https://doi.org/10.20965/ijat.2020.p0190
Murashima, M. et al.

Paper:New Droplet Removal Polishing Method for
Diamond-Like Carbon with Carbon Fiber Brush
Motoyuki Murashima† , Yusuke Imaizumi, Noritsugu Umehara, and Takayuki Tokoroyama
Department of Micro-Nano Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University
Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan† Corresponding author, E-mail: motoyuki.murashima@mae.nagoya-u.ac.jp[Received July 25, 2019; accepted November 13, 2019]

In this paper, we propose a new polishing methodfor diamond-like carbon (DLC) coatings using a carbon fiber brush (CFB). Surface finishing is an important process for DLC coating applications. A lapping
process is widely used for attaining tetrahedral amorphous carbon (ta-C) coatings, which are a type of DLC
coating containing many droplets, to obtain fine flat
surfaces. The lapping process removes protuberant
parts of droplets rather than the entire droplet. In thispaper, we propose a new polish brush material madeof carbon fiber, called CFB. Carbon fiber has both mechanical strength due to its hard carbonaceous material and flexibility due to its fiber structure. In polishing tests, CFB removed droplets from ta-C coatings
and the removal effect increased with the shorteningof the brush length. The surface profiles of the polished surfaces indicated that a shorter brush length
yielded deep scratch marks on ta-C surfaces. Consequently, the arithmetic average surface roughness ofthe polished ta-C surfaces, S a , had almost the samevalue as that of a non-polished surface. Here, we show
the ability of CFB to remove the droplets without anincrease in the surface roughness. The CFB with thelongest brush length in the present study (12 mm)showed a ten-point average roughness S Z J I S = 75 nm
and S a = 4.7 nm, which were 59% and 22% lower thanthose of the non-polished surface, respectively. Furthermore, the longest CFB removed the entire droplets
whereas a shorter CFB merely removed the protuberant part of the droplets. The result indicates that CFB
polishing can remove entire droplets, which result inabrasive wear or deterioration. From other polishingtests, the optimum polishing distance was determined.
Shorter polishing distances could not remove droplets
sufficiently whereas longer polishing distances caused
deep scratches on ta-C surfaces due to the materialtransferred to the CFB. Accordingly, the polishing distance of 600 m showed the best surface finishing withS Z J I N = 25 nm and R a = 0.43 nm, which were 86%
lower than and similar to those of the non-polishedta-C surface, respectively.

Keywords: carbon fiber brush, diamond-like carbon, polishing, droplet, tetrahedral amorphous carbon

190

1. Introduction
Recently diamond-like carbon (DLC) coatings have attracted increasing attention as a coating material. DLC
coatings, which have a combined structure of sp2 bondand sp3 bond, are one type of amorphous carbonaceous
coatings. They have several excellent properties such as
high hardness, low frictional coefficient, high wear resistance, and chemical stability [1–3]. Thus, they have been
increasingly applied to cutting tools, engine components,
etc. [4–8].
Among DLC coatings, extremely hard coatings containing no hydrogen are called tetrahedral amorphous carbon (ta-C) coatings. They are highly promising materials
for friction applications owing to their excellent mechanical properties [9]. The ta-C coatings deposited via physical vapor deposition are generally known to have someparticle residues, called droplets, on their surfaces, which
are formed with a nucleus of fine particles detached fromthe carbon target at the time of their deposition [10]. Numerous droplets are formed in ta-C coatings deposited via
vacuum arc vapor deposition, a method well known for
attaining good-quality ta-C coatings. The vacuum arc vapor deposition method can deposit fine coatings by using
high-energy ions discharged from the cathode spot to beformed on the cathode surface, but it discharges a large
quantity of droplets simultaneously [11]. Any droplets
present on a DLC coating will create a protuberant parton its surface to deprive it of smoothness. In addition,such droplets on DLC coatings could reportedly cause
their abrasive wear [12] and trigger their thermal degradation in high-temperature environments [13].
Recently, a new method called filtered cathodic vacuum arc (FCVA) deposition has been developed to reducethe droplets formed while depositing coatings [14–18].
The physical vapor deposition method using arc discharge
can deposit hard and high-density DLC coatings by efficiently ionizing carbon atoms through its high-energy arc
discharge. However, arc discharge generates sub-micronto several-micron-sized carbon fine particles, resulting indroplets, which will degrade the smoothness of DLC coatings. In contrast, in the FCVA method, a path of the ionized carbon is bent by the electromagnetic field, resulting
in the prevention of unionized carbon fine particles fromreaching a substrate. However, unlike liquid droplets from

