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Copyright copy 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 11 11176ndash11180 2011

Surface Nanomechanical Behavior ofZrN and ZrCN Films Deposited on NiTi Shape

Memory Alloy by Magnetron Sputtering

C L Chu1lowast H L Ji1 C Guo1 X B Sheng1 Y S Dong1 P H Lin1 T Hu2 and P K Chu21School of Materials Science and Engineering and Jiangsu Key Laboratory for Advanced Metallic Materials

Southeast University Nanjing 211189 China2Department of Physics and Materials Science City University of Hong Kong Tat Chee Avenue Kowloon Hong Kong China

Surface nanomechanical behavior under nanoindentation of ZrN and ZrCN film on NiTi substratewas studied The surface hardness and modulus of the films increase initially with larger nanoin-dentation depths and then reach their maximum values Afterwards they diminish gradually andfinally reaching plateau values which are the composite modulus and composite hardness derivedfrom the ZrNZrCN film and NiTi substrate They are higher than those of electropolished NiTi SMAdue to the properties of ZrN and ZrCN In comparison the surface nanomechanical properties ofelectropolished NiTi exhibit a different change with depths

Keywords NiTi Shape Memory Alloy ZrNZrCN Film Nanoindentation Mechanical Properties

1 INTRODUCTION

The functionalities of biomaterials depend very much ontheir surface properties and surface modification is becom-ing a hot research field in biomaterials1 For example for-mation of a titania or hydroxyapatite film can effectivelyblock the release of harmful Ni ions from biomedical NiTishape memory alloys in simulated body fluids and improvethe biocompatibility2ndash5 Recently ZrN film began to attractmore attention for its excellent biocompatibility high hard-ness good lubricity as well as ductility These propertiesmake it an attractive biomedical coating material Chenget al used PIIIampD technique to deposite ZrNZr coatingonto NiTi alloy and the corrosion resistance was greatlyimproved in Hankrsquos solution6 In our previous study7 wehave deposited ZrN film on NiTi shape memory alloy(SMA) by magnetron sputtering and improved blood com-patibility was obtained In addition it is necessary to inves-tigate the surface mechanical properties of the ZrN film forbiomaterials especially those used in hard tissue replace-ments Owing to the constraints imposed by the modifiedsurface layer many conventional testing methods cannotbe used to study the mechanical properties of biomedi-cal thin films In this aspect nanoindentation is a suitablemethod to determine the mechanical characteristics of thin

lowastAuthor to whom correspondence should be addressed

films because of its low load (1 N) and small displace-ment (1 nm)8 In this work the surface nanomechanicalproperties of the ZrN and ZrCN films were determinedby nanoindentation The surface deformation mechanismis proposed and compared to electropolished NiTi SMA

2 EXPERIMENTAL DETAILS

NiTi (508 at Ni) SMA plates for medical applicationswere cut into small rectangular blocks with dimensionsof 10 mmtimes 10 mmtimes 1 mm The samples were chemi-cally polished for several minutes using a solution con-taining H2O HF and HNO3 with a 514 ratio and thenelectropolished at a constant voltage of 10 V for 6 minat room temperature in an electrolytic cell with a mag-netic stirrer and graphite cathode The electrolyte consistedof 21 vol perchloric acid (HClO4 70sim72 vol) and79 vol acetic acid (CH3COOH 995) The sampleswere then ultrasonically washed in acetone for 10 min anddeionized water for 10 min They were divided into twogroups with the first group being the controlThe second group was used as the substrates to deposit

ZrN and ZrCN films using a reactive DC (direct current)magnetron sputtering system (JGP450A2) The vacuumchamber was pumped down to 1times 10minus3 Pa prior to filmdeposition The carrier gas (Ar) and reactive (N2) gas havea purity of 9995 The substrates were first cleaned by

11176 J Nanosci Nanotechnol 2011 Vol 11 No 12 1533-488020111111176005 doi101166jnn20114051

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Chu et al Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy

argon sputtering for 6 min During deposition the pres-sure in the vacuum chamber was 05 Pa and N2(Ar+N2)with a ratio of 625 was introduced at a total flux of32 cm3min The substrate temperature was 250 C TheDC power was kept at 150 W which was equivalent toa target DC offset voltage of 280 V The substrate cur-rent was 054 A for the substrate bias voltage of minus50 Vrespectively The distance between the target and substrateholder was 60 mm To deposit the ZrN and ZrCN filmsa high purity (999) Zr target and a Zr-C composite tar-get with Zr (999)C (999) area ratio of 11 were usedrespectively The diameter of the target was 80 mm andthe deposition time was about 8 minThe surface and cross section morphologies were evalu-

ated by field-emission scanning electron microscopy (SEMSirion 2000 FEI) at 20 kV after the surfaces were coatedwith gold X-ray diffraction (RAD IIA Rigaku) withCu K at 40 kV and 25 mA radiation was used to deter-mine the phase constituentsThe mechanical properties and deformation behavior

were investigated by nanoindentation using a nanoindenter(Nano Instruments XP MTS) with the continuous stiff-ness measurement (CSM) capability and the indentationwas carried out using a Berkovich (three-sided pyramid)indenter A small-harmonic high-frequency amplitude wassuperimposed on the indentation loading and the contactstiffness of the sample was measured from the displace-ment response at the excitation frequency The indentation

ZrCNfilm

NiTi

ZrNfilm

NiTi

(a)

(c) (d)

(b)

