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Behavior of SiC-Al coatings upon high dose irradiation with deuterium and helium ions
Surface and Coatings Technology, 64 (1994) 205—211
Behavior of SiC—Al coatings upon high dose irradiation with deuterium and helium ions M. Rubel, E. Franconi*, N. A1mqvist~,B. Emmoth and F. Brossa~ Royal Institute of Technology, Physics Department, Association EURATOM-NFR, S-I0405, Stockholm (Sweden) (Received March 20, 1993; accepted in final form December 9, 1993)
Abstract Al—SiC coatings obtained by vacuum plasma spraying co-deposition are considered as a new class ofplasma-facing materials (PFMs) for fusion devices. The technical requirements for PFMs are stringent, since they have to withstand severe operation conditions, including bombardment by high fluxes of particles escaping the plasma. A number ofAl—SiC coatings (containing 20 or SOwt.% SiC) deposited on Cu, stainless steel or graphite substrates were irradiated under laboratory conditions with high doses of deuterium or He ions. The surface properties of the materials were characterized before and after irradiation using several analytical techniques (Rutherford backscattering, nuclear reaction analysis, secondary ion mass spectrometry, Auger electron spectroscopy, energydispersive spectroscopy, microscopic methods and laser Exposure to low-energy deuterium ions orwere deuterium plasma 2 in the profilometry). near-surface layer. The initial steps ofblister formation also observed. resulted in the implantation of 7—9 x 1016 D cm Changes in the surface structure were noted following irradiation with 4He1 ions (1.7—2 MeV). Damage in the surface layer of the materials was dependent on the ion flux.
1. Introduction Materials surrounding the plasma in controlled-fusion machines undergo distinct modification and deterioration, caused by high thermal loads and bombardment by plasma particles [1]. The harmful effects of plasma— surface interactions are limiting factors in the design and construction of an efficient thermonuclear reactor for energy production. Studies have been carried out to manufacture and select the most appropriate materials for tokamak plasma-facing components (PFCs). Graphite has long been extensively used as a protective material and there exists a vast database on its behavior in tokamaks [2, 3]. Recently, a lot of attention has been focused on different classes of composites and layered materials, including carbon-fiber composites [4—6], as being possibly suitable for PFC production. There is also increasing interest in the application of Sicontaining materials: siliconized layers [7], silicon carbides [5, 8, 9] or Si—graphite mixtures [10]. Other candidate materials considered include metallic layers with imbedded SiC, obtained by vacuum plasma spraying (YPS) co-deposition [11, 12]. tLulek University of Technology, Department of Physics, 951 87 Lulea, Sweden. . . ~Commission of the European Communities, JRC Ispra Site, Institute for Advanced Materials, 21020 Ispra, Varese, Italy. ~Associazione EURATOM-ENEA sulla Fusione, CRE Frascati, CP65-0004, Frascati, Rome, Italy.
However, before being tested under real tokam conditions, the selected candidate materials have to extensively tested in the laboratory to evaluate thi properties. Therefore, this paper describes the charact ization of vacuum plasma-sprayed Al—SiC layers in initial state and after irradiation with low-energy deui rium (100—400 eV) or megaelectronvolt 4He ions. Ba deuterium [1] and He [1, 13] ions are expected interact with PFCs in controlled-fusion devices. The ai of the investigation was to recognize (1) the surfa structure and composition of the layers, (2) the deui rium retention in the materials after irradiation und different conditions, and (3) the radiation damage pt duced by the deuterium and helium ions.
2. Experimental details The investigation was carried out with Al—SiC (20 50 wt.% SiC) coatings obtained using the VPS techniqi which is based on the injection of Al and SiC so] particles into an Ar plasma to form a spray of molt metal and solid ceramic grains. The stream is direct onto a collecting surface or a shaped container, whe the material solidifies and forms a composite depot The ceramic particles do not melt during co-depositic therefore, the process of spraying the premixed powd can be considered as deposition in which the solid S impinges with high speed onto the semi-solid layers
206
Al. Rube!