Int. J. of Automation Technology Vol.14 No.2, 2020
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New Droplet Removal Polishing Method for
Diamond-Like Carbon with Carbon Fiber Brusha metal cathode, carbon fine particles are a solid material [19] and they do not stick quickly to the chamber’s
inner wall, making it difficult to remove droplets completely even using FCVA [20]. Hence, there is a need todevelop a technology for removing droplets formed during coating depositions.
As machine parts are generally subjected to polishingprocesses to improve their surface profiles [21–24], DLC
products also generally undergo a surface smoothing process to prevent their friction and wear properties from declining due to the presence of any droplets. The mostwidely applied method for removing droplets currently isa lapping process [11, 12]. In the lapping process, theprotuberant part of droplets is removed to improve thesurface roughness significantly. However, the part of thedroplets below the surface is not removed. Such residual parts of droplets could cause adverse effects such as
cracks/peeled-offs or accelerated wear in the same way as
in ordinary droplets [25]. Hence, there is a need to develop a new method for removing entire droplets.
In this study, we develop a new method for removing droplets from ta-C coatings with a carbon fiber brush(CFB) made of bound carbon fibers. Carbon fibers,a fibrous material of approximately 7 µ m in diameter,
have such mechanical properties as excellent strengthand elastic modulus. As they also have excellent selflubricity [26], new friction materials are being developedby binding carbon fibers in a loop shape to reduce starting torques [27]. In that study about the looped CFB,
authors consider that the flexibly self-deformable brushshape causes the unique friction property reducing starting torques.
Owing to their flexibility and multiple contacting
points, brush materials have been industrially used to remove particles without damaging the surface [28]. However, because of the relatively hard droplets present on
DLC coatings, no successful lapping examples using conventional brushes have been reported. In this study, we
have conducted experiments using a CFB having the combined properties of a flexibly deformable brush and a hardcarbon fiber to develop a new lapping method that attains
excellent removal of droplets on ta-C coatings and is low
in aggressiveness to the opposite face.
In this study, we reveal the basic properties of lapping
ta-C coatings with a CFB, which is a highly promisinglapping brush material, and discuss their lapping mechanism.

2. Experimental Method
2.1. Carbon Fiber Brush and ta-C Specimens
In this study, CFB specimens are manufactured fromcarbon fibers TOREKA TC-700SC-12000 made by Toray
Industries Inc. First, approximately 420,000 carbon fibersof diameter 7 µ m are bound and are fixed in a thermal
contraction tube to prepare a cylindrical brush of diameter 6.0 mm. Then, the cylindrical brush is inserted into
Int. J. of Automation Technology Vol.14 No.2, 2020

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Fig. 1. Schematic of CFB specimen.

Fig. 2. SEM image of as-deposited ta-C surface.

a cylindrical holder and tightened with a tightening ring
to complete the CFB (Fig. 1). Three kinds of CFBs
are manufactured with different brush lengths, 1.5, 3.0,
and 6.0 mm, and the effects of the brush length on thelapping performance are verified.
A ta-C coating, which is to be lapped, is deposited on asilicon wafer via the FCVA method. Fig. 2 shows a scanning electron microscopy (SEM) image of the ta-C coating used in this study. From this figure, we can observe
the white images of droplets scattered on the surface. In
this study, lapping experiments are conducted with a ta-C
coating having hardness of 38.3 GPa and arithmetic average surface roughness of Sa = 6.1 nm.

