International Colloquium Tribology
ict
expert verlag Tübingen
125
2022
231
Nanoscale war behavior of CVD grown monolayer WS2
125
2022
Himanshu Rai
Deepa Kumar
Deepak Kumar
Zhijiang Ye
Viswanath Balakrishnan
Nitya Nand Gosvami
In this study, the nanoscale wear behavior of chemical vapor deposition (CVD) grown tungsten disulfide (WS2) was examined using atomic force microscopy (AFM). Load-dependent controlled experiments were performed in the interior and at the edge of monolayer WS2 flakes using a diamond-like-carbon (DLC) coated silicon AFM probe. The critical load to initiate wear in the interior region of WS2 flake was found to be significantly higher than the edge regions. Experimental findings also elucidated significant variability in critical load values in both regions where particularly near the edge regions the different wear modes were observed including either sudden removal or gradual removal of the monolayer. The observed difference in wear behavior can be attributed to the presence of structural defects as confirmed via molecular dynamics simulations.
ict2310285
23rd International Colloquium Tribology - January 2022 285 Nanoscale wear behavior of CVD grown monolayer WS 2 Himanshu Rai Department of Materials Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India 110016 Corresponding author: himanshu.rai@mse.iitd.ac.in Deepa Thakur School of Engineering, Indian Institute of Technology Mandi, Himachal Pradesh, India 175075 Deepak Kumar Department of Materials Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India 110016 Zhijiang Ye Department of Mechanical and Manufacturing Engineering, Miami University, Oxford, OH 45056 Viswanath Balakrishnan School of Engineering, Indian Institute of Technology Mandi, Himachal Pradesh, India 175075 Nitya Nand Gosvami Department of Materials Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India 110016 Abstract In this study, the nanoscale wear behavior of chemical vapor deposition (CVD) grown tungsten disulfide (WS 2 ) was examined using atomic force microscopy (AFM). Load-dependent controlled experiments were performed in the interior and at the edge of monolayer WS 2 flakes using a diamond-like-carbon (DLC) coated silicon AFM probe. The critical load to initiate wear in the interior region of WS 2 flake was found to be significantly higher than the edge regions. Experimental findings also elucidated significant variability in critical load values in both regions where particularly near the edge regions the different wear modes were observed including either sudden removal or gradual removal of the monolayer. The observed difference in wear behavior can be attributed to the presence of structural defects as confirmed via molecular dynamics simulations. Keywords: WS 2 , Solid lubricant, 2D materials, Tribology, Atomic force microscopy 1. Introduction Friction and wear are a major concern as they result in significant energy loss and removal of materials, and eventually mechanical failures. Advanced sensing devices such as micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS) has led to the development of various modern devices. For these devices surface related issues including friction and wear are a crucial concern [1]. Tribological properties of these small-scale devices has an important role in achieving desired lifespan but unfortunately conventional lubricants have certain limitations at small-scale [2]. Therefore, solid lubricants are suitable in small-scale devices such as MEMS to achieve the desired tribological properties. Solid lubricants are more efficient under harsh conditions to a great extent [3]. Among various solid lubricants, two-dimensional (2D) materials have great potential due to their superior mechanical, chemical, and thermal properties [4]. 2D materials such as graphene and transition metal dichalcogenides (TMDs) are used extensively in tribological applications. But, graphene has poor lubrication behavior under dry conditions [5] which provides a way to explore TMDs including MoS 2 and WS 2 . Therefore, in the present study nanoscale wear behavior of CVD grown monolayer WS 2 has been explored and it was observed that the edge has lower load carrying capacity than the interior. Inconsistency in the wear behavior is attributed to the presence of structural defects as confirmed using molecular dynamics simulation study. 286 23rd International Colloquium Tribology - January 2022 Nanoscale wear behavior of CVD grown monolayer WS 2 2. Experimental methodology Atmospheric pressure chemical vapor deposition (AP- CVD) was used to grow the monolayer WS 2 on SiO 2 / Si substrate (300 nm SiO 2 on Si). WO 3 nanorods and sulfur powder (99.5% Alfa Aesar) were utilized as precursors and placed in a tube furnace. Further, the synthesis of monolayer WS 2 was accomplished at 850 °C (heating rate 8.5 °C/ min) for 10 min. To investigate the wear behavior of WS 2 , an atomic force microscopy (AFM, Flex Axiom, Nanosurf, Switzerland) was used. Experiments were performed using a sharp AFM probe (Multi75DLC, Budget-sensors, Bulgaria). Raman and photoluminescence (PL) microscopy (LabRAM HR evolution, Horiba Jobin Vyon, France) were used with a laser of 532 nm and spot size 1-2 micron (objective 100x, power ~ 0.2 mW8). 