34
NGHIÊN CỨU KHOA HỌC
Tạp chí Nghiên cứu khoa học - Đại học Sao Đỏ, ISSN 1859-4190 Số 1(60).2018
WEAR-RESISTING BEHAVIOUR OF GRAPHITE
DIAMOND-LIKE CARBON HETEROSTRUCTURE
ĐẶC TÍNH CHỐNG MÀI MÒN CỦA CẤU TRÚC DỊ THỂ
GRAPHITE - DIAMOND-LIKE CARBON
Cao Cuong Vu1, Duc Thang Le2
Email: cuongxavi@gmail.com
1Postgraduated Office, Le Quy Don Technical University
2Sao Do University
Date received: 27/10/2017
Date of post-review correction: 24/3/2018
Release date: 28/3/2018
Abstract
T
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he authors investigate friction and wear behaviours of atomic smooth graphite flake slides on
hydrogenated diamond-like carbon (DLC). In this paper, the authors propose a novel method to examine
if surfaces possess wear-resisting characteristic at micro-scale by measuring frictional force after a
number of sliding cycles. The current study is realized using an objective lens coupling into atomic force
microscope (AFM) head and a visible tip. Notably, frictional force between self-retracting motion (SRM)
graphite flake and hydrogenated DLC surface almost remains unchanged after a number of sliding
cycles in different conditions of temperature. This result suggests a potential of wear-resisting in the
case of atomic smooth graphite mesa slides on DLC surface. This exciting finding is promising for the
potential of utilization of using atomic smooth graphitic-based material as a wear-reducing material in
practical applications.
Keywords: Graphite; diamond-like carbon; heterostructure; wear-resisting.
Túm tắt
Nhúm tỏc giả nghiờn cứu vi ma sỏt và mũn của mảnh graphite mịn trượt trờn bề mặt carbon cú cấu trỳc
giống kim cương (DLC). Trong bài bỏo này, nhúm tỏc giả giới thiệu một phương phỏp mới để kiểm tra
cỏc bề mặt cú đặc tớnh chống mài mũn ở cấp độ micromet hay khụng, bằng cỏch đo lực ma sỏt sau
nhiều lần trượt tương đối giữa cỏc bề mặt. Nghiờn cứu này được thực hiện bằng cỏch sử dụng một thấu
kớnh hiển vi lắp thờm vào thiết bị kớnh hiển vi đo lực nguyờn tử và một đầu dũ nanomet. Điểm đỏng chỳ
ý là lực ma sỏt giữa mảnh graphite cực nhẵn cú khả năng tự chuyển động về vị trớ ban đầu và bề mặt
DLC duy trỡ hầu như khụng đổi sau rất nhiều lần trượt giữa hai bề mặt này trong cỏc điều kiện nhiệt độ
khỏc nhau. Kết quả này là gợi ý về khả năng chống mài mũn của gaphite mịn trượt trờn bề mặt DLC. Kết
quả cũng gợi ra triển vọng ứng dụng thực tế của việc sử dụng graphite mịn như là một vật liệu chống
mài mũn..
Từ khúa: Graphite; carbon cú cấu trỳc giống kim cương; cấu trỳc dị thể; chống mài mũn.
1. INTRODUCTION
Investigation concerning friction- and wear-
reducing material is a mandatory task for
tremendous scientists due to energy resources on
the earth are being depleted by human exploitation
at breakneck speed. One of the reasons leading
to the terrible demand of people for energy is the
energy consumption caused by friction and wear
of moving parts [1]. Besides, unexpected energy
dissipation consequently reduces durability
and reliability of devices. However, application
of superlubric phenomenon, undoubtedly, can
overcome this issues due to its extreme low
of friction and virtual zero wear. In the last two
decades, some researches have been reported
with respect to superlubric materials such as
MoS2 [2], graphite (graphene), [3, 5] CNT [6],
or respect to wear resistance [7, 8]. In practical
application field, especially at micro - and meso
-scale, an inevitable trend is using wear-reduced
heterostructures as solid lubricants. The former
also has been concerned recently [9, 12].
Otherwise, from the prospective of hard disk drive
(HDD) technology, the flying height should be as
small as possible for enhancing the recording
density. As the flying height gradually reduces,
the contact between slider and disk occurs,
namely contact recording [13]. However, contact
recording technique has many disadvantages
so far because of its unexpected friction and
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Tạp chớ Nghiờn cứu khoa học - Đại học Sao Đỏ, ISSN 1859-4190 Số 1(60).2018 35
wear. [14] Recently, scientists have focused on
friction and wear behaviours of graphene/DLC
heterostructure owing to its advantages of ability to
improve technique in HDD technology [7, 11, 12].
Although some works have been published, there
are still limitations because of wear reducing but
not surface - surface contact, which is the typical
feature of moving parts at micro- and meso-scale.
