Ching-cheh Hung and Kim K. de Groh

Glenn Research Center, Cleveland, Ohio

Bruce A. Banks

Alphaport, Inc., Cleveland, Ohio

Optical and Scanning Electron Microscopy of the 

Materials International Space Station Experiment 

(MISSE) Spacecraft Silicone Experiment

NASA/TM—2012-217678

August 2012
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Ching-cheh Hung and Kim K. de Groh

Glenn Research Center, Cleveland, Ohio

Bruce A. Banks

Alphaport, Inc., Cleveland, Ohio

Optical and Scanning Electron Microscopy of the 

Materials International Space Station Experiment 

(MISSE) Spacecraft Silicone Experiment

NASA/TM—2012-217678

August 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

10th International Space Conference on Protection of Materials and Structures From the Space

Environment (ICPMSE-10J)

cosponsored by ITL, MDA, CSA, JAXA, Kobe University Graduate School of Engineering, and the

Society for Promotion of Space Science

Bankoku-Shinryokan, Okinawa, Japan, June 12–17, 2011
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only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
Optical and Scanning Electron Microscopy of the Materials 

International Space Station Experiment (MISSE) 

Spacecraft Silicone Experiment 

Ching-cheh Hung and Kim K. de Groh 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Bruce A. Banks 

Alphaport, Inc. 

Cleveland, Ohio 44135 

Summary 

Under a microscope, atomic oxygen (AO) exposed silicone surfaces are crazed and seen as “islands” 

separated by numerous crack lines, much analogous to mud-tile cracks. This research characterized and 

compared the degree of AO degradation of silicones by analyzing optical microscope images of samples 

exposed to low Earth orbit (LEO) AO as part of the Spacecraft Silicone Experiment. The Spacecraft 

Silicone Experiment consisted of eight DC 93-500 silicone samples exposed to eight different AO fluence 

levels (ranged from 1.46 to 8.431021 atoms/cm2

) during two different Materials International Space 

Station Experiment (MISSE) missions. Image analysis software was used to analyze images taken using a 

digital camera. To describe the morphological degradation of each AO exposed flight sample, three 

different parameters were selected and estimated: (1) average area of islands was determined and found to 

be in the 1000 to 3100 m2

 range; (2) total length of crack lines per unit area of the sample surface were 

determined and found to be in the range of 27 to 59 mm of crack length per mm2

 of sample surface; and 

(3) the fraction of sample surface area that is occupied by crack lines was determined and found to be in 

the 25 to 56 percent range. In addition, average crack width can be estimated from crack length and crack 

area measurements and was calculated to be about 10 m. Among the parameters studied, the fraction of 

sample surface area that is occupied by crack lines is believed to be most useful in characterizing the 

degree of silicone conversion to silicates by AO because its value steadily increases with increasing 

fluence over the entire fluence range. A series of SEM images from the eight samples exposed to different 

AO fluences suggest a complex sequence of surface stress due to surface shrinkage and crack formation, 

followed by re-distribution of stress and shrinking rate on the sample surface. Energy dispersive spectra 

(EDS) indicated that upon AO exposure, carbon content on the surface decreased relatively quickly at the 

beginning, to 32 percent of the pristine value for the least exposed sample in this set of experiments 

(1.461021 atoms/cm2

), but then decreased slowly, to 22 percent of the pristine value for the most exposed 

sample in this set of experiment (8.431021 atoms/cm2

). The oxygen content appears to increase at a 

slower rate. The least and most AO exposed samples were, respectively, 52 and 150 percent above the 

pristine values. The silicone samples with the greater AO exposure (7.751021 atoms/cm2

 and higher) 

appear to have a surface layer which contains SiO2 with perhaps small amounts of unreacted silicone, CO 

and CO2 sealed inside. 

 

NASA/TM—2012-217678 1
Introduction 

In LEO, the AO environment causes silicones to shrink at the surface as the AO exposed surface 

converts to a glassy brittle silicate layer, which then cracks due to shrinkage giving the surface a crazed, 

or mud-tile cracked, appearance (Ref. 1). Because silicone is a commonly used pliable spacecraft material 

use for applications such as the use of DC 93-500 to bond cover glasses to solar cells for the ISS 

photovoltaic array blankets or as protective coatings on the back of solar arrays, characterization of its 

degradation during space exposure is essential for this material to be used properly.

Because AO exposure typically does not cause silicones to lose mass, its AO degradation is better 

characterized by other changes that can be more accurately measured than mass loss (which is used for the 

majority of other hydrocarbon based materials) (Ref. 2). For example, changes in surface hardness have 

been measured for silicone samples exposed to both ground-laboratory and space AO using nanoindentation 

techniques (Ref. 3). Silicone degradation in AO can also be characterized by changes in optical properties. 

This study was accomplished by measuring surface cracks in silicone samples exposed to LEO AO on the 

exterior of the International Space Station as part of the Spacecraft Silicones Experiment flown on both 

Materials International Space Station Experiment 2 and 4 (MISSE 2 & 4) (Ref. 4). 

This research characterized and compared the degree of AO degradation of silicones by analyzing 

optical microscope images of samples exposed to LEO AO as part of the Spacecraft Silicone Experiment. 

The Spacecraft Silicone Experiment consisted of eight DC 93-500 silicone samples exposed to eight 

different AO fluence levels (ranged from 1.46 to 8.431021 atoms/cm2

) during two different Materials 

International Space Station Experiment (MISSE) missions. 

The most obvious effect observed for silicones during AO environment exposure are changes in 

surface morphology. Under optical microscopy, all space exposed silicone surfaces contain “islands” 

separated by numerous crack lines (as shown in Fig. 1, Ref. 5) after they have been exposed to relatively 

low AO fluence exposures. Surface morphology changes, however, have not been used in the past to 

characterize silicone degradation during AO exposure. This is because of the difficulty in developing a 

standard method that can quickly, accurately and quantitatively be used to analyze changes in surface 

morphology. The problem of using numbers to characterize changes in pictures, however, is greatly 

alleviated due to the recent advancement of imaging analysis computer software. Consequently, using 

morphology changes to characterize silicone degradation due to AO exposure has become feasible. In 

this study, the morphology of retrieved MISSE 2 & 4 silicone samples were examined and quantified. 

