Materials Letters 211 (2018) 331–334

Contents lists available at ScienceDirect

Materials Lettersjournal homepage: www.elsevier.com/locate/mlblue

CAO bond enhancing direct bonding strength between plastic and puretitanium
Katsuyoshi Kondoh ⇑, Junko Umeda
Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaragi, Osaka 567-0047, Japana r t i c l ei n f oArticle history:
Received 16 August 2017
Received in revised form 10 October 2017
Accepted 14 October 2017
Available online 16 October 2017
Keywords:BondingInterface
Dissimilar material
C@O bondTitaniuma b s t r a c t
Direct bonding process by using the conventional hot pressing was applied to prepare the dissimilarmaterials consisting of the thermoplastic resin and pure titanium (Ti) material. In employing the pressing
temperature at the melting point of plastics, the interfacial bonding shear strength of the dissimilar material using polyamide66 (PA66) was drastically increased. Microstructure and phase analysis clarified thatC@O double bonds of the carbonyl group was thermally decomposed and oxygen atoms contributed todiffusion into pure Ti or oxides formation at the interface. In addition, the chemical reaction of CAH
and NATi formation was also effective to the interfacial bonding improvement. On the other hand, whenpolystyrene (PS) with no functional group including CAO/C@O bond was employed, for example, thePS-Ti dissimilar material showed no bonding strength even then the bonding temperature was close to300 °C.
Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction
Carbon fiber reinforced plastic (CFRP) is widely employed asengineering materials in the transportation industry such as airplanes, automotives, high-speed trains and marines because of its
low density, outstanding corrosion resistance, high specificstrength and Young’s modulus compared to the conventional
industrial metals such as steels and aluminum alloys [1]. In particular, CFRP material, using thermoplastic resins as a matrix, possibly reduces the processing cost due to its excellent formability inpressing and recyclability [2]. From high reliability and enhanced
safety points of view, however, the hybrid materials design using
both CFRP and metals called as ‘‘multi-materials design” is necessary for further lightweight of the components in the transportation industries. In joining the metal sheets and CFRP flat panels,
the use of adhesive agents [3,4] or mechanical joints and fastening
by bolts or rivets [5,6] is common in the industries because of theirease of assembly. They are, however, not complete in joining due toits insufficient reliability and weight increment of the components.
To establish a direct bonding process between CFRP and industrial
metals, the fundamental researches using ultrasonic welding [7,8],
friction welding [9] and laser welding [10,11] are recently carriedout, and successfully achieved a high bonding strength between
metal sheets and CFRP or thermoplastic materials by each process.
⇑ Corresponding author.
E-mail address: kondoh@jwri.osaka-u.ac.jp (K. Kondoh).
https://doi.org/10.1016/j.matlet.2017.10.042
0167-577X/Ó 2017 Elsevier B.V. All rights reserved.

Regarding the bonding mechanism in each process, for example,
Fourier-transform infrared spectroscopy (FTIR) analysis on thebonding interface indicated that a physicochemical interaction
based on hydrogen bonds originated in the polyamide caused achemical bonding of the plastic to aluminum materials [11]. Inaddition, a high gas pressure in gas bubbles generated from themolten plastic material assisted to directly bond the plastic to
the metal surface, and resulted in formation of thin oxide layers
at the interface [10]. In use of friction welding, the similar mechanism in using lase welding mentioned above was reported [9]. On
the other hand, the mechanical interlocking effect at the interface
was also necessary for a strong bonding strength in each process[7,9,11]. The dissimilar bonded materials between CFRP or plastic
and common metals prepared by the above processes showed an
enough bonding stress, and the fractures occurred on the plasticside, not at the bonding interface. The bonding mechanism, however, has not been completely clarified in these studies. In particular, the detail analysis at the local area near the interface is notenough to understand the chemical reaction between the plasticand metals by considering the molecular structural changes ofplastic materials at elevated temperature in bonding. This study
investigates the effect of C@O double bond of the carbonyl groupin polyamide66 on the direct bonding behavior between the plasticand pure titanium plate by using FTIR, Transmission Electron
Microscope with Energy dispersive X-ray spectrometry(TEM-EDS) and X-ray Photoelectron Spectroscopy (XPS), and establishes a chemical bonding model for the dissimilar materials.

332

K. Kondoh, J. Umeda / Materials Letters 211 (2018) 331–334

Fig. 1. Schematic illustration of direct bonding process between CFRP or plastic and metal plate (a) and shear bonding strength test using 50 kN hydraulic tensile testmachine (b). In (a), pure Ti material employed as a metal plate.