2.2. Lapping Equipment
In this study, the lapping equipment shown in Fig. 3 isused, where a CFB specimen is installed at the swing arm
tip via a jig and vertical load is applied by the dead weightof 1.0 N installed on the axis of the CFB. Lapping is performed by the relative motion to be generated between the
CFB and the disk specimen by rotating the rotary stage towhich the ta-C disk specimen is attached.
The lapping experiments are undertaken in the atmosphere. The experiments are conducted in the following conditions: lapping rate, 200 mm/s; vertical load,
1.0 N; lapping distance, 200 m. Three different lengthsof CFB specimens, 1.5, 3.0, and 6.0 mm, are prepared to191Murashima, M. et al.

(a) Side viewFig. 4. SEM images of ta-C surfaces (a) before polishing,
and after polishing with the brush lengths of (b) 1.5 mm,(c) 3.0 mm, and (d) 6.0 mm.
(b) Overview

Fig. 3. Schematics of pin-on-disk type polishing equipment.

check the effects of the brush length on the lapping quality of the ta-C coating. Experiments with a CFB of brush
length 6.0 mm are conducted by varying the lapping distance from 200 to 400, 600, 800, and 1000 m to check theeffects of the lapping distance on the lapping quality.

2.3. Surface Observations with SEM and AFM
In this study, the surface profiles of the ta-C coating
are observed with atomic force microscopy (AFM) andSEM before and after the lapping experiments. AFM canobserve the surface irregularities with resolutions of theorder of nanometers by scanning the specimen’s surface
with a probe fitted at the cantilever tip. SEM can observe
the surface profiles by detecting the secondary electrons tobe generated by exciting the electrons in the specimen material by scanning the specimen surface with the electron
beam emitted from an electron gun. In this study, AFM
is used to acquire the height-directional distributions of
the surface before and after lapping, and SEM is used toobserve the surface profiles in a wider vision before andafter lapping.
The ten-point average surface roughness, SZJIS , acquired from the AFM images is used to evaluate the removal of droplets. The heights of some large droplets are
sub-micron level. In this study, an extremely smooth ta-C
coating having an arithmetic average surface roughness
Sa = 6.1 nm is used. Hence, we may consider that theten-point average surface roughness is influenced only by
the droplet heights: an important indicator to evaluate thedroplet removal properties of CFB in this study. From the
AFM images, the arithmetic average roughness Ra for the
cross section with no droplets is calculated to evaluate thelapping properties of CFB in lapping the ta-C coating itself. For calculating the surface roughness, a cut-off valueof 3.3 µ m and an evaluation length of 17 µ m are used.
192

Fig. 5. AFM images of ta-C surfaces (a) before polishing,
and after polishing with the brush lengths of (b) 1.5 mm,(c) 3.0 mm, and (d) 6.0 mm.

3. Experimental Results
3.1. Effects of Brush Length on Lapped Surface
Figures 4–6 show the SEM images of the ta-C surface, the AFM images of the ta-C surface, and the crosssectional profiles of the droplet-less area, respectively,
before and after lapping with CFBs of different brush
lengths. We can observe from Fig. 4 that the droplets
present before lapping have been mostly removed by lapping with a CFB of brush length 1.5 mm and that a certain
quantity of droplets has been removed by lapping with
CFBs of brush lengths 3.0 mm and 6.0 mm. A comparison between Figs. 4(c) and (d) reveals that lapping with a
CFB of brush length 3.0 mm removes more droplets thanthat with a CFB of brush length 6.0 mm. Thus, CFB ofa shorter brush length can remove more droplets than thatof a longer brush length. As several post-lapping residual droplets are relatively small in diameter, the removal
Int. J. of Automation Technology Vol.14 No.2, 2020

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Fig. 8. Effect of brush length on the arithmetic averageroughness of droplet-less area.

