In this paper we use a photothermal imaging technique to characterize the damage caused to an imperfectly coated gold-coated Kapton sample exposed to successively increased fluences of atomic oxygen in a laboratory atomic source

Williams, A. W.

Wood, N. J.

Zakaria, A. B.

English

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ATOMIC OXYGEN DAMAGE CHARACTERIZATION BY PHOTOTHERMAL SCANNING
N94- 31051
_, N.J. Wood, and A.B. Zakaria
Department of Physics, University College of Swansea
University of Wales, Singleton Park, Swansea SA2 8PP U.K.
ABSTRACT
In this paper we use a photothermal imaging technique to characterize the damage caused to an
imperfectly coated gold-coated Kapton sample exposed to successively increased fluences of atomic
oxygen in a laboratory atomic source.
INTRODUCTION
One major problem associated with the flight of low Earth orbit (LEO) spacecraft is the damage
caused to various materials by bombardment with atomic oxygen (AO). AO will readily oxidize mater-
als with high erosion yield coefficients, such as polyimide Kapton, epoxy graphite, and Mylar. Materials
with low erosion yield coefficients such as aluminum, gold, and SiO2 may be used as barrier coatings to
prevent damage to the more vulnerable underlying materials mentioned above (ref. 1). However, manu-
facturing defects in the barrier coatings such as scratches and pin holes act as sites where the AO can
attack the substrate, and may cause undercutting of the protective layer (ref. 2).
Photothermal imaging of solids is a powerful technique which has been applied to numerous
problems involving the characterization of surface and subsurface, cracks inclusions, and delamination
in materials (ref. 3, 4). We have used this method to produce photothermal images of a gold-coated
Kapton sample at various stages of its exposure to AO in a laboratory AO source. The gold layer was
deliberately scratched prior to exposure of the sample, and the resultant damage around this site imaged
photothermally.
PHOTOTHERMAL IMAGING APPARATUS
A diagram of the apparatus is given in Figure 1.
In order to produce a photothermal image, a modulated and localized heat source is required
which is used to create thermal waves in the sample. In our system, we have used an argon ion laser as a
heat source whose beam is focused by means of a microscope objective onto the sample to be scanned.
The laser was operated on a wavelength of 488 nm in order to optimize the absorption in the gold (_63
percent at this wavelength) and thus generates "thermal waves" within the sample being illuminated.
The depth to which the thermal waves penetrate the sample/_s is given by:
547
https://ntrs.nasa.gov/search.jsp?R=19940026546 2020-06-16T12:37:55+00:00Z
where as is the thermal diffusivity of the material to be investigated, andfis the modulation frequency
of the laser light.
The value of/.ts and, hence, the penetration depth may be changed by altering the modulation fre-
quency.
Associated with the thermal waves are acoustic waves which are stresses generated in the sample
due to the thermal waves and which penetrate the whole sample and are detected by a piezoelectric
transducer coupled to the rear side of the sample. The voltage produced by the transducer, which
depends on the magnitude of these waves, is then amplified and detected by a lock-in amplifier
(Stanford SR530). The lock-in amplifier gives a reading of the magnitude of the photothermal signal
detected, together with its phase shift with respect to the light modulation. The sample together with its
piezodetector was mounted upon two orthogonal translation stages and raster scanned beneath the
focused laser beam of diameter =2.5 kun in steps of 3 _tm. The power density of the focused laser beam
was kept below the damage threshold of the sample.
The thermal waves traveling into the bulk of the sample are reflected and scattered by regions of
differing thermal properties within the sample. The photothermal signal, therefore, depends upon these
imperfections, and hence gives the imaging capability of this technique.
In order to produce a subsurface image, the X-Y scans consisting of 75 by 75 data points for
both the signal and its phase lag were recorded across a small area of the sample. The photothermal sig-
nal is sensitive to surface optical features which have differing optical absorption. However, the phase
lag is much less sensitive to surface features and is a better measure of subsurface features especially
delaminations (ref. 5). We have, therefore, concentrated upon the phase measurements in the results
given in this paper.
SAMPLE EXPOSURE
A 1- by 3-cm, 130-_trn thick Kapton sample was vacuum coated with 40 nm of gold and an area
of 1 by 1 cm selected. A strip of the gold about 35-l.tm wide and extending from one side of the sample
to the other was removed with a thin metal probe to expose the Kapton substrate beneath it. The sample
