Applied Surface Science 357 (2015) 721–727

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Friction behavior of network-structured CNT coating on puretitanium plate
Junko Umeda a,∗, Bunshi Fugetsu b, Erika Nishida c, Hirofumi Miyaji c, Katsuyoshi Kondoh aabcJoining and Welding Research Institute, Osaka University, Osaka 567-0047, Japan
Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan
Department of Periodontology and Endodontology, Hokkaido University Graduate School of Dental Medicine, Sapporo 060-8586, Japana r t i c l e

i n f oArticle history:
Received 21 May 2015
Received in revised form 1 September 2015
Accepted 6 September 2015
Available online 8 September 2015
Keywords:
Carbon nanotubes
Friction coefﬁcientTi plate
TiC interlayer
Abrasive wear
Carbon solid-solution strengtheninga b s t r a c t
Friction behavior of the network-structured CNTs coated pure Ti plate was evaluated by ball-on-disk
wear test using SUS304 ball specimen under dry condition. The friction coefﬁcient was signiﬁcantly
low and stable compared to the as-received Ti plate with no coating ﬁlm. CNTs coating ﬁlm had two
important roles; self-lubrication and bearing effects to reduce the friction coefﬁcient and carbon solidsolution hardening to improve the abrasive wear property of Ti plate. The annealing treatment at higher
temperature (1123 K) was more effective to reduce the friction coefﬁcient than that at lower temperature(973 K) because the Ti plate surface was uniformly covered with CNTs ﬁlm even after sliding wear test.
This is due to TiC interlayer formation via a reaction between Ti plate and carbon elements originatedfrom CNTs during annealing. As a result, a strong interface bonding between CNTs ﬁlm and Ti plate surface
was obtained by higher temperature annealing treatment, and obstructed the detachment of CNTs ﬁlm
during wear test.
© 2015 Elsevier B.V. All rights reserved.

1. Introduction
Carbon nanotube (CNT) has not only high mechanical strength
but also excellent self-lubrication performance due to its high modulus as well as other carbon materials [1]. Previous studies haveinvestigated CNTs tribological properties as lubricants on the surface of composites reinforced with CNTs. The results indicated that
CNTs have been effective to reduce both wear loss and friction
coefﬁcient of the composite materials [2–4]. On the other hand,titanium (Ti) and its alloy have been used in automotive, aeronautics and chemical industries due to their remarkable mechanicalproperties and excellent corrosion resistance. They have, however,poor tribological properties in contact with other metal materials or
itself due to their high reactivity [5]. The surface modiﬁcation treatments using carbon materials for the Ti matrix have been developed
to improve the wear resistance; for example, a laser cladding process to form titanium carbide (TiC) using Ti–CNTs mixture powders[6], in situ formed TiC dispersoids via reaction between Ti and CNTs
[7] and diamond-like carbon (DLC) surface coating [8]. In particular,a free energy of TiC formation is a large negative value compared∗ Corresponding author.
E-mail address: umedaj@jwri.osaka-u.ac.jp (J. Umeda).
http://dx.doi.org/10.1016/j.apsusc.2015.09.063
0169-4332/© 2015 Elsevier B.V. All rights reserved.

to the other metal carbides [9]. Then, TiC phases are easily synthesized from Ti–C system, and the thermally stable TiC compounds
are obtained on Ti substrate surface. These studies revealed thecoatings onto Ti materials have signiﬁcantly improved tribologicalproperties by TiC hard phases dispersion and low friction coefﬁcientof carbon elements.
In use of CNTs as raw materials, the main problem is to prevent their segregation and to uniformly disperse them into or onto
the materials. This is because their agglomeration easily occurs
due to the strong attractive or repulsive van der Waals forces
between carbon atoms at the most-surface layer of CNTs [10,11].
The wet process using the zwitterionic surfactant solution is oneof the effective methods to completely resolve CNT bundles withno damage and disperse individual CNTs in the solution [12]. The
metal powders such as magnesium (Mg) and aluminum (Al) aredipped into this solution containing un-bundled CNTs to prepare
their composite powders uniformly coated with CNTs [13,14]. Thiswet process is simple and valuable in comparison with the above
previous techniques which are generally applied under high temperature processing. In this study, CNTs coated at relatively low
temperatures to improve the interfacial coherence and metallurgical bonding of the MWCNTs ﬁlm to Ti substrate.
In this study, the network-structured CNT coating ﬁlm wasformed on the pure Ti substrate surface by using the above CNT

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Fig. 1. Surface morphology of network-structured MWCNTs ﬁlm on pure Ti plate after annealing at 1123 K observed by SEM (a) and AFM (b).

