Various reflector configurations were flown as part of the Long Duration Exposure Facility (LDEF) A0171 experiment. These reflectors consisted of nickel substrates with aluminum, enhanced aluminum (multiple layers of aluminum and silver), silver, and silver alloy coatings with glassy ceramic overcoatings. These samples have been evaluated for changes in reflectance due to 5.8 years in the space environment. The reflector materials have also been evaluated using angstrometer, Rutherford backscattering (RBS), and electron spectroscopy for chemical analysis (ESCA) techniques

Edwards, David

Finckenor, Miria M.

Kamenetzky, Rachel R.

Linton, Roger C.

Whitaker, Ann F.

English

NASA Technical Reports Server

" " N95 23912
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DURABILITY OF REFLECTOR MATERIALS IN THE SPACE ENVIRONMENT
Ann F. Whitaker
Project and Environmental Engineering
Marshall Space Flight Center, AL 35812
Miria M. Finckenor
David Edwards
Rachel R. Kamenetzky
Roger C. Linton
Space Environment Effects
Marshall Space Flight Center, AL 35812
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ABSTRACT
Various reflector configurations were flown as part of the Long Duration Exposure Facility
(LDEF) A0171 experiment. These reflectors consisted of nickel substrates with aluminum,
enhanced aluminum (multiple layers of aluminum and silver), silver, and silver alloy coatings with
glassy ceramic overcoatings. These samples have been evaluated for changes in reflectance due
to 5.8 years in the space environment. The reflector materials have also been evaluated using
angstrometer, Rutherford backscattering (RBS), and electron spectroscopy for chemical analysis
(ESCA) techniques.
INTRODUCTION
Multi-layered reflector materials, such as the samples flown on the LDEF satellite, have
been tailored for highly efficient reflectance in the near-ultraviolet through visible and near-infrared
wavelengths (approximately 350 nm- 1400 nm). Spacecraft applications include solar
concentrators and energy dispersion systems where reflection of solar radiation is required.
Analysis of the protective glassy ceramic layers utilized on these samples may have transferrable
applications in other areas, such as solar cell covers.
EXPERIMENT DESCRIPTION
The Solar Array Materials Passive LDEF Experiment (SAMPLE), A0171, was exposed to
the low Earth orbit space environment on the LDEF satellite. The duration of this exposure to
atomic oxygen, ultraviolet radiation, thermal cycling, meteoroid/debris impacts, and particulate
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radiation was approximately 69 months. The experiment was positioned 38 degrees off the RAM
direction at its location on Row 8 position A. The exposure conditions are given in Table 1.
The SAMPLE experiment tray is shown in Figure 1. Solar reflector materials were
included in the 300 flight specimen complement. Approximately half of the reflector samples
received full surface exposure while the remainder were configured with a mask that covered half
the surface area. These half-covered samples are hereat_er referred to as half-moon samples. These
samples flew on Plate IV of the A0171 experiment and are shown in Figure 2.
Nickel substrates were coated with either silver or aluminum then a glassy ceramic
protective layer. The samples exposed on LDEF were:
• Silver coated with SiO2
• Silver coated with MgF2 and sapphire
• Silver alloy coated with a multi-layer dielectric of MgF2 and A1203
• Aluminum coated with SiO2
• Aluminum coated with SiO
• Enhanced aluminum coated with SiO and SiO2
• Enhanced aluminum coated with MgF2 and sapphire
These flight samples and the control samples which were stored in the laboratory for the duration
of the mission have been evaluated for changes in visual and microscopic appearance, optical
properties, surface roughness, and layer thickness. Changes in the film morphology were
determined by precision angstrometer traces, ellipsometry measurements, and Rutherford
backscattering analysis. ESCA analysis was also performed to determine composition and extent
of contamination, if any.
High Vacuum
UV Radiation
Proton Fluence
Electron Fluence
Atomic Oxygen
Micrometeoroid/
Space Debris
Thermal Cycles
Table 1. Experiment A0171 Exposure Conditions
- 10 .5 - 10 -7 Torr (Estimated)
- 10,041 ESH
- 109 p+/cm 2 (0.05 - 200 MeV)
