Anodized coatings have been used as optical and thermal surfaces in spacecraft. Particulate contamination from cracked coatings is a concern for many applications. The major cause for the cracking is the difference in the coefficient of thermal expansion between the oxide coatings and the aluminum substrate. The loss of water when the coating is exposed to a vacuum also could induce cracking of the coating. Hot-water sealing was identified as the major cause for the cracking of the coatings because of the large temperature change when the parts were immersed in boiling water and the water was absorbed in the coating. when the hot-water sealing process was eliminated, the cracking resistance of the anodized coatings was greatly improved. Also, it was found that dyed black coatings were more susceptible than clear coatings to cracking during thermo-vacuum cyclings

He, Charles C.

Heslin, Thomas M.

English

NASA Technical Reports Server

NASA Technical Memorandum 104622
/,4, --- -_#
/__
Preventing Cracking of Anodized
Coatings
Charles C. He and Thomas M. Heslin
August 1995
(NASA-TM-104622) PREVENTING
CRACKING OF ANODIZED COATINGS
(NASA. Goddard Space Flight Center)
13 p
N95-31690
Unclas
G3/31 0059403
https://ntrs.nasa.gov/search.jsp?R=19950025269 2020-06-16T06:35:10+00:00Z

NASA Technical Memorandum 104622
Preventing Cracking of Anodized
Coatings
Charles C. He
Unisys Corporation
4700 Boston Way
Greenbelt, Maryland
Thomas M. Heslin
Goddard Space Flight Center
Greenbelt, Maryland
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland
1995
This publication is available from the NASA Center for AeroSpace Information,
800 Elkridge Landing Road, Linthicum Heights, MD 21090-2934, (301) 621-0390.
ABSTRACT
Anodized coatings have been used as optical and thermal surfaces in
spacecraft. Particulate contamination from cracked coatings is a concern for
many applications. The major cause for the cracking is the difference in the
coefficient of thermal expansion between the oxide coatings and the aluminum
substrate. The loss of water when the coating is exposed to a vacuum also
could induce cracking of hte coating. Hot-water sealing was identified as the
major cause for the cracking of the coatings because of the large temperature
change when the parts were immersed in boiling water and the water was
absorbed in the coating. When the hot-water sealing process was eliminated,
the cracking resistance of the anodized coatings was greatly improved. Also it
was found that dyed black coatings were more susceptible than clear coatings to
cracking during thermovacuum cycling.
iii

Table of Contents
I. INTRODUCTION ..................................................................................................................................... 1
II. BACKGROUND ...................................................................................................................................... 1
III. EXPERIMENT ....................................................................................................................................... 3
IV. RESULTS ................................................................................................................................................ 4
V. CONCLUSION ......................................................................................................................................... 5
VI. ACKNOWLEDGMENT ........................................................................................................................ 5
VII. REFERENCE ........................................................................................................................................ 5
_,4G-,_ BLAt4K NUT FN...M_I)
P_,'3E ! J ......
_,[;' t-,:__. E,_ ._.!,pr\

