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Table of Contents   
ORIGINAL RESEARCH  
Year : 2013  |  Volume : 24  |  Issue : 4  |  Page : 411-417
Effect of ion-implantation on surface characteristics of nickel titanium and titanium molybdenum alloy arch wires


1 Department of Dental Research and Implantology, Institute of Nuclear Medicine and Allied Sciences (INMAS), Defence Research and Development Organization (DRDO), Ministry of Defence, Govt of India, Timarpur, Delhi, India
2 School of Medicine and Paramedical Health Sciences, Indraprastha University, Delhi, India
3 Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology, Council of Scientific and Industrial Research, Industrial Estate, Thiruvananthapuram, Kerala, India
4 Department of Biostatistics, Mar Thoma College, Mahatma Gandhi University, Thiruvalla, Kerala, India

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Date of Submission11-Oct-2012
Date of Decision07-Feb-2013
Date of Acceptance05-Mar-2013
Date of Web Publication19-Sep-2013
 

   Abstract 

Aim: To evaluate the changes in surface roughness and frictional features of 'ion-implanted nickel titanium (NiTi) and titanium molybdenum alloy (TMA) arch wires' from its conventional types in an in-vitro laboratory set up.
Materials and Methods: 'Ion-implanted NiTi and low friction TMA arch wires' were assessed for surface roughness with scanning electron microscopy (SEM) and 3 dimensional (3D) optical profilometry. Frictional forces were studied in a universal testing machine. Surface roughness of arch wires were determined as Root Mean Square (RMS) values in nanometers and Frictional Forces (FF) in grams.
Statistical Analysis Used: Mean values of RMS and FF were compared by Student's 't' test and one way analysis of variance (ANOVA).
Results: SEM images showed a smooth topography for ion-implanted versions. 3D optical profilometry demonstrated reduction of RMS values by 58.43% for ion-implanted NiTi (795.95 to 330.87 nm) and 48.90% for TMA groups (463.28 to 236.35 nm) from controls. Nonetheless, the corresponding decrease in FF was only 29.18% for NiTi and 22.04% for TMA, suggesting partial correction of surface roughness and disproportionate reduction in frictional forces with ion-implantation. Though the reductions were highly significant at P < 0.001, relations between surface roughness and frictional forces remained non conclusive even after ion-implantation.
Conclusion: The study proved that ion-implantation can significantly reduce the surface roughness of NiTi and TMA wires but could not make a similar reduction in frictional forces. This can be attributed to the inherent differences in stiffness and surface reactivity of NiTi and TMA wires when used in combination with stainless steel brackets, which needs further investigations.

Keywords: Frictional forces, optical profilometry, root mean square roughness

How to cite this article:
Krishnan M, Saraswathy S, Sukumaran K, Abraham KM. Effect of ion-implantation on surface characteristics of nickel titanium and titanium molybdenum alloy arch wires. Indian J Dent Res 2013;24:411-7

How to cite this URL:
Krishnan M, Saraswathy S, Sukumaran K, Abraham KM. Effect of ion-implantation on surface characteristics of nickel titanium and titanium molybdenum alloy arch wires. Indian J Dent Res [serial online] 2013 [cited 2019 Jun 24];24:411-7. Available from: http://www.ijdr.in/text.asp?2013/24/4/411/118375

   Introduction Top


Surface topographic features of orthodontic arch wires crucially influence its clinical and biologic behavior. Presently four types of arch wire alloys are used in orthodontics: Stainless steel (SS), cobalt chromium (Co-Cr), nickel titanium (NiTi), and titanium molybdenum alloy/beta titanium (TMA). Surface roughness of these arch wires using laser spectroscopy and specular reflectance were first reported by Kusy et al., [1] in 1988 and was followed by several other experiments; [2],[3],[4],[5],[6],[7],[8] findings of which can be summarized as; surface roughness increases in the following order: SS < Co-Cr < TMA < NiTi. The parameters, which are related to surface roughness of orthodontic arch wires, include; esthetics, frictional forces at arch wire bracket interface, adhesion of bio-film and propensity for intra-oral corrosion. Since frictional forces are generated between arch wire and bracket slot at all stages of treatment starting from initial leveling and alignment through space closure to finishing, and exert significant influences in the biologic mechanisms of tooth movement, substantial focus has been put to this aspect.

