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Crater formation on the surface of metals and alloys during high power ion beam processing PDF

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NuclearInstrumentsandMethodsinPhysicsResearchB148(1999)154–158 Crater formation on the surface of metals and alloys during high power ion beam processing V.A. Shulov a,*, N.A. Nochovnaya b aMoscowAviationInstitute,4Volokolamskoyeshosse,125871Moscow,RussianFederation bAll-RussianInstituteofAviationMaterials,17RadioStreet,107005Moscow,RussianFederation Abstract Thee(cid:128)ectofhigh-power,pulsedion-beamirradiationandvariousmethodsofthepreliminarysurfacetreatmenton thecraterformationprocesswasexaminedusingAugerelectronspectroscopy,X-raydi(cid:128)ractionanalysis,andscanning electron microscopy. The crater distribution density, sizes and shape, along with their microhardness and chemical compositioninsideandoutsidethemweredetermined.Asaresultoftheseexperiments,themostprobablemechanisms of crater formation on the surface of refractory alloys were established. (cid:211) 1999 Elsevier Science B.V. All rights re- served. PACS: 34.50.D Keywords: Ionbeam; Craterformation; Serviceproperties; Surface 1. Introduction characteristics of irradiated components are the most important factors. The objective of the It is well known that crater formation on the present research is the investigation of such e(cid:128)ects solid surfaces, when irradiating them by high- as the preliminary surface treatment and irradia- power pulsed, ion-beams (HPPIB) [1] leads to the tion conditions (particularly pulsed ion current catastrophic deterioration of the property level density) on the crater formation process. (fatigue strength, oxidation resistance, salt corro- sion resistance etc.) [2–4] of irradiated compo- nents. As a result, the determination of crater 2. Experimental formation mechanisms and the development of methods allowing to decrease the negative influ- The studied samples were made of bars, pro- enceof already formed craters upon the operating duced of the following refractory alloys: VT8M (Ti–6Al–3.7Mo–0.2Fe–0.3Si), VT9 (Ti–7Al– 3.8Mo–0.2Fe–2.5Zr–0.3Si), VT18U (Ti–6A1– *Correspondingauthor.Tel.:+70951584424;fax:+7095 3.4Mo–0.2Fe–4.5Zr–1.5Nb–0.25Si–3Sn), VT25U 1582977;e-mail:roman@fipc.ru (Ti–7A1–2.5Mo–1.5W–0.2Fe–2.5Zr–0.25Si), 0168-583X/98/$–seefrontmatter (cid:211) 1999ElsevierScienceB.V.Allrightsreserved. PII:S0168-583X(98)00845-3 V.A.Shulov,N.A.Nochovnaya/Nucl.Instr.andMeth.inPhys.Res.B148(1999)154–158 155 EP866sh (Fe–2.5Ni–16.5Cr–1.6Mo–1W–0.3Nb– 5.5Co–0.6Si), and EP718ID (Ni–33Fe–2.4Ti– 16Cr–1.4A1–5.2Mo–3.5W–1.5Nb–0.6Mn–0.3Si) withtheuseofmachining.Theirradiationofthese targets (14·7·3 mm3) were carried out using the TEMPaccelerator[4]bycarbon(70%)andproton ion beams under the following conditions: ion energy – 300 keV (E), pulse duration – 50 ns (s), andtheioncurrent densityinapulse(j)aswellas thenumberofpulses(n)werevariedfromj(cid:136)20A cm(cid:255)2, n(cid:136)1 up to j(cid:136)220 A cm(cid:255)2, n(cid:136)100. After theirradiationsometargetswerestudiedbyAuger electron spectroscopy (AES), X-ray di(cid:128)raction analysis, scanning electron microscopy (SEM), opticalmicroscopy(OM),andmicrohardness(H ) l Fig. 1. SEM-micrographs of VT9 alloy surface treated by measurements. Other irradiated samples were HPPIB(j(cid:136)120–140Acm(cid:255)2;n(cid:136)3pulses):a,b–round-shaped