I. INTRODUCTION
Anodizing is a surface-treating process that creates a hard, corrosion- and abrasion-
resistant oxide coating on aluminum alloys. These properties, combined with superior
adhesion and dielectric strength of the anodized coating, allow aluminum to be
substituted for costlier materials in many applications. Clear anodic oxide coatings
have high emissivity and low absorptivity that result in efficient radiative heat transfer,
and the coating is stable in the space environment [1]. Black anodized coatings with
high emissivity are used in spacecraft to provide reference optical signals for the
calibration of instruments. Particulate contamination, however, from crazed coatings is
a concern in many applications. In addressing this concern for a NASA project,
information concerning the cracking of aluminum anodized coating was developed that
may be of general interest.
II. BACKGROUND
In the anodizing process, an electric current is introduced into an electrolytic bath in
which the aluminum part has been immersed. The current density and the operating
temperature of the bath for conventional anodizing ranges from 90 to 120 A/m-(8 - 11
A/ft -) and from 20 ° to 25 °C (70°-75 °F), respectively [2]. Increasing current density
and lowering operating temperatures usually result in a more dense coating. The
variables involved in the process include composition and surface finishing of the
aluminum alloy, current density, voltage, temperature of the bath, chemical composition
and concentration of the electrolyte, duration of the anodizing process, etc. During the
anodizing, aluminum reacts with the electrolyte and a layer of aluminum oxide is formed
on the surface of the aluminum specimen. This coating is highly porous and is subject
to attack from the environment and corrosive elements. Therefore, anodized aluminum
is normally processed with a sealing process as a final step after anodizing. A hot-
water sealing process is one of the widely used methods [1]. The specimen is
immersed in deionized water at or near the boiling point (100 °C or 212 °F). In this
process, the aluminum oxide reacts with water to form aluminum hydroxide, which fills
pores in the coating and seals the surface.
The coefficient of thermal expansion of aluminum is about five times greater than that
of aluminum oxide [3-5]. As a result, anodized coatings tend to craze or crack mainly
because of the thermal stresses. Cracking of the coating may occur during fabrication
and/or during operation. Any environmental change that causes differential expansion
or shrinking between the coating and the substrate will introduce stresses into the
coating. If the stresses are large enough, cracking will occur. The cyclic temperature
changes in vacuum in space applications are the potential factors that cause crazing
problems. The cracking of the coatings changes the optical and thermal properties of
the coating and may cause particulate contamination. Therefore, the crazing of
anodized coatings should be avoided in space applications.
Alwitt and his colleagues investigated the thermal stresses in the coating during the
anodizing process and under various environments [4,5]. It was found that after
anodizing, the stress in the coating was always compressive and independent of
coating thickness. The stress was about 16 MPa at 15 V of anodizing voltage and
increased with increasing voltage. When the specimen was immersed in boiling water
for sealing, the stress became tensile immediately and was large, arising from the
greater thermal expansion of the metal than that of the oxide (this could be the moment
that crazing of the coating occurs). This stress, however, was greatly reduced as the
seal reaction proceeded. At the end of the sealing, the stress in the coating became
compressive again. After the coating was dried at ambient temperature, the stress was
always compressive. When the coating was exposed in dry air, the stress changed to
tensile. The tensile stress in the coating increased with decreasing humidity in the
environment. The coating lost weight in dry air, and the weight loss increased with
decreasing humidity. It has been proposed that the anodic oxide consists of an array of
alumina crystallites [6] with typical dimensions of several nanometers [7].
Interpretations of both infrared (IR) [6] and nuclear magnetic resonance (NMR) [7]
spectra lead to the conclusion that the sealed oxide contains both molecular water and
water present as hydroxyl groups. Exposed alumina surfaces are presumed to be
hydroxylated during sealing, with physisorbed molecular water residing in the
intercrystallite space. The weight loss of 1.1% - 3.6% of coating weight can be
removed by heating at moderate temperature or under vacuum at room temperature
[4,7]. It was also found that the exchange of water with the atmosphere was a
reversible process, and the rate of water loss is much slower than the subsequent rate
of water recovery [4].
If the crazing does not occur after anodizing and sealing processes, heating the coating
can induce the cracking. The temperature at which the cracks first appear on the
anodized coatings, To,was evaluated by Alwitt and McClung [5]. It was found that there
was a general tendency for 25 pm thick coatings to crack at temperatures about 20° to
25 °C lower than for 12.5 pm coatings. This means that thick coatings are more
vulnerable to cracking than thin coatings [3,5]. Measurement of tensile failure strain of
the oxide scale on the surface of iron also confirmed that thin oxide scale failed at
