Surface erosion and re-emission behaviour of TiC coatings under hydrogen and helium ion bombardments

Surface erosion and re-emission behaviour of TiC coatings under hydrogen and helium ion bombardments

Journal of Nuclear Materials 1 I1 & 112 (1982) 848-X51 North-Holland Publishing Company 848 SURFACE EROSION AND RE-EMISSION BEHAVIOUR HYDROGEN AND H...

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Journal of Nuclear Materials 1 I1 & 112 (1982) 848-X51 North-Holland Publishing Company

848

SURFACE EROSION AND RE-EMISSION BEHAVIOUR HYDROGEN AND HELIUM ION BOMBARDMENTS M. SAIDOH,

R. YAMADA

OF TiC COATINGS

UNDER

and K. NAKAMURA

Japan Atomic Energy Research Institute, Tokal-mura, Naka-gun, Iharaki-ken, Japan

Extensive studies of surface erosion and re-emission behavior of candidate low-2 coatings have been performed under 200 keV Hz and He+ ion bombardments at temperatures ranging from 50°C to 5OO’C. The results on TIC coatings will be presented here, since TIC coatings are expected to be the most promising materials from the results of their heat load tests [ 1,2]. Target samples for bombardments were two kinds of TiC coatings with a thickness of 20 pm on molybdenum sub-

strates made by physical vapor deposition (PVD) and chemical vapor deposition (CVD), respectively. In the case of CVD-TIC two kinds of target surfaces were prepared, as-deposited and mechanically polished. Details of the coating processes are described elsewhere [3]. The re-emission rates of implanted species during 200 keV Hz and He+ bombardments were determined by measuring the partial pressure rises of H, and He in the target chamber with a quadrupole mass spectrometer

Table I Critical fluences for onset of re-emission and for appearance of surface deformation, number of exfoliated layers. thickness of exfoliated layer and deformation mode in TiC coatings bombarded with 200 keV Ht and 200 keV He+ ions at various target temperatures. respectively.

Particle

flux of 200 keV Hz

and 200 keV He+

ions were 4.4~ lOI

atoms/cm2/sec

r Sample sate

Coati metho

implanted ions

As-deposited As-deposited PVD As-deposited As-deposited



+ H2

5.0

10'"

As-deposited As-deposited As-deposited

WC

Polished

-

"HZ As-deposited

(a)

Values

were

obtained

2.0 1 10'"

50

2.1

2.5 ' 10'"

500

1.3

by dividing

0022-3115/82/0000-0000/$02.75

fluence

by number

I 10:. 10"

of exfulidted

Q 1982 North-Holland

2.2

layers.

10”

and 2.2X lOI

atoms/cm2/sec.

M. Saidoh et al. / Surface erosion and re-emission

and the sample surfaces were subsequently observed by a scanning electron microscope. Details of the bombardment and re-emission measurement apparatus have been described elsewhere [4]. The results are summarized in table 1. In case of hydrogen bombardment the re-emission behavior is strongly dependent on the target temperature but weakly on the kind of samples. The re-emission rate at 50°C is very low until the critical fluence is reached; above this fluence it increases rapidly and then, gradually, reaches saturation level. Above 300°C the re-emission rate begins to increase immediately from the start of the bombardment and reaches saturation level. The re-emission rate and its saturation level are higher for higher

849

temperatures at the fluence examined and the re-emission rate at 500°C is almost the same as the incoming flux from the calibration of the re-emission rate made by using the re-emission data for molybdenum at 500°C. Hydrogen bombardments at 50°C and 300°C cause blistering. The surfaces are eroded at 50°C more than at 3OO”C, namely the surfaces at 50°C are eroded with repetitive blistering for PVD- and polished CVD-TIC as shown in fig. 1. Blistered surfaces have also been observed in TIC coatings on POCO graphite bombarded with 20-60 keV D+ at room temperature [5]. Above 400°C no surface topography change occurs at the fluence examined in this study. This is to be expected from the re-emission data, since rapid diffusive loss of

Fig. 1. Scanning electron micrographs of as-deposited PVD-((a), (b) and (c)), as-deposited CVD-((d), (e) and (f)) and polished CVD-((g), (h) and (i)) TiC surfaces: (a), (d) and (g) non-irradiated; (b), (e) and (h) 200 keV Hz bombardment to a fluence of 5.0X 1Ol8 atoms/cm* at 50°C; (c). (f) and (i) 200 keV He+ bombardment to a fluence of 2.5 X IO’s atoms/cm2 at 500°C.

