Đặc tính chống mài mòn của cấu trúc dị thể graphite - Diamond-like carbon

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 LIấN NGÀNH CƠ KHÍ - ĐỘNG LỰC 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, LIấN NGÀNH CƠ KHÍ - ĐỘNG LỰC 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. 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