Commercially available image analysis software was used to analyze images of the samples taken by an 

optical microscope. Scanning electron microscope (SEM) was also used to examine features that were 

beyond the capability of the optical microscope. No attempts were made to use the image analysis 

software to analyze the SEM images. 

Images from the optical microscope were analyzed using commercially available software to 

determine the average area of “islands”, the density of crack lines, and the percent of total area that is 

occupied by the crack lines. Relationships between these morphological parameters and the AO fluences 

were then examined and discussed. SEM was used to analyze smaller features of the samples exposed to 

the LEO environment over several different fluence levels. Development of individual cracks as a 

function of fluence was examined. In addition, energy dispersive spectrum (EDS) was used to semiquantitatively analyze the relative concentrations of silicon, oxygen and carbon. Results of the SEM 

investigation were used to infer the kinetics of atomic oxygen-silicone reactions. 

 

NASA/TM—2012-217678 2
Methods 

Silicone Samples and LEO Exposure 

Eight DC 93-500 samples were exposed to LEO ram exposure during 2 different MISSE missions 

(MISSE 2 & 4) such that each sample received a different AO fluence level. Four samples were exposed 

during each of the MISSE missions. Three of the four samples in each mission were covered with 

different thickness layers of Kapton H film in order for each of the samples in the same mission to receive 

a different AO fluence, as the AO needs to erode through the cover-laying Kapton before attacking the 

underlying silicone. The samples were 0.025 cm (10 mil) thick and were placed on 0.16 cm (1/16 in.) 

thick fused silica slides to keep the samples from bending and hence inducing cracking in the glassy 

oxidized layer (Ref. 4). The samples in the experiment trays were photographed prior to, and after, space 

exposure as shown in Figures 2 and 3. Post-flight photos of the eight samples are shown in Figure 4 along 

with the sample ID, the Kapton H cover thickness and the resulting AO fluences. It should be noted that 

MISSE 4 sample 4-E22-2 was flown with an aluminum washer (with a 0.44 in. diameter opening), thus 

the exposed area is a smaller area than the other seven flight samples, as indicated in Figure 4. The edges 

of the samples are protected from AO exposure during flight, and hence the Kapton covers are still 

present at the edges of the six samples that contained Kapton covers. The sample space exposures are 

summarized in Table 1. The sunlight and temperature history and the detailed results of previous studies 

of these samples are summarized elsewhere (Refs. 4, 6 to 8). 

Optical Microscope 

Instruments 

The stereomicroscope used in this study was a MZ 16 by Leica Microsystems. It has a magnification 

power range of x7 to x110. The silicone samples were placed on a motorized microscope stage (by 

Semprex) which is operated by a controller (OASIS-4i with computer software by Objective Imaging 

Ltd). The controller precisely controls the movement of microscope stage in a simple, repeatable manner 

for reproducible results. This eliminates the need to manually adjust sample positions. Images of the 

samples are taken by a digital color camera (TRV3.3 by MicroPublisher). The camera is controlled by 

computer software (Image Pro Plus by Media Cybernetics), which acquires, processes, analyzes and 

measure the images from the camera. Image analysis used in this study is described in detail in the 

following sections. 

Image Description and Characterization 

The Leica Microsystems camera receives white light reflected from the sample surface. Since the 

crack region is less transparent and more reflective than the rest of the sample surface, the image obtained 

from the camera shows a network of white crack lines and black “island” areas. Figure 5(a) is an example 

of an optical microscope image This picture is from the MISSE sample exposed to the highest AO fluence 

(sample 2-E5-1, which did not contain a Kapton H cover and received an AO fluence of 

8.431021atoms/cm2 during the MISSE 2 mission). In this report, all the optical microscope images were 

taken from an 1110830 μm area at the center of the sample surface. This is equivalent to 100x 

magnification. 

Various parameters to characterize the optical microscope images were analyzed and compared. 

Specifically, three parameters were obtained from the image analysis software and studied in this 

research. These include: (1) the average area of the islands, (2) the fraction of sample surface area that is 

occupied by crack lines, and (3) the total length of crack lines per unit area of the sample surface. 

NASA/TM—2012-217678 3
Image Analysis 

The above three parameters were obtained using image analysis software. This was done by first 

modifying the original pictures so that the modified picture was compatible with the software, and then by 

actually doing the analysis measurement. Details of this process are described below using Figure 5 as the 

illustrating example. 

The original picture (Fig. 5(a)) was at first modified by removing the shades of grey, so that the 

region occupied by crack lines became 100 percent white and the rest of the areas became 100 percent 

black. The black and white in this picture was then inverted to give Figure 5(b), which can be used by the 

software to calculate individual and total white (island) areas. The black thick lines (crack lines) in 

Figure 5(b) were then reduced to thin lines by the software to result in Figure 5(c), from which the 

perimeter of individual areas as well as sum of perimeters of all areas in the picture can be calculated by 

the software. The total length of the crack lines in the picture were then calculated as half of the sum of 

perimeters of all areas in the picture. Individual, total and average island areas were also calculated from 

Figure 5(c). 

Noise reduction and signal enhancement were applied throughout the process. These were done in the 

same way to all samples, and were carried out by the computer software. When enhancing the very faint 

crack lines, the image enhancement process may erroneously create fabricated lines and many very small 

“islands” from two crack lines that are in very close proximity to each other. The effects of this type of 

error will be discussed later in this report. 

Scanning Electron Microscopes 

Sample Preparation 

A Hitachi S-3100 was used to take 350x pictures. A Hitachi S-4700II was used to take 1000x and 

higher magnification pictures. All silicone samples in this research are electrical insulators. A path of 

electricity from the sample holder to the sample surface is needed for the purpose of examining the 

sample by SEM. For this purpose, two sections of copper tape were taped to every sample to electrically 

connect the bottom of the sample (i.e., the fused silica slides used to protect the silicone sample) to the 

sample surface. The surfaces of samples were then coated with thin layers of 40 percent Pd-60 percent 

Au. During coating, half of the sample surface was protected by a piece of paper. Therefore, only half of 

the surface of every sample was coated. A small spot of carbon paint was then placed on the area between 

the copper tape and the coated surface. In this situation, the electron beam from the SEM travels to the 

surface, the carbon dot, the copper tape, the sample holder at the bottom of the sample, and then back to 

the instrument. 

