Author's personal copy

Tribological behavior of NiTi alloy against 52100 steel and WCat elevated temperatures

M. Abedini, H.M. Ghasemi!, M. Nili AhmadabadiSchool of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran

A R T I C L E D A T A A B S T R A C T

Article history:Received 28 July 2009Received in revised form27 March 2010Accepted 30 March 2010

The dry tribological behavior of a Ti–50.3 at.% Ni alloy at temperatures of 25 °C, 50 °C and200 °C was studied. The wear tests were performed on a high temperature pin-on-disktribometer using 52100 steel and tungsten carbide pins. The worn surfaces of the NiTi alloywere examined by scanning electron microscope. The results showed that in the wear testsinvolving steel pins, thewear rate of the NiTi decreased as thewear testing temperature wasincreased. However, for the NiTi/WC contact, a reverse trendwas observed. There was also alarge decrease in the coefficient of friction for the NiTi/steel contact with increasing weartesting temperature. The formation of compact tribological layers could be the main reasonfor the reduction of the wear rate and coefficient of friction of the NiTi/steel contact athigher wear testing temperatures.

© 2010 Elsevier Inc. All rights reserved.

Keywords:NiTi alloyHigh temperature wearFrictionWCSteel

1. Introduction

NiTi alloys are well known for the superelastic and shapememory effects. Since their first discovery in 1960s, NiTi alloyshave been extensively studied. Both shape memory effect andsuperelasticity have been exploited to design functional andsmart structures in mechanical and biomedical engineering[1–3]. Many more potential applications and mechanicalbehaviors of shape memory alloys have been investigated.For instance, shape memory and superelastic nickel–titaniumalloys have been increasingly used in medical surgery andidentified as possible materials for micro-electromechanicalsystems (MEMS) [4,5]. In these applications, the wear perfor-mance of the material plays a critical role [5]. Another field inwhich the wear behavior of NiTi alloys may have a majorimportance is that of mechanical machining [6].

The wear resistance of conventional tribo-materialsstrongly depends on their mechanical properties such ashardness, toughness, and work-hardening [7,8]. Ball [9]attributed the wear properties of NiTi simply to the work-

hardening. Singh et al. [10] showed that during dry slidingwear tests, the wear rate of Ti50Ni47Fe3 alloy against SAE 52100bearing steel was only 2–5% of that of the steel counterface.This was mainly attributed to the work-hardening of surfacelayers and formation of iron oxide layer at the worn surfaces.For traditional materials, both theory and experiments showthat the wear resistance increases with the hardness [7]. Therelationship betweenwear and hardness of five annealed NiTishape memory alloys was studied by Arciniegas et al. [11].They showed that the wear resistance of austenitic NiTi alloysis consistent with the hardness values of each alloy, higher ashardness increases.

Most of the previous work was concerned with the effect ofsuperelasticity on the wear behavior of NiTi alloys. It isbelieved that the super wear resistance of shapememory NiTiis mainly due to the recovery of the superelastic deformation[5,12–22]. However, the positive effect of the superelasticity onthe wear behavior of NiTi alloy was observed at workingtemperatures near austenitic transformation finish tempera-ture [22]. No studies have been found at higher working

M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 6 8 9 – 6 9 5

! Corresponding author. Tel.: +98 21 88012999; fax: +98 21 88006076.E-mail address: hghasemi@ut.ac.ir (H.M. Ghasemi).

1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2010.03.017

ava i l ab l e a t www.sc i enced i r ec t . com

www.e l sev i e r . com/ loca te /matcha r

Author's personal copy

temperatures where superelasticity and shape memoryeffects may not exist. Therefore, characterization of hightemperature wear behavior of NiTi alloy could be highlybeneficial for better understanding of the wear mechanism ofNiTi alloys. The wear mechanisms involved and the tribolog-ical layers formed may depend on the counterface materialduring sliding. The main aim of the present study is tocharacterize the dry sliding wear behavior of an austeniticNiTi alloy against 52100 steel and tungsten carbide (WC) pinsat wear testing temperatures of 25 °C, 50 °C and 200 °C.

