the effect of argon and nitrogen ion implantation on nickel ... joe (2).pdfthe effect of argon and...

Basic Research—Technology

The Effect of Argon and Nitrogen Ion Implantationon Nickel-Titanium Rotary InstrumentsCarlos Frederico Brilhante Wolle, MSc,* Marcos Antonio Zen Vasconcellos, PhD,†,‡

Ruth Hinrichs, PhD,‡,§

Alex Niederauer Becker, MSc,* and Fernando Branco Barletta, DMD*,k

AbstractIntroduction: This qualitative study investigated theeffect of N2

+ and Ar+ ion implantation on morphologicalterations and fatigue resistance in Pro Taper S1 NiTi(Dentsply-Maillefer, Ballaigues, Switzerland) rotary instru-ments. Methods: Instruments were divided into threegroups: N2

+ implanted, Ar+ implanted, and unmodifiedcontrol group. All instruments were used to prepare fivecurved canals in epoxy resin blocks with brushing motion.The instruments were examined in a scanning electronmicroscope (SEM) before use, after first use, and afterthe fifth use. A more demanding cyclic fatigue test wasundertaken, submitting the instruments to 15-secondperiods of continuous rotation inside the curved canalswithout a brushing motion. Crack formation was analyzedwith the SEM, and the number of 15-second periodsrequired to fracture each instrument was recorded.Results: No significant morphologic alterations wereobserved in the instruments after the preparation of fivecanals. Crack density was similar in all groups. In the subse-quent cyclic fatigue test, instruments implanted withnitrogen performed worse than those implanted withargon and the control group. Fracture faces show differ-ences in the fracture modes. Conclusions: Ar+ implanta-tion improved the performance of S1 files moderately,whereas nitrogen ion–implanted files performed worse inthe fatigue test. A reduction in file performance seems tobe caused by nitrogen diffusion in the grain boundaries,instead of the desired improvement caused by titaniumnitride formation. (J Endod 2009;35:1558–1562)

Key WordsCyclic fatigue resistance, Endodontics, Ion implantation,Nitinol, Scanning electron microscopy

From the *School of Dentistry, Universidade Luterana doBrasil (ULBRA), Canoas, RS, Brazil; †School of Physics, Universi-dade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, RS,Brazil; ‡Microanalysis Laboratory, School of Physics, UFRGS,Porto Alegre, RS, Brazil; §School of Geosciences, UFRGS, PortoAlegre, RS, Brazil; and kUniversidade de Santa Cruz do Sul(UNISC), Santa Cruz, RS, Brazil.

Supported in part by the Brazilian Agencies CAPES, CNPq,and FINEP.

Address requests for reprints to Dr Carlos Frederico Bril-hante Wolle, Av Nossa Senhora das Dores, 830/401, CEP:97050-330, Santa Maria, RS, Brazil. E-mail address:[email protected]/$0 - see front matter

Copyright ª 2009 American Association of Endodontists.doi:10.1016/j.joen.2009.07.023

1558 Wolle et al.

Since Walia et al (1) proposed the fabrication of endodontic instruments fromsuperelastic nickel-titanium (NiTi) alloys, it is possible to use rotary instrumentation

for root canal preparation. The flexibility, superelasticity, and resistance to torsion ex-hibited by this alloy allow continuous rotation inside root canals, even if they arestrongly curved. The drawback of these files is that they eventually break without priorindication of the impending catastrophic failure.

Over recent decades, there has been a great deal of development in surface modifi-cation techniques that intend to avoid microcrack formation that will give rise to failure. Thepurpose is to enhance surface strength without changing bulk properties, like superelas-ticity and toughness. Among these techniques, ion implantation is especially noteworthy. Inthis technique, gaseous atoms of different species are ionized and subsequently acceleratedwith very high voltages (up to several hundreds of kilovolts). When they hit the samplesurface, theses ions get buried below the surface (depth depending on acceleratingvoltage), leaving a track of dislocations that enhance materials toughness.

