Synthesis of Co-Cr-Mo Fluorapatite NanoComposite Coatings by Pulsed Laser Deposition for Dental Applications

Aim: The study was to fabricate FA nanopowder/Co-Cr-Mo dental alloy nanocomposite using pulsed laser deposition (PLD), and to evaluate bioactivity properties on simulated body fluid. Methods: In this work, the FA nanopowder was prepared by mixing calcium hydroxide (Ca(OH)2), phosphorouspent oxide (P2O5) and calcium fluoride (CaF2) in a planetary high energy ball mill using zirconium vial. Fluorapatite (FA) nanopowder was processed in the form of pellet for pulsed laser deposition process. The Co-Cr-Mo alloy was coated with FA nanopowder which was approximately 35 65 nm at various laser energy, pressure and time. The X-ray diffraction (XRD) was used to analyze phase, crystallinity and size distribution of Co-Cr-Mo/FA nanocomposite. The surface analysis was by scanning electron microscopy (SEM), Atomic Force microscopy (AFM) and Energy dispersive spectroscopy (EDS). Results: From the results obtained, It was shown that FA nanopowder deposited on Co-CrMo alloy was stable during 14 days of incubation on simulated body fluid. It was also observed that the FA nanopowder coated on the surface of the alloy was still intact after the deposition process, which indicated the bioactivity and biocompatibility of the material. Conclusions: The fabrication of FA nanocomposite based dental alloys (Co-Cr-Mo) using PLD was done successfully. This was confirmed by various characterization techniques, which included XRD, AFM, SEM and EDS.


Introduction
Dental alloys and metals play an important role in dentistry.Although the latest trend is towards the "metal free" dentistry, the metals remain the only clinically proven material for long term dental applications.Dental alloy is a mixture of metallic elements or compounds with other metalloic elements in varying proportions for use in restorative or prosthetic dentistry.The alloys are widely used in dentistry for many years.However, it is of paramount importance to understand the biocompatibility of alloys for their long term success in rendering successful dental treatment [1] [2] [3].
Metallic materials play an essential role in assisting with the repair or replacement of diseased or damage bone tissue.Metals are more suitable for load bearing applications compared with ceramics or polymeric materials because they combine high mechanical strength and fracture toughness [4].However, the main limitation of these metallic materials is the release of the toxic metallic ions that can lead to various adverse tissue reactions and/or hypersensitivity reactions [5].Corrosion resistance is also a very important property for dental alloys, in addition to other properties such as strength, ductility and casting accuracy.Corrosion of dental alloys in the oral environment not only results in the deterioration of restoration, but also involves release of ions that is considered unclear to their biocompatibility [6].
Metals had been used in dentistry for many years [7].The first metallic material used in stomatology was gold in foil form, used as loss and tooth decay fillers.Unfortunately this material could not be applied to customized crowns and bridges because of their low strength [8] [9] [10].Cobalt-chromium-molybdenum (Co-Cr-Mo) alloys have been widely used as removable partial dentures metal frames and porcelain-fused-to-metal crowns [11] [12] [13].The reason for this is that these materials are strong and resistant to corrosion.These alloys have also been used in other application such as in orthopedics, for bone fixation devices, total hip and knee replacements in both the cast and wrought forms.However, despite these properties, the fabrication processes, such as casting, cutting and plastic works are usually difficult due to their high melting point (1623 -1723 K), hardness and limited ductility [14] [15].Therefore, this contribution reports on the fabrication of FA/Co-Cr-Mo nanocomposite using Pulsed Laser Deposition method and a possible application in dentistry.

Materials and Methods
The Co-Cr-Mo dental alloys were purchased from American Elements (USA), and were provided as a small disk (ASTM F75).The Co-Cr-Mo alloy chemical composition is shown in Table 1 as provided by the manufactures.The Co-Cr-Mo dental alloys were purchased in the form of a cylinder of 2 cm diameter and a 2 cm height.

Samples Preparation
The dental alloys (Co-Cr-Mo) shown in Figure 1 were cut using Colchester 5 × 20 Chipmaster Machine (Beckman, Scotland) into 0.1 cm height, sized tablet which was then cut into four parts to form 1 cm in diameter each.The original and reduced sizes of dental alloys are indicated in Figure 1.

Preparation of Fluorapatite Nanopowder
The preparation of fluorapatite nanopowder was performed as described in the literature [6].The fluorapatite nanopowder was prepared by mixing calcium hydroxide (Ca(OH) 2 ), phosphorous-pent oxide (P 2 O 5 ) and calcium fluoride (CaF 2 ) in a laboratory scale ball-milling machine (Beckman, Scotland), using zirconium vial and zirconium balls with ball-to powder weight ratio of 35:1, rotational speed of 300 rpm, and a time of 6 h.The fluorapatite nanopowder particles were obtained at approximately 35 -65 nm.

Preparation of Fluorapatite Nanopowder Pellets
The pellets of diameter 20 mm and a 3 mm height were prepared using the lab pellet presses, (Beckman, Scotland) (Figure 2).This was done under the pressure of 12 tons for a period of 30 minutes.

