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Analysis of TiC and TiN coatings exposed to fusion plasmas
36/numbers in Great Britain
Vacuum/volume
Printed
1-3fpages
23 to 25/l
986
0042-207X/86$3.00 Pergamon
Analysis of TiC and TiN coatings fusion plasmas
exposed
E Taglauer. P Varga” and GarchinglMiinchen, FRG
EURATOM-Association,
K Ertl.
Max-Planck-lnstitut
fiir Plasmaphysik,
+ .OO Press Ltd
to 0-8046
received
Coatings with elemental constituents of low atomic number are used for first wall surfaces and special structures in fusion plasma devices. Therefore Tic and TiN coatings on graphite and molybdenum were tested by exposing appropriate samples to the divertor plasma of the ASDEX tokamak in Garching. In parall& experiments surface modifications under hydrogen ion bombardment were investigated. The coatings were examined by scanning electron microscopy and Auger electron spectroscopy together with Ar + sputter depth profiling. The results show that at high heat fluxes the coatings can be partly removed from the substrate. There is also a considerable effect of preferential sputtering of C and N, which, in the case of plasma exposed samples, occurs simultaneously with deposition of metals, oxygen, sulphur and carbon contaminants from the plasma. The heat flux during neutral beam injection results in sample temperatures of about 7000°C and therefore segregation of carbon can also occur on graphite samples.
Introduction
Experimental
The interaction of the plasma with material wall surfaces is a key problem in fusion devices. The development of a plasma discharge is to a large extent determined by fuel recycling and impurity release processes occurring by plasma-wall interaction’. In order to alleviate the problems connected with plasma contamination by impurity atoms, low atomic number materials are chosen for those structural parts which have strong plasma contact’. Graphite, for instance, is the favourite present day ‘limiter’ material and even beryllium is considered in this respect3. Coatings of low Z material offer the possibility of separating surface from bulk material properties. Coatings with titanium compounds such as TIC or TIN are in fact used for limiters4.5, wall coatings 6 or high frequency heating antennae’. The question is whether these coatings can withstand the high particle and energy fluxes occurring in the scrape-off layer of a fusion plasma and how these fluxes change the surface composition of these compounds. Surface compositional changes of titanium compounds due to light ion bombardment have been measured before in laboratory experimentss,9. In this paper we report on the analysis of surface composition and concentration depth profiles of TIC and TIN layers after exposure to the divertor plasma in the Garching tokamak ASDEX. In situ temperature measurements were done in some cases. The surface structure was examined in a scanning electron microscope (SEM). For comparison, simulation experiments were done by H+-ion bombardment in an uhv apparatus.
TiC and TIN coatings were deposited by reactive ion plating on 2 mm thick molybdenum and 3 mm thick graphite (PT 1116 Carbone Lorraine) samples 100 x 17 mm2 in size. The layer thickness of the coatings is about 5 pm. They have a rough granulated structure on graphite substrates with a grain size of about 2 pm; coatings on MO are much smoother, the grain size in that case appears to be below 1 pm (see also Figure 2). Each of these four types of samples was exposed to the plasma in the ASDEX divertor by means of a moveable manipulator”. The samples were positioned in front of the upper outer divertor plates, the magnetic field lines intersecting almost perpendicularly the sample surface, see Figure 1. The width of the scrape-off layer at this position is about 20 mm, visible by a clear mark on the samples after one exposure. During a discharge the plasma streams along magnetic field lines from the midplane onto the samples (or divertor plates). Each sample analysed in this work was exposed for one discharge in deuterium with neutral beam injection (3.4 MW D for MO substrate samples and 3.85 MW H for graphite substrate samples), see also Figure 4. The target temperature during and after the discharge was measured in some cases with a chromel-alumel thermocouple. After the exposure the samples were analysed using Auger electron spectroscopy (AES) and sputtering with 1 keV Ar+ ions to obtain depth profiles. Ar+ ions have a very small preferential sputtering effect on TIC and TiNI and therefore it can be assumed that Ar sputtering does not strongly modify the original depth profiles. The depth scale given in Figure 3(a),(b) is called ‘nominal’ because it is calculated using the experimental ion fluence and extrapolated literature values of the sputtering yield of Y= 1 for both coatings, the exact sputtering yield for our samples
* Humboldt-Fellow, permanent address: Institut fiir Allgemeine Physik, TU Wien, Austria.
