Carbon arc electrical discharges struck across the surfaces of metals such as Nb-1 percent Zr, alter the morphology to produce a high thermal emittance surface. Metal from the surface and carbon from the arc electrode vaporize during arcing, and then condense on the metal surface to produce a microscopically rough surface having a high thermal emittance. Quantitative spectral reflectance measurements from 0.33 to 15 microns were made on metal surfaces which were carbon arc treated in an inert gas environment. The resulting spectral reflectance data were then used to calculate thermal emittance as a function of temperature for various methods of arc treatment. The results of arc treatment on various metals are presented for both ac and dc arcs. Surface characterization data, including thermal emittance as a function of temperature, scanning electron microscopy, and atomic oxygen durability, are also presented. The ac arc texturing was found to increase the thermal emittance at 800 K from 0.05 to 0.70

Banks, Bruce A.

Barry, Jennifer

Behrend, Tracy

Difilippo, Frank

Hotes, Deborah

Kussmaul, Michael

Mirtich, Michael J.

Rutledge, Sharon K.

Stidham, Curtis

Stueber, Thomas

English

NASA Technical Reports Server

NASA Technical Memorandum 100894
Arc-Textured Metal Surfaces for High
Thermal Emittance Space Radiators
CMAL
SIRS ACES FOE ElGt lEEfc f iAL E E I l S A f c C E SPftCE
BADIA1CES (HIS A) 11 f CSCI 11F
Unclas
G3/26 01^17351
Bruce A. Banks, Sharon K. Rutledge,
and Michael J. Mirtich
Lewis Research Center
Cleveland, Ohio
Tracy Behrend
Wittenberg University
Springfield, Ohio
Deborah Hotes, Michael Kussmaul, Jennifer Barry,
Curtis Stidham, and Thomas Stueber
Cleveland State University
Cleveland, Ohio
Frank DiFilippo
Case Western Reserve University
Cleveland, Ohio
Prepared for the
International Conference on Metallurgical Coatings
sponsored by the American Vacuum Society
San Diego, California, April 11-15, 1988
NASA
https://ntrs.nasa.gov/search.jsp?R=19880015370 2020-03-20T07:09:30+00:00Z
ARC-TEXTURED METAL SURFACES FOR HIGH THERMAL EMITTANCE SPACE RADIATORS
Bruce A. Banks, Sharon K. Rutledge, and Michael J. Mirtich
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
Tracy Behrend*
Wittenberg University
Springfield, Ohio 45501
Deborah Hotes, Michael Kussmaul, Jennifer Barry,
Curtis Stldham, and Thomas Stueber
Cleveland State University
Cleveland, Ohio 44115
Frank D1F1lippo*
Case Western Reserve University
Cleveland, Ohio 44106
SUMMARY
Carbon arc electrical discharges struck across the surfaces of metals
such as Nb-1% Zr, alter the morphology to produce a high thermal emlttance sur-
face. Metal from the surface and carbon from the arc electrode vaporize dur-
ing arcing, and then condense on the metal surface to produce a microscopically
rough surface having a high thermal emlttance.
Quantitative spectral reflectance measurements from 0.33 to 15 pm were
made on metal surfaces which were carbon arc treated in an Inert gas environ-
ment. The resulting spectral reflectance data were then used to calculate
thermal emlttance as a function of temperature for various methods of arc
treatment.
The results of arc treatment on various metals are presented for both ac
and dc arcs. Surface characterization data, including thermal emlttance as a
function of temperature, scanning electron microscopy, and atomic oxygen dura-
bility, are also presented. Ac arc texturing was found to increase the ther-
mal emlttance at 800 K from 0.05 to 0.70.
INTRODUCTION
Advanced space power systems w i l l require high thermal emlttance radia-
tors which operate at high temperatures to efficiently reject waste heat with
a minimum radiator area and mass. Space nuclear and solar dynamic power sys-
tems may require such high temperature radiators for overall system efficiency
