Titanium perovskite (CaTiO3) formation in Ti6Al4V alloy using the electrical discharge machining process for biomedical applications

    Titanium perovskite (CaTiO 3 ) formation in Ti6Al4V alloy using the electrical discharge machining process for biomedical applications A.R.F. Oliveira, W.F. Sales, A.A. Raslan PII: DOI: Reference:

S0257-8972(16)31022-2 doi: 10.1016/j.surfcoat.2016.10.028 SCT 21673

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

11 July 2016 5 October 2016 10 October 2016

Please cite this article as: A.R.F. Oliveira, W.F. Sales, A.A. Raslan, Titanium perovskite (CaTiO3 ) formation in Ti6Al4V alloy using the electrical discharge machining process for biomedical applications, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.10.028

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ACCEPTED MANUSCRIPT Titanium perovskite (CaTiO3) formation in Ti6Al4V alloy using the electrical discharge machining process for biomedical applications

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Oliveira, A.R.F., Sales, W.F. and Raslan, A.A.

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Tribology Research Group, Uberlândia, MG, Brazil.

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Federal University of Uberlândia (UFU), Faculty of Mechanical Engineering (FEMEC),

Abstract

The use of the energy generated in machining processes by electrical discharge (EDM)

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to enrich metal surfaces with nitrogen is already well known, and following on from this

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research focus, the aim of this study is to evaluate the possibility of enrichment through this process, for the surface of Ti6Al4V alloy with titanium perovskite. The resulting

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surface shows high biocompatibility with human bone and teeth and becomes essential

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as a biomedical material. Tests were performed using a sink EDM process and as a dielectric fluid, an aqueous solution of calcium chloride was used. The samples

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characterizations were made using the scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), optical microscopy and

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Knoop microhardness. The results showed the formation of an uneven surface enriched with calcium, in the form of titanium perovskite. On the lateral section of the sample one notes the formation of a porous layer. Beneath this, a uniform hardened layer was formed, extending to a depth of approximately 200 m below the surface. Calcium insertion into the Ti6Al4V surface was attributed to the occurrence of re-solidification and ion implantation. It can be concluded that it is feasible to enrich, by EDM, the surface of a Ti6Al4V alloy with titanium perovskite. Key words: Titanium perovskite, EDM, Coatings, Ti6Al4V, CaTiO3, Biomedical material. *Wisley Falco Sales – [email protected] Corresponding author

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1. Introduction

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Machining by electrical discharge is essentially a thermal process and the energy is

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provided by the formation and dissipation of a plasma channel. It is formed from the passage of an electric current between two electrodes submerged in a dielectric fluid. One of the electrodes, the tool electrode is made of copper or graphite, usually, however

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it needs to be a good electrical conductor, as a principal property, followed by a high

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melting point free of porosity. The other electrode, the electrode work piece, is the metal material that is proposed for machining. Dielectric fluids are generally based on mineral oils, despite vegetable oils having gained prominence in several applications, in

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addition, they are environmental friendly. In the wire EDM process, deionized water is used.

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According to McGeough (1985), the high temperatures generated in the plasma channel, between 8.000 oC and 10.000 oC dissociate the molecules of the fluid, forming ions and

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a plasma channel in the sequence. The acceleration of electrons and ions, promoted by polarity and voltage involved in the system, force them to levels of high kinetic energy, and when collisions with electrodes are produced, temperatures reach values higher than 15.000 oC. Evaporation of the fluid at the interface of electrodes generates turbulence and pressure estimated around the 200 bar mark. There are instantly sublimation in the electrodes surfaces and consequently the removal of material, characterizing the machining process. The enrichment surfaces using the energy generated in the EDM process has been studied using three techniques: the tool electrode material transfer, powders diluted in the fluid or by combination of both techniques (Kumar et al., 2009).

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ACCEPTED MANUSCRIPT The tool electrode material transfer to the piece was investigated by Bleys et al., (2006), using copper tool electrode in the EDM process for dies and molds in hardened steels.

