Tribological Performance of DLC-Coated Stainless Steel, TMA and Cu-NiTi

8/12/2019 Tribological Performance of DLC-Coated Stainless Steel, TMA and Cu-NiTi

1/8International Orthodontics2008 ; 6 : 335-342 335

2008. CEO.dit par / Published by Elsevier Masson SAS.

Tous droits rservs/All rights reservedArticle originalOriginal article

Performances tribologiques de lacierinoxydable, du TMA et du Cu-NiTirecouverts de DLC

Tribological performance of DLC-coated stainlesssteel, TMA and Cu-NiTi

Zakaria BENTAHAR1, Michel BARQUINS2, Martial CLIN3, Nezha BOUHAMMAD4,Khalid EL BOUSSIRI5

1 DCD, SQODF, Professeur agrg en orthodontie, Laboratoire de biomatriauxmtalliques, Facult de mdecine dentaire de Casablanca.2 Professeur dhydrodynamique et mcanique physique, cole suprieure de physiquechimie de la ville de Paris (ESPCI).3 Professeur de physique, Laboratoire de physique de la matire condense, Facult dessciences dAmiens.4 DCD, Pratique prive, Agadir, Maroc.5Professeur de biomatriaux, Docteur dtat en physico-chimie des matriaux, Laboratoirede biomatriaux mtalliques, Facult de mdecine dentaire de Casablanca.

Correspondance et tirs part / Correspondence and reprints:Z BENTAHAR, 3 rue Zemamra, Anfa, 20050 Casablanca, [email protected]

Rsum Lamlioration de la friction et la rsistance lusure font partie des proprits des couches de carbone amorphe. Lobjectif de notre travailest dtudier les performances tribologiques des couches de carboneamorphe (DLC : Diamond-Like Carbon) dposes sur des fils ortho-dontiques : Cu-NiTi, acier et TMA.

Les substrats tudis (acier inoxydable, TiMo, Cu-NiTi) sont des alliages frquemment utiliss en orthodontie. Des tiges .019 x .025 inch de5 cm ont t fixes sur un support mtallique en acier inoxydable. Lesdpts de DLC ont t raliss selon la technique plasma enhancedchemical vapor deposition. Un tribomtre muni dune balance, dunmoteur et dun capteur lectronique a t utilis pour calculer le coeffi-cient de frottement. Pour vrifier la reproductibilit, chaque test a t repris 10 fois. Lensemble des donnes a t recueilli laide du logi-ciel WinDataq Lite pour calculer le coefficient de frottement et repr-

senter son volution en fonction du temps. Le coefficient de frottement du couple acier inoxydable recouvert de DLC/acier non recouvert a subi une augmentation par rapport aucouple dacier non recouvert, alors que pour les deux autres couples(acier/Cu-NiTi et acier/TMA), le coefficient de frottement a diminu de 20 % 30 %.

Mots-cls

DLC. Friction.

Acier inoxydable. TMA. Cu-NiTi.

SummaryAmorphous carbon films are characterized by lower friction and improved resistance to wear. The aim of our study was to investigate the tribological performance of diamond-like carbon (DLC) coated on CuNiTi, stainless steel and TMA orthodontic wires.

The substrates investigated (CuNiTi, stainless steel and TiMo) are alloys frequently used in orthodontics. Five centimetre long .019x.025 rods were attached to a stainless steel support. DLC depositions were performed using the plasma-enhanced chemical vapor deposition technique. A tribometer fitted with scales, a motor and an electronic sensor were used to calculate the friction coefficient. To check reproducibility, each test was repea- ted 10 times. All the data were gathered using WinDataq Lite software in order to measure the friction coefficient and to show any changes occurring over time.The friction coefficient of the DLC-coated stainless steel vs DLC- non-coated stainless steel pair showed an increase as compared with the non-coated stainless steel pair. In contrast, the friction coefficient of the two other pairs (stainless steel/CuNiTi and stainless steel/TMA) decreased by 20 to 30%.

Key-words DLC. Friction.

Stainless steel. TMA. CuNiTi.