Ct
all SiC Al coatings exposed
the materials already deposited, while the molten Al droplets cover the solid particles, embedding them. The aim of co-deposition is to obtain uniformly distributed SiC grains in the matrix. The composite layers were deposited on different substrates, including Cu, Al, graphite and stainless steel. Deposits up to 1.5—2 mm thick were produced. Both non-polished and polished surfaces of the coatings were studied. The materials were irradiated with deuterium or He ions. Irradiation with low-energy deuterium ions was performed using plasma generated in a glow-discharge facility. Some experiments were carried out with the direct implantation of D~ ions of energy 300 eV, using a low-energy accelerator (Danfysik). To bombard surfaces with 4He~ions of energy l.7—2 MeV, a Van de Graaff accelerator was used (AN 2000, High-Voltage Engineering Co.). Before and after irradiation, the coatings were characterized by a number of surface-sensitive techniques. The composition of the outermost layers was studied with Auger electron spectroscopy (AES, Varian equipment) and secondary ion mass spectrometry (SIMS, LeyboldHeraur/Riber equipment), while that of the near-surface layers was determined by means of Rutherford backscattering spectroscopy (RBS) using an analysing beam of 4He with energy 1.7 or 1.8 MeV. The areal distribution of the components in the surface region was traced using energy-dispersive X-ray spectroscopy (EDS, Link Analytical), and changes in the surface topography were observed using scanning electron microscopy (JEOL 5400) and atomic force microscopy (AFM, Nanoscope II). Images were obtained by operating the AFM equipment in both constant force and constant height modes, using silicon nitride cantilevers of 100 tim with a spring constant of 0.58 N m The surface roughness of the non-polished, “as delivered” surfaces was measured with a laser profilometer (Rodenstock). The deuterium concentration on surfaces exposed to deuterium ions was determined by means of nuclear reaction analysis (NRA) using a 3He~analysing beam at 770 keV and detecting the protons emitted in the 3He(d,p)4He process. ~
3. Results and discussions 3.1. Characterization of non-exposed layers
The scanning electron micrograph in (Fig. 1) shows the surface topography of the non-polished surface of Al—50%SiC deposited on Cu. The micrograph is representative of all the coatings under investigation. The as-delivered deposits are rough, which makes their real surface area very large. Measurements of the surface roughness made with the laser profilometer showed
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.
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an average roughness Ra of approximately 3 ~tm b there were areas with the maximum amplitude I approaching 100 ~m (distance from the top of a gm to the bottom of a pit). Removal of the outer rou~ layer of the deposit, by polishing it with diamond pas revealed the microstructure of the Al—SiC coati (Fig. 2). Irregular (5—15 l.tm) grains can be seen in matrix. Details of the Al—SiC microstructure and roug ness are discussed later. As detected using the E[ technique, the matrix consists of Al with admixtures Cu contaminant from the Cu substrate, whereas only together with C are detected in the grains. The SiMS, AES, and RBS measurements allow characterization of the overall composition of the surfa and subsurface layers of the coatings. The SIMS analys performed in a few spots on the polished specirn (Al—20% SiC), indicated a nearly homogeneous surfa composition. Very small variations in the compositi were found by depth profiling, performed using an gun with Ar~ ions of energy 5 keV with a curre of 2.5 ttA. These results were qualitatively confirmed by sputtt assisted AES analysis. After 1 h of sputter erosion wi Ar~ions of energy 5 keY, the ratio of atomic conce trations c 5~/c~was found to be in the range 0.11—0.] which is in good agreement with the atomic ral
M. Ruhel et al.
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SiC—Al coatings exposed to D~and He~irradiation
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-
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-
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3.2. Irradiation with high-energy 4He~ The irradiation was performed with doses 5 x l0~~—8 x 1017 cm2 and fluxes of up to 3 x 1013cm s~ For low fluxes, no significant changes in the surfa structure and composition of the layers were four independently of the ion dose, and the surface topogi phy remained unchanged when compared with the mit condition. However, intensive irradiation with fim exceeding 2x 1013 cm2 ~ induced morphologi reconstruction in the layers. The morphological changes are shown in Figs. 3 a 4(a) for the non-polished and the polished surfa respectively. A comparison between Figs. 1 and Fig indicates that the irradiated surface remains as rough the initial surface, but the grains on the irradiated tart are turned into spherically shaped structures. No forntion of large agglomerates or blisters and flakes can seen. Neither blistered nor flaked structures are fou: on the He-bombarded polished surface, but a cracke mud structure appears after irradiation. Distinct crac and grooves are visible between the SiC grains a: Al matrix. The most likely explanation for the surface rearrant ment is the distinct temperature rise (about 700—750 in the outer layers of the coating, resulting from energy deposition by the incoming megaelectronv 0
Fig. 2. Surface topography of polished Al—20% SiC coating.
