussion
Aluminium alloy, B51 SWP was used as the substrate. The adhesive employed was an epoxy resin supplied by M/s. Huntsman India pvt. ltd., cured at 100 °C with o,o0 Bis(2-aminopropyl) polypropylene glycol purchased from M/s. Sigma Aldrich, India. BR127, purchased from M/s. Cytec Industries Inc.,USA was used for priming the aluminium substrates. Diidomethane, 98% and water (HPLC grade), used as the reference liquids for measuring the surface energy of the aluminium substrates, was obtained from M/s. Spectrochem, India. Trichloroethylene 99.9% from M/s. Nice, India was used as the solvent. The FPL etch solution was prepared as per the standard procedure given elsewhere [27].
The substrate surfaces after various pre-treatments were analysed for contact angle, surface energy, surface topography and adhesive strength properties.
2.2. Methods 2.2.1. Substrate preparation All the substrate surfaces were initially abraded using P100 grade emery paper and any emery dust was removed with a clean forced air supply. The substrate surfaces were subjected to three different types of pre-treatment as follows. 2.2.1.1. Solvent degreasing (method a). Solvent degreasing was conducted by wiping the surface using a lint free cotton cloth soaked with trichloroethylene solvent followed by drying with a hot air stream which did not exceed a temperature of 60 °C. This surface was designated as Al-SD. 2.2.1.2. Method a followed by FPL etching (method b). This involved immersion in an FPL etch solution for 15 min. at 60 °C, followed by rinsing in tap water and drying with a hot air stream at a temperature not exceeding 60 °C. The resulting surface was designated as Al-SDFPL.
3.1. Surface energy The contact angles made by the reference liquids water and diiodomethane on an aluminium substrate surface after the three pre-treatment techniques are reported in Table 1. The variation of the dispersive and polar components of surface free energy and the total surface energy of the various surfaces studied are given in Fig. 1a–c respectively. Fig. 1a indicates that even though the surface energy contribution of the dispersive component is high in comparison to the polar component, the variation is minimal for the different pretreated surfaces. Fig. 1b shows that the variation in the polar contribution is more prominent in determining the total surface energy. The contribution by the polar component is the highest for the Al-SDFPL surface due to the freshly formed aluminium oxide layer on the surface and lowest for the primed surface. Hence the Table 1 Average contact angles in degrees shown by two reference liquids and the surface roughness average of variously pre-treated aluminium substrates. System
Contact angle (°) H2O
Al-SD Al-SDFPL Al-SDFPLP
76 32 93
Surface roughness Ra (nm)
CH2I2 63 56 54
530 620 390
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267
and creates a fresh and strong oxide layer by the FPL treatment. It has been reported that acid treatment generates a thin oxide layer of the order of 10 nm on the aluminium surface [1,4]. The Al-SD substrate showed a considerably lower surface energy (36 mN/m) than the Al-SDFPL substrate (64 mN/m). This can be attributed to solvent degreasing which removes oils and other organic contaminants from the substrate surface and does not contribute to the removal of the weakly bound thin aluminium oxide layer. The decreased surface energy of the AL-SDFPLP surface can be attributed to the lower surface energy of the epoxy based primer coating on the surface as indicated by a higher contact angle (93°) for the polar and lower contact angle (54°) for the non-polar reference liquids. 3.2. Contact angle measurements of epoxy adhesive The contact angle of the epoxy adhesive imaged 600 ms following application of a drop on each substrate surface is shown in Fig. 2. The trend in the variation of contact angle values as a function of time following application of the drop is depicted in Fig. 3. All substrates exhibited the same trend in the variation of contact angle with respect to time. Contact angle for the Al-SD substrate showed an initial value of 84°, then stabilised at 65°, whereas, Al-SDFPL substrate showed an initial contact angle of 72°, then stabilised at a lower value of 45°. The absorption of the adhesive on a substrate depends on many factors such as the surface energies of the adhesive and substrate, the geometry of the pores, rheology of the adhesive and the time since application. For the Al-SD substrate, even if the oils and organic contaminants are removed, the weakly bound oxide layer hinders the spreading of the adhesive. For Al-SDFPL substrate, the increased surface energy and the uniform porosity after the chromic acid etching resulted in more penetration of the adhesive into the surface pores resulting in good wetting and a lower contact angle. Al-SDFPLP substrates, even though they showed a lower surface energy, the epoxy adhesive showed good wetting on its surface, giving an initial contact angle of 68°, followed by stabilisation at 25°. This may be attributed to the increased interaction of the epoxy /primer interface thereby increasing the wetting of the resin on to the surface. 3.3. Theoretical work of adhesion from contact angle measurement
Fig. 1. Variation of dispersive part (a), polar part (b) and the total surface energy (c) of the pre-treated aluminium substrates.
