Relationship between electrochemical corrosion and the tribologic properties of orthodontic alloys [Relation corrosion électrochimique et propriétés tribologiques des alliages orthodontiques]

Summary

The aim of this study was to investigate the electrochemical properties of orthodontic alloys (SS, CuNiTi and TMA) and the impact of corrosion on their friction coefficient. Twelve pairs of 5-cm long 017 .025 rectangular wires (SS/SS, CuNiTi/SS, TMA/SS) were formed and SEM observations and friction tests were conducted on the surface of each pair of wires. Then, each type of alloy was dipped in artificial saliva during different immersion times (24 h, 48 h, 72 h and 1 week). Following the electrochemical tests, the samples were recovered for a second series of SEM observations and a second friction test. After 24 h of immersion, we observed corrosive attacks on the surface of the alloys and a reduction of the friction coefficients. After 1 week, we observed surface passivation and an increased friction coef- ficient which attained values identical to those prior to immer- sion. One can conclude that, at the onset of corrosion, the friction coefficient decreases but that this advantage diminishes after passivation of the alloy surfaces.

Ó 2010 CEO. Published by Elsevier Masson SAS. All rights reserved

R esum e

Les objectifs de ce travail etaient d’ etudier les propri et es electrochimiques des alliages orthodontiques (SS, CuNiTi et TMA), ainsi que l’influence de la corrosion sur le coefficient de frottement. Douze couples de fils rectangulaires (SS/SS, CuNiTi/SS, TMA/SS) 0,017 0,025 de 5 cm de longueur ont et e constitu es. Pour chaque couple, des observations de surface au MEB et des tests de frottement ont et e r ealis es.

Par la suite, chaque type d’alliage a et e tremp e dans la salive artificielle a` diff erents temps d’immersion (24 heures, 48 heures, 72 heures, une semaine). Apr es avoir r ealis e les essais electrochimiques, les echantillons ont et e r ecup er es pour une deuxi eme observation au MEB ainsi qu’un deuxi eme test de frottement. Apr es 24 heures d’immersion, on a observ e des attaques corrosives sur la surface des alliages ainsi qu’une diminution des coefficients de frottement. Apr es une semaine, on a eu une passivation de la surface et une augmentation du coefficient de frottement qui a atteint des valeurs identiques a` celles pr ec edant son immersion. On peut conclure qu’au d ebut de la corrosion, le coefficient de frotte- ment diminue mais cet avantage s’affaiblit apr es passivation de la surface des alliages.

Ó 2010 CEO. E´dite´ par Elsevier Masson SAS. Tous droits re´serve´s

Article original

Relationship between electrochemical corrosion and the tribologic properties of orthodontic alloys

Relation corrosion electrochimique et propri et es tribologiques des alliages orthodontiques

Zakaria BENTAHAR

, Meryem BELLAMINE

, Mustapha ZERTOUBI

, Abdellatif IRHZO

, Khalid EL BOUSSIRI

a

Facult e de m edecine dentaire de Casablanca, Casablanca, Morocco

b

D epartement de chimie, facult e de sciences de Casablanca, Casablanca, Morocco

c

Laboratoire de biomat eriaux m etalliques, facult e de m edecine dentaire de Casablanca, Casablanca, Morocco

Available online: 26 November 2010 /

* Correspondence and reprints /Correspondance et tires a` part : Zakaria BENTAHAR, 3, rue Zemamra, Anfa, 20050 Casablanca, Morocco.

e-mail address /Adresse e-mail :zbentahar@wanadoo.net.ma

Frictional forces constitute the main component of resistance to sliding since they are responsible for a loss of 12 to 60% of the intensity of the initial force applied [5].

Electrochemical corrosion is another phenomenon with which orthodontists are faced. Indeed, the continuous contact between orthodontic appliances and saliva results in the elec- trochemical breakdown or corrosion of the former, triggering the release into the body of toxic and allergenic substances [6,7].

Orthodontic alloys are thus subject to both friction and corro- sion. The aim of our study was to investigate the electroche- mical behavior of orthodontic alloys and the impact of corro- sion on friction.

Material and methods

To achieve the aim of our study, we used 5-cm long samples of rectangular 017 .025 wire made of stainless steel (SS), cop- per nickel-titanium (CuNiTi and titanium molybdenum (TMA).