dispersed solution, and the effects of heat treatment conditions
after CNTs coating on the microstructural and mechanical properties at the interface between CNT ﬁlm and Ti substrate, and thetribological behavior of the CNTs ﬁlms by wear test under dry conditions were investigated. In particular, the local microstructureevaluation to understand TiC formation effect at the interface after
heat treatment on the adhesion of CNTs ﬁlm to the substrate wascarried out.
2. Experimental details
A commercial pure Ti plate, ASTM Gr 2, was employed as a substrate, and multi-wall carbon nanotubes (MWCNTs), having 9 nm
mean diameter and 1.5 ␮m average length, were used as starting
materials. The isopropyl alcohol (IPA) based zwitterionic surfactant solution with concentration of 1.0 wt% CNTs was preparedaccording to the previous study [13]. Ti plate was dipped into thissolution, and subsequently annealed at 973 K and 1123 K for 1.8 ks
in a vacuum furnace to completely remove IPA solution elements
and solid zwitterionic surfactants [13]. Microstructure analysis on
CNTs and Ti plate surface was carried out by using X-ray diffraction (XRD, SHIMADZU, XRD-6100), optical microscope (OLYMPUS,
DSX-500), scanning electron microscope (SEM, JEOL, JSM-6500F)
equipped with energy dispersive X-ray spectrometer (EDS, JEOL,
JED-2300), transmission electronic microscopy (TEM, JEOL, JEM2010), and atomic force microscopy (AFM, SHIMADZU, SPM-9600).
Fig. 1 shows an example of microstructure analysis on CNTs coating ﬁlm of Ti plate surface (after annealed at 1123 K) by SEM
and AFM. The network-structure consisting of un-bundled individual CNTs was formed on the plate. Micro-hardness test was
conducted by using Vickers micro-hardness tester (SHIMADZU,
HMV-2T) with a loading weight of 0.025 N for 15 s. Ra of asreceived original Ti plate was 0.08 ␮m, by using a proﬁle meter(Tokyo Seimitsu, Surfcom1400D). In additionally, Ra of the MWCNTs ﬁlm coating substrates after annealing was 2.4 at 1123 K and
2.6 at 973 K, respectively. CNTs coated Ti plate and as-received
original Ti plate were served to ball-on-disk wear test equipment (RHESCA Co. Ltd., FPR-2100) under dry sliding conditionsat ambient temperature to investigate their tribological behavior.
SUS304 stainless steel ball with 4.76 mm diameter and 160–178 Hv
micro-hardness was used as a counterpart material. The sliding
speed was 31.4 mm/s, and the applied constant load from the
SUS304 ball was controlled at 9.8 N in 3.6 ks. The friction radius andsliding distance of this wear test were 5 mm and 113 m, respectively. Then, the friction coefﬁcient between the Ti plate surface
and SUS304 ball was automatically calculated during wear test.
Wear tracks and cross sectional view of CNTs ﬁlm of each specimen after wear test were investigated by optical microscope and
SEM–EDS.

3. Results and discussion
3.1. Microstructure analysis of CNTs ﬁlm after annealing
As shown in Fig. 2, XRD proﬁles of as-received pure Ti plate,
CNTs coated Ti plate after annealing 973 K and 1123 K in vacuum.
1123 K annealed specimen clearly indicates TiC diffraction peaks,
which are never detected in the original as-received Ti plate. The
patterns in the narrow scanning range (2 = 33–43◦ ) also showsa small but clear peaks of TiC in the 973 K annealed specimen. Itmeans that TiC formation via reaction between CNTs and Ti plate
was performed during vacuum heat treatment because the Gibbs’
standard free energy of TiC formation is −172 kJ/mol at 1073 K [15].
In addition, Carbon atoms originated in CNTs would diffuse into ␣Ti during annealing treatment at 973–1123 K. As shown in the XRD
proﬁles of the annealed specimens at 973 K and 1123 K in Fig. 2(a),␣-Ti peaks obviously shifted to a lower diffraction angle, comparedto the original Ti plate. That is, a few carbon atoms originated from
CNTs dissolved into ␣-Ti crystal during annealing, and resulted in
the enlarged lattices of ␣-Ti. It means that carbon solid solution
into ␣-Ti occurred because the carbon solubility is limited as about
0.5 wt% at 1273 K. In particular, when the annealing at 1123 K was
employed, not only the carbon solution but also TiC layer formation occurred at the interface between the CNTs coating ﬁlm and Ti
substrate as shown in Fig. 3.
Fig. 4 reveals SEM–EDS line scan analysis on the above CNTs
coated Ti plates after annealing treatment. In case of the specimen
annealed at 973 K (a), carbon (C) and Ti proﬁles from the outermost
surface are constant. The concentrated C elements, however, areclearly detected at the near surface area (about 1.5 ␮m depth) area
in case of 1123 K annealing (b), and the Ti concentration obviously
decreases at the same area. In addition, the higher temperatureannealing treatment accelerates the diffusion and solid solution ofcarbon atoms into pure Ti material as shown in Fig. 4(c), and resultsin the formation of TiC layer at the interface between CNTs ﬁlm andTi plate.