Fig 1 SEM images of the ZrN and ZrCN films (a) Surface morphology of the ZrN film (b) Cross section of the ZrN film (c) Surface morphologyof the ZrCN film (d) Cross section of the ZrCN film

experiments were conducted in displacement control to adepth of 2000 nm on each sample The modulus or hard-ness was derived instantaneously as a function of depthfrom the contact stiffness Load and hardness calibrationwas performed employing a fused silica reference At leastthree indentations were conducted for each group

3 RESULTS AND DISCUSSION

Figure 1 depicts the surface and cross section of the ZrNand ZrCN films under SEM There are many ball-shapeparticles on the two films The cross section morphologyindicates that the microstructure of the films has a fibrouscharacteristic The films adhere well to the NiTi substrateCompared to the ZrCN film the ZrN film surface has somepores and appears more porousFigure 2 shows the XRD spectra acquired from the ZrN

and ZrCN films The ZrN film exhibits preferred growthat the (111) crystal plane ZrN is the predominant phaseand there is small amount of ZrO2 secondary phase as aresult of the strong affinity between Zr and O leading tothe formation of the ZrO2 phase during depositing Brownet al9 have found N in ZrN phase can be substituted by Oin the ambient into ZrO2 In the XRD spectra of the ZrCNfilm the diffraction peak of the ZrCN phase (actually thecomposite diffraction peak from ZrN and ZrC phases) canbe observed The ZrO2 secondary phase cannot be foundDuring deposition of the ZrCN film the high temperature

J Nanosci Nanotechnol 11 11176ndash11180 2011 11177

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Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy Chu et al

Fig 2 XRD spectra of (a) ZrN film and (b) ZrCN film

from the discharge can deoxidize ZrO2 into ZrC10 therebymaking the former disappearsFigure 3 shows the representative load vs displacement

curves acquired from the different samples It can be foundthat the needed load increases with increasing depth fromthe top surface For each sample the load peaks of at adepth of 2000 nm are different from each other namely200 mN for the electropolished NiTi SMA 240 mN forthe ZrN film and 250 mN for the ZrCN film At the samenanoindentation depth the loads of the ZrN and ZrCNfilms are obviously higher than that of the electropolishedNiTi SMA It indicates that the deposited Zr(C)N filmscan enhance the load bearing ability of NiTiFigures 4 and 5 display the corresponding hardness val-

ues and Youngrsquos moduli obtained from the CSM systemThe hardness and modulus values here not only depend onthe surface films but also are influenced by the NiTi sub-strate because the total nanoindentation depth of 2000 nmis larger than or comparable to the thickness of the sur-face films (a native titania film about 10 nm thick onelectropolished NiTi SMA as the control and 2500 nmthick ZrN and ZrCN films) As shown in Figures 4 and 5the hardness and modulus determined from each sample

Fig 3 Representative load versus nanoindentation displacement(depth) curves acquired from different samples (1) Electropolished NiTisubstrate (2) ZrN film and (3) ZrCN film

display a similar trend with increasing depths However itshould be noted that the surface nanomechanical propertiesof different samples are different from each other therebysuggesting a different deformation mechanism during thenanoindentation processWith regard to the electropolished NiTi SMA as the con-

trol both the hardness and modulus reach the maximum

Fig 4 Representative apparent hardness versus nanoindentation dis-placement (depth) curves acquired from different samples (1) Electropol-ished NiTi substrate (2) ZrN film and (3) ZrCN film

11178 J Nanosci Nanotechnol 11 11176ndash11180 2011

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Chu et al Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy

Fig 5 Representative apparent Youngrsquos modulus versus nanoinden-tation displacement (depth) curves acquired from different samples(1) Electropolished NiTi substrate (2) ZrN film and (3) ZrCN film

values 42 GPa and 85 GPa respectively at the ini-tial stage of nanoindentation With increasing depths thehardness and modulus values decrease finally reaching aplateau near 27 GPa and 66 GPa respectively The factthat the maximum modulus value of 85 GPa observed atthe initial stage of nanoindentation is lower than that ofdense amorphous TiO2 between 130 and 150 GPa11 seemsreasonable because the thickness of the native titania filmon the NiTi substrate is much smaller than the total nanoin-dentation depthWith increasing indentation depths the modulus

diminishes as a result of the larger contribution from thesubstrate finally reaching a plateau resulting from thecomposite modulus encompassing the moduli of the densetitania film (TiO2 and NiTi substrate Considering that thethickness of the titania film (10 nm) on the electropolishedNiTi sample is much smaller than the total nanoindenta-tion depth of 2000 nm its composite modulus (66 GPa)should approach that of the NiTi substrateIn comparison the hardness and modulus determined

from the ZrN film increase initially with larger nanoinden-tation depths and then reach maximum values of 72 GPaand 120 GPa respectively Afterwards they graduallydecrease finally attaining plateau values of about 34 GPaand 68 GPa respectively which are different from thoseof the electropolished NiTi control Similar mechanicalbehavior has been observed in the nanoindentation study ofTiO2 nanotubes on Ti substrate12 It is reasonable becausethe ZrN film on NiTi has a porous structure on thenanometer scale (Fig 1(a)) similar to that of TiO2 nano-tubes on Ti Obviously the decrease in the porosity anddensification of the porous ZrN film can increase the elas-tic modulus and also affect the hardness

