INTRODUCTION

During fixed orthodontic treatment, friction is generated by the relative motion between the archwire and bracket. This friction must be overcome before force can be transferred from the oral device to the teeth.^1–3 Sliding mechanisms are now widely used in fixed appliances,⁴ which means that friction from the treatment process is becoming a crucial concern that affects the performance and efficiency of appliances.^5,6 Indeed, subtle changes in brackets, archwires, and ligation methods designed to reduce friction may improve anchorage control and shorten the orthodontic treatment period. Many orthodontists use austenitic stainless steel wires to close extraction spaces; however, a major shortcoming of this is the weak mechanical properties of the material.⁷ This has led to a global research focus on using mechanical processes to reduce the friction coefficient and improve overall efficiency of treatment.

The rapid development of biomedical engineering has resulted in researchers exploring a variety of surface modification methods to improve the performance of biomedical metallic materials.⁸ Very recently, owing to its extreme hardness and substantial wear resistance, diamondlike, carbon (DLC) coating has emerged as a new type of surface coating.^9,10 In particular, studies have shown DLC coating of orthodontic appliances may indeed improve their frictional properties.^11–14 Traditional DLC films, however, tend to shed from the substrate due to the large residual stress, which has greatly limited any industrialization or further application of DLC coating in orthodontics.

Nanomaterials with unique properties such as high strength and good plastic deformation have begun to move the field of orthodontic materials forward. Nanostructured DLC films, defined as a series of amorphous carbon materials with nanoparticles or a membrane layer, not only reduce the internal stress of DLC film, but also increase its adhesiveness.^15,16 Research into surface modification techniques using nanostructured DLC films and coatings, however, is still in its initial stages.

Another technique to harden the surface and reduce the friction coefficient of austenitic stainless steel is nitrocarburizing,^17,18 which uses two major methods: gas-plasma and salt-bath. The salt-bath method excels owing to its simplicity, low cost, and stable processing temperature. It has been reported that the gas nitriding technique has improved the frictional properties and wear resistance of dental titanium.¹⁹ However, use of salt-bath nitrocarburizing surface modification for orthodontic materials has yet to be investigated.

Therefore, the present study was undertaken to evaluate and compare the effects of a nanostructured DLC coating and salt-bath nitrocarburizing technique on the frictional properties and biocompatibility of orthodontic stainless steel archwires, to provide an experimental basis for the application of surface modification technology in orthodontics.

MATERIALS AND METHODS

As-received austenitic, stainless steel archwires with cross-sectional dimensions of 0.019 × 0.025 inch (3M Unitek, Monrovia, Calif) were used as controls. DLC coatings were deposited on these stainless steel wires using plasma-enhanced chemical vapor deposition (PECVD). All wires were fixed on a worktable, and deposition was carried out at an offset voltage of 60 V, under a pressure of 0.5 Pa, and a radio frequency power of 100 W. The substrate temperature was 200°C and the total time of deposition was 8 minutes. For the salt-bath nitrocarburizing procedure, the wires were finished in the salt-bath nitrocarburizing arc furnace at 565°C for 10 minutes. The specific medium for austenitic stainless steel was 16% wt K₂CO₃, 24% wt KCl, and 60% wt urea.

RESULTS

Raman Analysis and SEM Observation

For the DLC-coated wires, the Raman spectra and Lorenz curves show peaks D (sp3 bonding state of carbon for diamondlike) and G (sp2 bonding state of carbon for graphitelike), broadening significantly (Figure 1). The D peak appears at 1347 cm⁻¹ and the G peak at 1557 cm⁻¹, which is typical for DLC structures.²² SEM photomicrographs from a cross-section of the DLC-coated wires clearly show the coating to have a thickness of approximately 1 μm (Figure 2E). Diameters of the ultrafine crystal grains in the films were all below 100 nm (Figure 2D), and therefore, defined as nanocrystals. The surface of the nitrocarburized wires was greyish-black and relatively smooth compared with the control wire.

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Microhardness

After surface modification, microhardness of the DLC-coated wires (685.17 Hv) was increased 1.46 times compared with the control wires (468.42 Hv), while the microhardness of the nitrocarburized wires (1119.58 Hv) was increased nearly 2.39 times compared with the control wires (Table 1; P  =  .0001). The nanoindentation test revealed that the elastic modulus of the DLC-coated wires (74.17 GPa) obtained from the 6 mN load was significantly lower than that of the control wires (201.24 GPa; P  =  .0001). The elastic modulus of the nitrocarburized wires (208.03 GPa) was similar to that of the control wires (P  =  .845).

Table 1.  Microhardness and Elastic Modulus of Control, DLC-coated, and Nitrocarburized Wires

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Static and Kinetic Friction

Table 2 shows that the kinetic friction coefficient of the DLC-coated wires was reduced 40.71% compared with the control wires (P  =  .001), and the static friction coefficient of the nitrocarburized wires was reduced 15.24% compared with the control wires (P  =  .067). The maximum static friction of the nitrocarburized wires was significantly lower than those of the other two types of wires, and it decreased 22.2% compared with the control wires (P  =  .001).

