Project Report
2011 MVK160 Heat and Mass Transport
May 13, 2011, Lund, Sweden
Mass Transfer: A look at atomic oxygen erosion of spacecraft material in the
low earth orbit
Patrick Colvin
Dept. of Energy Sciences, Faculty of Engineering,
Lund University, Box 118, 22100 Lund, Sweden
ABSTRACT
The presence of atomic oxygen in the low earth orbit
contributes to degradation of spacecraft material. This
material experiences erosion that leads to mass loss,
while the remaining material is mechanically
compromised. Investigation is ongoing to improve the
resistance of spacecraft material to the harmful
effects of atomic oxygen. The ITO-coating and nanoSiO2 methods of material enhancement are discussed
and evaluated. Injection with nano-SiO2 is predicted
as being superior for it predicted to maintain
resistance throughout the material. Research should
be conducted to evaluate this prediction over long
term exposure of atomic oxygen.
NOMENCLATURE
𝐸𝑦
erosion yield (cm3 atom-1)
𝐸𝐾
erosion yield of Kapton H witness
sample (3 x 10-24 cm3 atom-1)
𝛥𝑀
mass loss (g)
𝐴
surface area exposed to AO (cm2)
F
fluence of atomic oxygen (atom cm-2)
Greek Symbols
𝜌
density of the sample material (g cm-3)
𝜌𝐾
density of Kapton H witness sample
(1.4273 g cm-3)
Subscripts
S
sample material
K
Kapton H witness sample
INTRODUCTION
Space is often thought of as a volume containing
nothing; an environment that exists in the essence of
nothingness. This, however, is not true. According to
Tori Woods of NASA’s Glenn Research Center “space
is considered an environment . . . filled with entities
that can be harmful to spacecraft.” Woods lists the
various physical and chemical threats: “ultraviolet rays
and x-rays from the sun; solar wind particle radiation;
thermal cycling (hot and cold cycles); space particles
(micrometeoroids and debris); and atomic oxygen.”
(Woods, 2011)
This report will focus on the last environmental
hazard: atomic oxygen (AO). According to a 2005
study by the Beijing University of Aeronautics and
Astronautics, atomic oxygen is the “predominant
component and most active species of the LEO
atmosphere” (Wang, Zhao, Wang, & Shen, 2005).
Atomic oxygen levels in the low earth orbit (LEO) may
vary with solar cycle, altitude, season of the year, etc.
A spacecraft travelling with a velocity of 7 or 8 km s-1
in the LEO (an altitude between 160 and 2000 km) can
experience an average atomic oxygen flux between
1012 and 1015 atoms cm-2 s-1 (Wang X. , Zhao, Wang, &
Shen, 2007). This flux of atomic oxygen poses possible
physical threats such as “elastic scattering, scattering
with partial or full thermal accommodation,
recombination, or excitation of ram species” (Groh,
Banks, McCarthy, Rucker, Roberts, & Berger, 2008).
However, studies are most greatly concerned with the
reactive nature of atomic oxygen and the chemical
reactions that occur with the surface material, leading
to material degradation.
For a society that has endeavored to explore space,
we place a great deal of stress on properly
manufacturing that which we send into the unknown.
Among those subjects highlighted is the choice of
spacecraft material. In modern day science, humanity
has reached a point where new materials may be
synthesized or made from a combination of known
materials. The possibilities for materials to own
different characteristics are thereby infinite. The
question remains: how do we select the proper
material with which to outfit a spacecraft in order to
reduce the harmful effects of this foreign
environment?
PROBLEM STATEMENT
Atomic oxygen found in the LEO is reactive with
polymers and will cause degradation and mechanical
failure from exposure. It has been shown that AO will
target material defects as illustrated in Figure 1. As AO
molecules strike the surface, a chemical reaction
occurs to release gases. When an AO molecule
encounters a material defect, there is simply more
surface area to interact with, which increases the rate
of chemical reaction. Any defect is an open target for
AO degradation.
Figure 1: Degradation of surface topography due to AO exposure;
a) material defects; b) AO-irradiated sample (Shimamura &
Nakamura, 2009)
A material exposed to atomic oxygen will suffer
mechanical property degradation. Not only will some
of the material react and the mass reduce, but the
sample as a whole will decrease in tensile strength
and elongation. The material surface becomes rough
when previously smooth, and the chance of a
discontinuity greatly increases (Shimamura &
Nakamura, 2009). This may have detrimental
consequences for a spacecraft experiencing
continuous AO exposure. As the outer surface remains
in exposure to the space environment, it will slowly
degrade and eventually discontinuities will appear
that expose sub-surface layers. These sub-surface
layers will then be exposed unintentionally to space
environment, and themselves be mechanically
compromised.
The erosion yield allows researchers and designers to
compare the resistance of various materials against
the impact of atomic oxygen. Equation 1 illustrates
how the atomic oxygen erosion yield is calculated
based on several variables.
