Surface Conditions of Nitinol Wires, Tubing, and As-Cast Alloys.The Effect of Chemical Etching, Aging in Boiling Water,and Heat Treatment

S. A. Shabalovskaya,1 J. Anderegg, 1 F. Laab,1 P. A. Thiel,1 G. Rondelli2

1 Institute for Physical Research and Technology and Ames Laboratory, Iowa State University, Ames, Iowa 50011

2 CNR - Institute of Energy and Interface, Milano, Italy

Received 25 March 2002; revised 3 September 2002; accepted 2 October 2002

Abstract: The surface conditions of Nitinol wires and tubing were evaluated with the use ofX-ray photoelectron spectroscopy, high-resolution Auger spectroscopy, electron backscatter-ing, and scanning-electron microscopy. Samples were studied in the as-received state as wellas after chemical etching, aging in boiling water, and heat treatment, and compared to amechanically polished 600-grit-finish Nitinol surface treated similarly. General regularities insurface behavior induced by the examined surface treatments are similar for wires, tubing,and studied as-cast alloy, though certain differences in surface Ni concentration were ob-served. Nitinol wires and tubing from various suppliers demonstrated great variability in Nisurface concentration (0.5–15 at.%) and Ti/Ni ratio (0.4–35). The wires in the as-receivedstate, with the exception of those with a black oxide originating in the processing procedure,revealed nickel and titanium on the surface in both elemental and oxidized states, indicatinga nonpassive surface. Shape-setting heat treatment at 500 °C for 15 min resulted in tremen-dous increase in the surface Ni concentration and complete Ni oxidation. Preliminary chemicaletching and boiling in water successfully prevented surface enrichment in Ni, initially resultingfrom heat treatment. A stoichiometric uniformly amorphous TiO2 oxide generated duringchemical etching and aging in boiling water was reconstructed at 700 °C, revealing rutilestructure. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 65B: 193–203, 2003

Keywords: 600-grit polished Nitinol and wire/tubing surface conditions; surface treatment;chemical etching; aging in boiling water and heat treatment; amorphous oxide

INTRODUCTION

Extensive use of Nitinol wire in the production of stents,especially delicate ones like vascular stents, imposes severerequirements on surface homogeneity and stability. Smallsurface defects tolerable in thick fixation plates can be of realdanger in the case of tiny wires whose surfaces are subjectedto complex manufacturing processes. Surface chemistry isanother aspect related to surface stability and biocompatibil-ity, in particular. Elimination of surface nickel and prepara-tion of completely passive surface layers that are structurallyand chemically uniform is a desirable target to pursue in thedevelopment of stable Nitinol surfaces for long-term implan-

tation. Nitinol wires currently offered commercially exhibitvariable corrosion resistance.1,2 The same observation appliesto the corrosion studies of an as-cast NiTi alloy with mechan-ically polished finish surface.3 In the absence of a standardprocedure for the treatment of Nitinol surface a 600-grit finishsurface is a final state in state–of-the-art studies on Nitinolimplantation.4 The surface conditions of Nitinol are alsomodified during heat treatment at � 500 °C required forshape setting of an implant or device or through sterilizationprocedures.

Previous studies on the surface of Nitinol as-cast alloyshave shown that surface chemistry critically depends onoxidation media (air, water/steam, hydrogen peroxide),though general tendency is toward the formation of Ti-basedoxide surface films.5,6 The effect of certain sterilization pro-cedures was a subject of a discussion in a review article.7 Inthe present study the surface conditions of Nitinol wires andtubing are evaluated with the help of X-ray photoelectronspectroscopy, Auger electron spectroscopy, electron back-scattering, and scanning electron microscopy in as-received,heat-treated, and chemically treated states and compared with

Correspondence to: S. A. Shabalovskaya, Institute for Physical Research andTechnology and Ames Laboratory, Iowa State University, Ames, IA 50011 (e-mail:shabalov@ameslab.gov)

Contract grant sponsor: National Institutes of Health (NHLBI); contract grantnumber: 1 R01 HL67632-01

Contract grant sponsor: U.S. Department of Energy; contract grant number:W-4705-ENG-82

© 2003 Wiley Periodicals, Inc.

