Nuclear Instruments and Methods in Physics Research B 108 (1996) 300-304

~a~lnter.act~s

with Materials & Atoms

EL"iEVlER

Structural changes and tribological behaviors ofnitrogen ion-implanted tantalum of:

W.l. Wang, T.M. Wang " X.J. Wang

Department ofMuteriuls Science, Lanzhou University. I...m~/lOu 730000. China

Received 3 January 1995; revised form received 28 July 1995

AbstractSingle-crystal tantalum sheets were implanted by 110 keV ntrogen ions [0 a dose of 5 x 1017 ions/cm 2 at a temperature

less than (OO°e. The structural changes and the concentration depth profiles of the implanted layers were characterized bygtancing-angle X-ray diffraction (GXRD). selected area ditfraction (SAD) and Auger electron spectroscopv (AES),respectively. The micrchardness, the friction coefficient and .ne wear rate of the specimens against hardened GCri5 steelballs were also determined before and after the implantatir a . Scanning electron microscopy (SEM) and electron probemicro-analyzer (EPMI\) were used to analyze the wear tracks, The results showed that there exist dev iations between thech;· .·acterizatio,~ results of AES and GXRD or SAD. The AES measurement suggests the formation of the tantalum nitrideswi rh a Il'~:dmum 'N/ Ta ratio of 112, while the GXRD and SAD reveal the formation of bee Ta(N), fcc TaN and a traceamount of hcp Ta:N. This can be explained by considering the inhomogeneous distribution of nitrogen atoms inmicro-rcgicns : the enrichment of nitrogen atoms in local micro-regions lead s to the formation of fcc TaN~ however. theunfavorable structural compatibility between bee Ta and hcp Ta 2N hinders the fo rmation of hcp Ta 2N in the regions wherethe NITa ratio reaches' 12. The detected trace amount of the hcp Ta 2N phase in the implanted layers can be considered asan after effect of nitrogen loss from the originally formed nitrides. The results also showed that the tribological properties ofthe To, surfaces were improved due to the implantation. II is believed that the implantation-induced Ta(N), fcc Ta"!, and hcpTa 2N phases are responsibi,' for this Improvement.

J. Introduction

It is well !~nown that the nitrogen ion (N +) implanta­tion can improve the surface properties of metals bychanging their compositions and structures in ~he surfacelayers. Because of this reason, the N I implantation hasbeen studied for many years and many nitrogen-metalsystems have been investigated in order to clarify theformat ion mechanisms of various metal nitrices and theirinfluences on surface properties [1.2]. Among these stud­ies, one of the well-investigated system is N-Ti system.The earlier reports for this system generally believed th?!only TiN yielded after N + implantation, although thecomposition range of TiN covers that of Ti 2N [3]. How-

" This research was partially supported by the National NaturalScience Foundation of China and the Open Laboratory of SolidLubrication, Lanzhou Institute of Chemical Physics. ChineseAcademy of Sciences.

• Correspondmg .author. Tel. +86 931 8843000 3806, Fax.. ~ '>'1 f<385076.

ever, very interestingly. Rauschenbach [4] and Zhou [5]claimed that Ti2N was detected at high dose N + implanta­tions, such as at 2 x 1018 ionsy'cm", This reverse sequenceof titanium nitride formation was attributed to the struc­tural compatibility between the matrix and the new phasebeing formed [5}.

A similar but less studied system is the N-Ta system.To the author's knowledge only several studies have beenreported in the past few years fc- this system. Wilson [6]identified the presence of tetragonal Ta 4Ns precipitatesafter N + implantation in thin Ta films . Belli et al. [7]reported the formation of Ta 2N an-I TaN phases at a doseof I X 1017 ions/cm2, while at a h:~her dose c 5 X 10 17

ions/ cm2, only fcc TaN was detected. Recently. Raole et

al. [8] reponed the N+ implantation induced phases inpolycrystalline Ta thin films. Fcc TaN, fcc riaNO O4 andhcp Ta sN6 phases were observed at a dose of I X 10;7ions/cm2