3.2. Micro-hardness distribution at near surface of Ti plate
Fig. 5 shows micro-hardness proﬁles from the outermost surface of Ti plates. CNTs coated Ti specimen with 1123 K annealing
treatment reveals extremely high values (473 Hv) compared to theother specimens. In addition, the hardness gradually decreases in
the inside direction due to the solid-solution strengthening by carbon diffusion under higher temperature annealing treatment as
mentioned in Fig. 4(c). In case of 973 K annealed specimen, a small
increment of micro-hardness at the near surface area (137 Hv) was
detected, comparing the original Ti plate (115 Hv). This is also due

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Fig. 2. XRD proﬁles of as-received pure Ti plate, CNTs coated Ti plate after annealing at 973 K and 1123 K in wide (a) and narrow (b) scanning range.

to a very small amount of carbon solid-solution into Ti crystal underlower temperature heat treatment.
3.3. Friction and wear properties of CNTs coating ﬁlm on Ti plate
The friction coefﬁcient evolution of the original and CNTs coatedTi plate specimens by using ball-on-disk wear test is shown in

Fig. 6. As shown in the original Ti plate (a), the friction coefﬁcient issigniﬁcantly large (average value; 0.95) and the remarkable variation during wear test is observed because of the typical severe
abrasive and adhesive wear damages between the metal ball anddisk specimens. Particularly, a large variation of the friction coefﬁcient would be caused by “stick-slip” sliding phenomenon in theadhesive wear. On the other hand, two specimens coated with CNTs

Fig. 3. SEM observation on the cross-section of MWCNTs ﬁlm coated Ti plate annealed at 1123 K (a). Dark ﬁeld of TEM observation on the cross-section of MWCNTs ﬁlm
coated Ti plate annealed at 1123 K (b) and high resolution TEM image of MWCNTs ﬁlm on Ti plate annealed at 1123 K (c).

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Fig. 4. SEM–EDS line scan analysis on CNTs coated Ti plate after annealing at 973 K (a) and 1123 K (b), and carbon content distribution at near outermost surface (c).

ﬁlm shown in (b) and (c) reveal the low and stable friction coefﬁcient compared to the original specimen with no coating ﬁlm. It
means the CNTs coating ﬁlm contributes to a mild sliding phenomenon under dry sliding condition. In particular, 0.19 averagevalue of CNTs coated Ti plate after annealing at 1123 K suggests aformation of very stable lubricant ﬁlms at the contacting interface
between the ball and plate specimens.
To clarify wear behavior of each specimen, the wear track observation on Ti plate and SUS304 ball was carefully carried out by
using optical microscope and SEM–EDS. The surface morphology
observation results are summarized in Fig. 7. As-received original
Ti specimen (a-1) shows severe damages and a lot of wear debris
at the wear track. The maximum wear depth of the Ti plate surface
is 7.0 ␮m. This is due to an abrasive wear phenomenon because
the micro-hardness of SUS304 ball (160–178 Hv) is much larger
than that of pure Ti plate (115 Hv) and digs up Ti specimen as wear
debris. In addition, the active Ti material easily sticks to the countermaterials (SUS304), and results in an adhesion wear phenomenon.
EDS analysis in (a-2) obviously indicates both Fe elements on Ti
plate and Ti elements on SUS304 ball. In case of Ti plate with CNTs
ﬁlms after annealed at 973 K (b-1), the smooth sliding surface isobserved at the wear track. EDS analysis result, however, means
the remained CNTs coating ﬁlm on the wear track is very limited,
and CNTs transfer to the contacting area of SUS304 ball. Furthermore, Fe and Cr elements originated from SUS304 ball are neverFig. 5. Micro-hardness distribution from outermost surface of as-received Ti plate
and with CNTs ﬁlm after annealing at 973 K and 1123 K.