2. Experimental procedure
Pure Ti plate with a purity of 99.5% was employed in this study.
It had a mirror-finished surface through various polishing treatments using fine alumina particles (mean diameter; 50 nm) anddiamond lapping paste compounds (mean diameter; 0.25 lm).
The plates of polyamide66 (PA66), polystyrene (PS) and polyethylene terephthalate (PET) were also used for bonding test after polishing treatment using #4000 abrasive paper. The melting point ofeach plastic was measured by differential thermogravimetric analysis (DTA). The direct bonding process of each plastic to metal plateis schematically illustrated in Fig. 1 (a). The metal plate was heatedby contact with a small soldering iron device, and its heat conduction effectively caused a temperature increase of the plastic. The
temperature profile at the interface between them was measured
by a thermocouple inserted in the interface. When the interfacial
temperature reaches the target, the thermocouple is pulled out,
and immediately 5 MPa pressure is applied to pure Ti by an upper
punch (4 mm  4 mm) for 8 s. Examples of the directly bonded
material of PA66 to pure Ti plate is also shown in Fig. 1 (a). TEMEDS was applied to analyze oxygen atoms behavior at the bonding

interface, and FTIR and XPS analysis was carried out to investigate
the molecular structural changes of the plastic materials before
and after bonding to the Ti plate. A shear bonding strength of thedissimilar material was measured as illustrated in Fig. 1 (b).
3. Results and discussion
Fig. 2 shows a dependence of shear bonding strength of the dissimilar bonded materials on the interfacial temperature, where themelting point of PA66, PET and glass transition temperature of PS
measured by DTA are 248 °C, 242 °C and 98 °C, respectively. They
are close to the data measured in the previous studies [12,13]. PureTi plate was used as a counter material. In case of the plastic contains a carbonyl group including C@O double bond such as PA66
and PET, their shear bonding strength of both dissimilar materials(Ti-PA66 and Ti-PET) drastically increases when the interfacial
temperature is over the melting point of each plastic material.
The average of the bonding strength in Ti-PET dissimilar material
is a little higher than that of Ti-PA66 bonded material. In addition,
the bonded materials easily separate and their shear strength iszero in employing the interfacial temperature in bonding less thanFig. 2. Shear bonding strength of dissimilar materials between pure Ti plate and PA66, PS and PET plastic materials prepared by changing bonding temperature.

K. Kondoh, J. Umeda / Materials Letters 211 (2018) 331–334333

Fig. 3. TEM-EDS and electron diffraction analysis on bonding interface of Ti-PA66 bonded material.

Fig. 4. IR spectra of raw pure Ti plate (a), as-received PA66 material (b), PA66 after heat treatment (HT) at 260 °C (c), PA66 after HT at 260 °C under 5 MPa pressure (d), andPA66 of the bonded material (e).

each melting point. On the other hand, Ti-PS dissimilar bonded
materials have no bonding and joining behavior even the interfacial temperature is over its melting point of 98 °C. It is concluded
that the carbonyl group with C@O double bond are necessary to
directly bond the plastic to pure Ti and the bonding temperaturemust be controlled over the melting point of the plastic material.
TEM-EDS analysis in Fig. 3 indicated oxygen concentration areaat the interface of the Ti-PA66 bonded material, and its electron
diffraction detects some oxides (TiO2 and TiO) formation in thelayer. By considering an interstitial solubility of oxygen in a-Ti is
33% in Ti-O binary phase diagram [14], the oxides formation suggests many oxygen atoms diffused from PA66 into pure Ti throughthe bonding interface during hot pressing.
As shown in Fig. 2, the shear bonding strength test already indicated C@O bond has an important role on the direct bonding ofPA66 to Ti plate. First of all, FTIR analysis was conducted to detect
C@O spectrum changes in bonding as shown in Fig. 4. As mentioned above, the conventional hot pressing process was appliedin the preparation of Ti-PA66 dissimilar bonded material. That is,
the effects of the thermal factor and pressure in hot pressing on
the chemical reaction at the interface should be considered. Therefore, as-received PA66 material, PA66 after heat treatment (HT) at260 °C, PA66 after HT at 260 °C under 5 MPa pressure, and PA66
separated from the bonded material were prepared for FTIR analysis. Each spectrum at wavenumbers 3300 cm1, 3100 cm1, 1633cm1, 1539 cm1 is amino group (NH2, stretch), aromatic CAH
group (stretch), C@O bond and NH2 (bending), respectively[15,16]. The specimens (c) and (d) show no change in C@O spectrum compared to the original PA66 (b), that is, the molecular

structural change of C@O bond never occurs by pressure and hightemperature over its melting point. The specimen (e), however,
obviously reveals a significant variation of C@O double bond. ThePA66 specimen separated from the dissimilar bonded materialshows an increment of wavenumber (j) compared to the reference
specimen (b). That is, it is obvious that the direct bonding behavior
between PA66 and pure Ti by hot pressing causes molecular structural changes of PA66 material. A wavenumber, j is expressed as amodified Hooke’s Law in the below by considering the bond as aspring.