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of large droplets present on the surface before lapping
has been prioritized. The droplet-less area, very smooth(Ra = 0.33 nm) before lapping, has been observed to havesome lapping scratches after lapping with CFBs.
Figure 7 shows the ten-point average surface roughness SZJIS before and after lapping with CFBs of different brush lengths as calculated from the AFM images inFig. 5. The ten-point average surface roughness of theta-C coating, which was 181 nm before lapping, is reduced to 47, 66, and 85 nm after lapping with CFBs ofbrush lengths 1.5, 3.0, and 6.0 mm, respectively. They
represent reductions of 74%, 63%, and 53%, respectively,from the value before lapping. Thus, the shorter brush
lengths of CFBs have evidently reduced the ten-point average surface roughness.
Figure 8 shows the arithmetic average roughness Ra as
calculated from the cross-sectional profiles of the dropletless area in Fig. 6. From the figure, we can observethat the arithmetic average roughness of the ta-C coating, which was 0.41 nm before lapping, is increased
to 1.8, 1.6, and 0.65 nm after lapping with CFBs of brush
lengths 1.5, 3.0, and 6.0 mm, respectively. They represent
increases of 350%, 290%, and 60%, over the value before lapping, respectively. Thus, the shorter brush lengthsof CFBs have evidently increased the arithmetic averageroughness of the droplet-less area.
Int. J. of Automation Technology Vol.14 No.2, 2020

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Fig. 9. Effect of brush length on the arithmetic averagesurface roughness of the entire ta-C surface area.

The AFM images in Fig. 5 show the droplets scatteredon the flat surface as demonstrated by their peak heights.
Therefore, any removal of such droplets from the surface
should lead to a reduction of the ten-point average surface
roughness. While Fig. 7 shows that CFBs of shorter brush
lengths can more efficiently remove droplets, they also
evidently degrade the surface profiles more as shown by
the cross-sectional profiles in Fig. 6 and the arithmetic
average roughness in Fig. 8.
Figure 9 shows that lapping with CFBs of different
brush lengths has no effect on the arithmetic average surface roughness of the entire ta-C surface area. That is,lapping with a CFB of brush length 6 mm has generated
the same arithmetic average surface roughness as that before lapping, probably because the smoothing effects ofremoving the droplets and the effects of generating lapping scratches have cancelled out each other. In otherwords, the experiments have proved that lapping with a
CFB can remove droplets and can achieve a degree ofarithmetic average surface roughness similar to that of thedeposited coating.

3.2. Effects of Lapping Time on Lapped Surface
Figures 10 and 11 show the SEM and AFM images,
respectively, of the ta-C coating surface before and after
lapping for the lapping distances of 200, 400, 600, 800,
193

Murashima, M. et al.

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Fig. 12. Effect of polishing distance on the ten-point average surface roughness.

Fig. 10. SEM images of ta-C (a) before polishing, andafter polishing using CFB with the polishing distances of(b) 200 m, (c) 400 m, (d) 600 m, (e) 800 m, and (f) 1000 m.

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Fig. 13. Cross-sectional profiles of droplet-less area before
polishing, and after polishing using CFB with the polishingdistances of 200 m, 400 m, 600 m, 800 m, and 1000 m.

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Fig. 11. AFM images of ta-C (a) before polishing, andafter polishing using CFB with the polishing distances of(b) 200 m, (c) 400 m, (d) 600 m, (e) 800 m, and (f) 1000 m.

and 1000 m. A CFB of brush length 6 mm is used forthese lapping experiments. We can observe from Fig. 10that lapping with a CFB has evidently removed dropletsfrom the ta-C coating surface. While lapping for the lapping distance of 200 m has left relatively small droplets
194