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They detected the presence of copper in quantities not exceeding 1% in the EDM

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machined surface, penetrated into the recast layer and the heat affected zone. In WEDM

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(wire-EDM) they observed the transfer of copper, zinc, and tungsten through the wires used as tool electrodes. Lee et al., (2004) observed in WEDM machining of titanium alloys with the use of copper and nickel as tool electrodes, forming a porous layer with

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a thickness of the order of 136 m. The Knoop microhardness suffered a wide range

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between 0.5 and 9.0 MPa. Gill and Kumar (2015) investigated the transfer material from an electrode composed of Cr-Cu-Ni to high-carbon steel surface and from the

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chemical surface analysis, they identified a significant increase in the amounts of Ni, Cu

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and C. There was an increase in the surface hardness of the steel, which is attributed to the formation of chromium carbides. Fredriksson and Hogmark (1995) used the

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electrode of copper and liquid nitrogen as a dielectric fluid in EDM machining of steel for hot working work pieces. They identified the presence of copper in the re-solidified

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layer of the piece. A gaseous dielectric under reduced pressure facilitates the transfer of material through the gap between the electrodes. In addition, with the fact that material is removed in small quantities from the surface, is a possible reason why the electrode transfers an amount of material to the re-solidified zone of the piece. Furutani et al. (2003) used titanium powder suspended in kerosene in the EDM machining of carbon steel. They obtained a layer enriched with titanium carbide with a hardness of 1600 HV. Furutani (2003) proposes a method for solid lubricant layer deposition, based on the suspension of the molybdenum disulphide powder in the dielectric for the production of parts used in an ultra-high vacuum. High open circuit voltage (320 V), low current discharge (2A), short pulse (2 μs), and pulse average

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ACCEPTED MANUSCRIPT interval (8 μs), are used to deposit layers of lubricants upon carbon steel and stainless steel. The process has some drawbacks, such as the difficulty of maintaining nano

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particles in suspension. It is easier to achieve this when using a more viscous dielectric,

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but loses the washing efficiency needed to clean the gap by removal of eroded material.

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The nitriding by electrical discharges using EDM equipment was proposed by Yang et al. (2005) using a solution of distilled water and urea as a dielectric fluid. As a workpiece electrode, the alloy Ti6Al4V was used. These researchers obtained a layer

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enriched with titanium nitride (TiN) with a thickness of 60 m. In addition to the

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increase in hardness, a gain in relation to sliding wear was noted. With the use of spectroscopic techniques based on glow discharge (Glow Discharge

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Optical Emission Spectroscopy - GDOES) and XRD. Santos et al. (2013) identified that

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the copper electrode is transferred to the surface of an AISI 4140 steel nitrided by electrical discharges during the sink EDM process. The depth reached was between 20

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to 25 m, which decays from the surface of the workpiece. Deionized water solution blended with urea was used as a dielectric and as a result, the nitrogen was transferred

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to the workpiece surface. Thus, it is possible that the electrode ions interfere with the thermodynamics of nitriding by EDM. The CaTiO3 (calcium titanate/bioceramic composite – CTBC) has been studied as a surface coating with the goal of improving the biocompatibility and osseointegration abilities of the Ti alloys (Wei et al., 2007). One method widely employed for the enrichment of Ti alloy with titanium perovskite is using the thermal spray method. Another method, according to Yerokhin et al. (1999), Hang et al. (2003), Yang et al. (2004), Wei et al. (2007) and Cheng et al. (2012) is the use of micro arc oxidation (MAO), which is an effective technique for depositing various functional coatings with porous structures on the surfaces of Ti, Al, Mg and their alloys. Tamura et al. (2013)

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using this technique.

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The main goal of this study is to investigate the deposition of calcium by EDM, on the

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surface samples of a Ti6Al4V alloy and observe the formation of titanium perovskite and at which depth it can take place.

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2. Methodology

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A semi-industrial sink EDM machine with 30A of electrical current was used for the tests. The operating parameters were selected to allow for the occurrence of electric

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discharges, which are shown on Table 1.

