Application of a Molybdenum and Tungsten Disulfide Coating To

nanomaterials

Article Application of a Molybdenum and Tungsten Disulﬁde Coating to Improve Tribological Properties of Orthodontic Archwires

Antonio Gracco 1, Martina Dandrea 2, Flavio Deflorian 3 , Caterina Zanella 3,4, Alberto De Stefani 1 , Giovanni Bruno 1,* and Edoardo Stellini 1 1 DDS, Faculty of Dentistry, University of Padua, 35100 Padua, Italy; [email protected] (A.G.); [email protected] (A.D.S.); [email protected] (E.S.) 2 DDS, Faculty of Dentistry, University of Rome, 00185 Rome, Italy; [email protected] 3 Department of Industrial Engineering, University of Trento, 38121 Trento, Italy; flavio.defl[email protected] (F.D.); [email protected] (C.Z.) 4 Department of materials and manufacturing, School of Engineering, Jönköping University, 553 18 Jönköping, Sweden * Correspondence: [email protected]

Received: 8 April 2019; Accepted: 14 May 2019; Published: 16 May 2019

Abstract: Coatings incorporating nanoparticles of molybdenum and tungsten disulﬁde (MoS2 and WS2)—known for their lubricating properties—are applied to orthodontic stainless steel wires to verify if there is an improvement in terms of tribological properties during the sliding of the wire along the bracket. To simulate in vitro sliding of the wire along the bracket and evaluate friction 0.019 0.025 × inches orthodontic stainless steel (SS) wires were subjected to the application, by electrodeposition, of Ni, Ni + MoS2, and Ni + WS2. The samples produced were analyzed with scanning electron microscopy and assessment of resistance to bending. Thirty-two test conditions have been analyzed, arising from the combination of four types of coatings (SS bare wires and strings with three types of coating), two types of self-ligating bracket (Damon Q, Ormco and In-Ovation R, GAC International), two bracket-wire angles (0◦ and 5◦), two environments (dry and wet). Analyses carried out on the samples show acceptable coatings incorporating MoS2 and WS2 and a resistance of coatings after a minimum bending. In “dry conditions” a statistically signiﬁcant decrease in friction occurs for wires coated with MoS2 and WS2 if associated with the In-Ovation bracket. In “wet conditions” this decrease is observed only in isolated test conditions. Analysis of the wires after sliding tests show little wear of the applied coatings. Nanoparticles are acceptable and similar in their behavior. Improvements in terms of friction are obtained pairing coatings incorporating MoS2 and WS2 with the In-Ovation bracket in dry conditions.

Keywords: friction; stainless steel archwires; inorganic nanoparticles

1. Introduction Orthodontic treatment involves the sliding of a tooth along an orthodontic archwire. Each time this happens, a friction force between archwire and bracket, which is opposed to the movement itself, is generated. For this reason, orthodontic force must exceed this resistance to carry out the displacement. More than 60% of orthodontic force applied to obtain dental movement is expected to be lost due to frictional forces [1], reducing the amount of forces employed by the fixed appliance. Friction reduction would allow the application of a lower orthodontic force, with significant benefits, ranging from a lower risk of root resorption, to the best anchorage control, and reduction of the treatment time [2]. Many years of materials engineering support orthodontic research in finding a solution to

Nanomaterials 2019, 9, 753; doi:10.3390/nano9050753 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 753 2 of 10 this problem. While in the past the focus was mainly on the characteristics of the bracket, these days, however, the most significant progress can be achieved with the application of nanotechnology to the materials science of orthodontic interest. In particular, coating orthodontic archwires with film incorporating nanoparticles of MoS2 and WS2 seems to be one of the best ways to achieve results in research aimed at the reduction of friction. In 1985 H.W. Kroto [3,4] discovered a new allotropic form of carbon organized in stable molecules of 60 atoms (C60) which was called “fullerene”. The discovery of this molecule gave birth to intensive research for producing other possible structures of carbon. In this way, later, nanotubes (NT) were discovered [5]. The great interest in the unique physical and chemical properties of fullerene-like nanoparticles pushed the research towards the development of inorganic fullerene-like nanoparticles (IF-NP). The attention was focused mainly on chalcogenides (binary salts of S, Se or Te) of the transition metals, in particular on disulfides MoS2 and WS2, which were described for the first time by Tenne in 1992. These compounds are derived from materials with a layered structure, consisting of a plane of hexagonal crystals (H) of the metal Mo/W intercalated with two floors of anionic sulfur atoms, synthetically 2H-MoS2 and 2H-WS2, respectively. These structures, potentially unstable, can, in certain conditions, bend and close on themselves, stitching the rim atoms together, becoming similar to graphite and giving birth to particles of 20 to 200 nm [6]. The particular structure of MoS2 and WS2 nanoparticles makes them excellent solid lubricants, allowing an improvement in friction and wear properties under wet and dry conditions at different load levels [7]. This probably happens due to the penetration of solid lubricant WS2 nanoparticles into the interface between rubbed surfaces. As the load between the bodies increases, the nanoparticles gradually deform and exfoliate. In this way the asperities at the interface are coated, allowing a very low shear force sliding motion between the two contacting bodies [6–8]. Previous studies of coating stainless steel (SS) or NiTi wires with a metal matrix (nickel-phosphorus, nickel or cobalt) impregnated with IF-WS2 nanoparticles showed a significant reduction in friction during in vitro sliding tests [9–12]. This work aims to reduce friction between orthodontic stainless-steel wires and brackets by coating the wires with self-lubricating nickel (Ni) films containing inorganic amorphous nanoparticles (NP) of tungsten disulfide (WS2) and molybdenum disulfide (MoS2), which are well-known lubricants.

