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Deuterium ion bombardment behaviour of alumina coatings
Thin Solid Films, 165 (1988)L107-L110
L107
Letter Deuterium ion bombardment behaviour of alumina coatings v. c. GEORGE, A. K. DUA AND R. P. AGARWALA Chemistry Division. Bhabha Atomic Research Centre, Bombay 400085 (India)
(ReceivedJuly 6, 1988;acceptedAugust 5, 1988)
Alumina is widely used because of its refractory nature, chemical inertness, abrasion resistance, thermal shock property and mechanical strength at high temperature. It is one of the potential candidate coating materials for reducing the plasma contamination and power loss in Tokamak-type controlled thermonuclear reactors. Its high electrical resistivity is expected to reduce arcing, which is a desirable feature. These coatings would be subject to the severe environment of a fusion reactor. In particular, they will be exposed to energetic deuterium. The study of deuterium ion bombardment of alumina coatings has yielded interesting results which are reported herein. Alumina coatings have been deposited 1 on metallographically polished 304 stainless steel substrates using electron beam evaporation of sintered alumina, the deposition parameters being as follows: pressure, about 1.3 x 10- 3_1.3 x 10- 4 Pa; substrate-to-environment distance, about 1.2 x 10 -1 m; substrate temperature, about 548 K; rate of deposition, about 0.9 nm s-1; in situ annealing at 548 K for 1.8 x 103 s. These coatings have been characterized by Auger electron spectroscopy, X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry and were found to be nearly stoichiometric. The coating thickness has been kept more than the range of the energetic projectiles and it has been measured using a mass difference method and the Planar surfometer. The deuterium ions were unanalysed and were obtained from the accelerator portion of a neutron generator. These principally consisted of D ÷ together with a small quantity o f D 2 ÷ and were incident normal to the specimen. The sample current was monitored as a function of time and was used to evaluate the dose. The base pressure in the target chamber was about 1.33 x 10 -4 Pa and was attained using a diffusion pump coupled with a liquid nitrogen trap. Deuterium was introduced in the system so that the pressure rose to about 6.65 x 10 - 4 Pa and the bombardment of the sample was carried out at this pressure. Figures 1, 2 and 3 show respectively scanning electron micrographs of the coatings bombarded by deuterium ions of energies 40 keV (4.5 x 1019 ions cm-2; coating thickness, about 2.8 ~tm), 80 keV (9.6 x 1019 ions cm-2; coating thickness, about 8.3 ~tm) and 120 keV (2.1 x 1020 ions cm-2; coating thickness, about 9.6 ~tm). Damage in the form of small exfoliated multilayers distributed around the main 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netherlands
Lt08
LETTERS
"pockmark" is a common observation. The pockmark itself is multilayered and its size seems to increase roughly as the projectile energy increases. In Fig. l(b), the skin of the central circular large "blister" is still intact and may be that a higher dose is required for its exfoliation and hence conversion into a pockmark. Pockmarks have also been observed by Iwamoto et al. 2 on plasma-sprayed alumina coatings bombarded by argon ions (energy, 100kV; dose about 1015-1017 ions cm-2). Bodhansky e t al. 3 bombarded alumina coatings prepared by different methods by H +, D + and He + ions (energy, 100 eV-8 keV) and observed, in general, smoothing of the bombarded region and sometimes the existence of craters similar to the pockmarks of Iwamoto et al. 2
(a) (b) Fig. 1. Scanning electron micrographs of deuterium-bombarded alumina coating on 304 stainless steel (deuterium energy, 40 keV; dose, 4.5 × 1019 ions cm-2; coating thickness, about 2.8 ~tm):(a) magnification, 120 × ; (b) one of the white spots in (a) magnifiedto 3000 ×.
Radiation damage processes in alumina are expected to be complex because the electrostatic effects and the different dpa rates on the sublattices of aluminium and oxygen alter the nature of the aggregated defects formed 4. At the moment, the damage mechanism is not fully understood. Of course, sputter etching and gas inclusions are expected to be present. These would be influenced by electrostatic charges and thermal spikes 5. Because the material lacks ductility, pressure build-up cannot deform it into a blister; rather, exfoliation takes place because of its brittle nature.
(b)
(c)
Fig. 2. Scanning electron micrographs of deuterium-bombarded alumina coating on 304 stainless steel (deulerium energy, 80 keV; dose, 9.6 x 1019 ions c m - Z ; coating thickness, about 8.3 ~tm): (a) magnification, 5000 x ; (b) same area as in (a) magnified 2500 x ;(c) another area magnified 5000 x .
(a)
L 110
(a)
LETTERS
(b)
(c) Fig. 3. Scanning electron micrographs of deuterium-bombarded alumina coating on 304 stainless steel (deuterium energy. 120 keV: dose, 2.1 x 10 2o ions cm -': coating thickness, about 9.6 ~mj: (aj magnification, 2500 × :(bj another area magnified 3000 x :lcl another area magnified 800 x. The authors thank the Neutron Physics Division of this centre for accelerator t i m e a n d t h e H i n d u s t a n L e v e r R e s e a r c h C e n t r e , B o m b a y , for s o m e o f t h e s c a n n i n g electron micrographs. 1 2 3 4 5
A.K. Dua, V. C. George and R. P. Agarwala, Thin SolidFilrns, (1988) in press. N. Iwamoto, Y. Makino, S. Endo, N. Itoh and N. Matsunami, J. Nucl. Mater., 133 134 (1985) 736. J. Bodhansky, J. Roth and F. Brossa, J. Nucl. Mater., 85-86 (1979) 1145. F.W. Clinard, J. Nucl. Mater., 85~86 (1979) 394. F. Seitz and J. S. Koehler, Prog. Solid State Phys., 2 (1957) 30.
