comparison of friction force between corroded and noncorroded titanium nitride plating of metal...

ORIGINAL ARTICLE

Comparison of friction force between corrodedand noncorroded titanium nitride plating ofmetal brackets

Chia-Tze Kao,a Jia-Uei Guo,b and Tsui-Hsien Huangc

Taichung, Taiwan

aProfevisitinTaichbPostMediccProfedeparDeparTaiwaThe aucts oReprinScienKuo Nedu.twSubm0889-Copyrdoi:10

594

Introduction: Titanium nitride (TiN) plating is a method to prevent metal corrosion and can increase the surfacesmoothness. The purpose of this study was to evaluate the friction forces between the orthodontic bracket, withor without TiN plating, and stainless steel wire after it was corroded in fluoride-containing solution. Methods: Intotal, 540 metal brackets were divided into a control group and a TiN-coated experimental group. Theelectrochemical corrosion was performed in artificial saliva with 1.23% acidulated phosphate fluoride (APF)as the electrolytes. Static and kinetic friction were measured by an EZ-test machine (Shimadazu, Tokyo,Japan) with a crosshead speed of 10 mm per minute over a 5-mm stretch of stainless steel archwire. Thedata were analyzed by using unpaired t test and analysis of variance (ANOVA). Results: Both the controland TiN-coated groups’ corrosion potential was higher with 1.23% APF solution than with artificial solution(P\0.05). In brackets without corrosion, both the static and kinetic friction force between the control and TiN-coated brackets groups showed a statistically significant difference (P\0.05). In brackets with corrosion, thecontrol group showed no statistical difference on kinetic or static friction. The TiN-coated brackets showeda statistical difference (P\0.05) on kinetic and static friction in different solutions.Conclusion: TiN-coatedmetalbrackets, with corrosion or without corrosion, cannot reduce the frictional force. (Am JOrthod Dentofacial Orthop2011;139:594-600)

Orthodontic tooth movement followed the alveo-lar bone remodeling theory. Orthodontists areconcerned with the friction characteristics of

orthodontic brackets. Most orthodontic metal bracketsare made of stainless steel.1 In an oral environment, or-thodontic brackets are exposed to potentially damagingphysical and chemical agents. These conditions may af-fect the amount of metal corrosion.2 The outcome of the

ssor, Institute of Oral Biology and Biomaterial, College of Oral Medicine;g staff of Orthodontic Department, Chung Shan Medical University,ung, Taiwan, ROC.graduate student, Institute of Oral Biology and Biomaterial, College of Oraline, Chung Shan Medical University Hospital, Taichung, Taiwan, ROC.ssor, School of Dentistry, College of Oral Medicine; visiting staff of Dentaltment, Chung Shan Medical University, Taichung, Taiwan, RO andtment of Dentistry, Chung Shan Medical University Hospital, Taichung,n, ROC.uthors report no commercial, proprietary, or financial interest in the prod-r companies described in this article.t requests to: Tsui Hsien Huang, Institute of Oral Biology and Biomaterialce, College of Oral Medicine, Chung Shan Medical University, 110 Chien-orth Road, Section 1, Taichung, Taiwan 402, ROC; e-mail, [email protected], February 2009; revised and accepted, June 2009.5406/$36.00ight � 2011 by the American Association of Orthodontists..1016/j.ajodo.2009.06.034

corrosion is that metallic ions are released and the metalbracket surface becomes rough.3

Physical vapor deposition (PVD)-type ion plating isa surface modification technique capable of coatingsubstrates with various thin-layer films.4 This procedureis currently being successfully applied to the coating ofdental instruments with titanium nitride (TiN), resultingin improved properties for rotary and endodontic cuttinginstruments.5 The TiN film demonstrates favorable char-acteristics with regard to corrosion resistance, wear resis-tance, and increased hardness.6,7 Except anticorrosioneffects of TiN plating, the friction effects between theTiN plating bracket and the orthodontic wire isimportant in orthodontic treatment.