At the National Center for Scientific and Technical Research in Rabat, Morocco, we performed observations and analyses on the surfaces of the samples using scanning electronic microscopy (SEM).

Each type of alloy (SS, CuNiTi and TMA) was attached to a segment of stainless steel of the same length in order to form the following pairs: SS/SS, CuNiTi/SS and TMA/SS.

Twelve pairs were created, i.e. four pairs per alloy. Each pair was kept in a labeled plastic box according to the type of alloy and the immersion time in artificial saliva (24 h, 48 h, 72 h and 1 week).

For each alloy pair, tribologic tests were conducted to deter- mine the friction coefficients. For this purpose, a tribometer equipped with scales, a motor and an electronic sensor was used. The applied usual force and speed were, respectively, 10 gm and 684 mm/s. We used digital acquisition to calculate changes in the friction coefficient over time.

reproductibilit e des m ecaniques de glissement [1–4].

Les forces de friction constituent la principale composante de la r esistance au glissement puisqu’elles sont responsables d’une perte de 12 a` 60 % de l’intensit e de la force initiale appliqu ee [5].

La corrosion electrochimique constitue un autre ph enom ene auquel les orthodontistes sont confront es. En effet, la pr esence de dispositifs orthodontiques constamment au contact de la salive entraıˆne leur d egradation electrochimique.

Cette corrosion provoque la lib eration, dans l’organisme, de substances toxiques et allergisantes [6,7].

Les alliages orthodontiques ob eissent donc aux ph enom enes de frottement et de corrosion. L’objectif de notre travail est d’ etudier le comportement electrochimique des alliages ortho- dontiques ainsi que l’influence de la corrosion sur le frottement.

Mat eriel et m ethodes

Pour r epondre a` l’objectif de notre travail, nous avons utilis e des echantillons de fils rectangulaires 0,017 0,025 pouces de 5 cm de longueur en acier inoxydable (SS), en cuivre nickel titane (CuNiTi) et en titane molybd ene (TMA).

Au Centre national pour la recherche scientifique et technique de Rabat (CNRST), nous avons proc ed e a` l’observation et a` l’analyse de la surface des echantillons au microscope electronique a` balayage (MEB).

Chaque type d’alliage (SS, CuNiTi et TMA) a et e coupl e a` une tige en acier inoxydable de m^ eme longueur pour obtenir les couples suivants : SS/SS, CuNiTi /SS et TMA/SS.

Douze couples ont et e constitu es, a` raison de quatre couples par alliage. Chaque couple a et e conserv e dans une boıˆte en plastique etiquet ee, en fonction du type d’alliage et du temps d’immersion dans la salive artificielle (24 heures, 48 heures, 72 heures et une semaine).

Pour chaque couple d’alliage, des tests tribologiques ont et e

r ealis es pour d eterminer les coefficients de frottement. Pour

cela, un tribom etre muni d’une balance, d’un moteur et d’un

capteur electronique a et e utilis e. La force normale et la

vitesse appliqu ees etaient respectivement de10 g et de

684 mm/s. Nous avons obtenu, par acquisition num erique,

l’ evolution du coefficient de frottement en fonction du temps.

Following the friction tests, we performed electrochemical tests at the Materials and Environment Interface Laboratory at the Casablanca Faculty of Science. The aim was to inves- tigate the electrochemical degradation and the impact of immersion time on the corrosion behavior of the different alloys (SS, CuNiTi and TMA).

The immersion solution was artificial saliva containing 0.9%

NaCL (Ringer solution). The pairs of alloy were dipped into a temperature-controlled water-bath at 37

C over four different time periods (24 h, 48 h, 72 h, 1 week) (fig. 1).

At the end of each immersion period, the wires were indivi- dually wrapped in Teflon in order to restrict the surface to be studied to 0.2 cm

. They were then attached to a clip acting as a working electrode. The working electrode was plunged into a test recipient in pyrex glass containing 300 mL of artificial saliva temperature-controlled at 37

C and containing a Ag/

AgCl reference electrode and a platinum counter-electrode with a large surface area to minimize polarization effects.

This chronopotentiometric technique was used to assess the stationary conditions of each alloy. The intensity/potential curves and the Tafel curves were recorded by linear scanning voltaperometry. The polarization curve was produced in the anodic direction from the corrosion potential. The same test was performed several times in order to check the reproduci- bility of the results.