Examination 

Morphologies of all AO exposed silicone surfaces were examined. Pictures of every sample were 

taken at three different magnifications: 350x, 1000x, and 5000x. From these images, the kinetics of 

shrink-crack interaction can be examined. 

EDS spectra of all AO exposed silicone surfaces, and a pristine sample surface, were also obtained 

and analyzed. All spectra were taken with the same instrument settings of 6 kV and 10 μA. From each 

spectrum, the ratios of C to Si peak height and O to Si peak height were measured and calculated. The 

peak height ratios of AO exposed samples were then compared to those of the pristine sample. From these 

results, a semi-quantitative analysis can be made to measure the changes of the chemical composition of 

silicone resulting from AO exposure over a long period of time. 

NASA/TM—2012-217678 4
Results and Discussions 

Optical Microscope 

Pictures 

The original and processed optical microscopy images of all eight samples examined in this research 

are presented in Appendix A. 

It was observed that the silicone sample with the lowest AO fluence (sample 4-E22-5) has a large 

amount of debris material on it, seen primarily as white spots and clusters (Fig. A8 in Appendix A). These 

are residual fragments of the Kapton H cover layer placed on top of the silicone for the purpose of 

adjusting AO exposure of the under-laying silicone to a lower level. Since the residue resulted in strong 

interference during image analysis, optical microscope data obtained from this sample was not further 

examined. This sample, however, was further examined by SEM which will be described later in this 

report. 

All of the samples had some debris on them also. These particles were examined using EDS, and will 

be described at the final part of the Scanning Electron Microscope Image Analysis Section. These 

particles also contribute to the interference of signals, especially during the process of signal 

enhancement. During this process a large number of very small “islands” were created around these 

particles at the same time when the very faint crack lines were enhanced and therefore preserved. The 

error cause by this interference will be discussed in Section “Possible error”. 

General Trend of Parameters to Characterize AO Exposure 

The three different characterization parameters (the average area of the islands, the fraction of sample 

surface area that is occupied by crack lines, and the total length of crack lines per unit area of the sample 

surface) for the samples that were degraded by AO exposure to seven different fluence levels are 

summarized in Table 2. Since average crack widths can be estimated from crack length and crack area 

described above, they were calculated and also shown in Table 2. Figure 6 contains plots and corresponding 

trend lines of these morphological parameters plotted against the AO fluence. Exponential equations were 

used to represent the trend lines because of the observation that changes of the morphological parameters 

were fast at the beginning, but slowed down continuously as the total fluence increased. 

Possible Error 

A review of Table 2 and Figure 6 revealed that, among the three morphological parameter-fluence 

relations, the fraction of sample surface area that is occupied by crack lines seems to have the least data 

scattering and most consistent trend, as shown in Figure 6(b). The other two plots had larger data 

scattering. These large data scattering may be an artificial effect resulting from the data processing. 

The data scattering for the average area of the islands-fluence relation may be an error resulting from 

the creation of a large number of very small “islands” during the process of image enhancement. Adding 

many small fabricated islands into the picture significantly reduces the average island area. 

The data scattering of the total crack line length per unit area is more difficult to explain. It may also 

be an artificial effect resulting from the image processing. 

Data scattering of the fraction of sample surface area that is occupied by crack lines is the least 

among these three. This may possibly be a result of it being the simplest data processing method, as it was 

calculated from images of crack areas (see picture (b) in all Figures in Appendix A), while the other two 

parameters were calculated from images where crack areas needed to be reduced to thin lines (picture (c) 

from all Figures in Appendix A). Regardless of the reason why some trends appears to be more 

complicated than others, the above results suggest that the best morphological parameter to characterize 

the degradation of silicone under AO exposure would be the crack line area as a percent of the total 

sample surface area because it has the least data scatter. 

NASA/TM—2012-217678 5
Possible Kinetics of Crack Line Development 

Figure 7 is a plot of crack length per area versus crack area per total area. It shows the surface area 

increases almost linearly with crack length, indicating that crack width does not change much. Newly 

formed cracks continue to have the same width as the old ones until the fluence reached a very high level. 

This suggests that crack propagation is mostly either by growth of new cracks or by lengthening of the 

old cracks until the island areas become thick, glassy, and small. 

As described previously, the image of AO exposed silicone samples obtained from reflected light 

shows white crack lines and black “island” areas. This illustrates that the island areas transmit light, and 

the crack areas are more reflective. A consequence of the growth of crack area would therefore be a loss 

of specular transmittance with fluence. This would be a problem for Fresnel concentrators made of 

DC 93-500, or similar behaving silicones, which could represent large losses for concentrator-based 

photovoltaic systems. Light can be specularly transmitted through the islands that are free of cracks, 

however, the bottom of cracks are not parallel to the pristine surface of the DC 93-500 and thus cause the 

light passing through them to be diffusely scattered. Assuming the specular transmittance (1) is zero at the 

crack area and (2) does not change due to AO exposure at the island area, then the percent of specular 

transmittance loss would be the same as the percent of crack area, or the y axis of Figure 6(b). From this 

point of view crack area as a percent of total surface area seems to be a more relevant parameter to 

characterize the transmittance loss due to AO exposure. 

For the AO fluence level studied here (1.72-8.431021 atom/cm2

), adding island area as percent of 

total area to crack area as percent of total area would result in 100 percent. Therefore, for the plot of 

percent island area (A) as a function of fluence (F), the data fit (Fig. 8) an exponential equation 

A = 100-(26.071F 0.3218) F1.72 (1) 

A = 100-(26.071) 1.720.3218 (F/1.72) F1.72 (2) 

The first equation is from the data fit in Figure 6(b). The second equation assumes a linear relation at 

a fluence level from zero to 1.721021 atom/cm2

, the lowest fluence examined in this research. 

Then, if the specular transmittance at the crack area is not zero, the predicted specular transmittance 

of the sample relative to initial transmittance as a function of AO fluence would be the area-based average 

of the specular transmittance at the crack and the specular transmittance at the island. It is calculated to be 

approximately 

 ST/STp = 1–0.26(1-(STc/STp))F1/3 for F  1.72 (3) 

 ST/STp = 1–0.26(1-(STc/STp))1.721/3 (F/1.72) for F  1.72 (4) 

where 

F is the total AO fluence to which the silicone sample is exposed (in 1021 atom/cm2). ST is specular 

transmittance of the AO exposed sample (0 ST  STp), STc is specular transmittance of the AO exposed 

sample at the crack area (0 STc  STp), and STp is specular transmittance of the pristine sample. 