Al-SDFPL substrate showed the highest total surface energy among the various substrates as shown in Fig. 1c. This is because chromic acid etching removes weakly bound oxide layer from the surface
The stabilised contact angles for each substrate were taken for computing the work of adhesion. The surface energy of the liquid epoxy resin used was determined as 40 mJ/m2 by the pendent drop method using the contact angle goniometer. The plot for the work of adhesion for each type of substrate is given in Fig. 4. The lowest work of adhesion for Al-SD, which has undergone a nonefficient surface preparation method, is attributed to the existence of a weak boundary layer on the surface. As expected, the Al-SDFPL showed a higher work of adhesion compared to Al-SD which is evident due to the presence of the fresh and porous aluminium oxide layer on the substrate after the pre-treatment. For AlSDFPLP, the good interfacial interaction of the epoxy primed substrate and the epoxy adhesive played a greater role than the surface energy of the substrate in improving the work of adhesion which was earlier proven by achieving the lowest contact angle of 25° by the epoxy adhesive. The trend obtained in the theoretical work of adhesion is further substantiated experimentally by measuring the single lap shear properties of the variously pre-treated aluminium substrates using the epoxy adhesive system. The results are given in Fig. 5. A positive correlation exists between the theoretical work of adhesion and the experimentally measured lap shear strength properties for Al-SD and Al-SDFPL substrates. The AL-SD substrates
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Fig. 2. Stabilised contact angles of epoxy adhesive on variously pre-treated aluminium surfaces (a) Al-SD (b) Al-SDFPL and (c) Al-SDFPLP.
Fig. 3. Dynamic contact angle vs. drop age for various pre-treated aluminium substrates.
Fig. 5. Lap shear strength of various pre-treated aluminium substrates.
showed the lowest and the Al-SDFPL substrate showed the highest shear strength properties. Even if Al-SDFPL substrate showed lower theoretical work of adhesion than Al-SDFPLP, their practical adhesive strength was comparable. This may be attributed to the other contributing factors such as surface roughness of the adherent, interfacial contact area, surface energy and chemical composition of adhesive and adherent; nature of the adhesive etc. in determining the practical adhesive strength.
3.4. Surface topography and roughness by AFM
Fig. 4. Theoretical work of adhesion vs. various surface pre-treatments.
One of the limitations encountered with surface studies was in quantifying the surface roughness of adherents and correlation to adhesion parameters. The recent development in AFM enables both imaging and the quantification of surface roughness to subnanometre resolution. In the present study, each of the pre-treated aluminium substrates was subjected to AFM imaging to understand the surface texture and surface roughness. The AFM images
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Fig. 6. Surface topography by AFM of (a) Al-SD (b) Al-SDFPL and (c) Al-SDFPLP substrates.
of the pre-treated substrates are depicted in Fig. 6 and the measured surface roughness are given in Table 1. The Al-SD substrate showed a compact, less random surface with uniform peaks visually. Al-SDFPL showed a perfectly random porous texture with predominant valleys or pits and has given a roughness average, Ra, of 620 nm. This is attributed to the oxide protrusions which serve the mechanical interlocking and very good interfacial interaction with the epoxy. This has resulted in the highest lap shear strength for aluminium substrates as discussed in the previous section. Al-SDFPLP substrate showed a perfectly random surface with predominant peaks but showed a lower surface roughness average of 390 nm. This may be due to the entrapment of the primer into the pits formed during the FPL etching as shown in the AFM images. The study confirms that a combination of high surface energy, high surface roughness and good wettability of the adhesive altogether contributed for the highest joint strength for the aluminium alloy substrates through the FPL etching pre-treatments.