- (10 TM - 1012) e-/cm 2 (0.05 - 3.0 MeV)
- 7.15 x 1021 atoms/cm 2
- 2 to 5 impacts per 25 cm 2, < 1mm
- -32,000 cycles (temp. unknown)
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VISUAL OBSERVATIONS
Visual observations of the reflector materials post-flight revealed few changes. The most
significant changes occurred in the reflector samples of silver overcoated with magnesium fluoride
and sapphire. These samples were more diffuse due to space environment exposure. The exposed
areas appeared scratchy and non-reflective. One sample in particular (fig. 3) was darker, with
increased oxidation in atmosphere. The flight silver samples with silicon dioxide coatings had a
few pits but remained reflective. The silver alloy reflectors overcoated with a dielectric multilayer
suffered some scratches but also remained reflective. The aluminum reflectors with silicon dioxide
or silicon monoxide coatings appeared unchanged (fig. 4), as did the enhanced aluminum
reflectors. Black light observations did not reveal any obvious molecular contaminant deposition.
OPTICAL PROPERTY MEASUREMENTS
A variety of instruments are available for optical measurements in the Engineering Physics
Division of the Materials and Processes Laboratory at the Marshall Space Flight Center. Post-
flight reflectance measurements were performed on both control and flight samples with several
instruments as follows - a Varian Cary model 2300 spectrophotometer, a Beckman model DK-2A
spectrophotometer, a Perkin-Elmer model Lambda 19 spectrometer, and an AZ Technology
Laboratory Portable Spectroreflectometer (LPSR). Thermal emittance measurements were made
with a Gier-Dunkel DB 100 portable infrared reflectometer.
Reflectance and thermal emittance measurements for selected samples are given in Table
2. The reflectance measurement given for each material is the integrated reflectance from 350 nm
to 1400 nm. The exposed reflectance measurements shown are for the worst case (highest AR 0
in each sample group. The range of reflectance change for each sample group is also given. Small
decreases in reflectance were noted for the majority of exposed samples. Changes in reflectance
were consistent within each sample group with the exception of the silver samples coated with
magnesium fluoride and sapphire. These reflectance changes are in agreement with the visual
observations, with the most obviously degraded sample experiencing a 17% drop in solar
reflectance (fig. 5). Thermal emittance measurements on all samples showed no significant
changes due to space exposure.
CHANGES IN SURFACE MORPHOLOGY
Precision angstrometer traces were made on all the samples using a Wyko Topo-3D surface
profiler. A decrease in film height was noted in the exposed areas of the flight samples. A typical
trace of a half-moon sample is shown in fig. 6. The changes in thickness of the exposed materials
are given in Table 3 and range from 25 up to 160 A. For applications of these materials where the
total coating thickness is 1000 to 2000 A, the percentage change is considerable, and the effect can
be substantial for space optics. If these changes in thickness are assumed to be the result of atomic
793
oxygen interaction, then the calculated reaction efficiencies, based on the angstrometer
measurements of a change in layer height, range from 0.4 to 2.3 x 10 -2gcm3/atom.
Roughness of the samples was measured using a TMA Technologies laScan instrument.
RMS roughness measurements are given in Table 3. Measurements of unexposed areas of the half-
moon samples agreed well with control surface measurements. These roughness measurements
also agree with visual observations, particularly of the oxidized silver/MgF2-sapphire samples.
Ellipsometry measurements were made using a Gaertner Waferskan model L115B. For
given N, the refractive index, and K, the extinction coefficient, calculations based on the Fresnel
equations indicate a reflectance equal to the measured reflectance for an aluminum reflector coated
with 1300 A of silicon monoxide. The top layer thickness measurements are nominally in
agreement with the measurements indicated by RBS analysis, as discussed in the next section.
RUTHERFORD BACKSCATTERING ANALYSIS
Rutherford Backscattering (RBS) analysis was performed on a number of the half-moon