• while at a dose of 5 X 1017 ionsj/crn", hcpTa 2 N and hep TaN phases were detected in addition to thephases present in the specimen with lower dose. As to theformation mechanism of the low stoichiometry phase hcpTa 2N at a high dose, the authors explained it in terrns of

Ol()l' .• •\/96/$15.00 CO 1996ElsevierScience B.V. All rights reservedSSDl OI68 -583X(95)00980·9

WJ. Wt/lIR et at. / Nucl. Instr. and Merh. in Phys. Res. F fJ8(1996) 300-304 301

Fig. .2. GXRD pattern typically observed for the implanted sur­faces.

SO 60 70 80 90 1002 THETA ldeg.)

nitrogen loss from the previously formed nitride. Thus wesee that the earlier reports on the N-Ta system have noagreement on the phase changes and a further study is stillneeded in order to explain the discrepancies.

We have implanted Ta sheets with 110 keV N+ andinvestigated the implantation induced compositional andstructural changes of the Ta surfaces. Tribological proper­ties of the Ta specimens were also measured before andafter the Nt-implantation. In the present paper, we willreport our primary results and compare them with theprevious reports.

VIa:w

20 30 40

11•

2. Experimental

The single-crystal Ta sheets \\ ith ( I II) orientation wereprepared by mechanical polishing with alumina powderfollowed by electropolishing in an acrd solution of Uf andH2S04, The purity of the Ta is 99.99%. After ultrasoniccleaning In a mixture of alcohol and acetone, the polishedspecimens. with a size of 0 22 x 1.8 rnm, were implantedby 110 keY Nt- to a dose of 5 X 1017 ionsycm? . The:angle between the incident ion beam and the [I III direc­tion of the crystals was 7° so as to prevent the channelingeffect from happening. During the implantation, the cham­ber pressure was maintained at 1.3 x 10-} Pa. The beamcurrent density was 30 JJ.A/cm 2

• The substrate holder wascooled by flowing water and the temperature rise of thespecimens was estimated not to be greater than -~ 100°C.

The concentration depth profiles in the implanted layerswere measured by AII,,"er electron spectroscopy (AES)with a 3 keV Ar + sputtering. The implantation inducedstructural changes of the surface layers were characterizedby both glancing-angle X-ray diffraction (GXRD) andselected area diffraction (SAD) witt a transmission elec­tron microscope. The specimen used in SAD was prepared

by backthinning with jet electropolishing followed by Ar 'ion beam sputtering thinning. The glancing angle betweenthe incident X-ray and the specimens' surface was 5'.

In order to study the implantation induced tribologicalchanges. a Vickers' microhardness tester by applying aload of 10 g was used to determine the change in micro­hardness due to the implantation. A ball-on-plate triborne­ter with " single directional sliding ball was used tomeasure the friction coefficient and the wear. The ballswere hardened GCrl5 steel (Hv = 710 kg/mm 2• for de­tailed composition of the steel, see Ref. [9]) and were 3rnm in diameter. The track length, the load, and the slidingspeed of the steel balls were 5 rnm, 300 g, and 90mmyrnin, respectively. The friction coefficient was auto­matically given by using a force transducer. After certainslidings, the wear area across the wear track was deter­mined by means of a surface profilometer and the wearrate was then obtained after plotting wear area vs. slidingnumber (in this study. the maximum sliding number was800). Scanning electron microscope (SEM) and electronprobe micro-analyzer (FPMA) were used to analyze the

Table IPhase identificanon results of GXRD pattern and SAD part..m

100 deAl deAl dlA)Peal Phase and t hkl)number GXRD SAD standard a identified

.... 80 2.5095 2.509 2.5030 fcc Ta.'II(l1 I)Z

2 2.3258 2.3380 bee Ta( I iO)wu

60 :l.3230 hcp Ta!:\l( 101)ex:w 3 2.1842 2.107 2.i690 fcc TaN(200)a.