4. Conclusion In this study, aluminium substrates were subjected to three different surface pre-treatment techniques. The substrate surfaces after various pre-treatments were analysed for contact angle, surface energy, surface topography and adhesive strength properties. The aluminium substrate which had undergone only the solvent degreasing pre-treatment technique showed a lower surface energy than the FPL etched surface. It showed a comparatively poor wetting of epoxy adhesive on its surface and lower surface roughness than the FPL etched surface which resulted in the lowest theoretical work of adhesion and practical adhesive strength. The FPL etching of the substrate resulted in good wetting by the epoxy adhesive, highest surface energy and surface
roughness thereby highest lap shear strength properties. The primed aluminium substrate showed very good interfacial interaction with the epoxy adhesive resulting in comparable lap shear strength properties to that of FPL etched surfaces.
Acknowledgement The authors thank the authorities of Vikram Sarabhai Space Centre for giving permission to publish the article. One of the authors, K.K. Athira is thankful to the National Institute of Technology, Calicut, Kerala, for permitting to conduct her academic project work on this topic. Thanks are also due to the Analytical and Spectroscopy Division of the Vikram Sarabhai Space Centre for analytical support.
References [1] Critchlow GW, Yendall KA, Bahrani D, Quinnand A, Andrews F. Strategies for the replacement of chromic acid anodising for the structural bonding of aluminium alloys. Int J Adhes Adhes 2006;26(6):419–53. [2] Barnes TA, Pashby IR. Joining techniques for aluminium space frames used in automobiles: Part II — adhesive bonding and mechanical fasteners. J Mat Process Tech 2000;99(1-3):72–9. [3] Digby RP, Packham DE. Pre-treatment of Aluminium– topography, surfacechemistry and adhesive bond durability. Int J Adhes Adhes 1995;15(2):61–71. [4] Lapique F, Redford K. Curing effects on viscosity and mechanical properties of a commercial epoxy resin adhesive. Int J Adhes Adhes 2002;22(4):337–46. [5] Lunder O, Olsen B, Nisancioglu K. Pre-treatment of AA6060 aluminium alloy for adhesive bonding. Int J Adhes Adhes 2002;22(2):143–50. [6] Hadavinia H, Kinloch AJ, Little MSG, Taylor AC. The prediction of crack growth in bonded joints under cyclic-fatigue loading I. Experimental studies. Int J Adhes Adhes 2003;23(6):449–61. [7] Bjørgum A, Lapique F, Walmsley J, Redford K. Anodising as pre-treatment for structural bonding. Int J Adhes Adhes 2003;23(6):401–12. [8] Bockenheimer C, Valeske B, Possart W. Network structure in epoxy aluminium bonds after mechanical treatment. Int J Adhes Adhes 2002;22:349–56.