0.09) inferred from the nominal composition. Also, RBS analysis carried out in 6—10 different spots on several Al—SiC samples (with 20% or 50% Si) showed only insignificant variations of the cs~/cAlratio characteristic for the surface and subsurface layers. Such small differences in the local surface composition can be interpreted in terms of stochastic distribution of the SiC particles during their deposition together with the molten Al. The fairly homogeneous composition and absence of segregation effects on a macroscale, as found using the three analytical methods, indicate that the SiC particles probably remain in their original impact positions in the matrix after the complete solidification of the composite. However, impurity atoms were also detected in the deposits. In particular, rough as-deposited layers contained some light impurities (C, N, 0, S and Cl) together with Cu or stainless steel components, depending on the substrate of the coating. As confirmed by RBS, the content of Cu on the rough surfaces was up to 4 x 1016 cm2. In a few cases, small quantities of W 14 (up to 3 x 10 cm —2 ) from the filament of the plasma generator were detected. A much lower content of heavy (up to 1 x 1015 Cu cm2) and light impurities was typical for the polished specimen. (cSI/cAl =
~
Fig. 3. SurI.ftc IopoL’rsph\ al Hc-Irr.IdIalcd .\! of 1800 keV 41-1e0 ions, 5 x i0~cm2).
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M. Ruhel et al.
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Fig. 4. (a) Surface topography and distribution (b) of Si and (c) Al on the He-irradiated Al—50%SiC coating (dose of 1800 keV 8 x 1017 cm2).
4He + ions. The energy deposited within the micrometer range induces surface melting of the non-actively cooled sample. However, the topographical changes are not accompanied by any significant change in the distribution of Al or SiC particles, as exemplified by the EDS micrographs in Figs. 4(b) and 4(c). The positive—negative character of the pattern generally proves that the separation of the Al and SiC phases is retained, and no macrosegregation of the components is induced by the megaelectronvolt 4He irradiation—at least with the doses and fluxes applied in the experiments performed here. Similar EDS patterns have been recorded for the irradiated rough surface. In addition, RBS analysis of the He-bombarded layers did not show any distinct
changes in the composition of the coatings compar with the initial composition. 3.3. Irradiation with deuterium The irradiation was performed using either a gb’ discharge plasma facility or by means of direct implar ation of D + ions from a low-energy accelerat Bombardment of the polished Al—50%SiC substra with a total dose of 5 x 1017 cm2 of D~of ions ener~ 300 eV resulted in the saturation of the implanted lay by 3 x 1016 cm2. Furthermore, NRA measurements pt formed immediately after irradiation showed that t implanted deuterium was deposited in the near-surfa layer of thickness not exceeding 100 mn, which is
M. Rubel et a!.
0 irradiation
/
agreement with the range predicted theoretically by TRIM calculations, During exposure of Al—SiC to the glow-discharge deuterium plasma, the samples were biased at 400 V,
and the total dose estimated from current measuremen was about 1 x 1018 cm2. Following exposure, sever different points on each sample were analysed by mea of NRA. The deuterium content was in the rang 7—10 x 1016 cm2 and 7—9 x 1016 cm2 for the coatin~ with 20% and 50% SiC respectively. The results indica fairly uniform areal of deuterium aton and the absence of distributions distinct differences between tI coatings containing different amounts of SiC. Figures 5(a) and 5(b) show the NRA spectrum ar
TOTAL AMOUNT IMPLANTED 86 i016 cm -2 - 1
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the depth profile of the deposited deuterium. The profi is relatively narrow, proving that most of the deuteriu occurs in the near-surface layer 100—150 nm thic
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However, there is also a contribution of deuterium fro deeper regions, extending to 600—700 nm, which cann be explained by the direct implantation of low-enerl deuterium ions from the plasma. Therefore, the surfa structure on a nanoscale should be considered. Detailed comparative studies of the surfaces we made with AFM for the polished coatings, non-expos
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and treated with glow-discharge deuterium plasm Figure 6(a) shows the SiC grain (area 1) and the
Fig. 5. (a) NRA spectrum and (b) depth profile of deuterium in the Al—50%SiC coating exposed to the plasma.
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area 2. and Fig. 7( hj ) is uniformI~ covered v~it Ii buhh or blisters 20 50 mu in diameter: v~hereas the SIC gn topograph~( Fig. 7(a). area I ) remains unchanged co pared with the initial topograph~. I Iowe~er.there no liakes or open blisters on the Al surface. Ibis indica that the initial stage of radiation damage has hL
detected b A FM. On the basis oF the A FM results. we can suggest tI the broad deuterium depth profiles ohser~ed
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M. Rube! et al. / SiC—Al coatings exposed to D on the coatings exposed to the deuterium plasma are caused by the surface roughness and also by the migration of deuterium into the bulk of the material along the Al—Al and Al—SiC grain boundaries. The suggestion of the deuterium migration along the Al grain boundaries in SiC—Al materials is supported by the observation that, for non-doped SiC substrates (treated with the deuterium plasma in the same manner as the SiC—Al composites), no deuterium migration and a fairly narrow depth distribution of deuterium have been found.