Friction is a function of the dynamic relationshipamong the archwire, bracket, and ligation type in theoral environment. In vitro studies on bracket and wirefriction were affected by the following factors: type ofarchwire and bracket materials8; their size and shape9;the width and slot dimensions10; the ligature mate-rials11; the type of ligation12; the surface composition,roughness, and cleanliness of the contacting surfaces13;the bracket-to-archwire positioning in a 3-dimensionalspace14; the interbracket distances15; and saliva or

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Kao, Guo, and Huang 595

solution, plaque, and the acquired pellicle.16 There weresignificant differences in friction noticed between dryand wet test conditions.17 When human saliva and drytesting were compared, the human saliva sometimes be-haved as an adhesive on steel-on-steel couples,18 but atother times it behaved as a lubricant on nickel titaniumarchwires on stainless steel brackets.19

To prevent tooth decay in orthodontic treatment,fluoride is usually applied. The fluoride level in the oralcavity varies according to the prophylactic treatment.Fluoride is used at concentrations of up to 1% in tooth-pastes and mouthwashes and about 2% when the aim isto eliminate enamel stains.20 These substances have a pHrange of about 3.5 to 7.0.21 In fluoride-containing solu-tions, the friction of orthodontic metal brackets can beaffected. Our previous study showed that friction forcesin fluoride-containing solutions is higher than that in pH6.5 artificial solutions.22

The TiN-plated metal surface has anticorrosion ef-fects.4 The purpose of the present study was to evaluatethe friction forces between the orthodontic bracket, withor without TiN plating, and stainless steel wire after itwas corroded in fluoride-containing solution.

MATERIAL AND METHODS

Sample preparation

In total, 540 bracket samples were used in the presentstudy. Each test condition contained 30 brackets (Fig 1).The metal brackets were selected from Tomy OPAK sys-tem bracket with a 0.022-in 3 0.028-in slot (Tomy Co.,L, Tokyo, Japan). The 0.019-in3 0.025-in stainless steelwire (3M Unitek, Monrovia, Calif) was used for the fric-tion test. The ligation between the bracket and the wirewas a clear AlastiK module (Quik-Stick Clear, A-1AlastiK, 3M Unitek [Monrovia, Calif). The bracket andarchwire were cleaned with alcohol wipes before the test.

The testing solutions used in the present study weredistilled water, artificial saliva (Sinphar PharmaceuticalCo., Ltd, Taipei, Taiwan) (Table I), and 1.23% acidifiedphosphate fluoride (APF) (0.2 mass % NaF 1 0.17mass % H3PO4, pH 3.5) solution.

Titanium nitride (TiN) ion plating

The TiN plating employed is according to the methodpreviously referred to by Hai et al.23 The metal bracketswere ultrasonically cleaned in both acetone and ethanolbaths. Specimens were first placed for 10 minutes in anion-plating apparatus and were exposed to argon gasplasma created under 400-W radiofrequency (rf) power,67-megapascal (mPa) argon gas pressure, and –400-Vbias voltage.11 After this and prior to TiN ion coating,a 100-nm primary layer of titanium undercoating was

American Journal of Orthodontics and Dentofacial Orthoped

applied to improve the bonding between the substrateand the subsequent TiN film. TiN ion plating was thenperformed under conditions of 400-W rf power,13-mPa argon gas pressure, 53-mPa nitrogen gas pressure,and –1-kV bias voltage. The substrate temperature wasmaintained at approximately 200�C by means of an infra-redheat source. The thickness of theTiNfilmwas controlledby changing the ion plating time with the use of a quartzcrystal microbalance.

Electrochemical corrosion

The method followed here was that first described byKuphasuk et al.24 A mixture of epoxy glue and silverpaste was used to attach the back surface of each spec-imen to a copper wire, which provided electrical contactto the electrochemical corrosion-testing apparatus. Arti-ficial saliva and 1.23% APF solution were chosen for cor-rosion testing. Electrochemical corrosion studies wereconducted at 37�C 6 1�C, and the test system incorpo-rated an atmosphere composed of nitrogen gas in orderto simulate the intraoral environment and reduce anypotential fluctuation in the oxygen concentration ofthe electrolyte. The g/AgCl reference electrode, platinumplate as counter electrode, and platinum wire for bracketligation as the working electrode were prepared. The3-electrode assembly was connected to CH InstrumentModel 600C Series Potentiostat/Galvanostat (CH Instru-ments, Inc., Austin, Tex). The data was controlled and re-corded by computer. Corrosion potential (Ecorr) wasmeasured over a 100-minute period. Voltammetry wasperformed at a scanning rate of 0.25 mV per second inthe anodic direction, with a range of –1000 to 500mV/saturated calomel electrode.

Friction test

After the corrosion potential was calculated, usingthe data on the Jiehan 5000 Electrochemical Worksta-tion (Jiehan Co, Taipei, Taiwan) the new sample wasallowed to corrode for 23 hours. After sample corrosion,the bracket and wire friction were measured in a wetsolution that contained artificial saliva (Fig 2).