The above empirical equation suggests the specular transmittance loss would be proportional to AO 

fluence when the fluence is low, and proportional to AO fluence’s one-third power (i.e., F1/3) when the 

fluence is high. 

The specular transmittance of the pristine and AO exposed samples were measured to be STp = 0.911 

for the pristine sample, ST = 0.644-0.666 for MISSE 2 samples, and ST = 0.788-0.864 for MISSE 4 

samples (Ref. 8). Using these values with the known AO fluences for these samples, the specular 

transmittance at the crack areas (STc) can be calculated from the above equations. The calculated value 

NASA/TM—2012-217678 6
for the four high AO fluence MISSE 2 samples was in 0.40 to 0.44 range. Using the average value of 0.42 

and the above equation for high fluence, the specular transmittance of DC93-500 samples after long LEO 

exposures can be simplified to 

ST = 0.91-0.13F1/3 (5) 

Both the high fluence equation and the low fluence equation give the four MISSE 4 samples STc

values in 0.45 to 0.78 range. Because of the debris on some of the MISSE 4 samples, the true STc is 

believed to be near the high end of this range. Using STc = 0.78 and the above equation for low fluence, 

the specular transmittance of a debris-free DC93-500 samples after short LEO exposures can be 

simplified to 

ST = 0.91-0.024F (6)

Scanning Electron Microscope Image Analysis 

Pictures 

The representative SEM images of silicone samples exposed to different AO fluences are presented in 

Figures 9(a) to (d). It can be seen that at lower AO exposure levels (Fig. 9(a) and (b)), the cracks are 

relatively simple and can be understood as the result of shrinkage of a thin layer of silicone surface due to 

silicone-AO reactions. 

It can also be seen that new crack lines split old crack lines, and some old crack lines seem to stop 

cracking after some time, as many new crack lines become wider and deeper than the old crack lines. This 

data suggests that the oldest and first cracks, which are narrow, occur on surfaces that are under only 

small tensile stress. After the event of a crack, there will be a re-distribution of stress and tensile stress 

parallel to the crack becomes reduced. However, in directions approximately perpendicular to the initial 

cracks, stress builds up to accumulate to greater than that at the time of the initial cracks. This causes the 

newest cracks to open wider and deeper than the initial cracks. In general, the process continues to cause 

the newest branch cracks to be wider and deeper than older cracks. Consequently, some of the old crack 

lines diminish their cracking because the stress on them decreases. The more mud tiled the surface 

becomes, the deeper the thickness of silicone oxidation, which grows slower with fluence because the 

atomic oxygen has a more difficult time migrating deep to unoxidized silicone. Although the surface 

gradually becomes very cracked, the islands remain well adhered because their surface layer is a gradual 

transition from completely oxidized silica to unoxidized silicone interior. It would only be for very thin 

surface layer where oxidation could occur through the full thickness causing the silicone to be 

significantly converted to silica that the mud tiles could break apart. This may be a durability threat for 

silicone Fresnel lenses, inflatable structures, or silicone coatings in low Earth orbit. 

In the case of dimishing stress, after long AO exposure, the part of the surface that has not yet cracked 

develops a thick glassy layer, which is structurally stronger than a newly formed very thin oxide layer. 

Under stress, this newly formed layer will crack first to release the stress, and, further cracking of the 

thick glassy layer can be prevented. In the case of intensifying stress, the cycle of brittle surface 

formation-shrink-crack will continue until the stress is reduced, or the cracks become too deep for AO to 

reach. There are also observed cases where part of the cracked surface become increasingly glassy and 

thick, but the rest of the cracked surface continues to crack into greater depth due to the cycles of forming 

thin oxide layers causing high stress-cracking (Fig. 10). 

The results give a complex overall picture of the morphology change of silicone surfaces during 

silicone-AO reactions. Shrinkage causes stress. Thin AO exposed silicone is brittle. After long AO 

exposure, the part of the surface that has not yet cracked develops a thick glassy layer, which is 

structurally stronger than a newly formed very thin oxide layer. Shrinkage and stress of thin AO exposed 

layers cause cracks. Cracks then cause sudden surface movements, which cause stress on the surface to 

NASA/TM—2012-217678 7
re-distribute, which result in a new shrink rate on the surface and new crack rate at the crack lines. The 

activity of cracking increases for some crack lines, but diminishes for some others. Above all of these, the 

AO continues to react with the entire silicone surface, most likely at a higher rate at the stressed crack 

lines because those are places where some interior silicone is newly exposed to AO. The result is the 

newly formed thin oxide layer which is more inclined to crack under stress. 

Figure 11(a) to (c) are pictures of the complex crack morphology from the highest AO fluence 

exposed sample in this research (fluence = 8.431021 atom/cm2

).

The representative SEM images of all eight flight samples are presented in Appendix B (1000x) and 

C (5000x). 

Energy Dispersive Spectrum 

Representative EDS spectra of all eight AO exposed flight samples at “island” regions are provided in 

Appendix D. The EDS of the pristine sample (Fig. D9) was taken from the part of sample 4-E22-2 

(Fig. 4) that was covered by the aluminum spacer. From these spectra, the semi-quantitative atomic C/Si 

and O/Si concentrations of the flight samples relative to the pristine sample can be calculated. Results are 

shown in Table 3. It was estimated that the C/Si ratio dropped to 28 to 35 percent and 16 to 22 percent of 

the pristine value when the AO fluences are 1.46 to 2.141021 (MISSE 4) and 7.75 to 8.431021

(MISSE 2), respectively. The O/Si ratio, on the other hand, increased to 152 to 195 percent and 206 to 

258 percent of the pristine value, respectively. The carbon content decreased relatively quickly at the 

beginning when the fluence level was lower than the MISSE 4 value, but leveled to 20 percent of the 

pristine value at the MISSE 2’s fluence. The oxygen content appears to increase at a slower rate. 

Assuming the pristine DC93-500 has a C/Si and O/Si values of 2 and 1, respectively, the above 

numbers would suggest the highest AO fluence exposed silicone has C/Si and O/Si values of about 0.4 

and 2.3, respectively. From these numbers, it is theorized that the AO exposed silicone will eventually 

become SiO2 with small amounts of unreacted silicone, CO and CO2 sealed inside. 

