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INTRODUCTION
Controlled mechanical forces are used in orthodontics to create a biologic response to move the teeth. Although several mechanical systems are available, edgewise mechanics, which involve the sliding of brackets or tubes through an archwire, are the most popular.1 As observed in all mechanical systems, friction is generated at the point of contact between the bracket and archwire when teeth are moved through the archwire during treatment using sliding mechanics. This friction is caused by the contact between the wire and the surface of the bracket slot, binding between the wire and the corners of the bracket, and notching of the wire as a result of permanent deformation at the wire–bracket corner interface.2 Friction can occasionally slow or even stop tooth movement, and orthodontic tooth movement occurs only when the applied forces can adequately overcome the frictional force between the bracket and archwire.3 Studies have shown that up to 60% of the applied force could be lost because of friction during sliding mechanics.4-6 For optimal tooth movement and tissue responses, the amount of applied force should be such that it maximizes tooth movement without irreversible side effects.5-7
In orthodontic treatment, the friction force is affected by several factors such as the width and depth of the bracket slot, bracket–archwire angulation, shape and size of the archwire, method of ligation, interbracket distance, level of bracket slots between adjacent teeth, forces applied for retraction, and biological variables.8,9 The surface characteristics of orthodontic materials also affect the friction force between the bracket and archwire during treatment with fixed appliances.10 Different materials with different frictional characteristics, such as stainless steel (SS), gold, chromium–nickel alloy, titanium, polycarbonate, aluminum oxide or alumina (Al2O3), and zirconia, are used to manufacture brackets.11-14 However, SS brackets and archwires have been shown to produce the least friction.12,15-17 Therefore, metal slots were placed in ceramic brackets to reduce the frictional resistance between the archwire and bracket.12-14 Research has accordingly targeted different approaches, including surface treatments and coatings to further reduce friction between the bracket and archwire.18-21
Surface treatment techniques such as electrolytic treatment and ion implantation have been used to modify the surfaces of orthodontic metal brackets and archwires.22,23 In one study, the surfaces of nickel–titanium (NiTi), beta titanium, and SS wires were modified by a polytetrafluoroethylene (teflon) coating applied using the ion implantation technique.24 In other studies testing friction and other mechanical properties, diamond-like carbon films were deposited on NiTi and SS archwires and SS brackets using the plasma-based ion implantation/deposition method.9,20,25
Several coating techniques and materials have been used with the objective of improving the surface properties of orthodontic appliances. For instance, a coating technology known as the electrostatic powder technique was used to apply epoxy paint to NiTi wires, and it was found that the coating resulted in reduced friction relative to that without coating.26 Another coating technique, physical vapor deposition, was used to coat beta titanium orthodontic archwires with titanium aluminum nitride and tungsten carbide/carbon for the assessment of frictional properties, the surface morphology, and the load deflection rate.18 Radio frequency (RF) magnetron sputtering is another technique used for coating orthodontic materials. This technique removes surface atoms from a solid cathode (target) by bombarding it with positive ions from an inert gas discharge; then, it deposits the surface atoms on the surface to form a thin film.27 RF magnetron sputtering is an effective technique because of the inherent versatility, low-temperature thin film deposition, and uniform surface coating.28,29 Using this technique, a thin layer of photocatalytic titanium dioxide was applied to SS orthodontic brackets in order to utilize the antiadhesive and antibacterial properties of this compound.30 This technique was also used to coat NiTi orthodontic wires with titanium in order to assess deterioration of the superelasticity of such wires, and it was concluded that this coating method has potential for application in the orthodontic field.31
Although some of the surface treatments and coatings described above can decrease the coefficients of friction (CoFs), most had other drawbacks such as low resistance to challenging intraoral conditions, high frictional wear, and delamination or wear of the coating.32 Thus far, attempts at improving the mechanical properties of orthodontic archwires using surface treatment and coating techniques have been somewhat satisfactory. However, limited studies have evaluated changes in the frictional properties of brackets and archwires coated with the same material. Therefore, the aim of this in vitro study was to evaluate the changes in friction between SS orthodontic brackets and NiTi and SS archwires coated with Al2O3, titanium nitride (TiN), or chromium nitride (CrN). Moreover, the resistance of the coatings in simulated intraoral conditions was evaluated. The null hypothesis was that coating of SS orthodontic brackets and NiTi and SS archwires with Al2O3, TiN, or CrN does not affect the in vitro CoFs for the bracket–archwire combination.