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[9] Lunder O, Lapique F, Johnsen B, Nisancioglu K. Effect of pre-treatment on the durability of epoxy-bonded AA6060 aluminium joints. Int J Adhes Adhes 2004;24(2):107–17. [10] Molitor P, Barron V, Young T. Surface treatment of titanium for adhesive bonding to polymer composites: a review. Int J Adhes Adhes 2001;21(2):129–36. [11] Guo Z, Wang Y, Wang R. Study on the durability of bonded joint of silanizing aluminium alloy. China Adhes 2006;5:9–13.9 in Chinese. [12] Domingues L, Fernandes JCS, Da Cunha Belo M, Ferreira MGS, Guerra-Rosa L. Anodising of Al 2024-T3 in a modified sulphuric acid/boric acid bath for aeronautical applications. Corros Sci 2003;45(1):149–60. [13] Critchlow GW, Brewis DM. Review of surface pretreatments for aluminium alloys. Int J Adhes Adhes 1996;16(4):255–75. [14] Underhill PR, Rider AN, Du Quesnay DL. Warm water treatment of aluminum for adhesive bonding. Int J Adhes Adhes 2003;23(4):307–13. [15] Zhang J, Zhao X, Zuoy Y, Xiong J, Zhang X. Effect of surface pretreatment on adhesive properties of aluminum alloys. J Mater Sci Technol 2008;24(2):236–40. [16] Bishopp JA. Surface pretreatment for structural bonding, Handbook of Adhesives and Sealants. Volume 1, Cognard, P. (ed.), Amsterdam; Elsevier: pp. 163–214. [17] Zain NM, Ahmmad SH, Ali ES. Green Polyurethane Adhesive Bonding of Aluminum: effect of Surface Treatment. Appl Mech Mater 2013;393:51–6. [18] Oliver JF, Huh C, Mason SG. Liquid spreading on rough metal surfaces. J Mater Sci 1980;15:431–7. [19] Tamai Y, Aratani K. Experimental study of the relation between contact angle and surface roughness. J Phys Chem 1972;76:3267–71.
[20] Oliver JF, Huh C, Mason SG. An experimental study of some effects of solid surface roughness on wetting. Colloids Surf 1980;1:79–104. [21] Hazlett RD. On surface roughness effects in wetting phenomena. J Adhes Sci Technol 1992;6:625–33. [22] Borsellinoa C, Di Bellab G, Ruisib VF. Adhesive joining of aluminium AA6082: The effects of resin and surface treatment. Int J Adhes Adhes 2009;29:36–44. [23] Packham DE. The adhesion of polymers to metals: the role of surface topography. Adhesion Aspects of Polymeric Coatings. Mittal KL (ed.), London: 1983; pp. 19–44. [24] Moth D. Factors affecting pretreatment design to optimise adhesive bonding of aluminium. Adhesion 14 Allen KW (ed.). London: Elsevier Applied Science; 1990. p. 15–35. [25] Venables JD. Adhesion and durability of metal-polymer bonds. J Mater Sci 1984;19:2431–53. [26] Bishopp JA, Sim EK, Thompson GE, Wood GC. The adhesively bonded aluminium joint: the effect of pretreatment on durability. J Adhes 1988;26:237–63. [27] Venables JD, McNamara DK, Chen JM, Sun TS. Oxide morphologies on aluminum prepared for adhesive bonding. Appl Surf Sci 1979;3:88–98. [28] Owens DK, Wendt RC. Estimation of the surface free energy of polymers. J Appl Polym Sci 1969;13:1741–7. [29] Kaelble DH. Dispersion-polar surface tension properties of organic solids. J Adhes 1970;2:66–81.
