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polymer-composites: Improvement of the adhesion between polymers and pseudoelastic shape memory alloys
Materials Science and Engineering A 481–482 (2008) 606–611
Surface engineering of shape memory alloy/polymer-composites: Improvement of the adhesion between polymers and pseudoelastic shape memory alloys K. Neuking ∗ , A. Abu-Zarifa, G. Eggeler Institut f¨ur Werkstoffe-Werkstoffwissenschaft, Ruhr-University Bochum, D-44780 Bochum, Germany Received 16 February 2007; received in revised form 9 May 2007; accepted 15 May 2007
Abstract In recent years, pseudoelastic applications of NiTi shape memory alloys have received considerable attention in the medical field due to the development of medical devices and implants. For such applications it can be beneficial to consider hybrid systems like polymer-coated shape memory metals. The objective of the present work is to show that surface treatments can strongly improve adhesion between a pseudoelastic NiTi shape memory ribbon and polyamide (PA6). In our study a pseudoelastic shape memory ribbon was subjected to different kinds of surface treatments, including mechanical, chemical and physical processing steps. We use injection moulding to produce a composite, consisting of a central NiTi ribbon half of which is fully contained in polyamide. Then, pull-out experiments are performed to characterize the adhesion of the NiTi ribbon in the polymer matrix. We investigate if depending on the surface condition, pull-out stresses can vary. © 2007 Elsevier B.V. All rights reserved. Keywords: NiTi shape memory alloys; Polymer coatings; Adhesion; Surface treatments
1. Introduction Over-stoichiometric NiTi shape memory alloys (SMAs) are used in medical technology for applications like stent-grafts and guide wires which exploit the pseudoelastic effect [1]. NiTi SMAs outperform many other systems in terms of structural strength and intensity of functional properties, but their Ni content is often critically discussed with respect to Ni allergies [2,3]. This is one reason for an increasing interest in polymer coatings on NiTi SMAs [4]. General aspects of hybrid systems consisting of polymers and NiTi SMAs were discussed recently [5] and new work has been performed in the last 5 years. Thus, Kim [6] developed a polymer-composite actuator with thin SMA strips. Tahiri et al. [7] and Winzek et al. [8] increased the functionality of a SMA actor by combining the SMA with a polymer and exploiting the difference between the phase transition temperature of the SMA and the glass transition temperature of the polymer. Yang et al. [9] and Murasawa et al. [10] investigated mechanical behaviour and failure of SMA/polymer-composites. And it
∗
Corresponding author. E-mail address: [email protected] (K. Neuking).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.05.118
was highlighted that surface quality strongly affects the adhesion between the polymer and the metal [11]. Good adhesion between polymer and NiTi shape memory alloys is a prerequisite for all potential applications of this type of hybrid system [12,13]. Smith et al. [14] reported recently, that they could increase the adhesive force between NiTi and a polymer matrix by using silane coupling agents. In the present study, we investigate the effect of mechanical, chemical and physical parameters on the adhesion between a NiTi SMA with 50.8 at.% Ni and a polyamide PA6. 2. Materials and experiments 2.1. Materials We use ribbons of binary commercial pseudoelastic NiTi with a Ni content of 50.8 at.%. Using differential scanning calorimetry (DSC), Ms and Af were identified as −31 and −4 ◦ C, respectively (Fig. 1). All details of the DSC procedure have been published elsewhere [15,16]. The pseudoelastic ribbons (cold deformed and straight annealed) with a cross-section of 0.63 mm × 3.30 mm were provided by Memory-Metalle GmbH, Weil am Rhein. The thermoplastic polymer PA6 (Ultramid B3S,
K. Neuking et al. / Materials Science and Engineering A 481–482 (2008) 606–611
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Fig. 1. DSC chart of the pseudoelastic NiTi material investigated in the present study.
BASF Ludwigshafen) was chosen as coating material, because of its good biocompatibility. It shows a high tensile strength, a good wear resistance, a low-friction coefficient [17–19] and has a proven track record in a number of medical applications [20]. Two coupling agents were considered in the present study. As a first coupling agent, Ultramid 1C (BASF, Ludwigshafen) was applied using a dip procedure. As a second coupling agent, Chemosil 597E (Henkel, D¨usseldorf) was applied using a fine brush. The coupling agents represent organic substances with a good adhesion to both, metallic surface and organic coating. 2.2. Injection moulding We use an Arburg Allrounder 270 M 500–210 for injection moulding. A special mould was used to produce the type of hybrid NiTi/PA6-specimen shown in Fig. 2. Fig. 2(a) shows the dimensions of the composite and Fig. 2(b) shows the NiTi ribbon sticking out of the polymer after injection moulding. Injection moulding was performed at 270 ◦ C, and since it was reported [21,22] that the parameters of injection moulding do not strongly affect the mechanical properties of the composite, the parameters of injection moulding listed in Table 1 were kept constant throughout this study.