MATERIALS AND METHODS
Materials
In this in vitro study, Al2O3, TiN, and CrN were used as coating materials for 50 0.016-inch round, heat-activated NiTi arch wires (3M Unitek, Monrovia, CA, USA), 50 0.019 × 0.025-inch rectangular SS archwires (3M Unitek), and 100 upper canine brackets with a slot dimension of 0.022 inches and zero torque and tip values (Victory series, 3M Unitek; Table 1).
Coating procedure
Samples were coated with a radio frequency/direct current (RF/DC) magnetron sputtering system (Bestec GmbH, Berlin, Germany). This computer-controlled system consists of vacuum units (vacuum chamber, pump, gauges), a sample heating system, sample holders, a sputtering gun, a gas flow system, thickness measurement instruments (instant in situ measurements), and RF/DC power supplies. After surface cleaning, the samples were loaded into the vacuum chamber with the help of sample holders, and the target coating material was placed in front of the material gun. The system was evacuated to a vacuum level of 10−8 mbar. Then, argon gas with high purity (99.998%) was input into the system to generate plasma on the surface of the target material.
The surfaces of the brackets and archwires to be coated were etched with argon-RF plasma using an applied power of 100 W for 10 minutes. Then, a pure titanium (99.95% purity) mid-layer with an approximate thickness of 60–80 nm was added to the substrate materials by argon-DC plasma with an applied power of 90 W for 5 minutes to enhance the adhesion of the coatings. TiN (99.5% purity) and Al2O3 (99.99% purity) discs with a diameter of 3 inches and a thickness of 0.125 inches were used as the target materials, while CrN coatings were formed by the reactive RF magnetron sputtering method using a disc-shaped Cr (99.95% purity) target material in the presence of argon and nitrogen gases (Table 1).
Surface characterization
The surfaces of the samples were examined using Dektak 8 Profilometer and Nanoscope IV Multimode Atomic Force Microscopy (AFM) (Veeco Metrology Inc., Santa Barbara, CA, USA) before and after the coating process. Sample surfaces were cleaned with ethyl alcohol in an ultrasonic cleaner for 10 minutes before surface profilometry and AFM. Four measurements were made along a line (400–2,000 µm) for two samples in each group, and surface roughness values were calculated from the average of these measurements by software supplied with the profilometer.
Intraoral simulation
Two samples from every coated and uncoated bracket group were subjected to 250 thermal cycles. Thermal cycling was performed in distilled water between −5°C and +55°C with a dwelling time of 30 seconds.
An in vitro toothbrushing test was applied to the coated samples in order to observe the mechanical damage resistance of the coatings after thermal cycling. A testing machine (Figure 1A) composed of a single electric motor, a container, a sample holder, and arms to hold the toothbrush was designed. The electric motor moved a metal arm connected to the toothbrush in an elliptical manner and transformed the elliptical movement into a swinging movement. The samples were placed in a container filled with tap water at room temperature, and tests were carried out using a medium-bristle toothbrush and fluoride toothpaste for 12 hours with a brushing pressure of 40 g. A pea-sized amount (0.25 g) of the fluoride toothpaste was released on the toothbrush every 15 minutes. After these tests, changes in the surfaces of the materials were investigated.
Friction tests
CoFs were measured using a modified CSM tribometer (CSM Instruments SA, Peseux, Switzerland; Figure 1B). The sample holder of the tribometer was modified to accommodate the orthodontic materials (Figure 1C). Furthermore, specific software was developed to simulate the slow motion of teeth, calibrate the tribometer, and record and analyze the data. Details regarding the modification and use of the tribometer have been presented in a previous study.12
Five samples for every archwire–bracket combination were used for the friction tests. Every bracket–archwire pair was used once, and the friction tests were performed under dry conditions. The samples were cleaned with acetone and dried before the tests. A 150-g load was applied through the vertical axis to simulate the ligation force during sliding mechanics. A standard crosshead speed of 17 µm/s (approximately 1 mm/min) was maintained. The bracket was moved 34 mm over the archwire during the tests, and approximately 10,000 CoF data points were collected and recorded by the software during each test. The mean value of these data points was used as the mean CoF for the tested sample. All tests were performed at room temperature (22 ± 2°C) and under controlled humidity (50% ± 10%). The testing apparatus was enclosed to prevent external effects on the test results.