was then mounted in a laboratory AO apparatus similar to the design described by Neely (ref. 6) and
exposed to an AO flux of =1.5×1017 atoms/cm2/s at a temperature of 200 °C.
The sample was exposed to AO for four successive exposure times, with cumulative fluences of:
(i)
(ii)
(iiJ)
(iv)
8.3x 1020 atoms/cm 2
17x 102o atoms/cm 2
25x 102o atoms/cm 2
50× 102o atoms/cm 2.
The sample was scanned photothermally before and after each exposure.
548
RESULTS
Five photothermal scans of the area containing the exposed Kapton, each comprising 75 by 75
data points were obtained giving a photothermal image 220 by 220 _m in area. The photothermal signal
was detected by a piezotransducer on the Kapton surface remote from the gold. An imperfection in the
gold/Kapton interface, such as delamination, which would introduce an air layer between the gold and
Kapton substrate will scatter the thermal waves before they are detected, modifying the photothermal
phase, since this is in effect the delay in generation of the photothermal signal with respect to the modu-
lation frequency, hence making the scan sensitive to subsurface imperfections. The data were repre-
sented in false colors in the images of the scans, using Unimap 2000, Uniras A/S (ref. 7).
A modulation frequency of 3 kHz was chosen for all the scans which correspond to a thermal
wave probe depth of =3.2 l.tm into the Kapton. Photothermal images of the phase lag of the signal are
shown in Figure 2. These images show undercutting and delamination around the bare Kapton section of
the sample. A greater phase lag is seen in the scans for the areas in which the gold is no longer in contact
with the Kapton. As the sample becomes progressively more damaged, the area around the scratch area
is seen to widen and the associated phase lag becomes greater.
Further photothermal analysis, performed in Swansea, will concentrate on coated samples that
are scanned while inside the At apparatus so that the progressive damage may be characterized in situ
and in real time.
ACKNOWLEDGMENTS
The work was supported by the Strategic Defense Initiation Organization Office of Innovative
Science and Technology (SDIO/IST) under contract number 60921-86-CA226 with the Naval Surface
Warfare Center. ABZ wishes to thank the Public Service Department of Malaysia and the Universiti
Pertanian Malaysia for the award of a postgraduate scholarship.
549
REFERENCES
.
o
3.
4.
.
.
Banks, B.A., Rutledge, S.K., Paulsen, P.E., and Streuber, T.J.: NASA Technical Memorandum
101971, 1989.
Pippin, H.G." Surface Coatings Technology, vol. 39, No. 40, 1989, p. 595.
Wong, Y.H.: in Scanned Image Microscopy, Academic Press, London, 1980.
Sawada, T., and Kasai, M.: in Photoacoustic and Thermal Wave Phenomena in Semiconductors, vol.
3, A. Mandelis (ed.), North Holland, New York, 1987.
Zhang, S., and Chen, L.: in Photoacoustic and Photothermal Phenomena in Semiconductors, vol. 27,
A. Mandelis (ed.), North Holland, New York, 1987.
Neely, W.C., Yang, T.C., Wey, J.P., Clothiaux, E.J., and Worley, S.D.: "Space Structures, Power,
and Power Conditioning," SPIE, vol. 871, 1988, p. 313.
7. Unimap 2000 User Manual, Uniras A/S, S_borg, Denmark.
Edge Filter 0(3570
[ Argon Ion _) ! OpticalLaser Modulator
Reference ]
Signal
Microscope Eyep_
)1 I
x_am Splitter
}_x_ Microscope
<_._-/ Objective
Sample. II Pi.ezoelectric
,_ _ ,¢
[ _o_o I 1_°_= I JAmplifier ] [ Interface
RS-232
Ii
I I
I ILow NoiseAmplifier ]
[._._.l I Translati°n Stage IDriver
Figure 1. Diagram of photothermal imaging apparatus.
550
U nex posed
0 50 100 150 200
Position on Sample [microns]
I st Exposure
Photothermal
Phase
[x18]
_ABOVE 8.7
7.9 8.7
7.1 7.9
6.3 7.1
5.5 6.3
4.7 5.5
l! 3.9 4.7
3.1 3.9
_-_ 2.3- 3.t
!__ 1.5- 2.3
! __. IBELOW 1.5
0 5O 100 150 200
Position on Sample [microns]
Figure 2. Photothermal phase images for cumulative AO exposures of gold-coated Kapton.
551
2nd Exposure
Photothermal
Phase
[x18]
8.7
7.9 8.7
7.1 7.9
6.3 7.1
5.5 6.3
4.7 5.5
3.9 4.7
3.1 3.9
2.3 3.1
1.5- 2.3
!- _ELOW 1.5
0 50 100 150 200
Position on Sample [microns]
3rd Exposure
Photothermal
Phase
[xl 8]
_ABOVE 8.7
7.9- 8.7
7.1- 7.9
6.3- 7.1
5.5- 6.3
4.7- 5.5
/ 3.9- 4.7
3.1- 3.9
2.3- 3.1
1.5-2.3
L__! BELOW 1.5
0 50 100 150 200
Position on Sample [microns]
Figure 2. Photothermal phase images for cumulative AO exposures of gold-coated Kapton (continued).
552
4th Exposure
c-
£
E
Photothermal
Phase cL
[x 18] _=
IABOVE 8,7
,t"-
I 719- 87 0
II 7_-_o
I 6.3- 71 ,.,,_
I 4,- 515 _ 5 o
319- 47
3.1- 3.9
_ ! 2.3- 3.1
----] 11 5 -- 2.3
: BELOW 1.5 0
0 50 100 150 200
Position on Sample [microns]
Figure 2. Photothermal phase images for cumulative AO exposures of gold-coated Kapton (continued).
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Atomic oxygen damage characterization by photothermal scanning

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