The test procedure was modified from our previousdesign.22 Testing was performed on an EZ-test machine(Shimadazu, Tokyo, Japan) with a crosshead speed of 10mm per minute over a 5-mm stretch of archwire. Aplumb line was hung to ensure that the bracket mountwas parallel with the vertical line scribed on the steelbar base of the bracket mount assembly. The 5-N loadcell was calibrated to between 0 and 5 N, and the arch-wire was drawn through the bracket as the crossheadmoved inferiorly at 10 mm per minute. This crossheadspeed was selected because a previous study found no

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Fig 1. Flow chart of test system. APF, acidic phosphate fluoride; CP, constant current corrosion;CVC, constant voltage corrosion.

Table I. Content of SaliLube* artificial saliva

Content Amount (mg)Sodium chloride 0.844Potassium chloride 1.2Calcium chloride anhydrous 0.146Magnesium chloride 6 H2O 0.052Potassium phosphate dibasic 0.34Sorbitol solution (70%) 60Methylparaben 2Hydroxyethyl cellulose 3.5

*Sinphar Pharmaceutical Co., Ltd, Taipei, Taiwan.

596 Kao, Guo, and Huang

significant difference with crosshead speeds of 0.5 to 50mm per minute.25 Care was taken to align the archwireso that the sample was parallel with the vertical frame-work of the machine. The bracket was pulled verticallyby a loop of 0.018-in stainless steel wire, and the forcerequired to initiate and maintain movement of thebracket over the 5-mm test distance was measured.The program was set to highlight the maximum fric-tional force at initial movement, which was taken to rep-resent the peak static frictional resistance.

Data acquisition and statistical analysis

The load cell registered the force levels needed tomove the wire through the bracket, and these valueswere stored on a computer hard disk. The data were

May 2011 � Vol 139 � Issue 5 American

then analyzed using a statistical package (Primer,McGraw Hill, New York, NY,). The corrosion potentialwas static by an unpaired t test. A completely random-ized design (2-way) analysis of variance (ANOVA) wasused to test for significant differences among the solu-tions. The Student-Newman-Keuls multiple comparisonof means procedure at P\0.05 was used to determinedifferences between the groups. An unpaired t test wasused to detect the frictional difference between theTiN-coated groups and control groups with P\0.05 ac-cepted as showing a significant statistical difference.

RESULTS

Corrosion potential measurement

The control group and TiN group corrosion potentialshowed a statistical difference in artificial saliva (–0.0560.01 vs –0.036 0.01, t5 4.47, P\0.05) and a nonstatis-tical difference in 1.23% APF solution (–0.25 6 0.05 vs–0.26 6 0.08, t 5 –0.355, P .0.05). Both the controland TiN-coated groups’ corrosion potential was higherwith 1.23% APF solution than with artificial solution(P\0.05) (Fig 3).

Friction force measurement

Bracket without corrosion. The friction forcewasmea-sured in artificial saliva by a universal testingmachine. The

Journal of Orthodontics and Dentofacial Orthopedics

Fig 2. Diagrammatic representation of experimental setup.

Fig 3. Corrosion potential measurement. TiN noncoatedand TiN-coated metal brackets in artificial saliva and1.23% acidic phosphate fluoride (APF) solutions. Samecharacters represent statistical difference at P\0.05.


static friction force between the control group and theTiN-coated brackets group showed a statistically signifi-cant difference (t 5 –3.6, P 5 0.002\0.05) (Fig 4) Thekinetic friction force between the control and TiN-coated brackets groups showed a statistically significantdifference (t 5 –3.042, P 5 0.007\0.05) (Fig 4) Bothcontrol and TiN-coated groups’ static friction was higherthan kinetic friction.

Bracket with corrosion. The static friction force of thecontrol group showed no difference (P 5 0.218 .0.05)in different solutions (Fig 5) The static friction force ofthe TiN-coated group showed a statistical difference(P 5 0.000 \0.05) in different solutions (Fig 5,Table IV). The highest static friction was in the TiN-coated bracket group, which corroded in distilled water(11.50 6 1.48 N).

The kinetic friction force of the control group showedno difference (P 5 0.228 .0.05) in different solutions(Fig 6). The kinetic friction force of the TiN-coated groupshowed a statistical difference (P5 0.045\0.05) in dif-ferent solutions (Fig 6).

DISCUSSION

It is reported that only human saliva can be used toquantify the magnitude or to rank the efficiency or re-producibility of orthodontic sliding.26 To assess frictionand its coefficients in the wet state, human saliva is suit-able. The present study was tested in a wet solution thatcontained human artificial saliva.

