Journal of Nuclear Materials

7/27/2019 Journal of Nuclear Materials

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Journal of Nuclear Materials 60 (1976) 321-3290 North-Holland Publishing Company

MEASUREMENTS OF THE EROSION OF STAINLESS STEEL, CARBON, AND SIC BY HYDROGENBOMBARDMENT IN THE ENERGY RANGE OF 0.5 - 7.5 keVR. BEHRISCH, J. BOHDANSKY 1, G.H. OETJEN 2, J. ROTH, G. SCHILLING,s and H. VERBEEKMax-Planck-Insti tut fi ir Plasmaphysik, EURA TOM -Associat ion, 8046 Garching b.Mi inchen, Fed. Rep. Germany

Received 23 December 1975

Total erosion yields by sputtering and blistering for 1 to 15 keV H: bombardment at normal Incidence have beenmeasured by weight loss of 304 stainless 7.teel, pyrolytic graphite, carbon fibres, glassy carbon and Sic. The erosionyields are in the range of 3 X 10e3 to 2.6 X lo- atoms per incident hydrogen atom. Observation in the scanningelectron microscope shows that blisters occur in stainless steel and Sic at doses of 5 X lo* particles/cm, but disap-pear at doses of 5 X 10 particles/cm*. The surface roughening observed depends largely on grain orientation. Oncarbon no blistering could be found. After bombardment the carbon surfaces are generally more smooth than before.

Les taux derosion et la formation de cloques par bombardment, sous incidence normale, avec des ions de 1 a 15 keVHi de lacier inoxydable 304, du graphite pyrolitique, de fibres de carbone, du carbone vitreux et de SE ont 6temesures par la methode de perte de poids. Les taux derosion sont de 3 X 10d3 a 2,6 X lo-* atome par atomedhydrogene incident. Des observations au microscope electronique a balayage montrent la formation de cloques ades doses de 5 X 10s particles/cm pour lacier inoxydable et SIC, mais elles disparaissent a des doses de 5 X 10particules/cm. La rugosite observde depend beaucoup de lorientation des grains. 11ny a pas de formation de cloquespour le carbone. Sa surface est, en g&t&al, plus lisse apres bombardement.

Edelstahl304, Siliciumcarbid, pyrolytischer Graphit, Kohlefasern und Glaskohlenstoff wurden mit 1 bis 15 keVH+?-Ionen Senkrecht einfallend beschossen und die totale Erosion durch Zerstaubung und Blasenbildung uber den Ge-wichtsverlust gemessen. Die Erosionsausbeuten liegen zwischen 3 X 10M3 und 2,6 X lo- Atomen pro einfallendemWasserstoffatom. Beobachtungen im Rasterelektronenmikroskop zeigen, dass auf Edelstahl und Sic bei Dosen von5 X 10 Teilchen/cm* Blasen (blisters) entstehen, aber bei 5 x 10 Teilchen/cm* wieder verschwinden. Es wurdeeine Aufrauhung der Obertichen beobachtet, die stark von der Kornorientierung abhangt. Auf Kohlenstoff konntenkeine Blasen beobachtet werden. Nach dem Beschuss sind die Kohlenstoffoberflachen im allgemeinen glatter als vorher.

1. IntroductionIn todays plasma experiments and in future fusion

reactors the first wall will be bombarded with plasmaions and charge exchange neutrals having a broad ener-gy distribution with the maximum between 1 and 10keV. This causes two serious problems: (1) The wall willbe eroded by sputtering and blistering [l-4]. (2) Theplasma will be contaminated by the sputtered atoms ofthe wall material [l , 5-71. Concerning the erosion by1 Guest scientist from CCR EURATOM, Ispra, Italy.2 Present address: Toledo Scales, Columbus, Ohio.3 Present address: Thermonuclear Division, ORNL, Oak RidgeTenn. 37830, USA.