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Effect of surface pre-treatment on surface characteristics and adhesive bond strength of aluminium alloy K. Leena a,n, K.K. Athira c,1, S. Bhuvaneswari b, S. Suraj a, V. Lakshmana Rao a a
Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala 695022, India Analytical and Spectroscopy Division, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala 695022, India c National Institute of Technology, Calicut, Kerala 673601, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 13 September 2015 Accepted 17 July 2016 Available online 22 July 2016
In this work aluminium alloy surfaces have been subjected to three different methods of surface pretreatments such as solvent degreasing, FPL (Forest Products Laboratory) etching and priming using an epoxy based primer. The treated surfaces were evaluated for surface energy, contact angle, surface topography, surface roughness and adhesive strength characteristics. The influence of surface pre-treatments on the variation of polar, dispersive and total surface energy of the surfaces is addressed. A wettability test was performed on the surfaces using an epoxy adhesive in order to assess the influence of the pre-treatment techniques on substrate/adhesive interaction. Theoretical work of adhesion values for the various pre-treated surfaces were calculated using the contact angle data and further tested experimentally by adhesive bond strength evaluation by tensile testing of a single lap aluminium-epoxyaluminium assembly. The method of surface pre-treatment showed a profound effect on the surface topography and roughness by AFM. This study reveals that a combination of high surface energy and high surface roughness of the substrate along with good wettability of the adhesive contributed to the highest joint strength for the aluminium alloy through the FPL etching pre-treatment. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Atomic Force Microscopy C Wettability D Surface energy Surface roughness
1. Introduction Adhesively bonded aluminium joints have wide spread applications in aerospace, automotive and general engineering sectors because of their high strength-to-weight ratio, excellent corrosion resistance and improved manufacturability compared to those made by traditional welding techniques [1–4]. The durability of the adhesive bond strength and the long service life under demanding conditions necessitates the pre-treatment of the surface before adhesive bonding. Aluminium surfaces are usually covered with a weakly bound naturally formed surface oxide layer and adsorbed contamination, which needs to be removed to establish a durable bond between the metal and the adhesive. Various mechanical, chemical and electrochemical surface pre-treatment methods have been reported for aluminium substrates like liquid or vapour degreasing, abrading, gritn
Corresponding author. Tel.: þ 91 4712564298. E-mail addresses: [email protected] (K. Leena), [email protected] (K.K. Athira), [email protected] (S. Bhuvaneswari), [email protected] (S. Suraj), [email protected] (V.L. Rao). 1 Present address: Four dimensional X-ray microscopy lab, Department of Mechanical Engineering, IIT Bombay, Powai, Mumbai-400076, India http://dx.doi.org/10.1016/j.ijadhadh.2016.07.012 0143-7496/& 2016 Elsevier Ltd. All rights reserved.
blasting, acid / alkaline etching, anodising etc. to name a few [5–12]. A full review on the surface pre-treatments for aluminium alloys has been reported elsewhere [13]. Alkaline etching removes the unstable aluminium oxide/hydroxide film and cleans the stubborn oils and greases off the bonding surfaces. Vapour degreasing consists of removing oils and other organic contaminants from the roughened surface using suitable solvents. Boiling water can act as a surface treating agent for aluminium alloy which results in durable adhesive strength with epoxy adhesive [14]. A combination of different pre-treatment techniques such as grit-blasting, acetone degreasing, alkali etching and phosphoric acid anodising, provides a better adhesive bonding property for aluminium alloys [15]. The most exploited chemical pre-treatment for aluminium adherents is based on chromic-sulphuric acid etching which generates a suitable oxide layer on the substrate surface and can produce strong and durable adhesive bonds [16]. The surface pre-treatment method adopted for the adherent would greatly influence the contact angle. An aluminium alloy surface after alkaline etching, dipping in warm water followed by treating with silane solution showed a decreased contact angle for water and a polyurethane adhesive on the surface [17]. A higher
266
K. Leena et al. / International Journal of Adhesion & Adhesives 70 (2016) 265–270
surface roughness of the adherent favoured the spreading of the adhesive drop on its surface. Studies conducted elsewhere have shown that surface roughness and chemical heterogeneities greatly influence the contact angle values of the adhesive formed on the adherent surface [18–21]. One of the factors deciding the durability of an adhesive joint is the extent of penetration of the adhesive into the pores of the surface film formed after the pre-treatment [22]. The penetration of the adhesive depends on many factors such as pore dimensions, contact angle between the adhesive and the substrate, viscosity of the adhesive and the viscosity–time characteristics at the temperature of applications [23]. In the case of adhesively bonded aluminium, there have been comparatively few studies on the influence of surface roughness on joint strength. It has been reported that a commonly used chromate pre-treatment improves the lap shear strength of bonded joints with optimal joint strength corresponding to a surface morphology consisting of etch pits of 1–5 mm in diameter [24]. Similar structures have been reported, where the etched and anodised aluminium surfaces were scanned using transmission electron microscopy (TEM) [25,26]. These workers have attributed much of the increase in joint strength of pre-treated aluminium to fine scale oxide structures. The present study is focused on the effectiveness of various surface pre-treatments for producing strong adhesive bonds on aluminium interfaces which is evaluated using the single lap shear test with an epoxy adhesive. The influence of surface energy, contact angle, surface topography and surface roughness on the experimental shear strength properties of aluminium substrates was explored.