and mass minimization. Although the radiator operating temperatures, configu-
rations, orbital environment, and lifetime w i l l depend upon the type of power
*Summer Intern at NASA Lewis Research Center.
system and the specific mission, there are two radiator characteristics which
need to be addressed for most low earth orbital (LEO) missions. These are
high thermal emlttance and environmental durability.
Atomic oxygen in the LEO environment appears to be the durability issue
of greatest concern. However, thermal cycling, UV radiation, mlcrometeroid/
debris, and space system effluents may be significant contributors to degrada-
tion depending upon the radiator surface material used to obtain high thermal
emittance.
The SP-100 space nuclear power system shown in figure 1 may.be required
to operate at temperatures as low as 500 K and as high as 950 K, depending
upon the specific mission. Techniques previously used to produce high thermal
emittance surfaces for lower temperature radiators include the use of anodized
aluminum, low solar absorptance and high thermal emlttance paints, and metal-
lized second surface polymer films. Such surfaces may not have the high tem-
perature, thermal cycling, and atomic oxygen durability required for space
nuclear applications. The emlttance properties are dominated by surface chem-
istry as opposed to surface morphology properties. For reasons of reliability
and durability, it would be desirable to obtain high thermal emlttance by sur-
face alteration of the radiator bulk material rather than the application of
paints or coatings that may spall or otherwise degrade in thermal emlttance
(ref. 1). This paper discusses a technique which alters the surface
morphology of potential radiator materials to develop high thermal emlttance.
APPARATUS AND PROCEDURE
Candidate high temperature radiator materials whose surfaces were altered
by means of arc texturing Include, [niobium with 1 percent ziconium (Nb-1%
Zr), titanium with 6 percent aluminum and 4 percent vanadium <T1-6% Al-4% V)],
titanium, copper, and type 304 stainless steel. Arc-texturing was performed
by manually moving the site of the discharge over the entire surface of the
test coupon. A carbon arc discharge was obtained between a 6.35 mm diameter
water-cooled, spectroscopic grade carbon rod and a test coupon of sample radia-
tor material. The electric arc was operated in a nitrogen or argon environ-
ment. The dc or ac arc current was varied between 11 and 16 A. By manually
moving the carbon arc electrode wand, as illustrated in figure 2, the entire
surface of the test radiator sample could be covered with arc sites. Because
of significant visual darkening of the arc-treated areas, it was easy during
sample preparation to discern which areas had been adequately arc-textured,
and which areas needed to be treated further.
The thermal emittance of the arc-textured surfaces was evaluated by means
of a Perkin-Elmer X - 9 spectrophotometer and a Hohllraum reflectometer. The
Perkin-Elmer spectrophotometer was used to obtain spectral emlttance data over
wavelengths ranging from 0.4 to 2.5 pm. The Hohlraum reflectometer was used
to obtain spectral emlttance data over wavelengths from 1.5 to 15 urn. The
data from both instruments was combined to make a smooth transition from UV
through the IR to allow calculation of the integrated thermal emittance from
room temperature up to 3000 K by Integration over the black body spectra for
each radiator temperature.
Some radiator materials, such as Nb-1% Zr, readily oxidize at high temper-
atures 1n an oxygen environment. Concepts to Inhibit oxidation of this mate-
rial by atomic oxygen through the application of sputter-deposited coatings of
MgF2 were evaluated. These thin film coatings were applied by ion beam sput-
ter deposition from targets composed of these materials. Techniques used for