4. Conclusions Composites of SiC—Al, produced using the YPS technique, have been studied before and after irradiation with deuterium and He ions. To our knowledge, this is the first report concerning the investigation of deuterium or helium ion simulated radiation damage of such composites. The most significant results obtained by means of complementary surface-sensitive spectroscopies and microscopies can be summarized as follows. (1) Irradiation with He + ions of energy 1.7—2 MeV induces rearrangement of the outer layer of the coating, resulting from the surface melting caused by energy deposition. The rearrangement occurs for doses exceeding 5 x 10’~cm2 and fluxes approaching 3 x lOis cm2 s’. No blisters or flakes are formed on the coatings irradiated with high-energy He. (2) Exposure of the surfaces to a glow-discharge deuterium plasma results in the deposition of 7—10 x 1016 D cm 2 Under such conditions, an initial stage of blistering is observed on the Al matrix, whereas the SiC grains remain uneroded. (3) Deposited deuterium is mostly retained in the surface layer 100—150 nm thick, but certain amounts of deuterium are also found in deeper layers of up to 600—700 nm. (4) Irradiation with deuterium or He ions does not simulate macrosegregation of the Al and SiC phases. The homogeneous distribution of the components of the coating remains unchanged following ion irradiation. The results obtained can be considered as preliminary, but they lead to rather unfavorable conclusions on Al—SiC coatings as candidate materials for PFCs. The
2
coatings undergo ion-induced erosion even under rel tively mild conditions. In addition, the deuterium depo ited accumulates not only in blisters in the Al matr but also penetrates beneath the surface, probably alor the Al—SiC grain boundaries. Such effects should I taken into account when the tritium inventory in futu fusion devices is expected.
Acknowledgments The authors are very grateful to L. Grobusch for tI exposure of the samples to deuterium plasma and 1 A. Möller for the implantation of the deuterium ions the Al—SiC substrate. The work was performed und the NFR Contract FF/FU 4639—311 and ENE Mobility Contract No. 13 1—83—7 FUCS-Suppl. 5 fi EURATOM staff.
References 1 R. JanevSupplement (ed.), Atomic Plasma-Material Da for K. Fusion, to and Nuclear Fusion, Vol. 1,Interaction IAEA, Viens 1991. 2 R. A. Causey, J. Nucl. Mater., 162—1 64 (1989) 151.
3 W. Moller, J. Nuci. Mater., 162—164 (1989) 138. 4 D. M. Goebel, J. Bohdansky, R. W. Conn, Y. Hirool B. LaBombard, W. K. Leung, R. E. Nygren, J. Roth and G. Tynan, NucI. Fusion, 28(1988)1041. 5 E. Franconi, M. Rubel, and B. Emmoth, Nucl. Fusion, 29(1989)7! 6 M. Rubel, B. Emmoth, H. Bergskker, P. Wienhold, V. Dunaev al V. Sukhomlinov, J. Nucl. Mater., 196—198 (1992) 285. 7 J. Winter, H. G. Esser, G. L. Jackson, L. Konen, V. Phitip~
A. Pospieszczyk, U. Samm, B. Schweer, B. Unterberg a~ P. Wienhold, Proc. 20th EPS Conf. on Controlled Fusion a Plasma Physics, Lisbon, July 1993, Europhys. Conf. Ahstr., i (1993) 279. 8 R. A. Causey, W. R. Wampler and 0. I. Buzhinshij, J. Nucl. Mats 98 (1992)Y.977.Hirooka, R. W. Conn, W. K. Leung a 9 196—1 E. Franconi, B. LaBombard, J. Nuci. Mater., 162—164 (1989) 89. 10 M. Rubel, B. Emmoth, P. Wienhold and C. H. Wu, Vacuum, (1994) 429. 11 A. Peretta. A. Camunzi, G. Giunta and A. Mazzarano, J. Mat (1991) 6057. 12 Set., T. S. 26 Srivatsan, I. A. Ibrahim, F. A. Mohamed and E. J. Lavern J. Mater. Sci., 26 (1991) 5965. 13 D. Post, Proc. 2nd mt. Workshop on Helium Transport and Exhai in Tokamak Experiments, Monterey, CA, USA, April 1992.