For comparative evaluation, the SEM surface analysis and observations as well as the tribologic tests were conducted on the samples following the electrochemical corrosion tests.

Results

Before immersion in the artificial saliva, the surface of the new stainless steel (SS) sample showed the presence of grooves and

Apr es les tests de frottement, des essais electrochimiques ont

et e effectu es au Laboratoire interface mat eriaux et environne- ment (LIME) de la facult e des sciences de Casablanca ; l’objectif etant d’ etudier la d egradation electrochimique ainsi que l’influence du temps d’immersion sur le comportement en corrosion des diff erents types d’alliage(SS, CuNiTi et TMA).

La solution d’immersion utilis ee constituait la salive artificielle a` 0,9 % de NaCl (solution de Ringer). Les couples d’alliage ont

et e plong es dans un bain-marie thermostat e a` 37

C pendant quatre intervalles de temps (24 heures, 48 heures, 72 heures, une semaine) (fig. 1).

Apr es l’ ecoulement de chaque dur ee d’immersion, chaque fil a

et e enroul e par du t eflon, pour limiter la surface d’ etude a` 0,2 cm

, puis a et e fix e par une pince jouant le r^ ole de l’ electrode de travail. Cette electrode de travail a et e plong ee dans une cellule d’essai en verre pyrex de 300 mL de salive artificielle thermostat ee a` 37

C contenant une electrode de r ef erence d’Ag/AgCl et une contre- electrode en platine de grande surface pour minimiser les effets de polarisation.

La technique chronopotentiom etrique a et e utilis ee pour evaluer les conditions stationnaires de chaque alliage. Les courbes intensit e/potentiel et les courbes de Tafel ont et e enregistr ees par voltamp erom etrie a` balayage lin eaire. Le trac e de la courbe de polarisation a et e effectu e dans le sens anodique a` partir du potentiel de corrosion. Le m^ eme essai a

et e r ealis e plusieurs fois pour v erifier la reproductibilit e des r esultats.

Pour l’ evaluation comparative, l’analyse et les observations de surface au MEB, ainsi que les tests tribologiques ont et e pratiqu es sur les echantillons apr es les tests de corrosion

electrochimique.

R esultats

Avant immersion dans la salive artificielle, la surface de l’acier inoxydable (SS) neuf est caract eris ee par la pr esence de

[(Fig._1)TD$FIG]

Fig. 1: Water-bath at 37

C containing the samples in the artificial saliva solution.

Fig. 1 :Bain-marie a` 37˚ C contenant lesechantillons dans la solution de salive artificielle.

notches in certain areas. The mean friction coefficient of the SS/SS pair was 0.07.

After immersion during 24 h, we observed a generalized cor- rosive attack in the crevasses. After a week, the surface had recovered a smooth and homogenous appearance (fig. 2).

The current (I

) and the corrosion potential (E

) of the stainless steel (SS) increased after 48 h of immersion. After 1 week, the intensity of the corrosion had considerably decreased while the polarization resistance (pR) had increased (Table I) (fig. 3).

After 24 h of immersion, the friction coefficient of the SS/SS pair had dropped to 0.04. Nevertheless, after 10 seconds the curves were finally superimposed. After 1 week, this

rainures et d’encoches localis ees au niveau de certaines zones. Le coefficient de frottement moyen du couple SS/SS etait de 0,07.

Apr es 24 heures d’immersion, on a not e une attaque g en eralis ee par crevasse. Apr es une semaine, la surface a repris un aspect plus homog ene et lisse (fig. 2).

Le courant (I

) et le potentiel de corrosion (E

) de l’acier inoxydable(SS) ont augment e apr es 48 heures d’immersion.

Apr es une semaine, l’intensit e de corrosion a diminu e consid erablement parall element a` une augmentation de la r esistance de polarisation (Rp) (Tableau I) (fig. 3).

Apr es 24 heures d’immersion, le coefficient de friction du cou- ple SS/SS a baiss e jusqu’a` une valeur de 0,04, n eanmoins apr es dix secondes les courbes finissent par se superposer.

Fig. 2: a–c: microphotographs of the state of the surface of the new stainless steel and after 14 h and 1 week of immersion.

Fig. 2 : a–c: microphotographies de l’etat de surface de l’acier neuf et apres 24 h et une semaine d’immersion.