2. Materials and Methods

2.1. Nanoparticles This study was carried out using NP supplied by Nanoshel LLC (Nanoshel LLC, Willmington DE, United States), which has provided the information reported in Table1.

Table 1. Properties of MoS2 and WS2.

MoS2 WS2 Purity 99.90% 99.90% Color Black Black Morphology Spherical Spherical APS 80–100 nm 40–80 nm Density 5.06 g/cm3 ND Melting Point 1185 ◦C ND Synthesis MOCVD MOCVD

As regards the advantages brought by the use of these materials, Nanoshel LLC states that a few tenths of a percent in weight of the product can improve tribological properties of the system. When the load between the sliding part is small (low load conditions), friction reduction is mainly ascribable to the bearing-like behavior of NP, that roll between the contact surface, keeping their shape intact. For high load conditions, a coating, induced by the presence of NP, is deposited on the crests of surface roughness and it can reduce direct contact between the asperities, thus, minimize wear. Nanomaterials 2019, 9, 753 3 of 10

2.2. Bracket To create a sliding movement, two kind of self-ligating bracket (interactive and passive bracket) were employed:

40 In-Ovation (Dentsply Sirona, New York, US) upper right central incisors bracket (tq 12 , tip 5 ). • ◦ ◦ 40 Damon Q (Ormco, 200 S. Kraemer Blvd., Building E Brea, California 92821) upper right central • incisors bracket (tq 15◦, tip 5◦).

2.3. Artiﬁcial Saliva To simulate an intraoral environment, setting up “wet” condition, a saliva substitute (Biotene™ Oral Balance gel) was employed. Since the product is available in a gel formulation, this was diluted with water, having a solution that in a liter of water contained 250 mg of product.

2.4. Coatings’ Deposition on Orthodontic Wires Sixty orthodontic stainless steel (SS) wires with a rectangular cross section (0.019 0.025 inches) × shape have been subjected to a stage of preparation, consisting in:

Mechanical abrasion with abrasive paper of silicon carbide (500 grains/unit area), to clean • orthodontic wires. Solvent degreasing with acetone in an ultrasonic cleaner for 2 min, to remove organic residues • and dust of abrasive paper. Rinsing with deionized water and drying. • Positioning of orthodontic wires on a special frame, with the aim of being able to handle more • easily without leaving traces on the substrates to be coated. Immersion of the frame with the wires in sulfuric acid at 0.5 M with cathodic polarization to 1 V. • − Rinsing with deionized water. • Orthodontic wires were then divided into 4 group, each of these made of 15 wires:

GROUP 1: control group; wires of this group were not applied any coating. • GROUP 2: wires of this group were coated with a Ni film by electrochemical co-deposition. • GROUP 3: wires of this group were coated with a Ni + MoS film by electrochemical co-deposition. • 2 GROUP 4: wires of this group were coated with a Ni + WS film by electrochemical co-deposition. • 2 Orthodontic wires that have to undergo the deposition of a coating (GROUP 2, GROUP 3, GROUP 4) were then inserted in a pre-nickel plating bath (Wood bath) at 25 ◦C, with a direct current of 0.04 A and starting the process of electrodeposition for 2 min. This procedure allowed the application of a Ni layer to the substrate to promote adhesion of the coating subsequently applied to SS wires. Afterward, wires of each of the three groups were treated in a different way:

GROUP 2: wires were inserted in a Watts bath, with a direct current of 0.04 A and starting the • process of electrochemical co-deposition for 25 min. GROUP 3: wires were inserted in a Watts bath in which 2 g/L of MoS NP was dispersed; since • 2 MoS2 NP are hardly wettable, a cationic surfactant (CTAB cetyl trimethyl aminobromide) was added to the bath (0.4 gr/L of CTAB). To minimize agglomeration of MoS2 NP, the bath was maintained in constant agitation with the use of a mechanical stirrer (activated 12 h before the deposition and during the entire deposition step) and ultrasound treatment. Electrochemical co-deposition was processed for 25 min with a direct current of 0.04 A. GROUP 4: wires were inserted in a Watts bath in which 2 g/L of WS NP was dispersed; since WS • 2 2 NP are hardly wettable, a cationic surfactant (CTAB cetyl trimethyl aminobromide) was added to the bath (0.2 gr of CTAB). To minimize agglomeration of WS2 NP, the bath was maintained in constant agitation with the use of a mechanical stirrer (activated 12 h before the deposition and Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 10

