A Theoretical Investigation

1
9th International Conference on Atomically
Controlled Surfaces, Interfaces and Nanostructures
2007/11/13
PS2-62
A Theoretical Investigation on the Wear
Prevention Mechanism of ZDDP Boundary Lubricating
Film
?Tasuku Onodera1, Yusuke Morita1, Ai
Suzuki2, Riadh Sahnoun1, Michihisa Koyama1,
Hideyuki Tsuboi1, Nozomu Hatakeyama1, Akira
Endou1, Hiromitsu Takaba1, Momoji Kubo1, Carlos
A. Del Carpio1, Clotilde Minfray3, Jean-Michel
Martin3 and Akira Miyamoto2, 1
1 Department of Applied Chemistry, Graduate
School of Engineering, Tohoku University, JAPAN 2
New Industry Creation Hatchery Center, Tohoku
University, JAPAN 3 Laboratory of Tribology and
Dynamical Systems, Ecole Centrale de Lyon, FRANCE
2
01
Tribology in the Automotive Industry
Real contact area
Metal contact leads gt High friction gt Severe
wear
Piston-cylinder system
Increment of fuel consumption and lowering of
engine output
Piston ring
Cylinder

• Control of a friction is the most important
  issue in the automotive industry, thus the
  surface control is important technology.
• The study of the TRIBOLOGY (friction, wear and
  lubrication) has been strenuously doing.

Automobile
3
02
Engine Oil Additives

• Addition of functionality

Zinc DialkylDithioPhosphate ZnDTP
Anti-wear additives
Dispersants
Friction modifier
Anti-oxidants
Detergents
R alkyl groups
etc
Without ZnDTP
Within ZnDTP
Abrasive wear produces a lot of debris and also
rougher surfaces.
ZnDTP forms zinc phosphate film which prevents
abrasive wear.
Zinc phosphate film
Wear particle
Oils
substrate
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03
The Aim of Our Study

• Sulfur and phosphorus in additives
• Poisoning of environmental catalysts in automobile

Sulfur
Phosphorus
Development of environmentally friendly (sulfur
and phosphorus-free) additives
For this purpose, the information of a wear
prevention mechanism of ZDDP additive at the
electronic and atomistic levels is needed.
In this study
We investigated atomistic wear prevention
mechanism of ZDDP additive by using
computational chemistry method.
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04
Molecular Dynamics Simulation Calculation Model
Fe substrate
Zinc thiophosphate (decomposition product of ZDDP)
Fe2O3 wear particle
which is produced during running-in process
We placed zinc thiophosphate layer between Fe
substrates to investigate how the ZDDP boundary
lubricating film prevents the abrasive wear
Fe substrate
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05
Molecular Dynamics Simulation Calculation Model
1 GPa
Calculation conditions
100 m/s
Integration time 0.5 fs/step Calculation
steps 1,000,000 Temperature 353 K
Fe substrate
Fe2O3 particle
Fe
O
Zn(PS0.5O2.5)2 film
?
Friction
To investigate the wear prevention mechanism of
ZDDP boundary lubricating film, we observed
dynamic behavior of Fe2O3 particle during
friction.
Fe substrate
Fixed
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06
Dynamic Behavior of Fe2O3 Particle
Pressure
The picture of the phosphate film is hidden for
clear understanding.
Friction
Deformation
Fe
O
0 ps
75 ps
50 ps
100 ps
Digested by the film
150 ps
Digestion of Fe2O3 wear particle was observed
during friction process.
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07
Dynamic Behavior of Fe2O3 Particle
Pressure
Friction
300 ps
Completely digested
Cause of wear Fe2O3 particle
Digestion of particle
Digestion of Fe2O3 particle
Friction
Abrasive wear prevention
ZDDP film
Substrate
The wear prevention mechanism by ZDDP boundary
lubricating film is elucidated.
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08
Effect of Sulfur to Wear Prevention
Pressure
The picture of the phosphate film is hidden for
clear understanding.
Friction
Deformation
Fe
O
0 ps
75 ps
50 ps
100 ps
Replacement of S atom by O atom
150 ps
Digested by the film
Fe2O3 wear particle was digested by the ZDDP
boundary lubricating film when the film contain
no sulfur.
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09
Effect of Sulfur to Wear Prevention
Wear prevention performance
MSD (vertical direction)
Estimated by diffusivity of Fe atoms
Means square displacement, MSD
w/o S
Nnumber of atom,Ri(t)position of atom
w/in S
MSD
Large
Fe diffusivity
High
Digestible
High
ZDDP boundary lubricating film without S atom
potentially digest the wear particle as well as
original ZDDP film. Sulfur is no need to achieve
the abrasive wear prevention.
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10
Tribochemical Reaction Dynamics - A Hybrid
Tight-Binding Quantum Chemical Molecular Dynamics
Study -
1 GPa
Calculation conditions
Fe substrate
100 m/s
Integration time 0.5 fs/step Calculation
steps 1,000,000 Temperature 353 K
Central part of reaction with 156 atoms
Zn(PO3)2 film
Zn
Fe
P
OZn
Fe substrate
OFe
Fixed
Tight-binding QCMD method was employed to a part
of Fe2O3 particle and its surrounding Zn(PO3)2.
Calculation model within deformed Fe2O3 particle
obtained by MD
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11
Fe O Bond Formation/Dissociation
Fe OTribofilm
Results indicate that Fe2O3 particle reacted with
zinc phosphate to form mixed iron and zinc
phosphate during friction process.
Bond overlap population -
Fe OParticle
Time ps
1.25 ps
0.93 ps
0.75 ps
0.00 ps
Dissociation of FeOParticle
Fe
OZn
OFe
Formation of FeOTribofilm
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12
Origin of the Fe O Bond Formation
PDOS of 0.75 ps (In this simulation time,
chemical bond between Fe and OZn was observed. )
Orbital overlap between Fe 3d, OZn 2p and P 3p
HOMO
DOS -
OZn
OZn
OFe
Fe
OFe
OFe
Energy level eV
Bond formation of FeOTribofilm is based on the
interaction between Fe 3d, OTribofilm 2p and P 3p
orbitals.
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Conclusion
Wear Prevention Mechanism
Sulfur Effect
Digestion of particle
Fe2O3 particle
ZDDP
Alternatives
Friction
Tribochemical reaction
Substrate
ZDDP boundary lubricating film without sulfur
potentially digest the wear particle as well as
original ZDDP film. Sulfur is no need to achieve
the abrasive wear prevention. This result
provides us the structure of alternative
anti-wear additive of ZDDP.
Prevention of abrasive wear
ZDDP tribofilm can digest Fe2O3 wear particles by
the tribochemical reaction between
tribofilm/particle.