2.2.1.3. Method b followed by primer application (method c). Aluminium substrates were primed immediately after method b. Substrates were dipped in primer BR127, kept in a vertical position for removal of excess primer at room temperature for 30 min. and further cured at 120 °C for 30 min. in a hot air oven. This surface was designated as Al-SDFPLP. 2.2.2. Substrate surface characterisation The substrate surface was analysed for surface energy, contact angle, surface topography, roughness and adhesive strength properties. The contact angles of the reference liquids and the dynamic contact angle of an epoxy adhesive on variously pretreated aluminium substrates were analysed using a sessile drop technique using a video based optical contact angle measuring equipment OCA20 (M/s. Data Physics, Germany). The surface energy of the substrates, which is calculated as the sum of the polar and dispersive components were measured by the Owens Wendt Rabel Kaelble (OWRK) method [28,29]. The theoretical work of adhesion of the epoxy adhesive on the variously pre-treated substrates was determined using the Dupre-Young equation. Single lap shear strength of the bonded joints was determined according to ASTM D1002-72. The testing was carried out with a universal testing machine (UTM) Instron Model 5569, where, five samples were tested in each case. The surface topography of the various pre-treated aluminium substrates was measured using a 300R atomic force microscope (M/s. WILec, Alpha, Germany) in a non-contact mode at a scanning speed of 1 μm/s. for an analysing area of 100 μ 100 μ. All the images had 256 data points with a scan rate of 1.0 line/s. Three individual scan areas were considered and the roughness values were averaged to calculate the surface roughness using WITec project plus software.
2. Materials and methods 2.1. Materials
3. Results and discussion
Aluminium alloy, B51 SWP was used as the substrate. The adhesive employed was an epoxy resin supplied by M/s. Huntsman India pvt. ltd., cured at 100 °C with o,o0 Bis(2-aminopropyl) polypropylene glycol purchased from M/s. Sigma Aldrich, India. BR127, purchased from M/s. Cytec Industries Inc.,USA was used for priming the aluminium substrates. Diidomethane, 98% and water (HPLC grade), used as the reference liquids for measuring the surface energy of the aluminium substrates, was obtained from M/s. Spectrochem, India. Trichloroethylene 99.9% from M/s. Nice, India was used as the solvent. The FPL etch solution was prepared as per the standard procedure given elsewhere [27].
The substrate surfaces after various pre-treatments were analysed for contact angle, surface energy, surface topography and adhesive strength properties.
2.2. Methods 2.2.1. Substrate preparation All the substrate surfaces were initially abraded using P100 grade emery paper and any emery dust was removed with a clean forced air supply. The substrate surfaces were subjected to three different types of pre-treatment as follows. 2.2.1.1. Solvent degreasing (method a). Solvent degreasing was conducted by wiping the surface using a lint free cotton cloth soaked with trichloroethylene solvent followed by drying with a hot air stream which did not exceed a temperature of 60 °C. This surface was designated as Al-SD. 2.2.1.2. Method a followed by FPL etching (method b). This involved immersion in an FPL etch solution for 15 min. at 60 °C, followed by rinsing in tap water and drying with a hot air stream at a temperature not exceeding 60 °C. The resulting surface was designated as Al-SDFPL.