1on beam sputter deposition are described in references 2 to 4.
Simulation of the LEO atomic oxygen..environment was provided by radio
frequency (13.56 MHz) plasma ashers operated on air plasma at pressures of
30 to 100 pm. Because the radiators w i l l be operating at high temperatures,
it Is necessary to simulate this, as well as atomic oxygen, 1n order to fully
assess LEO durability. A plasma asher was modified to contain both a room tem-
perature and heated substrate sample holder, as Illustrated 1n figure 3, so
that both sample holders would be exposed to identical fluences.
RESULTS AND DISCUSSION
Total reflectance data from the Perkin-Elmer X - 9 spectrophotometer and
the Hohlraum reflectometer for Nb-1% Zr are shown 1n figure 4. The data reveal
a significant reduction in both visible and IR reflectance of the arc-textured
surface, which has a very dark, or black-appearing surface. The cause of the
reduced reflectance (and therefore, Increased thermal emlttance) of the sur-
faces Is the rough morphology produced on a microscopic level, as Illustrated
1n the scanning electron microscope photograph in figure 5. It is believed
that the electric arc causes melting and vaporization.of the radiator surface,
as well as vaporization of the carbon arc electrode. Movement of the site of
the arc allows condensation of these materials over the sites of microscopic
arc craters. The net effect produces a highly irregular absorbing surface pre-
dominantly composed of bulk radiator material with a portion of the condensed
electrode carbon intermixed.
The thermal emittances as a function of radiator temperature for Nb-1% Zr,
Cu, T1, T1-6% Al-4% V, and type 304 S.S. are shown in figure 6 for untreated
and arc-textured surfaces. The untreated surfaces in this figure are the orig-
inal surface finishes from commercial sheet metal suppliers. An ac arc with a
current of 14 to 15 A was used to arc-texture the various materials. Figure 6
Illustrates results for temperatures up to either the melting points of the
various materials or 3000 K.
Obviously, the amperage of the arc plays a significant role in the degree
of alteration of the surface morphology of the radiator materials tested.
Figure 7 shows the dependence of the thermal emittance of radiator materials
at 800 K on ac arc current, based on data obtained from ac arcs of 11 to 16 A.
A comparlsion of thermal emlttance dependence on ac, dc, and polarity varia-
bles is shown 1n figure 8 for Nb-1% Zr radiator materials at 800 K. As can be
seen, ac arc-texturing produces the highest thermal emlttance. Sixty cycle
arc texturing may not represent the optical frequency. Efforts are underway
to Investigate frequency optimization for thermal emlttance of ac arc-textured
surfaces.
The results of atomic oxygen durability evaluation of arc-textured Nb-1%
Zr radiator surfaces is shown in figure 9. The arc-textured Nb-1% Zr surface
was Ion beam sputter deposited with 950 A of MgF£ in an attempt to provide a
barrier to oxidation by atomic oxygen in the asher environment. As can be
seen, the MgF£ thin film coating caused very little change In thermal emlt-
tance. However, there was a large accompanying reduction 1n emlttance when
the sample was heated to 900 K during exposure to atomic oxygen. A slight
reduction in thermal emlttance upon atomic oxygen exposure also occurs when
the radiator samples are oxidized in the plasma asher at room temperature. .;
This may be due in part to oxidation of the carbon that resulted from the con-
densation of the radiator surface metal vapor and the carbon vapor from the
electrode during arc-texturing. The sample was exposed to the RF plasma asher ^
environment for 24 hours. This represents an equivalent atomic oxygen damage
fluence (as measured by Kapton® polylmlde oxidation) of 6xl02^ atoms/cm2.
