Wear inhibitors and rheology of lubricants
confined to nano-separation between two surfaces
Yanhua Zhou, Tahir Cagin, William A. Goddard III
Materials and Process Simulation Center, Caltech
Elaine S. Yamaguchi, Andrew Ho
Chevron Chemical Company, Oronite Technology Group
Chevron Petroleum Technology Company
To understand the mechanism by which a wear inhibitor additive
in a base oil prevents
wear of an engine surface, we studied dithiophosphates (DTP) bound to the Fe2O3-(0001)
surface under a shear flow condition, using computer simulations. The shear flow is
generated by moving a top surface at a constant speed and keeping a bottom surface
stationary. Each surface is covered by a self-assembled monolayer of DTP molecules.
Between them filled with a lubricant film (Figure1, postscript).
We have carried out the
simulations at a speed of 1 A/ps and separation of 40 A.
The interesting results are summarized as follows.
- The steady-state velocity profile in the lubricant is different
for DTP molecules with different organic R groups. For R=isopropyl,
which is the best commercially available wear inhibitor, the shearing
occurs in the central region of the lubricant, whereas for R=isobutyl
and R=phenyl, whose wear protection performances are relatively poor,
the shearing takes place across the entire fluid region including the
(Figure 2, postscript).
The lubricant layer generated at the interfaces in the case of the
center-concentrated shearing should aid in protecting the surfaces.
- The result of the velocity profiles also indicates a slip
existing at the boundaries between the walls and lubricant. The degree
of the slip varies with the type of DTP molecules used. This property
is useful in validating the boundary conditions used in fluid dynamics problems.
- Lubricant molecules demonstrate microscopic stick-slip motions
for the three types of DTP molecules investigated, characterized by a
sequence of move (slip), pause (stick), and move (slip) again
(Figure 3, postscript).
- The viscosity of the lubricant in such confined geometry is a
few times larger than the viscosity of the lubricant in bulk at the
same temperature (~2.5 cP vs. 0.45 cP).
- The lubricants confined to such a nanoscale separation show
significant density oscillations (Figure 4, postscript).
This research was supported by the Chevron Chemical Company
(Oronite Technology Group) and grants from the DOE-BCTR and
NSF (CHE 95-12279). We thank Larry Smarr for access
to the NCSA system for some calculations.