A deformation mechanism similar to the one discussedin the literature12 is applicable There are three distinctregions defined by curve 2 in Figures 4 and 5 Region Ithat is characterized by an increase in the modulus withtime is primarily due to increased densification of the ZrNfilm Region II is characterized by a parabolic decreasein the indentation modulus where the indentation depth isgradually comparable to the thickness of ZrN film That isthe indenter is gradually approaching the substrate surfaceHence the variation in the moduli in this region is theresult of the increasing contributions from the substratethat can decrease the moduli Finally Region III describesthe region in which the ZrN film has become nearly fullydense It is characterized by a plateau in the indentationmodulus corresponding to the composite modulus of thedense film and NiTi substrate A similar interpretation isapplicable to the composite hardnessThe deformation behavior of the ZrCN film under inden-

tation has some similar characteristics as that of the ZrNfilm on NiTi The difference between them is that the hard-ness and modulus obtained from the former reach maxi-mum values of about 21 GPa and 230 GPa respectivelywith a larger increasing rate at the initial stage of nanoin-dentation With increasing depths the hardness and mod-ulus decrease finally reaching a plateau near 37 GPa and73 GPa respectively which are the composite hardnessand modulus of ZrCN film and NiTi substrate It appearsto be logical because the ZrCN film is denser than the ZrNfilm as shown in Figure 1 indicating that the latter needsa longer densification process under indentationIt is obvious that the composite modulus and composite

hardness values derived from the ZrNZrCN film and NiTisubstrate are higher than those of electropolished NiTiSMA The enhancement can be attributed to the intrinsicproperties of the ZrN and ZrCN films It is common todetermine the average properties of a thin film at indenta-tion depths of less than 10 of the film thickness in orderto minimize contributions from the substrate11 Hence inthis work the average hardness and modulus values ofthe ZrN film are about 55 GPa and 106 GPa respec-tively whereas those of the ZrCN film are higher at about168 GPa and 2028 GPa respectively That is to say theZrN and ZrCN films produced by magnetron sputteringcan enhance the surface load bearing ability of NiTi SMAwhile simultaneously offering distinct advantages in sup-pression of Ni ion release and enhancement of surfacebiocompatibility

4 CONCLUSION

Nanoindentation study on surface mechanical behaviorof ZrN and ZrCN films deposited on NiTi SMA bymagnetron sputtering indicates ZrN and ZrCN films canenhance the surface load bearing ability of NiTi SMA Thesurface deformation mechanism under nanoindentation is

J Nanosci Nanotechnol 11 11176ndash11180 2011 11179

RESEARCH

ARTIC

LE

Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy Chu et al

proposed and the phenomenon is compared to that on elec-tropolished NiTi SMA

Acknowledgments The work described in this paperwas supported by Program for New Century Excellent Tal-ents (NCET-06-0464) in University of Ministry of Edu-cation of China Natural Science Foundation of JiangsuProvince (Project No BK2007515) and Hong KongResearch Grants Council (RGC) General Research Funds(GRF) No CityU 112307

References and Notes

1 X Y Liu P K Chu and C X Ding Mater Sci Eng R 47 49(2004)

2 M F Chen X J Yang Y Liu S L Zhu Z D Cui and H CMan Surf Coat Technol 173 229 (2003)

3 J Choi D Bogdanski M Koumlller S A Esenwein D MuumlllerG Muhr and M Epple Biomaterials 24 3689 (2003)

4 C L Chu T Hu S L Wu Y S Dong L H Yin Y P Pu P H LinC Y Chung K W K Yeung and P K Chu Acta Biomaterialia3 795 (2007)

5 S K Wu H C Lin and C Y Lee Surf Coat Technol 113 13(1999)

6 Y Cheng and Y F Zheng IEEE Transactions on Plasma Science34 1105 (2006)

7 C L Chu H L Ji L H Yin Y P Pu P H Lin and P K ChuSurf Coat Technol 204 2841 (2010)

8 W C Oliver and G M Pharr J Mater Res 7 1564 (1992)9 R Brown M N Alias and R Fontana Surf Coat Technol 62 467

(1993)10 Q F Tong J L Shi Y Z Song X F Hu and L Liu Aerospace

Materials amp Technology 2 45 (2004)11 A C Fischer-Cripps Nanoindentation 2nd edn Springer New York

(2004)12 G A Crawford N Chawla K Das S Bose and

A Bandyopadhyay Acta Biomaterialia 3 359 (2007)

Received 30 December 2009 RevisedAccepted 30 August 2010

11180 J Nanosci Nanotechnol 11 11176ndash11180 2011

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ARTIC

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Chu et al Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy

argon sputtering for 6 min During deposition the pres-sure in the vacuum chamber was 05 Pa and N2(Ar+N2)with a ratio of 625 was introduced at a total flux of32 cm3min The substrate temperature was 250 C TheDC power was kept at 150 W which was equivalent toa target DC offset voltage of 280 V The substrate cur-rent was 054 A for the substrate bias voltage of minus50 Vrespectively The distance between the target and substrateholder was 60 mm To deposit the ZrN and ZrCN filmsa high purity (999) Zr target and a Zr-C composite tar-get with Zr (999)C (999) area ratio of 11 were usedrespectively The diameter of the target was 80 mm andthe deposition time was about 8 minThe surface and cross section morphologies were evalu-

ated by field-emission scanning electron microscopy (SEMSirion 2000 FEI) at 20 kV after the surfaces were coatedwith gold X-ray diffraction (RAD IIA Rigaku) withCu K at 40 kV and 25 mA radiation was used to deter-mine the phase constituentsThe mechanical properties and deformation behavior

were investigated by nanoindentation using a nanoindenter(Nano Instruments XP MTS) with the continuous stiff-ness measurement (CSM) capability and the indentationwas carried out using a Berkovich (three-sided pyramid)indenter A small-harmonic high-frequency amplitude wassuperimposed on the indentation loading and the contactstiffness of the sample was measured from the displace-ment response at the excitation frequency The indentation