The regions occupied by the crack lines were also examined by EDS. In general these areas contain 

less oxygen and more carbon than their neighboring islands do. This is expected, as the cracks result in 

the opening of the sample interior that was not exposed to AO at the beginning of the experiments. 

The 2.2 keV peaks in all EDS spectra are believed to be aluminum contamination, as all samples were 

near aluminum when they were exposed to AO. This includes the pristine sample, which was well 

protected by aluminum during the time of AO exposure. 

SEM pictures of the residual Kapton, SiOX nano-particles and contaminants on the sample surfaces 

are shown in Figures 12 and 13. These materials were also examined by EDS. In Figure 12 the EDS 

spectra of the MISSE flight sample with the lowest AO fluence exposure (1.461021 atoms/cm2

) indicates 

more carbon and oxygen than the pristine material. This is consistent with the belief that the debris is 

residual fragments of the Kapton H cover layer placed on top of the sample at the beginning of the 

experiment. In Figure 13 the numerous nano-sized particles were found to be SiOX. They were observed 

to be only on the samples that were covered by the over-laying Kapton. It is believed that during AO 

exposure, small particles of silicone were ejected from the samples, trapped by the over-laying Kapton, 

and eventually oxidized by the AO to form the nano-sized particles. Other particles such as the one shown 

in Figure 13(b) contain mostly C and O, but also contains numerous elements that include Na, Mg, Al, Si, 

S, P, Cl. These are believed to be organic contaminations. 

NASA/TM—2012-217678 8
Conclusions 

Eight DC 93-500 samples exposed to eight different AO fluence levels (that ranged from 1.46 to 

8.431021 atoms/cm2

) during two different MISSE missions (MISSE 2 and 4) were examined using 

optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Three 

morphological parameters were obtained for each image using computer image analysis software. These 

parameters were then used to characterize the morphological degradation of the AO exposed samples: 

1. The average area of the islands was found to range from 1000 to 3100 m2. Average island area is 

under-estimated because the software created a large number of small islands during image 

processing. 

2. The total length of crack lines per unit area of the sample surface was found to range from 27 to 

59 mm crack length per mm2

 of sample surface. The total crack length was observed to increase with 

increasing AO fluence until the AO fluence reached a high level where the glassy islands on the 

surface may become sufficient hard, thick and small to inhibit further length propagation. 

3. The fraction of sample surface area that is occupied by crack lines was found to range from 25 to 

56 percent for the fluence levels tested. This value increases steadily during the entire fluence range 

tested in this research. 

4. Average crack width can be estimated from crack length and crack area described above, and was 

calculated to be about 10 m. 

The fraction of sample surface area that is occupied by crack lines was found to be most useful 

morphological parameter in characterizing the degree of silicone degradation caused by AO exposure. 

The series of SEM images from the eight MISSE flight samples exposed to different AO fluences 

suggest a complex sequence of surface stress due to shrinkage, cracking, and then re-distribution of stress 

and shrinking on the sample surface. The data suggests that the oldest cracks, which are narrow, occur on 

surfaces that are under only small tensile stress. After a “crack event” there is a re-distribution of stress 

and the tensile stress parallel to the crack becomes reduced. However, in directions approximately 

perpendicular to the initial cracks, stress builds, which causes the newest cracks to be wider and deeper 

than the initial cracks. This cycle of shrinkage and cracking may be slowed in some areas if there is near 

area cracking sufficient to reduce the local stress. Then they will experience sufficient oxidation to 

develop a thick glassy layer, which not only significantly shields further oxidation, but also is structurally 

stronger than a newly formed very thin oxide layer. In that instance the reactions to AO will move to the 

stressed crack lines because those are places where some interior silicone is newly exposed to AO, 

resulting in a new thin oxide layer which is more inclined to crack again under stress. The energy 

dispersive spectra (EDS) indicated that upon AO exposure, carbon content decreased to 32 percent of the 

pristine value for the least exposed sample in this set of experiments (1.461021 atoms/cm2

), but leveled 

to 20 percent of the pristine value for the most exposed sample in this set of experiments 

(8.431021 atoms/cm2

). The oxygen content appears to increase at a slower rate. The least and most AO 

exposed samples were, respectively, 52 and 150 percent above the pristine values. The highest AO 

fluence exposed flight sample appears to have a surface layer which contains SiO2 with perhaps some 

unreacted silicone, CO and CO2 sealed in the SiO2 layer. 

References 

1. K.K. de Groh and T.A. McCollum, “Low Earth Orbit Durability of Protected Silicone for Refractive 

Photovoltaic Concentrator Arrays,” Journal of Spacecraft and Rockets, vol. 32, no. 1, Jan-Feb 1995, 

pp. 103-109. 

2. Ching-cheh Hung and Gidget Cantrell, “Reaction and protection of electrical wire insulators in 

atomic-oxygen environments,” NASA TM 106767 1994. 

NASA/TM—2012-217678 9
3. K.K. de Groh, B.A. Banks and D. Ma, “Ground-to-Space Effective Atomic Oxygen Fluence 

Correlation for DC 93-500 Silicone,” (Special Section: Space Environmental Effects on Materials) 

Journal of Spacecraft and Rockets, vol. 43, no. 2, March-April 2006, 414-420. 

4. de Groh K., Banks B., Dever J., Jaworske D., Miller S., Sechkar E., Panko S. 2008. “NASA Glenn 

Research Center’s Materials International Space Station Experiments (MISSE 1-7),” NASA/TM—

2008-215482. 

5. de Groh K. and McCollum T.1995 “Low Earth Orbit Durability of Protected Silicone for Refractive 

Photovoltaic Concentrator Arrays,” Journal of Spacecraft and Rockets, Volume 32 Number 1, Page 

103-109. 

6. Pippin G. 2008 “Summary Status of MISSE-1 & MISSE-2 Experiments and Details of Estimated 

Environmental Exposures for MISSE-1 & MISSE-2,” Final Report Air Force Contract 02-S470-011-

C1. 

7. Pippin G. 2008 “Summary Status of MISSE-3 & MISSE-4 Experiments and Details of Estimated 

Environmental Exposures for MISSE-3 & MISSE-4,” Final Report Universal Technology 

Corporation Contract 07-S568-030-C1-R1 (Government Prime Contract FA8650-050D-5807). 

