Simpkins, J.E.

Investigates the properties of 304L stainless steel for possible use as inner vacuum liner in ORNAK

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-PASSIVATION OF THE ORMAK VACUUM LINERJ. E. SispkinsOak Ridge national LaboratoryOak Ridge, Tennessee 37330AbstractThe inner vacuum liner in ORMAX, which wasconstructed of type 30UL stainless steel, has beencoated with a 1-2 micron gold film. The resultant sur-' face is considerably more inert than the bare stainlesssurface and reduces plasma-wall interactions. We found,however, that some of the constituents of the stainlesssteel diffuse through the gold film when the couple isheatedQto ~ 300°C- Further investigation indicated thata 500.A platinum filn between the gold and stainlessacts as a diffusion barrier for many of these constitu-ents. Diffusion profiles of oxygen, silicon, sulphur,and carbon suggest that platinum acts as a barrier toprevent diffusion of all these elements. Mass scans ofthe gold surface with platinum underlayer show greatlyreduced peaks for most of the impurities found en thebare stainless surface.Surface effects and plasma-wall interactions whichoccur in thermonuclear experiments deserve carefulevaluation since contaminants produced by these reac-tions cool the plasma. The percent cf power loss is afunction of impurity density, but also rises veryrapidly as the Z of the contaminant increases.1 Plasrr.a-wall interactions which contribute to plasma contamina-tion include:1. Sputtering. Energetic neutrals which are producedby charge-exchange near the plasma boundary andenergetic ions may bombard the wall with sufficientenergy to dislodge particles and eject then intothe plasma. Chemical sputtering may occur if thebombarding particle reacts chemically with aparticle on the wall. Enhanced sputtering may beobserved if adsorbed gases are present on the wall,due to the high sputtering yield of these looselybound particles.2. Electron Impact. Energetic electrons nay beproduced at the wall by photon emission from theplasma. These electrons will be turned back bythe magnetic field and stril-te the wall causingelectron-induced desorption and/or adsorption.3. Piffusion and Surface Migration. Interstitialsmay diffuse or migrate to the surface and mayeventually enter the gas phase.k. •Thermal Pesorption. As the vail temperature is" increased, interstitials, sorbed gases, or thewall material itself nay be desorbed into theplasma.We have investigated the properties of type 3ChLstainless steel for possible use as the inner vacuualiner in ORKAK. We also investigated the propertiesof this material coated with gold, since gold iscommonly used to achieve a passive surface. However,our investigation shows that a 500 A platinum under-layer significantly improves the performance of goldon stainless as a passivu surface.Research sponsored by the y.S. Atomic Energy Coaaissionunder contract with the Union Carbide Corporation.Perhaps one of the more serious disadvantages inthe use of gold is its relatively high sputtering yield.This depends, of course, on the type and energy of theparticle which bombards the surface. Figure 1 showsthe sputtering yield for gold bombarded by K + as afunction of energy as reported by Furr and Fingeld.^This yield is about two orders of magnitude below thatfor gold bombarded by Ar+ and about a factor of fivebelow the yield for stainless steel.Koyinsa3 observed that if a bare stainless sampleis bombarded by a 1.5 keV electron beam in the presenceof carbon monoxide, the Auger peak heights for carbonand oxygen increase as a function of tine, as illustratedby Fig. 2. He then turned off the primary beam andlooked at these peak heights only at the beginning andend of the time period. He found no increase in thiscase and concluded that the electron beam induced theadsorption of carbon monoxide on the bare stainlesssurface. He deposited 10 atom layers of gold on thesample and repeated the experiment. No increase in thepeak heights of carbon and oxygen was observed,indicating that no carbon monoxide was being adsorbedon the gold surface. He also observed that the peakheights of iron and sulphur, as well as carbon andoxygen, decreased after coating the sample with gold.Since our vacuua liner may be. subjected to tempera-tures as high as 2C0°C, we investigated diffusion ofthe constituents in type 30'H, stainless steel into thegold film. Calculations based on the bulk propertiesof gold indicated that diffusion through the gold filmwould be insignificant as long as the temperature doesnot exceed 200°C. It is well known, however, thatenhanced diffusion may occur along grain boundaries infilms and that the diffusion rate may be many orders ofmagnitude greater than for bulk