23
f Taglauer,
P Varga and K frtl:
Analysis
of TIC and TIN coatings
exposed
to fusion
plasmas
ASDEX
1’1
7
r
7 ‘1’T
TiN Probe
/
c ‘I’“1
‘I-
“I
on MO
x
exposed
.
not exposed
near
seporotr~x li
Divertor Plates
l x
b Multipole Coil
x
Separatrix Serape
Sk
- off
Layer
tix
*-x--x-x-_x_-x-
Radius (m)
)
. .. . . . .
a=-.
01’
Main Plasma
ii
-d’j-J_ 10L
FLUENCE
LL
-1
10-2
-l
I
(lOIL I
I
J 1oL
Ar*/cm2) LLLuiiLLuu.
1
NOMINAL
,-IO3
d
10-l
-x
-‘m-J_
’
10’
I$
101
DEPTH
102
(nm)
L
Figure I. Cross-section position
of the ASDEX in the upper divertor chamber.
tokamak
showing
the
x Prebombordment
probe
l
3 being unknown. The surface structure was examined by scanning electron microscopy (SEM). For comparison with well defined ion bombardment effects, identical samples were bombarded with I keV H: ions (which is equivalent to 500 eV H+) and subsequently analysed.
500 eV Hi
Unbomborded
a a 2 4
2
u &= 1
0 I
10’
10‘
FLUENCE
TIC
on
TIN
MO
on
MO
Figure 3. (a) AES depth profile on MO. (b) AES depth profile hydrogen pre-bombardment.
( 10IL
10J Are/cm2
)
(I keV Ar ’ ) ofa plasma-exposed TiN layer (I keV Art) of a TiC layer on MO with
Results and discussion
TiC
on
Graphite
Figure 2. SEM micrographs 24
TIN of plasma-exposed
on Graphite samples.
Inspection ofthe exposed samples reveals that the strongest effects of plasma interaction can be detected within the 20 mm wide region of the scrape-off intersection. Figure 2 shows SEM micrographs of these areas. The TiN and TIC layers show molten spots which are typical for unipolar arcs”. The layers also show damage, blister-like structures and are in some areas almost completely removed from the MO substrate. The adhesion to the graphite substrate appears to be stronger and at the same time the lateral cohesion is weaker due to the granulated structure of the
E Taglauer, P I/arga and K Ertl: Analysis
of TIC and TiN coatings
exposed
ASDEX D-DISCHARGE
IN H2
*
13908
600 600
0 ” % 5 m
m 200
0
0.5
15
10
_
_^ 1”
IO
t Is)
Figure 4. Plasma current I,, line averaged density 1,, and sample temperature during a divertor discharge in ASDEX with neutral beam injection.