2. Experimental Procedure

A NiTi alloy with composition of Ti–50.3 at.% Ni used for thepresent study was prepared in a Vacuum Induction Meltingfurnace (VIM). Disks, 5 mm in thickness, were wire-cut fromthe bar obtained in VIM. The disks were hot forged at 800 °Cinto a thickness of 3.5 mm (a 30% reduction), solutionannealed in a vacuum furnace at 1000 °C for 12h in acontrolled argon atmosphere and quenched in water. Finally,the disks were aged at 400 °C for 60 min, followed by waterquenching. The disks were then ground to a surface finish ofabout 0.9±0.1 !m.

The phase transformation temperatures weremeasured bya Mettler differential scanning calorimeter (DSC). Measure-ments were carried out at temperatures ranging from !120 °Cto 120 °C under a controlled cooling/heating rate of 10 °C/min.In order to characterize the mechanical behavior of thematerial, uni-axial compression tests were performed in anMTS universal testing machine at testing temperatures of25 °C and 200 °C. The diameter of the cylindrical compressionspecimens was 3 mm with a length of 4.5 mm, i.e., a length/diameter ratio of 1.5. In these tests, the specimenswere loadedto fracture.

The wear tests were performed on a high temperature pin-on-disk tribometer using the NiTi disk with a diameter of40 mm. SAE 52100 bearing steel and tungsten carbide (WC–6 wt.% Co) pins were used as slider. The diameter of the pinswas 5 mm with 3.1 mm radius on their tips. The wear testswere performed under a normal load of 20 N at a constantsliding speed of 0.3 m/s for a sliding distance of 1000 m at weartesting temperatures of 25 °C, 50 °C and 200 °C in dry condi-tions. Prior to each test, all contact surfaces were ultrasoni-cally cleaned in acetone, dried and weighed to a precision of0.1 mg. Three wear tests were performed for each conditionand the average weight loss was calculated. The coefficient offriction was calculated using the measured friction and thenormal load. Finally, the worn surfaces were studied with ascanning electron microscope (SEM).

3. Results and Discussion

The phase transformation temperatures, measured by differ-ential scanning calorimetry (DSC), are listed in Table 1. At thewear testing temperature of 25 °C, which was less than theaustenitic transformation finish temperature (Af) of 45 °C, themicrostructure of the alloy would be a mixture of martensiticand austenitic NiTi [23]. However, the frictional heating during

sliding and wear could raise the surface temperature of thecontact [7] and, therefore, the alloy could be transformed to anaustenitic structure. At the higher wear testing temperatures(50 °C and 200 °C, i.e., aboveAf), however, thematerial could becompletely in the austenitic state.

Fig. 1 shows the variation of wear rates of the NiTi samplesagainst steel and WC pins as a function of the wear testingtemperature under a normal load of 20 N. The figure showsthat as the wear testing temperature was increased the wearrate of the NiTi alloy against 52100 steel pin decreased.However, there was an increase in the wear rate of the NiTialloy against WC pin with an increase in the wear testingtemperature.

In Fig. 2 the compressive stress–strain curves of the NiTialloy at the testing temperatures of 25 °C and 200 °C areshown. At the testing temperature of 25 °C the stress–straincurve of the NiTi alloy showed a plateau region, which couldbe attributed to stress induced martensitic phase transforma-tion [23]. However, at the testing temperature of 200 °C, whichwas far above the Af temperature of 45 °C in Table 1, no suchplateau was observed. This could indicate that no phasetransformation occurred and, therefore, the alloy acted like anon-superelastic material. The yield strengths of the alloy atthe temperatures of 25 °C and 200 °C were measured from thestress–strain curves in Fig. 2. The figure shows that with anincrease in the testing temperature from 25 °C to 200 °C, theyield strength was decreased from a value of 940 MPa to560 MPa, resulting in a reduction of about 40%. The lower yieldstrength of the material at the temperature of 200 °C couldresult in a higher wear rate of the NiTi against WC at the weartesting temperature of 200 °C as expected [7]. However, thewear results of the NiTi against a steel pin in Fig. 1 showed areverse trend. The wear rates of the NiTi alloy against steel

Table 1 – Phase transformation temperatures (°C) of theNiTi alloy.