Table I

Electrochemical parameters of stainless steel after different immersion times.

Tableau I

Param etres electrochimiques de l’acier inoxydable a` diff erents temps d’immersion.

Immersion time/ Temps d’immersion E (mV) I

(mA/ ^cm

⁾ R

(kV.cm

)

Steel/ Acier 24 h 202.0 0.6372 35.54

Steel/ Acier 48 h 89.5 0.6918 41.05

Steel/ Acier 72 h 146.6 0.3694 65.20

Steel/ Acier 1W/S 93.9 0.1956 103.54

coefficient had increased back to the initial value before immersion (figs. 4 and 5).

Surface observations of the new CuNiTi revealed the presence of parallel, irregular, shallow grooves arranged in strips along- side areas exhibiting crevasses. After 24 h of immersion, the surface appearance of the CuNiTi had not changed a great deal compared with the new state. The surface had been pitted and the grooves appeared to be narrower and deeper. After 1 week, the surface appearance was homogenous with fewer crevasses (fig. 6).

Up to 72 h of immersion, the corrosion potential (E

) and intensity (I

) had increased from 298.1 mV to –144.2 mV and from 0.1791 to 0.1667 mA/cm

, respectively. We also

Apr es une semaine, ce coefficient a subi une augmentation qui a atteint la valeur initiale avant immersion (fig. 4 and 5).

L’observation de surface du CuNiTi a` l’ etat neuf a montr e la pr esence de rainures parall eles, irr eguli eres et peu profondes configur ees en bandes avec des zones de crevasses. L’aspect de la surface du CuNiTi apr es 24 heures d’immersion n’a pas trop chang e par rapport a` l’ etat neuf. La surface est attaqu ee par piquˆration, avec un aspect moins large et plus profond des rainures. Apr es une semaine, l’ etat de surface etait plus homo- g ene, avec moins de crevasses (fig. 6).

Jusqu’a` 72 heures d’immersion, le potentiel (E

) et l’intensit e (Icorr) de corrosion ont augment e de 298,1 mV a` 144,2 mV et de 0,1791 a` 0,1667 mA/cm

respectivement.

[(Fig._3)TD$FIG]

Fig. 3: a–c: evolution of the I

, E

and pR of the stainless steel.

Fig. 3 : a–c:evolution de Icorr, Ecorret Rp de l’acier inoxydable.

[(Fig._4)TD$FIG]

Fig. 4: Evolution of the friction coefficient of the SS/SS pair before and after 24 h immersion.

Fig. 4 :Evolution du coefficient de friction du couple SS/SS avant et apres 24 h d’immersion.

observed a decrease of the polarization resistance from 143.53 k V .cm

to 16.929 k V .cm

. After 1 week, E

and I

had decreased ( 157.1 mV ; 0.5825 mA/cm

) with an increase in pR (81.268 kV.cm

) (Table II) (fig. 7).

After 24 h of immersion, we noted a reduction of the friction coefficient of the CuNiTi/SS pair. Before and after results for this coefficient were almost identical (figs. 8 and 9).

Nous avons assist e par ailleurs, a` une diminution de la r esistance de polarisation (Rp) de 143,53 kV.cm

a` 16,929 kV.cm

. Apr es une semaine, E

et Icorr ont diminu e ( 157,1 mV ; 0,5825 mA/cm

) avec une augmenta- tion de la Rp (81,268 kV.cm

) (Tableau II) (fig. 7).

Apr es 24 heures d’immersion, on a not e une diminution du coefficient de friction du couple CuNiTi/SS. Avant et apr es une semaine, ce coefficient etait quasi identique (fig. 8 and 9).

[(Fig._6)TD$FIG]

Fig. 6: a–c: microphotographs of the state of the surface of the new CuNiTi and after 24 h and 1 week of immersion.

Fig. 6 : a–c: microphotographies de l’etat de surface du CuNiTi neuf et apres 24 h et une semaine d’immersion.

Fig. 5: Evolution of the friction coefficient of the SS/SS pair before and after 1 week of immersion.

Fig. 5 :Evolution du coefficient de friction du couple SS/SS avant et apres 1 semaine d’immersion.

SEM revealed that the surface of TMA, when new, displayed grooves and notches. Before immersion, the mean friction coefficient of the TMA/SS pair was 0.16, higher than that of the SS/SS and CuNiTi/SS pairs.

