nanotribology of mos 2 : microscopic simulations of oxidation and friction
DESCRIPTION
Nanotribology of MoS 2 : Microscopic Simulations of Oxidation and Friction. Tao Liang, W. Gregory Sawyer*, Scott S. Perry, Susan B. Sinnott and Simon R. Phillpot University of Florida Materials Science and Engineering *Mechanical and Aerospace Engineering. Experimental Context. - PowerPoint PPT PresentationTRANSCRIPT
Nanotribology of MoS2: Microscopic Simulations of Oxidation and Friction
Tao Liang, W. Gregory Sawyer*, Scott S. Perry, Susan B. Sinnott and Simon R. Phillpot
University of FloridaMaterials Science and Engineering
*Mechanical and Aerospace Engineering
MoS2 Structure
•Identify oxidation mechanisms
•Develop reactive bond-order (REBO) potential for MoS2
•MD Simulations of MoS2 tribology
ABA
BAB
Substitution O for S of Bulk Structure
DFT-LDA calculations show:
E ~ -1.7 eV (-39 kcal/mol)
Substitution of S for O strongly energetically favored
• Atomic oxygen prevalent in low earth-orbit conditions• On space station, each sulfur is hit by 1 atom oxygen per second
Oxygen
Oxidation Energies of MoS2 Edge Structures
Mo
Termination
S Termination
0% coverage 50% coverage 100% coverage
-1.7 -1.5
-1.1
-1.7 -2.1
-2.3
-1.7
-1.3
-1.8
-1.7
-1.6
-1.7
-1.7
-1.0
-1.4 -1.0
-1.7
-2.1
-1.7
5 nm 5 nm
1000 nm 500 nm
MoS2 MoO3
MoO3 island on MoS2 (AFM)
a)Oxidation conditions: 480 °C in the furnace with O2 flowing.
b)The MoO3 island surface is not flat.
Sheehan, Paul E.; Lieber, Charles M. Nanotribology and nanofabrication of MoO3 structures by
atomic force microscopy. Science (1996), 272(5265), 1158-1161.
Thermal Oxidation (AFM)
MoS2 vs. Graphite
• Directional bonding – angular terms• Layered structures with vdW interactions
• Captured for graphite in Adapted Intermolecular Reactive Empirical Bond Order (AIREBO) potential
• Adapt REBO for MoS2
Graphite
MoS2
S..
Mo.
.S …
...S
..M
o..S
REBO Potential for Mo-S Systems
i ij
ijA
ijijR
b rVbrVE )()(
)(2
1jiijij bbb Bond Order:
2/1,
,
...)]())(cos()(1[
Si
Moiijijk
jikikikij NNPGrfb
Cut-off function
Angular Term Coordination Term
• Each bond has one set of pair-wise parameters.
• Each element has one set of many body parameters, G and P.
ijrijij
cij
R AerQrfrV )/1)(()(
ijrij
cij
A BerfrV )()(
Repulsive Term:
Attractive Term:Pair-wise parameters: Q, A, α, B and β
Validation of Mo-S potentialE
xp.
a B c11 c12 a c B c11 c12
3.15
Å
173
GP
a
3.16
Å
12.3
Å
76 G
Pa
238
GP
a
52 G
Pa
450
GP
a
230
GP
a
Mo MoS2
Static Potential Energy Surface of MoS2
(nm)
Path I
0.0030.003
0.003
0.003
0.003
0.0030.003
0.003
0.001
0.001
0.001
0.001
0.287
0.287
0.287
0.287
0.287
0.287
0.287
X
YPath II
(nm)
Path I
0.030.03
0.03
0.03
0.03
0.030.03
0.03
0.01
0.01
0.01
0.01
0.15
0.15
0.15
0.15
0.15
0.15
0.15
X
YPath II
DFT REBO
MD Simulation of MoS2 Tribology
System size: 12071 atomsTemperature: ~100 KDynamic process
Fixed
Rig
id m
ovin
g
DFT
96 atoms0 KStatic process
MD
X
Z
Y Fixed
Rigid moving
Thermostat
Active
17.4 nm
18.9 nm
6.2
nm
Accomplishments• Thermodynamics for oxidation is strongly favorable
• Flexible REBO potential for MoS2
• MD simulation of sliding friction of MoS2
• Thermal-transport properties of MoS2 (with Andrey Voevodin, AFRL)
Opportunities• Oxidation kinetics• Elucidating nature of experimentally observed electronic defects• Role of step edges and oxidation on tribological performance