Conventionally, AFM instrument has been widely
used to measure friction of surfaces at nano-scale.
Scientists have attempted using modified AFM tip
to improve size range of samples in their works.
[15, 16]. However, using AFM device to measure
friction at micro-scale still remains challenge
[17]. Thus, using AFM instrument for measuring
friction between surfaces at micro-scale is a
novel contribution to investigation methods of
tribology field.
Here, for the first time, the authors have investigated
friction behaviour of SRM graphite flake slides on
hydrogenated DLC surface. The current study
reveals that frictional force of atomic smooth
graphite on hydrogenated DLC is extremely low.
Particularly, the frictional force virtually remains
unchanged after a specific number of sliding
cycles even at room temperature. These results
maybe propose a potential of utilization of graphite
DLC heterostructure for practical application in
micro electro-mechanical systems (MEMS) as
well as HDD technology fields in future.
2. PRINCIPLE THEORY
2.1. Methodology
Our experiment was carried out with three main
steps including fabrication of samples, transfer
SRM graphite flake on to DLC surface, and
measurement friction between SRM graphite flake
and DLC surface. Graphite flakes were fabricated
from high-quality, highly oriented pyrolytic graphite
(HOPG) substrates by means of reactive ion
etching, using silicon dioxide layer as self-aligned
shadow masks. The fabrication method can be
found elsewhere [18, 22]. Graphite mesas with a
size of 4 ì 4 àm square, 1 àm height with 200 nm
thickness of silicon dioxide cover were obtained.
Owning to SRM mesas represent the contact of
atomic smooth surfaces the authors then verified
if graphite flake possesses SRM behaviour
using optical microscope (OM, HiRox KH-3000)
and micro-manipulator MM-3A (Kleindiek MM-
3A). The authors used 3D micro-manipulator
and home-built tungsten tip (chemical etching)
to transfer SRM graphite flake on hydrogenated
DLC surface. The latter was fabricated by plasma
enhanced chemical vapor deposition technique,
which is similar to that of the method has been
presented in previous researches [13, 23, 24]. To
measure friction between the SRM graphite flake
and the DLC surface, an objective lens ( ,
Mitutoyo, Japan) coupling into the head of AFM
instrument (NT-MDT, Russia) and a visible tip
(VIT-P) were utilized. In our experiments, the AFM
tip was acted on the central area of the SiO2 cap.
The normal force, N, applied to the cap by the tip
can be precisely measured (in the accuracy on
the order of 3.98%) and is controlled through the
AFM feedback system. The lateral (shear) force,
F, was applied to the cap by the same tip through
the friction between the tip and the cap and can be
also precisely measured (in the accuracy on the
order of 0.7%) by the AFM.
2.2. Experimental model
The schematic diagram of experiment is illustrated
in Fig. 1. The deflection and torsion of the cantilever
are simultaneously obtained upon on a quadrant
photodiode of AFM device then are converted
to force unit through the well-known calibration
method for AFM cantilever [25, 27].
Fig. 1 (colour online). The schematic diagram of
the experimental model adopted in this study. An
objective lens ( , Mitutoyu, Japan) is coupled
onto AFM head combining with a special AFM
cantilever with an extruded tip mounted in front
in order to clearly observe the movement of SRM
graphite flake. The latter with 200 nm thickness of
SiO2 cap is transferred on the DLC surface then
both are placed on heat stage. XYZ piezoelectric
scanner tube is utilized to control the movement of
sample. Solid line schematic illustrates the track
of graphite flake in forward direction and dashed
line depicts the track in backward direction. Sliding
distance (x) is 1.2 àm, sliding velocity v is 1.2 àm
per second. N and F are the applied normal and
lateral forces are in situ calibrated through the
well-known calibration method for AFM cantilever
based on obtained bending and torsion of cantilever
upon a quadrant photodiode. Applied normal force
N adopted in the study: 7.267 to 17.702 àN for
wear-verifying experiments. Resolutions of normal
and lateral force are 1.3 and 0.51 nN, respectively.
36
NGHIấN CỨU KHOA HỌC
Tạp chớ Nghiờn cứu khoa học - Đại học Sao Đỏ, ISSN 1859-4190 Số 1(60).2018
Frictional force in SRM graphite flake and DLC
surface is defined since applied normal force
reaches a certain magnitude that can drive
SRM graphite flake to slides on DLC surface. At
present, AFM tip and SRM graphite flake with SiO2
cap simultaneously slide on DLC surface. As it can
be seen in Fig. 1, solid line indicates the trace of
AFM tip and SRM flake in forward direction and
dashed line shows the trace in reverse direction.
The friction between the SRM graphite flake and
the DLC surface is recorded after each certain
sliding cycles until reaching a specific number of
sliding cycles while the flake sliding on the DLC
surface all the time.