ZrCNfilm

NiTi

ZrNfilm

NiTi

(a)

(c) (d)

(b)

Fig 1 SEM images of the ZrN and ZrCN films (a) Surface morphology of the ZrN film (b) Cross section of the ZrN film (c) Surface morphologyof the ZrCN film (d) Cross section of the ZrCN film

experiments were conducted in displacement control to adepth of 2000 nm on each sample The modulus or hard-ness was derived instantaneously as a function of depthfrom the contact stiffness Load and hardness calibrationwas performed employing a fused silica reference At leastthree indentations were conducted for each group

3 RESULTS AND DISCUSSION

Figure 1 depicts the surface and cross section of the ZrNand ZrCN films under SEM There are many ball-shapeparticles on the two films The cross section morphologyindicates that the microstructure of the films has a fibrouscharacteristic The films adhere well to the NiTi substrateCompared to the ZrCN film the ZrN film surface has somepores and appears more porousFigure 2 shows the XRD spectra acquired from the ZrN

and ZrCN films The ZrN film exhibits preferred growthat the (111) crystal plane ZrN is the predominant phaseand there is small amount of ZrO2 secondary phase as aresult of the strong affinity between Zr and O leading tothe formation of the ZrO2 phase during depositing Brownet al9 have found N in ZrN phase can be substituted by Oin the ambient into ZrO2 In the XRD spectra of the ZrCNfilm the diffraction peak of the ZrCN phase (actually thecomposite diffraction peak from ZrN and ZrC phases) canbe observed The ZrO2 secondary phase cannot be foundDuring deposition of the ZrCN film the high temperature

J Nanosci Nanotechnol 11 11176ndash11180 2011 11177

RESEARCH

ARTIC

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Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy Chu et al

Fig 2 XRD spectra of (a) ZrN film and (b) ZrCN film

from the discharge can deoxidize ZrO2 into ZrC10 therebymaking the former disappearsFigure 3 shows the representative load vs displacement

curves acquired from the different samples It can be foundthat the needed load increases with increasing depth fromthe top surface For each sample the load peaks of at adepth of 2000 nm are different from each other namely200 mN for the electropolished NiTi SMA 240 mN forthe ZrN film and 250 mN for the ZrCN film At the samenanoindentation depth the loads of the ZrN and ZrCNfilms are obviously higher than that of the electropolishedNiTi SMA It indicates that the deposited Zr(C)N filmscan enhance the load bearing ability of NiTiFigures 4 and 5 display the corresponding hardness val-

ues and Youngrsquos moduli obtained from the CSM systemThe hardness and modulus values here not only depend onthe surface films but also are influenced by the NiTi sub-strate because the total nanoindentation depth of 2000 nmis larger than or comparable to the thickness of the sur-face films (a native titania film about 10 nm thick onelectropolished NiTi SMA as the control and 2500 nmthick ZrN and ZrCN films) As shown in Figures 4 and 5the hardness and modulus determined from each sample

Fig 3 Representative load versus nanoindentation displacement(depth) curves acquired from different samples (1) Electropolished NiTisubstrate (2) ZrN film and (3) ZrCN film

display a similar trend with increasing depths However itshould be noted that the surface nanomechanical propertiesof different samples are different from each other therebysuggesting a different deformation mechanism during thenanoindentation processWith regard to the electropolished NiTi SMA as the con-

trol both the hardness and modulus reach the maximum

Fig 4 Representative apparent hardness versus nanoindentation dis-placement (depth) curves acquired from different samples (1) Electropol-ished NiTi substrate (2) ZrN film and (3) ZrCN film

11178 J Nanosci Nanotechnol 11 11176ndash11180 2011

RESEARCH

ARTIC

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Chu et al Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy

Fig 5 Representative apparent Youngrsquos modulus versus nanoinden-tation displacement (depth) curves acquired from different samples(1) Electropolished NiTi substrate (2) ZrN film and (3) ZrCN film

values 42 GPa and 85 GPa respectively at the ini-tial stage of nanoindentation With increasing depths thehardness and modulus values decrease finally reaching aplateau near 27 GPa and 66 GPa respectively The factthat the maximum modulus value of 85 GPa observed atthe initial stage of nanoindentation is lower than that ofdense amorphous TiO2 between 130 and 150 GPa11 seemsreasonable because the thickness of the native titania filmon the NiTi substrate is much smaller than the total nanoin-dentation depthWith increasing indentation depths the modulus

diminishes as a result of the larger contribution from thesubstrate finally reaching a plateau resulting from thecomposite modulus encompassing the moduli of the densetitania film (TiO2 and NiTi substrate Considering that thethickness of the titania film (10 nm) on the electropolishedNiTi sample is much smaller than the total nanoindenta-tion depth of 2000 nm its composite modulus (66 GPa)should approach that of the NiTi substrateIn comparison the hardness and modulus determined

from the ZrN film increase initially with larger nanoinden-tation depths and then reach maximum values of 72 GPaand 120 GPa respectively Afterwards they graduallydecrease finally attaining plateau values of about 34 GPaand 68 GPa respectively which are different from thoseof the electropolished NiTi control Similar mechanicalbehavior has been observed in the nanoindentation study ofTiO2 nanotubes on Ti substrate12 It is reasonable becausethe ZrN film on NiTi has a porous structure on thenanometer scale (Fig 1(a)) similar to that of TiO2 nano-tubes on Ti Obviously the decrease in the porosity anddensification of the porous ZrN film can increase the elas-tic modulus and also affect the hardness