Fig. 2. Composite specimen after injection moulding. (a) Schematic drawing with dimensions and (b) photograph of specimen. Table 1 Parameters used for injection moulding Melt temperature of PA6 (◦ C) Mould temperature (◦ C) Injection rate (mm/s) Injection pressure (MPa) Holding pressure (MPa) Holding time (s)
270 60 26 60 55 15
and polishing to a grid size of 500 (EST2), electropolishing using an electrolyte of 21% perchloric and 79% acetic acid 900 s at 20 ◦ C and 10 V (EST3) [23]. The effect of low temperature plasma treatments with different processing gases (oxygen, hydrogen and argon) which affect the chemical surface composition was investigated (EST4). And finally, the effect of two coupling agents, Ultramid 1C (BASF AG, Ludwigshafen) and Chemosil 597E (Henkel KGaA, D¨usseldorf) was considered (EST5). All surface treatments studied in the present work are summarized in Table 2. Three different low-pressure plasma treatment conditions were used (Table 3). The individual elementary surface treatments EST1 to EST5 considered in the present study were used in different combinations. These combined surface treatments (CSTs) are summarized in Table 4.
2.3. Surface treatments 2.4. Surface characterization and mechanical testing We study the influence of a number of different elementary surface treatments (ESTs) of the NiTi ribbon prior to injection moulding. These include rinsing with acetone to remove macroscopic organic impurities (EST1), mechanical grinding
X-ray photoelectron spectroscopy (XPS) allows to analyze the elemental surface composition quantitatively [24]. Different binding states of the detected elements can be distinguished.
Table 2 Elementary surface treatments (ESTs) considered in the present study and a brief description of their effects (according to Ref. [26]) Type
Surface treatment
Procedure
Effect
EST1 EST2 EST3 EST4 EST5
Cleaning Mechanical Chemical Physical Interlayer
Applying detergent Grinding, polishing Electropolishing Low-pressure plasma treatment Applying coupling agent
Removes macroscopic impurities with low adhesion (e.g. grease) Changes surface roughness, removes surface layers (e.g. an oxide layer) Decreases surface roughness, changes chemical composition Activates surface, changes chemical composition Creates surface with different chemical and physical properties
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K. Neuking et al. / Materials Science and Engineering A 481–482 (2008) 606–611
Table 3 Processing parameters used in low-pressure plasma treatments (variations EST4) Type
Process gas
Generator power (W)
Generator frequency (MHz)
Pressure (Pa)
EST4-1 EST4-2 EST4-3
Oxygen Hydrogen 25 vol.% oxygen, 75 vol.% argon
100 100 40
0.04 0.04 13.56
0.05 0.05 10−5
Table 4 Combination of different surface treatments (CSTs) CST#
Combination of ESTs
1 2 3 4 5 6 7 8 9 10 11 12
EST3 + EST4-1 (600 s) EST3 + EST4-2 (600 s) EST2 + EST4-1 (1.8 ks) EST3 + EST4-3 (1.8 ks) EST3 EST2 + EST4-1 (300 s, 50 W) EST2 + EST1 EST3 + EST5 (Chemosil 597E) EST2 + EST1 EST3 + EST5 (Ultramid 1C) EST3 + EST5 (Ultramid 1C) + T4-2 (300 s) EST2 + EST4-1 (1.8 ks) + EST5 (Chemosil 597E) + EST4-1 (600 s)
In case of EST4 treatments the process time is added in brackets.
Table 5 Contact angle Θ and surface energy γ of NiTi SMA after different surface treatments determined with bidestilled water (γ = 72.80 mN/m) Surface treatment
Θ (◦ )
γ (mN/m)
EST3 EST3 + EST4-3 (1.8 ks)
64.75 24.50
45.12 66.95
and physico-chemical methods to modify the surface. In the present study, XPS was used to study the chemical condition of the surface before and after a low-pressure plasma treatment in a mixture of 25% oxygen and 75% argon (EST4-3). In the present study, we use an atomic force microscope of type Autoprobe CP from Park Scientific Instruments (now: Veeco) in contact mode to investigate the influence of a plasma treatment (EST4-3 from Table 3) on surface morphology. Con-
Angle-resolved XPS is a non-destructive method to investigate the distribution of elements or functional groups in a certain depth of the sample surface. The knowledge of the chemical surface composition and the kind of functional surface groups is the basis for evaluating surface reactivity and to apply chemical
Fig. 3. Contact angle measurements. (a) Schematic showing equilibrium of forces and identifying contact angle, (b) water droplet on electropolished NiTi (EST3) and (c) water droplet on NiTi surface after additional plasma treatment (EST4-3).
Fig. 4. Pull-out testing. (a) Schematic illustration of pull-out procedure and (b) three load vs. displacement curves after a combined surface treatment of type CST5.