unremoved, such residual droplets decrease in quantity inaccordance with the lapping distances.
The ten-point average surface roughness is calculated
as shown in Fig. 12 based on the AFM measurement results in Fig. 11. The ten-point average surface roughness before lapping is 181 nm, whereas that for the lapping distances of 200, 400, 600, 800, and 1000 m is reduced to 85, 44, 25, 33, and 34 nm, respectively. Thatis, compared with the ten-point average surface roughness before lapping, it is decreased by 53%, 76%, 86%,82%, and 81%, respectively. The ten-point average surface roughness has a tendency to decrease for the lapping
distances up to 600 m but it stagnates for longer lapping
distances.
Figure 13 shows the cross-sectional profiles of thedroplet-less areas in Fig. 11, and Fig. 14 shows their arithmetic average roughness. The arithmetic average roughness before lapping is 0.41 nm, whereas it is increased
to 0.65, 0.49, 0.43, 0.88, and 1.2 nm, after lapping for thelapping distances of 200, 400, 600, 800, and 1000 m, respectively. That is, compared with the arithmetic averageroughness before lapping, it is increased by 59%, 19%,
5.7%, 120%, and 200%, respectively. The arithmetic avInt. J. of Automation Technology Vol.14 No.2, 2020

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Fig. 14. Effect of polishing distance on the arithmetic
roughness of droplet-less area.

erage roughness remains approximately the same as that
before lapping for the lapping distances up to 600 m but
it has a tendency to increase for longer lapping distances.
The aforementioned experimental results reveal that, inlapping with a CFB, the lapping distance is an importantfactor having a significant influence on the lapped surface
profiles. As the lapping process generally depends heavily on the maximum surface pressure of a lapped surface,the efficiency of the lapping process decreases as the lapping time increases, because the entire processed surface
is so smoothed as to have no local high surface pressure
areas that it is not further lapped. Therefore, except for industrial productivity and field sags, generally, there are no
special disadvantages in making the lapping time longer.
However, in this study, lapping for the lapping distancesof longer than 600 m has degraded the surface profiles andarithmetic average roughness as the lapping distance increases as shown in Figs. 13 and 14. These experimental
results suggest that the mechanism of lapping with CFB
should be different from that of lapping with conventional
DLC coatings.

4. Discussions
4.1. Mechanism of Lapping with CFB
The surface profiles of the ta-C coatings are analyzed
before and after lapping with CFBs of brush lengths 1.5,
3.0, and 6.0 mm to reveal the effects of the brush length of
the CFB on the lapping properties of ta-C coatings. In the
spectral analyses, each line among 256 lines of the AFM
images in Figs. 5 and 11, which are composed of 256
× 256 numerical values, is Fourier transformed [29]. The
amplitude change ratio is calculated by dividing the amplitude value of each wavelength by its value before lapping as shown in Fig. 15, where any larger negative amplitude change ratio values are considered to indicate thatthe surface has been made smoother.
We can observe from Fig. 15 that, in the wavelength
range between 0 and 2 µ m, shorter brush lengths result inlarger negative amplitude change ratio values. In otherwords, given that the droplets present on the ta-C surInt. J. of Automation Technology Vol.14 No.2, 2020

Fig. 15. Fast Fourier transform result of the polished surface. Each value of the polished surface is divided by thevalue of the non-polished surface at the same wavelength.

face in the lapping experiments are between sub-microns
and several microns in size, Fig. 15 proves that shorterbrush lengths remove more droplets. This correspondswith the measurement results of the ten-point average surface roughness in Fig. 7, which indicate that shorter brush
lengths result in smaller ten-point average surface roughness values. With one piece of fiber taken for a beam,
when the same force vertically acts on the fiber, a shorter
fiber will become less bent, so that a shorter brush length
makes it stiffer, with a resultant increase in the load thatacts on the droplets.
In the wavelength range between 3 and 5 µ m, the CFBof brush length 1.5 mm produces a smaller amplitudechange ratio than the CFBs of longer brush lengths. In
addition, it produces an amplitude change ratio of +0.2
or more with the wavelength of 10 µ m. That is, lapping with a CFB of brush length 1.5 mm raises the amplitude of frequency components between 6 and 10 µ m.
Fig. 8 shows that the arithmetic average roughness of thedroplet-less area increases with a decrease in the brush
length. That is, lapping with a CFB of a shorter brush
length removes more droplets but its high stiffness generates some lapping scratches on the ta-C coating. The
carbon fibers used in this study have a diameter of 7 µ m.
Fig. 15 shows positive amplitude change ratio values in
the wavelength range of 6 to 10 µ m, which is almost samevalue of the fiber diameter, indicating that CFBs of shorterbrush lengths have such higher stiffness as to make thefibers themselves a cutting edge to micro-cut the ta-C surface (Fig. 16(a)).
As carbon fibers are fine fibers, it is difficult to measure
their hardness. However, during the processing of composite materials containing these fibers, they are known to
wear diamond and DLC-coatings heavily [30–32]. Carbon fibers are also reported to have the same Young’s
modulus as DLC coatings [33]. Therefore, in this study,
we consider that the ta-C coating has been worn by sliding
with the carbon fiber edge. The droplets, which are amorphous carbons containing many sp2 bonds, are consideredsofter than ta-C coatings [9, 11].
As compared with the brush length of 1.5 mm, the195Murashima, M. et al.