higher strains than thick oxide scale [8]. This means that thin surface layers are more
crack resistant than thick surface layers. The reason for this thickness-dependence of
crazing is associated with the critical flaw size in the surface layer [8].
In summary, the factors that affect cracking of the anodized coatings are as follows:
• Temperature: Temperature is the most important factor for cracking of anodized
coating• The tensile stress in the coating is proportional to the temperature change
[6]. Both hot-water sealing and thermal cycling may cause the formation and
propagation of cracks.
2. Humidity of environment.. Low humidity causes loss of water from the coating, which
generates large stresses in the coating. Tensile stress and the weight loss of
..
coatings increase with decreasing humidity [4]. For example, the stress in a typical
sealed coating was 20 MPa (3 ksi) compressive in ambient atmosphere at 25 °C.
After 2 days in vacuum, the stress was 110 MPa in tension (16 kpsi) [4].
Thickness of the coatings: Thicker coatings tend to crack at lower temperatures
[3,5].
Sealing techniques: Hot-water sealing can cause the crazing of anodized coatings.
The temperature change during hot-water sealing introduces thermal stresses. The
water absorbed in the coating during the sealing process escapes into vacuum or
low-humidity environments and causes crazing. Some sealing methods, such as
low-nickel seal and no-nckel seal, can result in higher crazing resistance of the
coating than hot-water sealing method [5].
The purpose of this study is to investigate the feasibility of eliminating the cracking of
anodized coatings by eliminating the hot-water sealing process. The purpose of hot-
water sealing is to seal the porous coatings to protect the substrate from attack by
environment. In the space environment, however, there are little moisture and other
chemicals that attack the coatings. Corrosion is not a major concern for space
applications of the anodized coatings.
III. EXPERIMENT
Four anodized plates (76 x 76 x 1.5 mm, or 3 x 3 x 1/16") of Aluminum 1100 with
different coating thicknesses were obtained from the Goddard Space Flight Center
Plating Shop (specimens B to E, Table 1). The anodizing was performed in a 15 vol%
H2SO4 solution at 68°F. Those specimens were not hot water sealed. The
characteristics of the specimens and the anodized conditions are listed in Table 1.
Table 1. The Characteristics of the Anodized Specimens
Specimen Coating Thickness
pm (mil)
A black, sealed 31 (1.5)
B clear, unsealed 18 (0.7)
C clear, unsealed 25 (1.0)
D clear, unsealed 61 (2.4)
E black, unsealed 31 (1.5)
Small specimens (about 10 x 10 mm, or 3/8" x 3/8") were cut from the received plates.
The specimens were cleaned with acetone and dried in air. The specimens were
thermocycled in a vacuum furnace. The vacuum was maintained below 10 .8 torr
through the test. The temperature was cycled between 25 °C and 115 °C for three
3
cycles. The dwell time is one hour at 115 °C and overnight at 25 °C. The heating and
cooling rates are less than 2 °C/min. The test took a total of about 100 hours during
which time the specimens were maintained in vacuum. The surfaces of the coatings
were examined using optical microscopy and scanning electron microscopy (SEM)
before and after the thermocycling.
IV. RESULTS
Specimen A is a commonly used anodized black coating sealed in hot water. Figure 1
shows the surface of specimen A in the as-received condition. Randomly distributed
cracks are observed on the surface. Figure 2 shows the surface of specimen E, which
is an unsealed black coating of the same thickness. No cracks are visible. Specimens
B through D also do not have any cracks in the as-received condition (prior to
thermocycling), although those specimens have varying thickness of from 18 to 61 IJm.
This suggests that the hot-water sealing could be the major cause for the crazing
problem, as inferred from searching the literature.
After thermocycling in vacuum, the specimens were observed again. Figure 3 shows
the surface of specimen A. The fine cracks flaked particles from 13 to 50 pm in size.
This poses a serious contamination problem for space application. As discussed earlier
in this paper, the major causes for this crack enhancement include the thermal stress
induced by the temperature changes and the loss of water from the coating caused by
the vacuum environment.
For the unsealed clear coatings (specimens B, C, and D), no cracking occurs during the
combination of thermocycling and vacuum. Figure 4 shows the surface of specimen B
after thermocycling.
For unsealed black coating (specimen E), there are no microcracks observed after
thermocycling, as shown in Figure 5. However, several large cracks are observed
(Figure 6). Five cracks are observed on a 16 x 22 mm (5/8" x 7/8") area. This is still
much better than the sealed black coating (Figure l a) where small-particulate
contamination generated by small sizes of cracks presents a potential danger. The
long cracks in the unsealed black coating are unlikely to shed particles and to cause
contamination problems.
The above observation suggests that the dyeing process changes the structure and
properties of the coating somewhat. Since the black dye is most likely to penetrate into
the pores in the coating, the dyeing process may increase the apparent density of the
coating. This reduces the effectiveness of stress relaxation by pores and increases the
possibility of cracking.
Although the cracking resistance of anodized coatings can be improved by elimination