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Fig. 6. AFM data documenting different surface roughnesses after (a) electropolishing (EST3) and (b) electropolishing and plasma treatment (CST4). Fig. 5. X-ray photoemission spectroscopy results from (a) electropolished NiTi surface (EST3) and (b) electropolished and plasma treated NiTi surface (CST4).
tact angle measurements (CAMs) were also used to describe the shape of a liquid droplet resting on a solid surface. When drawing a tangent line from the droplet to the solid surface, the contact angle is the angle between the tangent line and the solid surface. The starting point is always a simplified balance of forces in the so-called three-phase point between liquid, solid and vapour. This force balance is schematically illustrated in Fig. 3(a), and can be expressed as γSV = γSL + γLV cosθ The smaller the measured contact angle, the better is the wettability and the smaller is the interfacial energy between liquid and solid. In the present study, we use a droplet of distilled water (surface energy between distilled water and air: γ = 72.80 mN/m) in a CAM apparatus of type DSA 10 Mk 2 (Kr¨uss, Hamburg) to evaluate the wetting behaviour and to determine the surface energy between the solid surface and the vapour phase (Table 5). The calculated value for the energy γ SV was obtained using so called equation of state as described in Ref. [25]. Pull-out testing was performed using an electromechanical tensile test machine of type Zwick Z100 (Zwick, Ulm). A schematic illustration of the pull-out procedure is shown in Fig. 4(a). The polymer part of the specimen, shown in Fig. 2(b), is constrained and the NiTi ribbon is gripped and pulled out
in vertical direction. As a result we obtain force versus displacement curves of the type shown in Fig. 4(b). This diagram shows three tests which were performed for the same CST condition (CST5). Pull-out events are accompanied by a sudden drop in the P()-curves (Fig. 4(b)). We use the maximum load just before the pull-out event and calculate an adhesive stress by dividing this maximum load by the total contact area A between the NiTi ribbon and the polymer. This yields an adhesive strength with the unit of a stress which has the character of a combined shear strength (elements of A parallel to the pull-out direction; 98.7%) and normal strength (element of A perpendicular to the pull-out direction; 1.3%). For each surface treatment between 3 and 5 pull-out tests were performed and the error bars are given as the mean deviation of the resulting mean value. 3. Results and discussion The results which are presented in this section do not cover all possible combinations of surface treatments. From the previous section it is clear that this would require too much effort. Instead, we present some representative examples which document typical changes related to the surface treatments. In Fig. 5 we present XPS results which document the chemical changes when an electropolished surface (EST3, Fig. 5(a)) is subjected to a low-pressure plasma treatment of type EST4-3
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Fig. 8. Results from pull-out testing after different combined surface treatments. Ordinate: Mean value of the adhesive strength (see text) of in between three and five experiments with mean deviation from this mean value. Abscissa: Combined surface treatments (as listed in Table 4).
Fig. 7. Scanning election micrographs of (a) NiTi ribbon with a thin layer of coupling agent Ultramid 1C (as applied) and (b) after additional plasma treatment (EST4-3).
(Table 3). Fig. 5 shows peaks which indicate the presence of Zn and C which seem not to be affected by the plasma treatment. Zn and C are impurities which are probably related to our electropolishing procedure. No effort was made to clarify the presence of Zn and C. But it can be clearly seen that a Ni-peak near 800 eV which characterizes the electropolished condition (Fig. 5(a)) vanishes after a plasma treatment of type EST4-3 (Fig. 5(b)). This is a first indication of chemical changes in the surface region associated with plasma treatments. The results of atomic force microscopy (AFM) shown in Fig. 6 demonstrate how in addition to the chemical effect (disappearance of Ni-peak) there is a significant change in surface topography when an electropolished surface (EST3, Fig. 5(a)) is subjected to a low-pressure plasma treatment of type EST4-3. The plasma treatment affects the surface roughness in creating finer and sharper peaks (Fig. 6). The AFM data were evaluated in terms of a mean depth of roughness. The mean depth of roughness changes from 1.59 nm (after electropolishing) to 2.57 nm (after the low-pressure plasma treatment). This shows that plasma treatments do not only affect surface chemistry but moreover surface topography. A coupling agent treatment does not result in significant features which can be detected in the scanning electron microscope (Fig. 7(a)). Fig. 7(a) shows a NiTi surface after the combined surface treatment CST10. A subsequent plasma treatment, in