The high frictional force associated with titanium molybdenum alloy and NiTi arch wires have prompted many researchers to investigate whether this is related to its high surface roughness, but no definitive association between the two could be derived hitherto and the current understanding is that frictional forces of orthodontic arch wires increase in the order: SS < Co-Cr < NiTi < TMA. [2],[9],[10] A direct relationship between surface roughness and frictional forces holds good only for stainless steel and Co-Cr wires but not for NiTi and TMA. It, therefore, appears paradoxical that NiTi with an inherent high surface roughness produces less friction than TMA with a smoother exterior.

NiTi and TMA wires have important roles in contemporary pre-adjusted orthodontic mechanotherapy. NiTi alloy possess unique properties like shape memory, super-elasticity, large spring back and low stiffness which make them suitable for initial stages of treatment. [11] On the other hand, TMA wires with a modulus of elasticity approximately twice that of NiTi and less than half of SS, display adequate spring back, and ability to impart light continuous forces over long duration. With its added formability and weldability features, it is useful in the intermediate stages of treatment. [12] Many studies have pointed to the high frictional forces associated with NiTi and TMA arch wires compared to SS and Co-Cr. [9],[13]

It is in this context that ion-implanted/surface modified TMA and NiTi wires are introduced by quite a few manufacturers. TMA wires with ion-implantation have shown to reduce friction to some extent with varying claims, [14],[15] whereas evidences lack for surface-modified NiTi wires. Ion-implantation involves physical vapor deposition of desirable elements on a substrate from a plasma state at energies ranging from 100-1000 electron volts and temperatures up to 700°C, mediated under vacuum conditions. [16] The deposited material form compounds with the base material and creates a hard surface with high compressive forces. It thus rectifies the surface defects of substrate; improve its fatigue resistance, ductility and coefficient of friction. With advancements in surface metrology, non-invasive/destructive optical methods like ellipsometry, interferometry, atomic force microscopy, and optical profilometry are available for studying surface roughness of orthodontic arch wires. [4] Optical profiling offers versatility due to the ease in handling samples and in its ability to scan a wide area in short time. In this method, white light is passed through a beam filter, which directs the light to the sample surface and a reference mirror. When the light reflected from two surfaces recombine, a pattern of interference 'fringes' emerge from the sample surface that helps in deducing surface roughness to a resolution of 0.1 nm. [17]

With the launch of surface modified NiTi and TMA wires, it is therefore prudent to find out the relationship between surface roughness and frictional forces for such products and to evaluate its clinical utility. Therefore, aim of this study was to determine the surface roughness of ion-implanted NiTi and TMA wires with its conventional counterparts using scanning electron microscopy and optical profilometry and the corresponding frictional forces in an in-vitro set up.


   Materials and Methods Top


Following arch wires were selected for the study: Bioforce Sentalloy Ionguard Nickel Titanium; Ion-implanted NiTi of 0.018 × 0.025 inches (0.457 × 0.635 mm) GAC International, Inc; Central Islip, NY, and Low friction, Ion-implanted beta titanium, TMA arch wire of 0.017 × 0.025 inches (0.431 × 0.635 mm), Ormco Corp, South Lone Hill Avenue, Glendora, CA. Respective dimensions of conventional NiTi and TMA (C-NiTi and C-TMA) arch wires without ion-implantation from the same manufacturers were used as controls [Table 1].
Table 1: Study design (n = 10)

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Surface features of arch wires were initially assessed with surface electron micrographs at 500X magnification; Hitachi scanning electron microscope, S-2400, Hitachi Instruments, Inc, San Jose, CA. Wyko NT 1100 series optical profilometer; Veeco instruments, Inc, Woodbury NY, USA, was used for assessing the surface roughness of arch wires. The x, y, range of the contour map was determined by the size of the microscope objective and the z range or height was determined by the mechanical movement of the sample stage. Ten different regions in a 0.5 mm × 0.05 mm area of arch wire surface was scanned to study its topography and averages were taken. The 'Wyko vision software' of the equipment was used for data processing to represent surface roughness as Root Mean Square Roughness (RMS) value in nanometers.

Frictional testing was done as per an in-vitro test protocol described by Tidy. [18] Frictional forces were measured in grams (g) using a universal testing machine (Autograph AG 2000G, Schimadzu, Kyoto, Japan). Ten arch wire samples from each group were pulled for a distance of 16mm at a crosshead speed of load cell at 5 mm/minute. A conventional stainless steel pre-adjusted bracket; 0.022 inch slot (0.558 mm) and an elastomeric module (Gemini series, 3M Unitek Monrovia, CA) was used for friction testing to ensure uniformity among all groups. Study was done under dry conditions and with proprietary soft ware of the machine, kinetic frictional force was determined.