of the sealing process, it may cause other problems. Dye bleeding could occur if the
4
coating needs to be machined using coolant. Individual circumstances should be taken
into account when eliminating the sealing process.
V. CONCLUSION
Hot-water sealing process was identified as a major cause for the cracking of anodized
coatings. Elimination of the hot-water sealing greatly improves the cracking resistance
of the coatings. Dyed black coatings are more susceptible than clear coatings to the
cracking during thermocycling in vacuum.
VI. ACKNOWLEDGMENT
This work was done under the auspices of the Earth Observing System (EOS)
Moderate Resolution Imaging Spectro-Radiometer (MODIS) in the process of resolving
a contamination concern.
VII. REFERENCE
° Dever, J. A., E. Rodriguez, W. S. Siemp, and J. E. Stoyack, 1991, "Evaluation of
thermal control coatings for use on solar dynamic radiators in low Earth orbitk,"
NASA TM 104335.
2. Abbott, J. S., 1994, "Hardcoat Anodizing: Low-Cost Coating for Aluminum,"
Advanced Materials & Processes, No. 9, p. 29.
3. Basaly, M., 1994, "Sealing of Anodized Aluminum," presentation from Novamax
Technologies (US) Inc..
4. AIwitt, R. S., J. Xu, and R. C. McClung, 1993, "Stresses in Sulfuric Acid Anodized
Coatings on Aluminum," J. Electrochem. Soc., 140(5), p. 1241.
5. Alwitt R. S. and R. C. McClung, 1993, "Cracking of Anodized Coatings on
Aluminum," Plating and Surface Finishing, No 8, p. 48.
6. O'Sullivan, J. P., J. A. Hockey, and G.C. Wood, 1969, "Infra-Red Spectroscopic
Study of Anodic Alumina Films," Trans. Faraday Soc., 65, p. 535.
7. Baker B. R. and R. M. Pearson, 1972, "Mechanical Properties of Metal Oxide Films,"
J. Electrochem. Soc., 116, p. 1232.
8. Nagl M. M. and W. T. Evans, 1993, "Review: The Mechanical Failure of Oxide
Scales under Tensile or Compressive Load," J. Mater. ScL, 28, p. 6247.
5
Figure 1. SEM micrograph of Sample A (31 IJm or 1.5 mil
black anodized coating with hot-water sealing) in as-
received condition. The coating cracked severely.
Figure 2. Sample E (31 pm or 1.5 mil black anodized
coating without hot-water sealing) in as received condition.
No cracks are present.
6
Figure 3. Optical micrograph of Sample A after
thermovacuum cycling tests. Magnification is
the same as in Figure 1 (200x).
Figure 4. Sample B (18 pm or 0.7 mil clear coating
without hot-water sealing) after thermovacuum cycling
test. No cracks are observed.
7
Figure 5. Sample E (31 #m or 1.5 mil black coating without
hot-water sealing) after thermovacuum cycling test. There
are no small cracks as observed in Figures 1 and 3.
Figure 6. Sample E after themocycling. Several large
cracks are present. These large cracks should not cause
any particulate contamination.
8
REPORT DOCUMENTATION PAGE For'm"Approved
OMB No. 0704-0188
Public reporting burden for this collection of informalion is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,
gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspecl of this
collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for tnformation Operations and Reports, 1215 Jefferson
Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503,
1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE
August 1995
4. TITLE AND SUBTITLE
Preventing Cracking of Anodized Coatings
6. AUTHOR(S)
Charles C. He and Thomas M. Heslin
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS (ES)
Goddard Space Flight Center
Greenbelt, Maryland
9. SPONSORING I MONITORING AGENCY NAME(S) AND ADDRESS (ES)
National Aeronautics and Space Administration
Washington, DC 20546-0001
3. REPORT TYPE AND DATES COVEREDTechnical Memorandum
5. FUNDING NUMBERS
Code 313
8. PEFORMING ORGANIZATION
REPORT NUMBER
95B00106
10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
NASA TM- 104622
11. _UPPJ.I_MENTA_Y NOTES.
He: Unlsys tgorporatlon, 4700 Boston Way, Greenbelt, Maryland, (Currently stationed with Govern-
ment Systems Group, NASA/Goddard Space Flight Center, Greenbelt, Maryland); Heslin: NASA/
Godd, ard Space Flight Center, Greenbelt, Mar_,land
12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE
Unclassified - Unlimited
Subject Category 31
Availability: NASA CASI (301) 621-0390.
13. ABSTRACT (Maximum 200 words)
Anodized coatings have been used as optical and thermal surfaces in spacecraft. Particulate contami-
nation from cracked coatings is a concern for many applications. The major cause for the cracking is
the difference in the coefficient of thermal expansion between the oxide coatings and the aluminum
substrate. The loss of water when the coating is exposed to a vacuum also could induce cracking of the
coating. Hot-water sealing was identified as the major cause for the cracking of the coatings because
of the large temperature change when the parts were immersed in boiling water and the water was
absorbed in the coating. When the hot-water sealing process was eliminated, the cracking resistance of
the anodized coatings was greatly improved. Also, it was found that dyed black coatings were more
susceptible than clear coatings to cracking during thermo-vacuum cyclings.
14. SUBJECT TERMS
Anodized coating, cracking, crazing, thermal cycling, space application
17. SECURITY CLASSIFICATION 18. SECURITY CLASSIRCATION
OF REPORT OF THIS PAGE
Unclassified Unclassified
_ISN 7540-01-280-5500
19. SECURITY CLASSIFICATION
OF ABSTRACT
Unclassified
15. NUMBER OF PAGES
18
16. PRICE CODE
20. LIMITATION OF ABSTRACT
UL
:::;tanaara I-orm z_ (Hev. z-uu)
Prescribed by ANSI Std Z3918



Preventing Cracking of Anodized Coatings

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server