A studies performed by Lee et al (2) used x-ray diffraction to detect alterations inthe microstructure of endodontic instruments after boron ion implantation, whichincreased surface hardness. It has been shown that nitrogen ion implantation increasescutting efficiency with a consequent enhancement in instruments endurance (3). Otherstudies report an increment in wear resistance (4). These results have been attributed tothe formation of a titanium nitride (TiN) layer, which prevents crack formation. The TiNconfers special properties to the surface, like high hardness and increased resistance tofatigue, wear, and corrosion (5).

Several attempts have been made to obtain safer, more efficient, and more reliableinstruments inducing changes to their external surface with different treatments (6, 7).However, the variables involved in the procedures of surface modification have not yetbeen entirely elucidated. Up to now, it was not clear which of the macroscopic prop-erties of endodontic instruments react to the microscale processes.

In this qualitative study, rotary NiTi instruments were subjected to nitrogen andargon ion implantation. The instruments then underwent instrumentation of simulatedcurved canals and cyclic fatigue testing to observe the effect of the different surface treat-ments on morphologic alterations, crack formation, crack growth, and the instrumentsresistance to fatigue fracture.

Materials and MethodsSamples

Three batches of 10 rotary S1 instruments (length 25 mm, Pro Taper System;Dentsply-Maillefer, Ballaigues, Switzerland) were tested to obtain a qualitative concep-tion of the influence of ion implantation on file performance. The samples in batch 1 and2 were implanted with N2

+ and Ar+ ions, respectively, in a 500-kV High Voltage Engi-neering Europa (Amersfoort, The Netherlands) accelerator. For each implantation,10 files were mounted on a cylindrical rotating base to expose the entire file circumfer-ence to the ion beam. Implantation parameters were previously calculated by usingtransport of ions in matter (TRIM) code (8), with the purpose of depositing the differentions at similar depths below the surface, around 50-nm deep. Accelerating voltage was100 kV, ion dose was 1017 ions/cm2, and current density was 1 mA/cm2. The sampletemperature during implantation was monitored and was always below 100�C. A pol-ished NiTi plate was implanted simultaneously with each batch of files for subsequentnuclear reaction analysis (NRA). This analytic technique allows quantification of the

JOE — Volume 35, Number 11, November 2009

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Figure 1. Tip of the instruments (left) and the region 3 to 5 mm away from the tip (right), from instruments that were (A) nitrogen ion–implanted, (B) argon ion–implanted, and (C) control groups. No morphologic alterations were observed in the instruments after instrumentation of five curved canals. Scale bars are 200 mmon the left and 500 mm on the right.

nitrogen content in the near surface layer. The sample is irradiated withfast protons, and the gamma-rays that are produced as a result of theproton-induced nuclear reaction in nitrogen nuclei are counted, butNRA needs a flat surface area of several square millimeters to be per-formed. The third group of rotary instruments consisted of controlsamples and did not undergo any type of treatment.


Instrumentation (Clinical Simulation with a BrushingMotion)

One hundred fifty pristine epoxy resin blocks containing simulatedcurved canals were instrumented using the brushing motion proposedby Webber and Machtou (9). The blocks had hardness between 78 and80 shore D similar to dentine, which has 94 shore D. The canal length

Figure 2. SEM micrographs of (A) N2+-implanted, (B) Ar+-implanted, and (C) control group file cutting edges. Few microcracks after the first fatigue test in all

samples. Scale bars 20 mm.

Effect of Ar+ and N2+ Ion Implantation on NiTi Rotary Instruments 1559


Figure 3. SEM micrographs of microcracks in the cutting edges of the (A) N2+-implanted instrument after nine fatigue tests (broken sample). (B) Ar+-implanted

and (C) control group instruments, both after 12 fatigue tests. Scale bars 20 mm.

was 17 mm, canal diameter compatible with #10 caliber instruments,canal curvature of 64�, and 6-mm curvature radius (Dentsply Maille-fer). An Endo Pro Torque (VK Driller, Sao Paulo, Brazil) motor wasused at 300 rpm and 2 N/cm torque, according to the instrumentationsequence of Webber and Machtou (9). The simulated canal was keptfull of a low surface tension anionic detergent solution, with irrigationvolume set to 2 mL per instrument change.