4 1.5469 1.558 1.5310 fcc TaN(220)u 40 1.5220 hep Ta 2N( 110)-%:5 1.4099 1.3910 hcpTa 2N(l03)f:

20 6 1.3492 1.3500 bee Ta(21 I)«7 13014 1316 1.J070 ICcTaNOI J)

0 8 1.2910 1.2940 hcp Ta2N( 112)9 1.254 1.255 1.2510 fcc TaN(222)

0 10 20 3Q 10 1.1896 1.156 1.1687 bee Ta(220)

SPUTTER TIME (min,) II 1.0314 1.0453 bee Ta(310)12 0.967 0.9343 bee Ta(222)

Fig. I. AES depth profiles of N. Ta, C. and 0 for the N~

implanted Ta specimen. a The standard values from JCPDS files.

302 WJ. Wang etal./NlKl.llIStr. andMeth. in Phys. Res. 8 JOB (1996) }OO-J04

Fig. 3. SAD pattern of Ta after the implantation of 5 X 1017

ions/cm2 dose.

Tal,le 2Implantation induced changes in microhardness, friction coeffi­cient and wear rate of the 11 surfaces

Specimen Mi .rohardu: ,s friction Wear rate[kg!mm21 coefficient [10- 5 mm3/Nm]

Unimplanted 134 -0.9 11.6Implanted 208 0.20 4.1

morphologies and the compositional distributions of thewear tracks,

3. Results

3./. AES. GXRD and SA.D results

Fig. ! shows the AES depth profiles of N. Ta, C. and 0for the N~ implanted Ta specimen. It can be seen that a

Fig. 4. SEM and EPMA examinations for the wear tracks of unimplanted Ta: (a) micrograph of a track (sliding number. Ii(Wi); (b) Fecartography corresponding to (a). Magnification: 4L X .

Fig. 5. SEM and EPMA examinations for the wear tracks of implanted Ta: (a) micrograph of a track (slidin~' number: f~)I); (b) Fecartography corresponding to (a). Magnificat-on: 415 x .

WJ. Wani: et ul./Nucl.lnslr. and Melfi. in Phys. Res. B iOB( 1(96) 300-304 303

Gaussian distribution of N is formed, This is in agreemen:with the results predicted by LSS theory [IOj. This al;oimplies that the channeling effect is not evident under .iurimplantation conditions. The higher C content and thesmall 0 peak in the near-surface region are considered tobe a result of contamination during the ion implantation.From this figure. the maximum NITa atomic ratio in theimplanted layer can be estimated to be approximately 1/2.which corresponds to the stoichiometric composition ofTa2N.

During the GXRD study. the specimen holder did notrotate, while specimen-position-independent peaks wereobserved except for the specimen-dependent diffractionp.aks of bee Ta, A typical GXRD pattern for the ionif.planted surfaces is shown in Fig. 2. The pattern wasobtained under a certain specimen position at which thediffraction peaks of the Ta substrate (and also of the solidsclution bee Ta(N») were the lowes'. in intensities. Theidentification results of the peaks are listed in Table I. It isinteresting to note that. except for the diffraction peaks ofbee Ta (and of bee Ta(N». there exist strong peakscontributed by fcc TaN - a phase that is not in agreementwith (N/Ta)m:u = 112 predicted by our AES results.Mean·...hile, although the corresponding peaks of hcp Ta 2 Nare ve.y weak (No.5 and 8) or ove rlap with that of bee Ta(No.2) and fcc TaN (No.4). we cannot rule out their traceexistence in the layer. Thus. from the GXRD results. thebee Ta (and also bee Ta(N». fcc TaN and a trace amountof hcp T32N phases may exist in the implanted layers.

Fig. 3 shows a SAD pattern of the implanted layers .The identification results for the SAD pattern are alsolisted in Tab le I. Most diffraction rings in the SAD patternalso originate from fcc TaN. only very weak diffractionrings of bee 'fa (and of bee Ta(N» can be detected. For thehcp Ta2N phase . only one possible diffraction ring (No.4)appears. but the ring overlaps with that of fcc TaN.Similarly. from the data. we cannot rule out the traceexistence of the hcp Ta 2N phase . Because the thinnedsample came from the ion implanted region where most Tahad converted into N-Ta phases due to the high doseimplantation. the intensity decreases of the diffractionrings of bee Ta is understandable.