3. Results and discussion 3.1 Initial characterization CVD grown monolayer WS 2 flakes were identified by optical microscopy as shown in Figure 1a. Monolayer nature of the WS 2 flakes was confirmed by performing Raman and PL emission and the results are shown in Figure 1b & c. Figure 1: (a) Optical micrograph showing the monolayer WS 2 flakes (b) Raman intensity, and (c) corresponding PL intensity spectra. 3.2 Nanoscale friction and wear measurements To observe the wear behavior, load-dependent (~ 0.02 µN to ~ 1.35) sliding experiments were performed using AFM. When the experiment was performed within the interior of the WS 2 flake, complete removal of the flake was observed after a certain normal load which leads to the abrupt increase in friction as shown in Figure 2a. Figure 2b & c are the zoomed-out topographic image and its line profile, respectively, which confirms the wear in the experimental region. Zoomed-out friction force maps and its line profile are shown in Figure 2d & e, respec- 23rd International Colloquium Tribology - January 2022 287 Nanoscale wear behavior of CVD grown monolayer WS 2 tively, which shows an increase in the friction force in the experimental region. Figure 2: (a) Friction force variation with the applied normal force measured on the CVD grown monolayer WS 2 . (b) Zoomed-out topographic image of the experiment region and (c) its line profile. (d) Zoomed-out friction map of the experiment region and (e) its line profile. These experiments were performed in the interior of WS 2 . When the experiment was performed at the edge of the WS 2 flake, most experiments revealed gradual removal of the flake initiating at much lower load as shown in Figure 3a. Figure 3b & c are the zoomed-out topographic image and its line profile, respectively, that shows the removal in the form of small fragments. Zoomed-out friction force maps and its line profile are shown in Figure 3d & e, respectively. Figure 3: (a) Friction force variation with the applied normal force measured on the CVD grown monolayer WS 2 . (b) Zoomed-out topographic image of the experiment region and (c) its line profile. (d) Zoomed-out friction map of the experiment region and (e) its line profile. These experiments were performed at the edge of WS 2 . In this work, two distinct wear behavior were observed i) complete removal of the WS 2 flake when the experiments were performed within the interior of WS 2 flake, ii) gradual removal of the WS 2 flake with the increasing load when the experiments were performed at the edge of the flake. Removal of the flake at the edge was initiated at a significantly lower load and persisted up to the maximum applied normal load. Qi et al. [6] systematically studied the wear behavior of mechanically exfoliated monolayer graphene and noticed different wear behavior within the interior and at the edge, similar to present work. They also observed comparatively low load-carrying capacity at the edge than the interior. However, in contrast to exfoliated graphene, buckling of the monolayer near the edge region was not observed. Moreover, experiments in the present work were carried out in the air, so the anticipation of a certain level of organics and adsorbed water on the surface cannot be neglected [7]. 288 23rd International Colloquium Tribology - January 2022 Nanoscale wear behavior of CVD grown monolayer WS 2 4. Conclusions In conclusion, wear behavior of CVD grown WS 2 on a SiO 2 / Si substrate was investigated using AFM. Our experiments revealed that monolayer WS 2 is significantly stronger in the interior region than the edge of the WS 2 . In the interior region, below a critical normal load wear of monolayer WS 2 was not observed. Beyond this load, there is sudden removal of the WS 2 layer and leads to the abrupt increase in the friction. At the edge gradual removal of the WS 2 layer with the increasing normal load was observed in majority of experiments. However, it was also observed that partially removed WS 2 layer can still provide lubrication efficiently. Significantly low wear strength at the edge can be due to the easy bond formation between the DLC coated tip and the edge of WS 2 or due to the presence of structural defects. This study throws light on the wear behaviour of the CVD grown monolayer WS 2 , which is critical for various tribological applications of this atomically thin 2D material as a solid lubricant. References [1] D. Boer, T.M. Mayer, Society 26 (2001) 302-304. [2] A. Erdemir, C.D.-J. of P.D.A. Physics, undefined 2006, Iopscience.Iop.Org (n.d.). [3] P. Sutor, MRS Bull. 16 (1991) 24-30. [4] S. Zhang, T. Ma, A. Erdemir, Q. Li, Mater. Today 26 (2019) 67-86. [5] B.Y.- Wear, undefined 1996, Elsevier (n.d.). [6] Y. Qi, J. Liu, J. Zhang, Y. Dong, Q. Li, ACS Appl. Mater. Interfaces 9 (2017) 1099-1106. [7] J. Gao, B. Li, J. Tan, P. Chow, T.M. Lu, N. Koratkar, ACS Nano 10 (2016) 2628-2635.