Scanning electron microscopy (SEM)
In order to investigate the effects of friction and brushing tests on the coating materials, SEM analysis was performed using the Philips XL30 FEG device (FEI Company, Hillsboro, OR, USA) before and after the coating process. The coating thickness and homogeneity were determined during surface characterization of the coated materials.
Statistical analysis
In this study, a blind experimental research design was adopted to eliminate the experimental biases that could arise from observers. After the coating procedures, each sample and its group were randomly numbered by a researcher (S.A.) so that the other researchers who performed the thermal cycling, brushing, and friction tests (B.S.A., N.A., and A.A.O.) and the characterization processes (Y.G. and M.T.) were blinded to the group to which the sample belonged. Randomization was performed by computer-generated random codes (Research Randomizer V 4.0). These numbers were kept in a closed envelope until all the tests were completed.
SPSS 15.0 (SPSS Inc., Chicago, IL, USA) was used for statistical analysis of the CoF values. The data were examined for normality using the Kolmogorov–Smirnov test. The data for the mean CoF values were normally distributed; thus, one-way analysis of variance (ANOVA) was used for statistical comparison of the data. Statistically significant differences between groups were evaluated using Dunnett’s test.
RESULTS
Reduction in surface roughness
The surface roughness measurements obtained before and after coating of the brackets and archwires are shown in Table 2. Surface roughness decreased with the coating process in all groups, with the greatest reduction (almost 60%) observed for the SS archwires with the Al2O3 coating. The lowest surface roughness values were obtained for the brackets and NiTi archwires coated with CrN (Ra = 276.85 nm and Ra = 354.35 nm, respectively).
As shown in Figure 2, the roughest surface was observed for the uncoated NiTi archwire (Ra = 439.34 nm), and all three coatings resulted in significant reductions in the surface roughness of this wire. In contrast, the uncoated SS archwire exhibited a very low surface roughness value (Ra = 234.44 nm).
The SEM and AFM images of the coated and uncoated archwires clearly showed a reduction in surface roughness after the coating process (Figure 2). Moreover, SEM, AFM, and surface profilometry showed that the coated brackets had homogenous and smooth surfaces. The pits and irregularities on the bracket edges were filled with coating materials (Figure 3). Measurements from different regions of the bracket and slot base showed that the coating had a homogeneous distribution, with an average film thickness of 5,600 Å (0.56 ± 0.03 µm).
Reduction in coefficients of friction
The results of the friction tests were grouped according to the archwire material. Table 3 shows the descriptive statistics and differences between the coated and uncoated metal brackets and archwires. The values are also graphically presented in Figure 4.
The mean CoF for the uncoated bracket and NiTi archwire combination was 0.316. The lowest CoF was identified for the Al2O3-coated bracket and Al2O3-coated NiTi archwire combination (CoF 0.238), followed by the Al2O3-coated bracket and uncoated NiTi archwire combination (CoF 0.251). The decreases in friction in these two groups were statistically significant (p < 0.05; Table 3).
Table 3 also shows the mean CoF values and statistical differences between the coated and uncoated brackets and SS archwires. The CoF for the control group was 0.552. The Al2O3-coated bracket and SS archwire combination showed a significant decrease in friction (p < 0.05), with the lowest mean CoF (0.207). There was a significant decrease in friction (CoF 0.237) between the TiN-coated bracket and uncoated SS archwire. However, the CoF (0.818) for the uncoated bracket and TiN-coated SS archwire combination was the highest among all groups. The CrN coating, on the other hand, was ineffective in decreasing friction; the mean CoF for nearly all the bracket–archwire pairs showed an increase. Remarkably, an increase in CoFs was observed in almost all scenarios involving a combination of a coated NiTi or SS archwire and an uncoated bracket (Figure 4).
Coating resistance to physical tests
Wear associated with the friction tests was apparent in the SEM images of all uncoated and coated samples. The friction, thermal, and brushing tests did not affect the overall quality of the Al2O3 coatings; however, peeling of TiN was observed in some small areas (Figures 5 and 6). The CrN coating showed the poorest results in all tests. Large areas of peeling could be seen on the bracket surfaces, and there was significant material transfer between the bracket base and archwire as a result of frictional wear (Figures 5G, 5H, and 6D).