Five blocks were instrumented with each file. Morphologicchanges to file surfaces were evaluated before use, after first use, andafter fifth use in accordance with Troian et al (10) using a scanning elec-tron microscope (SEM) in secondary electron imaging mode withmagnifications of 100� and 250�. The tip of the file and a portionof the cutting edge, 4 to 6 mm away from the tip, were analyzed. Beforeeach observation, instruments were cleaned brushing them with thedetergent solution for 3 minutes and then sterilized in an autoclave tokeep the conditions as similar to real clinical use as possible.

Cyclic Fatigue TestsAfter the preparation of five simulated canals, all instruments were

subjected to a more demanding fatigue test consisting of 15 seconds ofcircular motion (75 rotations without brushing motion) of the instru-ment completely inserted in the curved canal followed by visual inspec-tion to detect incipient failure. These steps were repeated as many timesas necessary to break the instruments. During the 15-second periods

Figure 4. A histogram showing the number of 15-second periods of fatiguetests until failure of the instruments in ascending performance order. Meanvalues (standard errors) were 10.9 (1.7) for the N2

+-implanted, 20.0 (2.2)for the Ar+-implanted, and 18.4 (3.0) for the control files (mean shown onthe right).

1560 Wolle et al.

inside the canal, the resin block and handpiece were stabilized in fixedpositions to prevent involuntary movements (11). The instruments wererun at 300 rpm with a torque of 2 N/cm (9, 12).

In this test, the files suffered cyclic bending because of the rotationin the curved canal. Because no brushing motion was used, maximalflexure always occurred at the same location of the file. Preliminary testsshowed that 15-second steps were adequate for periodic inspectionbefore breakage and that the most probable fracture point was locatedapproximately 4 mm away from the tip. SEM images were taken ofcutting edges of this region after repetitions 1, 2, 4, 9, and 12 to observeearly crack formation at the approximate location in which failure wasbound to happen. Two instruments of each group were also examinedin greater detail at higher magnifications. Fracture faces were examinedat magnifications around 300�.

ResultsCharacterization of the Ion Implantation

NRA showed that the nitrogen ions were implanted at a depth ofapproximately 60 nm in good agreement with TRIM simulations thathad indicated 65 nm range with a distribution width (straggling) of28 nm. NRA also showed that the implanted nitrogen amount gaverise to a maximum concentration below stoichiometric TiN.

Morphologic Changes after Five Cycles of BrushingMotion Instrumentation

All files withstood the five clinical instrumentations of simulatedcanals without significant morphologic changes. Figure 1 shows repre-sentative SEM images in low magnification from the instrument tip andfrom the region 3 to 5 mm away from the tip after the clinical instrumen-tation. Neither material loss nor distortion of flutes was observed at thetip of the file and at the cutting edge next to the tip where maximumflexure occurred. None of the instruments exhibited fractures in thistest.

Cyclic Fatigue ResistanceSEM analysis of all instruments revealed a low density of micro-

cracks after the first fatigue test (Fig. 2A-C). The sample implantedwith N2

+ shown in Figure 3A fractured after the ninth fatigue test,whereas the Ar+ implanted file and the control group showed no signif-icant difference in the number of microcracks after 12 fatigue tests(Fig. 3B and C).

The fatigue tests were continued until fracture of all instruments.Figure 4 shows a histogram of the number of 15-second periods neces-sary to fracture each file. The dispersion of this number is very high;some files broke after five periods, whereas others from the same groupwithstood up to 31 periods. Mean values (standard errors) of thenumber of periods necessary until fracture were 10.9 (1.7) for theN2

+-implanted files, 20.0 (2.2) for the Ar+-implanted files, and 18.4



Figure 5. Scanning electron fractograph showing the angular sudden fracture surfaces of (A) N2+-implanted, (B) Ar+-implanted, and (C) control group instru-

ments. The slow growth pattern can be seen at the smooth areas on the outer rim of b and c.

(3.0) for the control group. Instruments implanted with Ar+ hada similar performance as the instruments of the control group. Theinstruments implanted with N2

+ performed significantly worse, withtheir mean number of fatigue cycles around half the value of the othergroups.