850

M. Saidoh et al. / Surface erosion and re-emission

hydrogen generally prevents the accumulation of implanted hydrogen to form bubbles and hence prevents blister formation. In case of helium bombardment the re-emission behavior depends strongly on the kind of samples, particularly on the surface topography. Typical helium re-emission data at 5OO’C are shown in fig. 2. Successive occurrences of gas bursts are observed in PVD- and polished CVD-TiC after the critical fluence is attained. The re-emission curves show periodic structures, each of which consists of numerous short duration gas bursts, at the fluence interval of l-2 X lOI atoms/cm2. This periodic nature of re-emission curves becomes less prominent at higher ffuences. The number of these periodic structures of the re-emission curve agrees well with the number of exfoliated layers, giving clear evidence that the repetitive large scale surface exfoliation is accompa-

nied by a substantial helium burst. The average thickness of the exfoliated layer was -0.4 pm as seen in table 1. The surface erosion rate due to the repetitive surface exfoliations mentioned above, therefore, becomes - 10 Tic molecules per incident He+. In the case of as-deposited CVD-TIC, on the other hand, the re-emission rate increases rapidly, and then gradually, without gas bursts after the critical fluence is reached and the surface is eroded due to blister formation. Qualitatively similar results of surface erosion and reemission behavior are also observed for the bombardments at 50°C indicating that helium induced surface erosion of TiC coatings is not influenced by the target temperature examined in this study but strongly by the sample surface. As seen in fig. 1, the reduction of surface erosion observed in as-deposited CVD-TIC for both hydrogen

PVD-TIC

23

_ CVD-Tic

25

5

As-deposited o Polished

l

3

24

( I017 He /cm2 )

FLUENCE

5 B A

%012345

FLUENCE Fig. 2. Helium

re-emission

_

( toi7 He/cm”

rates as a function

1 of fluence for (a. top) PVD-and

(b) CVD-TiC hombarded

at WW’C.

M. Saidoh et al. / Surface erosion and re-emission

and helium bombardments indicates clearly that the surface roughness is very effective on the reduction of blister formation/exfoliation, since as-deposited CVDTiC reveals rough surfaces (see fig. Id). The usefulness of rough surface on the reduction of blister formation has been demonstrated previously [6]. The present results indicate the possibility of preventing surface erosion due to blistering for hydrogen bombardments at temperatures above 300°C. TIC, therefore, can be a useful coating material for JT-60 first wall, since the JT-60 tokamak will produce hydrogen or deuterium plasmas. The fact that the prevention of large exfoliations is found in as-deposited CVD-TiC bombarded with helium implies that TIC coatings may also be applicable for the future D-T tokamak first walls.

851

Acknowledgements The authors express their gratitude to Drs. Y. Murakami, Y. Obata and Y. Iso for their continuous encouragements.

References [I] K. Nakamura, R. Yamada, M. Saidoh and Y. Murakami, these proceedings. [2] R. Yamada, K. Nakamura, M. Saidoh and Y. Murakami, these proceedings. [3] Y. Murakami, T. Abe and H. Nakamura, these proceedings. [4] M. Saidoh, R. Yamada and K. Nakamura, J. Nucl. Mater. 102 (1981) 97. [S] A.S. Rao and M. Kaminsky, Thin Solid Films 83 (1981) 93. [6] K. Sone, M. Saidoh, R. Yamada and H. Ohtsuka, J. Nucl. Mater. 76/77 (1978) 240.