As reported, the TiN film demonstrates favorablecharacteristics with regard to corrosion resistance, wearresistance, and increased hardness.4,5 The present


study showed that corrosion potential was similarbetween the control and TiN-coated groups in artificialsaliva and 1.23% APF solutions (Fig 3). When bracketswere electrochemically corroded in artificial saliva, thecontrol group had a higher corrosion potential thandid the TiN-coated group (P \0.05). It demonstratedthat TiN-coated brackets have good anticorrosion prop-erties in chloride-containing artificial saliva. However,the result was controversial for TiN-coated brackets in1.23% APF solution (Fig 3). The electrochemical corro-sion potential of the control and TiN-coated groupsshowed no difference. It demonstrated that TiN platingof brackets did not increase bracket anticorrosion abilityin 1.23% APF solution. The reason is presumably that


Fig 4. Friction force measurements were taken in artificial saliva solution in contained system. In staticfriction comparison, noncorroded control group and TiN-coated brackets group showed statistically sig-nificant difference (t5 –3.6, P5 0.002\0.05). In kinetic friction comparison, noncorroded control groupand TiN-coated brackets group showed statistically significant difference (t5 –3.042, P5 0.007\0.05).

Fig 5. Static friction measurements of control group and TiN-coated brackets after electrochemicalcorrosion in different solutions. APF, acidic phosphate fluoride; CP, constant current corrosion;CVC, constant voltage corrosion.


corrosion of titanium and its alloys are enhanced in anacidic environment. The F� ions in the solution combinewith H1 ions to form HF, even at low fluoride concentra-tions.27 The other studies showed similar results—thatthe fluoride ions in the prophylactic agents were re-ported to cause corrosion and discoloration of titaniumand its alloys.28,29 The natural characteristics of theTiN coating caused corrosion failure mainly by theautocatalytic effect in the test solution.30 Thus, the pres-ent result found TiN plating of brackets did not increasecorrosion resistance in fluoride-containing solutions.

In the brackets without corrosion, the control and TiN-coated groups’ static and kinetic friction forces wereshown to have a statistical difference (Fig 4). The TiN-


coated group had higher friction force than did the controlgroup. It demonstrated that uncorroded TiN-coatedbrackets do not reduce the friction. The reason might bebe the irregular morphology of the bracket, which causedthe TiN plating to be uneven. The TiN-coated bracket sur-face becamemore rough than the surface without the TiNcoating. The present study used a single-layered TiN plat-ing method. It is reported that multilayered Ti/TiN coat-ings can have a larger impedance and lower porosity.31

In a metal-coated system, the porosity and adhesionstrength of coatings are 2 main factors affecting corrosionresistance.32,33 The porosity is associated with thecompactness of the coating, and the lower thecalculated porosity, the higher the packing factor.


Fig 6. Kinetic friction measurements of control group and TiN-coated brackets after electrochemicalcorrosion in different solutions. APF, acidic phosphate fluoride; CP, constant current corrosion;CVC, constant voltage corrosion.


The ligation of bracket and wire used the elastic mod-ule in the present study. It showed that steel ligaturesgenerated greater frictional forces than plastic modulesand that moistening caused an insignificant increase infriction for steel ligatures and was irrelevant to the plasticmodules.34 To prevent too many side effects that mightaffect the frictional forces of the wire and bracket, an elas-tic module was used as the ligation method in this study.

After corroding of the bracket in different solutionsby electrochemistry, the static friction result did notshow a difference from the control group. However,the electrochemical treatment of the TiN-coated bracketgroup showed a statistical difference (P\0.05) on thestatic friction result (Fig 5).

The kinetic friction result showed findings similar tothose of the static friction result in the control group andTiN-coated group (Fig 6). The bracket corroded in artifi-cial saliva and 1.23% APF showed no difference in itsfriction measurements. However, the static friction forceis still higher than the kinetic friction force in both con-ditions.

In one study that treated the surface of wire by ionimplantation, the outcome showed questionable bene-fits to the frictional properties of the wire when in clin-ical use.35 In the present study, the stainless steel wiredoes not have corrosion or plating. The stainless steelwire is changed every time after doing the friction test,as there are reports that many factors impact the friction.Besides the alloy composition of the archwire, the wiresize, the elasticity, and the surface structure, includingsurface treatments, also play important roles. In thepresent study, the tests were more similar to clinical ap-plication by using the same wire size. The final result


showed that the TiN-coated bracket was expected to re-duce the friction, but results were controversial.

CONCLUSIONS

Because the oral environment is complex, the metalcan be corroded. The TiN ion plating method can im-prove the metal defect. However, in the present study,applying TiN plating on the metal bracket does not pro-duce an outcome that meets the study purpose. The cor-rosion potential does not increase and friction forceapparently does not reduce. Further study on improvingthe metal surface smoothness and reducing surface fric-tion is needed.

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comparison of friction force between corroded and noncorroded titanium nitride plating of metal...

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