Comparison of Frictional Properties of CVD-Grown MoS₂ and Graphene Films under Dry Sliding Conditions.

Dae-Hyun Cho,Chan Kim,Se-Doo Oh,Kwang-Seop Kim,Jinhwan Lee,Jaehyuck Jung,Changgu Lee

doi:10.3390/nano9020293

Abstract

In the present study, dry friction and wear properties of atomically thin CVD-grown graphene and MoS2 films on SiO2/Si substrates were compared at low (72 MPa) and high (378 MPa) contact pressures. Analysis of atomic force microscopy images of these films verified that the MoS2 films, which were directly grown on the SiO2/Si substrates, had clean surfaces and made conformal contacts with the substrates. In contrast, the graphene film showed many contaminants on its surface and was loosely bonded with its SiO2/Si substrate due to its wet transfer from a Cu foil to the substrate. The MoS2 film exhibited friction and wear properties superior to those of the graphene film both at low and high contact pressures. We found that the clean sliding surface and strong bonding with SiO2/Si were the main causes of the superiority of the MoS2 film compared to the graphene film. Mild wear occurred in a layer-by-layer fashion at low contact pressure for the MoS2 film. At high contact pressure, severe wear occurred due to failure at the boundary between the MoS2 films and the underlying substrates. At both contact pressures, friction did not increase immediately after the removal of the MoS2 film from the SiO2/Si substrate because the film transferred onto the counter sliding surface and served as a lubricant.

Highlights

Solid lubricants have been used to make sliding systems effective
We speculated that the wear under high contact pressure occurred as a result of the repeated frictional stresses that exceeded the limit of the bonding strength between the 12L MoS2 films and the underlying substrates
Regardless of thickness, the MoS2 films showed lower friction than did the graphene film both at low and high contact pressures

Summary

Introduction

Solid lubricants have been used to make sliding systems effective. In the past few decades, solid lubricants made up of layered crystalline structures and displaying low-friction properties have been reported. Even single-layer graphene, hexagonal boron nitride and molybdenum disulfide (MoS2) have been reported to exhibit low friction when strongly anchored onto an underlying substrate [2,3] These observations imply that the low shear strength of the atomic shear plane is not the only determinant of the outstanding lubricities of these materials. When sliding surfaces make a micro-scale contact area, contaminants would have an undesirable effect on the tribological performance of the graphene Another issue limiting the application of graphene as a solid lubricant is the difficulty of controlling its thickness over a large area. Lubrication engineers can control the clearance between the contacting surfaces in MEMS and NEMS applications In this context, CVD-grown MoS2 would be considered to be an excellent atomically thin solid lubricant for tribological applications if it can be made to have frictional properties surpassing those of CVD-graphene. A transition from mild to severe wear of MoS2 was observed

Sample Preparation

Conclusions