DISCUSSION
In the present study, we evaluated the effectiveness of TiN, Al2O3, and CrN coatings in decreasing friction between orthodontic brackets and archwires. The results revealed that the TiN and Al2O3 coatings could successfully decrease the CoFs.
Initially, Al2O3, TiN, silicon carbide (SiC), and CrN, which are biocompatible and used for coating medical instruments and implants, were selected as coating materials for this study.33-36 Single-crystal silicon with an orientation of (100), glass, and AISI 316L SS surfaces were coated with these four target materials using the RF/DC magnetron sputtering system under different experimental conditions. The goal was to determine the formability of the coating materials, optimize the coating conditions, and obtain the necessary reference information. After the pilot study, the coated surfaces were characterized by performing SEM and X-ray diffraction. Furthermore, a 20-h brushing test was performed to evaluate the resistance of the coating against mechanical factors. In the SEM examinations, it was observed that TiN, Al2O3, and CrN formed a homogenous coating on SS, without any major adhesion problems. However, SiC did not adhere well to the SS plate. Because this adhesion problem was commonly observed on the surfaces of the samples, the SiC coating was excluded from this study.
Various coatings are used on orthodontic attachments, mostly for esthetic reasons. Teflon coatings are used to change the natural dark color of NiTi archwires to a lighter color. Such wires are primarily used with esthetic brackets. A teflon coating is known to have a relatively low CoF, and some studies have shown that it decreases the frictional resistance between brackets and archwires;19 however, any mechanical disturbance can easily cause the coating to peel off from the surface. The peeled coating materials then cause mechanical locking between the archwire and bracket, which in turn increases the CoF.24
Coatings with better mechanical properties and low CoFs have recently been introduced, and some are already in clinical use. TiN is one such material. Its greatest advantage for clinical use is its accreditation by the US Food and Drug Administration. TiN coatings have been used to achieve improved performance of surgical instruments and implants, and they provide an inert barrier that protects the surface from corrosion and increases wear resistance.34,35,37 Additionally, it has been shown that TiN coatings can reduce the adverse effects of nickel allergies and decrease the friction caused by NiTi wires.38 Kao et al.39 asserted that TiN coatings were useful for decreasing surface roughness, but unlike the present study, their study found that a TiN coating had no effect on the CoF. Although they used a similar but slightly thicker titanium mid-layer under the coating on SS brackets as well as uncoated SS wires, the other coating conditions and friction measurement techniques were different from those used in the present study. Our results showed that a TiN coating on orthodontic brackets could decrease the CoF by almost 50% if SS archwires were used. Our results, and those of previous studies, show that the coating technique and conditions play an important role in determining the frictional characteristics of the end product, and that TiN coatings can be beneficial in orthodontic applications, particularly for decreasing surface roughness and friction.
Al2O3 has been used as a coating material in the medical field because of its high corrosion resistance.40 Liu et al.41 tested the corrosion resistance and frictional properties of biomedical NiTi plates coated using a micro-arc oxidation technique involving an electrolyte-containing sodium aluminate solution. They claimed that Al2O3 was synthesized on the surface of NiTi. Although they reported a significant increase in surface roughness, they also reported improved frictional behavior and a decrease in corrosion resistance. The results of our study align with those of Liu et al.41 in terms of frictional properties, although the Al2O3 layer in our study was very smooth, and its surface roughness and frictional properties were much better than those reported by Liu et al.41
CrN coatings were previously used on SS instruments. They enhanced the surface properties of tool steels, considering CrN provides good wear resistance and enhanced resistance to corrosion.42 Although the CrN coating showed the lowest surface roughness values, it increased the CoF in the present study. SEM images obtained after the friction tests showed that the coating peeled off from the surface; thus, the residual coating material at the contact point between the surfaces could be the reason for the increased CoF. Although the same problem was observed for the TiN coating, the peeling was not as severe as that observed for the CrN coating, and it did not seem to affect the CoF as much as the peeling of the CrN coating. This problem with TiN and CrN coatings can be attributed to thermal expansion differences between the base material and coating materials.