reflector samples. RBS is based on the concepts of coulomb scattering and heavy ion energy loss
in matter. In most applications, alpha particles are used as the incident beam. A collimated beam
of mono-energetic alpha particles impinges on a target, and a small fraction of these particles is
scattered due to coulomb interactions with atomic nuclei in the target. Scattering may occur at the
front surface of the target or at depths within the target. If the alpha particle penetrates the target
instead of scattering from the front surface, the alpha particle loses energy continuously during the
penetration due to coulomb interactions with atomic electrons. This measured energy loss
produces information related to the depth within the sample at which a scattering event takes place.
During this investigation, a 2 MeV beam energy, a 0.015" beam diameter, and 2
microcoulomb total charge to the sample were used. The incident alpha particles impinged normal
to the sample, with the scattered alpha particles measured through an angle of 160 degrees. RBS
spectra were obtained from both the exposed and unexposed areas of the reflector samples.
The nickel substrate, being massive relative to the coating materials, dominates the RBS
spectrum up to approximately 1200 KeV. The presence of this nickel signal complicates the
analysis with respect to the lighter elements.
Results from the RBS analysis are given in Table 4. A typical RBS spectra of the exposed
and unexposed areas of an apparently unaffected silver/silicon dioxide reflector is shown in Figure
7. Slight diffusion was apparent at the silicon dioxide-silver and silver-nickel substrate interfaces
in both spectra, but the thickness of the SiO2 layer was not significantly changed. Enhanced
aluminum with magnesium fluoride-sapphire overcoating showed the most significant change in
RBS spectra. Figure 8 shows the diffusion of the silver in the enhanced aluminum into the nickel
substrate and into the coating layer. However, this observation was not consistent among the
samples. This diffusion was observed in sample #51 but not #60. These samples received the
same amount of space exposure, therefore the cause of this variation is not obvious. Analysis of
these samples is continuing.
To sum up the Rutherford backscattering analysis results, the silver and silver alloy
samples, in general, revealed some diffusion between the silver-substrate and silver-protective
coating interfaces for both exposed and unexposed material. Space environment effects on these
794
materials, as indicated by surface profllometry and other measurements, were not obviously
revealed by RBS analysis. The aluminum and enhanced aluminum samples with SiOx coatings
appeared to be stable in the space environment. The enhanced aluminum with a magnesium
fluoride-sapphire protective coating appeared to be affected by space exposure, but this observation
was not consistent among the samples nor in agreement with angstrometer traces.
ESCA ANALYSIS
An aluminurn/SiO2 reflector, a silver/MgF2-sapphire reflector, and other material samples
from this experiment plate were selected for ESCA analysis. The aluminum reflector was half-
exposed to the environment, so three measurements were made on the unexposed and exposed
areas. Only one of the three measurements on the exposed area has a significantly higher carbon
content (C ls peak) than that of the control area (see Table 5). The silicon to oxygen ratio is near
0.5, in agreement with the SiO2 top layer chemistry.
The silver/MgF2-sapphire reflector was fully exposed to LEO, therefore only exposed
ESCA data is given in Table 5. Aluminum, from the sapphire layer, was found in atomic
percentages of 12 and 16% in the two measured areas. Silver was also found in atomic percentages
of 3 and 7%. Magnesium and fluoride peaks were not reported. The level of oxygen found is in
agreement with the sapphire A1203 layer and formation of silver oxide and/or silicon oxide during
the flight. This oxidation layer was not significant enough to appear on RBS spectra. For the
silver reflector, carbon levels are typical of LDEF flight samples, though some of the carbon may
have been deposited during post-flight storage before ESCA analysis was performed. From these
measurements and results from other Plate IV materials, there is some variability in contaminant