193

the surface of Nitinol as-cast alloy. Wires and tubing withcharacterized surfaces were further used in corrosion-resis-tance studies.8

MATERIALS AND METHODS

As-cast alloy samples were prepared from a TiNi50.8 alloyproduced by six-fold fusion in an electrolytic arc furnace inhelium atmosphere. After melting, the ingot was spark cut indistilled water. ICP-AES chemical analysis of the alloy com-position revealed several impurities such as carbon, oxygen,and nitrogen in the amounts of 33, 235, and 18 ppm, respec-tively. Mechanical polishing was performed by means of wetgrinding with 180- and 320-grit SiC paper, and finishing with600-grit paper. Samples underwent prolonged washing indeionized distilled water, were etched in a 1HF � 4 NHO3 �5 H2O solution for at least 4 min, ultrasonically cleaned indeionized distilled for 5 min, and aged in distilled boilingwater for 20 min. Wire (two black and two silver colored) andblack tubing (from two sources) samples were prepared fromthe material obtained from various suppliers at exhibitions ofthe SMST conferences in Asilomar, CA, as well as fromMemry, Inc. The as-received specimens were ultrasonicallycleansed for 5 min in successive baths of acetone, methanol,and distilled water and dried with an air gun after each step.Chemical etching of the wires was performed in the mannerdescribed above. The shape-setting treatment was simulatedby heating the samples in a muffle furnace in air at 500 °C for15 min.

X-Ray Photoelectron Spectroscopy (XPS)

X–ray photoelectron spectroscopy was used to determine thesurface elemental and chemical composition of the studiedsamples. The XPS analysis was conducted with a PHI 5500spectrometer at a take-off angle of 45° from a 1-mm2 areawith a base pressure in the spectrometer of less than 1 � 10�9

Torr and monochromatized Al k�1 radiation. Survey scanswere taken at pass energy of 187 eV and a binding energy inthe range 0–1100 eV. Atomic concentrations were deter-mined with the use of both survey and multiplex high-reso-lution scans with a pass energy of 29.3 eV. Three–four spotsper sample were examined. Deconvolution of elementalpeaks was performed with the use of Gaussian–Lorenziancurve fitting.

Auger Electron Spectrometry (AES)

Auger electron spectrometry combined with sputter etchingwas used to obtain depth profiles and distribution of elementson the surface. These experiments were performed in a JeolJamp 7830F field emission Auger microprobe with a basepressure of less than 1 � 10�9 Pa. The system uses ahigh-resolution hemispherical electrostatic analyzer. SurveyAuger spectra were taken with the use of an electron beamhaving 10-kV kinetic energy and a current of 2.5 nA with an

energy resolution of 0.75%, minimal probe size less than 20�m, and tilt angle of 40° corresponding to a 85° electronescape angle in XPS geometry. Auger pixel maps represent-ing distribution of elements on the surface were constructedusing Ni LMM, Ti LMM, C KLL, and O KLL Auger spectrawith the intensity being represented by a peak–backgroundratio. Depth profiling was performed with the use of Ar� ionbombardment at 3-kV energy. In contrast to reference 5, itwas suggested that the NiTi surface oxide removal rate issimilar to that of SiO2. A standard SiO2 on Si substrate wasused to determine the Ar� sputtering rate, 11 nm per min. TheNiTi surface oxide thickness was estimated as the point atwhich the oxygen peak reached half of its maximal concen-tration.

Electron Back Scattering (EBS) and Scanning-ElectronMicroscopy (SEM)

The crystalline state of the oxide surface films was deter-mined with the use of an Amray 1845FE scanning electronmicroscope with a TexSemLaboratory OIM (TSL–V 6.3)attachment for the detection of diffracted backscattered elec-trons of 5–25 kV primary electron energy at 70° tilt angle.Surface morphology was evaluated with the use of a 15-kVelectron beam.

RESULTS AND DISCUSSION

As-Received Wires and Mechanically PolishedAs-Cast Alloy

In this section results on the surface chemistry of Nitinolwires and tubing in as-received state and as-cast alloy with a600-grit-finish grinded surface are presented. The wire withsandblasted surface is discussed in more detail because sand-blasting is often used as a procedure to strip off the blackoxide from the surface of Nitinol wires after their processing.

