JOURNAL OF ENDODONTICS
`Copyright © 2003 by The American Association of Endodontists
`
`Printed in U.S.A.
`VOL. 29, NO. 2, FEBRUARY 2003
`
`Fabrication Of Hard Coatings On NiTi Instruments
`
`Teresa Roberta Tripi, DHD, Antonio Bonaccorso, DHD, and Guglielmo Guido Condorelli, DHD
`
`The present study was designed to compare the
`nature of modified surface layers obtained by two
`different procedures on endodontic files made of
`NiTi alloy: the procedures were arc evaporation
`physical vapor deposition and thermal metal or-
`ganic chemical vapor deposition (MOCVD). Exper-
`imental samples were GT Rotary Instruments. The
`first method was based on the physical deposition
`of elemental titanium in the presence of nitrogen.
`The second technique is a typical MOCVD proce-
`dure which adopts Ti(Et2N)4 as a titanium and ni-
`trogen precursor. Control samples were not ex-
`posed to any process. The chemical composition
`of the surface and in-depth layers of each sample
`were examined by X-ray photoelectron spectros-
`copy and X-ray diffraction measurements. The in-
`struments showed surface chemical compositions
`that were different from those seen in the control
`group; samples treated with the first method show
`a surface Nitrogen/Titanium ratio of 1; MOCVD in-
`struments show a surface Nitrogen/Titanium ratio
`of 1.7; control samples show a Nitrogen/Titanium
`ratio of 0.2. Both techniques can produce a high
`nitrogen concentration on the surface. However,
`data showed that the morphologies, the in-depth
`nitrogen distribution, and the chemical nature of
`the coatings obtained with the two procedures
`were different. The paper also reports the effects
`of the two deposition procedures on the nickel/
`titanium ratio of the surface.
`
`Root canal preparation has been considered the most important
`step in root canal therapy. In root canal preparation, the use of
`rotary nickel-titanium instruments is gaining more acceptance.
`Therefore, the surface quality of the cutting surfaces and cutting
`heads after repeated use is of clinical interest (1). A variety of
`surface engineering techniques have brought about improvements
`in hardness and wear resistance by the production of hard surface
`coatings such as titanium nitride (2). Lee et al. (3) implanted doses
`of 4.8 ⫻ 1017 per ion/cm2 of boron to successfully increase the
`surface hardness of nickel-titanium instruments. Some authors
`have increased the surface hardness and the wear resistance of
`
`nickel-titanium instruments through ionic implant and thermal
`nitridation (4). In both cases, most of the nitrogen is essentially
`located on the surface, suggesting the presence of layers of Ti2N in
`the case of thermal nitridation or TiN in the case of ionic implan-
`tation. Preliminary studies have demonstrated that the metal or-
`ganic chemical vapor deposition (MOCVD) can be used to in-
`crease the Nitrogen/Titanium ratio up to 2 on the surface (5).
`Previous studies have shown that the nickel concentration on the
`surface is also strongly affected by surface treatment. In this
`respect, the evaluation of the surface nickel amount is important to
`characterize the chemical composition of NiTi instruments. Sha-
`balovskaya and Anderegg (6) have examined by spectroscopic
`techniques the NiTi alloy surface exposed to sterilization pro-
`cesses. They noticed that nickel concentration on the surface of the
`instrument decreases after autoclave sterilization processes. Simi-
`larly, it was found that nickel surface concentration is strongly
`reduced by ionic implantation and thermal treatment under N2 (4).
`The aim of this study was to investigate the applicability of
`MOCVD in the fabrication of surface layers with different physical
`and chemical properties (i.e., composition and thickness). More-
`over, an alternative method based on the arc physical vapor dep-
`osition (Arc PVD) of titanium and nitrogen was tested on rotary
`endodontic NiTi instruments. The chemical nature of the coatings
`obtained with the two methods was compared.
`
`MATERIALS AND METHODS
`
`Two methods of deposition were adopted. The first technique is
`a MOCVD procedure which deposits Ti(Et2N)4 as a titanium and
`nitrogen precursor (7, 8). Depositions were performed under re-
`duced pressure (1 Torr). Ti(Et2N)4 was evaporated at 25°C. The
`deposition zone was kept at 300°C. The MOCVD deposition
`system has already been reported in a previous paper (5).