8. D.L. Waters, K.K. de Groh, B.A. Banks, and K.C. Cameron, “Changes in Optical and Thermal 

Properties of the MISSE 2 PEACE Polymers and Spacecraft Silicones,” Proceedings of the 

International Symposium on Materials in a Space Environment (ISMSE-11), September 15-18, 2009, 

Aix-en-Provence, France, 2009. 

TABLE 1.—MISSE 1 AND 2 MISSION AND SPACE EXPOSURE SUMMARY 

MISSE 

PEC 

Launch 

Mission 

Date placed 

outside ISS 

Location on ISS Orientation Retrieval 

mission 

Date 

retrieved 

from ISS 

LEO 

exposure 

duration 

1 & 2 STS-105 8/16/2001 PEC 1: High pressure 

gas tank 

PEC 2: Quest airlock 

Ram & Wake STS-114 7/30/2005 3.95 years 

3 & 4 STS-121 8/3/2006 PEC 3: High pressure 

gas tank 

PEC 4: Quest airlock 

Ram & Wake STS-118 8/18/2007 1 year 

TABLE 2.—MORPHOLOGICAL PARAMETERS OF SILICONE SAMPLES EXPOSED TO 

DIFFERENT AO FLUENCES DURING MISSE 2 AND 4 MISSIONS 

Fluence 

xE21 

Total crack length in 

1 mm2

 of sample, 

mm 

Crack area to 

sample surface area ratio 

Average island 

area, 

μm2

Approx. crack 

width, 

μm 

8.43 52 0.56 979 11 

8.18 59 0.53 696 9 

8.01 46 0.45 1191 10 

7.75 48 0.49 1146 10 

2.14 42 0.38 1234 9 

1.89 41 0.35 1472 9 

1.72 27 0.25 3089 9 

TABLE 3.—CALCULATED VALUES OF THE SEMI-QUANTITATIVE 

ATOMIC C/Si AND O/Si CONCENTRATION OF THE AO EXPOSED 

SAMPLE RELATIVE TO THE PRISTINE SAMPLE 

AO fluence 

X1021 atom/cm2

(O/Si)/(O/Si)p (C/Si)/(C/Si)p 

1.46 1.52 0.32 

1.72 1.73 0.28 

1.89 1.65 0.35 

2.14 1.95 0.34 

7.75 2.58 0.16 

8.01 2.47 0.21 

8.18 2.06 0.17 

8.43 2.50 0.22 

NASA/TM—2012-217678 10
Figure 1. Post-flight photograph of MISSE 2 DC 93-

500 sample 2-E5-1 showing the crazed surface.

NASA/TM—2012-217678 11
a.

Figure 2. MISSE 2 Tray E5with the Spacecraft Silicone Samples highlighted, a). Prior to flight, b). MISSE 2 on the ISS Quest Airlock (the Spacecraft Silicone Experiment samples are outlined), and c). After space flight.

b.

C

NASA/TM—2012-217678 12
Figure 3: MISSE 4 with the Spacecraft Silicone Experiment samples highlighted, a). Prior 

to flight, and b). On-orbit just prior to retrieval (August 2007).

a b

NASA/TM—2012-217678 13
Figure 4: Spacecraft Silicone Experiment Samples

MISSE 2 & MISSE 4 (Ram Exposures)

2-E5-1 

(No Kapton H Cover)

AO= 8.43 E21

2-E5-2 

0.3 mil Kapton H Cover

AO= 8.18 E21

2-E5-3 

0.5 mil Kapton H Cover

AO= 8.01 E21

2-E5-4 

0.3+0.5 mil Kapton H Cover

AO= 7.75 E21

4-E22-2 

(No Kapton H Cover)

AO= 2.14 E21 

4-E22-5 

0.3+0.5 mil Kapton H Cover 

AO= 1.46 E21

4-E22-4 

0.5 mil Kapton H Cover 

AO= 1.72 E21

4-E22-3 

0.3 mil Kapton H Cover 

AO= 1.89 E21

Covered with Al 

spacer with 

0.44” diameter 

opening

NASA/TM—2012-217678 14
National Aeronautics and Space Administration

Figure 5: Optical micrograph of silicone DC93-500 exposed to 8.43 E21 fluence atomic oxygen in Low Earth orbit during MISSE 2 mission(a) Original picture (Upper left)(b) Converted version of original picture (black 

crack and white non-crack area) (above)

(c) Converted version with all cracks made into thin 

lines (left)

100 μm

NASA/TM—2012-217678 15
6(a) Total length of cracks in mm within 1 mm2

for atomic oxygen exposed silicone samples

6(b) Crack area as a % of area for atomic oxygen exposed silicone samples

6(c) Average island area

Figure 6: Plots and the trend lines of morphological parameters as a function of the fluence

NASA/TM—2012-217678 16
Figure 7: Crack surface area increases almost linearly with crack length, 

indicating the crack width does not change much during the entire time of atomic oxygen exposure

NASA/TM—2012-217678 17
Figure 8: Island area as a % of area of atomic oxygen exposed silicone 

samples. This would be the theoretical specular transmittance due to crack area assuming the specular transmittance at the crack is zero

NASA/TM—2012-217678 18
Figure 9: SEM pictures of silicone exposed to different level of AO fluences (a) 1.72 x 1021 atom/cm2

(b) 2.14 x 1021 atom/cm2 (c) 7.75 x 1021 atom/cm2 (d) 8.43 x 1021 atom/cm2

(a) (b)(c) (d)

NASA/TM—2012-217678 19
Figure 10: Pictures show part of the cracked surface stopped cracking as it became 

increasingly glassy and thick, but the rest of the cracked surface continues to crack into greater depth due to the cycles of forming thin oxide layer- causing high stress-cracking(Fluence = 8.01 x 1021 atom/cm2) 

NASA/TM—2012-217678 20
National Aeronautics and Space Administration

Figure 11: An illustration of the complex surface morphology after repeated shrink, crack and stress redistribution (AO fluence = 8.43 x 1021 atom/cm2

NASA/TM—2012-217678 21
National Aeronautics and Space Administration

Figure 12: SEM image of residual Kapton on the MISSE flight sample with the lowest 

AO fluence exposure (1.46 x 1021atom/cm2). EDS show it contains more carbon and 

oxygen than the pristine silicone.