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Table 1. Parameters adjusted in the sink EDM machine. Parameter

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Current (TS = 10)

30

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Pulse time (Ton)

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µs

Ration between the pulse time and the total time in each

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Erosion time (open plasma arc)

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Periodic removal tool (distance)

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mm

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Tension

Positive

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Tool polarity

Unity

cycle (Toff)

The dielectric fluid used was deionized water with the addition of calcium chloride (CaCl2) in a concentration 0.1 mol/l. The electrical conductivity was higher than 2000 S/cm. As a tool, a graphite electrode was used with a 12 mm diameter. As a workpiece electrode, an ASTM B348 6AL-4V Grade 5 Ti6Al4V alloy with a 10 mm diameter was

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ACCEPTED MANUSCRIPT used. The surface morphology was analyzed using SEM. The presence of calcium in the coating was investigated using EDS.

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The changes in the structure and mechanical properties were evaluated in the cross

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section of the sample by optical microscopy and 3 Knoop microhardness profiles with

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10gf load and applied for 15s.

The observation of the cross section of samples for analysis by EDS required a special preparation process. The cutting using an abrasive disc followed by grinding and

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polishing promotes the removal of powder of perovskite-enriched layer. This powder

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dispersed and adheres to the titanium alloy matrix. Therefore, the EDS results are affected by this contamination. To avoid it, an abrasive disc was used to cut followed by electrolytic polishing. Struers Electropol Mark 5 equipment was used. The electrolyte

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used was based solution of perchloric acid with 60% concentration (78 ml), distilled water (120 ml), ethanol (700 ml) and butyl glycol (100 ml). The electrical current

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employed was 2A for a time of 5 minutes. The sample surface was evaluated by DRX with a low angle of 2.5° using the Shimadzu XRD-6000 equipment. This technique

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allows characterizations at depths of 2 m order. Three profiles of calcium concentration were drawn from the uniform enriched surface into the volume of the part.

3. Results and discussions

The modification on the surface of Ti6Al4V alloy after the deposition of calcium is shown in Figure 1. Note the formation of an uneven surface, typical of EDM machined materials, with the presence of cracks, blisters and craters.

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Figure 1 - Ti6Al4V alloy surface after deposition of calcium.

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In the XRD pattern of Figure 2, obtained by EDS, the presence of calcium was recorded. The presence of carbon is due to ion transfer from the electrode. Oxygen is due to the dissociation of the water molecule. Aluminum is one of Ti6Al4V alloy

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elements and Vanadium was not registered, probably due to its very low concentration. Through x-ray diffraction, it was possible to identify the CaO3Ti formation, or titanium perovskite as shown in Figure 3. This is made possible due to the dissociation of calcium, thus forming a CaCl2 molecule with oxygen from water and titanium from the alloy. In subsequent solidification, this was the combination of elements for forming the perovskite. This transformation occurs under conditions outside of thermodynamic equilibrium, therefore, the time elapsed in the formation and dissipation of the plasma channel is of the order 10-6s. Although the solidification occurs at high speeds, due to the sample being immersed in water, there were no restrictions placed upon the crystal formation.

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Figure 2. Graph showing the presence of calcium upon the Ti6Al4V surface (Spectrum

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1, highlighted in the fig. 1).

Figure 3. Diffractogram obtained by x-ray diffraction indicating the formation of titanium perovskite (CaO3Ti).

The microstructure of the cross section shows the formation of two layers with different morphologies, as seen in Fig. 4. A surface layer with an approximate thickness of 150

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ACCEPTED MANUSCRIPT m and porous appearance. Gases evaporated by the high temperatures generated by the plasma channel and captured after the rapid solidification cause porosity. The named

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“uniform layer” in this picture is approximately 65 m thick. The sub-surface layer

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morphology and hardness profiles are shown in figure 4.

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remained uniform. Knoop microhardness profiles were obtained from this layer, and the

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Porous Layer

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Bulk

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Uniform Layer

Figure 4 - Cross section of the Ti6Al4V sample, and uniformly porous layers and marks of Knoop microhardness indentations.

The Knoop microhardness profile allows for tracing, in order to estimate the thickness of the HAZ (heat affected zone) as 225 m, shown in Figure 5. At the depth of 165 m, hardness is shown to be significantly higher than the hardness of the matrix, which indicates the formation of perovskite. Between 185 m and 225 m, along a 40 m range, there is a sudden drop in toughness, although it shows higher hardness than the matrix. However, despite being uniform, this layer was not homogeneous. The rough

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ACCEPTED MANUSCRIPT and porous layer is desirable to enhance the osseointegration process and improves the formation of a bone layer locked and connected to the metal. In addition, Ti which has a

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surface smooth, with low Ra, average roughness less than 1m, is normally undesirable

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for implants due to the fact that the connections between osteoblasts cells and the oxide

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layer is not sufficient to guarantee the success of the implant.

