Evaluation of Space Enviroment
and Effects on Materials
(ESEM) Archive System
NASA Langley Research Center
Hampton, Virginia

Final Report
United States Developed ESEM Experiments
Evaluation of Space Environment and Effects on Materials

Appendix E

Optical-Witness-Plates Experiment Results

Gale A. Harvey
National Aeronautics and Space Administration (NASA)
Langley Research Center (LaRC)
Hampton, VA 23681-0001

OBJECTIVE

The Optical Witness Plate Experiment was part of the Evaluation of the Space Environment on Materials cooperative experiment between NASA LaRC, Boeing Aerospace, William and Mary University Physics Department, and NASDA. A preflight photograph of the ESEM experiment is shown in Figure 1. The Optical Witness Plates Experiment is in the left quarter of the ESEM sample tray. Pre- and postflight photographs of the optical witness plates holder are shown in Figures 2 and 3. The primary objective of the ESEM Optical Witness Plates Experiment was to obtain cleanliness measurements, particularly of molecular depositions, in the orbiter payload bay during the STS 85 mission. A secondary objective was to demonstrate several complimentary passive optical techniques for measurement of molecular depositions.

Figure 1. Preflight photograph of the ESEM experiment.

A pronounced change from specular to diffuse reflection accompanied by a blue color shift was measured on a 1/4 mil aluminized Kapton thermal control film.

PROCEDURE

The procedure was to measure pre-space-exposure optical characteristics (UV-visible and mid IR transmission), particle cleanliness and microscopic appearance, and scanning electron microscope energy dispersive spectra (SEM-EDS) of the ESEM optical witness plates. Four optical witness plates: sapphire (AL2O3), calcium fluoride (CaF2), AR coated germanium, and a green (500 nm ) bandpass (70 nm) interference filter were used. Preflight characterization of the optical witness plates were documented in Test Report 110 (2-28-97). The optics were mounted in a holder covered with 1/4 mil aluminized Kapton to thermally isolate them from radiative heating of the material sample holder by the SUN. Three of the optical plates are radiative-environment cold (low A/e), and the germanium plate is radiative-environment hot (high A/e). The predicted on-orbit temperatures of the optical witness plates and thin films were documented in Test Report 112 (4-4-97).

Figure 2. Preflight photograph of optical witness-plate holder.

RESULTS

The optical witness plates were examined visually by optical microscopy (10x to 40x) to measure particle cleanliness. The pre-and post flight cleanliness is listed in Table 1. The particle cleanliness levels (MIL-STD-1246C) are plotted in Figure 2. These cleanliness levels are based on the numbers of largest particles on the optical witness plates. Microphotographs of particles on the sapphire and green witness plates are shown as Figures 5 and 6. The halos around the larger particles in Figures 5 and 6 are out of focus reflections of the particles from the back surface of the witness plates. The postflight cleanliness level (~CL=500) is about 100 times dirtier than the preflight cleanliness level (CL=200, or about one 50 micron diameter particle/square inch). The postflight particles and fibers are typical of those in the Orbiter Processing Facility(OPF). The pre- and post cleanliness levels verify the importance of providing protective covers during orbiter and experiment processing, and during ascent and reentry for high particle cleanliness measurements. The particle cleanliness of an experiment (PPMD), returned by STS 86 from Mir after 18 months external deployment, had protective covers and maintained a particle cleanliness of CL~250.

No indication of thin films of molecular residue on any of the optical witness plates was seen by microscopic inspection at 10x to 500x. Likewise no indication of thin films of molecular residue were indicated by comparison of pre- and post flight UV-visible spectra (Figure 7) or IR spectra (Figure 8). A very small amount of silicone deposition is indicated by SEM-EDS on the CaF₂ witness plate (Figure 9).

Figure 3. Post-flight photograph of optical witness-plate holder.

The AO exposed 1/4 mil aluminized Kapton thermal control film was noticeably brighter, and more yellow than the unexposed film. Figures 2 and 3 are photographs of the 1/4 mil thermal control film before and after 56 hours (6 x 10¹⁹ atoms/cm2) of ram direction atomic oxygen exposure during the STS 85 mission. Figures 10 and 11 are specular (90 deg.) reflection and diffuse (45 deg.) reflection of exposed and unexposed areas of the film. About 3% of yellow (600 nm) light is diffusely reflected (Figure 10) from the AO exposed area compared to zero percent measured yellow light diffusely reflected (Figure 11). Atomic oxygen erosion of this film could probably be reliably measured at 10% (6 x 10¹⁸ atoms/cm²) of the ESEM atomic oxygen exposure by this method of spectral change in reflectance.

CONCLUSIONS

1. The particle contamination on the witness plates increased by about two orders of magnitude to levels and types of particles typical of payload processing and the orbiter payload bay (CL~=500).

2. The molecular cleanliness of the optical witness plates remained very high (i.e., class A or better). This amount of molecular contamination is below the level of the optical measurement techniques used, A very small amount silicone, probably return flux of outgassing from orbiter thermal protection system RTV adhesives was indicated by electron microscope energy dispersive spectroscopy.

3. The ¼ mil aluminized Kapton was eroded by atomic oxygen and changed from primarily specular reflection to primarily diffuse reflection. The visual color also shifted toward the blue region of the spectrum i.e. the color changed from brown to yellow.

Figure 4. ESEM optical witness-plate cleanliness.


Figure 5. Photograph of particle on sapphire optical witness plate.


Figure 6. Photograph of particle on green (500 nm) witness plate.


Figure 7. UV - isible post-flight spectra of calcium flouride witness plate.


Figure 8. IR post-flight spectra of CaF₂ witness plate.


Figure 9. SEM-EDS spectrum of clacium flouride witness plate.


Figure 10. Specular relection from 1/4 mil Al Kapton.


Figure 11. Diffuse reflection from 1/4 mil Al Kapton.

Final Report | Appendix A | Appendix B | Appendix C | Appendix D | Appendix E

---

AORP | Clementine | EuReCa | ESEM | Hubble
LDEF | MDIM | MEEP | MIS | MPID