NASA/TM—2012-217678 22
National Aeronautics and Space Administration

Figure 13: (a) The nano-sized particles on the sample surface are SiOx where x is 

close to 2 (EDS data). It is believed to be fragments of silicone trapped by the KaptonH cover layer and then oxidized by atomic oxygen (8.18x 1021 atom/cm2). (b) 

External materials believed to be organic contamination (from sample exposed to AO fluence of 8.43 x 1021 atom/cm2). EDS show it contains C, O Na, Mg, Al, Si, S, P, Cl. 

NASA/TM—2012-217678 23

Appendix A.—Optical Microscope Pictures for the MISSE2 and MISSE4 

Silicone Samples 

 

NASA/TM—2012-217678 25

National Aeronautics and Space Administration

Figure A1 Optical micrograph of silicone DC93-500 exposed to 8.43 E21 fluence atomic oxygen in in Low Earth Orbit during MISSE 2 mission(a) Original picture (Upper left)(b) Converted version of original picture (black 

crack and white non-crack area) (above)

(c) Converted version with all cracks made into thin 

lines (left)

NASA/TM—2012-217678 27
National Aeronautics and Space Administration

Figure A2: Optical micrograph of silicone DC93-500 exposed to 8.18 E21 fluence atomic oxygen in in Low Earth Orbit during MISSE 2 mission(a) Original picture (Upper left)(b) Converted version of original picture (black 

crack and white non-crack area) (above)

(c) Converted version with all cracks made into thin 

lines (left)

NASA/TM—2012-217678 28
National Aeronautics and Space Administration

Figure A3: Optical micrograph of silicone DC93-500 exposed to 8.01 E21 fluence atomic oxygen in in Low Earth Orbit during MISSE 2 mission(a) Original picture (Upper left)(b) Converted version of original picture (black 

crack and white non-crack area) (above)

(c) Converted version with all cracks made into thin 

lines (left)

NASA/TM—2012-217678 29
National Aeronautics and Space Administration

Figure A4: Optical micrograph of silicone DC93-500 exposed to 7.75 E21 fluence atomic oxygen in in Low Earth Orbit during MISSE 2 mission(a) Original picture (Upper left)(b) Converted version of original picture (black 

crack and white non-crack area) (above)

(c) Converted version with all cracks made into thin 

lines (left)

NASA/TM—2012-217678 30
National Aeronautics and Space Administration

Figure A5: Optical micrograph of silicone DC93-500 exposed to 2.14 E21 fluence atomic oxygen in Low Earth Orbit during MISSE 4 mission(a) Original picture (Upper left)(b) Converted version of original picture (black 

crack and white non-crack area) (above)

(c) Converted version with all cracks made into thin 

lines (left)

NASA/TM—2012-217678 31
National Aeronautics and Space Administration

Figure A6: Optical micrograph of silicone DC93-500 exposed to 1.89 E21 fluence atomic oxygen in Low Earth Orbit during MISSE 4 mission(a) Original picture (Upper left)(b) Converted version of original picture (black 

crack and white non-crack area) (above)

(c) Converted version with all cracks made into thin 

lines (left)

NASA/TM—2012-217678 32
National Aeronautics and Space Administration

Figure A7: Optical micrograph of silicone DC93-500 exposed to 1.72 E21 fluence atomic oxygen in Low Earth Orbit during MISSE 4 mission(a) Original picture (Upper left)(b) Converted version of original picture (black 

crack and white non-crack area) (above)

(c) Converted version with all cracks made into thin 

lines (left)

NASA/TM—2012-217678 33
National Aeronautics and Space Administration

Figure A8: Optical micrograph of silicone DC93-500 exposed to 1.46 E21 fluence atomic oxygen in Low Earth Orbit during MISSE 4 mission(a) Original picture (Upper left)(b) Converted version of original picture (black 

crack and white non-crack area) (above)

(c) Converted version with all cracks made into thin 

lines (left)

NASA/TM—2012-217678 34
Appendix B.—SEM Pictures for the Eight MISSE2 and MISSE4 Samples 

Exposed to Different Level of AO Fluences (x1000) 

 

NASA/TM—2012-217678 35

National Aeronautics and Space Administration

Figure B1: SEM pictures for the MISSE2 sample exposed to 8.43 x 1021 AO fluences ( x1000) 

NASA/TM—2012-217678 37
National Aeronautics and Space Administration

Figure B2: SEM pictures for the MISSE2 sample exposed to 8.18 x 1021 AO fluences ( x1000) 

NASA/TM—2012-217678 38
National Aeronautics and Space Administration

Figure B3: SEM pictures for the MISSE2 sample exposed to 8.01 x 1021 AO fluences ( x1000) 

NASA/TM—2012-217678 39
National Aeronautics and Space Administration

Figure B4: SEM pictures for the MISSE2 sample exposed to 7.75 x 1021 AO fluences ( x1000) 

NASA/TM—2012-217678 40
National Aeronautics and Space Administration

Figure B5: SEM pictures for the MISSE4 sample exposed to 2.14 x 1021 AO fluences ( x1000) 

NASA/TM—2012-217678 41
National Aeronautics and Space Administration

Figure B6: SEM pictures for the MISSE4 sample exposed to 1.89 x 1021 AO fluences ( x1000) 

NASA/TM—2012-217678 42
National Aeronautics and Space Administration

Figure B7: SEM pictures for the MISSE4 sample exposed to 1.72 x 1021 AO fluences ( x1000) 

NASA/TM—2012-217678 43
National Aeronautics and Space Administration

Figure B8: SEM pictures for the MISSE4 sample exposed to 1.46 x 1021 AO fluences ( x1000) 

NASA/TM—2012-217678 44
Appendix C.—SEM Pictures for the Eight MISSE2 and MISSE4 Samples 

Exposed to Different Level of AO fluences (x5000) 

 

NASA/TM—2012-217678 45

National Aeronautics and Space Administration

Figure C1: SEM pictures for the MISSE2 sample exposed to 8.43 x 1021 AO fluences ( x5000) 

NASA/TM—2012-217678 47
National Aeronautics and Space Administration

Figure C2: SEM pictures for the MISSE2 sample exposed to 8.18 x 1021 AO fluences ( x5000) 