XPS chemical analysis of Nitinol wires obtained fromvarious suppliers revealed significant surface compositionvariability. In Figure 1 a typical survey spectrum of sand-blasted wire is presented. In addition to carbon and oxygen,inevitable surface components, there were also traces of N(up to � 3 at.%) Cl, S, Si (total up to � 10 at.%), and Mo (upto 1%). Sometimes elements such as Mg, P, Ca as well as Cu,Zn, and even Cd were also observed. Some of those elements,Cu, Zn, and P, could be observed even after chemical etchingor electropolishing. All these impurities originate in the lu-bricants and processing equipment.

The concentrations of the major surface components ofstudied Nitinol wires and tubing in as-received state as wellas polished plate of as-cast alloy are listed in Table I. Only themajor surface components, carbon, oxygen, titanium, andnickel are provided, so that the tendencies of surface chem-istry can be exposed. Carbon and oxygen were always thedominant surface species (total 80–90 at.%), in agreementwith the results obtained on the surfaces of as-cast alloys. The

194 SHABALOVSKAYA ET AL.

observed Ni surface concentrations of Nitinol tubing andwires varied greatly (0.4–15 at.%) in this study. The lowestNi concentration and the highest Ti/Ni ratios were obtainedon the surfaces of silver color wire II and polished plate. Theelemental concentrations on the surfaces of the wires/tubingwith a black oxide (an original oxide resulting from multipledrawing and annealing in air during wire processing) did notdiffer from each other. The only exception was the black wireII (Table I) exhibiting �15 % of Ni on the surface, in therange of the Ni concentrations mentioned in one of the studieson the corrosion resistance of Nitinol wires (25 at.%).9 Thesurface composition in all cases of studied wires with theexception of sandblasted ones was uniform. In the case ofsandblasted wire, titanium concentration could vary from 5 to10 at.% and nickel from 2 to 5 at.% on the surface of the samewire sample.

Typical XPS spectra of carbon, oxygen, nitrogen, tita-nium, and nickel, as well as molybdenum, characteristic tothe wire with the sand-blasted surface are presented in Figure2. Carbon, a major surface contaminant, is shown to be bound

with oxygen (� 289-eV, � 286-eV, and � 284-eV peaks).10

The 1s peak of oxygen consists of a major peak characteristicof Ti oxide (530.5 eV) with a small contribution from Nioxide (529.6 eV)11 that is not easy to separate unless Ni oxideis dominant.7 The high-energy contributions into the 1s ox-ygen spectrum at energy � 531 eV originate in the oxygen insurface organic compounds (C—O—C and CAO) as well asin metal hydroxides. These high-energy contributions to the

Figure 1. A representative survey XPS spectrum of a sand-blastedwire at 45° escape electron angle.

TABLE I. Elemental Surface Concentrations in As-Received NiTi Samples (� 5–10 at.%).

Material/Element Carbon Oxygen Titanium Nickel Ti/Ni ratio

Tubing I 41.5 � 6.9 45.5 � 6.9 10.9 � 0.67 2.1 � 0.3 5Tubing II 48.5 39.3 6.8 5.4 1Black wire I 36.1 48.0 10.6 5.3 2Black wire II 32.8 46.1 5.8 15.3 0.4Silver wire I 40.3 49.7 7.9 2.1 4Silver wire II 38.3 46.1 15.2 0.4 35Sandblasted 35.1 � 1.9 52.7 � 5.4 8.2 � 7.4 4.1 � 2.1 2As-cast alloy 53.0 38.0 8.1 0.9 9

Figure 2. High-resolution XPS spectra (dots) of major surface com-ponents of sandblasted wire: carbon, oxygen, titanium, nickel, nitro-gen, and molybdenum. Spectra were fitted with the use of Gaussian–Lorenzian curves (solid lines).

195SURFACE CONDITIONS OF NITINOL WIRES

1s oxygen spectrum associated with surface contaminantswere significantly reduced after chemical etching, heat treat-ment, and Ar� ion sputtering.7 The high-energy shoulder stillremaining in the 1s oxygen spectra after heat treatment can beassigned to the metal hydroxides. It must be pointed out thatthe intensity of the 1s oxygen high-energy peaks in the wiresand polished Nitinol was variable. Occasionally these peaksbecame dominant in the 1s oxygen spectra of the same wireor plate sample, indicating a variable level of organic con-taminants that is very difficult to control.