Preparation of FA Nanopowder on Co-Cr-Mo Alloy by PLD
The dental alloys were cut into four quarts section and were cleaned by acetone and distilled water prior to PLD analysis.The target-to-substrate distance was maintained at 3.8 cm.Various samples were prepared at different deposition time laser energy and pressure.All of the depositions were performed at room temperature following procedure indicated in Table 2.

Preparation of Simulated Body Fluid (SBF)
The simulated body fluid was prepared in the laboratory with the ionic concentration nearly similar to human blood plasma.The fluid was prepared according to a procedure developed by Kokubo, as reported in the literature [9], and as summarized in    of various inorganic ions similar to those of the human blood plasma.The reagents were added, one after another, ensuring complete dissolution.HCl was added to maintain the pH of the solution at 7.4.The temperature of the solution was maintained at 37˚C.
The SBF was then prepared according to Table 4, following the procedure described in the literature [12].Reagents were added, one after another, ensuring complete dissolution.HCl was added to maintain the pH of the solution at 7.4.
The temperature of the solution was maintained at 37˚C.

Pulsed Laser Ablation
Pulsed laser deposition (PLD) is a growth technique in which the photon, laser energy is characterized by pulse duration and laser frequency, interacting with a bulk material [2].As a result, material is removed from the bulk depending on the absorption properties of the target materials.

Scanning Electron Microscope (SEM)
Scanning electron microscopy (SEM) is a type of electron microscope, which is used for various purposes: Topographic studies, Microstructure analysis, Phase morphology, Chemical composition, Elemental mapping and Elemental analysis if equipped with appropriate detector (energy/wavelength dispersive X-rays).

Energy-Dispersive Analysis of X-Rays Spectroscopy (EDAX)
The EDAX system is mostly connected to the electron microscopes such as SEM, FE-SEM and HR-TEM.EDAX spectra of the corresponding elements of the sample are obtained by measuring the energy of X-rays emitted from the sample during e-beam bombardment.X-rays are produced as a result of ionization of an atom when the incident electrons have removed an inner shell electron.The data were measured with a silicon ultra-thin window (SUTW).

Results and Discussion
The exposure time in the specimen preparation was 20 minutes.The EDAX analysis was obtained by irradiating the specimen with a beam of electrons of energy 20 keV.The irradiation live-time was 120 seconds.The data were measured with a silicon ultra-thin window (SUTW).
The exposure time in the specimen preparation was 10 minutes.The EDAX analysis was obtained by irradiating the specimen with a beam of electrons of energy 20 keV.The irradiation live-time was 120 seconds.The data were measured with a silicon ultra-thin window (SUTW).
In the specimen preparation the laser energy was 170 mJ and the pressure was The data were measured with a silicon ultra-thin window (SUTW).The surface area scanned was 10 μm × 10 μm and where necessary, a larger area was selected.

Instrumental Analysis of the Specimens after Immersion into
The laser energy was 130 mJ and the pressure was maintained at 7 × 10 −9 MPa.The laser energy was 170 mJ and the pressure was maintained at 7 × 10 −9 MPa.The exposure time was 15 minutes.The micrograph was obtained by irradiating the specimen with a beam of electrons of energy 5 keV.The irradiation live-time was 120 seconds and the working distance was kept maintained at 4.2 ± 0.2 mm.The surface area scanned was 10 μm × 10 μm and where necessary, a larger area was selected.

Energy Dispersive Analysis of X-Rays (EDAX) Analysis
The energy dispersive analysis of X-rays (EDAX) analysis (shown in Figures 18-22) of the samples were fluorapatite nanopowder was coated onto the Co-Cr-Mo alloy are shown in Figures 3-7.The laser energy was set at 130 mJ and 170 mJ and the pressure was maintained at 7 × 10 −9 MPa.The exposure times varied from five to 20 minutes.The micrographs were obtained by irradiating the specimens with a beam of electrons of energy 5 keV.The irradiation live-time was 120 seconds.The surface area scanned was approximately 10 μm × 10 μm.
In the specimen preparation the laser energy was 130 mJ and the pressure was maintained at 7 × 10 −9 MPa.The exposure time in the specimen preparation was five minutes.The EDAX analysis was obtained by irradiating the specimen with a beam of electrons of energy 20 keV.The irradiation live-time was 120 seconds.The data were measured with a silicon ultra-thin window (SUTW).
In the specimen preparation the laser energy was 130 mJ and the pressure was maintained at 7 × 10 −9 MPa.The exposure time in the specimen preparation was 10 minutes.The EDAX analysis was obtained by irradiating the specimen with a beam of electrons of energy 20 keV.The irradiation live-time was 120 seconds.The data were measured with a silicon ultra-thin window (SUTW).
In the specimen preparation the laser energy was 130 mJ and the pressure was maintained at 7 × 10 −9 MPa.The exposure time in the specimen preparation was 20 minutes.The EDAX analysis was obtained by irradiating the specimen with a  The data were measured with a silicon ultra-thin window (SUTW).
In the specimen preparation the laser energy was 170 mJ and the pressure was maintained at 7 × 10 −9 MPa.The exposure time in the specimen preparation was 10 minutes.The EDAX analysis was obtained by irradiating the specimen with a beam of electrons of energy 20 keV.The irradiation live-time was 120 seconds.
The data were measured with a silicon ultra-thin window (SUTW).
In the specimen preparation the laser energy was 170 mJ and the pressure was maintained at 7 × 10 −9 MPa.The exposure time in the specimen preparation was  The data were measured with a silicon ultra-thin window (SUTW).

