Annual Report on Contract NAS8-36645 

entitled 

PARTIAL ANALYSIS OF EXPERIMENT LDEF A-0114 

for the period ending May 31st, 1986 

[NASA-C8-179236) PARTIAL ANALPSIS OF 188-14787 

EXPEBIBENT LDEP a-0 11 4 annual Report, for 

period ending 31 May 1986 (Alabaaa Univ.) 

6P CSCL 20P Unclas 

G3/74 0114847 

Submitted to 

George C. Marshall Space Flight Center 

Huntsville. AL 35812 

National Aero nautics and Space Administration 

Dr. J. C. Gregory, Principal Investigator 

Institute for Surface Science 

and Chemistry Department 

Huntsville, AL 35899 

The University of Alabama In Hunstville 
I -- 

- Introduction 

Due to delays in manifesting the return of the Long Duration Exposure 

Facility from space, attention has remained concentrated on extracting the maximum information from the EIOM-2 experiment flown on STS-8 in September 1983. 

During the period covered by this report an analysis was made of the optical 

surfaces exposed during that flight and an assessment made of the effect of the 

5eV atomic oxygen upon their physical and chemical properties. This work, which 

has now appeared in the journal "Applied Optics" Vol. 25, page 1290 on April 15, 

1986 was entitled "Effects on Optical Systems from Interactions with Oxygen 

Atoms in Low Earth Orbits". Its authors were P. N. Peters, J. C. Gregory, and 

J. T. Swann. 

EXDerimental Measurements 

Surfaces of various samples were exposed in the orbital direction as part of 

The University of Alabama in Huntsville module of the oxygen interaction with 

materials experiment (EOIM-2) which flew on Shuttle Flight STS-8 in September 

1983. 

Approximately 95% of the total atom fluence of -3.5 x 1020 oxygen atoms cmV2 

was produced while surfaces were oriented within lo of the orbital direction. 

The surfaces studied were of two types: high-purity thin films sputtered or 

evaporated onto 2.54-cm diam h/20 fused silica optical flats, and highly 

polished bulk samples. Films were prepared with optical densities of -2.5 or 

less, as listed in Table 11. Measurement of optical densities using a 

Perkin-Elmer PDS scanning microdensitometer allowed sensitive determination of 

any changes to these thin films produced by the exposure. By masking one-half 

of each sample during flight as a control it was possible to measure changes in 

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optical density of -0.01, corresponding to a few percent change in thickness of 

-10 nm thick film. (No increase in scattering was assumed, since most films 

became smoother; changes in reflection due to very small oxide thickness 

increases were also neglected.) In several cases the sensitivity of the method 

was fl monolayer of surface atoms. Total film thicknesses were accurately measured by step height changes in stylus traces of the fildsubstrate surfaces 

using a Tencor Alpha-Step 200 stylus profiloaeter with a nominal 2 pm radius 

diamond stylus. Decreases in both the optical density and total film thickness 

were attributed to metal film removal. A decrease in optical density but an 

increase in total film thickness was attributed to film expansion due to formation of a nonvolatile oxide of lower density than the original film. To obtain 

optimum resolution of step heights from stylus measurements, fine lines were 

scraped to the bare substrates, except for the niobium and nickel films, which 

could not be removed without damaging the substrate. The arithmetical average 

of the step plateaus (average absolute distance from a center line level for up 

to 2000 data points) gave indicated surface roughness of the order of 0.5 nm for 

the smoothest surfaces. Although some smoothing probably occurred from the stylus riding over microscopic defects and compressing the surface somewhat, 

changes of fl nm in relative film heights between exposed and unexposed surfaces 

were measurable when averaging was used on both levels of sharp steps. 

Topographic measurements were also made on a number of surfaces with an optical 

heterodyne instrument using recent techniques. These measurements provided an 

optical determination of surface roughnesses for comparison to stylus results as 

well as several correlation functions from the same measurement. 

Estimates of metallic film thicknesses in exposed and unexposed areas were 

calculated from the optical density measurements using equations for transmission and refle~tion.~ Published optical constants for metals3t4 were used 

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forLcalcQlations except for noted cases, such as for niobium, where a value of 

28 nm thickness per unit optical density change had been accurately determined 

for our sputtered niobium films. Air, metal, and fused silica substrate media 

were assumed. This neglected the effect of very thin transparent oxides on 

absorption and reflection for normal incidence on these samples. Neglecting 

such effects on metals forming thick oxides, such as for silver, would introduce 

unacceptable errors, however. 

Osmium and iridium films were purchased, the platinum overcoatings on the 

osmium were electron-beam evaporated, and most of the other films were sputtered. 

Sensitive step height measurements on bulk samples such as Os, C, and optical plastics were obtained with the diamond stylus. Most of the bulk sample 

surfaces had 10 nm arithmetic roughness and were not very flat. In most cases 

this lack of precision for bulk sample measurements was unimportant since large 

effects were produced. Step height determination was enhanced on most bulk samples by sputtering a protective niobium film -220 nm thick in a resolution pattern of small rectangles spaced over the whole sample surface before mounting 

for flight. Although both knife-edges and film masks were used, much sharper 

steps were obtained with films than with mask knife-edges, because intimate contact between substrate and film mask was assured, and effects due to shadowing 

of areas of the substrate next to taller mask structures did not occur. 

Accurate niobium mask thicknesses were obtained in the unexposed areas and subtracted from the step heights in the exposed areas. Typically, large negative 

steps were produced on combustible materials by oxygen exposure: however, in the 

case of silver, a large expansion occurred. 

The polycarbonate (CR-39) and polymenthyl methacryalte (Lucite) bulk plastics were cut into disks and cleaned with ethanol. The other bulk materials in 

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', 

Table 11'1, except for the diamond sample, were ground and polished reasonably 

flat with progressively finer abrasives (aluminum oxide or diamond) to -10 nm 

arithmetic roughness and cleaned with strong solvents followed with alcohol. 

Results 

Rapid etching of carbon and carbonaceous surfaces was observed with the 

polycarbonate CR-39 showing the largest etch (over 22 pm) of any substrate yet 

flown and measured. 

the former being heavily etched and the latter forming a very thick layer of 

oxide. The first measurable (but small) effects on iridium. aluminum, nickel, 

tungsten and niobium thin films were reported. Except for iridium the change in 

optical properties was ascribed to a small increase in the thickness of the 

metal oxide layer which covers these metals after they have been exposed to the 

normal atmosphere. Iridium appears to have been very lightly etched and this 

effect will be studied further as other exposures become available. Full 

details of the observations are available in the Applied Optics paper and are 

not repeated here. 

Of the metals, only osmium and silver showed large effects, 

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+ +* ' I.. 

1. 

. . , i. 

1. J. C. Gregory and P. N. Peters, "The Production of Glow Precursors by 

Oxidative Erosion of Spacecraft Surfaces," NASA CP-2391, Second Workshop on 

Spacecraft Glow (1985), pp. 174-179. 

2. G. E. Sonmargren, "Optical Heterodyne Profilometry," Appl. Opt. 20, 610 

(1981). 

3. G. Hass and L. Hadley, "Optical Properties of Metals," in AIP Handbook, D. 

E. Gray, Ed. (McGraw-Hill, New York, 1963). Sec. 69. 

4. D. W. Lynch and W. R. Hunter, "Comments on the Optical Constants of Metals 

and an Introduction to the Data for Several Metals," in Handbook of Optical 

Constants of Solids, E. D. Palik, Ed. (Academic, Orlando, 1985). 

5. P. N. Peters, private communication. 

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