Therefore, lateral stresses cannot build up to the same as for the MO substrates, and thus no blistering and peeling off is observed for graphite substrate samples. Depth profiles of TIN and TIC on MO are shown in Figure 3(a),(b). The TIN sample was exposed in ASDEX, the TiC sample bombarded with H+ ions. Both profiles show a depletion of the N or C component into a certain depth. This depletion has been observed before and is typical for preferential sputtering effects13. The analysis of our samples demonstrates that the ASDEX-exposed samples exhibit a high depletion to a much larger depth than those bombarded by 500 eV H+ in the simulation experiment. The ASDEX samples also show overlayer deposits of C, 0, Fe and S which must have developed during the discharge. A summary of the analysis of our samples is given in Table 1. It shows the maximum value for the depletion given by the ratio (Ti/X),,, and the depth of the altered layer characterized by the value T as defined in Figure 3(b). The fact that all ASDEX samples on MO show a higher depletion than those bombarded by 500 eV H+ indicates that the average energy of the D+ ions from the plasma is well below 500 eV, because lower energies result in higher depletion13. This is in accordance with measurements of the plasma parameters in the
coating. extent
Table 1. Maximum depletion of the lighter element, measured by the Auger peak-to-peak ratio (Ti/X),,, and depth of the altered layer T given in 1 keV Ar + ion fluence for samples exposed in ASDEX and samples bombarded by 500 eV Hi. TiC on MO
TIC on graphite
TiN on MO
TiN on graphite
ASDEX (separatrix) (Ti/X),,, T(Ar+ cm-‘) 500eV H+
4 1.0 x 10”
0.2-2 1.7 x 10”
12-15 1.7 x 10”
8 5.2 x lOI
(Ti/X),,, T(Ar cm-‘)
2.2 2.8 x lOI
4.4 3.2 x lOl6
::: x 1016
to fusion
plasmas
divertor 14, even taking the sheath potential into account. The total deuterium fluence must be of the order of magnitude required for equilibrium surface concentrations, i.e. above about 10” cm-’ 13. The comparison is complicated by the fact that TIN layers do not show an equilibrium surface concentration even after a fluence of 10” H+ cm-‘. For graphite substrates the diffusion of carbon to the surface at high temperatures could obscure the depletion effects caused by sputtering. The larger depth T in spite of the low particle energy is explained by the overlayer of impurity atoms (C, 0, Fe, S). The origin of these atoms is not entirely clear at this point. A large amount can be deposited as contaminants with the plasma, impurity fluxes of that kind have been reported before”. There is also the possibility of an additional contribution due to segregation from the bulk through the coating, e.g. S from MO or C from graphite. The sample surface temperature during the discharge can become very high, during neutral beam injection close to the separatrix a value of about 950°C was measured as a lower limit, see Figure 4. It is intended to clarify the role of segregation in future experiments. In summary it has been shown that significant changes are observed in TIN and TIC coatings due to the action of the divertor plasma. The depletion of the lighter constituent can be explained by preferential sputtering effects using particle fluxes and energies which are compatible with the divertor plasma parameters. In addition to the erosion, deposition of C, 0, Fe and S overlayers is observed. The high heat flux during neutral beam injection can result in removal of most of the coating on molybdenum in the area near the separatrix intersection.
Acknowledgements assistance of K Gehringer, G Nagleder, F Schuster and M-L Hirschinger is gratefully acknowledged as is the fruitful collaboration with Dr Riidhammer.
The technical
References ’ Proc of the Int Conf Series on Plasma Surface Interaction (PSI) and hoc VIth PSI in Nagoya, JApAn, 1984; J Nucl Mater: 1281129 (1984). ’ D M Meade, Nucl Fusion, 14, 289 (1974). 3 K J Dietz ef al, J Nucl Mater, 128/129, 10 (1984). 4 Y Hirohata et al, J Nucl Mater, 122/123, 1160 (1984). 5 J L Cecchi and TFTR Group, J Nucl Mater, 128/129, 1 (1984). ’ Y Shimomura et al, J Nucl Mater, 128/129, 19 (1984). ’ M Still and F Wesner, Proc 10th Symp on Fusion Engineering, Philadelphia, USA, lP20 (1983). 8 P Varga and E raglauer, J Nucl Mater, lll/llt, 726 (1982). 9 T Yamashina, Proc 9th Int Vacuum Conyr and 5th Int ConfSurJScience, Madrid, p 614 (1983). lo Metallwerke Plansee, Reutte, Tirol Austria. ‘I E Taglauer, J Nucl Mater, 128/129, 141 (1984). I2 K Ertl R Behrisch and B Jiittner, Proc 1 lth Europ Conf Contr Fusion and Plastk Physics, Aachen, 11, 385 (1983). l3 E Taglauer, Appl Surf Sci, 13, 80 (1982). l4 Y Shimomura, M Keilhacker, K Lackner ahld H Murmann, Nucl Fusion, 23, 869 (1983).