`The second method is an arc evaporation physical vapor depo-
`sition (PVD) performed by CRT-Milano using ION-BOND威 tech-
`nology (9). In this method, titanium is simultaneously evaporated
`at microscopically small areas and accelerated all in one single
`work stage. Deposition was performed under nitrogen at 10⫺6
`Torr. Pure titanium was used as the source target. The NiTi
`instruments were kept at 300°C.
`The instruments used for experimental models were 15 GT
`Rotary instruments 0.6 taper n. 20 (Maillefer, Ballaigues, Switzer-
`land). The instruments were divided into three groups (A, B, and
`C) of 5 instruments each.
`
`Group A: Samples were treated via MOCVD.
`Group B: Samples were treated via PVD.
`
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`Vol. 29, No. 2, February 2003
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`NiTi Coatings
`
`133
`
`Group C: Samples were not exposed to any process and were
`utilized as a control group.
`
`All instruments were analyzed with X-ray photoelectron spec-
`troscopy (XPS), X-ray diffraction (XRD), and scanning electron
`microscopy (SEM). XRD measurements were made with a Bruker
`AXS D5005, an X-ray Diffractomer equipped with a copper anode.
`20°⬍2␪⬍70° scans were used. XPS alternating with sputter etch-
`ing was used to determine the chemical composition of the surface
`and near the surface to a depth of 60 nm XPS analysis was carried
`out with a PHI ESCA/SAM 5600 Multy Technique system unit
`(PHI, Ismaning, Germany).
`The X-ray source, operating at 350 W, induced the emission
`of photoelectrons from the sample. Kinetic energies of photo-
`electrons emitted from the atoms were characteristic for each
`element. In-depth distribution (depth profile) was obtained by
`alternating XPS analysis and sputter etching with an argon ion
`gun operated at 4 KeV. Material was removed by sputter etching
`at approximately 2 nm per minute (sputter rate). Sputter rate
`was estimated from theoretic values of nickel and titanium
`sputter yields.
`Before the observation under the SEM, all the instruments were
`cleaned in an ultrasonic dish and were not autoclaved. The en-
`largements for all the samples were as follows: I micrograph at
`53/170 ⫻ for the apical and middle portion of the instrument; II
`and III micrographs respectively at 2400 ⫻ and 5000 ⫻ for the
`surface of the instrument.
`
`RESULTS
`
`Possible alterations of the crystalline structure of the samples were
`checked by XRD. XRD patterns of all samples show the typical
`crystalline phase of NiTi material, thus showing that the deposition
`processes do not affect the bulk structure of the instruments.
`The surface compositions were analyzed by XPS. Results are
`shown in Fig. 1. The three groups show a different Nitrogen/
`Titanium ratio. Control group C has a surface Nitrogen/Tita-
`nium ratio of 0.2; samples of group A have a surface Nitrogen/
`Titanium ratio of approximately 1.7;
`the Nitrogen/Titanium
`ratio of those of group B is approximately 1. In order to evaluate
`the nitrogen concentration in the in-depth layers,
`the XPS
`analysis was performed after 5 min of sputtering processes.
`After sputter the Nitrogen/Titanium ratio in control group sam-
`ples was near 0. The Nitrogen/Titanium ratio of sample of group
`A decreased markedly to 0.25, whereas group B decreased
`slightly to 0.9. XPS analysis and sputter etching were also used
`to evaluate the amount of nickel on the surface and in the
`in-depth layers.
`In Fig. 2 the Nickel/Titanium ratios are reported on the surface
`and in the in-depth layers. In the control group C the highest value
`of the Nickel/Titanium surface ratio (0.25) was observed. The
`lowest surface value (0.1) was observed in group B samples. After
`sputter the ratio decreased in all samples. Also, the lowest ratio was
`observed for group B samples in this case.