contrast, does produce surface changes which can be clearly detected in a scanning electron microscope (Fig. 7(b)). The results from pull-out testing are presented in Fig. 8. This diagram shows that different combined surface treatments result in different adhesive strengths. The lowest adhesive strength was observed for a composite where the metal ribbon was electropolished and the plasma treatment was performed in a pure oxygen atmosphere (CST1). This adhesive force is smaller than the adhesive strength of a composite with a NiTi ribbon which is only electropolished (CST5). However, there are surface treatments which can improve adhesive strength significantly. This is the case for the combined surface treatment (CST12), where a mechanically polished surface was subjected to a first plasma treatment of type EST4-1, then covered with the coupling agent Chemosil 597E and then again subjected to the same type of plasma treatment (EST4-1). We cannot provide a physical explanation for this effect. But our result shows that a combination of mechanical, chemical and physical surface treatment steps can significantly improve adhesion. Further work is required to understand this effect. It is clear that one important factor is related to the change in surface energies during surface treatments (Fig. 3(b and c)). 4. Summary and conclusions In the present study, we investigated the adhesion of a polyamide PA6 (Ultramid B3S) on a pseudoelastic NiTi ribbon which was subjected to different surface treatments. Our results show that appropriate combinations of sequential mechanical (grinding and polishing), chemical (electropolishing and application of coupling agents) and physical (plasma treatments) surface treatments can result in good adhesive strength. Interface engineering is the key technology for obtaining good adhesion between polymers and metals. The objectives of the present study were (i) to provide examples for the effect of different surface treatments in this respect and (ii) to identify one surface treatment which provides good adhesive strength. This condition was identified as (i) mechanically polishing followed
K. Neuking et al. / Materials Science and Engineering A 481–482 (2008) 606–611
by (ii) a plasma treatment, (iii) then application of a coupling agent again (iv) followed by a plasma treatment. Further work is required to identify the elementary mechanisms which lead to higher adhesive strength between polymers and metals as a basis for a more systematic approach in surface engineering of metal/polymer-interfaces. Acknowledgements The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) and the MIWFT-NRW through project B3 of SFB 459 (Shape Memory Technology) and support by BASF AG (Ludwigshafen) and Henkel KGaA (D¨usseldorf). Help from Dipl.-Ing. S. Youcheu-Kemtchou (surface treatments), Dr. O. Shekhah (XPS), Dr. S.H. Hong (CAM) and A. Westphalen (AFM) is also gratefully acknowledged. References [1] V. Michaud, Scripta Mater. 50 (2004) 249–253. [2] S. Trigwell, R.D. Hayden, K.F. Nelson, G. Selvaduray, Surf. Interface Anal. 26 (1998) 483–489. [3] R.W.Y. Poon, J.P.Y. Ho, X.Y. Liu, et al., Mater. Sci. Eng. A Struct. 390 (2005) 444–451. [4] W.R. Yang, K.S. Chen, S.K. Wu, J.C. Lee, Thermec ‘2003’ PTS 1–5 (426-4) (2003) 3055–3060. [5] K. Neuking, A. Abu-Zarifa, S. Youcheu-Kemtchou, G. Eggeler, Adv. Eng. Mater. 7 (2005) 1014–1023.
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[6] C. Kim, Int. J. Mod. Phys. B 20 (25–27) (2006) 3733–3738. [7] V.L. Tahiri, E. Patoor, A. Eberhardt, J. Phys. IV 115 (2004) 195–203. [8] B. Winzek, T. Sterzl, H. Rumpf, E. Quandt, J. Phys. IV 112 (2003) 1163–1168. [9] S.Y. Yang, B.C. Goo, H.J. Kim, Key Eng. Mater. PTS 1–4 (297–300) (2005) 1551–1555. [10] G. Murasawa, K. Tohgo, H. Ishii, J. Compos. Mater. 38 (2004) 399– 416. [11] C. Bischof, Materialwiss. Werkstofftech. 24 (1993) 33–41. [12] C.K. Poon, K.T. Lau, L.M. Zhou, Compos. Part B Eng. 36 (2005) 25–31. [13] G.W. Ehrenstein, Kunstst. Plast. Eur. 94 (2004) 28–34. [14] N.A. Smith, G.G. Antoun, A.B. Ellis, W.C. Crone, Compos. Part A Appl. S. 35 (2004) 1307–1312. [15] J. Khalil Allafi, X. Ren, G. Eggeler, Acta Mater. 50 (2002) 793–803. [16] J. Khalil Allafi, A. Dlouhy, G. Eggeler, Acta Mater. 50 (2002) 4255– 4274. [17] C.A. Harper, Handbook of Plastics, Elastomers & Composites, McGrawHill, New York, 2002. [18] G.W. Ehrenstein, Polymeric Materials, Hanser, Munich, 2004. [19] T.A. Osswald, G. Menges, Materials Science of Polymers for Engineers, Hanser, Munich, 2003. [20] N. Hause, Dissertation, Eberhard-Karls-Universit¨at T¨ubingen, T¨ubingen, 2003. [21] V.M. Giese, Dissertation, Universit¨at Erlangen-N¨urnberg, Erlangen, 1995. [22] C. Ziegmann, Dissertation, RWTH Aachen, Aachen, 2001. [23] M. Pohl, C. Hessing, J. Frenzel, Mater. Sci. Eng. A Struct. 378 (2004) 191–199. [24] J.F. Watts, J. Wolstenholme, An Introduction to Surface Analysis by XPS and AES, Wiley, Chichester, 2003. [25] E.L. Decker, B. Frankl, Y. Suo, S. Garoff, Colloid Surf. A 156 (1999) 177–189. [26] J.R.J. Wingfield, Int. J. Adhes. Adhes. 13 (1993) 151–156.