321

hydrogen and helium ions, there are up to now onlyvery few experimental data available for stainless steel,carbon and Sic. These materials recently gained high \interest for the vacuum vessel and limiters especiallyin tokamak type plasma machines. After bombardmentof Nb with 500 keV He ions [8] and stainless steelwith 300 keV He ions [9] exfoliation and the forma-tion of new blister generations had been observed.From these observations serious erosion of the wallwas anticipated [8]. On the other hand it has beenshown [lo] that for Nb bombardment with He ionswith less than 15 keV blistering does not contributesignificantly to erosion. Many more measurements ofthe erosion yields of relevant materials after bombard-


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322 R. Behrisch et al. /Erosion of stainless steel, carbon, and Sic

ment doses which are much larger than the critical do-ses for blistering are necessary especially with energiesbelow 15 keV. In this work the high current ion sourcebuilt for neutral particle injection into the stellaratorW VII [I I] was used to achieve bombardment dosesas large as 2.5 X 1020 particles/cm2.

2. Experimental methodThe experimental set up is shownschematically in

fig. 1. The central part of the ion beam from an OakRidge type high current injector [ 111 was magnetical-ly analyzed and entered a viton sealed stainless steelbombarding chamber. This was evacuated by a turbo-molecular pump with 120 Qlsec pumping speed.For the bombardment, instead of protons, generally$ or H; ions with the corresponding velocities havebeen used. Molecular ions give much higher currentdensities at the low energies. Theoretical estimatesshow, that for these light ions the collision cascadesinitiated by the two atoms impinging simultaneouslydo not overlap. Thus the yields should not be differentfor atomic and molecular ions with equal energies peratom. During bombardment with particle current den-sities up to 5 mA/cmZ (achieved with Hl and Hi ions)the residual gas pressure rose to 3 x 10-6 torr, predo-minantly hydrogen. The beam hit the target at normalincidence. It was confined by an aperture to a diameterof 5 to 9 mm. Three additional apertures with largerdiameters were used to suppress secondary electrons.The ion doses onto the target were determined by acurrent integrator. For bombarding energies below 4

Fig. 1. Experimental setup (schematic). 5 Obtained by courtesy from R. Cow, University of Wisconsin.

keV a retarding field could be applied to the target andthe apertures in front of it.

In each experiment up to five targets could bemounted on a water cooled copper holder. They weremoved one after the other into the ion beam and bom-barded with different doses. The targets were 0.5 to 2mm thick discs with a diameter of 12 mm and a weightof 0.4 to 0.5 g. They were mechanically polished. Somestainless steel targets were also electrolytically polished.But no significant difference in the erosion yield wasfound. The carbon targets were mounted on stainlesssteel holders which were then screwed on the targetholder to avoid breaking away of small carbon piecesduring handling.

The stainless steel targets were cut from 304 material.Carbon was investigated in the form of pyrolytic gra-phite from two suppliers (Union Carbide and Le Car-bone) glassy carbon (Le Carbone), and graphite fibres*.The Sic-targets were cut from Refel specimens de-livered by J. Crimson & Co.

The temperature of the targets was dependent onthe incident beam power. Measurements with a thermo-couple and a Thermodotinfrared pyrometer showedthat the targets generally reached temperatures between50 and 150C. In some cases for stainless steel the tar-get temperature went up to 5OOC, when the highestfluxes were used.

To determine the total erosion yield the targetswere weighed before and after bombardment on a mi-crobalance Mettler M5 (sensitivity


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R. Behri sch et al. /Erosion of st ain less steel, carbon, and Sic 323

(3)That means that S can be determined by weight lossmeasurements only if I is known. On the other hand

this method can be used to determine r(ni), if the sput-tering yield is known [ 121.

Trapping of the bombarding ions depends stronglyon the ion target combination. Efficient trapping of highdoses of the injected gas takes place only if the heat ofsolution is positive and the temperature is high enoughto allow the implanted gas to diffuse into the bulk, butlow enough that thermal release is negligible [ 131. If theheat of solution is negative, diffusion may be very fastin perfect crystals, as was shown for He in well annealedW [14]. In this case however, lattice defects alwayscreated within the range of the incident ions are veryeffective sinks for the implanted gas. Thus the implantedions are generally trapped only within their range untilthe concentration reaches a certain level (-20 to 50atomic percent) where the surface layer is damaged tosuch an extent that release through the surface as wellas blistering occurs [ 151. Beyond the critical dose forblistering the gas concentration has saturated and trap-ping reaches zero [ 161. Under these conditions themethod of measuring erosion yields by weight lossworks at very high bombardment doses, when theweight gain by the trapped gas can be neglected. Aswill be justified later this latter situation was assumedto apply for these measurements, i.e. trapping has beenneglected in determining the erosion yield.