3.1. Surface energy The contact angles made by the reference liquids water and diiodomethane on an aluminium substrate surface after the three pre-treatment techniques are reported in Table 1. The variation of the dispersive and polar components of surface free energy and the total surface energy of the various surfaces studied are given in Fig. 1a–c respectively. Fig. 1a indicates that even though the surface energy contribution of the dispersive component is high in comparison to the polar component, the variation is minimal for the different pretreated surfaces. Fig. 1b shows that the variation in the polar contribution is more prominent in determining the total surface energy. The contribution by the polar component is the highest for the Al-SDFPL surface due to the freshly formed aluminium oxide layer on the surface and lowest for the primed surface. Hence the Table 1 Average contact angles in degrees shown by two reference liquids and the surface roughness average of variously pre-treated aluminium substrates. System
Contact angle (°) H2O
Al-SD Al-SDFPL Al-SDFPLP
76 32 93
Surface roughness Ra (nm)
CH2I2 63 56 54
530 620 390
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and creates a fresh and strong oxide layer by the FPL treatment. It has been reported that acid treatment generates a thin oxide layer of the order of 10 nm on the aluminium surface [1,4]. The Al-SD substrate showed a considerably lower surface energy (36 mN/m) than the Al-SDFPL substrate (64 mN/m). This can be attributed to solvent degreasing which removes oils and other organic contaminants from the substrate surface and does not contribute to the removal of the weakly bound thin aluminium oxide layer. The decreased surface energy of the AL-SDFPLP surface can be attributed to the lower surface energy of the epoxy based primer coating on the surface as indicated by a higher contact angle (93°) for the polar and lower contact angle (54°) for the non-polar reference liquids. 3.2. Contact angle measurements of epoxy adhesive The contact angle of the epoxy adhesive imaged 600 ms following application of a drop on each substrate surface is shown in Fig. 2. The trend in the variation of contact angle values as a function of time following application of the drop is depicted in Fig. 3. All substrates exhibited the same trend in the variation of contact angle with respect to time. Contact angle for the Al-SD substrate showed an initial value of 84°, then stabilised at 65°, whereas, Al-SDFPL substrate showed an initial contact angle of 72°, then stabilised at a lower value of 45°. The absorption of the adhesive on a substrate depends on many factors such as the surface energies of the adhesive and substrate, the geometry of the pores, rheology of the adhesive and the time since application. For the Al-SD substrate, even if the oils and organic contaminants are removed, the weakly bound oxide layer hinders the spreading of the adhesive. For Al-SDFPL substrate, the increased surface energy and the uniform porosity after the chromic acid etching resulted in more penetration of the adhesive into the surface pores resulting in good wetting and a lower contact angle. Al-SDFPLP substrates, even though they showed a lower surface energy, the epoxy adhesive showed good wetting on its surface, giving an initial contact angle of 68°, followed by stabilisation at 25°. This may be attributed to the increased interaction of the epoxy /primer interface thereby increasing the wetting of the resin on to the surface. 3.3. Theoretical work of adhesion from contact angle measurement
Fig. 1. Variation of dispersive part (a), polar part (b) and the total surface energy (c) of the pre-treated aluminium substrates.
Al-SDFPL substrate showed the highest total surface energy among the various substrates as shown in Fig. 1c. This is because chromic acid etching removes weakly bound oxide layer from the surface
The stabilised contact angles for each substrate were taken for computing the work of adhesion. The surface energy of the liquid epoxy resin used was determined as 40 mJ/m2 by the pendent drop method using the contact angle goniometer. The plot for the work of adhesion for each type of substrate is given in Fig. 4. The lowest work of adhesion for Al-SD, which has undergone a nonefficient surface preparation method, is attributed to the existence of a weak boundary layer on the surface. As expected, the Al-SDFPL showed a higher work of adhesion compared to Al-SD which is evident due to the presence of the fresh and porous aluminium oxide layer on the substrate after the pre-treatment. For AlSDFPLP, the good interfacial interaction of the epoxy primed substrate and the epoxy adhesive played a greater role than the surface energy of the substrate in improving the work of adhesion which was earlier proven by achieving the lowest contact angle of 25° by the epoxy adhesive. The trend obtained in the theoretical work of adhesion is further substantiated experimentally by measuring the single lap shear properties of the variously pre-treated aluminium substrates using the epoxy adhesive system. The results are given in Fig. 5. A positive correlation exists between the theoretical work of adhesion and the experimentally measured lap shear strength properties for Al-SD and Al-SDFPL substrates. The AL-SD substrates
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Fig. 2. Stabilised contact angles of epoxy adhesive on variously pre-treated aluminium surfaces (a) Al-SD (b) Al-SDFPL and (c) Al-SDFPLP.