Surface engineering of shape memory alloy/polymer-composites: Improvement of the adhesion between polymers and pseudoelastic shape memory alloys K. Neuking ∗ , A. Abu-Zarifa, G. Eggeler Institut f¨ur Werkstoffe-Werkstoffwissenschaft, Ruhr-University Bochum, D-44780 Bochum, Germany Received 16 February 2007; received in revised form 9 May 2007; accepted 15 May 2007
Abstract In recent years, pseudoelastic applications of NiTi shape memory alloys have received considerable attention in the medical field due to the development of medical devices and implants. For such applications it can be beneficial to consider hybrid systems like polymer-coated shape memory metals. The objective of the present work is to show that surface treatments can strongly improve adhesion between a pseudoelastic NiTi shape memory ribbon and polyamide (PA6). In our study a pseudoelastic shape memory ribbon was subjected to different kinds of surface treatments, including mechanical, chemical and physical processing steps. We use injection moulding to produce a composite, consisting of a central NiTi ribbon half of which is fully contained in polyamide. Then, pull-out experiments are performed to characterize the adhesion of the NiTi ribbon in the polymer matrix. We investigate if depending on the surface condition, pull-out stresses can vary. © 2007 Elsevier B.V. All rights reserved. Keywords: NiTi shape memory alloys; Polymer coatings; Adhesion; Surface treatments
1. Introduction Over-stoichiometric NiTi shape memory alloys (SMAs) are used in medical technology for applications like stent-grafts and guide wires which exploit the pseudoelastic effect [1]. NiTi SMAs outperform many other systems in terms of structural strength and intensity of functional properties, but their Ni content is often critically discussed with respect to Ni allergies [2,3]. This is one reason for an increasing interest in polymer coatings on NiTi SMAs [4]. General aspects of hybrid systems consisting of polymers and NiTi SMAs were discussed recently [5] and new work has been performed in the last 5 years. Thus, Kim [6] developed a polymer-composite actuator with thin SMA strips. Tahiri et al. [7] and Winzek et al. [8] increased the functionality of a SMA actor by combining the SMA with a polymer and exploiting the difference between the phase transition temperature of the SMA and the glass transition temperature of the polymer. Yang et al. [9] and Murasawa et al. [10] investigated mechanical behaviour and failure of SMA/polymer-composites. And it
∗
Corresponding author. E-mail address: [email protected] (K. Neuking).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.05.118
was highlighted that surface quality strongly affects the adhesion between the polymer and the metal [11]. Good adhesion between polymer and NiTi shape memory alloys is a prerequisite for all potential applications of this type of hybrid system [12,13]. Smith et al. [14] reported recently, that they could increase the adhesive force between NiTi and a polymer matrix by using silane coupling agents. In the present study, we investigate the effect of mechanical, chemical and physical parameters on the adhesion between a NiTi SMA with 50.8 at.% Ni and a polyamide PA6. 2. Materials and experiments 2.1. Materials We use ribbons of binary commercial pseudoelastic NiTi with a Ni content of 50.8 at.%. Using differential scanning calorimetry (DSC), Ms and Af were identified as −31 and −4 ◦ C, respectively (Fig. 1). All details of the DSC procedure have been published elsewhere [15,16]. The pseudoelastic ribbons (cold deformed and straight annealed) with a cross-section of 0.63 mm × 3.30 mm were provided by Memory-Metalle GmbH, Weil am Rhein. The thermoplastic polymer PA6 (Ultramid B3S,
K. Neuking et al. / Materials Science and Engineering A 481–482 (2008) 606–611
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Fig. 1. DSC chart of the pseudoelastic NiTi material investigated in the present study.
BASF Ludwigshafen) was chosen as coating material, because of its good biocompatibility. It shows a high tensile strength, a good wear resistance, a low-friction coefficient [17–19] and has a proven track record in a number of medical applications [20]. Two coupling agents were considered in the present study. As a first coupling agent, Ultramid 1C (BASF, Ludwigshafen) was applied using a dip procedure. As a second coupling agent, Chemosil 597E (Henkel, D¨usseldorf) was applied using a fine brush. The coupling agents represent organic substances with a good adhesion to both, metallic surface and organic coating. 2.2. Injection moulding We use an Arburg Allrounder 270 M 500–210 for injection moulding. A special mould was used to produce the type of hybrid NiTi/PA6-specimen shown in Fig. 2. Fig. 2(a) shows the dimensions of the composite and Fig. 2(b) shows the NiTi ribbon sticking out of the polymer after injection moulding. Injection moulding was performed at 270 ◦ C, and since it was reported [21,22] that the parameters of injection moulding do not strongly affect the mechanical properties of the composite, the parameters of injection moulding listed in Table 1 were kept constant throughout this study.