NASA/TM—2012-217678 48
National Aeronautics and Space Administration

Figure C3: SEM pictures for the MISSE2 sample exposed to 8.01 x 1021 AO fluences ( x5000) 

NASA/TM—2012-217678 49
National Aeronautics and Space Administration

Figure C4: SEM pictures for the MISSE2 sample exposed to 7.75 x 1021 AO fluences ( x5000) 

NASA/TM—2012-217678 50
National Aeronautics and Space Administration

Figure C5: SEM pictures for the MISSE4 sample exposed to 2.14 x 1021 AO fluences ( x5000) 

NASA/TM—2012-217678 51
National Aeronautics and Space Administration

Figure C6: SEM pictures for the MISSE4 sample exposed to 1.89 x 1021 AO fluences ( x5000) 

NASA/TM—2012-217678 52
Figure C7: SEM pictures for the MISSE4 sample exposed to 1.72 x 1021 AO fluences ( x5000) 

NASA/TM—2012-217678 53
National Aeronautics and Space Administration

Figure C8: SEM pictures for the MISSE4 sample exposed to 1.46 x 1021 AO fluences ( x5000) 

NASA/TM—2012-217678 54
Appendix D.—EDS for Pristine Silicone and the 

Eight MISSE2 and MISSE4 Samples Exposed 

to Different Level of AO Fluences 

 

NASA/TM—2012-217678 55

National Aeronautics and Space Administration 

Figure D1: Energy Dispersive Spectra from two clean spots of the silicone DC93-500 sample exposed to 8.43 E21 fluence atomic oxygen in lower Erath orbit during MISSE 2 mission 

NASA/TM—2012-217678 57
Figure D2: Energy Dispersive Spectra from two clean spots of the silicone DC93-500 sample exposed to 

8.18 E21 fluence atomic oxygen in lower Erath orbit during MISSE 2 mission 

NASA/TM—2012-217678 58
Figure D3: Energy Dispersive Spectra from two clean spots of the silicone DC93-500 sample exposed to 

8.01 E21 fluence atomic oxygen in lower Erath orbit during MISSE 2 mission 

NASA/TM—2012-217678 59
Figure D4: Energy Dispersive Spectra from two clean spots of the silicone DC93-500 sample exposed to 

7.75 E21 fluence atomic oxygen in lower Erath orbit during MISSE 2 mission 

NASA/TM—2012-217678 60
National Aeronautics and Space Administration 

Figure D5: Energy Dispersive Spectra from two clean spots of the silicone DC93-500 sample exposed to 2.14 E21 fluence atomic oxygen in lower Erath orbit during MISSE 2 mission 

NASA/TM—2012-217678 61
National Aeronautics and Space Administration 

Figure D6: Energy Dispersive Spectra from two clean spots of the silicone DC93-500 sample exposed to 1.89 E21 fluence atomic oxygen in lower Erath orbit during MISSE 2 mission 

NASA/TM—2012-217678 62
Figure D7: Energy Dispersive Spectra from two clean spots of the silicone DC93-500 sample exposed to 

1.72 E21 fluence atomic oxygen in lower Erath orbit during MISSE 2 mission 

NASA/TM—2012-217678 63
National Aeronautics and Space Administration 

Figure D8: Energy Dispersive Spectra from two clean spots of the silicone DC93-500 sample exposed to 1.46 E21 fluence atomic oxygen in lower Erath orbit during MISSE 2 mission 

NASA/TM—2012-217678 64
National Aeronautics and Space Administration 

Figure D9: Energy Dispersive Spectra from two spots of the pristine silicone DC93-500 sample 

NASA/TM—2012-217678 65

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4. TITLE AND SUBTITLE

Optical and Scanning Electron Microscopy of the Materials International Space Station 

Experiment (MISSE) Spacecraft Silicone Experiment 

5a. CONTRACT NUMBER

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Hung, Ching-cheh; de Groh, Kim, K.; Banks, Bruce, A. 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

Under a microscope, atomic oxygen (AO) exposed silicone surfaces are crazed and seen as “islands” separated by numerous crack lines, much analogous to mud-tile cracks. 

This research characterized and compared the degree of AO degradation of silicones by analyzing optical microscope images of samples exposed to low Earth orbit (LEO) AO 

as part of the Spacecraft Silicone Experiment. The Spacecraft Silicone Experiment consisted of eight DC 93-500 silicone samples exposed to eight different AO fluence levels 

(ranged from 1.46 to 8.43×1021 atoms/cm2) during two different Materials International Space Station Experiment (MISSE) missions. Image analysis software was used to 

analyze images taken using a digital camera. To describe the morphological degradation of each AO exposed flight sample, three different parameters were selected and 

estimated: (1) average area of islands was determined and found to be in the 1000 to 3100 mm2 range; (2) total length of crack lines per unit area of the sample surface were 

determined and found to be in the range of 27 to 59 mm of crack length per mm2 of sample surface; and (3) the fraction of sample surface area that is occupied by crack lines 

was determined and found to be in the 25 to 56 percent range. In addition, average crack width can be estimated from crack length and crack area measurements and was 

calculated to be about 10 mm. Among the parameters studied, the fraction of sample surface area that is occupied by crack lines is believed to be most useful in characterizing 

the degree of silicone conversion to silicates by AO because its value steadily increases with increasing fluence over the entire fluence range. A series of SEM images from the 

eight samples exposed to different AO fluences suggest a complex sequence of surface stress due to surface shrinkage and crack formation, followed by re-distribution of stress 

and shrinking rate on the sample surface. Energy dispersive spectra (EDS) indicated that upon AO exposure, carbon content on the surface decreased relatively quickly at the 

beginning, to 32 percent of the pristine value for the least exposed sample in this set of experiments (1.46×1021 atoms/cm2), but then decreased slowly, to 22 percent of the 

pristine value for the most exposed sample in this set of experiment (8.43×1021 atoms/cm2). The oxygen content appears to increase at a slower rate. The least and most AO 

exposed samples were, respectively, 52 and 150 percent above the pristine values. The silicone samples with the greater AO exposure (7.75×1021 atoms/cm2 and higher) 

appear to have a surface layer which contains SiO2 with perhaps small amounts of unreacted silicone, CO and CO2 sealed inside.

15. SUBJECT TERMS

Silicone; low Earth orbit (LEO); Space environment; Environmental degradation; Characterization 

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