A deformation mechanism similar to the one discussedin the literature12 is applicable There are three distinctregions defined by curve 2 in Figures 4 and 5 Region Ithat is characterized by an increase in the modulus withtime is primarily due to increased densification of the ZrNfilm Region II is characterized by a parabolic decreasein the indentation modulus where the indentation depth isgradually comparable to the thickness of ZrN film That isthe indenter is gradually approaching the substrate surfaceHence the variation in the moduli in this region is theresult of the increasing contributions from the substratethat can decrease the moduli Finally Region III describesthe region in which the ZrN film has become nearly fullydense It is characterized by a plateau in the indentationmodulus corresponding to the composite modulus of thedense film and NiTi substrate A similar interpretation isapplicable to the composite hardnessThe deformation behavior of the ZrCN film under inden-

tation has some similar characteristics as that of the ZrNfilm on NiTi The difference between them is that the hard-ness and modulus obtained from the former reach maxi-mum values of about 21 GPa and 230 GPa respectivelywith a larger increasing rate at the initial stage of nanoin-dentation With increasing depths the hardness and mod-ulus decrease finally reaching a plateau near 37 GPa and73 GPa respectively which are the composite hardnessand modulus of ZrCN film and NiTi substrate It appearsto be logical because the ZrCN film is denser than the ZrNfilm as shown in Figure 1 indicating that the latter needsa longer densification process under indentationIt is obvious that the composite modulus and composite

hardness values derived from the ZrNZrCN film and NiTisubstrate are higher than those of electropolished NiTiSMA The enhancement can be attributed to the intrinsicproperties of the ZrN and ZrCN films It is common todetermine the average properties of a thin film at indenta-tion depths of less than 10 of the film thickness in orderto minimize contributions from the substrate11 Hence inthis work the average hardness and modulus values ofthe ZrN film are about 55 GPa and 106 GPa respec-tively whereas those of the ZrCN film are higher at about168 GPa and 2028 GPa respectively That is to say theZrN and ZrCN films produced by magnetron sputteringcan enhance the surface load bearing ability of NiTi SMAwhile simultaneously offering distinct advantages in sup-pression of Ni ion release and enhancement of surfacebiocompatibility

4 CONCLUSION

Nanoindentation study on surface mechanical behaviorof ZrN and ZrCN films deposited on NiTi SMA bymagnetron sputtering indicates ZrN and ZrCN films canenhance the surface load bearing ability of NiTi SMA Thesurface deformation mechanism under nanoindentation is

J Nanosci Nanotechnol 11 11176ndash11180 2011 11179

RESEARCH

ARTIC

LE

Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy Chu et al

proposed and the phenomenon is compared to that on elec-tropolished NiTi SMA

Acknowledgments The work described in this paperwas supported by Program for New Century Excellent Tal-ents (NCET-06-0464) in University of Ministry of Edu-cation of China Natural Science Foundation of JiangsuProvince (Project No BK2007515) and Hong KongResearch Grants Council (RGC) General Research Funds(GRF) No CityU 112307

References and Notes

1 X Y Liu P K Chu and C X Ding Mater Sci Eng R 47 49(2004)

2 M F Chen X J Yang Y Liu S L Zhu Z D Cui and H CMan Surf Coat Technol 173 229 (2003)

3 J Choi D Bogdanski M Koumlller S A Esenwein D MuumlllerG Muhr and M Epple Biomaterials 24 3689 (2003)

4 C L Chu T Hu S L Wu Y S Dong L H Yin Y P Pu P H LinC Y Chung K W K Yeung and P K Chu Acta Biomaterialia3 795 (2007)

5 S K Wu H C Lin and C Y Lee Surf Coat Technol 113 13(1999)

6 Y Cheng and Y F Zheng IEEE Transactions on Plasma Science34 1105 (2006)

7 C L Chu H L Ji L H Yin Y P Pu P H Lin and P K ChuSurf Coat Technol 204 2841 (2010)

8 W C Oliver and G M Pharr J Mater Res 7 1564 (1992)9 R Brown M N Alias and R Fontana Surf Coat Technol 62 467

(1993)10 Q F Tong J L Shi Y Z Song X F Hu and L Liu Aerospace

Materials amp Technology 2 45 (2004)11 A C Fischer-Cripps Nanoindentation 2nd edn Springer New York

(2004)12 G A Crawford N Chawla K Das S Bose and

A Bandyopadhyay Acta Biomaterialia 3 359 (2007)