material. Electro-migration is also a well known phenomenon which mayoccur in films carrying an electrical current. In thiscase, material is transported from one area of the filmto another in the direction of current flow, resultingin the destruction of the film. We have not observedany effects due to electromigration.Trisone and DroTjek* observed diffusion in as-deposited films of Au-Ti and Pt-Ti. Diffusion occurredduring deposition with substrate temperatures between100-150°C. Howevar, they found that in the case of thePt-Ti couples, isostructural compounds of TiPt andTioPt were present at the interface after annealing.After the formation of TijPt, they observed that furtherdiffusion of Ti into Pt ceased for temperatures up to500°C. This indicates that a diffusion barrier hadbeen formed. Annealing the couple at 200°C was suf-ficient to produce this barrier. Figure 3 illustratesa schematic of the diffusion profile which they observedfor the Ti-Pt couple.We were interested in determining the extent ofdiffusion from the stainless steel liner into the goldcoating at moderate temperatures (~ 300°C, 1 hr). Ifsignificant diffusion occurred, we were interested Indetermining the effectiveness of platinum as a possiblediffusion barrier for the gold/stainless system. Asillustrated in Fig. U, three sets of 50 mm square type30^1, stainless substrates were prepared for analysis.The first group was bare stainless: the second groupwas coated with a gold film, 2000 A in thickness; andTu;:1:the third group was sequentially coated with 500 Aplatimum and 2000 A gold. The three groups, bare stain-less, gold/stainle;;3, and gold/platinum/stainles?.,were subsequently heat treated simultaneously in vacuumfor 1 hr at 300°C.These samples were investigated, utilizing the IonMicroprobe Kass Analyzer. This instrument detects allelements and permits the measurement of isotopic ratios.The detection limit is about 0.1 ppn, surface resolutionis approximately 20,000 A and depth resolution a fewatomic layers near the surface. A snail area on thesample surface is bombarded by a 20 keV beam of mono-isotopic nitrogen ions, which causes atoms to be sput-tered from this surface. Some fraction of these atomsare ionized and are subsequently analyzed according totheir mass-to-charge ratio utilising a mass spectrometer.Both negative and positive secondary ion spectrawere obtained from each sample, covering the mass range0-300 amu. Figure 5 shows the positive spectra obtainedfrom the surface of the bare stainless sample. Majorpositive ions detected include iron, chromium, nickel,•sodium, potassium, and calcium. The major ions detectedin the negative spectra from this sample are carbon,oxygen, flourine, sulphur, and chlorine.Both the positive and negative spectra from thegold/stainless sample surface are very similar to thoseobtained from the bare sample. Figure 6 illustratesthe positive spectra obtained from this saaple. A goldpeak occurs at mass 197? Au0+ at 213, and some higherpeaks are also present, e.g., Au-Fe at 253- Theseresults indicate that many of the constituents from thestainless steel diffused through the gold film.Figure 7 exhibits the positive spectra obtainedfrom the gold/platinum/stainless sample. In the posi-tive spectra, the 23 (Ka) ana 39 (X) peaks are reduced,and the major peak occurs at mass 197 (Au). Smallpeaks occur at 191* and 195> indicating platinum dif-fusion into gold. In the negative spectra, only oxygenand CK were present but in reduced amounts. The absenceof significant amounts of sulphur, silicon, and carbonindicates that the platinum underlayer produces a bar-rier to these elements in this sample.Profiles were obtained by sputtering through thefilms and into the substrate, while monitoring a pre-selected mass peak. Figure 8 shows the profilesobtained from the gold/stainless sample in this mannerfor positive secondary ion intensities of masses197 (Au), 195 (Pt), 56(Fe),and 29 (Si) as a functionof sputtering time. Also shown here are the negativesecondary ion intensities of masses 16 (0), 28 (Si),32 (S), and 12 (C). The intensities of iron, oxygen,sulphur, silicon, and carbon all increase before thegold/stainless interface is reached, indicating dif-fusion of these elements.Figure 9 illustrates the profiles obtained fromthe gold/platinum/stainless sample by monitoring thesame ion intensities. The sputtering rate was increasedfor this series, so that the ion intensities wereincreased and the sputtering time decreased. As theprimary beam bombards the gold surface and progressesthrough the film, the secondary ion intensities of 0,S, Si, and C do not increase until the beam approachesthe platinum/stainless interface. Some diffusion ofiron and platinum into the gold is indicated. A