The 2p3/2 spectrum of Ti consists of two peaks; the majorone, located at � 458.6 eV, corresponds to titanium in theTiO2 oxide. A small peak resolved at the low-energy side at454.6 eV originates in Ti in NiTi. This contribution thatconstituted less than 5% of the total intensity of the 2p3/2 Tispectrum was revealed in all silver-color wires and it indi-cated noncomplete oxidation of titanium on the surface. Incontrast to silver-color wires, Nitinol wires with black oxidedid not reveal any other Ti species but Ti in � 4 state (TiO2).

The Ni 2p3/2 spectrum of the surface of sandblasted wireconsists of three major peaks located at 856.5, 862, and 852.8eV. This spectrum closely resembles the one obtained by theauthors of this article from the surface of polished samples ofthe as-cast alloy. The two major peaks observed at � 856.5and 862 eV represent Ni spectrum in oxide (Ni�3 state),12

presumably Ni2O3, though the contribution from the Ni hy-droxides, like Ni(OH) and Ni(OH)2 cannot be excluded ei-ther. These latter compounds have slightly different shape ofthe Ni 2p3/2 spectrum that is shifted toward the low-energyside by 0.5–1 eV relative to the Ni2O3 spectrum.12 An intense852.8-eV peak in the Ni 2p3/2 spectrum indicates the presenceof � 25% nickel in elemental state (Ni0) on the surface, againpointing at the fact that the surface is not completely passive.

All types of Ni 2p3/2 spectra observed on the surfaces ofNitinol wires and tubing examined in this study are presentedin Figure 3. The black lustrous oxide resulting from process-ing (c) reveals Ni only in an oxidized state (� 856.2 eV and862-eV peaks). Silver-color wires (d), and sandblasted wire(b), however, always have some Ni in the elemental state aswell (� 852.8 eV). Even when the total Ni surface concen-tration is close to zero (� 0.4 at.%) one can also recognize aNi0 peak at corresponding energy [852.8 eV(d)].

In addition to the previously discussed elements, spectra ofnitrogen and molybdenum are also presented. Nitrogen iscommonly observed on the surface of Nitinol, especially afterchemical etching. However, it is usually in the organic state(400.6 eV). Sandblasted wire, however, revealed nitrogenthat can be assigned to a metal compound complexed withwater Ni (NO3) � 6 H2O.10 A weak 3d spectrum of molyb-denum resolves the 3d3/2 and 3d5/2 peaks at � 235 and 232eV, respectively. These peaks can be assigned to molybde-num in the MoO3 oxide.13 Because Mo was observed only onthe surface of wires, it may be concluded that it originates inthe lubricants used in wire drawing.

Depth profiles revealed very different elemental distribu-tions in the cases of silver color and black wires. Althoughsilver-color wire exhibited gradual Ni increase, starting from

� 2 at.%, and oxide thickness of � 10 nm, black wiresdemonstrated steady Ni concentration (10–20 at.%) into thedepth of at least �200 nm.

Corrosion studies, to be discussed in detail in future pub-lications, revealed the fact that the breakdown potential ofas-received wires and tubing in potentiodynamic tests vary inthe range from � 120 mV to � 400 mV.

The Effect of Heat Treatment

The effect of heat treatment on the conditions of Nitinolsurface was studied with both samples of as-cast alloy(plates) and wires in as-received and treated states. Regular-ities in surface alteration induced by heat treatment of Nitinolas-cast alloy were similar to those obtained on wire andtubing samples. (Table II and Fig. 4)

Heat treatment of Nitinol at 500 °C for 15 min resulted insignificant change of surface chemistry. Data presented inTable II exemplify alterations in surface composition inducedby the heat treatment in the case of Nitinol tubing withoriginal black oxide (Tubing I). The major observation is asignificant decrease in carbon and a fivefold increase insurface nickel concentration. A little change, however, in thecontent of titanium and oxygen was noticed. Significant de-crease in carbon concentration is a result of decontaminationof the surface upon heating. The loss of intensity observed onthe high-energy side of the 1s oxygen spectra associated withorganic compounds is in correspondence with the reductionin carbon surface concentration after heat treatment. Afterheat treatment both titanium and nickel are present on thesurface only in the oxidized � 4 and � 3 states, respectively.