AustenitestartAs

AustenitefinishAf

MartensitestartMs

MartensitefinishMf

25 45 16 !1

Fig. 1 –Wear rate of the NiTi alloy against 52100 steel andWCpins as a function of wear testing temperature.

690 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 6 8 9 – 6 9 5

Author's personal copy

and WC pins were almost equal at the wear testing temper-ature of 25 °C; however, at the testing temperature of 200 °Cthewear rate of the NiTi alloy against 52100 steel was about anorder of magnitude lower than for the NiTi/WC contact.

The mechanisms responsible for the observed wearkinetics have been investigated through the analysis of thewear tracks. Fig. 3 shows the micrographs of the worn surfaceof the NiTi disk against a steel pin at the wear testingtemperature of 25 °C. No oxide layers were formed on thewear track at this temperature. Fig. 3a shows that ploughingand abrasive mechanisms could be important in the slidingwear of the NiTi alloy. The SEM micrograph in Fig. 3b alsoshows surface fatigue cracks on the worn surface of the NiTialloy along with plastic deformation. Due to repeated cyclicloading at the sliding interface, the surface or subsurfacecracks could be formed. These could result in the breakup ofthe surface into large fragments, leaving large pits on thewornsurface [24] as shown in Fig. 3b.

Figs. 4 and 5 show the micrographs of the worn surface ofthe NiTi, which ran against a steel pin at the wear testingtemperatures of 50 °C and 200 °C, respectively. Fig. 4a showsthat some patches were formed on the worn surface of theNiTi alloy at the wear temperature of 50 °C. Results of theEnergy Dispersive X-ray Spectroscopy (EDS) analysis of twodifferent regions of the wear track in Fig. 4b are listed inTable 2. The results revealed that the tribological layer formedin region A mainly contained iron and oxygen, which couldindicate formation of iron based oxides on the wear surface atthe wear testing temperature of 50 °C. In region B, the EDSanalysis could probably suggest a mixture of nickel andtitanium oxides and no formation of iron base oxides. Fig. 5ashows that a larger region of the worn surface of the NiTi alloywas covered with tribological compacted layer at the weartesting temperature of 200 °C compared to 50 °C. Results of theEDS analysis of two different regions of the wear track inFig. 5b are listed in Table 3. The results also revealed that thecompacted oxide region was mainly iron oxides.

During sliding at higher temperatures, the rate of oxidegeneration of the steel pin in the interface could increase. Theformation of wear debris and transfer of the oxides to thewearsurface of the NiTi sample [16] and their subsequent compac-

tion at the higher temperatures under high loads and stressescould form a mechanically mixed layer (MML) or a solidcompact oxide layer on the surface of theNiTi alloy. Therefore,the formation of this layer on thewear surface of the NiTi alloyin contact with a steel pin could decrease the wear damage byabout 80% at the testing temperature of 200 °C compared to25 °C (Fig. 1). A stable layer could lower the wear damage by

Fig. 2 – Stress–strain curves of the NiTi alloy obtained fromuni-axial compression tests.

Fig. 3 – SEM micrographs of worn surfaces of the NiTi alloyagainst 52100 steel pin under a load of 20 N for slidingdistance of 1000 m at wear testing temperature of 25 °C: (a) ata low magnification, and (b) at a higher magnification.

691M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 6 8 9 – 6 9 5

Author's personal copy

acting as a solid lubricant as well as reducing the metalliccontact. In fact, the formation of tribological oxide layers couldcause a transition from amore severe wear mechanism at thetesting temperature of 25 °C to a milder wear at the testingtemperature of 200 °C.

SEM micrographs of the worn surfaces of the NiTi againstWC pins at the wear testing temperatures of 25 °C and 200 °Care shown in Fig. 6. The figure shows that, at both testingtemperatures, the main wear mechanisms of the NiTi alloy

Fig. 4 – SEM micrographs of worn surfaces of the NiTi alloyagainst 52100 steel pin under a load of 20 N for slidingdistance of 1000 m at wear testing temperature of 50 °C, (a) ata low magnification, and (b) at a higher magnification.

Fig. 5 – SEM micrographs of worn surfaces of the NiTi alloyagainst 52100 steel pin under a load of 20 N for slidingdistance of 1000 m at wear testing temperature of 200 °C,(a) at a low magnification, and (b) at a higher magnification.