Received 30 December 2009 RevisedAccepted 30 August 2010

11180 J Nanosci Nanotechnol 11 11176ndash11180 2011

RESEARCH

ARTIC

LE

Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy Chu et al

Fig 2 XRD spectra of (a) ZrN film and (b) ZrCN film

from the discharge can deoxidize ZrO2 into ZrC10 therebymaking the former disappearsFigure 3 shows the representative load vs displacement

curves acquired from the different samples It can be foundthat the needed load increases with increasing depth fromthe top surface For each sample the load peaks of at adepth of 2000 nm are different from each other namely200 mN for the electropolished NiTi SMA 240 mN forthe ZrN film and 250 mN for the ZrCN film At the samenanoindentation depth the loads of the ZrN and ZrCNfilms are obviously higher than that of the electropolishedNiTi SMA It indicates that the deposited Zr(C)N filmscan enhance the load bearing ability of NiTiFigures 4 and 5 display the corresponding hardness val-

ues and Youngrsquos moduli obtained from the CSM systemThe hardness and modulus values here not only depend onthe surface films but also are influenced by the NiTi sub-strate because the total nanoindentation depth of 2000 nmis larger than or comparable to the thickness of the sur-face films (a native titania film about 10 nm thick onelectropolished NiTi SMA as the control and 2500 nmthick ZrN and ZrCN films) As shown in Figures 4 and 5the hardness and modulus determined from each sample

Fig 3 Representative load versus nanoindentation displacement(depth) curves acquired from different samples (1) Electropolished NiTisubstrate (2) ZrN film and (3) ZrCN film

display a similar trend with increasing depths However itshould be noted that the surface nanomechanical propertiesof different samples are different from each other therebysuggesting a different deformation mechanism during thenanoindentation processWith regard to the electropolished NiTi SMA as the con-

trol both the hardness and modulus reach the maximum

Fig 4 Representative apparent hardness versus nanoindentation dis-placement (depth) curves acquired from different samples (1) Electropol-ished NiTi substrate (2) ZrN film and (3) ZrCN film

11178 J Nanosci Nanotechnol 11 11176ndash11180 2011

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ARTIC

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Chu et al Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy

Fig 5 Representative apparent Youngrsquos modulus versus nanoinden-tation displacement (depth) curves acquired from different samples(1) Electropolished NiTi substrate (2) ZrN film and (3) ZrCN film

values 42 GPa and 85 GPa respectively at the ini-tial stage of nanoindentation With increasing depths thehardness and modulus values decrease finally reaching aplateau near 27 GPa and 66 GPa respectively The factthat the maximum modulus value of 85 GPa observed atthe initial stage of nanoindentation is lower than that ofdense amorphous TiO2 between 130 and 150 GPa11 seemsreasonable because the thickness of the native titania filmon the NiTi substrate is much smaller than the total nanoin-dentation depthWith increasing indentation depths the modulus

diminishes as a result of the larger contribution from thesubstrate finally reaching a plateau resulting from thecomposite modulus encompassing the moduli of the densetitania film (TiO2 and NiTi substrate Considering that thethickness of the titania film (10 nm) on the electropolishedNiTi sample is much smaller than the total nanoindenta-tion depth of 2000 nm its composite modulus (66 GPa)should approach that of the NiTi substrateIn comparison the hardness and modulus determined

from the ZrN film increase initially with larger nanoinden-tation depths and then reach maximum values of 72 GPaand 120 GPa respectively Afterwards they graduallydecrease finally attaining plateau values of about 34 GPaand 68 GPa respectively which are different from thoseof the electropolished NiTi control Similar mechanicalbehavior has been observed in the nanoindentation study ofTiO2 nanotubes on Ti substrate12 It is reasonable becausethe ZrN film on NiTi has a porous structure on thenanometer scale (Fig 1(a)) similar to that of TiO2 nano-tubes on Ti Obviously the decrease in the porosity anddensification of the porous ZrN film can increase the elas-tic modulus and also affect the hardness

A deformation mechanism similar to the one discussedin the literature12 is applicable There are three distinctregions defined by curve 2 in Figures 4 and 5 Region Ithat is characterized by an increase in the modulus withtime is primarily due to increased densification of the ZrNfilm Region II is characterized by a parabolic decreasein the indentation modulus where the indentation depth isgradually comparable to the thickness of ZrN film That isthe indenter is gradually approaching the substrate surfaceHence the variation in the moduli in this region is theresult of the increasing contributions from the substratethat can decrease the moduli Finally Region III describesthe region in which the ZrN film has become nearly fullydense It is characterized by a plateau in the indentationmodulus corresponding to the composite modulus of thedense film and NiTi substrate A similar interpretation isapplicable to the composite hardnessThe deformation behavior of the ZrCN film under inden-

tation has some similar characteristics as that of the ZrNfilm on NiTi The difference between them is that the hard-ness and modulus obtained from the former reach maxi-mum values of about 21 GPa and 230 GPa respectivelywith a larger increasing rate at the initial stage of nanoin-dentation With increasing depths the hardness and mod-ulus decrease finally reaching a plateau near 37 GPa and73 GPa respectively which are the composite hardnessand modulus of ZrCN film and NiTi substrate It appearsto be logical because the ZrCN film is denser than the ZrNfilm as shown in Figure 1 indicating that the latter needsa longer densification process under indentationIt is obvious that the composite modulus and composite

hardness values derived from the ZrNZrCN film and NiTisubstrate are higher than those of electropolished NiTiSMA The enhancement can be attributed to the intrinsicproperties of the ZrN and ZrCN films It is common todetermine the average properties of a thin film at indenta-tion depths of less than 10 of the film thickness in orderto minimize contributions from the substrate11 Hence inthis work the average hardness and modulus values ofthe ZrN film are about 55 GPa and 106 GPa respec-tively whereas those of the ZrCN film are higher at about168 GPa and 2028 GPa respectively That is to say theZrN and ZrCN films produced by magnetron sputteringcan enhance the surface load bearing ability of NiTi SMAwhile simultaneously offering distinct advantages in sup-pression of Ni ion release and enhancement of surfacebiocompatibility