aminobromide) was added to the bath (0.2 gr of CTAB). To minimize agglomeration of WS2 NP, the bath was maintained in constant agitation with the use of a mechanical stirrer Nanomaterials(activated2019, 9, 753 12 h before the deposition and during the entire deposition step) and ultrasound4 of 10 treatment. Electrochemical co-deposition was processed for 25 min with a direct current of 0.04 A. during the entire deposition step) and ultrasound treatment. Electrochemical co-deposition was Once the coating process was done, wires were washed with deionized water and dried with processed for 25 min with a direct current of 0.04 A. compressed air. Once the coating process was done, wires were washed with deionized water and dried with compressed2.5. Analysis air.of Coated Orthodontic Wires The morphology and chemical composition of the film deposited on orthodontic wires were 2.5. Analysis of Coated Orthodontic Wires analyzed with scanning electron microscopy (SEM) in association with energy-dispersive X-ray spectrometryThe morphology (EDS). These and analyses chemical were composition carried out of theboth ﬁlm in surface deposited and onin section. orthodontic wires were analyzedTo evaluate with scanning the adherence electron of microscopy the coating (SEM)upon bending in association of the withorthodontic energy-dispersive wire (a common X-ray spectrometryclinical procedure) (EDS). orthodontic These analyses wires were coated carried with out Ni, both Ni + inMoS surface2, and and Ni + in WS section.2 were gradually hand- bent Tousing evaluate two spindles, the adherence respectively, of the coating of 4 mm upon and bending 32 mm. ofThe the procedure orthodontic was wire always (a common performed clinical by procedure)the same operator. orthodontic wires coated with Ni, Ni + MoS2, and Ni + WS2 were gradually hand-bent using two spindles, respectively, of 4 mm and 32 mm. The procedure was always performed by the same2.6. Friction operator. Measurements of the Wires

2.6. FrictionTo simulate Measurements a sliding ofmovement, the Wires a universal device for mechanical measurements (Instron 4502) was employed to carry out friction measurements of the coated and uncoated orthodontic wires. To performTo simulate tests, the a slidingDamon movement, Q (passive) a universaland In-Ovation device for(interactive) mechanical brackets measurements were employed. (Instron 4502)Measurements was employed were carried to carry out out both friction in dry measurements and wet conditions, of the using coated a sali andva uncoated substitute orthodontic (Biotene™ wires.Oral Balance) To perform to simulate tests, the the Damon intraoral Q (passive) environmen and In-Ovationt. A dry condition (interactive) was bracketsused to obtain were employed. reference Measurementsvalues. were carried out both in dry and wet conditions, using a saliva substitute (Biotene™ Oral Balance)A support to simulate on which the intraoral mounting environment. the bracket Ausing dry conditionthe same type was usedof the to resin obtain used reference in orthodontic values. treatmentA support (Transbond™ on which PLUS mounting Color the Change, bracket 3M using Unitek, the same US) was type fixed of the on resin the usedbody in of orthodontic the device. treatmentThe bracket’s (Transbond support™ couldPLUS be Color rotated Change, varying 3M th Unitek,e angle US) between was fixed bracket on the and body wire of to the simulate device. Thedifferent bracket’s levels support of frictional could forces. be rotated Two angulati varyingons the between angle between bracket bracketand wire and were wire chosen: to simulate 0° and di5°.ff Theerent orthodontic levels of frictional wire was forces. fixed Two above angulations to a 100 N between load cell bracket of Instron and 4502 wire and were lowered chosen: to 0 a◦ weightand 5◦. Theof 136 orthodontic g required wire to hold was it fixed taut abovealong the to a vertical. 100 N load The cell temperature of Instron was 4502 21 and °C. lowered to a weight of 136 gIn required this way, to holdthe wire it taut engaged along the the vertical. slot of Thethe temperatureself-ligating bracket. was 21 ◦ C.The load cell was put into motionIn thisat a way, speed the of wire 5 mm/min engaged for the a slotdistance of the of self-ligating 5 mm. The bracket. test began The with load cella steady was put increase into motion in the atforce a speed and ofreached 5 mm/ mina maximum for a distance with of the 5 mm. beginnin The testg of began movement with a steadyon the increase wire. This in the maximum force and reachedrepresents a maximum the static withfriction the force beginning that is of needed movement to initiate on the movement. wire. This maximum represents the static frictionConsidering force that is4 neededkinds of to coatings, initiate movement. 2 types of bracket, 2 types of environment, and 2 different angles,Considering 32 test conditions 4 kinds ofwere coatings, created 2 types(Figure of bracket,1). Each 2test types condition of environment, was repeated and 25 ditimes.fferent For angles, each 32repetition, test conditions after a were run-in created period (Figure of repeated1). Each back test conditionand forth was movements repeated 5(5 times. times) For of eachthe wire repetition, in the afterbracket, a run-in which period is recommended of repeated back by andprevious forth movementsstudies [11], (5 times)5 values of theof wirethe frictional in the bracket, force which were ismeasured. recommended The data by previouswere recorded studies in [ 11the], 5operatin valuesg of system the frictional of the forceInstron were and measured. then analyzed The dataand wereplotted. recorded Data inanalysis the operating was systemconducted of the consider Instron anding then“dry” analyzed and “wet” and plotted. conditions Data analysisin parallel, was conductedindependently considering of one another. “dry” and “wet” conditions in parallel, independently of one another.