25
Vacuum/volume
Printed
1-3fpages
23 to 25/l
986
0042-207X/86$3.00 Pergamon
Analysis of TiC and TiN coatings fusion plasmas
exposed
E Taglauer. P Varga” and GarchinglMiinchen, FRG
EURATOM-Association,
K Ertl.
Max-Planck-lnstitut
fiir Plasmaphysik,
+ .OO Press Ltd
to 0-8046
received
Coatings with elemental constituents of low atomic number are used for first wall surfaces and special structures in fusion plasma devices. Therefore Tic and TiN coatings on graphite and molybdenum were tested by exposing appropriate samples to the divertor plasma of the ASDEX tokamak in Garching. In parall& experiments surface modifications under hydrogen ion bombardment were investigated. The coatings were examined by scanning electron microscopy and Auger electron spectroscopy together with Ar + sputter depth profiling. The results show that at high heat fluxes the coatings can be partly removed from the substrate. There is also a considerable effect of preferential sputtering of C and N, which, in the case of plasma exposed samples, occurs simultaneously with deposition of metals, oxygen, sulphur and carbon contaminants from the plasma. The heat flux during neutral beam injection results in sample temperatures of about 7000°C and therefore segregation of carbon can also occur on graphite samples.
Introduction
Experimental
The interaction of the plasma with material wall surfaces is a key problem in fusion devices. The development of a plasma discharge is to a large extent determined by fuel recycling and impurity release processes occurring by plasma-wall interaction’. In order to alleviate the problems connected with plasma contamination by impurity atoms, low atomic number materials are chosen for those structural parts which have strong plasma contact’. Graphite, for instance, is the favourite present day ‘limiter’ material and even beryllium is considered in this respect3. Coatings of low Z material offer the possibility of separating surface from bulk material properties. Coatings with titanium compounds such as TIC or TIN are in fact used for limiters4.5, wall coatings 6 or high frequency heating antennae’. The question is whether these coatings can withstand the high particle and energy fluxes occurring in the scrape-off layer of a fusion plasma and how these fluxes change the surface composition of these compounds. Surface compositional changes of titanium compounds due to light ion bombardment have been measured before in laboratory experimentss,9. In this paper we report on the analysis of surface composition and concentration depth profiles of TIC and TIN layers after exposure to the divertor plasma in the Garching tokamak ASDEX. In situ temperature measurements were done in some cases. The surface structure was examined in a scanning electron microscope (SEM). For comparison, simulation experiments were done by H+-ion bombardment in an uhv apparatus.
TiC and TIN coatings were deposited by reactive ion plating on 2 mm thick molybdenum and 3 mm thick graphite (PT 1116 Carbone Lorraine) samples 100 x 17 mm2 in size. The layer thickness of the coatings is about 5 pm. They have a rough granulated structure on graphite substrates with a grain size of about 2 pm; coatings on MO are much smoother, the grain size in that case appears to be below 1 pm (see also Figure 2). Each of these four types of samples was exposed to the plasma in the ASDEX divertor by means of a moveable manipulator”. The samples were positioned in front of the upper outer divertor plates, the magnetic field lines intersecting almost perpendicularly the sample surface, see Figure 1. The width of the scrape-off layer at this position is about 20 mm, visible by a clear mark on the samples after one exposure. During a discharge the plasma streams along magnetic field lines from the midplane onto the samples (or divertor plates). Each sample analysed in this work was exposed for one discharge in deuterium with neutral beam injection (3.4 MW D for MO substrate samples and 3.85 MW H for graphite substrate samples), see also Figure 4. The target temperature during and after the discharge was measured in some cases with a chromel-alumel thermocouple. After the exposure the samples were analysed using Auger electron spectroscopy (AES) and sputtering with 1 keV Ar+ ions to obtain depth profiles. Ar+ ions have a very small preferential sputtering effect on TIC and TiNI and therefore it can be assumed that Ar sputtering does not strongly modify the original depth profiles. The depth scale given in Figure 3(a),(b) is called ‘nominal’ because it is calculated using the experimental ion fluence and extrapolated literature values of the sputtering yield of Y= 1 for both coatings, the exact sputtering yield for our samples
* Humboldt-Fellow, permanent address: Institut fiir Allgemeine Physik, TU Wien, Austria.