Table 2 – Chemical compositions (at.%) of two regions inthe worn surface of NiTi alloy against steel pin at weartesting temperature of 50 °C (Fig. 4b).

Ti Ni Fe O

Region A 5.88 5.94 25.63 62.56Region B 36.54 37.06 – 26.40

692 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 6 8 9 – 6 9 5

Author's personal copy

against WC were ploughing and abrasive wear. The higherwear rate of the alloy at the wear testing temperature of 200 °Cin Fig. 1 could be the result of lower yield strength of the alloyat this temperature as inferred from Fig. 2. The EDS analysis ofthe wear track in Fig. 6b is listed in Table 4. The resultsrevealed that the patches formed on the worn surface of theNiTi alloy in region A contained a certain amount of nickel,titanium and oxygen. This might indicate formation of amixed nickel and titanium oxide layer on the worn surface ofthe NiTi at the wear testing temperature of 200 °C. However,the layerwas not compact enough to help decrease thewear ofthe alloy in contact with WC pin at the higher temperature.

The average coefficient of friction for the NiTi/steel and theNiTi/WC contacts at different wear testing temperatures isshown in Fig. 7. A small decrease in the average coefficient offriction was observed for the NiTi/WC contact. This reductioncould be attributed to the formation of tribolayer (oxidepatches in Fig. 6b) on the worn surface of the NiTi alloy athigher temperatures. However, there was more than 35%decrease in the average coefficient of friction for the NiTi/steelas the testing temperature increased from 25 °C to 200 °C. Atthe wear testing temperature of 25 °C the average coefficientof friction for the NiTi/steel contact is higher than that for theNiTi/WC contact, which could indicate greater adhesionbetween the steel pin and the NiTi disk. However, at thewear testing temperature of 200 °C the average coefficient offriction for the NiTi/steel contact became lower than that forthe NiTi/WC contact. This could be the result of the formationof compact iron oxide based tribological layer as shown inFig. 5. Variations of coefficient of friction for the NiTi/steel andtheWC/steel contacts with sliding distance at the wear testing

Table 3 – Chemical compositions (at.%) of two regions inthe worn surface of NiTi alloy against steel pin at weartesting temperature of 200 °C (Fig. 5b).

Ti Ni Fe Cr O

Region A 5.48 5.33 26.75 0.47 61.97Region B 36.54 37.06 – – 26.40

Fig. 6 – SEM micrographs of worn surfaces of the NiTi alloyagainst WC pin under a load of 20 N for sliding distance of1000 m at wear testing temperature of: (a) 25 °C, and(b) 200 °C.

Table 4 – Chemical compositions (at.%) of two regions inthe worn surface of NiTi alloy against WC pin at weartesting temperature of 200 °C (Fig. 6b).

Ti Ni O

Region A 35.26 36.44 28.29Region B 51.03 48.97 –

Fig. 7 –Variation of average coefficient of friction as a functionof wear testing temperature for the NiTi/steel and theNiTi/WC contacts under a normal load of 20 N for slidingdistance of 1000 m.

693M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 6 8 9 – 6 9 5

Author's personal copy

temperatures of 25 °C and 200 °C are shown in Figs. 8 and 9,respectively. At the wear testing temperature of 25 °C, Figs. 8aand 9a show a higher instability in the coefficient of friction ofboth the NiTi/steel and theWC/steel contacts compared to the

wear testing temperature of 200 °C (Figs. 8b and 9b). At thewear testing temperature of 200 °C, the formation of tribolog-ical oxide layer could lower the adhesion between matingsurfaces, resulting in a lower instability in the coefficient offriction.

4. Conclusions

1. The yield strength of the NiTi alloy was decreased about40% as the testing temperature increased from 25 °C to200 °C.

2. The wear rate of the NiTi alloy in contact against WC pinincreased with the wear testing temperature. However, thewear rate of the alloy in contact against steel pin decreasedwith increasing in the wear testing temperature.

3. The reduction of the wear rate and the average coefficientof friction of the NiTi/steel contact as the wear testingtemperature increased were attributed to the formation ofcompact tribological layer on the wear surface of the NiTialloy. The layer mostly contained iron oxides, which couldlimit adhesion between the mating surfaces.