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Droplet sweeping ability with long CFB.
(a) No droplet was swept away with CFB of brush
length 3.0 mm, (b) a droplet was swept away with CFB of
brush length 6.0 mm, and (c) the longest CFB with the brush
length of 12 mm swept away more droplets.

Fig. 16. Polish mechanism of CFB with (a) short brush
length and (b) long brush length.

brush length of 6.0 mm exhibits a relatively small stiffness and can flexibly deform, so that it can buckle relatively so largely that it can behave less as a cutting edge.
The amplitude change ratio in the wavelength range between 6 and 10 µ m appears to have taken a negative value
for these reasons. The brush length of 6.0 mm, despite
having a lower droplet removal effect than shorter brush
lengths, removes some droplets to contribute to the amplitude reduction in the low-wavelength range.
In view of the aforementioned discussions, using a CFB
of a longer brush length is considered appropriate to realize an excellent lapping process. Accordingly, a CFB of
brush length 12 mm is newly manufactured and lapping
experiments are conducted with it to obtain the ten-point
average surface roughness SZJIS = 75 nm and the arithmetic average surface roughness Sa = 4.7 nm, which represent reductions of 59% and 22%, respectively, as compared with those before lapping. These results demonstrate excellent lapping properties. The arithmetic average roughness of the droplet-less area Ra = 0.70 nm
is increased by approximately 0.3 nm as compared withthat before lapping (0.41 nm). However, the droplet-lessarea after lapping has a high level of smoothness withRa = 1.0 nm or less. From the AFM images, we can observe several spots after lapping where the droplets havebeen removed (Fig. 17). They are not observed from the
AFM images after lapping with CFBs of brush lengths up
to 3.0 mm. Lapping with CFBs of short brush lengths
with high stiffness appears to have merely removed the
protuberant part of the droplets in the same way as theconventional lapping methods.
It is experimentally established that the interface between the droplets and DLC coating contains more sp2bonds than bulk part [12] and that the presence of such
weakly bonded layers triggers the detachment of the196

Fig. 18. Fast Fourier transform result of polished surface fordifferent polishing distances. Each value of polished surface
is divided by the value of the non-polished surface at thesame wavelength.

droplets [11]. The use of CFBs of longer brush lengths
bends the fibers due to their flexibility and sweeps out theentire droplets without breaking them up (Fig. 16(b)).
This study revealed that the use of long brushes is essential for developing CFBs that are low in opponent
aggressiveness and excellent in the ability to sweep out
droplets. However, as any increase in the brush length
could decrease the droplets removal effect, it would also
be important to design an optimum brush length thatwould match the ta-C to be lapped.