23
f Taglauer,
P Varga and K frtl:
Analysis
of TIC and TIN coatings
exposed
to fusion
plasmas
ASDEX
1’1
7
r
7 ‘1’T
TiN Probe
/
c ‘I’“1
‘I-
“I
on MO
x
exposed
.
not exposed
near
seporotr~x li
Divertor Plates
l x
b Multipole Coil
x
Separatrix Serape
Sk
- off
Layer
tix
*-x--x-x-_x_-x-
Radius (m)
)
. .. . . . .
a=-.
01’
Main Plasma
ii
-d’j-J_ 10L
FLUENCE
LL
-1
10-2
-l
I
(lOIL I
I
J 1oL
Ar*/cm2) LLLuiiLLuu.
1
NOMINAL
,-IO3
d
10-l
-x
-‘m-J_
’
10’
I$
101
DEPTH
102
(nm)
L
Figure I. Cross-section position
of the ASDEX in the upper divertor chamber.
tokamak
showing
the
x Prebombordment
probe
l
3 being unknown. The surface structure was examined by scanning electron microscopy (SEM). For comparison with well defined ion bombardment effects, identical samples were bombarded with I keV H: ions (which is equivalent to 500 eV H+) and subsequently analysed.
500 eV Hi
Unbomborded
a a 2 4
2
u &= 1
0 I
10’
10‘
FLUENCE
TIC
on
TIN
MO
on
MO
Figure 3. (a) AES depth profile on MO. (b) AES depth profile hydrogen pre-bombardment.
( 10IL
10J Are/cm2
)
(I keV Ar ’ ) ofa plasma-exposed TiN layer (I keV Art) of a TiC layer on MO with
Results and discussion
TiC
on
Graphite
Figure 2. SEM micrographs 24
TIN of plasma-exposed
on Graphite samples.
Inspection ofthe exposed samples reveals that the strongest effects of plasma interaction can be detected within the 20 mm wide region of the scrape-off intersection. Figure 2 shows SEM micrographs of these areas. The TiN and TIC layers show molten spots which are typical for unipolar arcs”. The layers also show damage, blister-like structures and are in some areas almost completely removed from the MO substrate. The adhesion to the graphite substrate appears to be stronger and at the same time the lateral cohesion is weaker due to the granulated structure of the
E Taglauer, P I/arga and K Ertl: Analysis
of TIC and TiN coatings
exposed
ASDEX D-DISCHARGE
IN H2
*
13908
600 600
0 ” % 5 m
m 200
0
0.5
15
10
_
_^ 1”
IO
t Is)
Figure 4. Plasma current I,, line averaged density 1,, and sample temperature during a divertor discharge in ASDEX with neutral beam injection.