Student's 't' test was used for statistical comparisons of mean surface roughness values and frictional forces. Analysis of variance (one-way ANOVA) was done to compare all tested groups together. A probability value, P < 0.05 was considered as significant.


   Results Top


[Figure 1], [Figure 2], [Figure 3] and [Figure 4] shows the SEM images of conventional and ion-implanted NiTi and TMA wires. Ion-implanted groups demonstrated a smooth surface texture compared to controls. Surface topographic and 3D profilometric images [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11] and [Figure 12] showed raised and irregular peaks in conventional where as ion-implanted types exhibited more or less even peaks.
Figure 1: Scanning electron micrograph of conventional NiTi wire at 500X

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Figure 2: Scanning electron micrograph of ion-implanted NiTi wire at 500X

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Figure 3: Scanning electron micrograph of conventional TMA wire at 500X

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Figure 4: Scanning electron micrograph of low friction TMA wire at 500X

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Figure 5: Optical profilograph of conventional NiTi wire

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Figure 6: 3D optical profilograph of conventional NiTi wire

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Figure 7: Optical profilograph of ion-implanted NiTi wire

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Figure 8: 3D optical profilograph of ion-implanted NiTi wire

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Figure 9: Optical profilograph of conventional TMA wire

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Figure 10: 3D optical profilograph of conventional TMA wire

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Figure 11: Optical profilograph of low friction TMA wire

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Figure 12: 3D optical profilograph of low friction TMA wire

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RMS values in nanometers and frictional force in gram of conventional and ion-implanted NiTi and TMA arch wires are depicted in [Table 2]. Surface roughness of conventional NiTi wire was 795.95 nm and that of TMA was 463.28 nm. Ion-implanted NiTi and TMA registered roughness values of 330.87 nm and 236.35 nm respectively. Statistical analysis between conventional and ion-implanted groups recorded highly significant (P < 0.001) differences. The surface roughness of ion-implanted NiTi and TMA was reduced by 58.43% and 48.90%, respectively, from control groups [Figure 13]. Similarly, mean frictional forces (gm) also showed significant (P < 0.001) differences among study groups. Mean frictional force of conventional NiTi and TMA were 68.36 gm and 99.81 gm, both of which were found reduced to 48.41 gm (29.18%) and 77.81 gm (22.04%), respectively, for ion-implanted groups [Figure 13].
Table 2: Comparison of root mean surface roughness (RMS) values in nm and frictional forces (FF) in g between conventional and ion - implanted arch wires

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Figure 13: Percentage reduction of root mean square roughness and frictional forces in ion-implanted NiTi and TMA wires from controls

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   Discussion Top


The effects of bracket-arch wire material, nature of their surfaces: Smooth/rough, size, shape, angulations, ligation modes, and interactions with saliva on orthodontic frictional forces have been well documented. [9],[13] This is clinically important because frictional forces can influence the effectiveness of orthodontic materials in delivering light continuous forces; that are so essential for eliciting an optimum biologic response. [19] Classically, friction between two surfaces is directly related to the features of opposing surfaces; like surface roughness. [20],[21] Indeed, it is determined by the peaks of surface irregularities termed as 'asperities' and to its deformation occurring under impact of applied load. So there is a continuous binding/notching and release or 'stick-slip' phenomenon between arch wire, bracket slot and ligature interface [19] which makes it a complex entity, difficult to be explained by the laws of friction. [9],[18]

Beta titanium or TMA alloy has a composition of titanium (79%), molybdenum (11%), zirconium (6%), and tin (4%). [21] The high content of Ti (79%) in TMA wires compared to NiTi (45-50%) increases its surface reactivity and has been attributed to its increased adhesion to bracket slot causing higher frictional forces. [19] The same is said to make it more vulnerable to action of fluoride prophylactic agents. [5] Nickel-titanium alloys (Ni: 52%, Ti: 45%, and Co: 3%) exhibit an austenitic-martensitic phase transformation either as a function of temperature or stress that makes the basis for its shape memory and super elasticity properties. [22] In spite of its useful properties, the high surface roughness has been cited as a factor for increased adhesion of biofilm and susceptibility to intra-oral degradation/corrosion. [23]

Ion-implantation process was originally tried to modify the electronic properties of semiconductor devices and is now widely used for several tribological applications and for improving the surface features of titanium and titanium alloy implants. [24] This is normally done with reactive species like carbon and nitrogen, which occupy the pores and dislocations of substrate to form a uniform coating over it. The process of ion-implantation initiates a disorder in the topography of material by breaching its grain boundaries and, thereby, decreases friction. [25] This method, therefore, finds applications in orthodontics with potential for altering surface roughness and frictional forces of arch wires.