MgF2 is known to be atomic oxygen durable. However, at 900 K, It is not
clear whether or not there may be pathways through defects in the coating to
allow oxidation of the underlying Nb-1% Zr. In addition, Nb-1% Zr is a
well-known bulk absorber of a wide range of sizes of atomic species. Thus,
the integrity of the MgF2 thin film might be in doubt at these temperatures.
If no oxygen barrier coating 1s used In a LEO environment, Nb-1% Zr may readi-
ly transport atomic oxygen directly Into the liquid metal heat pipe working
fluid and cause it to be oxidized. The Nb-1% Zr may also become embrittled by
atomic oxygen attack.
Although arc-texturing has been shown to produce high thermal emlttance
surfaces by manually manipulating the arc site across ample radiator material,
the technique has potential for being performed automatically. The electric
arc can be rastered across the surface of flat sheets, tubes or tubes with
fins by means of an electrically driven arc wand manipulator and a moving bed
to support a radiator tub or fin.
CONCLUSIONS
Arc-texturing has been shown to increase the thermal emlttance of Nb-1%
Zr, Cu, T1, T1-6% Al-4% V, and type 304 S.S. radiator surfaces as measured by
the reflectance properties of these surfaces and subsequent integration over
the black body spectra to calculate thermal emittances at radiator temperatures
from room temperature to either 3000 K or the melting point of each material.
The highest thermal emittances for NB-1% Zr and Cu radiator surfaces were
obtained with ac arcs of 14 to 16 A. However, the optimum arc current and arc
frequency have not yet been established. The arc-texturing surface modifica-
tion technique has potential for being applied automatically to large area
radiator surfaces. The application of 950 A of MgF£ to arc-textured Nb-1% Zr
causes a slight reduction in thermal emi.ttance. However, It does not fully
provide a barrier for thermal emlttance durability in a high temperature
(900 K) atomic oxygen environment. Arc-texturing appears to be a viable tech-
nique to produce high thermal emlttance radiator surfaces with properties domi-
nated by surface microscopic morphology as opposed to chemically controlled
optical properties.
REFERENCES
1. S.K. Rutledge, B.A. Banks, M.J. Mirtich, R. Lebed, J. Brady, D. Hotes,
and M. Kussmaul, "High Temperature Radiator Materials for Applications in
Low Earth Orbital Environment," NASA TM-100190, 1987.
2. B.A. Banks, "Ion Beam Applications Research - A 1981 Summary of Lewis
Research Center Programs," AIAA Paper 81-0669, Jan. 1981. (NASA
TM-81721.)
3. B.A. Banks, M.J. Mlrtich, S.K. Rutledge, D.M. Swec, and H.K. Nahra, "Ion
Beam Sputter-Deposited Thin Film Coatings for Protection of Spacecraft
Polymers in Low Earth Orbit," AIAA Paper 85-0420, Jan. 1985. (NASA
TM-87051.)
4. B.A. Banks, M.J. Mlrtich, S.K. Rutledge, and D.M. Swec, "Sputtered Coat-
ings for Protection of Spacecraft Polymers," NASA TM-83706, 1983.
ELECTRICAL AND
THERMAL
INSULATION^
\
\
H2° ."1 H20 OUT
TO
VACUUM
PUMP
1 ATMOSPHERE
Ar OR N2
ENVIRONMENT
FIGURE 1. - SP-100 SPACE NUCLEAR POWER SYSTEM. FIGURE 2. - TECHNIQUE FOR ARC-TEXTURING SURFACES.
ORIGINAL PAGE
QUALITY
rSHIELDING SCREEN
/-THERMAL
TESTING UNIT
-MATER COOLING
TUBE
SAMPLE HOLDER WITH
HEATER AND THERMOCOUPLE
^AMBIENT TEMPERATURE SAMPLE
/ HOLDER WITH THERMOCOUPLE
LRF PLASMA
/ CHAMBER
^ PLASMA ASHER
UNIT
FIGURE 3. - HIGH TEMPERATURE RF PLASMA ASHER.
1.0 I—
.8
I
.1)
.2
UNTREATED
——— ARC-TEXTURED
6 9
WAVELENGTH, p
1? IS
FIGURE M. - TOTAL REFLECTANCE VERSUS WAVELENGTH FOR UN-
TREATED AND ARC-TEXTURED Nb-1Z r.
DSIG»TAD PAGS IS
POOR QUALITY
(A) UNTREATED.
(B) ARC-TEXTURED.
FIGURE 5. - SCANNING ELECTRON MICROSCOPE PHOTOGRAPHS OF UNTREATED AND ARC-
TEXTURED Nb-1Zr.
1.0
.8
.6
.14
.2
ARC-TEXTURED
UNTREATED
. r
500 900 1500
(A) Nb-1Zr.
2100 2700
1.0
.8
.6
.M
300 600 900
(B) Cu.
1200
