Date of Award

7-7-2020

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Defects, First Principles Calculation, Friction, Graphene, H2O, MoS2

Supervisor

Ahmet T. Alpas

Rights

info:eu-repo/semantics/embargoedAccess

Abstract

This dissertation focuses on studying sliding friction mechanisms of two-dimensional (2D) materials, namely graphene and MoS2, and delineating the effects of structural defects on their coefficient of friction (COF) values under different test atmospheres. Raman, SEM and cross-sectional TEM studies of samples and counterfaces before and after the wear tests in inert and air atmospheres with different relative humidity (RH) levels were used to identify initial microstructures and formation of sliding induced defects at the wear tracks and within the transferred layers. Using density functional theory (DFT) calculations the roles of undissociated and dissociated H2O molecules at defect sites of graphene and MoS2 layers on the interlayer binding energies (EB) were determined. It was shown that the formation of microstructural defects, including vacancies, as well as the changes in the layer structures of the worn surfaces and transfer layers would modify the EB and change the COF, increasing the COF of MoS2 in high humidity but decreasing that of graphene. Sliding friction tests of graphene conducted in ambient air and under a dry N2 atmosphere showed that in both cases a high running-in COF occurred initially but a low steady-state COF (μS) of 0.05 was reached only when the sliding was continued in air with moisture. DFT calculations indicated that the energy barrier (Eb) of 1.27 eV for dissociative adsorption of H2O was significantly lower in case of reconstructed graphene with a monovacancy compared to pristine graphene (3.53 eV). Cross-sectional TEM of graphene transferred to the counterface revealed a partly amorphous structure incorporating damaged graphene layers with d-spacings larger than that of the original layers. DFT calculations on the reconstructed bilayer AB graphene systems revealed an increase of d-spacing due to the chemisorption of H, O, and OH at the vacancy sites and a reduction in the EB by 30% to 0.21 J/m2 between the bilayer graphene interfaces compared to pristine graphene. Thus, sliding induced defects facilitated dissociative adsorption of water molecules and reduced COF of graphene for sliding tests under ambient and humid environments but not under an inert atmosphere. To advance the application that the H, OH passivation of graphene is an essential part for low adhesion for low friction, 5×10-4 wt.% graphene nanoplates (GNP) were vi dispersed in ethanol to lubricate the friction between DLC coated and uncoated tool steel, where a low μS of 0.06 was achieved and the wear rates of the DLC-coated steel were decreased by 70%. Formation of graphene tribolayers on top of steel contact surfaces and sliding induced bending and occasional fragmentation of graphene layers were observed by cross-sectional FIB-TEM. Similarly, addition of carbon nanotubes (CNTs) into ethanol was used to achieve low friction and low adhesion between an Al-alloy engine block material (319 Al) and a common piston ring coating (CrN). The sliding-induced bending and curling of the CNT tribolayers with formation of cylindrical morphology on the Al contact surface were identified by high resolution SEM. Unlike the defect free CVD graphene, magnetron sputtered MoS2 film exhibited a defect structure incorporating misoriented, fragmented layers. The sliding of MoS2 against Ti-6Al-4V showed low friction in vacuum and inert atmosphere but not in humid air. The formation of reoriented MoS2 tribolayers that were parallel to the sliding surfaces produced an increased μS of 0.13 in air with 82% RH instead of an ultra-low value of 0.007 in dry N2. The Raman and HRTEM depicted the formation of MoO3 and the reduced layer spacing of MoS2 in the tribolayers than the value prior to test, which correlated to increased EB. According to the climbing images nudged elastic band (CI-NEB) simulations, H2O dissociated into H/OH at a triple vacancy (a unit MoS2 missing) site of MoS2 (D-MoS2) with an Eb of 0.08 eV, and further to H/O/H again with a low Eb of 0.24 eV. Dissociated H2O and formation of Mo-O-Mo bonds on the MoS2 surface did not change the EB of the MoS2. The undissociated water molecules placed between the bilayer defected MoS2 formed hydrogen bonds with S atoms and increased EB by 20% to 0.37 J/m2, a process that increased the COF. The methodology developed in this study can be used to investigate the friction mechanisms of other 2D layered materials. Rationalization of these friction mechanisms offers guidance for the use of 2D materials in demanding environments in order to reduce friction and mitigate adhesion of engineering surfaces.

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