FigureFigure 1.1. Flow chart of the test condition.

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Nanomaterials2.7. Data Analysis2019, 9, 753 5 of 10 Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 10 Descriptive statistic of the frictional forces was accomplished including mean and standard 2.7.deviation Data Analysis for each of 32 test conditions. Regarding inferential statistic, data were analyzed using a mixed effects model considering coating, bracket, and angle as fixed factors, with wire as a random Descriptive statistic of the frictional forces was accomplished including mean and standard factor.Descriptive The multiple statistic comparisons of the frictional were madeforces wiwasth Tukey’saccomplished method. including Analyses mean were and performed standard deviation for each of 32 test conditions. Regarding inferential statistic, data were analyzed using a deviationemploying for R statisticaleach of 32 environment test conditions. (R Core Regarding Team, 2015),inferential using statistic, the nlme data package were (Pinheiroanalyzed &using Bates a, mixed effects model considering coating, bracket, and angle as fixed factors, with wire as a random mixed2000) for effects the adaptation model considering of models coating, and lsmeans bracket, (Lenth and angle& Hervé as fixed, 2015) factors, for multiple with wire comparisons. as a random factor. The The multiple multiple comparisons comparisons were were made withwith Tukey’sTukey’s method.method. Analyses were performedperformed employing R statistical environment (R Core Team, 2015), using the nlme package (Pinheiro & Bates, employing2.8. Analysis R of statistical Coated and environment Uncoated Orthodontic (R Core Team, Wires 2015), after Friction using the Test nlme package (Pinheiro & Bates, 2000) for the adaptation of models and lsmeans (Lenth(Lenth && HervHervéé,, 2015)2015) forfor multiplemultiple comparisons.comparisons. Twelve of the 32 test conditions were considered the most significant ones and were analyzed 2.8. Analysis of Coated and Uncoated Orthodontic Wires after Friction Test 2.8.after Analysis the Instron of Coated test. andThe Uncoated morphology Orthodontic and chemical Wires after composition Friction Test of these orthodontic wires were analyzedTwelve with of thescanning 32 test conditionselectron microscopy were considered (SEM the) in most association significant with ones energy-dispersive and were analyzed X-ray after Twelve of the 32 test conditions were considered the most significant ones and were analyzed thespectrometry Instron test. (EDS). The morphology and chemical composition of these orthodontic wires were analyzed after the Instron test. The morphology and chemical composition of these orthodontic wires were with scanning electron microscopy (SEM) in association with energy-dispersive X-ray spectrometry analyzed3. Results with scanning electron microscopy (SEM) in association with energy-dispersive X-ray (EDS). spectrometry (EDS). 3.3.1. Results Analysis of Coated Orthodontic Wires 3. Results Scanning electron microscopy analysis shows, for each wires’ group, well-defined continuous 3.1. Analysis of Coated Orthodontic Wires 3.1.coatings. Analysis From of Coat “in edsurface” Orthodontic images Wires we can say that coatings’ adhesion is more than good in each of the threeScanning groups electron (Figure microscopy 2). Coatings analysis are homogeneou shows, fors even each if wires’ the grain group, is much well-defined more evident continuous in the coatings.case Scanningof films From electronwith “in surface” nanoparticles. microscopy images we analysisFigure can say 2b,cshows, that show coatings’ for theeach adhesionmorphology wires’ group, is more of well-defined thanfilms good that in incorporatecontinuous each of the threecoatings.nanoparticles groups From (Figure of “in MoS surface”22)., in Coatings one images case, are and we homogeneous WScan2 ,say in the that other, even coatings’ if with the grainadhesionspherical is much isor morecylindrical-like more than evident good instructures. in the each case of ofthe films threeSectional with groups nanoparticles. images (Figure show 2). Coatings Figurethe thickness2b,c are show homogeneou of the the coatings, morphologys even which if of the filmsis grainabout that is 10 incorporatemuch μm formore the nanoparticlesevident wires coated in the ofcasewith MoS Ni,of2 , films20 in oneμm with for case, those nanoparticles. and products WS2, in the with Figure other, Ni + with2b,c MoS sphericalshow2, 15 μ mthe for or morphology cylindrical-likethose with Ni of + structures.filmsWS2 (Figure that incorporate 3). nanoparticles of MoS2, in one case, and WS2, in the other, with spherical or cylindrical-like structures. Sectional images show the thickness of the coatings, which is about 10 μm for the wires coated with Ni, 20 μm for those products with Ni + MoS2, 15 μm for those with Ni + WS2 (Figure 3).