Fig. 3. Dynamic contact angle vs. drop age for various pre-treated aluminium substrates.
Fig. 5. Lap shear strength of various pre-treated aluminium substrates.
showed the lowest and the Al-SDFPL substrate showed the highest shear strength properties. Even if Al-SDFPL substrate showed lower theoretical work of adhesion than Al-SDFPLP, their practical adhesive strength was comparable. This may be attributed to the other contributing factors such as surface roughness of the adherent, interfacial contact area, surface energy and chemical composition of adhesive and adherent; nature of the adhesive etc. in determining the practical adhesive strength.
3.4. Surface topography and roughness by AFM
Fig. 4. Theoretical work of adhesion vs. various surface pre-treatments.
One of the limitations encountered with surface studies was in quantifying the surface roughness of adherents and correlation to adhesion parameters. The recent development in AFM enables both imaging and the quantification of surface roughness to subnanometre resolution. In the present study, each of the pre-treated aluminium substrates was subjected to AFM imaging to understand the surface texture and surface roughness. The AFM images
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Fig. 6. Surface topography by AFM of (a) Al-SD (b) Al-SDFPL and (c) Al-SDFPLP substrates.
of the pre-treated substrates are depicted in Fig. 6 and the measured surface roughness are given in Table 1. The Al-SD substrate showed a compact, less random surface with uniform peaks visually. Al-SDFPL showed a perfectly random porous texture with predominant valleys or pits and has given a roughness average, Ra, of 620 nm. This is attributed to the oxide protrusions which serve the mechanical interlocking and very good interfacial interaction with the epoxy. This has resulted in the highest lap shear strength for aluminium substrates as discussed in the previous section. Al-SDFPLP substrate showed a perfectly random surface with predominant peaks but showed a lower surface roughness average of 390 nm. This may be due to the entrapment of the primer into the pits formed during the FPL etching as shown in the AFM images. The study confirms that a combination of high surface energy, high surface roughness and good wettability of the adhesive altogether contributed for the highest joint strength for the aluminium alloy substrates through the FPL etching pre-treatments.
4. Conclusion In this study, aluminium substrates were subjected to three different surface pre-treatment techniques. The substrate surfaces after various pre-treatments were analysed for contact angle, surface energy, surface topography and adhesive strength properties. The aluminium substrate which had undergone only the solvent degreasing pre-treatment technique showed a lower surface energy than the FPL etched surface. It showed a comparatively poor wetting of epoxy adhesive on its surface and lower surface roughness than the FPL etched surface which resulted in the lowest theoretical work of adhesion and practical adhesive strength. The FPL etching of the substrate resulted in good wetting by the epoxy adhesive, highest surface energy and surface
roughness thereby highest lap shear strength properties. The primed aluminium substrate showed very good interfacial interaction with the epoxy adhesive resulting in comparable lap shear strength properties to that of FPL etched surfaces.
Acknowledgement The authors thank the authorities of Vikram Sarabhai Space Centre for giving permission to publish the article. One of the authors, K.K. Athira is thankful to the National Institute of Technology, Calicut, Kerala, for permitting to conduct her academic project work on this topic. Thanks are also due to the Analytical and Spectroscopy Division of the Vikram Sarabhai Space Centre for analytical support.
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