In the present study, ion-implanted NiTi showed considerable reduction (P < 0.001) in RMS values (795.95-330.87 nm) from conventional group. Nitrogen ion bombardment alters the NiTi arch wire surface to titanium nitride that can possibly facilitate smooth sliding mechanics. Even though the surface roughness reduction achieved was 58%; frictional forces diminished only by 29%. This disproportionate change in friction might have happened due to the binding of NiTi arch wires with stainless steel bracket slot, given the gross difference in modulus of elasticity between the two. [21] It implied that surface roughness of NiTi wires can be favorably altered by ion-implantation but a concomitant improvement in friction may not be feasible unless its mechanical properties are augmented to prevent binding. For TMA group, we found a fall of approximately 48% in surface roughness for ion-implanted group displaying a highly significant difference similar to the change seen with NiTi group. However, the corresponding variation in friction was only 22%. This almost followed the pattern of NiTi wires. The results proved that ion-implantation of NiTi and TMA wires had contributed only to a partial correction of surface roughness and frictional values, probably due to manufacturing differences.

Kusy et al., [1] reported RMS values of 0.235 μm and 0.175 μm for two brands of NiTi wires where as TMA showed values of 0.154 μm. The high roughness of NiTi surface was in accordance with our findings. However, it is known that laser spectroscopy measurements involve extensive mathematical calculations but optical profilometry measurements are easy with modern computer algorithms. Laser spectroscopy also restricts analysis of data that is presentable only in a Gaussian profile and RMS values need to be less than the wavelength of the testing laser. [4] Conversely, optical profilometry used in this study does not have such limitations and allow easy computation of RMS value. [17]

Surface roughness of TMA wires was successfully altered by Burstone and Farzin-Nia [14] using ion-implantation. With ion-implanted TMA wires against stainless steel flats at different loads, they observed lower static and kinetic coefficient of frictions. In fact, they found values as low as stainless steel both in the dry and wet conditions. However, this sweeping reduction was not seen in the present study and can be ascribed to the differences in testing modes used by us with stainless steel pre-adjusted brackets.

Bourauel et al., [4] in a comprehensive review showed RMS values of 0.21 μm for TMA and 0.10-1.3 μm for NiTi, which was less than the present results. They had used methods like laser specular reflectance, atomic force microscopy (AFM) and profilometry for calibrating surface roughness and differences were expected as manufacturer variations and measuring methods can influence RMS values. AFM and specular reflectance use a certain region of the wire surface while profilometry uses a single line to determine the surface properties which is time consuming and difficult for samples like orthodontic wire. It also can damage the scanned surface and often mislead interpretation. [4] Optical profiling or white light interferometry on the other hand ensures no damage to samples and facilitates accurate understanding. [17]

In a major investigation involving TMA wires with laser specular reflectance, Kusy and co-workers [6] could not find any difference in surface roughness between colored-low friction and normal TMA wires. They found almost similar surface roughness (0.195 μm) for conventional and low friction types. The frictional coefficients also showed negligible relationship with its surface roughness. Cash et al., [15] too noticed no advantages for low friction, colored TMA over conventional but ion-implanted types showed a reduction in frictional forces which were in par with the present findings. There was a discernible reduction (RMS value: 0.463-0.236 μm) in this study for TMA groups that did not transform to an analogous reduction in frictional forces. Doshi and Bhad-Patil, [8] however, reported reduced RMS and frictional values for low friction TMA with ceramic brackets but could not establish a direct correlation between two factors which was true for the present study also.

Considering the different methods used for surface roughness estimation and wide variation in data available with the literature, it would be better not to take these values on a universal scale for comparison. This can be probably due to the differences in the types of samples and methods followed. Percentage changes in RMS and frictional force values may be a better option for judgment, until we have uniform means in this aspect. The results suggested that the currently marketed ion-implanted arch wire products cannot contribute to a total reduction in surface roughness. More so, its relationship with frictional forces tends to be uncertain, even though differences were highly significant. The low stiffness of TMA and NiTi wires with respect to stainless steel slot bracket slot can cause bracket slot binding and possibly influence their frictional characteristics. This may lessen the expected advantages of ion-implantation.


