May_AMP_Digital

M.T. Dugger*

T.W. Scharf*

S.V. Prasad, FASM*

Materials Science and

Engineering Center,

Sandia National

Laboratories

Albuquerque, N.M.

ADVANCED MATERIALS & PROCESSES •

MAY 2014

rom the 1960s through the early 1990s,

large government investments in space

exploration by both the U.S. and the for-

mer Soviet Union emphasized performance,

while factors such as cost and aging require-

ments were secondary. Spacecraft were

launched soon after they were completed and

tested, and traveled for relatively short mis-

sions. With today’s increased commercial and

military use of space, performance is still the

primary factor, but satellites may spend signif-

icant time awaiting launch opportunities. Mis-

sions often require longer time periods than

early exploration, so lubricants are expected to

perform in space for years rather than days or

weeks, and must do so after operating and

being stored in terrestrial environments for

months or years. While there have been several

notable missions within the solar system, many

space missions remain near Earth. Among

these missions are those associated with the In-

ternational Space Station (ISS), and polar-or-

biting satellites for Earth observation, in an

environment known as low earth orbit (LEO).

LEO corresponds to altitudes of roughly

200 to 1000 km. Objects in orbit experience

thermal cycles associated with cyclic sun expo-

sure and radiative heat transfer to space as well

as solar UV exposure, which can degrade poly-

meric materials. However, the primary element

present in LEO is atomic oxygen (AO)

[1]

. AO is

created by photodissociation of oxygen mole-

cules in the upper atmosphere by UV radiation

from the sun

[2]

. At orbital velocities near 8 km/s,

AO has energy of approximately 5 eV/atom and

flux density of 10

-10

atoms/cm

.s, depending

on altitude

[3]

. At this energy, AO can break bonds

and induce chemical reactions with many mate-

rials, posing a concern for long-term degrada-

tion of materials in LEO.

Satellite components—specifically those

designed for mechanical hold and release func-

tions—rely on solid lubricant coatings to miti-

gate adhesion, friction, wear, and debris

generation. Thin film solid lubricants have been

investigated for use in space for almost 30

years. Some of their advantages include lack of

migration, which eliminates extra mass associ-

ated with lubricant delivery and containment

systems, consistent frictional behavior from

cryogenic temperatures to hundreds of degrees

centigrade, and lack of velocity dependence on

friction coefficient over a wide range of sliding

speeds. Even with these advantages, space sys-

tems designers are hesitant to employ new ma-

terials without some flight history and an

understanding of how the space environment

impacts these materials.

This article considers the effect of AO ex-

posure on thin film solid lubricants of molybde-

num disulphide (MoS

) and diamond-like

carbon nanocomposites (DLN), a silica-con-

taining diamond-like carbon film. Advances in

deposition methods over the past few decades

enable dense films to be created that are more

resistant to oxidative degradation and the ef-

fects of adsorbed moisture than their predeces-

sors. Further, incorporating additional phases

improves the films’ tribological performance in

a range of atmospheres.

MoS

films

Sputtered thin films of MoS

were first ex-

posed to atomic oxygen in LEO as part of the

NASA Evaluation of Oxygen Interactions with

Materials-3 (EOIM-3) experiment during the

STS-46 mission aboard the shuttle Atlantis,

launched July 31, 1992

[4]

. The total AO fluence

for EOIM-3 was estimated to be 2.2 to

2.5×10

atoms/cm

over a period of 42.25

hours. The specimens also experienced an es-

timated 22±4 equivalent solar hours of UV ex-

posure. The tray reached a temperature of

Materials in Space:

Exploring the Effect of Low Earth Orbit

on Thin Film Solid Lubricants

Challenges

facing

materials used

in space have

not changed

significantly

over the past

few decades,

but today’s

requirements

have a

different focus

than those

used in the

heyday of

space

exploration.

This article

considers the

effect of

atomic oxygen

exposure on

thin film solid

lubricants.

*Member of ASM International

Fig. 1

—

MISSE-7 solid lubricant films (denoted by

red arrow) placed in a “suitcase-style” passive

experiment container (PEC) mounted externally on

the ISS (shown in inset with red arrow pointing to

the PEC). Courtesy of NASA.