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Introduction

Lengouement actuel pour lutilisation des couches minces decarbone nanostructur dans le domaine biomdical est justifipar les proprits remarquables de ces matriaux : bonne bio-compatibilit, inertie chimique, grande duret associes lacapacit des couches limiter la diffusion des mtaux dans lesang et une excellente proprit lubrifiante [1-8]. En outre, destests dusure ont rvl des ordres de grandeur similaires ceuxdes cramiques utilises dans le domaine des prothses mdica-les. Cependant, les couches de carbone nanostructur prsententun risque de dcohsion en raison des contraintes internes quilimitent leur application dans le domaine du traitement de sur-face des prothses mdicales.En odontologie, dautres applications des DLC semblent treintressantes [9, 10]. En effet, le traitement orthodontique arecours, pour la correction des malpositions dentaires, des

appareillages qui peuvent tre en mtal, en cramique ou enplastique. Le dplacement dentaire se fait par glissement de ladent sur un arc mtallique. La friction qui a lieu entre le verrou etlarc fait partie des facteurs qui augmentent la dure du traite-ment.Pour faire face cette friction, lindustrie essaye de dvelopper des mtaux qui amliorent le glissement. Lamlioration de lafriction et la rsistance lusure font partie des proprits descouches de carbone amorphe.Lobjectif de notre travail est dtudier les performances tribo-logiques des couches de carbone amorphe (DLC) dposes sur desfils orthodontiques en acier inoxydable, en Cu-NiTi et en TMA.

Matriel et mthodes

Les substrats tudis (acier inoxydable, TMA, Cu-NiTi) sontdes alliages frquemment utiliss en orthodontie. Des tiges.019 x .025 inch de 5 cm ont t fixes sur un support mtalliqueen acier inoxydable.Les dpts de couche de carbone ont t raliss au Laboratoirede physique de la matire condense de la Facult des sciencesdAmiens (France). Le support contenant les tiges a t nettoyaux ultrasons en trois temps chacun de 16 mn. Le premier net-toyage a t ralis laide du trychlorothylne, suivi de lac-tone et, enfin, un dernier nettoyage lthanol. Les dpts deDLC ont t raliss selon la technique plasma enhanced che-

mical vapor deposition (fig. 1) [11]. Dans un premier temps, unetching a t ralis avec une pression de 3 Pa, une puissance de250 W et une nergie de -770 V. Les couches de carbone amor-phe ont t dposes selon les paramtres suivants : 1 Pa,250 W, -454 V pendant 95 min.Un tribomtre muni dune balance rgle en quilibre, dunmoteur et dun capteur lectronique a t utilis pour les tests defrottement et pour raliser les tests dusure. Lalimentation dumoteur tait assure laide dun gnrateur branch un stabi-lisateur de courant. Sur le moteur tait connecte une cartedacquisition numrique DATAQ DI-194RS branche sur un

Introduction

The current popularity of thin films of nanostructured carbon usedfor biomedical purposes is based on the remarkable properties of

these materials, namely, good biocompatibility, chemical inertiaand extreme hardness combined with the ability of the coatings tolimit the diffusion of the metals into the blood, as well as theirexcellent lubricating properties [1-8]. In addition, wear tests haverevealed orders of magnitude similar to those of ceramics usedfor medical prostheses. However, nanostructured carbon filmsinvolve a risk of rupture on account of internal stresses, thusrestricting their usage to the field of surface treatment of medicalprostheses.

In dental science, other DLC applications appear to offer benefits[9, 10]. Orthodontic treatment uses appliances which can bemade of metal, ceramic or plastic in order to correct crooked

teeth. Tooth displacement is generated by sliding the tooth alonga metal archwire. The friction occurring between the brackets andthe archwire is one of several factors responsible for increasingtreatment duration.

To counter this friction, industry is striving to develop metalswhich improve the sliding movement. Lower friction andenhanced resistance to wear are two of the characteristics ofDLCs.The aim of this study is to investigate the tribological performanceof DLCs deposited on stainless steel, CuNiTi and TMA orthodon-tic archwires.