Table 2.  Kinetic and Static Friction Coefficients and Maximum Static Friction of Control, DLC-coated, and Nitrocarburized Wires

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Electrochemical Corrosion Experiments

The mean value of the corrosion potential and corrosion current of the control wires were 1.325 V and 3.41e-5, respectively. For the DLC-coated wires, the values were −0.487 V and 5.89e-7, respectively, with the polarization current decreasing by two orders of magnitude compared with the control wires. The mean corrosion current of the nitrocarburized wires obtained from the Tafel curve was 1.65e-4, an increase of one order of magnitude relative to the control wires (Figure 3). Therefore, our results demonstrate that DLC coating improves the corrosion resistance of the archwires, while such is true to a lesser extent for nitrocarburizing.

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Cytotoxicity

An inverted microscope for culture observations at 24, 48, and 72 hours was used, revealing an increase in cell numbers from the DLC-coated, nitrocarburized, and control groups. The means of the OD values are listed in Table 3. Results of the MTT colorimetric assay showed that cell proliferation rates for the DLC-coated group were between 91.92% and 115.32%, while for the nitrocarburized group they were between 84.76% and 110.11%. The toxicity level of both modification groups was 0, indicating no significant cytotoxicity.

Table 3.  Optical Density (OD) Values From the MTT^a Assay Test and Toxicity Levels of Control, DLC-coated, and Nitrocarburized Wires

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DISCUSSION

Orthodontic archwires must meet certain requirements in order to be considered effective oral devices, for example, high strength, low stiffness, high plasticity, excellent frictional properties, corrosion resistance, abrasion resistance, and biocompatibility.^23,24 Many techniques have been investigated to modify the surface of metallic materials to improve their overall performance. Surface nanoengineering might enhance both the surface properties and performance of materials by changing surface organization and structure.²⁵ In this study, we successfully deposited even and dense nanostructured DLC films onto orthodontic stainless steel archwires. The ultrafine grains and wider atomic spacing resulting from the nanostructured DLC films, provided excellent characteristics compared with traditional archwires.

In the salt-bath nitrocarburizing technique, CNO effectively removed the passivation film from the archwire surface, permitting the nitrogen and carbon atoms to infiltrate the internal wire to achieve surface hardening without special pretreatment. With no ion-etching effect in the salt-bath procedure, the nitrocarburized wires acquired a smooth surface that benefited their frictional properties.¹⁸

The primary task in achieving effective and efficient orthodontic treatment is to decrease friction between the bracket and the wire. Factors affecting friction include the material used, archwire size, and ligation methods, to name a few.^11,26,27 Surface roughness has a positive correlation with friction, while archwire hardness is negatively correlated with friction. In the present study, our DLC-coated wires displayed a smoother and harder surface compared with the control wire (Table 1), which possibly resulted from the larger proportion of sp3 bonds in the DLC coating. In support of this argument, the D peak from Raman spectra moved to lower wavenumbers (Figure 1). The harder surface of the nanostructured DLC-coated wires reduced not only kinetic friction (Table 2), but also the effects of binding and notching.

This study also clearly demonstrated that the nitrocarburizing process decreased maximum frictional force (Table 2), which is related to the solid-solution strengthening effect in the salt-bath procedure. Therefore, initial movement of the teeth should be accelerated efficiently. The DLC coating on the stainless steel wires revealed a lower elastic modulus compared with the surface layers on the control wires (Table 1). This result is similar to that obtained by Muguruma et al., who deposited a DLC film onto stainless steel archwires using a plasma-based ion implantation/deposition method.¹² DLC-coated wires with a lower elastic modulus show superior flexibility, a desirable characteristic of orthodontic wires.

Various chemical substances in the saliva at mouth temperature lead to electrochemical corrosion reactions on the wire surfaces, resulting in possible adverse reactions such as allergies, tooth discoloration, and toxic effects. The study of Kobayashi et al. showed that DLC coating on nickel-titanium archwires greatly reduced the release of nickel ions.¹³ In this study, Tafel curves obtained from the electrochemical test indicate that DLC coating significantly reduces the corrosion rate of stainless steel wire in artificial saliva (Figure 3). As the DLC-coated wires exhibited an even microstructure composition and smoother surface, the oxide layer of the wires were better protected. By contrast, the corrosion resistance of the nitrocarburized archwire declined to some extent, which may have been due to the destroyed passive film on the wire surface.

Primary acute toxicity screening provides a fast, simple, reproducible, and inexpensive method to detect material biocompatibility.²⁸ Our MTT assay results showed that no significant cytotoxicity was associated with either the DLC-coated or the nitrocarburized wires. After 72 hours of cell culture, cell proliferation rates in different concentrations of cell-impregnating solution were slightly increased relative to the control. The reason for this might be that the DLC coating suppressed release of the nickel ions into the cell culture, which should be further investigated.

Some limitations are raised in the present in vitro study since this investigation was conducted under ideal laboratory conditions, whereas in the oral cavity the presence of saliva, plaque, corrosion, and other variables could influence the frictional properties of wires. Moreover, in order to simulate what occurs clinically during orthodontic tooth movement, further research on the corrosion resistance of wires is needed to evaluate the changes in different situations.

CONCLUSIONS

• Nanostructured DLC coating and nitrocarburizing processes both significantly improved the surface hardness and reduced the friction of stainless steel archwires as well as exhibiting good biocompatibility.

• Nanostructured DLC coating provided excellent corrosion resistance and improved elasticity.

• Though the nitrocarburizing technique was outstanding in improving frictional properties, it reduced the corrosion resistance of the stainless steel wires to a lesser extent.