peakin the silicon profile at the platinum/stainless inter-face -indicates migration of this ion to the surface ofthe stainless during heat treatment. There is an un-explained peak in the oxygen profile well beyond thisinterface.Figure 10 shows a liner segment vhich has teencoated with gold/platinun. Each segment io baked invaccum for 12 hrs at 300°C- It is further cleaned ina glow discharge, then eoa£ed sequentially withplatinum and gold by an ion plating process in a singlepump-down cycle. These segments are*then weldedtogether to form the torus.ConclusionsThe results of this investigation indicate thatmany of the constituents in stainless steel diffuseinto gold films at a rate which is significantly greaterthan for bulk material.Platinum appears to act as a barrier between thegold film and stainless steel substrate after the heattreatment described here, although the nature andcomposition of this harrier are not known at this time.Mass scans of the gold platinum/stainless sample showgreatly diminished peaks for the elements between mass50 and 60, e.g., Fe and Cr (Fig. 7)- These sconsalso indicated that the gold surface is relativelyfree of impurities.The gold/platinum coating should provide an-improvement over a stainless steel surface for plasmaexperiments where the maximum energy o.? the particleswhich bombard the walls are a few hundred eV. Platinumalone would not be desirable because of its sorpticnproperties. For more energetic particles in futureexperiments, some other liner material may be moredesirable because of the high Z of gold and its rela-tively high sputtering yield. We ai-e presently lookingat some ether materials for possible use in High FieldOHMAK.AcknowledgmentThe author wishes to thank J. F. McLaughlin andSteve Ciisty of the Y-12 Development Division for theiranalysis of the samples with the Ion Microprobe MassAnalyzer.References1. M. Kaminsky, "Surface Phenomena Leading to Plasma.Containment and Vacuum Wall Erosion in FusionReactors and Devices," Proc. Intl. Working Sessionson Fusion Reactor Technology, Oak Ridge NationalLaboratory (1971) p. 86.2. A. K. Furr and C. R. Fingelc., J. Appl. Phys. Ill,1739 (1970).3* S. Koyima, T. Narusawa, and C. Hayashi, J. Vac.Sci. Technol. % 302 (1972).k. 1. C. Tisone and jr. Drobek, J. Vac. Sci. Technol.2, 271 (1972).- M O T I C t -Thii report wii prepared as an account of worksponsored by the United States Government. Neitherthe United States nor the United States Atomic EnergyCommission, nor any of their employees, nor my oftheir contractors, subcontractors, or their employees,makes any warranty, express or implied, or assumes anylegal liability or responsibility for the accuracy, com-pleteness or usefulness or any Information, apparatus,product or process disclosed, or represents that its usewould not infringe privately owned rights.0.03? 0.02aCOBoaUJ 0.0100RNL- DWG 7 3 - 339084 2 3 4 5 6 7PROTON ENERGY (keV)Yields os o Function of Energy for H* on Gold. A. K. Furr ond C. R.Finfgeld, J.AP., 4 1 , 4739, 1970.40FIGURE 1ORNL-OWG 73-3392BARE STAINLESS STEEL BARE STAINLESS STEELxsUfXUJo.ceuiGOLD DEPOSITED" 6 Torr60L0 OEPOSITEOIxfO"8 Torr "* •'," , n ::Au. . , ,C -8 12 16 0 20TIME EXPOSED TO BEAM (minj40 60Electron Induced Adsorption from S. Komiya,!. Narusawoand C. Hoyoshi, Jour not Vac. Set', and Tech. 9,302,1972.FIGURE 2GRAIN BOUNDARY0&NL-DW6 73-3391Pt GRAINDIFFUSIONZONETi3 Ptat 50-100 JS)Schematic of Diffusion Profile in the TiPt DiffusionCouple. From T. C. Tisone and J. Drobek, Journal Vac.Sa\ and Tech. 9 , 271, 1972.FIGURE 3ORNL-DWG 73-3389BARESTAINLESSAu ONSTAINLESS5Ox5OxO.25mrnTYPE 304 STAINLESSSTEEL SUBSTRATES500A PLATINUM(SPUTTERED)2000 A GOLD(SHUTTEREO)HEAT TREATMENT INVACUUM (2XKT 6 Torr)1hr AT 300°CAu/PtON STAINLESSPreparation of Samples for Diffusion Studies.FIGURE 4ORNL-DWG 73-33O3AO.OO1Moss Scan of Bore Stainless Steel Surface.Mass Range: 0-15C amu.FIGURE 5QRNL-DWG 73-3302A0.001Mass Scan of Gold Surface, Gold/Stainless Sample.Mass Range 0 - 300 amu.FIGURE 63I1010.,0.01nr\tmr ITTHIII1 23 3 9IULILJUJL Li..197ORNL-0W6 73-3304AIIIIIII 1 lillllIllilll 1 IIMass Scan of Gold Surface, Gold/Platinum/Stainless Sample.Mass Range 0 - 3 0 0 amu.FIGURE 7ORNL-DWG 73-33O7A10 150 290 430 570 710SPUTTERING TIME (sec)850 990Selected Ion Intensities as a Function of SputteringTime-Gold/Stainless Steel.FIGURE 8ORNL-DWG 73-3301A10 150 290 430 570 710SPUTTERING TIME (sec)850 990Selected Ion Intensities as a Function of SputteringTime- Gold/Platinum/Stainless Steel.FIGURE 9%>. , - • " ••vVV- •>' * - i *y ••••..•.V.-.1.'n..."fiftI;;wA SEGMENT OF THE ORMAK VACUUM LINER WHICH HAS BEENCOATED WITH GOLD/PLATINUM.FIGURE 10

Passivation of the Ormak vacuum liner

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