Material and methods

The substrates studied (stainless steel, TMA, CuNiTi) are alloysfrequently used in orthodontics. Five-centimeter long .019x.025rods were attached to a stainless steel support.

The DLC depositing was performed at the Laboratory of con-densed matter physics at the Faculty of Science in Amiens,France. The support holding the rods was cleaned by ultrasoundin three 16 minutes stages. The first cleaning was done usingtrichloroethylene, the second with acetone and the last with etha-nol. The DLC depositing was performed using the plasmaenhanced chemical vapor deposition technique (fig. 1) [11]Firstly, etching was performed at 3 Pa pressure, 250 W powerand -77V energy. The DLC was deposited using the followingparameters: 1Pa; 250W, -454V for 95min.

A tribometer fitted with scales set at balance, a motor and anelectronic sensor was used for the friction tests and to performthe wear tests. The motor was powered by a generator hookedup to a current stabilizer. To the motor we connected a DATAQDI-194RS digital data acquisition card plugged into a computer. Ahygrometer and a thermometer were placed in the work space in


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Performances tribologiques de lacier inoxydable, du TMA et du Cu-NiTi recouverts de DLCTribological performance of DLC-coated stainless steel, TMA and Cu-NiTi

International Orthodontics2008 ; 6 : 335-342 337

ordinateur. Un hygromtre et un thermomtre, dposs dans lechamp de travail, ont t utiliss pour calculer lhygromtrie et latemprature. Les fils en tige taient fixs sur les porte-chan-tillons du moteur et de la balance.Avant chaque manipulation, les chantillons non recouverts deDLC taient nettoys lactone. Plusieurs couples de frottementforms de tiges recouvertes et non recouvertes de DLC ont t tes-ts. Lacier inoxydable non recouvert de carbone composait tou- jours une surface du couple. La force normale choisie tait de 10 get le moteur tournait une vitesse de 684 m/s. Des tests dtalon-nage ont t raliss avant et la fin de chaque manipulation.Lvolution du coefficient de frottement en fonction du temps at enregistre. Pour vrifier la reproductibilit, chaque test a trepris 10 fois. Enfin, un test dusure a t ralis pour chaquecouple.

Rsultats

Pour le couple form par des tiges en acier inoxydable non recou-vert par du carbone amorphe, le coefficient de frottement a suiviune volution rgulire tout au long du trajet(fig. 2). Le coeffi-cient de frottement statique ( s) dpendant de la force ncessairepour amorcer le glissement est situ 0,13 et le coefficient defrottement dynamique ( d) est situ le long du trajet 0,07. Lerecouvrement de la surface par du DLC a modifi le comporte-ment tribologique(fig. 3). Le coefficient de frottement statique( s) est pass de 0,13 0,42. Deux zones irrgulires ont apparu,au dbut du mouvement et entre 13 et 16 secondes. Le coeffi-cient de frottement dynamique ( d) est situ 0,14. Aprs107 passages(fig. 4), nous avons assist une accentuation de ladtrioration de la surface matrialise par une trajectoire acci-dente. Le coefficient de frottement dynamique a chang devaleur trois reprises entre 0,28 et 0,36.

Fig. 1 : Sputtering.Fig. 1: Sputtering.

Fig. 2 : volution du coefficient de frottement du couple acier/anon recouvert.Fig. 2: Changes in the friction coefficient of the non-coated stainlesssteel/stainless steel pair.

order to measure hygrometry and temperature. The rods wereattached to the specimen-holders on the motor and scales.

Prior to each pass, the non-DLC-coated samples were cleanedwith acetone. Several friction pairs including DLC-coated andnon-DLC-coated rods were then tested. One surface in the pairalways consisted of a non-carbon-coated stainless steel rod. Theusual force chosen was 10 gr and the motor operated at a speedof 684 m/s. Calibration tests were performed before and after

each pass.Changes in the friction coefficient over time were recorded. Inorder to check reproducibility, each test was performed 10 times.Lastly, a wear test was performed on each pair.