deposition in exposed sample areas. Visual inspection indicate these reflectors appear to be
reasonably clean.
CONCLUSIONS
Degradation of the solar reflectors, when it occurred, was not uniform across the material.
When designing for the low Earth orbital environment, solar reflector materials require careful
selection. The best performance of a solar reflector material in terms of property maintenance was
obtained from a fully oxidized coating over aluminum. All of the coatings experienced
densification, decreased thickness, and increased surface roughness, but the measured solar
reflectivity values were better than expected based on visual observations. Inter_zoating diffusion
was observed with Rutherford backscattering analysis.
795
REFERENCES
Whitaker, A.F., Kamenetzky, R.R., Finckenor, M.M., and Norwood, J.K. Atomic Oxygen Effects
on LDEF Experiment A0171. Second LDEF Post-Retrieval Symposium, San Diego, CA, June
1992.
ACKNOWLEDGMENTS
The authors gratefully acknowledge Bob Patterson of TRW, for supplying the samples from
Cascade Optical Coating, Inc. and Optical Coating Laboratory, Inc. The authors also wish to thank
Ms. Whitney Hubbs and Mr. John Reynolds of the Physical Sciences Branch for performing
spectrophotometer and ellipsometer measurements, and Mr. J.K. Norwood of the Nondestructive
Evaluation Branch and Mr. Perry Gray of Microcraft, Inc. for Wyko analysis.
796
Table 2. Optical Property Changes of Solar Reflectors
Solar Reflector/Coating Control Rs Exposed Rs A Rs (%) Control em Exposed eIR
Ag/SiO2 0.94 0.93 -1 0.03 0.03
Ag/MgF2-Sapphire 0.93 0.76 -4 to 17 0.02 0.02
Ag Alloy/Dielectric 0.92 0.90 -1 to 2 0.02 0.02
A1/SiO2 0.87 0.87 <- 1 0.03 0.03
AI/SiO 0.87 0.85 -2 0.02 0.02
Enhanced A1/SiO-SiO2 0.88 0.86 -2 0.02 0.02
Enhanced AI/MgF2- 0.89 0.88 <- 1 0.05 0.05
Sapphire
Table 3. Changes in Film Morphology
Solar Reflector/Coating Decrease in Film Control RMS Exposed RMS
Thickness (A) Roughness (A) Roughness (A)
Ag/SiO2 40 45 60
Ag/MgF2-Sapphire 150 25 75
Ag Alloy/Dielectric 160 30 40
A1/SiO2 50 45 40
A1/SiO 150 30 35
Enhanced AI/SiO-SiO2 125 30 30
Enhanced AI/MgF2- 25 25 30
Sapphire
797
Table4. RutherfordBackscatteringAnalysis Results
Solar Reflector/Coating
Ag/SiO2
Ag/MgF2-Sapphire
Ag Alloy/Dielectric
Al/Si02
A1/SiO
Enhanced AI/SiO-SiO2
Enhanced AFMgF2-Sapphire
Observations
No appreciable change
Slight diffusion at interfaces
Visually apparent damage is not uniform enough to appear
on RBS spectra
Slightly increased diffusion at layer interfaces where
exposed to space environment
No appreciable change
No appreciable change
Little difference between exposed and unexposed spectra.
Protective layer appears to be mainly SiO 2
Silver of enhanced aluminum material diffused into
surrounding layers.
Results not consistent in sample group.
Locaaon
Aluminum/SiO2
Unexposed #1 32
#2 33
#3 31
Table 5. ESCA Survey Results
Ratio
Q (at%) SFO
54 0.59 14
51 0.65 16
53 0.58 16
Exposed#1 27 47 0.57 26
#2 31 50 0.62 19
#3 30 55 0.55 15
Silver/MgF2-Sapphire
Exposed #I 15
#2 14
50 0.30 20
41 0.34 22
798
Figure 3
Silver reflector with MgF2-sapphire coating
Figure 4
Aluminum reflector with SiO2 coating
800
Plate IV
,_, ,o
Figure 1
LDEF Experiment A0171 (SAMPLE)
Figure 2
A0171 Plate IV - Thermal Control Coatings
799
Silver Reflector
Magnesium Fluoride-Sapphire Coating
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0.6
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950 11 O0 1250 1400
Wavelength (nm)
Unexposed Exposed
Figure 5
FIOI 7 I IV79
RHS: 5.73nm
RR: 5.42nm
Time: LL:L9 Date: 0[/09/9 [
PROFILE PV: 19.9nm
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Dimtance on Surface in Ht lllmeters (2.5X)
NYKO
Figure 6
Angstrometer trace of
silver reflector with SiO 2 coating
Transition between exposed/unexposed areas
801
250
-Q 2O0
"6
_ 150
N
_ 100
Z 50
0
100
Energy (MeV)
0.5 1.0 1.5
I I I
__.....,...L./. 0 -- Control
...... Exposed
""". Si
..... .: / . Ag
I I I
200 300 400 5OO
Channel
_Figure 7: RBS Spectra of Ag/SiO21
20O
Energy (MeV)
0.5 1.0 1.5
I I I
2.0
"O
-_ 150
7-
N 100
_ 5o
Z
0
100
Control
_ ......... Exposed-
'{-.'_,, 0, F
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1 I I
200 300 4430 500
Channel
Figure 8: RBS Spectra of Enhanced AI/MgF2-Sapphire Sample Indicating
Interlayer Diffusion
8O2


Durability of reflector materials in the space environment

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