Figure 2.2.Scanning Scanning electron electron microscopy microscopy images images (a) Ni magnification(a) Ni magnification 2000 ,(b) Ni2000×,+ MoS (b2)magnification Ni + MoS2 × 2000magnification, and (c) 2000×, Ni + WS and2 magnification (c) Ni + WS2 magnification 2500 . 2500×. × × µ SectionalFigure 2. imagesScanning show electron the thicknessmicroscopy of theimages coatings, (a) Ni which magnification is about 102000×,m for(b) theNi wires+ MoS coated2 µ µ withmagnification Ni, 20 m for 2000×, those and products (c) Ni + with WS2 magnification Ni + MoS2, 15 2500×.m for those with Ni + WS2 (Figure3).

Figure 3. Sectional images of the coating (a) Ni, (b) Ni + MoS2, and (c) Ni + WS2.

In the case of wires coated with Ni + MoS2 and Ni + WS2, energy-dispersive x-ray spectrometry Figure 3. Sectional images of the coating (a) Ni, (b) Ni + MoS , and (c) Ni + WS . (EDS) shows fairlyFigure good 3. Sectional percentages images by of weightthe coating of MoS (a) Ni,2 and (b) NiWS + 2MoS. 22, and (c) Ni + WS22. Figure 4 is obtained through an optical microscope and shows three orthodontic wires coated, In the case of wires coated with Ni + MoS2 and Ni + WS2, energy-dispersive x-ray spectrometry respectively,In the case with of wiresNi (4a), coated Ni + withMoS 2Ni (4b), + MoS and2 Niand + NiWS +2 WS(4c)2 ,and energy-dispersive subjected to a minimumx-ray spectrometry bending. (EDS) shows fairly good percentages by weight of MoS and WS . (EDS)Overall, shows no zones fairly goodof breakage percentages can beby foundweight in of eachMoS2 2 typeand WSof 2coatings,. maintaining their quality unchanged.Figure 4 is obtained through an optical microscope and shows three orthodontic wires coated, respectively, with Ni (4a), Ni + MoS2 (4b), and Ni + WS2 (4c) and subjected to a minimum bending. Overall, no zones of breakage can be found in each type of coatings, maintaining their quality unchanged.

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Figure4 is obtained through an optical microscope and shows three orthodontic wires coated, respectively, with Ni (4a), Ni + MoS2 (4b), and Ni + WS2 (4c) and subjected to a minimum bending. Overall,Nanomaterials no zones2019, 9 of, x FOR breakage PEER REVIEW can be found in each type of coatings, maintaining their quality unchanged.6 of 10

FigureFigure 4. 4.Coating Coating at at optical optical microscope microscope ( a()a) Ni, Ni, ( b(b)) Ni Ni+ +MoS MoS22,, andand ((cc)) NiNi+WS+WS22.

OnOn thethe otherother hand,hand, FigureFigure5 5,, always always obtained obtained through through an an optical optical microscope, microscope, show show damages damages on on NiNi ++ MoS22 (5b) and Ni + WSWS22 (5c)(5c) coatings coatings as as a a consequence consequence of of maximum bending ofof thethe wires,wires, whilewhile thethe wirewire coatedcoated withwith NiNi appearsappears unchanged unchanged (5a). (5a).

FigureFigure 5. 5.Fractures Fractures at at optical optical microscope microscope ( a()a) Ni, Ni, ( b(b)) Ni Ni+ +MoS MoS22,, andand ((cc)) NiNi+ + WS22.

3.2.3.2. FrictionFriction MeasurementsMeasurements ofof thethe WiresWires DataData analysisanalysis waswas conductedconducted consideringconsidering “dry”“dry” andand “wet”“wet” conditionsconditions independentlyindependently ofof oneone another,another, soso resultsresults will will be be presented presented separately. separately.

3.2.1. “Dry” Conditions 3.2.1. “Dry” Conditions For each test condition performed in a dry environment, Table2 shows the mean and standard For each test condition performed in a dry environment, Table 2 shows the mean and standard deviation of five measurements of the frictional force. From a descriptive point of view, values of the deviation of five measurements of the frictional force. From a descriptive point of view, values of the friction coefficient are significantly higher for the wires coated with nickel compared to all other types friction coefficient are significantly higher for the wires coated with nickel compared to all other types of wires. Bartlett’s test shows a violation of the assumption of homogeneity of variances between the of wires. Bartlett’s test shows a violation of the assumption of homogeneity of variances between the four coatings (Bartlett’s K2 (3) = 160.75, p < 0.0001). The adaptation of the mixed effects model showed four coatings (Bartlett’s K2 (3) = 160.75, p < 0.0001). The adaptation of the mixed effects model showed a significance of all fixed effects included in the model. From the analysis of multiple comparisons a significance of all fixed effects included in the model. From the analysis of multiple comparisons performed by Tukey’s method and the average values some conclusions can be drawn in terms of performed by Tukey’s method and the average values some conclusions can be drawn in terms of friction coefficient: friction coefficient: • No significantNo significant diff erencesdifferences were were found found between between Ni Ni+ MoS + MoSand2 and Ni Ni+ +WS WS2films films even evenif, if, fromfrom aa • 2 2 descriptive point of view, wires coated with Ni + MoS2 show lower friction values; descriptive point of view, wires coated with Ni + MoS2 show lower friction values; • Wires Wires coated coated with with Ni + NiMoS + MoSand2 and Ni Ni+ WS + WSfilms2 films have have friction friction values values significantly significantly lower lower than • 2 2 wiresthan coated wires with coated Ni; with Ni; • If paired to the In-Ovation bracket, SS uncoated wires differ from all others, both from If pairedIf paired to the to the In-Ovation In-Ovation bracket, bracket, SS SS uncoated uncoated wires wires di differffer from from all all others, others, both both from fromwires • wires coated with Ni (whose friction coefficient is greater) and wires coated with Ni + MoS2 coated with Ni (whose friction coefficient is greater) and wires coated with Ni + MoS2 and Ni + and Ni + WS2 (whose friction coefficients are lower); WS2 (whose friction coefficients are lower); • If paired to the Damon Q bracket, SS uncoated wires have lower friction coefficient only for wires coated with Ni, while they do not significantly differ from wires coated with Ni + MoS2 and Ni + WS2.