4.2. Accumulation of Lapping Debris
The surface of the ta-C coating is spectrally analyzed before and after lapping with a CFB of brush
length 6.0 mm for the distances between 200 and 1000 m
to reveal the effects of the lapping distances on the surface profiles of the ta-C coating. Fig. 18 shows the spectral analysis results, which indicate that, in the lapping
Int. J. of Automation Technology Vol.14 No.2, 2020

New Droplet Removal Polishing Method for
Diamond-Like Carbon with Carbon Fiber Brush

itself to cause large wear scratches. The lapping experiments revealed that brush-lapping hard ta-C coatings requires hard fibers such as carbon fibers and that, becauseof the aggression by the transferred material derived fromthe ta-C coating, brush-lapping for a long time degrades
the surface profiles. The aforementioned experimental results show that CFB-lapping requires a process such as
dressing a grind stone and that it is important to control
its lapping distance appropriately.

5. Conclusions
Fig. 19. SEM images of CFB end (a) before polishing, andafter polishing for the distances of (b) 200 m, (c) 600 m, and(d) 1000 m.

distance section between 200 and 600 m, the amplitudechange ratio takes larger negative values for longer lapping distances. This suggests that a larger quantity ofdroplets has been removed. The spectral analysis resultscorrespond with the reductions in the ten-point averagesurface roughness in Fig. 12. This may be attributed tothe increase in the number of the contacts of the carbon
fibers with the droplets as the lapping distance increases.
In particular, the experimental results for the lapping distance of 600 m show the smallest amplitude change ratio in the entire wavelength range even if combined withthe graph in Fig. 15, suggesting that a large quantity ofdroplets has been removed and that the surface has been
made smoother. A comparison of Figs. 7, 8, 12, and 14
reveals the smoothest surface roughness for the lapping
distance of 600 m.
The experimental results for the lapping distances of
800 and 1000 m show smaller amplitude change ratios
than those for the lapping distance of 600 m, provingthat the surface has become rougher, which correspondswith the arithmetic average roughness of the droplet-lessarea in Fig. 14. These experimental results also correspond with the AFM images in Fig. 11, where we canobserve some deep lapping scratches for the lapping distances of 800 and 1000 m.
Figure 19 shows the SEM images of the CFB surface
before and after lapping. From this figure, we can observethat an increase in the lapping distance increases the transferred material, assumed to be derived from the ta-C coating, to cover the brush surface. Generally, clogging in thegrinding process reduces the grind efficiency. However,
the increase in the transferred material generates deep lapping scratches as shown in Fig. 11. ta-C coating, which
is an extremely hard material, is known to produce a large
amount of wear by its own worn debris in the friction process [34]. In this study, too, a similar phenomenon appears to have occurred, in which the transferred material
derived from the hard ta-C coating cut the ta-C coating
Int. J. of Automation Technology Vol.14 No.2, 2020

This study revealed the lapping properties of a lappingbrush made of carbon fibers, which is a material having
excellent mechanical strength and flexibility due to its
high aspect ratio. In this study, we revealed the lapping
properties of CFB in lapping DLC coatings where, in particular, the removal of fine particles called droplets hasremained an issue to be addressed.
The lapping experiments with CFBs of different brush
lengths revealed that shorter brush lengths have a betterdroplet removal efficiency. However, we observed somedeep lapping scratches on the surface lapped with CFBs
of shorter brush lengths. The results of surface roughness measurement indicate that a CFB with excellent
droplet removal properties and low opponent aggressiveness could be developed by increasing the brush length.
Lapping with a CFB of brush length 12 mm, which is
the longest in the experiments, generated a lapped surface with the ten-point average surface roughness SZJIS =75 nm and the arithmetic average surface roughness Sa =4.7 nm, which are 59% and 22% of those of the surface
before lapping, respectively. This demonstrates the excellent lapping properties of the CFB. The arithmetic averageroughness of the droplet-less area is Ra = 0.70 nm, which
is approximately 0.3 nm larger than that before lapping(0.41 nm) but the area maintained a high level of smoothness of Ra = 1.0 nm or less even after lapping. The lapping experiments also revealed that a longer brush length
can more effectively discharge the entire mass of droplets,demonstrating the effectiveness of CFB in lapping DLC
coatings.
The lapping experiments for different lapping distancesindicate that lapping for a lapping distance of 600 m
produced the best lapping properties. We observed thatshorter lapping distances left some droplets unremoved
because the lapping time was not sufficiently long. On
the other hand, longer lapping distances degraded surface
profiles by accumulating a transferred material, which
derived from the hard ta-C coating, on CFB surface.
The smoothest surface generated by the lapping distanceof 600 m exhibited the ten-point average surface roughness SZJIS = 25 nm, which is 86% better than that before
lapping. The arithmetic average roughness of the dropletless area Ra = 0.43 nm is nearly the same as that of thesurface before lapping. Thus, this study demonstrated theexcellent lapping properties of the CFB, which can selectively remove droplets.
197

