Erosion rates of organic materials are measured during a recent flight of the shuttle (STS-8). Several forms of carbon and a variety of thermosetting and thermoplastic polymers are exposed to the ram beam of atomic oxygen. Arrhenius energies of about 1000 to 2000 cal/mole were measured from the rate dependencies on temperature. If some simple assumptions are made about the chemical nature of the desorbed species, the data can be used to estimate production rates at surfaces in orbit under different conditions of temperature, oxygen atom flux, and material surface conditions

Gregory, J. C.

Peters, P. N.

English

NASA Technical Reports Server

THE PRODUCTION OF GLOW PRECURSORS BY OXIDATIVE 
EROSION OF SPACECRAFT SURFACES 
John C. Gregory 
The University of Alabama in Huntsville 
Palmer N. Peters 
NASA, Marsall Space Flight Center 
Abstract. Erosion rates of organic materials were measured during a 
recent flight of the shuttle (STS-8). Several forms of carbon and a 
variety of thermosetting and thermoplastic polymers were exposed to the ram 
beam of atomic oxygen. Arrhenius energies of about 1000-2000 cal molem1 
were measured from the rate dependencies on temperature. If some simple 
assumptions are made about the chemical nature of the desorbed species, the 
data can be used to estimate production rates at surfaces in orbit under 
different conditions of temperature, oxygen atom flux and material surface 
conditions. 
Introduction 
Erosion of carbon and organic polymer surfaces by "active" or atomic 
oxygen has been known for decades [e.g., Blackwood and McTaggart, 1959; 
Hansen et al., 19651. Gregory and Peters [1975] pointed out that the 
atmosphere in orbit at a few hundred km altitude would be at least as 
reactive and proposed the use of the returnable Long Duration Exposure 
Facility (LDEF) for a study of the effects. Although, at the time of 
writing, this spacecraft is still in orbit, a version of the atomic oxygen 
instrument was flown in 1983 on the STS-8 mission. The objectives of the 
experiment were to begin an investigation of the mechanisms of interaction 
of 5 eV oxygen atoms with solid surfaces by measuring the rate of reaction 
as a function of temperature and oxygen flux, and by examining the 
scattered distribution of atoms re-emitted from certain surfaces. In 
addition, a number of optical quality metal films were prepared and exposed 
to investigate the growth of their oxide films or any morphological changes 
induced by the oxygen atom beam. In this paper only the results on carbon 
and solid polymers is discussed. 
In the context of glow, emission of reaction products from the surface 
may be relevant in two ways. 
states which subsequently decay emitting radiation, or products may suffer 
reactive collision with another 5 eV oxygen atom producing new products in 
excited states. 
Oxidation products may be desorbed in excited 
Evidence favoring the latter mechanism includes: 
(a) New calculations [Rantanen, 1985; Brock, 19851 of mean-free-path 
of emitted species indicate large probability of collision with the ambient 
s t r eam . 
(b) Energetic collisions of organic molecules with oxygen atoms are 
17 4 
https://ntrs.nasa.gov/search.jsp?R=19860003789 2020-03-20T15:58:04+00:00Z
expected to have a signfficant probability for reaction producing excited 
products. 
19851 on Spacelab 1. 
unresolved infrared was observed [Witteborn, 19841 from the ground during a 
recent shuttle flight. 
(6) Emission lines from C species were observed [Torr and Torr, 2 
(d) An extended (tens of meters) glow around the orbi 
Experimental Approach 
The experimental approach used was necessarily very simple as no 
electromechanical devices such as lids, shutters, etc. were available, and 
no intermediate measurements could be made; i.e., only a single integral 
effect could be measured for each sample. Samples were nominally L-inch 
discs3 with the hot-plate discs only being %-inch diameter. For the case 
of the erodible materials discussed here, a bar-pattern of small rectangles 
of niobium was deposited on the surface using a photo-resist technique 
(Figure 1). The niobium was sputtered on as a uniform film %2000 1 
thick. Although it oxidized heavily, it still served to protect the 
underlying carbonaceous material. The scanning electron microscope 
picture in Figure 2 shows both the eroded portion of a vitreous carbon 
disc and the part covered and protected by t e niobium film. The height 
of the step on this sample was about 503000\ (5 urn). The bar-pattern 
allowed multiple measurements of the step-height to be made using a Dektak 
stylus profilometer. Amplitudes were checked using the SEM micrographs. 
Half of each sample was covered at all times before and during the flight, 
and served as a control. 
Erodible surfaces studied included single crystal graphite (basal and 
prismatic planes), vitreous (or glassy) carbon from various manufacturers, 
polymethyl methacrylate (lucite) , bisallyl diglycol carbonate (CR-39 , a 
high quality optical plastic), and diamond. 
The exposure obtained during the STS-8 mission was ideal from the point 
of view of the materials experiment in that about 95% of the atom fluence 
was accumulated while the vehicle attitude was held so that the ambient 
atom beam was within 1" of the normal to the material surfaces. The rest 
of the mission was spent at much higher altitudes or with the payload bay 
doors closed. Pertinent exposure data are shown in Table 1. 
Table 1. STS-8 ATOMIC OXYGEN EXPOSURE DATA 
Payload bay forward facing: t = 41.2 hrs. 
A1 titude : 120 nautical mi. (225 km) 
Velocity : 7.8 km s'l 
Mean oxygen atom density (calculated): 
Surface impact frequency: 
Integral fluence: 
2.65 x lo9 cm-9 
2.07 x 1015 cm-' s'l 
3.5 x 1020 atoms cm -2 
175 
Erosion observed by this experiment ranged from a thousand angstroms 