After 24 h, the surface appeared less pitted than the steel sample. The grooves and crevasses were completely sealed, giving a more homogenous surface after 1 week (fig. 10).

Unlike the steel and CuNiTi, the corrosion potential of TMA decreased considerably (from 77.3 mV to 126.2 mV) with an increase in the polarization resistance which increased from 41.61 kV.cm

to 89.09 kV.cm

after 48 h of immersion.

After 72 h, E

( 89.8 mV) and I

(0.9551 mA/cm

) had increased and the pR had decreased (25.85 kV.cm

).

After a week, we observed a clear increase of the pR (131.71 kV.cm

) and a decrease of the E

and I

( 93.9 mV, 0.1060 m A/cm

) (Table III) (fig. 11).

A ` l’ etat neuf, la surface du TMA a pr esent e au MEB des rai- nures et des encoches. Avant l’immersion, le coefficient de friction du couple TMA/SS est en moyenne de 0,16 ; valeur sup erieure a` celle des couples SS/SS et CuNiTi/SS.

Apr es 24 heures, la surface a subi une attaque par piquˆration moins importante que celle de l’acier. Les rainures et les cre- vasses etaient parfaitement colmat ees donnant une surface plus homog ene apr es une semaine (fig. 10).

Contrairement a` l’acier et au CuNiTi, le potentiel de corrosion du TMA a diminu e consid erablement (de 77,3 mV a`

126,2 mV) avec une augmentation de la r esistance de pola- risation qui est pass ee de 41,61 kV.cm

a` 89,09 kV.cm

apr es 48 heures d’immersion. Apr es 72 heures, E

( 89,8 mV) et I

(0,9551 mA/cm

) ont augment e et la Rp a diminu e (25,85 kVcm

).

Apr es une semaine, nous avons assist e a` une augmentation manifeste de la Rp (131,71 kV.cm

) et a` une diminution de E

et I

( 93,9 mV, 0,1060 mA/cm

) (Tableau III) (fig. 11).

[(Fig._7)TD$FIG]

Fig. 7: a–c: evolution of the I

, E

and pR of CuNiTi.

Fig. 7 : a–c:evolution de Icorr, Ecorret Rp du CuNiTi.

Table II

Electrochemical parameters of CuNiTi after different immersion times.

Tableau II

Param etres electrochimiques du CuNiTi a` diff erents temps d’immersion.

Immersion time/ Temps d’immersion E (mV) I

(mA/ cm

) R

(kV.cm

)

CuNiTi 24 h 298.1 0.1791 143.53

CuNiTi 48 h 207.1 0.1667 95

CuNiTi 72 h 144.2 0.6744 16.929

CuNiTi 1W/ S 157.1 0.5825 81.268

The friction coefficient of the TMA/SS pair decreased after 24 h, reaching a value of 0.07 before returning to its initial values after a week of immersion (figs. 12 and 13).

Discussion

Despite the smooth appearance of the surfaces of the new orthodontic wires, SEM revealed the presence of grooves and notches. These defects are due to imperfections created during the manufacture and milling processes and constitute areas which are liable to corrosion. In addition, the surface of the new CuNiTi wire showed that, due to the passivation process, some of the imperfections were beginning to be smoothed out (fig. 6). It is known that this copper-containing alloy oxidizes rapidly in the presence of air and forms a

Le coefficient de friction du couple TMA/SS a baiss e apr es 24 heures pour atteindre une valeur de 0,07 puis reprendre ses valeurs initiales apr es une semaine d’immersion (fig. 12 and 13).

Discussion

Malgr e l’aspect lisse de la surface des fils orthodontiques neufs, l’analyse au MEB a montr e la pr esence de rainures et d’encoches. Ces d efauts correspondent a` des imperfections de fabrication et d’usinage constituant des zones pr edispos ees aux attaques corrosives. La surface du CuNiTi neuf s’est caract eris ee en outre par un d ebut de colmatation de certaines br eches, gr^ ace au ph enom ene de passivation (fig. 6). Il est admis que cet alliage, qui contient du cuivre, s’oxyde rapidement au contact de l’air et forme une couche

[(Fig._9)TD$FIG]

Fig. 9: Evolution of the friction coefficient of the Steel/CuNiTi pair before and after 1 week of immersion.

Fig. 9 :Evolution du coefficient de friction du couple SS/CuNiTi avant et apres une semaine.