Therefore, lateral stresses cannot build up to the same as for the MO substrates, and thus no blistering and peeling off is observed for graphite substrate samples. Depth profiles of TIN and TIC on MO are shown in Figure 3(a),(b). The TIN sample was exposed in ASDEX, the TiC sample bombarded with H+ ions. Both profiles show a depletion of the N or C component into a certain depth. This depletion has been observed before and is typical for preferential sputtering effects13. The analysis of our samples demonstrates that the ASDEX-exposed samples exhibit a high depletion to a much larger depth than those bombarded by 500 eV H+ in the simulation experiment. The ASDEX samples also show overlayer deposits of C, 0, Fe and S which must have developed during the discharge. A summary of the analysis of our samples is given in Table 1. It shows the maximum value for the depletion given by the ratio (Ti/X),,, and the depth of the altered layer characterized by the value T as defined in Figure 3(b). The fact that all ASDEX samples on MO show a higher depletion than those bombarded by 500 eV H+ indicates that the average energy of the D+ ions from the plasma is well below 500 eV, because lower energies result in higher depletion13. This is in accordance with measurements of the plasma parameters in the
coating. extent
Table 1. Maximum depletion of the lighter element, measured by the Auger peak-to-peak ratio (Ti/X),,, and depth of the altered layer T given in 1 keV Ar + ion fluence for samples exposed in ASDEX and samples bombarded by 500 eV Hi. TiC on MO
TIC on graphite
TiN on MO
TiN on graphite
ASDEX (separatrix) (Ti/X),,, T(Ar+ cm-‘) 500eV H+
4 1.0 x 10”
0.2-2 1.7 x 10”
12-15 1.7 x 10”
8 5.2 x lOI
(Ti/X),,, T(Ar cm-‘)
2.2 2.8 x lOI
4.4 3.2 x lOl6
::: x 1016
to fusion
plasmas
divertor 14, even taking the sheath potential into account. The total deuterium fluence must be of the order of magnitude required for equilibrium surface concentrations, i.e. above about 10” cm-’ 13. The comparison is complicated by the fact that TIN layers do not show an equilibrium surface concentration even after a fluence of 10” H+ cm-‘. For graphite substrates the diffusion of carbon to the surface at high temperatures could obscure the depletion effects caused by sputtering. The larger depth T in spite of the low particle energy is explained by the overlayer of impurity atoms (C, 0, Fe, S). The origin of these atoms is not entirely clear at this point. A large amount can be deposited as contaminants with the plasma, impurity fluxes of that kind have been reported before”. There is also the possibility of an additional contribution due to segregation from the bulk through the coating, e.g. S from MO or C from graphite. The sample surface temperature during the discharge can become very high, during neutral beam injection close to the separatrix a value of about 950°C was measured as a lower limit, see Figure 4. It is intended to clarify the role of segregation in future experiments. In summary it has been shown that significant changes are observed in TIN and TIC coatings due to the action of the divertor plasma. The depletion of the lighter constituent can be explained by preferential sputtering effects using particle fluxes and energies which are compatible with the divertor plasma parameters. In addition to the erosion, deposition of C, 0, Fe and S overlayers is observed. The high heat flux during neutral beam injection can result in removal of most of the coating on molybdenum in the area near the separatrix intersection.
Acknowledgements assistance of K Gehringer, G Nagleder, F Schuster and M-L Hirschinger is gratefully acknowledged as is the fruitful collaboration with Dr Riidhammer.
The technical
References ’ Proc of the Int Conf Series on Plasma Surface Interaction (PSI) and hoc VIth PSI in Nagoya, JApAn, 1984; J Nucl Mater: 1281129 (1984). ’ D M Meade, Nucl Fusion, 14, 289 (1974). 3 K J Dietz ef al, J Nucl Mater, 128/129, 10 (1984). 4 Y Hirohata et al, J Nucl Mater, 122/123, 1160 (1984). 5 J L Cecchi and TFTR Group, J Nucl Mater, 128/129, 1 (1984). ’ Y Shimomura et al, J Nucl Mater, 128/129, 19 (1984). ’ M Still and F Wesner, Proc 10th Symp on Fusion Engineering, Philadelphia, USA, lP20 (1983). 8 P Varga and E raglauer, J Nucl Mater, lll/llt, 726 (1982). 9 T Yamashina, Proc 9th Int Vacuum Conyr and 5th Int ConfSurJScience, Madrid, p 614 (1983). lo Metallwerke Plansee, Reutte, Tirol Austria. ‘I E Taglauer, J Nucl Mater, 128/129, 141 (1984). I2 K Ertl R Behrisch and B Jiittner, Proc 1 lth Europ Conf Contr Fusion and Plastk Physics, Aachen, 11, 385 (1983). l3 E Taglauer, Appl Surf Sci, 13, 80 (1982). l4 Y Shimomura, M Keilhacker, K Lackner ahld H Murmann, Nucl Fusion, 23, 869 (1983).
25