for diamond (which appears to be particularly resistant to oxidation under 
these conditions), to about 200,000 A for the polycarbonate resin, CR-39, 
which was the most heavily eroded sample reported on any space flight 
exposure. 
The temperature dependence of the oxidative effects was measured by 
conducting the. erosion measurements at three temperatures spanning about 
12OoC. The Arrhenius activation energy, AE, was estimated, assuming: 
-AE r = A e  /RT 
where r is the rate of the reaction and A is a constant assumed independent 
of temperature T. These studies were performed for six materials, vitreous 
carbon, two graphites, CR-39, silver, and osmium. All activation energies 
were smal.1 and positive. 
Results 
The conclusions from the measurements on various forms of carbon exposed 
in the STS-8 mission appear applicable to organic solids in general. 
may be summarized as follows: 
They 
1. Measured erosion was linear with total fluence, 
2. No induction time observed before onset of erosion. 
3. Erosion rate linear with oxygen flux (i.e., reaction probability 
independent of flux) measured over a small range 1.5 to 2.5 x 10l5 atoms 
cm-' s-1. 
These results show that erosion and production of gas molecules at the 
surface can proceed indefinitely at such surfaces as long as oxidizable 
material remains exposed, in contrast with outgassing behavior which 
generally shows exponential decay of evolution rate with time. It should 
be emphasized, however, that our experiment used very pure materials and 
real or engineering materials may be contaminated either on the surface or 
by inclusions in the bulk. Such contamination, if less oxidizable than the 
matrix itself, may then serve to protect the rest of the matrix material 
from erosion. As erosion proceeds the density of these screening particles 
or films on the surface grows and the erosion rate may drop from its prior 
value. 
4. 
follows : 
Arrhenius activation energies forthe reactions were measured as 
Vitreous Carbon 1200 (cal mole-1) 
Graphite (basal plane) 1400 
CR-39 1050 
5. Reaction probabilities depend on temperature as shown in ( 4 )  
above. Reaction probabilities for carbons exposed at %300°K ranged from 
0.1 to 0.15 where reaction probability equals the number of carbon atoms 
lost divided by the number of incident oxygen atoms. 
6. Reaction probabilities for more complex organic materials have 
been expressed in cm3 x 
present purposes we use two examples at approximately: 300" K: 
of material l o s t  per incident 0 atom. For 
CR-39 (Cl2Hl8O7ln (this work) : %6 x 10 24 cm3 atom-l 
17 6 
Kapton [Leger et al, , 19841: 3 x 10’24 cm3 atom-1 
organic surfaces, the chemistry is e 
the data from 5 and 6 above we may then calculate the emission rates of 
product molecules from the eroding surfaces in the idealized case. The 
emitted flux of CO from carbon is given simply by the product of the 
reaction probability, P, times the flux of oxygen. Calculation of the 
molecular loss rate from polymers, on the otherhand, requires some 
assumption of mean molecular weight of the desorbed species. 
For the polymers we assumed a mean molecular weight of 28 amu of solid- 
derived component (i.e., not including the atomospheric component). Since 
the product mixture may well be complex, and different components will 
differ radically in their capacity for reactive collision excitation to a 
glow condition, this level of understanding is clearly inadequate. 
For the STS-8 mission, with an oxygen atom flux of 2 x IOi5 cm-2 s-l, 
we have : 
Substrate Emitted Rate of Emission 
Carbon co 3 x 101’) 
Species (molecules- s-1 cm-2) 
Kapton (28+) amu 1.9 1014 
CR-39 I1 3.0 1014 
Before these rates are used in any modeling of the gas cloud surrounding 
the shuttle it should be emphasized that they apply only to unprotected and 
rather pure organic surfaces. The extent to which impurities reduce the 
rate of mass loss requires further definition, and it is already quite 
clear that simple procedures of overcoating with oxidation-resistant 
materials can reduce mass loss by orders of magnitude. 
of the dynamics of scattering of surface-desorbed species with ambient 
atmospheric species have suggested that the probability of collision with a 
streaming oxygen atom is quite high. Under these conditions a large 
fraction of all organic molecules leaving this shuttle surface may emit one 
or more photons, contributing to an extended ball of glowing gas around the 
vehicle, with the upper limit r 1 mol f 1 photon) for the integral 
number of photons emitted per second equaling the total number of glow 
precursors leaving the entire surface per second. 
Model calculations 
177 
References 
1985. 
Spacecraft Glow, NASA/MSFC, 1984. 
Witteborn, F., unpublished results presented at the First Workshop on 
Acknowledgements 
We are grateful to the University of Alabama Research Institute for a 
grant to build the instrument hardware. Part of the analysis at the 
University of Alabama in Huntsville was supported by Contract NAS8-36189. 
178 
Fig. 1. One-inch-diameter vitreous 
carbon disc exposed to the 
atomic oxygen stream normal 
to its surface in a 225-km 
orbit. Rectangles are thin 
films of niobium. The 
lighter annulus round the 
edge of the sample iS pro- 
tected by it5 holder and re- 
mains highly specular, while 
the inner circle shows the 
light-absorbing nature of the 
eroded surface. 
Fig. 2. Scanning electron micrograph 
of a sample similar to Figure 
1 which has been cut through 
one of the niobium rectangles. 
Viewing direction is in the 
plane of the sample. The 
height of the step (a few um) 
indicates the amount of carbon 
eroded. 
179 


The production of glow precursors by oxidative erosion of spacecraft surfaces

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