I
1500 300 600 900 1200 1500
(D) TI-6AI-W.
1800 2100
1.0 I—
J L
300 600 900 1200 1500 1800 2100 300
TEMPERATURE, K
600 900 1200 1500 1800
(C) Ti . (E) TYPE 30<4 STAINLESS STEEL.
FIGURE 6. - THERMAL EMITTANCE VERSUS RADIATOR TEMPERATURE FOR VARIOUS UNTREATED AND ARC-TEXTURED SURFACES.
8
9 I—
10
O
1.0
.9
.8
.7
11 12 13 11 15
AC ARC CURRENT, AMPS
FIGURE 7. - DEPENDENCE OF THERMAL EMITTANCE AT 800 K ON AC
ARC-TEXTURING CURRENT.
16
ARC
CURRENT,
AMPS
11
n
rn D
1
AC ARC-TEXTURED DC+ ARC-TEXTURED DC- ARC-TEXTURED
FIGURE 8. - AC, DC, AND POLARITY DEPENDENCE OF THERMAL EMIT-
TANCE OF Nb-1Zr AT 800 K.
k
1.0
.9
.8
.7
.6
.5
.1
.3
.2
300
AC ARC-TEXTURED Nb-IZr
COATED WITH 950 A MgF2
AC ARC-TEXTURED Nb-1Zr
AC ARC-TEXTyRED Nb-1Zr COATED
WITH 950 A NgF2 THEN ASHED
21 MRS AT 900 K
UNTREATED Nb-1Zr
700 1100 1500 1900
TEMPERATURE, K
2300 2700
FIGURE 9. - Nb-1Zr THERMAL EMITTANCE VERSUS RADIATOR TEMPER-
ATURE FOR UNTREATED, ARC-TEXTURED AND 950 A MgF2 COATED,
AND ARC-TEXTURED THEN MgF2 COATED, THEN 900 K RF PLASMA
ASHER TREATED SURFACES.
NASA
National Aaronautics and
Space Administration
Report Documentation Page
1 Report No.
NASA TM-100894
2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle 5. Report Date
Arc-Textured Metal Surfaces for High Thermal
Emlttance Space Radiators 6. Performing Organization Code
7. Author(s)
Bruce A. Banks, Sharon K. Rutledge, Michael J. Mirtich,
Tracy Behrend, Deborah Hotes, Michael Kussmaul,
Jennifer Barry, Curtis Stidham, Thomas Stueber,
and Frank OiFilippo
8. Performing Organization Report No.
E-4135
',0. Work Unit No.
586-01-11
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
11. Contract or Grant No.
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the International Conference on Metal!urgial Coatings sponsored by the American Vacuum
Society, San Diego, California, April 11-15, 1988. Bruce A. Banks, Sharon K. Rutledge, and Michael
J. Mirtich, NASA Lewis Research Center; Tracy Behrend, Wittenberg University, Springfield, Ohio 45501
and Summer Student Intern at NASA Lewis Research Center; Deborah Hotes, Michael Kussmaul, Jennifer
Barry, Curtis Stidham, and Thomas Stueber, Cleveland State University Cleveland, Ohio 44115; Frank
OiFilippo, Case Western Reserve University, Cleveland, Ohio 44106 and Summer Student Intern at NASA
Lewis Research Center.
16. Abstract
Carbon arc electrical discharges struck across the surfaces of metals such as
Nb-1%, alter the morphology to produce a high thermal emlttance surface. Metal
from the surface and carbon from the arc electrode vaporize during arcing, and
then condense on the metal surface to produce a microscopically rough surface
having a high thermal emlttance. Quantitative spectral reflectance measurements
from 0.33 to 15 vim were made on metal surfaces which were carbon arc treated In
an inert gas environment. The resulting spectral reflectance data were then
used to calculate thermal emlttance as a function of temperature for various
methods of arc treatment. The results of arc treatment on various metals are
presented for both ac and dc arcs. Surface characterization data, Including
thermal emlttance as a function of temperature, scanning electron microscopy,
and atomic oxygen durability, are also presented. Ac arc texturing was found
to Increase the thermal emlttance at 800 K from 0.05 to 0.70.
17. Key Words (Suggested by Author(s))
Radiators
Emlttance
Space
19. Security Classif. (of this report)
Unclassified
16. Distribution Statement
Unclassif ied - Unlimited
Subject Category 26
20. Security Classif (of this page)
Unclassified
21. No of pages 22. Price'
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Arc-textured metal surfaces for high thermal emittance space radiators

Arc-textured metal surfaces for high thermal emittance space radiators

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