3. ResultsIn fig. 2 erosion yields of stainless steel after bom-

bardment with 5 and 7.5 keV protons (measured with10 keV and 15 keV H ions) are plotted for differentdoses up to 2.5 X 102 a particles per cm2. From fig. 2we see that for doses above 3 X 10lg particles/cm2there is no dependence of the erosion yield from theion dose within the experimental errors, The pointsshown were measured with fluxes of 0.5 to 2.4 mA/cm2, which resulted in target temperatures between50 and 500C. No visible temperature effect in theerosion yield could be observed.

The mean erosion yields determined by weight loss

1O-2t; 8 75 keV

4 .*Ft 6c mean val ue- - - - - - - -~_ _ _- * - - _ _ - -. .3$ 10- Z1 SkcV .s l-neon value ._________-----------._:: . .i i

I 1 . *0 5 10 15 20 25 30 35 LO I COUI I0

3 69 12 15 16 21 l d~pZbj t I t l eS/ cm21

bombardment doseFig. 2. Dose dependence of the erosion yield of 304 stainlesssteel by hydrogen bombardment with particle energies of 0.5and 1.5 keV.

using different ion doses are summarized in table 1for all investigated materials. For stainless steel theenergy range from 0.5 to 7.5 keV was covered. Theerrors indicated are estimated from the reproducibility.For Sic erosion yields for 5 and 7.5 keV are included.The yields for different forms of carbon - pyrolyticgraphite from two different sources, glassy carbon, andgraphite iibres - differ considerable from each other.Especially the different values for pyrolytic graphitefrom the two different suppliers indicate that the sput-tering yields depend strongly on the preparation of thematerial. During the bombardments of C, reported inthis work, the temperature did never exceed 150C.At higher temperatures the influence of chemical sput-tering had been observed [ 171. For comparison theHe-erosion yields of pyrolytic graphite at 6 keV is alsoincluded.

The surfaces were investigated also in the scanningelectron microscope @EM) after bombardment withdifferent doses. Some results are shown in figures 3 to


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324 R. Behri sch et al. /Erosion oj stai nless steel, carbon, and SicTable 1Average values for the erosion yields in target atoms per incident particle of stainless steel, different kinds of carbon and siliconcarbide. _____ ___-__ -____Material Ions Bombardment

energy per atom[kevlsatoms per incident particle

ss 304

SE (Refel) Hipyrolytic graphite(Le Carbone)pyrolytic graphite(Union carbide)

glassy carbongraphite fiberspyrolytic graphite

ti;H+3tffHSH;H;

He+

1.5 ( 5.9 f 1) x lo+5 ( 7.3 f 1) x 10F33 ( 7.6 ?I 3) x 10K33 8.8 x 10-3220.50.57.557.5332141333 2.6 x lo-* 16

5.9 x 10K3(10 i 1) x 1o-3( 3.2 i 0.5) x 1O-3

3.1 x .10-3( 7 f 0.9) x 1o-3

9 x 1o-3( 1.5 f 0.3) x lo-*2.4 x lo-*

2.6 x 1O-34.8 x 1O-32.3 x 1O-34 x 1o-31.4 x lo-*

1.8 x lo-

Number of experiments

--8841

21

1

1

6. As reported earlier [ 181 the formation of blisterswas observed on stainless steel (fig. 3a) and on Sic(fig. 4b) at doses of 4 X 1018 part/cmz. In no case therupture or the loss of blister covers was seen. At higherdoses no further generations of blisters were observed.

The blister covers are sputtered away (fig. 3b, 4c,4d). This disappearance of blisters had been reportedalready for He bombarded Nb [10,19]. With increasingbombardment doses the surface structure changesfurther and ridges and grooves appear (fig. 3b, c, d, 4c,d). Especially for stainless steel the development ofthe rough surface structure depends on the grain orien-tation. It develops at different doses for differentgrains (fig. 3), indicating that the sputtering yieldsmust be different for different crystallite orientations.

This effect is more pronounced for lower energies asshown by comparison of fig. 3c and 3d.

These observations in the SEM and the fact that theerosion yield does not depend on the dose demonstratethat the formation of blisters does not contribute tothe erosion in this energy range.For the different kinds of carbon in no case blisterformation after hydrogen bombardment could befound. After an initial roughening (fig. 5b) a smoothe-ning of the surface was always observed (fig. SC, Se).The surface becomes even smoother than the mechanic-ally polished surface. This smoothening effect of theproton beam was more pronounced at low energies(fig. 5c, 5e). For He bombardment a roughening ofthe surface was found (fig. 50.