L107
Letter Deuterium ion bombardment behaviour of alumina coatings v. c. GEORGE, A. K. DUA AND R. P. AGARWALA Chemistry Division. Bhabha Atomic Research Centre, Bombay 400085 (India)
(ReceivedJuly 6, 1988;acceptedAugust 5, 1988)
Alumina is widely used because of its refractory nature, chemical inertness, abrasion resistance, thermal shock property and mechanical strength at high temperature. It is one of the potential candidate coating materials for reducing the plasma contamination and power loss in Tokamak-type controlled thermonuclear reactors. Its high electrical resistivity is expected to reduce arcing, which is a desirable feature. These coatings would be subject to the severe environment of a fusion reactor. In particular, they will be exposed to energetic deuterium. The study of deuterium ion bombardment of alumina coatings has yielded interesting results which are reported herein. Alumina coatings have been deposited 1 on metallographically polished 304 stainless steel substrates using electron beam evaporation of sintered alumina, the deposition parameters being as follows: pressure, about 1.3 x 10- 3_1.3 x 10- 4 Pa; substrate-to-environment distance, about 1.2 x 10 -1 m; substrate temperature, about 548 K; rate of deposition, about 0.9 nm s-1; in situ annealing at 548 K for 1.8 x 103 s. These coatings have been characterized by Auger electron spectroscopy, X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry and were found to be nearly stoichiometric. The coating thickness has been kept more than the range of the energetic projectiles and it has been measured using a mass difference method and the Planar surfometer. The deuterium ions were unanalysed and were obtained from the accelerator portion of a neutron generator. These principally consisted of D ÷ together with a small quantity o f D 2 ÷ and were incident normal to the specimen. The sample current was monitored as a function of time and was used to evaluate the dose. The base pressure in the target chamber was about 1.33 x 10 -4 Pa and was attained using a diffusion pump coupled with a liquid nitrogen trap. Deuterium was introduced in the system so that the pressure rose to about 6.65 x 10 - 4 Pa and the bombardment of the sample was carried out at this pressure. Figures 1, 2 and 3 show respectively scanning electron micrographs of the coatings bombarded by deuterium ions of energies 40 keV (4.5 x 1019 ions cm-2; coating thickness, about 2.8 ~tm), 80 keV (9.6 x 1019 ions cm-2; coating thickness, about 8.3 ~tm) and 120 keV (2.1 x 1020 ions cm-2; coating thickness, about 9.6 ~tm). Damage in the form of small exfoliated multilayers distributed around the main 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netherlands
Lt08
LETTERS
"pockmark" is a common observation. The pockmark itself is multilayered and its size seems to increase roughly as the projectile energy increases. In Fig. l(b), the skin of the central circular large "blister" is still intact and may be that a higher dose is required for its exfoliation and hence conversion into a pockmark. Pockmarks have also been observed by Iwamoto et al. 2 on plasma-sprayed alumina coatings bombarded by argon ions (energy, 100kV; dose about 1015-1017 ions cm-2). Bodhansky e t al. 3 bombarded alumina coatings prepared by different methods by H +, D + and He + ions (energy, 100 eV-8 keV) and observed, in general, smoothing of the bombarded region and sometimes the existence of craters similar to the pockmarks of Iwamoto et al. 2
(a) (b) Fig. 1. Scanning electron micrographs of deuterium-bombarded alumina coating on 304 stainless steel (deuterium energy, 40 keV; dose, 4.5 × 1019 ions cm-2; coating thickness, about 2.8 ~tm):(a) magnification, 120 × ; (b) one of the white spots in (a) magnifiedto 3000 ×.
Radiation damage processes in alumina are expected to be complex because the electrostatic effects and the different dpa rates on the sublattices of aluminium and oxygen alter the nature of the aggregated defects formed 4. At the moment, the damage mechanism is not fully understood. Of course, sputter etching and gas inclusions are expected to be present. These would be influenced by electrostatic charges and thermal spikes 5. Because the material lacks ductility, pressure build-up cannot deform it into a blister; rather, exfoliation takes place because of its brittle nature.
(b)
(c)
Fig. 2. Scanning electron micrographs of deuterium-bombarded alumina coating on 304 stainless steel (deulerium energy, 80 keV; dose, 9.6 x 1019 ions c m - Z ; coating thickness, about 8.3 ~tm): (a) magnification, 5000 x ; (b) same area as in (a) magnified 2500 x ;(c) another area magnified 5000 x .
(a)
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(a)
LETTERS
(b)
(c) Fig. 3. Scanning electron micrographs of deuterium-bombarded alumina coating on 304 stainless steel (deuterium energy. 120 keV: dose, 2.1 x 10 2o ions cm -': coating thickness, about 9.6 ~mj: (aj magnification, 2500 × :(bj another area magnified 3000 x :lcl another area magnified 800 x. The authors thank the Neutron Physics Division of this centre for accelerator t i m e a n d t h e H i n d u s t a n L e v e r R e s e a r c h C e n t r e , B o m b a y , for s o m e o f t h e s c a n n i n g electron micrographs. 1 2 3 4 5
A.K. Dua, V. C. George and R. P. Agarwala, Thin SolidFilrns, (1988) in press. N. Iwamoto, Y. Makino, S. Endo, N. Itoh and N. Matsunami, J. Nucl. Mater., 133 134 (1985) 736. J. Bodhansky, J. Roth and F. Brossa, J. Nucl. Mater., 85-86 (1979) 1145. F.W. Clinard, J. Nucl. Mater., 85~86 (1979) 394. F. Seitz and J. S. Koehler, Prog. Solid State Phys., 2 (1957) 30.