Wear-resisting behaviour of graphite diamond-like carbon heterostructure

The 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.

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 Vu 1 , Duc Thang Le 2

Email: cuongxavi@gmail.com

1 Postgraduated Office, Le Quy Don Technical University

2 Sao Do University

Date received: 27/10/2017 Date of post-review correction: 24/3/2018

Release date: 28/3/2018

Abstract

The 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.

Tóm tắt

Nhóm tác giả nghiên cứu vi ma sát và mòn của mảnh graphite mịn trượt trên bề mặt carbon có cấu trúc giống kim cương (DLC) Trong bài báo này, nhóm tác giả giới thiệu một phương pháp mới để kiểm tra các bề mặt có đặc tính chống mài mòn ở cấp độ micromet hay không, bằng cách đo lực ma sát sau nhiều lần trượt tương đối giữa các bề mặt Nghiên cứu này được thực hiện bằng cách sử dụng một thấu kính hiển vi lắp thêm vào thiết bị kính hiển vi đo lực nguyên tử và một đầu dò nanomet Điểm đáng chú

ý là lực ma sát giữa mảnh graphite cực nhẵn có khả năng tự chuyển động về vị trí ban đầu và bề mặt DLC duy trì hầu như không đổi sau rất nhiều lần trượt giữa hai bề mặt này trong các điều kiện nhiệt độ khác nhau Kết quả này là gợi ý về khả năng chống mài mòn của gaphite mịn trượt trên 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 mòn

Từ khóa: Graphite; carbon có cấu trúc giống kim cương; cấu trúc dị thể; chống mài mòn.

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

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 × 4 µm square, 1 µm 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 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 SiO 2 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 µm, sliding velocity v is 1.2 µm 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 µN for wear-verifying experiments Resolutions of normal and lateral force are 1.3 and 0.51 nN, respectively.

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 50 O C, 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 (~ 27 O C, black), 50 (red), 100 (blue), and 120 O C (violent), respectively Sliding distances

(x) are 1.2 µm 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,

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

1000

1500

2000

2500

3000

3500

4000

4500

1164 1470

2136

3012

F fr

Room Temp 50 o 100 o 120 o

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 (F fr ) 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 µm; (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

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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