Fig. 2. Composite specimen after injection moulding. (a) Schematic drawing with dimensions and (b) photograph of specimen. Table 1 Parameters used for injection moulding Melt temperature of PA6 (◦ C) Mould temperature (◦ C) Injection rate (mm/s) Injection pressure (MPa) Holding pressure (MPa) Holding time (s)
270 60 26 60 55 15
and polishing to a grid size of 500 (EST2), electropolishing using an electrolyte of 21% perchloric and 79% acetic acid 900 s at 20 ◦ C and 10 V (EST3) [23]. The effect of low temperature plasma treatments with different processing gases (oxygen, hydrogen and argon) which affect the chemical surface composition was investigated (EST4). And finally, the effect of two coupling agents, Ultramid 1C (BASF AG, Ludwigshafen) and Chemosil 597E (Henkel KGaA, D¨usseldorf) was considered (EST5). All surface treatments studied in the present work are summarized in Table 2. Three different low-pressure plasma treatment conditions were used (Table 3). The individual elementary surface treatments EST1 to EST5 considered in the present study were used in different combinations. These combined surface treatments (CSTs) are summarized in Table 4.
2.3. Surface treatments 2.4. Surface characterization and mechanical testing We study the influence of a number of different elementary surface treatments (ESTs) of the NiTi ribbon prior to injection moulding. These include rinsing with acetone to remove macroscopic organic impurities (EST1), mechanical grinding
X-ray photoelectron spectroscopy (XPS) allows to analyze the elemental surface composition quantitatively [24]. Different binding states of the detected elements can be distinguished.
Table 2 Elementary surface treatments (ESTs) considered in the present study and a brief description of their effects (according to Ref. [26]) Type
Surface treatment
Procedure
Effect
EST1 EST2 EST3 EST4 EST5
Cleaning Mechanical Chemical Physical Interlayer
Applying detergent Grinding, polishing Electropolishing Low-pressure plasma treatment Applying coupling agent
Removes macroscopic impurities with low adhesion (e.g. grease) Changes surface roughness, removes surface layers (e.g. an oxide layer) Decreases surface roughness, changes chemical composition Activates surface, changes chemical composition Creates surface with different chemical and physical properties
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K. Neuking et al. / Materials Science and Engineering A 481–482 (2008) 606–611
Table 3 Processing parameters used in low-pressure plasma treatments (variations EST4) Type
Process gas
Generator power (W)
Generator frequency (MHz)
Pressure (Pa)
EST4-1 EST4-2 EST4-3
Oxygen Hydrogen 25 vol.% oxygen, 75 vol.% argon
100 100 40
0.04 0.04 13.56
0.05 0.05 10−5
Table 4 Combination of different surface treatments (CSTs) CST#
Combination of ESTs
1 2 3 4 5 6 7 8 9 10 11 12
EST3 + EST4-1 (600 s) EST3 + EST4-2 (600 s) EST2 + EST4-1 (1.8 ks) EST3 + EST4-3 (1.8 ks) EST3 EST2 + EST4-1 (300 s, 50 W) EST2 + EST1 EST3 + EST5 (Chemosil 597E) EST2 + EST1 EST3 + EST5 (Ultramid 1C) EST3 + EST5 (Ultramid 1C) + T4-2 (300 s) EST2 + EST4-1 (1.8 ks) + EST5 (Chemosil 597E) + EST4-1 (600 s)
In case of EST4 treatments the process time is added in brackets.
Table 5 Contact angle Θ and surface energy γ of NiTi SMA after different surface treatments determined with bidestilled water (γ = 72.80 mN/m) Surface treatment
Θ (◦ )
γ (mN/m)
EST3 EST3 + EST4-3 (1.8 ks)
64.75 24.50
45.12 66.95
and physico-chemical methods to modify the surface. In the present study, XPS was used to study the chemical condition of the surface before and after a low-pressure plasma treatment in a mixture of 25% oxygen and 75% argon (EST4-3). In the present study, we use an atomic force microscope of type Autoprobe CP from Park Scientific Instruments (now: Veeco) in contact mode to investigate the influence of a plasma treatment (EST4-3 from Table 3) on surface morphology. Con-
Angle-resolved XPS is a non-destructive method to investigate the distribution of elements or functional groups in a certain depth of the sample surface. The knowledge of the chemical surface composition and the kind of functional surface groups is the basis for evaluating surface reactivity and to apply chemical
Fig. 3. Contact angle measurements. (a) Schematic showing equilibrium of forces and identifying contact angle, (b) water droplet on electropolished NiTi (EST3) and (c) water droplet on NiTi surface after additional plasma treatment (EST4-3).
Fig. 4. Pull-out testing. (a) Schematic illustration of pull-out procedure and (b) three load vs. displacement curves after a combined surface treatment of type CST5.