Behavior of SiC—Al coatings upon high dose irradiation with deuterium and helium ions M. Rubel, E. Franconi*, N. A1mqvist~,B. Emmoth and F. Brossa~ Royal Institute of Technology, Physics Department, Association EURATOM-NFR, S-I0405, Stockholm (Sweden) (Received March 20, 1993; accepted in final form December 9, 1993)
Abstract Al—SiC coatings obtained by vacuum plasma spraying co-deposition are considered as a new class ofplasma-facing materials (PFMs) for fusion devices. The technical requirements for PFMs are stringent, since they have to withstand severe operation conditions, including bombardment by high fluxes of particles escaping the plasma. A number ofAl—SiC coatings (containing 20 or SOwt.% SiC) deposited on Cu, stainless steel or graphite substrates were irradiated under laboratory conditions with high doses of deuterium or He ions. The surface properties of the materials were characterized before and after irradiation using several analytical techniques (Rutherford backscattering, nuclear reaction analysis, secondary ion mass spectrometry, Auger electron spectroscopy, energydispersive spectroscopy, microscopic methods and laser Exposure to low-energy deuterium ions orwere deuterium plasma 2 in the profilometry). near-surface layer. The initial steps ofblister formation also observed. resulted in the implantation of 7—9 x 1016 D cm Changes in the surface structure were noted following irradiation with 4He1 ions (1.7—2 MeV). Damage in the surface layer of the materials was dependent on the ion flux.
1. Introduction Materials surrounding the plasma in controlled-fusion machines undergo distinct modification and deterioration, caused by high thermal loads and bombardment by plasma particles [1]. The harmful effects of plasma— surface interactions are limiting factors in the design and construction of an efficient thermonuclear reactor for energy production. Studies have been carried out to manufacture and select the most appropriate materials for tokamak plasma-facing components (PFCs). Graphite has long been extensively used as a protective material and there exists a vast database on its behavior in tokamaks [2, 3]. Recently, a lot of attention has been focused on different classes of composites and layered materials, including carbon-fiber composites [4—6], as being possibly suitable for PFC production. There is also increasing interest in the application of Sicontaining materials: siliconized layers [7], silicon carbides [5, 8, 9] or Si—graphite mixtures [10]. Other candidate materials considered include metallic layers with imbedded SiC, obtained by vacuum plasma spraying (YPS) co-deposition [11, 12]. tLulek University of Technology, Department of Physics, 951 87 Lulea, Sweden. . . ~Commission of the European Communities, JRC Ispra Site, Institute for Advanced Materials, 21020 Ispra, Varese, Italy. ~Associazione EURATOM-ENEA sulla Fusione, CRE Frascati, CP65-0004, Frascati, Rome, Italy.
However, before being tested under real tokam conditions, the selected candidate materials have to extensively tested in the laboratory to evaluate thi properties. Therefore, this paper describes the charact ization of vacuum plasma-sprayed Al—SiC layers in initial state and after irradiation with low-energy deui rium (100—400 eV) or megaelectronvolt 4He ions. Ba deuterium [1] and He [1, 13] ions are expected interact with PFCs in controlled-fusion devices. The ai of the investigation was to recognize (1) the surfa structure and composition of the layers, (2) the deui rium retention in the materials after irradiation und different conditions, and (3) the radiation damage pt duced by the deuterium and helium ions.
2. Experimental details The investigation was carried out with Al—SiC (20 50 wt.% SiC) coatings obtained using the VPS techniqi which is based on the injection of Al and SiC so] particles into an Ar plasma to form a spray of molt metal and solid ceramic grains. The stream is direct onto a collecting surface or a shaped container, whe the material solidifies and forms a composite depot The ceramic particles do not melt during co-depositic therefore, the process of spraying the premixed powd can be considered as deposition in which the solid S impinges with high speed onto the semi-solid layers
206
Al. Rube!