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R. Behri sch et al. /Erosion of stai nless steel, carbon, and SC 325

Fig. 3. SEM-microgra~hs of steel surfaces after hydrogen bombardment with (a,b,c) 7.5 keV and (d) 0.5 keV. Bombardment waswith 15 and 1 keV H, ions.

4. DiscussionThe neglection of the trapping of the injected gas

in these measurements is justified by the following ob-servations:

1) Similar to the observations.for He bombarded Nbwe find that on the surfaces of stainless steel and Sicblisters appear in the initial stage of the bombardmentand disappear at higher doses indicating low diffusibili-ties.


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Fig, 4, SEWmicragraphs of %C-surfaces bombarded with 7.5 keV hydrogen to diffment doses. Bombardment was with 1J keVHG hns. The mar@ bright arms In b and c are, probably pure Si, as Rafekts fabricated with a surplus of Si.

2) For staimlless steel no temperature?: dependence of For all kinds of carbon no blisters were cYbeervedthe measured erosion yielda could be observed. any dose. Scherzer et al, f21], however, showed tha

3) Final& spufier&zg yie& measatred receuffy in our also in th& ease de&erium is trapped at $0~ doses F3aboratory f2Qt by ~~~~r f ~r ~~~~~~~~~~~~~whi ch re in the range of the injected ions.independent of trapping ar@ n good agreement ~4th For He bombardment of stainless steel at energiethe results of this work. above 100 ke\F severe erosion due to exfoliatioE of

I at.tk4&!Sse*


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328 R. Behri sch et al. /Erosion of t ain less steel, carbon, and Sicveral generations of blisters has been reported [22].These results are not necessarily in contradiction withthe measurements reported here: At higher energiesblistering and exfoliation are expected to be more severe,as the ranges of the ions are larger and the sputteringyields are lower. However, also in this case the blistersmay disappear at much higher doses and a rough sur-face may finally be reached which is also eroded onlyby sputtering.

In fig. 6 the erosion yields of stainless steel areplotted versus the energy of the bombarding particlestogether with the results of other investigations. Inthese plots also the yields for D+, He, and Ar ionsknown from the literature [23-271 are included. Asthese data are needed for the design of high tempera-ture plasma experiments and fusion reactors [l-7]we tried to connect the points by lines and extrapo-late these guided by theoretical trends. The arrows atthe abscissae in figs. 6 indicate lower limits for thethreshold energies as calculated by kinematic consi-derations.

For the special case of light ions there are a numberof older theoretical approaches to determine sputteringyields [28-311 which are reviewed in ref. [23]. Themost general sputtering theory has been worked out by

I:ig. 6. Hydrogen sputtering yields of stainless steel as a func-tion of energy from this work compared with data from otherauthors 123-26). He and Ar yields 126, 221 are shown forcomparison.

Sigmund [33,33]. According to this theory the sputteringyield is proportional to the energy deposited by the in-coming particles as nuclear motion in the surface layerand inversely proportional to the surface binding energy.The calculated yields are, however, generally higher thanthe measured values [33 1. This can be explained bythe approximations still used in the calculations and un-certainties in the surface binding energy. But the ob-served dependences on energy and on the ion speciesshould follow the theory. This predicts that the sput-tering yields for protons, deuterons, and tritons SH, St,,and ST at equal energies should follow the relationSH : SD : ST = 1 : 2 : 3. Using this formula we relatedthe yields for deuterons to the measured yields for pro-tons and that is why we draw the dashed line for deu-terons in fig. 6 below the rather old experimentalvalues of Guseva [24].

The data for carbon are strongly dependent on thespecific material. They disagree up to a factor of 10for specimens from different sources while they are re-producable within 50% when measured with one ma-terial. This indicates that the erosion of carbon is in-fluenced by factors involved with the fabrication ofthe carbon samples which are not understood untilnow. Carbon sputtering yields published recently byother authors [34,35] are within the range of datafound in this work. Since the origins of their samplesare different, it is hard to compare their results withours. According to Littmark [36] the maximum inthe yield for physical sputtering of C can be expectedat about 0.2 keV.