Murashima, M. et al.

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Name:
Motoyuki Murashima

Affiliation:
Assistant Professor, Department of Micro-Nano
Mechanical Science and Engineering, Graduate
School of Engineering, Nagoya University

Address:
Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan

Brief Biographical History:
2012- Ph.D. Student, Graduate School of Engineering, Nagoya University
2014- Visiting Ph.D. Student, MADL Lab., University of Toronto
2016- Assistant Professor, Nagoya University

Main Works:• “Intelligent tribological surfaces: from concept to realization using
additive manufacturing,” Int. J. Mech. Mater. Des., pp. 1-10, 2019.
• “Effect of oxygen on degradation of defects on ta-C coatings depositedby filtered arc deposition,” Surf. & Coat. Technol., Vol.362, pp. 200-207,
2019.

Membership in Academic Societies:• Japanese Society of Tribologists (JAST)• Japan Society of Mechanical Engineers (JSME)• Japan Society for Precision Engineering (JSPE)

Int. J. of Automation Technology Vol.14 No.2, 2020

New Droplet Removal Polishing Method for
Diamond-Like Carbon with Carbon Fiber Brush

Name:

Name:

Yusuke Imaizumi

Takayuki Tokoroyama

Affiliation:

Affiliation:

Master Course Student, Department of MicroNano Mechanical Science and Engineering,
Graduate School of Engineering, Nagoya University

Associate Professor, Department of Micro-Nano
Mechanical Science and Engineering, Graduate
School of Engineering, Nagoya University

Address:

Address:

Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan

Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan

Brief Biographical History:

Brief Biographical History:

2014- Department of Mechanical and Aerospace Engineering, Nagoya
University
2018- Department of Micro-Nano Mechanical Science and Engineering,
Graduate School of Engineering, Nagoya University

2006- Assistant Professor, Nagoya University
2014- Associate Professor, Akita University
2017- Associate Professor, Nagoya University

Membership in Academic Societies:• “Collecting micrometer-sized wear particles generated between
DLC/DLC surfaces under boundary lubrication with electric field,” JSME
Mechanical Engineering Letters, Vol.4, 18-00089, 2018.

• Japan Society of Mechanical Engineers (JSME)

Main Works:

Membership in Academic Societies:

Name:
Noritsugu Umehara• Japan Society of Mechanical Engineers (JSME)• Japanese Society of Tribologists (JAST)• Japan Society of Applied Physics (JSAP)• Japan Society of Refrigerating and Air Conditioning Engineers (JSRAE)• Japan Society for Abrasive Technology (JSAT)

Affiliation:
Professor, Department of Micro-Nano Mechanical Science and Engineering, Graduate Schoolof Engineering, Nagoya University

Address:
Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan

Brief Biographical History:
1995- Associate Professor, Tohoku University
2001- Professor, Nagoya Institute of Technology
2003- Professor, Nagoya University

Main Works:• “Clarification of relationship between friction coefficient andtransformed layer of CNx coating by in-situ spectroscopic analysis,”
Tribology Int., Vol.93, Part B, pp. 660-665, 2016.

Membership in Academic Societies:• Japanese Society of Mechanical Engineers (JSME)• Japanese Society of Tribologists (JAST)• Japanese Society for Precision Engineering (JSPE)

Int. J. of Automation Technology Vol.14 No.2, 2020

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