Conclusion
In this study, the FA/Co-Cr-Mo nanocomposite was successfully synthesized using pulsed laser deposition technique.The bioactivity assay of the nanocomposite after 14 days test on SBF, showed no corrosion but intact stability and biocompatibility.These were confirmed using various characterizing techniques From these results, it can be deduced that the increase in exposure time as well as the energy of the laser beam, which should be greater or equivalent to 170 mJ must be taken into consideration during laser deposition.

Figure 1 .
Figure 1.Illustration of Co-Cr-Mo alloy before and after the cutting.

Figure 4 .
Figure 4. Electron micrograph of the microstructural topology of fluorapatite nanopowder-Co-Cr-Mo alloy coatings by Pulsed Laser Deposition.

Figure 5 .
Figure 5. Electron micrograph of the microstructural topology of fluorapatite nanopowder-Co-Cr-Mo alloy coatings by Pulsed Laser Deposition.

Figure 6 .
Figure 6.Electron micrograph of the microstructural topology of fluorapatite nanopowder-Co-Cr-Mo alloy coatings by Pulsed Laser Deposition.

Figure 7 . 4 . 1 . 2 .Figure 8 .
Figure 7. Electron micrograph of the microstructural topology of fluorapatite nanopowder-Co-Cr-Mo alloy coatings by Pulsed Laser Deposition.4.1.2.Energy Dispersive Analysis of X-Rays (EDAX) AnalysisFigures 8-12 indicate the EDAX analysis of the deposited fluorapatite on the Co-Cr-Mo alloy at energy 130 mJ.It is noticeable that the density of the deposition increased significantly and therefore correlated to the exposure time of FA deposited on the Co-Cr-Mo alloy.In the instance where the energy was 170 mJ, a significant in the density was also observed.The laser energy was 130 mJ and 170 mJ and the pressure was maintained at 7 × 10 −9 MPa.The exposure times were varying from five to 20 minutes.The micrographs were obtained by irradiating the specimens with a beam of electrons of energy 5 keV.The irradiation live-time was 120 seconds.The surface area

Figure 10 .
Figure 10.The energy dispersive X-rays (EDAX) analysis of fluorapatite nanopowder Co-Cr-Mo alloy coatings by Pulsed Laser Deposition.In the specimen preparation the laser energy was 130 mJ and the pressure was maintained at 7 × 10 −9 MPa.

Figure 11 .
Figure 11.The energy dispersive X-rays (EDAX) analysis of fluorapatite nanopowder Co-Cr-Mo alloy coatings by Pulsed Laser Deposition.In the specimen preparation the laser energy was 170 mJ and the pressure was maintained at 7 × 10 −9 MPa.

Figure 14 .
Figure 14.Electron micrograph of the microstructural topology of fluorapatite nanopowder-Co-Cr-Mo alloy coatings by pulsed laser deposition.

Figure 15 .
Figure 15.Electron micrograph of the microstructural topology of fluorapatite nanopowder-Co-Cr-Mo alloy coatings by pulsed laser deposition.

Figure 16 .
Figure 16.Electron micrograph of the microstructural topology of fluorapatite nanopowder-Co-Cr-Mo alloy coatings by pulsed laser deposition.

Figure 17 .
Figure 17.Electron micrograph of the microstructural topology of fluorapatite nanopowder-Co-Cr-Mo alloy coatings by pulsed laser deposition.

Figure 18 .
Figure 18.The energy dispersive analysis of X-rays (EDAX) analysis of the specimen in which fluorapatite nanopowder was coated onto Co-Cr-Mo alloy by pulsed laser deposition.

Figure 19 .
Figure 19.The energy dispersive analysis of X-rays (EDAX) analysis of the specimen in which fluorapatite nanopowder was coated onto Co-Cr-Mo alloy by pulsed laser deposition.

Figure 20 .
Figure 20.The energy dispersive analysis of X-rays (EDAX) analysis of the specimen in which fluorapatite nanopowder was coated onto Co-Cr-Mo alloy by pulsed laser deposition.

Figure 21 .
Figure 21.The energy dispersive analysis of X-rays (EDAX) analysis of the specimen in which fluorapatite nanopowder was coated onto Co-Cr-Mo alloy by pulsed laser deposition.

Figure 22 .
Figure 22.The energy dispersive analysis of X-rays (EDAX) analysis of the specimen in which fluorapatite nanopowder was coated onto Co-Cr-Mo alloy by pulsed laser deposition.

Table 3 .
[9] concentrations (mmol/dm 3 ) of SBF and Human Blood Plasma.The reagents were prepared as indicated in the literature[9].

Table 4 .
[12]ents used for preparing SBF.The reagents were prepared as indicated in the literature[12].