Ct
all SiC Al coatings exposed
the materials already deposited, while the molten Al droplets cover the solid particles, embedding them. The aim of co-deposition is to obtain uniformly distributed SiC grains in the matrix. The composite layers were deposited on different substrates, including Cu, Al, graphite and stainless steel. Deposits up to 1.5—2 mm thick were produced. Both non-polished and polished surfaces of the coatings were studied. The materials were irradiated with deuterium or He ions. Irradiation with low-energy deuterium ions was performed using plasma generated in a glow-discharge facility. Some experiments were carried out with the direct implantation of D~ ions of energy 300 eV, using a low-energy accelerator (Danfysik). To bombard surfaces with 4He~ions of energy l.7—2 MeV, a Van de Graaff accelerator was used (AN 2000, High-Voltage Engineering Co.). Before and after irradiation, the coatings were characterized by a number of surface-sensitive techniques. The composition of the outermost layers was studied with Auger electron spectroscopy (AES, Varian equipment) and secondary ion mass spectrometry (SIMS, LeyboldHeraur/Riber equipment), while that of the near-surface layers was determined by means of Rutherford backscattering spectroscopy (RBS) using an analysing beam of 4He with energy 1.7 or 1.8 MeV. The areal distribution of the components in the surface region was traced using energy-dispersive X-ray spectroscopy (EDS, Link Analytical), and changes in the surface topography were observed using scanning electron microscopy (JEOL 5400) and atomic force microscopy (AFM, Nanoscope II). Images were obtained by operating the AFM equipment in both constant force and constant height modes, using silicon nitride cantilevers of 100 tim with a spring constant of 0.58 N m The surface roughness of the non-polished, “as delivered” surfaces was measured with a laser profilometer (Rodenstock). The deuterium concentration on surfaces exposed to deuterium ions was determined by means of nuclear reaction analysis (NRA) using a 3He~analysing beam at 770 keV and detecting the protons emitted in the 3He(d,p)4He process. ~
3. Results and discussions 3.1. Characterization of non-exposed layers
The scanning electron micrograph in (Fig. 1) shows the surface topography of the non-polished surface of Al—50%SiC deposited on Cu. The micrograph is representative of all the coatings under investigation. The as-delivered deposits are rough, which makes their real surface area very large. Measurements of the surface roughness made with the laser profilometer showed
D
H
and He
irradiation
. .‘
.
I
I Ic I. ~~LIrLILc h’pocraph~ Al SO ~SK e~ctting.
‘I
n~~n—poIi’.Iie.I
a~,—dc!iscrc
an average roughness Ra of approximately 3 ~tm b there were areas with the maximum amplitude I approaching 100 ~m (distance from the top of a gm to the bottom of a pit). Removal of the outer rou~ layer of the deposit, by polishing it with diamond pas revealed the microstructure of the Al—SiC coati (Fig. 2). Irregular (5—15 l.tm) grains can be seen in matrix. Details of the Al—SiC microstructure and roug ness are discussed later. As detected using the E[ technique, the matrix consists of Al with admixtures Cu contaminant from the Cu substrate, whereas only together with C are detected in the grains. The SiMS, AES, and RBS measurements allow characterization of the overall composition of the surfa and subsurface layers of the coatings. The SIMS analys performed in a few spots on the polished specirn (Al—20% SiC), indicated a nearly homogeneous surfa composition. Very small variations in the compositi were found by depth profiling, performed using an gun with Ar~ ions of energy 5 keV with a curre of 2.5 ttA. These results were qualitatively confirmed by sputtt assisted AES analysis. After 1 h of sputter erosion wi Ar~ions of energy 5 keY, the ratio of atomic conce trations c 5~/c~was found to be in the range 0.11—0.] which is in good agreement with the atomic ral
M. Ruhel et al.
!
SiC—Al coatings exposed to D~and He~irradiation
-
-
‘
-
15KV
X2
3.2. Irradiation with high-energy 4He~ The irradiation was performed with doses 5 x l0~~—8 x 1017 cm2 and fluxes of up to 3 x 1013cm s~ For low fluxes, no significant changes in the surfa structure and composition of the layers were four independently of the ion dose, and the surface topogi phy remained unchanged when compared with the mit condition. However, intensive irradiation with fim exceeding 2x 1013 cm2 ~ induced morphologi reconstruction in the layers. The morphological changes are shown in Figs. 3 a 4(a) for the non-polished and the polished surfa respectively. A comparison between Figs. 1 and Fig indicates that the irradiated surface remains as rough the initial surface, but the grains on the irradiated tart are turned into spherically shaped structures. No forntion of large agglomerates or blisters and flakes can seen. Neither blistered nor flaked structures are fou: on the He-bombarded polished surface, but a cracke mud structure appears after irradiation. Distinct crac and grooves are visible between the SiC grains a: Al matrix. The most likely explanation for the surface rearrant ment is the distinct temperature rise (about 700—750 in the outer layers of the coating, resulting from energy deposition by the incoming megaelectronv 0
Fig. 2. Surface topography of polished Al—20% SiC coating.