`SEM microphotographs showed that the morphologies of in-
`strument surfaces in groups A and B were slightly changed with
`respect to group C (Fig. 3). The surfaces of all instruments in
`groups A appear covered by a deposited material (Fig. 4) with a
`few small grains embedded in a smooth layer. The surface of
`instruments in group A appeared homogeneous and flat. The sur-
`face of instruments in group B appeared more rough than those in
`
`FIG 1. Plot of the atomic nitrogen/titanium on the surface of instru-
`ments of groups A, B, and C. Group A: N/Ti ratio ⫽ 1.7, group B: N/Ti
`ratio ⫽ 1, group C: N/Ti ratio ⫽ 0.2.
`
`FIG 2. Plot of the atomic nickel/titanium on the surface of instruments
`of groups A, B, and C. Group A: Ni/Ti ratio ⫽ 0.17, group B: Ni/Ti
`ratio ⫽ 0.1, group C: Ni/Ti ratio ⫽ 0.25.
`
`FIG 3. Surface of GT Rotary size 20 0.6 taper of Group C. Typical
`longitudinal features of NiTi instruments. Original magnification
`2400 ⫻.
`
`the others two groups. In this case the longitudinal features, which
`are evident in new instruments, were not present (Fig. 5). Grains of
`approximately 100 nm constituted the deposited layers (Fig. 6). A
`few large grains of approximately 1 ␮m are visible in the
`microphotograph.
`
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`134
`
`Tripi et al.
`
`Journal of Endodontics
`
`FIG 4. Surface of GT Rotary size 20 0.6 taper of Group A. The surface
`appears covered by layers of deposited materials with few crystal-
`line grains. The surface of instrument appears more homogeneous
`and smooth. Original magnification 2400 ⫻.
`
`FIG 5. Surface of GT Rotary size 20 0.6 taper of Group B. The surface
`appears covered by layers of deposited materials with the presence
`of small grains. Original magnification 2400 ⫻.
`
`DISCUSSION
`
`XPS analysis shows a different in-depth concentration of nitro-
`gen among samples from groups A, B and C. This observation
`indicates that the nitrogen-rich layers have a different thickness. In
`the case of group B the nitrogen/titanium ratio is approximately 1
`after sputter, thus showing that the layers under the surface are also
`nitrogen-rich. This ratio suggests that the deposited material is
`TiN. This layer of titanium nitride created on the surface of the
`instrument can increase the cutting efficiency and wear resistance
`in that the instrument becomes harder on the surface and thus more
`effective in its shaping ability (10).
`the deposition processes
`The SEM observations show that
`change the surface morphology. The nitrogen-containing film de-
`posited on the instruments of group A is not constituted by well-
`defined crystalline grains, but mainly by continuous layers. This
`morphology suggests the presence of amorphous materials with a
`poor crystalline structure. Samples from group B show a deposited
`
`FIG 6. Presence of grains of 100 nm size on the surface of a sample
`of Group B. Original magnification 5000 ⫻.
`
`film composed of well-defined grains. This different morphology
`can be an indication of the presence of TiN film with a crystalline
`structure.
`Finally, XPS analysis seems to confirm that the nitrogen dep-
`osition process moves the nickel element from the surface towards
`the bulk. Note that the lower amount of Ni on the surface was
`found in the samples from group B. This effect of the PVD process
`can be of interest for NiTi biomedical application.
`MOCVD and PVD processes have been proved able to fabricate
`hard coating on NiTi instruments. However, further studies are
`underway to optimize the process conditions to obtain hard coat-
`ings of various thicknesses and compositions. Moreover, studies
`are needed to evaluate the possible increase in resistance to wear
`and the cutting ability of instruments covered by these hard
`coatings.
`
`Drs. Tripi and Bonaccorso are Staff Specialists, Cattedra di Odontoiatria
`Conservatrice, Corso di Laurea in Odontoiatria e Protesi Dentaria, Catania
`University. Dr. Condorelli is a Staff Specialist, Dipartimento di Scienze Chim-
`iche, Catania University. Address requests for reprints to Dr. Antonio Bonac-
`corso, via Vagliasindi n.51, 95126 Catania, Italy.
`
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