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Fig. 6. AFM data documenting different surface roughnesses after (a) electropolishing (EST3) and (b) electropolishing and plasma treatment (CST4). Fig. 5. X-ray photoemission spectroscopy results from (a) electropolished NiTi surface (EST3) and (b) electropolished and plasma treated NiTi surface (CST4).
tact angle measurements (CAMs) were also used to describe the shape of a liquid droplet resting on a solid surface. When drawing a tangent line from the droplet to the solid surface, the contact angle is the angle between the tangent line and the solid surface. The starting point is always a simplified balance of forces in the so-called three-phase point between liquid, solid and vapour. This force balance is schematically illustrated in Fig. 3(a), and can be expressed as γSV = γSL + γLV cosθ The smaller the measured contact angle, the better is the wettability and the smaller is the interfacial energy between liquid and solid. In the present study, we use a droplet of distilled water (surface energy between distilled water and air: γ = 72.80 mN/m) in a CAM apparatus of type DSA 10 Mk 2 (Kr¨uss, Hamburg) to evaluate the wetting behaviour and to determine the surface energy between the solid surface and the vapour phase (Table 5). The calculated value for the energy γ SV was obtained using so called equation of state as described in Ref. [25]. Pull-out testing was performed using an electromechanical tensile test machine of type Zwick Z100 (Zwick, Ulm). A schematic illustration of the pull-out procedure is shown in Fig. 4(a). The polymer part of the specimen, shown in Fig. 2(b), is constrained and the NiTi ribbon is gripped and pulled out
in vertical direction. As a result we obtain force versus displacement curves of the type shown in Fig. 4(b). This diagram shows three tests which were performed for the same CST condition (CST5). Pull-out events are accompanied by a sudden drop in the P()-curves (Fig. 4(b)). We use the maximum load just before the pull-out event and calculate an adhesive stress by dividing this maximum load by the total contact area A between the NiTi ribbon and the polymer. This yields an adhesive strength with the unit of a stress which has the character of a combined shear strength (elements of A parallel to the pull-out direction; 98.7%) and normal strength (element of A perpendicular to the pull-out direction; 1.3%). For each surface treatment between 3 and 5 pull-out tests were performed and the error bars are given as the mean deviation of the resulting mean value. 3. Results and discussion The results which are presented in this section do not cover all possible combinations of surface treatments. From the previous section it is clear that this would require too much effort. Instead, we present some representative examples which document typical changes related to the surface treatments. In Fig. 5 we present XPS results which document the chemical changes when an electropolished surface (EST3, Fig. 5(a)) is subjected to a low-pressure plasma treatment of type EST4-3
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Fig. 8. Results from pull-out testing after different combined surface treatments. Ordinate: Mean value of the adhesive strength (see text) of in between three and five experiments with mean deviation from this mean value. Abscissa: Combined surface treatments (as listed in Table 4).
Fig. 7. Scanning election micrographs of (a) NiTi ribbon with a thin layer of coupling agent Ultramid 1C (as applied) and (b) after additional plasma treatment (EST4-3).
(Table 3). Fig. 5 shows peaks which indicate the presence of Zn and C which seem not to be affected by the plasma treatment. Zn and C are impurities which are probably related to our electropolishing procedure. No effort was made to clarify the presence of Zn and C. But it can be clearly seen that a Ni-peak near 800 eV which characterizes the electropolished condition (Fig. 5(a)) vanishes after a plasma treatment of type EST4-3 (Fig. 5(b)). This is a first indication of chemical changes in the surface region associated with plasma treatments. The results of atomic force microscopy (AFM) shown in Fig. 6 demonstrate how in addition to the chemical effect (disappearance of Ni-peak) there is a significant change in surface topography when an electropolished surface (EST3, Fig. 5(a)) is subjected to a low-pressure plasma treatment of type EST4-3. The plasma treatment affects the surface roughness in creating finer and sharper peaks (Fig. 6). The AFM data were evaluated in terms of a mean depth of roughness. The mean depth of roughness changes from 1.59 nm (after electropolishing) to 2.57 nm (after the low-pressure plasma treatment). This shows that plasma treatments do not only affect surface chemistry but moreover surface topography. A coupling agent treatment does not result in significant features which can be detected in the scanning electron microscope (Fig. 7(a)). Fig. 7(a) shows a NiTi surface after the combined surface treatment CST10. A subsequent plasma treatment, in