5. ConclusionsThe main results of this work may be summarized:

1n the energy range below 7.5 keV blistering does notcontribute to surface erosion of stainless steel and Sic.At high doses an equilibrium surface is reached withgrooves and ridges which is eroded only by sputtering.The observed sputtering yields for hydrogen on all ma-terials investigated here are below 3 X 1OF2 atoms perincident particle. The surface structure which is deve-loped after high dose bombardment should have thesame effects as.the honeycomb wall proposed recentlyfor fusion machines 1371.For all kinds of carbon a smoothening was observed.The erosion yields are not reproduceable for carbon


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R. Behri sch et al. [Erosion of stai nless steel, carbon, and SC 329

from different sources. The influence of uncontrolledchemical effects is very likely.

Acknowledgements

[ 161 R. Behrisch, J. Bottiger, W. Eckstein, J. Roth, B.M.U.Schemer, J. Nucl. Mat. 56 (1975) 365.(171 J. Roth, J. Bohdansky, W.P. Poschenrieder, M.K. Sinha,presented at the Int. Conf. on Surface Effects in Contr.Nucl. Fusion Devices, San Francisco, Febr. 1976, to bepublished in J. Nucl. Mat.

his skillful technical assistance. The SEM micrographswere made by Labor Dr. Klingele, Munchen, Adelgunden-

We are indebted to H. Vernickel for his constant sup-port of these measurements and to W. Ottenberger for

1191 J.G. Martel, k.St.Jaques, B. Terreault, G. VeiUeux, J.Nucl. Mat. 53 (1974) 142.

[ 181 H. Verbeek, W. Eckstein, Applications of Ion Beams to Metals eds. ST. Picreaux, E.P. EerNisse F.L. Vook (PlenumN.Y.. 19741 v. 597.

(201 H.v. Seefeld, H. Schmidl, R. Behrisch, B.M.U. Scherzer,presented (s. f 171).tr. 8.

References[ 1 H. Vernickel, Proc. 1st Topical Meeting on the Techno-

logy of Controlled Nuclear Fusion, April 74, San Diego,Vol. II, p. 347.[2] R. Behrisch, Nucl. Fusion 12 (1972) 695.

[3] I-1.Vernickel, Nucl. Fusion 12 (1972) 386.[4] B.M.U. Scherzer, Proc. 22. Nat. Symp. AVS, Ph~adelphia1975; J. Vat. Sci. Techn. 13 (1976) 368.[.!I] E. Hinnov, J. Nucl. Mat. 53 (1974) 9.[6] M. Kaminsky, IEEE Trans. N.S. 18 (1971) 208.(7) R. Behrisch, B.B. Kadomtsev, Plasma Physics and Con-

trolled Nuclear Fusion Research 1974, Vol. 2, p. 229,(I.A.E.A., Vienna 197.5).[8] S.K. Das, M. Kaminsky, J. Appl. Phys. 44 (1973) 25.[Y] W. Bauer, G.J. Thomas, J. Nucl. Mat. 47 (1973) 241.1101 J. Roth, R. Behrisch, B.M.U.Scherzer, J. Nucl. Mat. 57(1975) 36.5.

[Ill G. Schllling, Kerntechnik 16 (1974) 309.(121 J. Bohdansky, J. Roth, W.P. Poschenrieder, Int. Conf. onthe Application of Ion Beams to Materials, Sept. 1975,W~wiek, England.[32] P. Sigmund, Phys. Rev. 184 (1969) 383.1331 R. Wei~mann, P. Sigmund, Radiat. Eff. 19 (1973) 7.[34] M.I.Cuseva, V.M. Gusev, Conf. on Eng. Problems at[13] G.M.McCracken, Rep. Prog. Phys. 38 (1975) 241. and G.M.

McCracken, D.K. Jefferies, P. Goldsmith, Proc. 4th Int. Thermonucl. Reactors Leningrad, July 1974.Vat. Congress Manchester 1968, Vol. I, p. 149. [35] A. Wane, J.A. Wesson, Proc. 2nd Int. Conf. Atom. Ener-[14] E.V. Kornelsen, Radiat. Eff. 13 (1972) 227. gy 32 (1958) p. 414.1151 R. Behrisch, J. Bottiger, W. Eckstein, U. Littmark, J. [36] U. Littmark, private communi~tion.

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