Figure 3. Ni 2p3/2 spectra observed in NiTi wires: (a) heat treated at500 °C for 15 min; (b) as received sanblasted wires; (c)as receivedwith black oxide, and (d) as-received silver-color wires.

196 SHABALOVSKAYA ET AL.

Comparison of the surface conditions of studied materialindicates the two- to tenfold increase in the Ni surface con-centration (up to � 20 at %) on the as-received wires andpolished plates after short heat treatment (Figure 4 [Seriesb]). Data for all wire samples, except for sandblasted ones(Sample 5), were very consistent. Sandblasted wire and the600-grit finish plate (Sample 6), however, demonstrated greatvariability. Regarding sandblasted wires, the observed vari-ability could be assigned to extremal surface roughness,though data on the as-received state revealed less scatteringthan after heat treatment. The great variability of Ni surfaceconcentration observed on 600-grit finish polished plates isattributed to the chemical heterogeneity of the surface asso-ciated with residual deformation (discussed in the section onAuger maps). The tremendous increase of Ni surface concen-tration upon heating is a result of Ni diffusion from the bulkto the surface of Nitinol. Ni diffusion to the surface istriggered by the intensive oxidation of Ti in the internalsurface layers due to the supply of oxygen from the externalsurface layers to the depth. There is also a well-known

inevitable amount of oxygen in the bulk of Nitinol, � 200ppm in the lab alloys, and � 500 ppm in the industrial ones.

A depth profile of a polished NiTi plate heated for 15 minat 500 °C is presented in Figure 5(a). This figure demon-strates accumulation of Ni in external layers of the surface. Asimilar shape of Ni depth profile was obtained from Nitinolwires (Stentor devices) that had failed after implantation inthe human body because of severe corrosion. The surfacesublayers of Stentor wire were enriched by Ni up to � 20at.%, a concentration equal to that of Ti.14 Repeated heattreatments are obviously not beneficial for the quality ofNitinol surface aimed at medical applications. For this reasonthe duration of the exposure to high temperatures should becarefully dosed. Oxide films obtained after 15-min heat treat-ment at 500 °C have intensive blue color. The thickness of thefilm determined at the point when the intensity of oxygen isreduced twice compared to the original intensity is 100 nm(550 s). There is a possibility that this oxide thickness couldbe enough to induce significant surface stresses, especiallywhen Nitinol is superelastically or plastically deformed. Po-tentiodynamic and potentiostatic corrosion studies of heat-treated wires are in progress.

Chemical Etching and Boiling in Water

Chemical etching in a HF/HNO3 solution was used to cleanthe surface physically, to eliminate surface scratches, to re-move highly deformed defect material, to oxidize the surface,and to leach nickel. The observed green color of the etchingsolution is an indication of the presence of the Ni�2 ions.Aging in boiling water promotes the diffusion of Ni atomsfrom the oxidized Ti-based surface layers to the surface/waterinterface and provides conditions for Ni release. The resultingNitinol surface is depleted in Ni into the depth of � 5–10nm.7 Boiling in water also provides conditions for oxygendiffusion promoting formation of a more compact and stoi-chiometric TiO2 oxide. No traces of Ti in metallic state weredetected after aging in water, though it has been detected afterchemical etching.7 It is evident that aging of NiTi in boiling

TABLE II. ELEMENTAL SURFACE CONCENTRATIONS IN TREATED NITI SAMPLES (� 5–10 AT.%).

Material/Element Treatment Carbon Oxygen Titanium NickelTi/Niratio

As-cast alloy Polished 53.0 38.0 8.1 0.9 9As-cast alloy Chem. etch. 26.6 48.6 16.4 9.0 2Tubing I As received 41.5 45.5 10.9 2.1 5Tubing I Heat treated 25 53 11.1 10.9 1Tubing I Chem. etch. 31 53 2.0 14.0 0.14Tubing I Chem. etch. 28.8 49.2 20.2 1.8 11

Heat treat.Tubing I Chem. etch. 40.9 45.0 11.3 2.8 4

Water agedTubing I Chem. etch. 28.3 50.6 20.6 0.5 41

Water agedHeat treat.