contrast, does produce surface changes which can be clearly detected in a scanning electron microscope (Fig. 7(b)). The results from pull-out testing are presented in Fig. 8. This diagram shows that different combined surface treatments result in different adhesive strengths. The lowest adhesive strength was observed for a composite where the metal ribbon was electropolished and the plasma treatment was performed in a pure oxygen atmosphere (CST1). This adhesive force is smaller than the adhesive strength of a composite with a NiTi ribbon which is only electropolished (CST5). However, there are surface treatments which can improve adhesive strength significantly. This is the case for the combined surface treatment (CST12), where a mechanically polished surface was subjected to a first plasma treatment of type EST4-1, then covered with the coupling agent Chemosil 597E and then again subjected to the same type of plasma treatment (EST4-1). We cannot provide a physical explanation for this effect. But our result shows that a combination of mechanical, chemical and physical surface treatment steps can significantly improve adhesion. Further work is required to understand this effect. It is clear that one important factor is related to the change in surface energies during surface treatments (Fig. 3(b and c)). 4. Summary and conclusions In the present study, we investigated the adhesion of a polyamide PA6 (Ultramid B3S) on a pseudoelastic NiTi ribbon which was subjected to different surface treatments. Our results show that appropriate combinations of sequential mechanical (grinding and polishing), chemical (electropolishing and application of coupling agents) and physical (plasma treatments) surface treatments can result in good adhesive strength. Interface engineering is the key technology for obtaining good adhesion between polymers and metals. The objectives of the present study were (i) to provide examples for the effect of different surface treatments in this respect and (ii) to identify one surface treatment which provides good adhesive strength. This condition was identified as (i) mechanically polishing followed
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by (ii) a plasma treatment, (iii) then application of a coupling agent again (iv) followed by a plasma treatment. Further work is required to identify the elementary mechanisms which lead to higher adhesive strength between polymers and metals as a basis for a more systematic approach in surface engineering of metal/polymer-interfaces. Acknowledgements The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) and the MIWFT-NRW through project B3 of SFB 459 (Shape Memory Technology) and support by BASF AG (Ludwigshafen) and Henkel KGaA (D¨usseldorf). Help from Dipl.-Ing. S. Youcheu-Kemtchou (surface treatments), Dr. O. Shekhah (XPS), Dr. S.H. Hong (CAM) and A. Westphalen (AFM) is also gratefully acknowledged. References [1] V. Michaud, Scripta Mater. 50 (2004) 249–253. [2] S. Trigwell, R.D. Hayden, K.F. Nelson, G. Selvaduray, Surf. Interface Anal. 26 (1998) 483–489. [3] R.W.Y. Poon, J.P.Y. Ho, X.Y. Liu, et al., Mater. Sci. Eng. A Struct. 390 (2005) 444–451. [4] W.R. Yang, K.S. Chen, S.K. Wu, J.C. Lee, Thermec ‘2003’ PTS 1–5 (426-4) (2003) 3055–3060. [5] K. Neuking, A. Abu-Zarifa, S. Youcheu-Kemtchou, G. Eggeler, Adv. Eng. Mater. 7 (2005) 1014–1023.
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[6] C. Kim, Int. J. Mod. Phys. B 20 (25–27) (2006) 3733–3738. [7] V.L. Tahiri, E. Patoor, A. Eberhardt, J. Phys. IV 115 (2004) 195–203. [8] B. Winzek, T. Sterzl, H. Rumpf, E. Quandt, J. Phys. IV 112 (2003) 1163–1168. [9] S.Y. Yang, B.C. Goo, H.J. Kim, Key Eng. Mater. PTS 1–4 (297–300) (2005) 1551–1555. [10] G. Murasawa, K. Tohgo, H. Ishii, J. Compos. Mater. 38 (2004) 399– 416. [11] C. Bischof, Materialwiss. Werkstofftech. 24 (1993) 33–41. [12] C.K. Poon, K.T. Lau, L.M. Zhou, Compos. Part B Eng. 36 (2005) 25–31. [13] G.W. Ehrenstein, Kunstst. Plast. Eur. 94 (2004) 28–34. [14] N.A. Smith, G.G. Antoun, A.B. Ellis, W.C. Crone, Compos. Part A Appl. S. 35 (2004) 1307–1312. [15] J. Khalil Allafi, X. Ren, G. Eggeler, Acta Mater. 50 (2002) 793–803. [16] J. Khalil Allafi, A. Dlouhy, G. Eggeler, Acta Mater. 50 (2002) 4255– 4274. [17] C.A. Harper, Handbook of Plastics, Elastomers & Composites, McGrawHill, New York, 2002. [18] G.W. Ehrenstein, Polymeric Materials, Hanser, Munich, 2004. [19] T.A. Osswald, G. Menges, Materials Science of Polymers for Engineers, Hanser, Munich, 2003. [20] N. Hause, Dissertation, Eberhard-Karls-Universit¨at T¨ubingen, T¨ubingen, 2003. [21] V.M. Giese, Dissertation, Universit¨at Erlangen-N¨urnberg, Erlangen, 1995. [22] C. Ziegmann, Dissertation, RWTH Aachen, Aachen, 2001. [23] M. Pohl, C. Hessing, J. Frenzel, Mater. Sci. Eng. A Struct. 378 (2004) 191–199. [24] J.F. Watts, J. Wolstenholme, An Introduction to Surface Analysis by XPS and AES, Wiley, Chichester, 2003. [25] E.L. Decker, B. Frankl, Y. Suo, S. Garoff, Colloid Surf. A 156 (1999) 177–189. [26] J.R.J. Wingfield, Int. J. Adhes. Adhes. 13 (1993) 151–156.