2.3. Results and discussions
Fig. 2a shows representative lateral and normal
forces (inset) in both forward and backward
directions of sliding process at room temperature.
In general, friction is measured by placing the tip
under an applied force and driving the sample
beneath it [28]. This motion leads to an effective
twist of the cantilever as torsion and is induced
in opposite directions for the different sliding
directions. For a single sliding line, the resulting
friction data generate in the form of a friction
loop. The vertical parts of the loop correspond
to the regions of static frictional force before
the movement of graphite flake occurs, while
the horizontal parts correspond to the kinetic
friction during sliding process. In practice, friction
forces are most often reported as average of
kinetic friction. In return, average kinetic friction
is calculated as a half of difference between the
mean lateral forces during forward and backward
motions. Generally, friction loops in Fig. 2 must be
drifted in the zero point, but in our experiments,
friction loop shift in the point larger than zero point.
This effect may be due to crosstalk effect caused
by applied normal force, however [25]. Obtained
lateral force loop is similar to the dynamical friction
loop of AFM device’s principle that has been
reported elsewhere [29, 30]. The Inset in Fig. 2(a)
generates variation trend of applied normal force
at entire sliding process of forward and backward
directions. Obviously, the normal load in forward
direction is almost the same in comparison with its
magnitude in reverse direction, namely the normal
load is invariant in the entire sliding process.
Fig. 2(b) shows typical lateral force loops at
different conditions of temperature corresponding
to room temperature ( ~ 27°C), 50, 100 and 120°C
(black, red, blue, and violet curves, respectively).
Obviously, obtained curves in Fig. 2(b) indicate
both static and kinetic friction decreases since
temperature increases. In other words, when
temperature increases, dynamic friction decreases
due to reducing of water-related substance at high
temperature. Additionally, thermal activation also
effects on variation of friction through the decrease
of energy barrier as well [31, 32].
(a)
(b)
Fig. 2 (colour online). Representative lateral force
(F) and applied normal force (N) loops at different
temperature (t) conditions. (a) Lateral force and
applied normal force loops at temperature condition
of 50OC, blue curves exhibit lateral force and
applied force (inset) in forward direction, red curves
represent lateral force and applied force in reverse
direction. (b) Summary lateral force loops at different
conditions of temperature corresponding to room
temperature (~ 27OC, black), 50 (red), 100 (blue),
and 120OC (violent), respectively. Sliding distances
(x) are 1.2 àm for all experiments
The authors next characterized the variation trend
of frictional force when SRM graphite flake slides
on DLC surface from initial movement to after a
specific number of cycles at different conditions of
temperature. Variation of frictional force in Fig. 3
shows a surprising trend, that is, frictional force
is almost unchanged after a number of sliding
cycles even at room temperature (blue) and this
trend become obviously at higher temperature
conditions (olive, orange, and pink colour curves
in Fig. 3). The blue, olive, orange, and pink dashed
line in Fig. 3 indicate average magnitudes - 3012,
2136, 1470, and 1164 nN (corresponding to 0.195,
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Tạp chớ Nghiờn cứu khoa học - Đại học Sao Đỏ, ISSN 1859-4190 Số 1(60).2018 37
0.1335, 0.093, and 0.07 MPa per unit area) - of
frictional forces at room temperature, 50, 100,
and 120°C, respectively. Obviously, these data of
frictional forces are well approximated compared
with the average magnitudes with an acceptable
statistical offset. Despite of these obtained results
are slightly higher in compared with the result of
super-lubricity phenomenon reported by Liu et
al [4], but significant lower in compared with the
shear-strength magnitude have been presented
with respect to superlubricity phenomenon in
previous research [3, 33].
The invariant friction after a number of sliding
cycles suggests a prediction that no-wear
state hold when SRM graphite flake is taken
into contact with DLC surface. In case of the
wear exists, the occurrence of wear causes the
increase of surface’s roughness. In its return,
the latter subsequently leads to a gradual rise
in the frictional force [34, 35]. In contrast, when
the wear does not occur, frictional force is, of
course, almost unchanged when the applied force
remains constant. In our experiments, the applied
force was controlled as an invariant value by the
AFM feedback system itself. Taken together, our
obtained results thus propose one conclusion that
there is almost zero wear in case of graphite flake
slides on DLC surface under experience a certain
applied normal force in a range not lead to plastic
deformation of surfaces.