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If paired to the Damon Q bracket, SS uncoated wires have lower friction coefficient only for wires • coated with Ni, while they do not significantly differ from wires coated with Ni + MoS2 and Ni + WS2.

Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 10 Table 2. Mean and standard deviation of ﬁve measurements of the frictional force in dry condition. Table 2. Mean and standard deviation of five measurements of the frictional force in dry condition. SS SS + Ni SS + Ni + MoS2 SS + Ni + WS2 SS SS + Ni SS + Ni + MoS2 SS + Ni + WS2 Damon Q/0◦ 0.58(0.12) 3.22(0.92) 0.42(0.08) 0.50(0.08) DamonDamon Q/5◦ Q/0°1.27(0.18) 0.58(0.12) 9.25(4.01) 3.22(0.92) 0.94(0.11) 0.42(0.08) 0.50(0.08) 1.19(0.14) In-OvationDamon/0◦ Q/5°1.24(0.12) 1.27(0.18) 3.35(0.57) 9.25(4.01) 0.64(0.06) 0.94(0.11) 1.19(0.14) 0.79(0.06) In-OvationIn-Ovation/0°/5◦ 1.43(0.06) 1.24(0.12) 7.96(0.59) 3.35(0.57) 1.06(0.07) 0.64(0.06) 0.79(0.06) 1.06(0.13) In-Ovation/5° 1.43(0.06) 7.96(0.59) 1.06(0.07) 1.06(0.13) FigureFigure6 summarizes 6 summarizes the the results results of statisticalof statistical analysis. analysis.

FigureFigure 6. Statistical6. Statistical analysis. analysis. 3.2.2. “Wet” Conditions 3.2.2. “Wet” Conditions For each test condition performed in a wet environment, Table3 shows the mean and standard For each test condition performed in a wet environment, Table 3 shows the mean and standard deviation of five measurements of the frictional force. From a descriptive point of view, values of the deviation of five measurements of the frictional force. From a descriptive point of view, values of the friction coefficient are significantly higher for the wires coated with nickel compared to all other types friction coefficient are significantly higher for the wires coated with nickel compared to all other types of wires. Bartlett’s test shows a violation of the assumption of homogeneity of variances between the of wires. Bartlett’s test shows a violation of the assumption of homogeneity of variances between the four coatings (Bartlett’s K2 (3) = 29.03, p < 0.0001). The adaptation of the mixed effects model showed four coatings (Bartlett’s K2 (3) = 29.03, p < 0.0001). The adaptation of the mixed effects model showed a significance of all fixed effects included in the model. From the analysis of multiple comparisons a significance of all fixed effects included in the model. From the analysis of multiple comparisons performed by Tukey’s method and the average values some conclusions can be drawn in terms of performed by Tukey’s method and the average values some conclusions can be drawn in terms of friction coefficient: friction coefficient: • Comparing Comparing Ni + MoSNi + MoSand2 Niand+ NiWS + WS, from2, from a descriptive a descriptive point point of view, of view, wires wires coated coated with with Ni +Ni • 2 2 MoS2+always MoS2 always show lowershow lower friction friction values. values. Tukey’s Tukey’s test results test results state state that Nithat+ NiMoS + MoS2 presents2 presents a lowera friction lower friction coefficient coefficient for both for Damon both Damon 0◦ and In-Ovation0° and In-Ovation 5◦. 5°. • Wires Wires coated coated with Niwith+ MoSNi + MoSand2 Niand+ NiWS + WSfilms2 films have have friction friction values values significantly significantly lower lower than • 2 2 wiresthan coated wires with coated Ni except with for Ni Damonexcept for 5◦. Damon 5°. • If paired If paired to the to In-Ovation the In-Ovation bracket, bracket, SS uncoated SS unco wiresated wires differ differ from from wires wires coated coated with with Ni + NiMoS + • 2 (whoseMoS friction2 (whose coe ffifrictioncient is coefficient lower). is lower). • If paired to the Damon Q bracket, SS uncoated significantly differ from wires coated with Ni + MoS2 and Ni + WS2 (whose friction coefficients are lower) only for angulation of 5°.