subjected to vacuum annealing for 10–60 min at craterwiththeconvexityandconcavityinthecentre;c–round- the given operating temperature for each alloy, in shapedmulti-ringcrater;d–adjoiningcrater. particular, VT8M, VT9-500(cid:176)C; VT18U, VT25U- 550(cid:176)C; EP866sh-600(cid:176)C, and EP718ID-650(cid:176)C). Aftercompletingofthepointedheattreatmentthe (Fig. 1(a)) and the concave centre (Fig. 1(b)); studying cycle was repeated again. In this case it round-shaped multi-rings craters (Fig. 1(c); ad- should be noted that some of targets were also joining craters (Fig. 1(d)); craters in the form of subjected to various treatments as grinding, pol- ellipse, crack and disk including also faceted and ishing, diamond smoothing, surface plastic defor- dent-shapedones.Analysisoftheresultspresented mation, chemical etching, ion implantation, and in Tables 1 and published in Refs. [5–7] gives a oxidation prior to irradiating by HPPIB. The possibility to trace the e(cid:128)ect of irradiation condi- craterdistributiondensity(q),theirdiameter(D ) tions on the type, size, and distribution density of av and form along with the microhardness and craters through the surface. It is obvious that the chemical composition inside and outside them ioncurrentdensity(j)increasefrom40upto110A were determined during the performed studies. In cm(cid:255)2leadstotheincreaseofthecraterdistribution addition to these studies, there were some pre- density (q) from 0 up to 1100–1400 mm(cid:255)2. The scratched samples, which were also irradiated and tendency to the decrease of j down to 200–500 investigated. mm(cid:255)2 and the growth of craters takes place with the further increasing j (120fi180 A cm(cid:255)2). How- ever,avalueofqisfirstlyincreasedwithariseofa 3. Results and discussion pulses number (n(cid:136)1, 2 ,3) and then it decreased (n(cid:136)5–7). It is impossible to establish any depen- The results of the performed studied allow to dence between q and n values if the ion current divideallcratersintotwogroupsdependingonthe density achieves high values (j>160–180 A cm(cid:255)2). stage of their formation. They are the primary On the base of the fixed experimental results one craters formed after the single-pulse irradiation more important conclusion can be made, that the and the secondary craters fixed either after the formation of round-shaped and faceted craters multiple-or single-pulse irradiation and at the occurs mainly when irradiation with the ion cur- stage of the final heat treatment (annealing after rentdensityof100–150Acm(cid:255)2 andtheformation HPPIB irradiation). Depending on the crater of dent-shaped and faceted craters and cracks shape they can be classified into the following takes place at very high values of the ion current types:round-shapedcraterswiththeconvexcentre density (Fig. 2(a)). The depth of the primary 156 V.A.Shulov,N.A.Nochovnaya/Nucl.Instr.andMeth.inPhys.Res.B148(1999)154–158 Table 1 DistributiondensityandaveragediameterofcratersformedonVT9alloysurface(E(cid:136)300keV;s(cid:136)50ns) Ioncurrentdensity Pulsenumber Craterdistributionden- Averagediameterofcra- Averagedepthof j,Acm(cid:255)2 n,pulses sityq,mm(cid:255)2 tersD ,lm cratersh ,lm av av 60 1 0 0 0 80 1 420(cid:139)20 5–10 1.5(cid:139)0.2 100 1 580(cid:139)40 5–40 1.5(cid:139)0.2 120 1 340(cid:139)30 15–50 2.0(cid:139)0.4 160 1 260(cid:139)20 28–38 2.2(cid:139)0.8 