Figure 4. Ni concentration on NiTi surfaces. Samples: 1, black tubingI; 2, silver wire I; 3, black wire II; 4, silver wire II; 5, sandblasted wire;6, polished plate. Series: (a) as-received wires and polished plate; (b)heat treated at 500 °C for 15 min in as-received state; (c) chemicallyetched � heat treated; (d) chemically etched � aged in water for 20min � heat treated.

197SURFACE CONDITIONS OF NITINOL WIRES

water is much more effective for surface oxidation thanexposure to pure oxygen.15

The elemental surface compositions obtained after chem-ical etching of one of the tubing and as-cast alloy samples arepresented in Table II. It is obvious that the Ni concentrationis lower on the surface of as-cast alloy, where titanium is thedominant surface metal. The same is true for the etchedsurfaces of silver-color wires, revealing a low Ti/Ni ratio, �

2. In contrast, on the surface with black oxide Ni is adominant metal after chemical etching. This observation is inagreement with the results described in the previous section.It indicates Ni enrichment of the external surface layers ofwires during processing that includes multiple annealing at700–800 °C.

In the as-cast alloy as well as sandblasted wire, the majorpart of Ni on the surface is in the elemental state afterchemical etching. However, processed wires with black oxiderevealed surface Ni mostly in oxidized state even after chem-ical etching, which is probably the result of deep Ni oxidationduring multiple annealing in air.

Aging in water restores metal atomic concentration on thesurface to the original low level and reverses the Ti/Ni ratioto the original one characteristic of the as-received state(Tubing I, Table II). The beneficial effect of aging in water onsurface improvement becomes especially obvious after heattreatment to set shape. As one can see in the diagram pre-sented in Figure 4, the tremendous Ni enrichment in thesurface after heat treatment can be prevented with the use ofchemical etching (Series c). Moreover, the surface Ni con-centration can be reduced to a negligible level after heattreatment if samples are preliminarily chemically etched andaged in boiling water (Figure 4, Series d). The very consistenteffect of aging in water on Ni surface concentration observedamong all studied samples was a result of the growth of newsurface oxide layers on a TiNi substrate depleted in Ni.

Depth profiling of a heat-treated (500 °C for 1 h) platesample preliminarily chemically etched and aged in boilingwater indicates that Nitinol surface consists of a thick, � 150nm, TiO2 oxide with nickel content �2 at.% into the depth of� 90 nm [Figure 5(b)]. In contrast to a polished finish surface(Fig. 5a), no accumulation of Ni in external surface layerswas observed after a 1-h heat treatment of chemically etchedand aged in water Nitinol samples. The color of the oxideformed during a 1-h exposure to 500 °C was gold–green.

A recent study of Nitinol clamps retrieved after 4–12months in the human body15 should now be mentioned. Incontrast to Reference 14, the authors15 did not observe anycorrosion marks on the surface of explanted samples, thoughthe material was subjected to repeated extrusion, drawing,and annealing cycles as well as to a final shape-setting heattreatment. Despite the multiple heat treatments a depth profileobtained from a retrieved Nitinol clamp did not show Niaccumulation in the external surface sublayers.15 The resultsof the present study suggest that Ni, inevitably accumulatedin the external surface sublayers during processing of shapememory clamps, was leached during chemical etching per-formed directly before surgery.

Auger Maps

The differences in surface quality resulting from varioussurface-preparation protocols can be easily demonstratedwith the use of Auger maps obtained by scanning the surface.These maps present distribution of the elements on the sur-face. The change of color from black to blue, yellow, red, and

Figure 5. Auger depth profiles of the surfaces of NiTi plate samples.(a) polished � heat treated (500 °C � 15 min, blue-color oxide); (b)polished � chemically etched � boiled in water and heat treated (500°C for 1 h, gold/green oxide).

198 SHABALOVSKAYA ET AL.

white corresponds to an increase of elemental surface con-centration from zero to 100%. SEM images of mechanicallypolished 600-grit finish surface and Auger maps of carbon,nickel, and titanium are shown in Figure 6. A surface frag-ment (Fig. 6a) demonstrating very poor surface quality wasintentionally selected. It exhibits a texture induced by theabrasion procedure as well as particles of various sizes.Auger analysis indicates that these particles are either oforganic origin or are the pieces of material stripped off fromthe surface, mostly titanium oxide.