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If paired to the Damon Q bracket, SS uncoated significantly differ from wires coated with Ni + • MoS2 and Ni + WS2 (whose friction coefficients are lower) only for angulation of 5◦.

Nanomaterials 2019, 9, x FOR PEER REVIEW 8 of 10 Table 3. Mean and standard deviation of five measurements of the frictional force in wet condition. Table 3. Mean and standard deviation of five measurements of the frictional force in wet condition. SS SS + Ni SS + Ni + MoS2 SS + Ni + WS2 SS SS + Ni SS + Ni + MoS2 SS + Ni + WS2 Damon Q/0 0.95(0.09) 1.01(0.07) 0.66(0.13) 0.94(0.07) Damon◦ Q/0° 0.95(0.09) 1.01(0.07) 0.66(0.13) 0.94(0.07) Damon Q/5◦ 1.87(0.10) 1.33(0.10) 1.16(0.07) 1.27(0.18) Damon Q/5° 1.87(0.10) 1.33(0.10) 1.16(0.07) 1.27(0.18) In-Ovation/0◦ 1.45(0.17) 1.75(0.11) 0.91(0.19) 1.13(0.16) In-OvationIn-Ovation/0°/5◦ 2.52(0.09) 1.45(0.17)4.06(0.45) 1.75(0.11) 0.91(0.19) 1.46(0.13) 1.13(0.16) 2.35(0.19) In-Ovation/5° 2.52(0.09) 4.06(0.45) 1.46(0.13) 2.35(0.19) 3.3. Analysis of Coated and Uncoated Orthodontic Wires after Friction Test 3.3. Analysis of Coated and Uncoated Orthodontic Wires after Friction Test Considering coated and uncoated wires, SEM images have highlighted that the condition with Considering coated and uncoated wires, SEM images have highlighted that the condition with greater wear is “Damon Q–5 ”. Wires coated with Ni show a greater amount of damage if compared greater wear is “Damon Q–5°”.◦ Wires coated with Ni show a greater amount of damage if compared to the other wires. In contrast, Ni + MoS2 and Ni + WS2 films seem to have better integrity (Figure7). to the other wires. In contrast, Ni + MoS2 and Ni + WS2 films seem to have better integrity (Figure 7). Moreover,Moreover, there there is no is dinoff erencedifference in wear in wear between between coatings coatings that incorporate that incorporate nanoparticles. nanoparticles. EDS analysis EDS confirmanalysis the confirm presence the of presence nanoparticles of nanoparticles in the areas in the of wearareas ofof thewear wires of the after wires friction after friction tests. tests.

FigureFigure 7. 7.Morphology Morphology ofof damagedamage ofof thethe coatingcoating material.material. ( (aa) )400× 400 magnification,magniﬁcation, (b ()b )1000× 1000 × × magniﬁcation.magnification.

4. Discussion4. Discussion

FromFrom a qualitativea qualitative point point of of view, view, coating coating stainless stainless steel steel orthodontic orthodontic wires wires with with Ni Ni+ +MoS MoS2 2and andNi + WSNi 2+ hasWS2 provided has provided a homogeneous a homogeneous result. result. Overall, Overall, scanning scanning electron electron microscopy microscopy (SEM) (SEM) analysisanalysis in associationin association with with energy-dispersive energy-dispersive X-ray X-ray spectrometry spectrometry (EDS) (EDS) allowed allowed to to state state that: that: • Films’ adhesion was homogeneous, with few uncoated areas. Films’ adhesion was homogeneous, with few uncoated areas. • • The amount of MoS2 and WS2 in the films is acceptable, confirming that incorporating The amount of MoS and WS in the films is acceptable, confirming that incorporating lubricant • lubricant compounds2 within2 a metallic matrix is one of the best ways to apply a lubricant. compoundsThis strategy within has a metallic already matrix been suggested is one of theby several best ways studies to apply in the a literature lubricant. [9–12]. This strategy has already• The been coatings’ suggested thickness by several was contained studies inwithin the literaturevalues that [9 would–12]. allow the insertion of the The coatings’wires into thickness the slot and was the contained sliding along within the values bracket. that would allow the insertion of the wires • intoThese the slotresults and can the be sliding considered along positive, the bracket. since the manufacture of the coatings was conducted in the laboratory, and not with industrial production methods. These results can be considered positive, since the manufacture of the coatings was conducted in Good adhesion of Ni + MoS2 and Ni + WS2 films to SS substrates and limited and constant thethickness laboratory, of andthe notcoatings, with industrialas evidenced production by SEM methods. images, have confirmed the effectiveness of electrodeposition,Good adhesion ofin Niagreement+ MoS2 and with Ni similar+ WS2 triafilmsls tocarried SS substrates out by other and limited authors and [10]. constant Compared thickness to of theelectroless coatings, deposition as evidenced [9,11], by which SEM images,is frequent havelyconfirmed used, electrodeposition the effectiveness has of made electrodeposition, the coatings’ in agreementapplication with more similar “controllable” trials carried and out manageable, by other authors providing [10]. Comparedconsistent repeatability. to electroless deposition [9,11], which isBending frequently orthodontic used, electrodeposition wires coated with has Ni, made Ni the+ MoS coatings’2, and Ni application + WS2 has more highlighted “controllable” the andacceptability manageable, of providingthe coatings consistent produced repeatability. for minimum bending, but also the need to improve the resistance to greater bending. A further development may be represented in research for possible treatments that improve the properties of the coatings.