An important fact to consider is that there is no standardization in terms of test setups among previously published studies evaluating friction between the bracket and archwire in orthodontics. The diversity of results obtained is based on the differences among experimental setups and systems created, the force application points, and the angles formed between the bracket and archwire.43 Of course, scientifically important information is obtained from every study. However, because of the abovementioned factors, it becomes impossible to make comparisons between the results of different studies. Testing the same bracket and archwire in different setups could result in the acquisition of data with several differences. Moreover, the existence of several brackets and archwires of different types and brands in the market, and the feasibility to perform friction tests with thousands of different bracket/archwire combinations, further complicate such comparisons, which become even more difficult when friction tests are performed with archwires and/or brackets coated with different materials and techniques. Thus, comparisons among the numerical values obtained in the present study and those obtained in other studies involving friction tests for coated brackets and/or archwires were avoided.
In the present study, two different archwire materials were used to evaluate the effects of the coatings during different phases of orthodontic treatment. Round, heat-activated NiTi archwires were selected because they are used during the leveling phase of orthodontic treatment, while rectangular SS wires were selected because they are used for space closure.
The purpose of performing the thermal cycling test in the present study was to simulate accelerated aging by thermally induced stresses similar to those observed when the coated brackets and archwires would be exposed to moisture and thermal changes in the oral environment. This thermal cycling test is mainly used for testing the bond strength of adhesives, and there is no standardization in terms of the number of thermal cycles among different studies. Although the number of cycles vary between 100 and 20,000, 250 cycles were used in the present study because the heat exchange level in each thermal cycle are very low (−5°C to +55°C), and it was thought that this would not have a significant effect on the coatings. Our aim was to check whether thermal stresses delaminated the coatings from the surfaces, and this effect could be observed even with just a few thermal cycles.
The effects of abrasive dentifrices on coated brackets and archwires were evaluated by conducting a brushing test in the present study, and adverse effects were observed. A 12-h brushing test was performed using a specially designed device; this duration corresponds to approximately 1 year of toothbrushing based on the amount of time that the brush will act on a group of teeth during an average daily brushing session.44 Changes in the surface morphology were investigated, especially with close magnification of the outer surfaces of the bracket wings that came in contact with the toothbrush. Changes in areas where there was no toothbrush contact were attributed to the precipitation of ions from the toothpaste and tap water during the thermal cycling and brushing tests.
The results of clinical studies on the frictional properties of mechanical systems used for orthodontic treatment can be easily affected by various factors in the oral environment and personal differences between patients. On the other hand, in vitro studies use a standard testing environment and larger sample size. Thus, in vitro friction tests were performed in this study to minimize environmental effects and determine the effect of the coating on friction between the bracket and archwire. These tests were performed in a dry environment. Previous studies regarding the role of natural or synthetic saliva during orthodontic friction tests have reported mixed results; some reported that artificial saliva increased friction,45,46 while others claimed that wet conditions have no effect on the frictional properties of various materials.47,48
The results of the present study and previous studies indicate that the coating technique and conditions are the most important factors determining the physical properties of material surfaces. More studies are needed to determine the optimum coating material and technique. Future studies should also focus on the effect of the same coating techniques on the CoFs of titanium orthodontic brackets, which are known to produce relatively high friction and surface roughness.
An important limitation of this study is that the changes in the coated surfaces of archwires and brackets after the thermal cycling, brushing, and friction tests were primarily examined under a scanning electron microscope, which provides only qualitative results. However, we believe that sufficient data about the condition of the coatings can be obtained with the use of not only constant parameters but also different experimental variables at each step (after and before thermal cycling, brushing, and friction tests for each bracket–archwire pair [those with and without coating and those supplied by different companies]).
As stated before, the aim of this in vitro study was to investigate combinations of surface coatings considered suitable for reducing friction between orthodontic brackets and archwires. However, it should be kept in mind that a low CoF value for a bracket–archwire pair is not always desirable for all patients and all clinical applications. At the same time, clinicians should consider individual patient requirements, such as the type of treatment mechanics, need for anchorage, type of tooth movement, mechanics used for space closure, and amount of space, when selecting the ideal bracket–archwire combination.
CONCLUSION
Al2O3, TiN, and CrN coatings were successfully applied by the RF/DC magnetron sputtering system.
Microstructural characterization revealed that the Al2O3 and TiN coatings were resistant to intraoral conditions and could not be peeled off easily.
The CoF for metal brackets combined with round NiTi or rectangular SS archwires could be decreased by coating with Al2O3 and TiN, but not by coating with CrN.
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