0.09) inferred from the nominal composition. Also, RBS analysis carried out in 6—10 different spots on several Al—SiC samples (with 20% or 50% Si) showed only insignificant variations of the cs~/cAlratio characteristic for the surface and subsurface layers. Such small differences in the local surface composition can be interpreted in terms of stochastic distribution of the SiC particles during their deposition together with the molten Al. The fairly homogeneous composition and absence of segregation effects on a macroscale, as found using the three analytical methods, indicate that the SiC particles probably remain in their original impact positions in the matrix after the complete solidification of the composite. However, impurity atoms were also detected in the deposits. In particular, rough as-deposited layers contained some light impurities (C, N, 0, S and Cl) together with Cu or stainless steel components, depending on the substrate of the coating. As confirmed by RBS, the content of Cu on the rough surfaces was up to 4 x 1016 cm2. In a few cases, small quantities of W 14 (up to 3 x 10 cm —2 ) from the filament of the plasma generator were detected. A much lower content of heavy (up to 1 x 1015 Cu cm2) and light impurities was typical for the polished specimen. (cSI/cAl =
~
Fig. 3. SurI.ftc IopoL’rsph\ al Hc-Irr.IdIalcd .\! of 1800 keV 41-1e0 ions, 5 x i0~cm2).
4
F
4 5I.l°oSI( eOdting Id1
M. Ruhel et al.
208
0 and He0 irradiation
SiC—Al coatings exposed to D
a
lb
1RI
~
C
15KV
X2
1580
18~
4He~ in
Fig. 4. (a) Surface topography and distribution (b) of Si and (c) Al on the He-irradiated Al—50%SiC coating (dose of 1800 keV 8 x 1017 cm2).
4He + ions. The energy deposited within the micrometer range induces surface melting of the non-actively cooled sample. However, the topographical changes are not accompanied by any significant change in the distribution of Al or SiC particles, as exemplified by the EDS micrographs in Figs. 4(b) and 4(c). The positive—negative character of the pattern generally proves that the separation of the Al and SiC phases is retained, and no macrosegregation of the components is induced by the megaelectronvolt 4He irradiation—at least with the doses and fluxes applied in the experiments performed here. Similar EDS patterns have been recorded for the irradiated rough surface. In addition, RBS analysis of the He-bombarded layers did not show any distinct
changes in the composition of the coatings compar with the initial composition. 3.3. Irradiation with deuterium The irradiation was performed using either a gb’ discharge plasma facility or by means of direct implar ation of D + ions from a low-energy accelerat Bombardment of the polished Al—50%SiC substra with a total dose of 5 x 1017 cm2 of D~of ions ener~ 300 eV resulted in the saturation of the implanted lay by 3 x 1016 cm2. Furthermore, NRA measurements pt formed immediately after irradiation showed that t implanted deuterium was deposited in the near-surfa layer of thickness not exceeding 100 mn, which is
M. Rubel et a!.
0 irradiation
/
agreement with the range predicted theoretically by TRIM calculations, During exposure of Al—SiC to the glow-discharge deuterium plasma, the samples were biased at 400 V,
and the total dose estimated from current measuremen was about 1 x 1018 cm2. Following exposure, sever different points on each sample were analysed by mea of NRA. The deuterium content was in the rang 7—10 x 1016 cm2 and 7—9 x 1016 cm2 for the coatin~ with 20% and 50% SiC respectively. The results indica fairly uniform areal of deuterium aton and the absence of distributions distinct differences between tI coatings containing different amounts of SiC. Figures 5(a) and 5(b) show the NRA spectrum ar
TOTAL AMOUNT IMPLANTED 86 i016 cm -2 - 1
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SiC—Al coatings exposed to D~and He
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the depth profile of the deposited deuterium. The profi is relatively narrow, proving that most of the deuteriu occurs in the near-surface layer 100—150 nm thic
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However, there is also a contribution of deuterium fro deeper regions, extending to 600—700 nm, which cann be explained by the direct implantation of low-enerl deuterium ions from the plasma. Therefore, the surfa structure on a nanoscale should be considered. Detailed comparative studies of the surfaces we made with AFM for the polished coatings, non-expos
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Depth (nm) (b)
and treated with glow-discharge deuterium plasm Figure 6(a) shows the SiC grain (area 1) and the
Fig. 5. (a) NRA spectrum and (b) depth profile of deuterium in the Al—50%SiC coating exposed to the plasma.