   Conclusion Top


  • Scanning electron microscopic and optical profilometric views of ion-implanted NiTi and TMA wires exhibited smoother surfaces than conventional control groups.
  • Ion-implantation reduced the surface roughness of NiTi and beta titanium wires by 58.43% and 48.90%, respectively, which suggested need for improving ion- implantation procedures for smoother topography.
  • The resultant reduction in frictional forces was only 29.18% for NiTi and 22.04% for TMA that showed an uncertain relationship between surface roughness and frictional forces, even after ion-implantation for the two materials.
  • Binding of NiTi and TMA wires with stainless steel bracket slot due to fundamental differences in stiffness and surface reactivity of titanium may be the probable factors that hamper the advantages of ion-implantation of NiTi and TMA wires in minimizing friction. This warrants more investigations.


 
   References Top

1.Kusy RP, Whitley JQ, Mayhew MJ, Buckthal JE. Surface roughness of orthodontic arch wires via laser spectroscopy. Angle Orthod 1988; 58:33-45.  Back to cited text no. 1
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2.Porosoki RR, Bagby MD, Erickson LC. Static frictional force and surface roughness of NiTi arch wires. Am J Orthod Dentofacial Orthop 1991; 100:341-8.  Back to cited text no. 2
    
3.Kusy RP. A review of contemporary arch wires: Their properties and characteristics. Angle Orthod 1997;67:197-207.  Back to cited text no. 3
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4.Bourauel C, Fries T, Drescher D, Plietsch R. Surface roughness of orthodontic wires via atomic force microscopy, laser specular reflectance, and profilometry. Eur J Orthod 1998;20:79-92.  Back to cited text no. 4
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6.Kusy RP, Whitley JQ, Gurgel JA. Comparisons of surface roughnesses and sliding resistances of 6 titanium-based or TMA-type arch wires. Am J Orthod Dentofacial Orthop 2004;126:589-603.  Back to cited text no. 6
    
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11.Andreasen GF, Heilman H, Krell D. Stiffness changes in thermodynamic nitinol with increasing temperature. Angle Orthod 1985;55:120-6.  Back to cited text no. 11
    
12.Burstone CJ, Goldberg AJ. Beta titanium: A new orthodontic alloy. Am J Orthod 1980;77:121-32.   Back to cited text no. 12
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13.Kapila S, Angolkar PV, Duncanson M, Nanda RS. Evaluation of friction between edgewise stainless steel brackets and orthodontic wires of four alloys. Am J Orthod Dentofacial Orthop 1990;98:117-26.  Back to cited text no. 13
    
14.Burstone C, Farzin-Nia F. Production of low friction and colored TMA by ion implantation. J Clin Orthod 1995;29:453-61.  Back to cited text no. 14
    
15.Cash A, Curtis R, Garrigia-Majo D, McDonald FM. A comparative study of the static and kinetic frictional resistance of titanium molybdenum alloy arch wires in stainless steel brackets. Eur J Orthod 2004;26:105-11.  Back to cited text no. 15
    
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17.Wyko NT1100 Optical Profiling System [Internet] 2012 Jan 15. Available from: http://ppewww.physics.gla.ac.uk/~williamc/NT1100 spec.pdf. [Last cited on 2012 April 5].  Back to cited text no. 17
    
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21.Kapila S, Sachdeva R. Mechanical properties and clinical applications of orthodontic wires. Am J Orthod Dentofacial Orthop 1989;96:100-9.  Back to cited text no. 21
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22.Andreasen GF, Morrow RE. Laboratory and clinical analysis of nitinol wire. Am J Orthod 1978;73:142-51.  Back to cited text no. 22
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23.Eliades T, Eliades G, Athanasiou AE, Bradley TG. Surface characterization of retrieved NiTi orthodontic arch wires. Eur J Orthod 2000;22:317-26.  Back to cited text no. 23
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24.Son SY, Nishida S, Hattori N, Jang HD, Son YJ. The effect of surface treatment of TI-6Al-4V alloy specimens. Key Eng Mater 2005;297-300:2429-34.  Back to cited text no. 24
    
25.Sioshansi P. improving the properties of titanium alloys by ion implantation. J Miner Met Mater Soc 1990;42:30-1.  Back to cited text no. 25
    

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Correspondence Address:
Manu Krishnan
Department of Dental Research and Implantology, Institute of Nuclear Medicine and Allied Sciences (INMAS), Defence Research and Development Organization (DRDO), Ministry of Defence, Govt of India, Timarpur, Delhi
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.118375

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