Results

Regarding the pair of non-DLC-coated stainless steel rods, thefriction coefficient changed steadily throughout the trajectory(fig. 2). The static friction coefficient ( s) reflecting the forcerequired to commence the sliding movement was 0.13 and thedynamic friction coefficient ( d) along the length of the trajectorywas 0.07. Coating the surface with DLC modified the tribologicalbehaviour (fig. 3). The static friction coefficient ( s) increasedfrom 0.13 to 0.42. Two irregular areas were observed at the onsetof the movement and between 13 and 16 seconds. The dynamicfriction coefficient ( d) was 0.14. After 107 passes (fig. 4), wewitnessed a deterioration of the surface as evidenced by jerkymovements. The dynamic friction coefficient changed value threetimes between 0.28 and 0.36.


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Le glissement dune tige en acier sur une tige en Cu-NiTinon recouverte de DLC a montr des valeurs du coefficientde frottement suprieur au couple prcdent. Le coefficientstatique ( s) est situ 0,37 et le coefficient dynamique( d) se situe 0,24(fig. 5). La trajectoire tait plus oumoins rgulire avec des pics de petite taille. Le recouvre-ment de la surface du Cu-NiTi par du DLC a amlior lecoefficient de frottement dynamique puisquil a t presquedivis de moiti, passant de 0,24 0,14 entre 2 et 7 secon-des (fig. 6). Une zone irrgulire est apparue entre 7 secon-des et 23 secondes. Dans cet intervalle de temps, lecoefficient a subi une augmentation jusqu 0,28 puis unediminution pour reprendre sa valeur initiale 0,14. Aprs96 passages, linterface a subi une dtrioration avec uneaugmentation du coefficient du frottement 0,33(fig. 7).La mme amlioration a t obtenue avec le couple acier/Cu-NiTi et le couple acier/TMA. Sans recouvrement, lecoefficient de frottement statique ( s) a t de 0,24 et lecoefficient dynamique ( d) est situ environ 0,20. La tra-

jectoire de variation est presque linaire, except linter-

valle de temps entre 16 et 23 secondes o nous avons eudeux valeurs minimales 16 et 22 secondes (fig. 8). Lerecouvrement de la tige TMA par DLC a diminu les coeffi-cients de frottement statique ( s) et dynamique ( d)puisquils sont passs respectivement de 0,24 0,14 et de0,20 0,18 (fig. 9). Lendommagement de linterface aprs61 passages est bien matrialis par une augmentation ducoefficient de frottement pour atteindre une valeur de 0,34(fig. 10).

Fig. 3 : volution du coefficient de frottement du couple acier/acierrecouvert de DLC.Fig. 3: Changes in the friction coefficient of the DLC-coated stainlesssteel/stainless steel pair.

Fig. 4 : volution du coefficient de frottement du couple acierecouvert de DLC aprs un test dusure.Fig. 4: Changes in the friction coefficient of the DLC-coated stainlesssteel/stainless steel pair after wear test.

A stainless steel rod sliding along a non-DLC-coated rod providedfriction coefficient values higher than those of the previous pair.The static coefficient ( s) was 0.37 and the dynamic coefficient( d) 0.24 (fig. 5). The trajectory was more or less even with onlyminor peaks. Coating the surface of CuNiTi with DLC improvedthe dynamic friction coefficient which almost halved from 0.24 to0.14 between 2 and 7 seconds (fig. 6). An irregular zone wasobserved between 7 and 23 seconds. During this interval, thecoefficient increased to 0.28 before returning to its initial value of0.14. After 96 passes, the interface deteriorated and the frictioncoefficient increased to 0.33 (fig. 7).

Similar improvement was achieved with the stainless steel/ CuNiTi and the stainless steel TMA pairs. Uncoated, the staticfriction coefficient ( s) was 0.24 and the dynamic coefficient( d) was approximately 0.20. The variation trajectory wasalmost linear apart from the interval between 16 and 23 seconds

when we observed two minimum values at 16 and at 22 seconds(fig. 8). Coating the TMA rod with DLC reduced the static friction( s) and dynamic friction ( d) coefficients as they decreased,respectively, from 0.24 to 0.14 and from 0.20 to 0.18 (fig. 9).The damage to the interface after 61 passes was evidenced bythe increase in the friction coefficient which increased to 0.34(fig. 10).