4 CONCLUSION

Nanoindentation study on surface mechanical behaviorof ZrN and ZrCN films deposited on NiTi SMA bymagnetron sputtering indicates ZrN and ZrCN films canenhance the surface load bearing ability of NiTi SMA Thesurface deformation mechanism under nanoindentation is

J Nanosci Nanotechnol 11 11176ndash11180 2011 11179

RESEARCH

ARTIC

LE

Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy Chu et al

proposed and the phenomenon is compared to that on elec-tropolished NiTi SMA

Acknowledgments The work described in this paperwas supported by Program for New Century Excellent Tal-ents (NCET-06-0464) in University of Ministry of Edu-cation of China Natural Science Foundation of JiangsuProvince (Project No BK2007515) and Hong KongResearch Grants Council (RGC) General Research Funds(GRF) No CityU 112307

References and Notes

1 X Y Liu P K Chu and C X Ding Mater Sci Eng R 47 49(2004)

2 M F Chen X J Yang Y Liu S L Zhu Z D Cui and H CMan Surf Coat Technol 173 229 (2003)

3 J Choi D Bogdanski M Koumlller S A Esenwein D MuumlllerG Muhr and M Epple Biomaterials 24 3689 (2003)

4 C L Chu T Hu S L Wu Y S Dong L H Yin Y P Pu P H LinC Y Chung K W K Yeung and P K Chu Acta Biomaterialia3 795 (2007)

5 S K Wu H C Lin and C Y Lee Surf Coat Technol 113 13(1999)

6 Y Cheng and Y F Zheng IEEE Transactions on Plasma Science34 1105 (2006)

7 C L Chu H L Ji L H Yin Y P Pu P H Lin and P K ChuSurf Coat Technol 204 2841 (2010)

8 W C Oliver and G M Pharr J Mater Res 7 1564 (1992)9 R Brown M N Alias and R Fontana Surf Coat Technol 62 467

(1993)10 Q F Tong J L Shi Y Z Song X F Hu and L Liu Aerospace

Materials amp Technology 2 45 (2004)11 A C Fischer-Cripps Nanoindentation 2nd edn Springer New York

(2004)12 G A Crawford N Chawla K Das S Bose and

A Bandyopadhyay Acta Biomaterialia 3 359 (2007)

Received 30 December 2009 RevisedAccepted 30 August 2010

11180 J Nanosci Nanotechnol 11 11176ndash11180 2011

RESEARCH

ARTIC

LE

Chu et al Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy

Fig 5 Representative apparent Youngrsquos modulus versus nanoinden-tation displacement (depth) curves acquired from different samples(1) Electropolished NiTi substrate (2) ZrN film and (3) ZrCN film

values 42 GPa and 85 GPa respectively at the ini-tial stage of nanoindentation With increasing depths thehardness and modulus values decrease finally reaching aplateau near 27 GPa and 66 GPa respectively The factthat the maximum modulus value of 85 GPa observed atthe initial stage of nanoindentation is lower than that ofdense amorphous TiO2 between 130 and 150 GPa11 seemsreasonable because the thickness of the native titania filmon the NiTi substrate is much smaller than the total nanoin-dentation depthWith increasing indentation depths the modulus

diminishes as a result of the larger contribution from thesubstrate finally reaching a plateau resulting from thecomposite modulus encompassing the moduli of the densetitania film (TiO2 and NiTi substrate Considering that thethickness of the titania film (10 nm) on the electropolishedNiTi sample is much smaller than the total nanoindenta-tion depth of 2000 nm its composite modulus (66 GPa)should approach that of the NiTi substrateIn comparison the hardness and modulus determined

from the ZrN film increase initially with larger nanoinden-tation depths and then reach maximum values of 72 GPaand 120 GPa respectively Afterwards they graduallydecrease finally attaining plateau values of about 34 GPaand 68 GPa respectively which are different from thoseof the electropolished NiTi control Similar mechanicalbehavior has been observed in the nanoindentation study ofTiO2 nanotubes on Ti substrate12 It is reasonable becausethe ZrN film on NiTi has a porous structure on thenanometer scale (Fig 1(a)) similar to that of TiO2 nano-tubes on Ti Obviously the decrease in the porosity anddensification of the porous ZrN film can increase the elas-tic modulus and also affect the hardness

A deformation mechanism similar to the one discussedin the literature12 is applicable There are three distinctregions defined by curve 2 in Figures 4 and 5 Region Ithat is characterized by an increase in the modulus withtime is primarily due to increased densification of the ZrNfilm Region II is characterized by a parabolic decreasein the indentation modulus where the indentation depth isgradually comparable to the thickness of ZrN film That isthe indenter is gradually approaching the substrate surfaceHence the variation in the moduli in this region is theresult of the increasing contributions from the substratethat can decrease the moduli Finally Region III describesthe region in which the ZrN film has become nearly fullydense It is characterized by a plateau in the indentationmodulus corresponding to the composite modulus of thedense film and NiTi substrate A similar interpretation isapplicable to the composite hardnessThe deformation behavior of the ZrCN film under inden-

tation has some similar characteristics as that of the ZrNfilm on NiTi The difference between them is that the hard-ness and modulus obtained from the former reach maxi-mum values of about 21 GPa and 230 GPa respectivelywith a larger increasing rate at the initial stage of nanoin-dentation With increasing depths the hardness and mod-ulus decrease finally reaching a plateau near 37 GPa and73 GPa respectively which are the composite hardnessand modulus of ZrCN film and NiTi substrate It appearsto be logical because the ZrCN film is denser than the ZrNfilm as shown in Figure 1 indicating that the latter needsa longer densification process under indentationIt is obvious that the composite modulus and composite