R E F E R E N C E S

[1] Funakubo H. Shape memory alloys. New York: Gordon andBleach; 1987.

[2] Van Humbeeck J. Non-medical applications of shape memoryalloys. Mater Sci Eng A 1999;273–275:134–48.

[3] Duerig T, Pelton A, Stoeckel D. An overview of nitinol medicalapplications. Mater Sci Eng A 1999;273–275:149–60.

[4] Kahn H, Huff MA, Heuer AH. The TiNi shape memory alloyand its applications for MEMS. J Micromech Microeng 1998;8:213–21.

[5] Li DY. Exploration of TiNi shape memory alloy for potentialapplication in a new area: tribological engineering. SmartMater Struct 2000;9:717–26.

[6] Gialanella S, Ischia G, Straffelini G. Phase composition andwear behavior of NiTi alloys. J Mater Sci 2008;43:1701–10.

[7] Hutchings IM. Tribology; friction and wear of engineeringmaterials. London: Edward Arnold; 1992.

[8] Glaser WA. Materials for tribology. Amsterdam, theNetherlands: Elsevier Science Publishers; 1992.

[9] Ball A. On the importance of work hardening in the design ofwear-resistant materials. Wear 1983;91:201–7.

[10] Singh J, Alpas AT. Dry sliding wear mechanisms in aTi50Ni47Fe3 intermetallic alloy. Wear 1995;181–183:302–11.

[11] Arciniegas M, Casals J, Manero JM, Pena J, Gil FJ. Study ofhardness and wear behaviour of NiTi shape memory alloys.J Alloys Compod 2008;460:213–9.

[12] Li DY. A new type of wear-resistant material: pseudo-elasticTiNi alloy. Wear 1998;221:116–23.

[13] Li DY, Liu R. The mechanism responsible for high wearresistance of pseudo-elastic TiNi alloy—a noveltribo-material. Wear 1999;225–229:777–83.

[14] Li DY. Development of novel wear-resistant materials:TiNi-based pseudoelastic tribomaterials. Mater Des 2000;21:551–5.

[15] Liu R, Li DY. Indentation behavior of pseudoelastic TiNi alloy.Scr Mater 1999;41(7):691–6.

[16] Liu R, Li DY. Experimental studies on tribological propertiesof pseudoelastic TiNi alloy with comparison to stainless steel304. Metall Mater Trans A 2000;31:2773–83.

Fig. 8 – Variation of coefficient of friction of the NiTi/steelcontact with sliding distance under a load of 20 N at the weartesting temperature of: (a) 25 °C, and (b) 200 °C.

Fig. 9 – Variation of coefficient of friction of the NiTi/WCcontact with sliding distance under a load of 20 N at the weartesting temperature of: (a) 25 °C, and (b) 200 °C.

694 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 6 8 9 – 6 9 5

Author's personal copy

[17] Li DY. Wear behaviour of TiNi shape memory alloy. Scr Mater1996;34(2):195–200.

[18] Jin J, Wang H. Wear resistance of Ni–Ti alloy. Acta MetallSinica 1988;1:76–81.

[19] Shida Y, Sugimoto Y. Water jet erosion behaviour of Ti–Nibinary alloys. Wear 1991;146:219–28.

[20] Yan W. Mechanics examination on the wear behaviour ofshape memory alloys. Fourth Australasian Congress onApplied Mechanics, Melbourne, Australia; 2005.

[21] Yan W. Theoretical investigation of wear-resistancemechanism of superelastic shape memory alloy NiTi. MaterSci Eng A 2006;427:348–55.

[22] Qian L, Sun Q, Xiao X. Role of phase transition in the unusualmicrowear behavior of superelastic NiTi shapememory alloy.Wear 2006;260:509–22.

[23] Otsuka K, Ren X. Physical metallurgy of Ti–Ni-based shapememory alloys. Prog Mater Sci 2005;50:511–678.

[24] Lin HC, Liao HM, He JL, Chen KC, Lin KM. Wear characteristicsof TiNi shape memory alloys. Metall Mater Trans A 1996;28:1871–7.

695M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 6 8 9 – 6 9 5