Fig. 8: Evolution of the friction coefficient of the Steel/CuNiTi pair before and after 24 h of immersion.

Fig. 8 :Evolution du coefficient de friction du couple Acier/CuNiTi avant et apres 24 h d’immersion.

homogenous oxidized layer even before it is dipped in saliva, thus helping to protect it in this corrosive environment.

After 24 h, the surfaces of the steel, TMA and CuNiTi had degraded (figs. 2, 6 10) and became homogenous and smooth after a week. This was due to the formation of an oxide layer on the surface of the alloys. This layer is the result of the filling in and smoothing out of the irregularities on the alloy surfaces by the products of the degradation process. It is known that passivation at the surface of materials increases their resis- tance and stability in a corrosive environment [8,9]. Several studies have demonstrated that corrosion of orthodontic alloys is dependent upon the properties of the passive film itself, which is liable to be dissolved [8].

d’oxyde homog ene, avant m^ eme son immersion dans la salive lui procurant ainsi une protection dans ce milieu corrosif.

Apr es 24 heures, la surface de l’acier, du TMA et du CuNiTi a

et e d et erior ee (fig. 2, 6 10), pour devenir homog ene et lisse apr es une semaine ; cela etant duˆ a` la formation d’une couche d’oxyde a` la surface de ces alliages. Cette couche est la cons equence de la colmatation des br eches de la surface par les produits de d egradation. En effet, cette passivation a` la surface des mat eriaux leur conf ere une propri et e de r esistance et de stabilit e dans un environnement corrosif [8,9]. Plusieurs etudes ont d emontr e que la corrosion des alliages orthodontiques d ependait des propri et es du film passif lui-m^ eme susceptible d’^ etre dissous [8].

Table III

Electrochemical parameters of TMA after different immersion times.

Tableau III

Param etres electrochimiques du TMA a` diff erents temps d’immersion.

Immersion time/ Temps d’immersion E (mV) I

( m A/ cm

) R

(k V .cm

)

TMA 24 h 77.3 0.2870 41.61

TMA 48 h 126.2 0.2347 89.09

TMA 72 h 89.8 0.955 125.85

TMA 1W/ S 93.9 0.1060 131.71

[(Fig._10)TD$FIG]

Fig. 10: a–c: microphotographs of the state of the new TMA surface and after 24 h and 1 week of immersion.

Fig. 10 : a–c: microphotographies de l’etat de surface du TMA neuf et apres 24 h et une semaine d’immersion.

Variations over time of E

, I

and pR of the three alloys provide an accurate reflection of the surface observations obtained by SEM. Regarding stainless steel, changes in the corrosion potential over time demonstrated the presence of an active material. After 48 h, the E

and I

had increased, indicating that the steel had been corroded in the medium (fig.

3). When the immersion time was increased, the stainless steel became less and less reactive, on account of the surface passivation.

TMA underwent fluctuations of E

, I

and pR depending on the duration of immersion (fig. 11). After 48 h, polarization resistance increased, pointing to inactivation of the alloy in the solution resulting from the formation of a protective tita- nium oxide layer on its surface [6,8,9]. After 72 h, the marked reduction of the pR combined with an increase in the E

and

Les variations en fonction du temps de E

, I

et de Rp des trois alliages refl etent fid element les observations de surface au MEB. Pour l’acier inoxydable, l’ evolution du potentiel de corrosion en fonction du temps a montr e qu’on est en pr esence d’un mat eriau actif. Apr es 48 heures, E

et I

ont augment e, traduisant l’attaque de l’acier dans le milieu (fig. 3). Avec l’augmentation du temps d’immersion, l’alliage concern e devient de moins en moins r eactif, gr^ ace a` la passi- vation de surface.

Le TMA a d ecrit une evolution fluctuante de E

, I

et de Rp en fonction du temps d’immersion (fig. 11). En effet, apr es 48 heures, la r esistance de polarisation a augment e, ce qui correspond a` une inactivit e du mat eriau dans la solution par formation d’une couche protectrice d’oxyde de titane a` la sur- face [6,8,9]. Apr es 72 heures, la diminution franche de Rp

[(Fig._12)TD$FIG]

Fig. 12: Evolution of the friction coefficient of the Steel/TMA pair before and after 24 h of immersion.