hardness values derived from the ZrNZrCN film and NiTisubstrate are higher than those of electropolished NiTiSMA The enhancement can be attributed to the intrinsicproperties of the ZrN and ZrCN films It is common todetermine the average properties of a thin film at indenta-tion depths of less than 10 of the film thickness in orderto minimize contributions from the substrate11 Hence inthis work the average hardness and modulus values ofthe ZrN film are about 55 GPa and 106 GPa respec-tively whereas those of the ZrCN film are higher at about168 GPa and 2028 GPa respectively That is to say theZrN and ZrCN films produced by magnetron sputteringcan enhance the surface load bearing ability of NiTi SMAwhile simultaneously offering distinct advantages in sup-pression of Ni ion release and enhancement of surfacebiocompatibility

4 CONCLUSION

Nanoindentation study on surface mechanical behaviorof ZrN and ZrCN films deposited on NiTi SMA bymagnetron sputtering indicates ZrN and ZrCN films canenhance the surface load bearing ability of NiTi SMA Thesurface deformation mechanism under nanoindentation is

J Nanosci Nanotechnol 11 11176ndash11180 2011 11179

RESEARCH

ARTIC

LE

Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy Chu et al

proposed and the phenomenon is compared to that on elec-tropolished NiTi SMA

Acknowledgments The work described in this paperwas supported by Program for New Century Excellent Tal-ents (NCET-06-0464) in University of Ministry of Edu-cation of China Natural Science Foundation of JiangsuProvince (Project No BK2007515) and Hong KongResearch Grants Council (RGC) General Research Funds(GRF) No CityU 112307

References and Notes

1 X Y Liu P K Chu and C X Ding Mater Sci Eng R 47 49(2004)

2 M F Chen X J Yang Y Liu S L Zhu Z D Cui and H CMan Surf Coat Technol 173 229 (2003)

3 J Choi D Bogdanski M Koumlller S A Esenwein D MuumlllerG Muhr and M Epple Biomaterials 24 3689 (2003)

4 C L Chu T Hu S L Wu Y S Dong L H Yin Y P Pu P H LinC Y Chung K W K Yeung and P K Chu Acta Biomaterialia3 795 (2007)

5 S K Wu H C Lin and C Y Lee Surf Coat Technol 113 13(1999)

6 Y Cheng and Y F Zheng IEEE Transactions on Plasma Science34 1105 (2006)

7 C L Chu H L Ji L H Yin Y P Pu P H Lin and P K ChuSurf Coat Technol 204 2841 (2010)

8 W C Oliver and G M Pharr J Mater Res 7 1564 (1992)9 R Brown M N Alias and R Fontana Surf Coat Technol 62 467

(1993)10 Q F Tong J L Shi Y Z Song X F Hu and L Liu Aerospace

Materials amp Technology 2 45 (2004)11 A C Fischer-Cripps Nanoindentation 2nd edn Springer New York

(2004)12 G A Crawford N Chawla K Das S Bose and

A Bandyopadhyay Acta Biomaterialia 3 359 (2007)

Received 30 December 2009 RevisedAccepted 30 August 2010

11180 J Nanosci Nanotechnol 11 11176ndash11180 2011

RESEARCH

ARTIC

LE

Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy Chu et al

proposed and the phenomenon is compared to that on elec-tropolished NiTi SMA

Acknowledgments The work described in this paperwas supported by Program for New Century Excellent Tal-ents (NCET-06-0464) in University of Ministry of Edu-cation of China Natural Science Foundation of JiangsuProvince (Project No BK2007515) and Hong KongResearch Grants Council (RGC) General Research Funds(GRF) No CityU 112307

References and Notes

1 X Y Liu P K Chu and C X Ding Mater Sci Eng R 47 49(2004)

2 M F Chen X J Yang Y Liu S L Zhu Z D Cui and H CMan Surf Coat Technol 173 229 (2003)

3 J Choi D Bogdanski M Koumlller S A Esenwein D MuumlllerG Muhr and M Epple Biomaterials 24 3689 (2003)

4 C L Chu T Hu S L Wu Y S Dong L H Yin Y P Pu P H LinC Y Chung K W K Yeung and P K Chu Acta Biomaterialia3 795 (2007)

5 S K Wu H C Lin and C Y Lee Surf Coat Technol 113 13(1999)

6 Y Cheng and Y F Zheng IEEE Transactions on Plasma Science34 1105 (2006)

7 C L Chu H L Ji L H Yin Y P Pu P H Lin and P K ChuSurf Coat Technol 204 2841 (2010)

8 W C Oliver and G M Pharr J Mater Res 7 1564 (1992)9 R Brown M N Alias and R Fontana Surf Coat Technol 62 467

(1993)10 Q F Tong J L Shi Y Z Song X F Hu and L Liu Aerospace

Materials amp Technology 2 45 (2004)11 A C Fischer-Cripps Nanoindentation 2nd edn Springer New York

(2004)12 G A Crawford N Chawla K Das S Bose and

A Bandyopadhyay Acta Biomaterialia 3 359 (2007)

Received 30 December 2009 RevisedAccepted 30 August 2010

11180 J Nanosci Nanotechnol 11 11176ndash11180 2011