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and a step of I Ut) nm on the houndarv bct~~eenthe phases. The sur!~ice roughness can he interred from Figs. 6( h) and 6(c) for SiC and Al respec— 1ivel~ The ~i( grain is l~iiri~ smooth ~ ithin 20 urn from top to alley). ~ith ouR a le~~ spikes of 11) I S nrn: ho’~ever, the t\ pical roughness of the .\l niatrix exceeds 1 50 200 urn. Bombardment of the coating ~sith lo~~ enei g’ deuterium ions induces structural changes. ~ hich are pronounced on the \l su rlitce. 1 lie matrix ( I i0. 7a). .
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area 2. and Fig. 7( hj ) is uniformI~ covered v~it Ii buhh or blisters 20 50 mu in diameter: v~hereas the SIC gn topograph~( Fig. 7(a). area I ) remains unchanged co pared with the initial topograph~. I Iowe~er.there no liakes or open blisters on the Al surface. Ibis indica that the initial stage of radiation damage has hL
detected b A FM. On the basis oF the A FM results. we can suggest tI the broad deuterium depth profiles ohser~ed
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M. Rube! et al. / SiC—Al coatings exposed to D on the coatings exposed to the deuterium plasma are caused by the surface roughness and also by the migration of deuterium into the bulk of the material along the Al—Al and Al—SiC grain boundaries. The suggestion of the deuterium migration along the Al grain boundaries in SiC—Al materials is supported by the observation that, for non-doped SiC substrates (treated with the deuterium plasma in the same manner as the SiC—Al composites), no deuterium migration and a fairly narrow depth distribution of deuterium have been found.
4. Conclusions Composites of SiC—Al, produced using the YPS technique, have been studied before and after irradiation with deuterium and He ions. To our knowledge, this is the first report concerning the investigation of deuterium or helium ion simulated radiation damage of such composites. The most significant results obtained by means of complementary surface-sensitive spectroscopies and microscopies can be summarized as follows. (1) Irradiation with He + ions of energy 1.7—2 MeV induces rearrangement of the outer layer of the coating, resulting from the surface melting caused by energy deposition. The rearrangement occurs for doses exceeding 5 x 10’~cm2 and fluxes approaching 3 x lOis cm2 s’. No blisters or flakes are formed on the coatings irradiated with high-energy He. (2) Exposure of the surfaces to a glow-discharge deuterium plasma results in the deposition of 7—10 x 1016 D cm 2 Under such conditions, an initial stage of blistering is observed on the Al matrix, whereas the SiC grains remain uneroded. (3) Deposited deuterium is mostly retained in the surface layer 100—150 nm thick, but certain amounts of deuterium are also found in deeper layers of up to 600—700 nm. (4) Irradiation with deuterium or He ions does not simulate macrosegregation of the Al and SiC phases. The homogeneous distribution of the components of the coating remains unchanged following ion irradiation. The results obtained can be considered as preliminary, but they lead to rather unfavorable conclusions on Al—SiC coatings as candidate materials for PFCs. The
2
coatings undergo ion-induced erosion even under rel tively mild conditions. In addition, the deuterium depo ited accumulates not only in blisters in the Al matr but also penetrates beneath the surface, probably alor the Al—SiC grain boundaries. Such effects should I taken into account when the tritium inventory in futu fusion devices is expected.
Acknowledgments The authors are very grateful to L. Grobusch for tI exposure of the samples to deuterium plasma and 1 A. Möller for the implantation of the deuterium ions the Al—SiC substrate. The work was performed und the NFR Contract FF/FU 4639—311 and ENE Mobility Contract No. 13 1—83—7 FUCS-Suppl. 5 fi EURATOM staff.
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A. Pospieszczyk, U. Samm, B. Schweer, B. Unterberg a~ P. Wienhold, Proc. 20th EPS Conf. on Controlled Fusion a Plasma Physics, Lisbon, July 1993, Europhys. Conf. Ahstr., i (1993) 279. 8 R. A. Causey, W. R. Wampler and 0. I. Buzhinshij, J. Nucl. Mats 98 (1992)Y.977.Hirooka, R. W. Conn, W. K. Leung a 9 196—1 E. Franconi, B. LaBombard, J. Nuci. Mater., 162—164 (1989) 89. 10 M. Rubel, B. Emmoth, P. Wienhold and C. H. Wu, Vacuum, (1994) 429. 11 A. Peretta. A. Camunzi, G. Giunta and A. Mazzarano, J. Mat (1991) 6057. 12 Set., T. S. 26 Srivatsan, I. A. Ibrahim, F. A. Mohamed and E. J. Lavern J. Mater. Sci., 26 (1991) 5965. 13 D. Post, Proc. 2nd mt. Workshop on Helium Transport and Exhai in Tokamak Experiments, Monterey, CA, USA, April 1992.