𝐸𝑦 =
𝛥𝑀𝑆
𝐴𝑆 𝜌𝑆 𝐹
Equation 1: Erosion Yield (Groh, Banks, McCarthy, Rucker,
Roberts, & Berger, 2008)
Because space is an environment, the fluence of
atomic oxygen (F) may vary significantly from one
location to another, and from one time to another. As
such, a control material—Kapton H witness sample—
is exposed to the same environment as all other
samples, allowing for comparative analysis. Kapton H
has a known and stable erosion yield in the LEO.
Equation 2 uses the measured mass loss of Kapton H
witness sample to calculate the fluence of atomic
oxygen.
𝐹=
𝛥𝑀𝐾
𝐴𝐾 𝜌𝐾 𝐸𝐾
Equation 2: Fluence of Atomic Oxygen according to Kapton H
witness sample
(Groh, Banks, McCarthy, Rucker, Roberts, & Berger, 2008)
Equations 1 and 2 may be combined to give an explicit
equation (Equation 3) for the erosion yield of any
material exposed to the same environment as Kapton
H witness sample.
𝐸𝑦 = 𝐸𝐾
𝐴𝐾 𝜌𝐾 𝛥𝑀𝑆
𝛥𝑀𝐾 𝜌𝑆 𝐴𝑆
Equation 3: Explicit Erosion Yield based on Kapton H witness
sample (Groh, Banks, McCarthy, Rucker, Roberts, & Berger,
2008)
These equations provide the rate at which atomic
oxygen is expected to degrade a specific material. The
equations follow from logical reason that an increase
in material density will lower the erosion yield as
atomic oxygen will encounter greater resistance to
penetration; an increase in exposed surface area
would mathematically decrease the erosion yield,
however, due to a greater exposure surface the mass
loss would inevitably be increased. As such, a balance
can be afforded between the exposed surface area
and mass loss due to erosion.
LITERATURE SURVERY
An ongoing struggle exists to evaluate and improve
materials used in spacecraft construction. NASA
performs practical in-environment experiments to
evaluate the erosion yield of various applied and
newly invented materials. Other independent
organizations perform specific laboratory testing of
methods to improve materials. Each experimental
resource helps us to optimize the performance of
space materials.
NASA’s MISSE
Among the highly practical, NASA performs periodic
material tests called Materials International Space
Station Experiment (MISSE). Currently, NASA has
completed 7 of these tests, with 2 on a return journey
and 2 more to have just begun (Woods, 2011). During
these tests, a spacecraft orbits at a specified altitude
from earth. Attached to the spacecraft are either one
or two Passive Experiment Containers (PECs). A
square of proportions approximately 60 cm by 60 cm,
the PEC is layered with hundreds of material samples
and exposed to the space environment for a
predetermined time; sometimes years. (Woods, 2011)
Data is collected throughout the experiment and the
results provide some basis on how to improve current
material, or which characteristics are desirable for
future material design. NASA’s experiments provide
critical data that encompasses all factors of space
environment; even those we may not be aware exist.
The erosion yields measured on the second MISSE
mission varied between 1-9 x 10-24 cm3 atom-1 and
contributed to estimated mass losses of ~10%,
although some mass losses were as low as 1% over a
period of 3.95 years (Groh, Banks, McCarthy, Rucker,
Roberts, & Berger, 2008).
Lab-based research
The great advantage of lab-based research is that
results can be deduced by way of relating a specific
reaction resulting from a known cause. Two primary
methods will be discussed for reducing the mass loss
of material exposed to atomic oxygen: (1) coated
polyimides and (2) filled polyimides.
Coated Polyimides
Hokkaldo University in Japan conducted a study in
2009 to evaluate the protective characteristics of
indium tin oxide (ITO) against atomic oxygen
degradation. Polyimide films of 125 µm thickness
were coated with 25 µm-thick ITO and compared
against the pristine samples. Table 1 below illustrates
the erosion yields of the ITO coated polyimide and
pristine polyimide samples. Erosion yields are
significantly reduced for the ITO-coated polyimide,
which indicates a high durability for AO erosion
(Shimamura & Nakamura, 2009). An increase in the
AO fluence caused the erosion yield to increase; this
demonstrates the limited resistance of the ITO
coating.
Sample
Pristine
Polyimide
ITO-coated
Polyimide
AO
fluence
(atoms
cm-2)
0.3 x 1021
0.85 x
1021
1.30 x
1021
0.3 x 1021
0.85 x
1021
1.30 x
1021
Erosion
yield
(cm3 atom1
)
1.7 x 10-24
1.7 x 10-24
< x < 3.0 x
10-24
3.0 x 10-24
Percent
Reduction
(%)
0.1 x 10-24
0.1 x 10-24
< x < 0.8 x
10-24
0.8 x 10-24
5.8
5.8 < x <
26.6
---
--
26.6
Table 1: Atomic oxygen erosion yield of pristine and ITO-coated
polyimide (Shimamura & Nakamura, 2009)
Using the indium tin oxide coating method has some
promise, as results have shown lesser material
degradation. The method of application is also rather
simple as it involves direct application to the exposed
polyimide surface. One concerning issue with this
method however, is that any exposed weakness in the
ITO coating may eliminate its effectiveness. If for
example, a hole appears in the coating due to AO
erosion, or some form of physical abrasion, the
sample material will be once again directly exposed to
AO in the environment. AO would then degrade the
material from this source location and erosion yield
might approach that of the pristine polyimide. Thus is
the significant weakness of this protective layer.