Figure 5 shows representative SEMs of the fracture surfaces ofN2

+-implanted samples, Ar+-implanted samples, and control groupinstruments. The nitrogen-implanted sample showed the characteristicirregular features of sudden fracture, with breakage at the grainboundaries. The other two groups showed both the irregular fracturepatterns of catastrophic fracture and the smooth areas of slow crackgrowth. The slow growth pattern can be seen at the smooth areas onthe outer rim of these instruments.

DiscussionSeveral authors have tried to enhance the overall behavior of rotary

instruments with ion implantation treatments that, although expensive,could result in longer tool life and safer employment of the instrument.

The good performance of NiTi rotary instruments without anysurface modification when used in the recommended way to preparea limited number of curved canals in epoxy resin blocks wasconfirmed in this study, corroborating the results of Troian et al(10). Ankrum et al (13) observed distortion in two of 84 filesfor the Pro Taper system after they were used to instrument 15severely curved root canals of extracted molars. Schafer and Schlin-gemann (14) observed that several NiTi instruments suffered frac-tures during the first use, but this difference to our results isexplained by their very demanding endurance test; the authorsopened the canals straightly to the complete working length andobserved the most frequent fracture in large caliber instruments.A novel model for in vitro analysis has been proposed to mimicoperator’s forces in three dimensions, which may contribute tothe understanding of clinical mishaps (15). Also, Sevec and Powers(16) observed metal losses from the cutting angles of ProFile flutesand deformations after just one use, but no details were given on theharshness of their test. The propagation of previous cracks anddefects is a possible explanation for instrument fracture (17, 18).Additionally, the tip design, angle, and instrument cross-sectionare factors that influence flexibility and torsional strength (19).The proportion of shank and flute of the instrument cutting areamay also be considered (20).

In the present study, instruments implanted with argon exhibitedslightly better results in the cyclic fatigue resistance tests than the instru-ments in the control group. The instruments implanted with nitrogenperformed significantly worse than the other groups. Despite the factthat nitrogen and argon had been implanted into similar depths, thereare crucial differences between them, particularly the difference of theiratomic masses. Nitrogen causes much less point defects in the crystal-line structure. Computer simulations of the effects of ion implantation in


the metal structure performed with the TRIM software (8) show that theargon ion produces four times more vacancies than the nitrogen ion.Another important difference is that nitrogen can react with the metalto form the very hard titanium nitride, whereas the noble gas argonproduces only mechanical defects. However, the concentration ofnitrogen necessary to form stoichiometric TiN could not be reachedwithin reasonable implantation times.

This study did not find a significant reduction in crack formationand growth due to Ar+ or N2

+ ion implantation during the simulatedclinical use in curved canals. This is in contrast with results from Rap-isarda et al (4), who found enhanced tool life because of implantationcompared with a control group that exhibited more microfracturing.Their results may be because of the different implantation parametersand different testing procedures.

With respect to the cyclic fatigue tests, instruments implanted withAr+ were able to bear a greater number of fatigue cycles before fracture,but the enhancement caused by argon ion implantation was slight, and wesuppose that it was caused by work hardening in the volume of maximumion concentration in the outermost surface layer. In this work, we did notfind improvements with the use of nitrogen implantation. Mechanicalhardening rather than TiN formation seems to have the more favorableeffect on instrument performance improvement. In nitrogen-implantedinstruments, no slow growth pattern was observed at the failure surface,whereas in the argon-implanted and the control group smooth areasslow growth occurred. After a large number of bending cycles, the cata-strophic failure occurred for all samples as a result of material fatigue(21). The more pronounced intergranular behavior from samples im-planted with nitrogen can be caused by weakening induced by diffusionof the small nitrogen ion at the grain boundaries.

AcknowledgmentThe authors thank the Ion Implantation Laboratory, Instituto

de Fısica, Universidade Federal do Rio Grande do Sul. Helpfuldiscussions with Dr Oli Dors are acknowledged.