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Discussion

Les alliages acier, Cu-NiTi et TMA recouverts par des couchesDLC hydrognes dposes par la technique de Plasma Enhan-ced Chemical Vapor Deposition et testes dans lair ambiantavec une humidit contrle ont montr un comportement tribo-logique variable dun couple lautre.Conformment aux rsultats obtenus dans dautres tudes sint-ressant aux films DLC, le recouvrement des alliages Cu-NiTi etTMA a permis davoir une amlioration importante de leur comportement tribologique.

Fig. 5 : volution du coefficient de frottement du coupleacier/CuNiTi non recouvert.Fig. 5: Changes in the friction coefficient of the uncoated stainlesssteel/CuNiTi pair.

Fig. 6 : volution du coefficient de frottement du couple acier/Curecouvert de DLC.Fig. 6: Changes in the friction coefficient of the DLC-coated stainlesssteel/CuNiTi pair.

Fig. 7 : volution du coefficient de frottement du coupleacier/CuNiTi recouvert de DLC aprs un test dusure.Fig. 7: Changes in the friction coefficient of the DLC-coated stainlesssteel/CuNiTi pair after wear test.

Fig. 8 : volution du coefficient de frottement du couple acier/TMnon recouvert.Fig. 8: Changes in the friction coefficient of the uncoated stainless steel/ TMA pair.

Discussion

Stainless steel, CuNiTi and TMA alloys coated with hydrogenatedDLC using the Plasma Enhanced Chemical Vapor Depositiontechnique and tested in ambient air in controled humidity demon-strated variations of tribological behavior between the differentpairs.The results agreed with those obtained in other studies related toDLC coating and showed that coating CuNiTi and TMA alloyswith DLC provided enhanced tribological behavior.


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Le recouvrement des tiges en Cu-NiTi a permis de rduire, depresque la moiti, le coefficient de frottement. Cette rductionimportante peut sexpliquer par les proprits lubrifiantes descouches DLC. Lintervalle de temps compris entre 7 et 23 secon-des est marqu par une augmentation du coefficient de frotte-ment pour atteindre le double de sa valeur initiale suivie dunediminution(fig. 8). Laugmentation du coefficient de frottementdynamique ( d) peut tre explique par une dtrioration de lacouche DLC au niveau de certains endroits mettant ainsi encontact direct les surfaces des deux composants du couple. Aprscette augmentation, le coefficient a diminu pour revenir de nou-veau sa valeur initiale. Sachant quune diminution du coeffi-cient de friction implique la prsence dun lubrifiant, on peutdire que cette baisse indique quun mcanisme dautolubrifica-tion sest produit. Il peut bien sagir de la graphitisation de sur-face [12-17].Ce processus impliquant la libration dhydrogne au niveau dela surface [18] va permettre une rduction de la friction grce une rserve constante de matriel sous forme de graphite obtenue partir dune transformation structurale de phases Sp3 en Sp2intressant les produits dusure [17].Paralllement la graphitisation, on peut galement supposer laformation dun film de transfert au niveau de la contreface rsul-tant de la compaction dune partie des dbris dusure transfre partir de la surface DLC [19, 20]. Ce film assure la protection dela contreface contre lusure et rend plus efficace le mcanismedautolubrification [12-15, 17]. Aprs 96 passages, le coefficientde frottement est pass 0,33. Cette augmentation peut tre due lusure tendue et llimination dune partie de la coucheDLC.Aprs recouvrement par DLC, on a not, sur le couple acier/TMA, une amlioration des coefficients de frottement mettant en

Fig. 9 : volution du coefficient de frottement du coupleacier/TMA recouvert de DLC.Fig. 9: Changes in the friction coefficient of the DLC-coated stainlesssteel/TMA pair.s

Fig. 10 : volution du coefficient de frottement du couple acierecouvert de DLC aprs un test dusure.Fig. 10: Changes in the friction coefficient of the DLC-coated stainlesssteel/TMA pair after wear test.