Filled Polyimides
An alternative to applying a layer of coating has been
evaluated by the Beijing University of Aeronautics and
Astronautics. Unlike the ITO-coating method, which
only protects the exposed surface of the material
from AO erosion, this alternate method enriches the
whole material with AO resistant nano-particles.
Through a process of impregnation, clipping,
superposing, and cure molding that will not be
described here, the material is embedded with nanoSiO2 particles. The particles are chosen for their AO
resistant characteristics.
The specific experiment conducted by the Beijing
University injected 0, 5, 10, and 15 phr nano-SiO2 into
a polyimide resin. An AO fluence of 1.036 x 1021 atoms
cm-2 was applied and the mass of each sample
weighed every 10 hours. After 40 hours of AO
exposure the polyimide resins injected with 5, 10, and
15 phr nano-SiO2 reduced their erosion yields to
58.2%, 34.3%, and 16.4% of the pristine polyimide
resin. As all other variables are maintained constant,
these percentages directly relate to the mass loss as
per Equation 1. These results verify that injection with
AO-resistant nano-particles significantly reduces the
degradation of the polyimide resin. Figure 2
compares the polyimide resistance to AO erosion
before and after injection with nano-SiO2. Some of the
atomic oxygen molecules that come in contact with
the polyimide resin undergo a chemical reaction to
release gases (CO and RO), while others are cast away.
The nano composite material effectively reduces the
quantity of AO molecules that undergo these
damaging chemical reactions. The AO molecules that
do not undergo chemical reaction will not contribute
to the release of gases or to the loss of material mass.
Figure 2: Schematic diagram of resistance to AO (Wang X. , Zhao,
Wang, & Shen, 2007)
Method Comparison
The ITO-coating method provides AO resistance to the
exposed surface of a material while the nano-particle
injection method enriches the entire material with
AO-resistant characteristics. The main purpose of
either method is to reduce the erosion yield caused by
atomic oxygen and thereby reduce the material
degradation and mass loss. Based on the experimental
studies performed by Hokkaldo University and Beijing
University, both methods are comparable for reducing
the erosion yield. The ITO-coating method reduces the
erosion yield of its respective polyimide resin to 26.6%
with an AO fluence of 1.30 x 1021 atoms cm-2. The
nano-SiO2 injection method reduces the erosion yield
to 16.4% with an AO fluence of 1.036 x 1021.
Unfortunately, the AO fluence for each experiment is
not the same, so direct comparison cannot be made.
However, both methods do achieve high reduction.
Research should be conducted to analyze how a
material reacts to prolonged AO exposure. Once the
exposed layer of the ITO-coating has been
compromised, the material may begin to exhibit
erosion yields closer to that of the pristine material.
The nano-SiO2 injection method may maintain
resistance for the lifetime of the material. With
further research, it may be proved if the nano-SiO2
injection method is superior for long term application.
After applying the ITO-coating, the polyimide resin
would be exposed to AO until significant
discontinuities appear on the material surface. The
erosion yield would then be compared against time to
determine the effectiveness of this coating over the
lifetime of the spacecraft material.
CONCLUSION
NASA continues to provide valuable data regarding
the exposure limits of spacecraft material in space,
and particularly in the low earth orbit. Their
experiments provide comparative data for material
manufacturers and designers as well as engineers.
Independent researchers are investigating the
prospect of enhancing materials to become more
resistant to AO exposure. Current experiments
validate the application of AO resistant layers to
polyimide resin and injection of AO resistant nanoparticles. Both methods show promising reduction of
material mass loss. Further research is recommended
in order to evaluate material behavior during long
term exposure.
REFERENCES
Groh, K. K., Banks, B. A., McCarthy, C. E., Rucker, R. N.,
Roberts, L. M., & Berger, L. A. (2008). MISSE 2 PEACE
Polymers Atomic Oxygen Erosion Experiment on the
International Space Station. High Performance
Polymers , 388-409.
Shimamura, H., & Nakamura, T. (2009). Mechanical
properties degradation of polyimide films irradiated
by atomic oxygen. Polymer Degradation and Stability ,
1389-1396.
Wang, X., Zhao, X., Wang, M., & Shen, Z. (2007). An
Experimental Study on Improving the Atomic Oxygen
Resistance of Epoxy Resin/Silica Nanocomposites.
WorldWideWeb: Wiley InterScience.
Wang, X., Zhao, X., Wang, M., & Shen, Z. (2005). The
effects of atomic oxygen on polyimide resin matrix
composite containing nano-silicon dioxide. Beijing:
Institute of Fluid Mechanics, Beijing University of
Aeronautics and Astronautics.
Woods, T. (den 28 April 2011). Materials: Out of this
World. Hämtat från NASA:
http://www.nasa.gov/centers/glenn/shuttlestation/st
ation/misse.html den 11 May 2011