3.2. Tribological properties

Table 2 shows the changes of microhardness. frictioncoefficient and wear rate due to the N + implantation. ;tcan be seen mat. after the N ' imnlanta .ion . the microhard­ness of the Ta surfaces increase" while me friction coeffi­cient and the wear rate of the 1 a surfaces decrease. TheSEM and EPMA resul-s for the .~ eilr tracks of unimplantedTa specimens showed that the tracks have the cnaracteris­tics of adhesion. plastic deformation and microcracks. Ar :1

higher sliding number. rhe feature of the wear track isnearly the same as that of the lower slid ing number exceptfor less microcracks. A typical SEM micrograph for the

unimp lar ted tracks is shown in Fig . 4a. Cartography offeindicated that the component transfer of Gerl5 steel ontoth.: wear (racks of the specimen occurred (Fig. 4b). Theseimply that the wear mechanism is mainly adhesive type forthe unimplsnted specimens. For the N + implanted speci­mens . however, the feature of the wear tracks changesobviously: no matter how big the sliding number is. thewear tracks always display parallel grooves and localcracks along the grooves (Fig . Sa). N(\ component transfercar. be observed from the cartography of Fe (fig. 5b). Thissuggests th:lt the ..Jhesive tendency is restrained great!ydue to the ir.•plantation. and the wear mechanism is mainlylight abrasive type for the ':;.planted specimen.

4. Discussions

In ',he AES measurement, the Auger electrons comefrom an area of -:3 X 3 mrrr', then . the obtained AESdepth profiles are macro-scale characterization results.Suppose the micro-distribution feature of the nitrogenatom s is the same as that shown in the AES depth profile.then the maximum atomic ratio of NITa in the implantedlayers should be 1/2. and the formed phase with thehighest nitrogen content should be Ta2~ " However, asindicated in the GXRD and SAD patterns, except for thepresence of bee Ta (and of interstitial solution bee Ta{N»,the dominant phase present in the implanted layers is fccTaN. which corresponds to NITa ~ III. This stronglysuggests that. the AES depth profiles are macro-scalecharacterization result s only . they cannot reveal themicro-scale distribution feature of the nitrogen atoms. Thedistribut ion of the nitrogen atoms in micro-reg ions may beinhonr-geneous actually, that is. nitrogen might enrich insome local regions. promoting the formation of fcc TaNphase which corresponds to NITa = III.

Since the nitrogen content varies in IOCOlI micro-regions.then in addition to nitrogen enriched regions, there mustbe some regions where the nitrogen concentration is muchlower to favor the formation of TOl, N phase with It> I"However. our GXRD and SAD patterns demonstrate thatthe ex '<ting phase with a low stochiometry is hcp 1 :\21':only a-d its amount seem s to be trace in the implantedlayers . This imr-!;es that. although the micro-fluctuation ofnitrogen atoms has created legion.. with lower nitrogenconn nt. it is not sufficient enough for the formation ofphases vith low stoichiometries. In other words. except forthermodynam ic connitions, such as an appropriate NITaratio, the growth kinetic conditions may be more importantfor the growth of hcp T32N phase and of other Ta,Nphases with x > I ; under our impla ntation conditions. theformation (If large amounts of N- Ta phases with lowerstoichiometr ies is hindered due to insufficient growth ki­netic factors . It is interesting to note that the present phasechanging feat ure is totally th, same as that observed in theN-Ti system [5J. thus we reno to believe that the important

.104 WJ W:I1';et al./Nucl. Instr. and Melh. in Phy«. Res. B 108 (19961300-304

growth kinetic factor for our 1'-Ta system mentionedabove is also the same as that in the N-Ti system, that is,the unfavorable structural compatibility between bee Taand hcp Ta 2 N.