Coating CuNiTi rods reduced the friction coefficient by almosthalf. This major reduction can be accounted for by the lubricatingproperties of the DLC film. The time interval between 7 and23 seconds was marked by an increased friction coefficient whichreached twice the initial value followed by a fall (fig. 8). The increased dynamic friction coefficient ( d) was probablydue to the deterioration of the DLC film at certain points, thusbringing into direct contact the surface of the two components

forming the pair. Following this increase, the coefficientdecreased, returning to its initial value. Given that a drop in thefriction coefficient implies the presence of a lubricant, it can besurmised that this decrease points to the presence of a self-lubricating mechanism which might involve surface graphitiza-tion [12-17].

This process involves the release of hydrogen at the surface [18]which triggers a reduced rate of friction because of the constantpresence of a pool of graphite material resulting from the struc-tural transformation of Sp 3 phases into Sp 2 and involving thematerial produced by wear and tear [17].In addition to the graphitization process, one can assume that atransfer film formed on the opposing surface as a result ofcompaction of some of the wear debris transferred from the DLCsurface [19, 20]. This film ensures the protection of the opposingsurface against wear and enhances the auto-lubrication process[12-15, 17]. Following 96 passes, the friction coefficient increasedto 0.33, possibly as a result of prolonged wear and tear and to theelimination of some of the DLC film.

Following coating with DLC, we observed, on the stainless steel/ TMA pair, an improvement in the friction coefficients, thus demons-


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vidence le rle lubrifiant des couches DLC. La figure 9 montreune lgre augmentation progressive avec le temps du coefficientde frottement sans pour autant dpasser 0,20. Cette augmenta-tion peut avoir comme cause un dtachement du film DLC. Pro-

bablement, ladhsion des couches DLC sur TMA ntait pasbonne et suffisante pour pouvoir rsister aux contraintes. Lam-lioration de ladhsion DLC/TMA doit tre aussi recherche dansles futures tudes afin de mieux claircir le comportement tribo-logique de ces alliages recouverts par DLC. Plusieurs approchesont t dcrites [21-27]. Lobjectif commun est doptimiser ladhsion de DLC sur son substrat. Aprs 61 passages, le coeffi-cient de frottement a augment 0,26 marquant une liminationdune partie du DLC.Pour le couple acier/acier, contrairement ce qui a t rapportpar dautres tudes, le coefficient de frottement aprs recouvre-ment par DLC a subi une augmentation. Or, il est important desouligner que les proprits tribologiques des DLC dpendenttroitement des paramtres de dpt et de ceux de lenvironne-ment o elles sont testes [17, 18, 20, 22, 28-32]. Deux filmsdposs par la mme technique et sur un mme alliage peuventavoir des comportements tribologiques diffrents. Il est donc pos-sible que les paramtres utiliss dans notre tude ne fussent pasoptimaux pour obtenir une couche idale permettant damliorer le glissement du couple form par les tiges en acier.

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trating the lubricating role played by the DLC film. Figure 9 dis-plays a slight gradual increase over time of the friction coefficientalthough never exceeding 0.20. This increase may be due to flak-ing of the DLC coating. It is likely that the adherence of the DLC

on the TMA was inadequate to withstand stress. In future studies,means of improving DLC/TMA adherence should be sought inorder to shed light on the tribological behaviour of these DLC-coated alloys. Several approaches have already been described[21-27]. Their common goal was to optimize DLC adherence onthe substrate. After 61 passes, the friction coefficient rose to 0.26,thus pointing to the elimination of some of the DLC film.

As regards the stainless steel/stainless steel pairing, and in con-trast with findings reported in other studies, the friction coefficientincreased after coating with DLC. It is important then to empha-size that the tribological properties of DLC are highly dependentupon the parameters governing the coating process as well asthe setting in which they are tested [17, 18, 20, 22, 28-32]. Twofilms deposited using the same technique on the same alloy canhave different tribological behaviors. It is possible, therefore, thatthe parameters used in our study were not optimal to obtain anideal film allowing an improvement of the sliding properties of thepair of stainless steel rods.


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