pﬃﬃ

j ¼ f=c ¼ 1=ð2pcÞ ðk=lÞ; lðreduced mass of atomsÞ
¼ m1 m2 =ðm1 þ m2 Þð1Þ

where k; spring constant for the bond, m; mass of atom, f; frequency and c; velocity of light (=3  1010 cm/s).
When the bonding strength between two atoms (m1 and m2)
increases, the increment of a spring constant occurs, and resultsin the increase of a wavenumber (jvalue). The commercial PA66
contains two kinds of amides; XAC(@O)AY and XAC(AO)AY, andhas a resonance structure with both amides [17]. In general thebonding strength of C@O bond is much larger than that of the latterwith a covalent bond. As shown in Fig. 4, the wavenumber increment in the PA66 of Ti-PA66 bonded material means the numberof XAC(@O)AY relatively becomes much large compared to XAC(AO)AY amide. It means C@O double bonds in the functional group(carbonyl group) are dominant in the PA66 specimen bonded topure Ti plate. When a polarity change occurs by the electronegativity due to the bonding of another atom to X or Y atom linked up

334

K. Kondoh, J. Umeda / Materials Letters 211 (2018) 331–334

with C atom in the functional group, the number ratio of C@O relatively increases [17,18]. In considering TEM observation result inFig. 3, Ti atoms would chemically bond to C atoms to form metal
complex during hot pressing, and resulted in C@O spectrum variation near the interface of the bonded material as shown above.
4. Conclusions
In summary, the role of CAO/C@O bonds contained in the plastic materials on the direct bonding behavior of the plastic to pureTi, which was one of the industrial light metals by using hot pressing was investigated. Dissimilar bonded materials between pure Ti
and PA66 or PET, having C@O double bonds, showed high shear
bonding strength while Ti-PS bonded one had no bonding behavior. C@O bond in the carbonyl group is essential for chemical bonding between plastic and pure Ti by hot press. The oxygen atoms
thermally separated from C@O bond diffused into Ti specimen,
and resulted in the oxides formation at the bonding interface.
Acknowledgment
Authors would additionally like to thank Mr. Hisashi Imai andMr. T. Izawa for their technical supports in the experiment.
References[1] R.F. Gibson, A review of recent research on mechanics of multifunctional
composite materials and structures, Compos. Struct. 92 (2010) 2793–2810.

[2] K.K. Chawla, Composite Materials, Science and Engineering, Springer, NewYork, 1998.
[3] L. Tong, Failure of adhesive-bonded composite single lap joints with embedded
cracks, AIAA J. 36 (1998) 448–456.
[4] P.T. Cheuk, L. Tong, Failure of adhesive bonded composite lap shear joints withembedded precrack, Compos. Sci. Tech. 62 (2002) 1079–1095.
[5] L.J.H. Smith, Bonded-bolted composite joints, J. Aircraft. 22 (1985) 993–1000.
[6] G. Marannano, B. Zuccarello, Numerical experimental analysis of hybrid double
lap aluminum-CFRP joints, Compos. Part B 71 (2015) 28–39.
[7] F. Balle, G. Wagner, D. Eifler, Joining of Aluminum 5754 Alloy to carbon fiber
reinforced polymers (CFRP) by ultrasonic welding, Proc TMS2009 Conf, In
Aluminum Alloys, San Francisco, CA, 2008, 191–196.
[8] F. Balle, G. Wagner, D. Eifler, Ultrasonic metal welding of aluminum sheets tocarbon fiber reinforced thermoplastic composites, Adv. Eng. Mater. 11 (2009)35–39.
[9] F.C. Liu, J. Liao, K. Kanaka, Joining of metal to plastic using friction lap welding,Mater. Des. 54 (2014) 236–244.
[10] S. Katayama, Y. Kawahito, Laser direct joining of metal and plastic, Scr. Mater.
59 (2008) 1247–1250.
[11] C. Lamberti, T. Solchenbach, P. Plapper, W. Possart, Laser assisted joining of
hybrid polyamide-aluminum structures, Phys. Proc. 56 (2014) 845–853.
[12] L.A. Utracki, Polymer alloys and blends, Thermodynamics and Rheology,
Hanser Pub., NY, 1989.
[13] M. Evstatiev, S. Petrovich, B. Krasteva, Calorimetric studies on PET/PA6 andPA66/PA6 drawn blends with various thermal histories, Bulgarian J. Phys. 30(2003) 70–79.
[14] J.L. Murray, H.A. Wriedt, The O-Ti (oxygen-titanium) system, Bull. Alloy Phase
Diag 8 (1987) 148–165.
[15] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification ofOrganic Compounds, 4th ed., John Wiley and Sons, New York, 1981.
[16] H. Domininghaus, P. Elsner, P. Eyerer, T. Hirth, Kunststoffe-Eigenschaften undAnwendungen, 8th ed., Springer Verlag, Berlin, 2012.
[17] E.J. Hartwell, R.E. Richards, H.W. Thompson, The vibration frequency of thecarbonyl linkage, J. Chem. Soc. 293 (1948) 1436–1441.
[18] R.N. Jones, C. Sandorfy, Chemical Applications of Spectroscopy, Interscience
Pub, New York, 1956.