The distribution of carbon and titanium on the surfacereflected surface texture caused by grinding, though this isnot obvious in the case of Ni (Figure 6). After heat treatmentsurface heterogeneity induced through abrasion procedurebecame more obvious: Both Ni and Ti maps revealed strip-like patterns that exactly correspond to the surface texture(Figure 7). It is important to note that these strip-like patternswere complementary: dark blue strips on Ni image (very lowin Ni) overlap with the yellow–red strips on the Ti image(high in Ti). These complementary patterns indicated elemen-tal segregation on the surface. The same patterns can betraced also in the images of oxygen and, to a lesser degree, of

carbon. The way Ni and Ti were distributed on the surfaceafter heat treatment is an indication of the formation of singleNi and Ti surface oxides rather than a complex NixTiyOz

oxide.The yellow–red colors observed after heat treatment on the

Auger map of nickel correspond to the increase of surface Niconcentration that is in agreement with the data obtained inthe XPS studies described above. In contrast, a very low Nisurface concentration, on the level of the method sensitivityand uniform distribution of elements, was obtained on thesurface after heat treatment of samples that were preliminar-ily chemically etched and aged in boiling water (Figure 7,lower left corner). Chemical etching removes deformed ma-terial and eliminates the conditions for further elementalsegregation upon heat treatment.

Surface grinding to a 600-grit finish resulted in micron-scale surface texture. The peak-and-valley structure reflectsthe profile of nonuniform deformation of external surfacesublayers. The depth of the material disturbed through theabrasion procedure is a function of surface roughness (orabrasion particle size) and the depth of deformed material.16

Nonuniform deformation of surface sublayers induced by

Figure 6. SEM and Auger maps of elemental distribution on the surface of polished NiTi plate (600-gritfinish). (b) Carbon, (c) nickel, (d) titanium.

199SURFACE CONDITIONS OF NITINOL WIRES

600-grit finish grinding was the cause of chemical heteroge-neity observed after heat treatment: Ni diffusion to the sur-face was made easier through the channels of defects inducedin highly deformed regions of the material.

Another aspect of nonuniform surface deformation causedby abrasion procedure is a nonuniform structural state. It hasbeen shown that plastic deformation of � 28% causes amor-phization of NiTi.17 Thus, the nonuniformly deformed sur-face is composed of severely deformed surface regions inamorphous and less-deformed regions of the material in crys-talline states. Lattice defects resulting from plastic deforma-tion and the nonuniform character of deformation associatedwith the abrasion procedure reduce surface stability and cor-rosion resistance. Based on the presented study the poorreproducibility of the results on mechanically polished sam-ples in corrosion tests by Villermoux et al.3 when the pittingpotentials ranged in the 240–1000-mV interval can be attrib-uted to the residual deformation and nonuniform structuralsurface state.

Obviously, a polished 600-grit finish surface should not bean option for Nitinol implantation. The temperature of heat

Figure 8. Backscattering electron micrograph diffraction patterns ofNiTi surface: polished � chemically etched � heat treated (700 °C �1 h) obtained at 20-kV electron beam.

Figure 7. (a) SEM image and Auger maps of (b) nickel and (d) titanium of a polished and heat-treated(at 500 °C � 15 min) NiTi plate sample. For comparison the lower left panel (c) represents an Augermap of Ni distribution on the surface of a heat-treated sample that was preliminarily polished andchemically etched.

200 SHABALOVSKAYA ET AL.

Figure 9. SEM images of the surface of NiTi (� 2000 magnification): polished � chemically etched �heat-treated: (a) –500 °C � 1 h; (b) –700 °C � 1 h (deep gray oxide).

201SURFACE CONDITIONS OF NITINOL WIRES

treatment that helped reveal the heterogeneous surface state isnot a crucial parameter. Because nickel on the surface ofpolished Nitinol samples was in the elemental state (Ni0), iteasily diffused to the surface. That is why a similar effect ofNi and Ti segregation was also observed in our study afterEtO sterilization when the temperature did not exceed 60 °C.The presented results demonstrate differences between NiTiand Ti surfaces. The well-established fact that NiTi, due topreferential oxidation of Ti, has basically a Ti-based surfaceoxide does not automatically mean that this surface oxidedominates under variable conditions.