0 500 1000 1500 2000
1000
1500
2000
2500
3000
3500
4000
4500
1164
1470
2136
3012
F f
r (
nN
)
Room Temp. 50 o 100o 120o
n (cycles)
1200
1500
1800
2100
2400
2700
3000
3300
3600
3900
4200
4500
1200
1500
1800
2100
2400
2700
3000
3300
3600
3900
4200
4500
1200
1500
1800
2100
2400
2700
3000
3300
3600
3900
4200
4500
Fig. 3 (colour online). Variation trend of frictional
force (Ffr) when graphite flake slides on DLC surface
at different conditions of temperature (t) after a
certain number of sliding cycles (n). Each data point
corresponds to average in a dozen subsequent
sliding lines, error bar demonstrates the standard
deviation. Dashed lines indicate average
magnitudes in each temperature condition
To certify above conclusion, the authors further
confirm if there are wear-related traces at DLC
surface and SRM graphite flake by conducting
AFM topography image area and Raman spectra
of several selected points in the sliding area after
SRM graphite mesa is controlled to move away.
The authors speculate that the roughness is
significantly increased if DLC surface damaged
after a number of cycles - the occurrence of
wear at DLC surface. Similarly, the 2D peak
corresponding to the featured peak of graphene
layer(s) will be observed if wear-related traces
occurs in SRM graphite flake.
(a)
(b)
Fig. 4. AFM topography and Raman spectra
images indicate topography and featured peaks
of surface in the sliding area when GF slides on
DLC surface after a certain number of cycles. (a)
AFM topography image indicates nearly atomic-
scale smoothness of DLC after a number of sliding
cycles both within (dashed yellow lines area) and
without the sliding area with the RMS deviation of
profiles (R q) are less than 0.2 nm (bottom curves).
Scale bar is 1 àm; (b) Raman spectra of two
selected random points in the sliding area (blue
square area in Fig. 4a) exhibit D and G peaks at of
~1340 and 1577 cm-1 of Raman shift (RS) positions
(vertical axis indicates Raman Intensity, RI)
As it is depicted in Fig. 4(a), the DLC surface under
experienced sliding beneath the SRM graphite
flake after a number of sliding cycles indicates
38
NGHIấN CỨU KHOA HỌC
Tạp chớ Nghiờn cứu khoa học - Đại học Sao Đỏ, ISSN 1859-4190 Số 1(60).2018
nearly atomic-scale smoothness surface with the
root-mean-square deviation of profiles (Rq) are
less than 0,2 nm both within (dashed yellow lines
area) and without the sliding area. This value is
the same order as the roughness of the intrinsic
DLC surface reported in previous research. [36]
This result proves that DLC surface is preserved
without wear when it is controlled to slide
beneath the SRM graphite flake after a number of
sliding cycles.
Raman spectroscopy shows many differing
features in the Raman shift region of 800 – 3000
cm-1, which can be utilized to distinguish the
nature of the bonding between carbon atoms.
In particular, the so-called D, G, and 2D peaks,
which correspond to the Raman shift (RS) of
around 1360, 1560, and 2700 cm-1 have been
widely used to confirm if a surface is DLC or
graphene/graphite [37]. Our obtained Raman
spectra of two selected random points in sliding
area are plotted in Fig. 4(b), in which mere D and
G peaks presenting hydrogenated DLC material
[11] are observed. It simply means that the sliding
area is purely hydrogenated DLC. This evidence
obviously indicates that there is no any debris of
graphene/graphite in the DLC surface after sliding,
thus there is virtually zero wear at both graphite
and DLC surfaces when the SRM graphite flake is
taken into contact with DLC surface.
3. CONCLUSIONS
Our results with a virtual unchanged of friction after
a number of sliding cycles have provided a reliable
foundation to certify wear-resisting behaviour
of atomic smooth graphite/hydrogenated DLC
heterostructure. Although there are some worth
attention results revealed by these studies, there
are also limitations that can improve in further
work. (i) Contaminations may be absorbed into
the gap between graphite flake and DLC surface
caused by the transfer and experiment processes
are realized at ambient air. This issue will lead
to extra frictional force compared with the result
of intrinsic SRM graphite mesa on DLC surface.
(ii) Self-clean effect maybe not clearly observed
in this instance due to sliding distance is quite
small compared with the size of graphite mesa.
(iii) Square mesa might not absolutely move
along its edges while sliding on DLC surface. This
unexpected factor will lead to extra frictional force
by the reason of unexpected edge effect.
To conclude, the authors have investigated
variation of friction when SRM graphite flake
slides on DLC surface after a specific number of
cycles at different conditions of temperature using
an objective lens coupling into AFM head of AFM
device and a visible tip. Our study reveals that
frictional force remains almost unchanged after a
number of sliding cycles in all cases of different
temperature conditions. The current study serves
as a proof-of-concept that atomic smooth graphite
flake slides on DLC coating surface could be
used as friction-reduced and wear-resisting of
moving parts on device as well as a possibility of
utilization of atomic smooth graphitic material in
MEMS applications. This study may offer a new
strategy to treat shortcomings in contact recording
technology of HDD technique in future as well.
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