60 2 0 0 0 80 2 200(cid:139)20 5–7 0.5(cid:139)0.2 100 2 1200(cid:139)200 5–7 0.5(cid:139)0.2 120 2 1060(cid:139)200 10–40 1.0(cid:139)0.4 60 3 0 0 0 80 3 1480(cid:139)200 5–15 1.0(cid:139)0.4 100 3 1100(cid:139)200 5–15 1.0(cid:139)0.4 120 3 1100(cid:139)200 10–40 1.0(cid:139)0.4 160 3 160(cid:139)20 20–60 2.0(cid:139)0.2 60 5 0 0 0 80 5 920(cid:139)100 5–10 0.5(cid:139)0.2 100 5 1100(cid:139)200 10–20 1.0(cid:139)0.2 120 5 500(cid:139)100 25–60 1.0(cid:139)0.2 160 5 260(cid:139)30 30–70 2.0)0.5 60 7 0 0 0 80 7 660(cid:139)70 10–40 0.5(cid:139)0.2 100 7 900(cid:139)100 20–50 1.0(cid:139)0.2 120 7 680(cid:139)50 20–60 1.5(cid:139)0.3 160 7 460(cid:139)30 15–25 2.0)0.5 craters achieves 1–2 lm while the secondary cra- bombarding the already melted surface of targets ters have the depth of not more, than 0.1–0.5 lm. byerodedparticles.Table1illustratesthee(cid:128)ectof The microhardness values near craters are signifi- theioncurrentdensityontheaveragesizeofformed cantly lower than the appropriate H values, fixed craters.Itisobviousthattheriseofjfrom80upto l on the ‘‘free’’ surface. H (craters) is equal to H 180 Acm(cid:255)2 leads to theaverage diameter increase l l (surface) only after the final annealing of irradi- (D (cid:136)5fi100 lm) of formed craters, however, if a ated samples. j(cid:136)160–180 A cm(cid:255)2 the surface craters sizes are The chemical composition of craters being changeddependingonanumberofpulses.Thein- formed as a result of high-current irradiation tensivegrowthofnewphases(carbides,oxides,and (j>160–180Acm(cid:255)2)andalsothesecondarycraters oxicarbidesofSn,Zr,Mo,W,Si,Al)withincraters are similar to the chemical composition of the with the changed composition is observed during studied alloys. Meanwhile, if HPPIB processing the final annealing. It is explained by the rise of wascarriedoutattheaveragevaluesofj(100–150A microhardness within craters as a result of the cm(cid:255)2),craterscontainedaconsiderableamountof vacuum annealing. In this case the formation of Mo,Zr,Si(titaniumalloys)andAl,Mo,W(steels separate1–20lmcrystalscanoccurwithincraters. and nickel-base alloys). Moreover, it was revealed These crystals can have various forms (dendrites, thatsomecraterscontainedtheelements,typicalof cubicandpyramidal crystals,Fig.2(b).Theinten- a structural material of the cathode system (for sive growth of crystals depends on the di(cid:128)erent example, Sn). It points to the fact of the cathode conditionsofheattransferatthecratersboundaries erosion under HPPIB irradiation e(cid:128)ect, and when andthehighresidualstressesnearcraters. V.A.Shulov,N.A.Nochovnaya/Nucl.Instr.andMeth.inPhys.Res.B148(1999)154–158 157 Some results of the second stage of experi- ments are presented in Table 2 and Fig. 3. It is evident, that the preliminary diamond smoothing of sample surfaces allows to obtain the ‘‘crater- free’’ surface when irradiating by using any ion current density (Fig. 3(a)). The surface of the sim- ilar prescatched samples after the irradiation con- tainscratershowever,theyareconcentratedalong the scratch (Fig. 3(b)). One more important result of these experiments is the formation of craters, separate areas of which were not melted if the targetsweresubjected tothe preliminarydiamond smoothing and final chemical etching(Fig. 3(c)). According to our experimental and published data [5–7] the variety of the following crater