Electron Backscattering Diffraction Microscopy

Using EBSD the structural state of the Nitinol as-cast alloysurfaces after various surface treatments was evaluated. TheEBSD images obtained at low voltages, 5 kV, did not revealany patterns that could be associated with crystallinity eitherin the mechanically polished surface or in chemically etchedand aged in water samples. The latter indicated an amorphousstate of surface oxide at a depth of � 0.2 �m. At 20–25-kVthe electron beam penetrates deeper (� 1-�m depth), and thediffraction patterns observed at these voltages could be fitwith the use of the cubic lattice parameter (0.302 nm) char-acteristic to the high-temperature austenitic phase of Nitinol.The latter was in agreement with the composition of the bulkalloy.

Nitinol samples that were chemically etched and aged inboiling water did not reveal crystalline patterns after heattreatment at 500 °C at either 5- or 20-kV primary beams. Thismeans that new surface layers formed during heat treatmentat 500 °C and also the original oxide layers that were sensedby the 20-kV beam were still either amorphous or nanocrys-talline. The first very weak crystalline patterns at 5-kV elec-tron beam were observed only after heat treatment at 700 °Cfor 30 min. These weak patterns that became obvious after 1and especially 2 hours of annealing at 700 °C indicatedsurface reconstruction due to intensive atomic diffusion.These patterns could be fit with the use of the parameters forTiO2 in rutile (Figure 8).

Evaluation of surface morphology with the use of SEMshowed that in general morphology does not change after heattreatment at 500 °C when the surface revealed a slight po-rosity induced by chemical etching, though it became slightlyrougher [Figure 9(a)]. The crater-like pores have variousdiameters from 1 to 10 �m and can be either single or inclusters. After heat treatment at 700 °C the surface morphol-ogy changed tremendously [Figure 9(b)]. This surface did notreveal any pores. It became extremely rough due to recon-struction and growth of new external surface layers.

It should be pointed out that surface porosity induced bychemical etching does not affect surface stability. Nitinolsamples that were chemically etched and aged in boilingwater did not reveal pitting at least up to 1200–1400 mV. Thedetails on the effect of shape-setting heat treatment andchemical etching on the corrosion performance of Nitinolwires and tubing will be published later.8

SUMMARY

Based on the conducted study it can be concluded that com-mercial Nitinol wires differ significantly in their surfacechemistry and the level of surface contaminants. Surfacesublayers of studied wires with flack oxide are enriched in Nidue to multiple annealings at high temperatures.

The 600-grit finish surfaces revealed chemical heteroge-neity upon heat treatments. The latter is associated with anon-uniform plastic deformation induced during surfacepreparation. It is not recommended to use 600 grit finishsurfaces for Nitinol implantation because chemical heteroge-neity caused by the diffusion of free Ni atoms to the surfacecan also be induced at body temperatures. It is probablyunfair to compare the results of corrosion and implantationstudies for the 600-grit polished Nitinol with the resultsobtained on electropolished stainless-steel surfaces. In thelatter case, the surface layers deformed during grinding/pol-ishing procedure are completely removed and the factorscontributing to the chemical and structural heterogeneitycaused by plastically deformed surface layers are absent. Forthis reason, the fact that the 600 grit Nitinol surfaces areperforming at the level of electropolished stainless steel incorrosion studies in vitro and also during implantation in thebody is remarkable by itself.

Shape-setting heat treatment of polished material and as-received wires was accompanied by a tremendous increase insurface Ni concentration, which is undesirable, if not unac-ceptable, for material intended for use in the body. Specialsurface treatments must be adopted to prevent surface enrich-ment in Ni upon heat treatment.

A cost-effective surface treatment combining chemicaletching and aging in boiling water results in porous, amor-phous, Ni-depleted surface built exclusively from TiO2

highly corrosion-resistant oxide. These oxide films remaindepleted in Ni and amorphous at the temperatures requiredfor shape setting and maintain their stability in physiologicalsolutions.8

The authors acknowledge Ames Laboratory for its contributionsto surface analysis of NiTi samples, and Memry Corp. for helpingwith material for the study. We also thank A. Roytburd for theproductive discussions. This work was performed under the 1 R01HL67632-01 grant from the National Institute of Health (NHLBI).

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