References1. Walia HM, Brantley WA, Gerstein H. An initial investigation of the bending and

torsional properties of Nitinol root canal files. J Endod 1988;14:346–51.2. Lee DH, Park B, Saxena A, et al. Enhanced surface hardness by boron implantation

in Nitinol alloy. J Endod 1996;22:543–6.3. Rapisarda E, Bonaccorso A, Tripi TR, et al. The effect of surface treatments of nickel-

titanium files on wear and cutting efficiency. Oral Surg Oral Med Oral Pathol OralRadiol Endod 2000;89:363–8.

4. Rapisarda E, Bonaccorso A, Tripi TR, et al. Wear of nickel-titanium endodonticinstruments evaluated by scanning electron microscopy: effect of ion implantation.J Endod 2001;27:588–92.

5. Simon G. Acos Nitretados a Plasma [thesis]. Porto Alegre (RS): UniversidadeFederal do Rio Grande do Sul 1995.

6. Boessler C, Paque F, Peters AO. The effect of electropolishing on torque and forceduring simulated root canal preparation with ProTaper shaping files. J Endod 2009;35:102–6.

Effect of Ar+ and N2+ Ion Implantation on NiTi Rotary Instruments 1561

7. Vinothkumar TS, Miglani R, Lakshminarayananan L. Influence of deep dry cryogenic
treatment on cutting efficiency and wear resistance of nickel-titanium Rotaryendodontic instruments. J Endod 2007;33:1355–8.

8. Ziegler JF. Simulation Code TRIM. 2003. Available at: http://www.srim.org. AccessedMay 31, 2006.

9. Webber J, Machtou P. ProTaper: endodontics has never been easier. Dynamics2001;91:45–6.

10. Troian CH, So MV, Figueiredo JA, et al. Deformation and fracture of RaCe and K3endodontic instruments according to the number of uses. Int Endod J 2006;39:616–25.

11. Chaves Craveiro de Melo M, Guiomar de Azevedo Bahia M, Lopes Buono VT. Fatigueresistance of engine-driven rotary nickel-titanium endodontic instruments. J Endod2002;28:765–9.

12. Plotino G, Grande NM, Sorci E, et al. Influence of a brushing working motion on thefatigue life of NiTi rotary instruments. Int Endod J 2007;40:45–51.

13. Ankrum MT, Hartwell GR, Truitt JE. K3 Endo, ProTaper, and ProFile systems:breakage and distortion in severely curved roots of molars. J Endod 2004;30:234–7.

14. Schafer E, Schlingemann R. Efficiency of rotary nickel-titanium K3 instrumentscompared with stainless steel hand K-Flexofile. Part 2. Cleaning effectiveness and

1562 Wolle et al.

shaping ability in severely curved root canals of extracted teeth. Int Endod J2003;36:208–17.

15. Diop A, Maurel N, Oiknine M, et al. A novel platform for in vitro analysis of torque,forces, and three-dimensional file displacements during root canal preparations:Application to ProTaper rotary files. J Endod 2009;35:568–72.

16. Svec TA, Powers JM. The deterioration of rotary nickel-titanium files undercontrolled conditions. J Endod 2002;28:105–7.

17. Shen Y, Haapasalo M, Cheung GS, et al. Defects in nickel-titanium instruments afterclinical use. Part 1: relationship between observed imperfections and factors leadingto such defects in a cohort study. J Endod 2009;35:129–32.

18. Shen Y, Cheung GS, Peng B, et al. Defects in nickel-titanium instruments after clinicaluse. Part 2: fractographic analysis of fractured surface in a cohort study. J Endod2009;35:133–6.

19. Camara A, Martins RC, Viana ACD, et al. Flexibility and torsional strength of ProTaperand ProTaper universal Rotary instruments assessed by mechanical tests. J Endod2009;35:113–6.

20. Biz MT, Figueiredo JAP. Morphometric analysis of shank-to-flute ratio in rotarynickel-titanium files. Int Endod J 2004;37:353–8.

21. Alapati SB, Brantley WA, Svec TA, et al. SEM observations of nickel-titanium rotaryendodontic instruments that fractured during clinical use. J Endod 2005;31:40–3.


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