Raole et ,II. [8} characterized the N + implantationinduced phases rn Ta foils. They chimed that ihe hcpTa 2N phase Wl~ formed after high dose Implantation of5 X 10 17 iom/cm 2

• In tact, a detailed observation for-theirGXRD pattern shows that the diffraction peaks of the hcpTa 2N phase are very weak, then we believe that therelative amount 0;' the hcp T:l2N in Raole's specimen maybe also very 511"11. which is in agreement w-th our GXRDand SAD observations.

As to the f"·.mation mechanism of the hcp Ta 2N phaseat high dose impl mtations. Raole suggested that it shouldbe an after effect of nitrogen oss from the originallyformed nitrides wi.h a higher nitrogen ratio [8]. the loss ofnitrogen is caused by further implantation. ln the N-Tisystem. a similar mechanism was also supposed. that is.the Ti 2N phase can precipitate from the ..Iready formedTiN matrix at high dose implantations through the move­ment of nitrogen atoms [5]. Thus we think that, in ourexperimental conditions, the appearance of the trace hcpTa 2N should be an after effect of nitrogen loss from theoriginally formed nitrides. but not a phase formed from theregions where nitrogen content is smaller.

Because hard phases like fcc TaN and hcp Ta2 N areformed after the implantation, the increase in rnicrohard­ness is reasonable. Of course, the formation vf interstitialsolution Ta(N) and ion-induced defects is also responsiblefor this hardness increase. but it is difficult to describethese contributions separately. Similar to the N-Ti system,the for-nation of dispersive hard phases should reduce theadhesive tendency of pllTP ::~etals. thus the reductions bothin friction coefficiert and in wear rate are much morepredictable, and the change of wear mechanism fromoriginal adhesive type to implanted light abrasive type canabo be expected. Because the implanted layer is han: ... rthan the Ta substrate, it could not fit the ...:'"JI derormatir nof the substrate during the sliding. so. in order to releisethe stress. local cracks were formed 011 thr worn surtu .C··.

(Fig. Sa). Detailed analysis indicates that there exist ahigher Fe: content in these cracks (Fig. 5bl. This C'1n beexplamed in tenus of the scraping effect of the cracks onthe GCrlS steel balls.

5. Conclusion-

I) The AES measurement reveals that the depth profileof the implanted nitrogen a oms has a Gaussain distribu­tion with a maximum NIT~ ratio of I,' .,. which r."rre­"pond<: to the sDichiomehc compcsition of Ta 1':.

2) The GXRD and SAD patterns agree well 1!1 identify-

ing the structure of the N +- implanted layers. They suggestthe formation of bee Ta(N), fcc TaN, and a trace amountof hcp Ta 2N phases.

3) The deviations between the AES and the GXRD orSAD results suggest that the nitrogen distribution inmicro-regions is inhomogeneous actually. The enrichmentof nitrogen atoms in local micro-regions leads to theformation of fcc TaN phase.

4) The unfavorable structural compatibility betweenbel' Ta and hcp Ta2N hinders the formation of hep Ta 2N

in the regions where the N/Ta ratio reaches 1/2. Thetrace amount of hcp Ta2N phase in the implanted layers isconsidered to be an after effect of nitrogen loss from theoriginally formed nitrides.

5) Because of the formation of bee Ta(N), fcc TaN andtrace hcp Ta 2N phase .., the rnicrohardness of the Tasurfaces increases: the friction coefficient and the wearrate of the Ta surfaces against hardened GCrl5 steel ballsdecrease: the predomina.it wear mechanism of the Taagainst the hardened GC,IS steel balls changes from theoriginal adhesive type to {b~ light abrasive type.

I\cknowlcdgemcnt~

Prof. Z.L. Xiao and Mr. J.Y. Wang of the Institute ofModem Physics, Chinese Academy of Sciences are thankedfor their help in performing the nitrogen ion implantarion.Prof. X.S. Zhang and Dr. J. Zhang of the Open Laboratoryof Solid Lubrication. Lanzhou Institute of ChemicalPhysics. Chinese Arademv of Sciences are also acknowl­edged for their help in measuring the tribological proper­ties.

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