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ple. The EDS map of the film acquired in the transmission
electron microscope shows a layer of silicon, oxygen, and
fluorine a few nm thick on this sample, similar to the
MoS
2
/Sb
2
O
3
/Au film. Because carbon erosion by AO results
in formation of volatile CO and CO
2
, the surface layer on
the DLN film is probably the result of both surface contam-
ination and the oxidized silicon that is naturally part of the
film remaining behind after erosion of the carbon phase.
Summary and outlook
Frictional behavior of solid lubricant films is highly in-
fluenced by phenomena that occur at the nanoscale. Sur-
face oxidation of just a few nanometers creates a
significantly higher initial friction coefficient for all MoS
2
films compared to the steady-state value, but lasts only a
few cycles until the oxidized layer is worn away. When slid-
ing in environments containing water vapor and oxygen,
dynamic oxidation and water vapor effects result in a
higher steady-state friction coefficient for MoS
2
films com-
pared to that exhibited in inert atmospheres or vacuum.
DLN films exhibit similar behavior to composite MoS
2
films, including a run-in period associated with removal
of surface contaminants and layers created by AO expo-
sure, as well as the development of a transfer film on the
mating surface.
Composite MoS
2
and DLN coatings have now been
flown in space to determine changes in surface composi-
tion and frictional behavior that accompany exposure to
AO in Low Earth Orbit. Samples exposed during MISSE-
7 experienced a fluence of AO about 14 times greater than
that during EOIM-3, but effects of LEO exposure on sur-
face chemistry and tribological behavior were very similar
for both missions. Given that oxide layers a few nm thick
are easily worn away exposing virgin lubricant underneath,
an intriguing possibility for protecting thin film solid lu-
bricants after deposition, but before use, may be to inten-
tionally cap the solid lubricant film with a layer of oxide or
metal to act as a barrier to the diffusion of water and oxy-
gen into the film.
For more information:
Michael T. Dugger and Somuri V.
Prasad, Materials Science and Engineering Center, Sandia Na-
tional Laboratories, Albuquerque, NM 87185-0889,
mt- dugge@sandia.govand
svprasa@sandia.gov. T.W. Scharf is on
sabbatical from the Dept. of Materials Science and Engineer-
ing, The University of North Texas, Denton, TX 76203-5310.
Acknowledgments
The authors thank Paul Kotula for TEM analysis and
Andre Claudet for supporting the MISSE-7 tribological analy-
sis. MoS
2
/Sb
2
O
3
/Au and DLN coatings were provided by
Andy Korenyi-Both (Tribologix Inc.) and Chandra Venkatra-
man (Bekeart Advanced Coatings Technologies), respectively.
The MISSE-7 project for Sandia National Laboratories was
led by Gayle Thayer. Sandia National Laboratories (SNL) is a
multiprogram laboratory managed and operated by Sandia
Corp., a wholly owned subsidiary of Lockheed Martin Corp.,
for the U.S. DOE’s National Nuclear Security Administration
under Contract DE-AC04-94AL85000.
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