for- mationmechanismscanbepossible:theformation of a gas-plasma phase during the ablation of sur- face volatiles; the bombardment of the melted surface by the accelerator structure elements; and the availability of nonstable conditions during meltingduetothedi(cid:128)erenceinphaseandchemical compositions, impurity distributions and disloca- tion densities of targets. The formulated crater formation mechanisms give a possibilitytoproposeanumberof methods to reduce this surface phenomenon including: changes in ion accelerator electrode material (in particular, the target material instead of steel); Fig. 2. SEM-micrographs of VT18U alloy surface treated by target surface preparation such as fine polishing; HPPIB(a–j(cid:136)180–200Acm(cid:255)2,n(cid:136)3pulses);b–j(cid:136)120–140A follow-on HPPIB treatment at low ion current cm(cid:255)2, n(cid:136)3 pulses) with the following annealing (b – density (j<60 A cm(cid:255)2) for titanium alloys and T(cid:136)550(cid:176)C). Table 2 E(cid:128)ectoftheinitialsurfacetreatmentonthecraterformationprocessduringtheirradiationbyHPPIB(E(cid:136)300keV;j(cid:136)120–140A cm(cid:255)2;s(cid:136)50ns;n(cid:136)1pulse) Treatment Craterdensity,q,mm(cid:255)2,(cid:139)40mm(cid:255)2 VT8M VT9 VT18U EP866sh EP718ID Grinding 290 320 270 120 150 Polishing 60 90 70 70 90 Diamondsmoothing ) ) ) ) ) Plasticdeformation 480 460 420 210 24 Chemicaletching 450 500 430 290 310 Oxidation 420 380 410 280 330 IonimplantationbySm 330 340 360 140 160 158 V.A.Shulov,N.A.Nochovnaya/Nucl.Instr.andMeth.inPhys.Res.B148(1999)154–158 4. Conclusion The major conclusions from this study can be summarized as follows: 1. Craterformationprocesstakesplaceonthesur- faceofsolidsduringirradiationbyHPPIB.This phenomenonisduetothecathodematerialero- sion (technological factor) and the nonstability in the physical and chemical state of the irradi- ated surface. 2. In order to reduce the negative e(cid:128)ect of crater formation upon the properties of targets can be proposed: changing the accelerator diode system material; to use the fine polishing when preparingthesurfacefortheirradiation;tocar- ryoutthefinalHPPIBtreatmentatlowioncur- rent density (crater-free irradiation); and to perform the final heavy ion implantation. Acknowledgements This work was supported by the Russian State Committee of Higher Education. References [1] G.E.Remnev,V.A.Shulov,J.LaserandParticleBeams11 (1993)707. [2] V.A.Shulov,N.A.Nochovnaya,G.E.Remnev,F.Pelleran, P.Monge-Cadet,J.Surf.andCoat.Tech.99(1998)74. [3] V.A.Shulov,N.A.Nochovnaya,G.E.Remnev,Proceedings of the Eighth Internernational Conference on Titanium, UK,vol.3,1996,p.2126. [4] D.J. Rej, H.A. Davis, J.S. Olson, G.E. Remnev, M.A. Fig. 3. SEM-micrographs of VT25U alloy surface treated by Opekunov, V.A. Shulov, N.A. Nochovnaya, K. Yatsui, J. HPPIB (j(cid:136)120–140A cm(cid:255)2; n(cid:136)3 pulse (a, b), n(cid:136)1 (c) pulse Vac.Sci.Technol.A15(3)(1997)1089. afterchemicaletching. [5] K.Yatsui,J.LaserandParticleBeams7(1989)733. [6] V.A. Shulov, G.E. Remnev, N.A. Nochovnaya, I.G. Polykova,I.F.Isakov,J.Surface(Russia)7(1994)117. (j<80 A cm(cid:255)2) for nickel-base alloys and steels); [7] V.A. Shulov, G.E. Remnev, N.A. Nochovnaya, I.G. and the final implantation by heavy ions at low Polykova, V.N. Kosheev, I.G. Karpova, I.F. Isakov, J. angles. Surface(Russia)11(1995)24.

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