Nanomaterials 2019, 9, 753 9 of 10

Bending orthodontic wires coated with Ni, Ni + MoS2, and Ni + WS2 has highlighted the acceptability of the coatings produced for minimum bending, but also the need to improve the resistance to greater bending. A further development may be represented in research for possible treatments that improve the properties of the coatings. In vitro simulation of clinical conditions that occur with an orthodontic fixed appliance has produced mixed results among themselves. As regards the tests carried out in “dry” conditions, from a descriptive point, there is a decrease in the coefficient of friction for wires coated with Ni + MoS2 and Ni + WS2 compared to SS wires and Ni-coated wires. Regarding inferential statistic, however, results provide different conclusions. Comparing wires coated with Ni + MoS2 and Ni + WS2 and SS wires, only testing the In-Ovation bracket, changes in terms of friction coefficient are obtained. This result on the efficiency of Ni + WS2 film for the In-Ovation bracket agrees with other researches in the literature [10]. Regarding this, we remind briefly the mechanism for which nanoparticles perform their lubricating action: in low load conditions, friction reduction is primarily due to the “buffer”-like behavior carried out by the nanoparticles, which flow between the surfaces, maintaining their shape; in high load conditions, nanoparticles induce the formation of a deposit at the level of surface roughness, reducing the direct contact between the asperities and minimizing wear. Damon Q behavior (which does not generate statistically significant differences between wires incorporating NP and SS wires) is not comparable with other data from the literature, because similar experiments have not been carried out with the use of such passive self-ligating brackets. Discussing the comparison between Ni + MoS2 and Ni + WS2 coated wires and Ni coated wires, there was always a statistically significant variation of the friction coefficient, in favor of the sample products incorporating NP. These outcomes for Ni + WS2 coating agree with other results in the literature [10]: The improvement in terms of friction coefficient is due to the lubricant behavior of WS2, but also to the mechanism of prevention of the formation of an oxide layer deployed by such compounds [10]. In regard to this, a further development of such research may be represented by coating NiTi orthodontic wires to verify if the remarkable properties of MoS2 and WS2 compounds are confirmed even with these substrates. Comparing Ni + MoS2 and Ni + WS2 does not lead to statistically significant differences, although from a descriptive point of view Ni + MoS2 has values of coefficient of friction lower than Ni + WS2. No other studies in the literature compare the lubricating behavior of MoS2 and WS2 NP in this way. As regards tests carried out in “wet” conditions, from a descriptive point of view, there is a decrease in friction coefficient for wires coated with Ni + MoS2 and Ni + WS2 compared to SS wires and Ni-coated wires. Regarding inferential statistics, however, results are extremely heterogeneous with each other. We cannot determine that a particular type of coating provides better results than the others or that the use of one of self-ligating bracket gives better outcomes than the other. A single study in the literature [9] has tested the use of IF-WS2 as a lubricant simulating the oral cavity with the use of deionized water: A decrease in the friction coefficient was observed for coated wires. According to the results of this trial and to the limited evidences provided by the literature, further investigations are needed to understand the behavior of Ni + MoS2 and Ni + WS2 coatings in “wet” conditions. If we compare SEM images of coated and uncoated wires after friction tests, Ni + MoS2 and Ni + WS2 coatings show limited areas of damage. This happens both for the minor (wire-slot angle = 0◦) and for the major (wire-slot angle = 5◦) tribological stress conditions. Chemical composition of Ni + MoS2 and Ni + WS2 coatings after friction tests remains substantially similar to that obtained with the analysis before Instron measurements.

5. Conclusions This study has allowed to develop coatings qualitatively acceptable: If there is an improvement in the stages of production, in the future they could make a change in orthodontic materials. Positive feedback has occurred using “commercial” NP: In terms of cost/eﬀectiveness they can be considered fairly good, and both disulﬁdes were comparable as regards their characteristics and their behavior. Nanomaterials 2019, 9, 753 10 of 10

Testing Ni + MoS2 and Ni + WS2 coated wires with In-Ovation brackets gives better performances in terms of friction than Damon brackets. The comparison between Ni + MoS2 and Ni + WS2 coatings did not show any substantial diﬀerences: As well as evidenced by sliding tests, SEM images and EDS analysis do not show a better behavior by one coating over the other. Additional investigations should be carried out to establish the behavior of Ni + MoS2 and Ni + WS2 coatings in conditions that simulate the oral cavity.

Author Contributions: Methodology: A.G., F.D.F. Investigation: M.D., C.Z. Data curation: M.D., A.D.S., G.B. Writing the manuscript: M.D., A.D.S., G.B. Project administration and supervision: A.G., F.D.F., E.S. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare that there is no competitive ﬁnancial interest.

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