detected at the wear tracks of Ti plate, and a very small amount of
Ti elements are found at SUS304 ball surface as shown in Fig. 7(b-2).
That is, the main wear mechanism of this specimen is an abrasive
phenomenon, not adhesive behavior. As shown in Fig. 7(c-1), CNTs
coated Ti plate annealed at 1123 K obviously shows the remained
CNTs ﬁlm at the wear track, and a few CNTs transfer to the contacting area of SUS304 ball. The maximum wear depth of the Ti
plate is 1.5 ␮m. This is because severe abrasive damages of the Ti
plate surface contacting with SUS304 ball specimen is completely
obstructed because of both the CNTs coating ﬁlm and surface hardening due to carbon solid-solution shown in Fig. 5, and results invery small and stable friction coefﬁcient as previously shown in
Fig. 6. When comparing the wear track width of the specimen (a)with no coating ﬁlm, that of the specimens (b) and (c) is muchsmall. This is also because of the mild abrasive wear behavior
due to the self-lubricant [16–18] and bearing effects [19–21] of
the network-structured CNTs coating ﬁlm existing at the sliding
interface.
Fig. 8 shows the detail observation results at the wear tracksof each Ti plate surface. In case of as-received Ti plate (a), a lotof ﬁne debris with 0.2–1 ␮m diameter and abrasive wear damages by contacting with SUS304 ball are obviously observed on
the wear track. As shown in (a-1), EDS point analysis on the debris(point 001 and 002) dispersed at the wear track indicates the debris
contains Fe and Cr elements originated from SUS304 ball specimen because the severe abrasive wear phenomenon takes placein both pure Ti plate and SUS304 ball specimens. As shown in
CNTs coating ﬁlms on Ti plate after annealing at 973 K (b), thecoating ﬁlms are locally detached from the Ti plate surface andsome debris are observed at the wear track. As shown in (b-2),
the debris (point 01) contains a very small amount of Fe and Cr
elements (Fe; 0.70 at.%, Cr; 0.19 at.%). It suggests that the mild abrasive wear behavior occurs due to the existence of CNTs ﬁlms as
solid lubricant layers at the dry sliding interface. On the other hand,the network-structured CNTs ﬁlm completely remains on the wear
track of Ti plate in use of the annealing treatment at 1123 K (c),and no debris is also observed at the sliding track. Therefore, as
shown in Fig. 6, the friction coefﬁcient of the Ti specimen (c) completely coated with the CNTs ﬁlm is much low and stable comparedto the specimen (b) with the local coating ﬁlm. The surface morphology difference of CNTs coating ﬁlms between (b) and (c) Ti
plate specimens depends on the afﬁnity of CNTs ﬁlm with Ti platesurface. With increase in the annealing temperature, the acceleration of solid-state diffusion of the carbon atoms originated from
CNTs into Ti becomes more active, and results in the formation of
TiC interfacial layer having an important role to strongly bond theCNTs to the Ti plate. Then, the CNTs coated Ti specimen annealed
at 1123 K shows no detachment of CNTs after wear test shownin Fig. 8(c).

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Fig. 6. Friction coefﬁcient changes of as-received Ti plate (a) and with CNTs ﬁlm after annealing at 973 K (b) and 1123 K (c) by using ball-on-disk wear test under dry sliding
condition.

Fig. 7. Surface morphology observation on wear tracks of Ti plate and SUS304 ball using original Ti (a), Ti with CNTs ﬁlm after annealing at 973 K (b) and 1123 K (c).

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Fig. 8. Detail observation on wear tracks of Ti plates after wear test by SEM, and EDS point analysis on wear tracks of Ti plate and debris dispersed at sliding surface;
as-received Ti plate (a-2) and CNTs coated Ti plate after annealing at 973 K (b-2).

4. Conclusions
A severe abrasive wear phenomenon occurred and a lot of debris
was observed at the wear track when as-received Ti plate with nocoating ﬁlm was employed under dry sliding condition. In case of
the network-structured CNTs ﬁlm coated on Ti plate specimens, avery low and stable friction coefﬁcient was obtained. The averagevalue of the friction coefﬁcient was 0.19 of CNTs coated Ti plate after
annealing at 1123 K, in contrast a value of no coating ﬁlm on Ti plate
was 0.95. CNTs ﬁlm existing at the interface between the Ti plateand SUS304 ball specimens contributed to a formation of mild sliding conditions because of their self-lubricant and bearing effects.
In addition, micro-hardness increment of the outermost surface of
CNTs coated Ti plate caused by carbon solid-solution strengthening
was also effective to obstruct the abrasive wear phenomenon by
SUS304 ball and to reduce the wear loss. SEM–EDS analysis clariﬁed that CNTs ﬁlms completely existed on the Ti plate surface
and no ﬁlm was detached from the surface after wear test due toits strong bonding to Ti plate via carbon solid-state diffusion andformation of TiC interfacial layer by higher temperature annealing
treatment. The frictional behavior of the MWCNTs ﬁlms formed on
the Ti plates after annealing was attributable not only to the specimens bearing effect and self-lubrication but also to TiC formation

at the interface between the MWCNTs ﬁlm and the Ti plate, which
was signiﬁcantly effective in improving the interfacial bonding
strength.
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