Fig. 12 :Evolution du coefficient de friction du couple Acier/TMA avant et apres 24 h d’immersion.

Fig. 11: a–c: evolution of the TMA I

, E

and pR.

Fig. 11 :Evolution de I corr, Ecorret Rp du TMA.

I

can be accounted for by the dissolution of the passivation layer. After 1 week, the increase in the pR indicated repassi- vation of the alloy surface (fig. 11).

What one observes, therefore, is alternating repassivation and depassivation of the TMA surface.

The friction coefficient of the SS/SS, SS/TMA and SS/CuNiTi pairs (figs. 4, 8, 12) decreased after 24 h of immersion, during which time the alloys underwent corrosive attack confirmed both by SEM observations and by the findings of the electro- chemical tests. After 1 week, passivation of the alloys increased the friction coefficient which reached levels close to the initial coefficient value for each alloy when new (figs. 5, 9, 13).

We can conclude, therefore, that when an alloy is attacked, there is a reduction of the friction coefficient. During passiv- ation, the products of the corrosion process form on the surface of the material thus changing its external morphology and increasing its friction coefficient, which can revert as far as its initial friction coefficient.

Some studies [3] claim that corrosion increases the friction coefficient and that the oxide layer formed is responsible for this increase.

The increase after 1 week of the friction coefficient of the SS/

SS, SS/TMA and SS/CuNiTi pairs can be explained by the fact that the oxide layer formed by corrosion on the surface of these wires probably increases the friction force.

In contrast, other studies [10] have reached different conclu- sions and claim that the oxide films protect and reduce the adhesion between two metals in contact with one another.

However, there is a trend towards lower friction forces with increased immersion times.

More research is needed to determine the behavior of the surfaces of these materials as well as their friction coefficients when subjected to electrochemical degradation over long periods approximating, as far as possible, the time these metal alloys are likely to remain in the mouth.

associ ee a` une augmentation de E

et I

peut ^ etre expliqu ee par la dissolution de la couche de passivation.

Apr es une semaine, l’augmentation de Rp traduit la repassi- vation de cette surface (fig. 11).

On assiste donc a` une alternance de ph enom enes de repas- sivation et de d epassivation de la surface du TMA.

Le coefficient de friction des couples SS/SS, SS/TMA, SS/

CuNiTi (fig. 4, 8, 12) a diminu e apr es les 24 heures d’immer- sion, durant lesquels les alliages ont subi une attaque corrosive confirm ee aussi bien par les observations au MEB que par les r esultats des tests electrochimiques. Apr es une semaine, la passivation des alliages est responsable d’une augmentation du coefficient de friction qui atteint des valeurs voisines du coefficient initial du mat eriau a` l’ etat neuf (fig. 5, 9, 13).

Nous pouvons dire que lorsqu’il y a une attaque du mat eriau, nous avons une diminution de son coefficient de friction. Au cours de la passivation, les produits de corrosion se forment en surface en modifiant sa morphologie externe et en aug- mentant ainsi son coefficient de friction jusqu’a` atteindre le coefficient de friction initial du mat eriau.

Certaines etudes [3] confirment que la corrosion augmente le coefficient de friction et que la couche d’oxyde form ee est responsable de cette augmentation.

L’augmentation du coefficient de friction des couples SS/SS, SS/TMA et SS/CuNiTi apr es une semaine, s’expliquerait par le fait que la couche oxyde form ee par la corrosion de surface au niveau de ces fils augmenterait la force de friction.

D’autres etudes [10], en revanche, sont parvenues a` des conclusions diff erentes. Selon ces travaux, les films d’oxydes prot egent et r eduisent l’adh esion entre deux m etaux en contact ; il existe cependant une tendance a` la baisse de la force de friction avec l’augmentation du temps d’immersion.

D’autres travaux sont n ecessaires pour d eterminer le compor- tement de l’ etat de surface des mat eriaux ainsi que leur coef- ficient de friction, lorsque ceux-ci sont soumis a` une d egradation electrochimique de longue dur ee qui s’approche le plus possible de la dur ee de s ejour des alliages m etalliques en milieu buccal.

[(Fig._13)TD$FIG]

Fig. 13: Evolution of the friction coefficient of the SS/TMA pair before and after 1 week of immersion.

Fig. 13 :Evolution du coefficient de friction du couple SS/TMA avant et apres une semaine.

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