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Figure 5 - Microhardness Knoop profile from porous layer to bulk.

The heterogeneity of the uniform layer can result from the change in concentration of CaO3Ti, as can be observed in the Figures 6 to 8. At higher concentrations near the surface, the hardness is higher. As it moves away from the surface to the bulk material, the concentration of CaO3Ti decreases, causing a drop in hardness. Then, observing these microhardness values, one can estimate that this oxide layer has an effective concentration from the surface to a 200 μm depth, a considerably thick layer for a material dedicated to biomedical applications. The formation of a calcium-enriched layer on the surface of a Ti6Al4V alloy can be explained by the occurrence of an ion implantation process, or re-solidification. In the 10

ACCEPTED MANUSCRIPT case of steel AISI 4140 nitrided by EDM, Santos et al. (2014) have justified the formation of a hardened layer with a thickness of 20 μm to be due to nitrogen

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incorporation by ion implantation and re-solidification. Nitrogen has a relatively small

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ionic radius of 0.13 Å, which enables its implantation into the steel structure. Calcium,

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however, has a relatively large ion radius of 0.99 Å, with its implementation across the full extent of the layer being very unlikely, which reaches values of the order 400 μm, when considering both porous and uniform layers. Therefore, what is most likely to

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have occurred is the dissociation of water, CaCl and metal fusion. With subsequent

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cooling, there was a combination of Ca, O and Ti that led to the formation of the perovskite. One should not completely discard the hypothesis of the ion implantation of calcium, although unlikely to exert considerable influence in the formation of the layer,

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in view of its large thickness thereof.

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ACCEPTED MANUSCRIPT Fig. 6 – Relative intensity of the spectra when evaluated at different depth from the surface. Series 2

Series 3

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Series 1

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Fig. 7 – HAZ and the three lines (series) where the micro-hardness were measured.

Fig. 8 – Perovskite quantity measured beneath of the surface in the three lines (series) highlighted in the Fig. 7.

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ACCEPTED MANUSCRIPT Figures 7 and 8 show the perovskite layer formed with the respective three positions, which were obtained calcium concentration profiles and the curves of concentration

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versus depth from the surface. In general, we note the decay in calcium concentration in

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both the microstructure and in the calcium concentration. In the re-solidified layer, the

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average calcium concentration is between 0.63 and 1% Ca, which coincides with the depth determined by micro-hardness profile. In the intermediate layer, the concentration

with the decrease in micro-hardness.

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drops to values of the order of 0.5% Ca, to a depth of 500 m, approximately coinciding

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The three techniques, showed the existence of a concentration gradient of calcium occurring in this one concentration decay from the surface. The layer thickness is relatively large when compared to the thickness of the nitride layer obtained by Santos

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et al. (2013) when EDM’ing the AISI 4140 steel. However, should consider differences in materials, from steel to titanium. Moreover, the ions implanted in the steel was the

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nitrogen. The calcium ions are relatively larger, 0.99 Angstroms and at first analysis more difficult to implant. However, these ions must transfer more kinetic energy to the

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titanium alloy, increasing the heat input and enabling the formation of thicker layers than those observed with nitrogen in steel. The physical and mechanical properties of Ti6Al4V alloy can be favorably influenced also in obtaining these thicker layers.

4. Conclusions After developing this study, the following conclusions can be drawn: 1.

The energy generated in the machining process by electrical discharges has been used to coat a calcium surface upon a Ti6Al4V alloy workpiece from the use of a dielectric fluid comprising of calcium chloride and deionized water.

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The results showed that there was a formation of titanium perovskite with an uneven surface showing the presence of bubbles voids and cracks, typical of EDM

There is a gradual decrease in the calcium concentration from the surface,

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machined surfaces.

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confirmed by the micrographic analysis, micro-hardness profile and EDS results in the cross section of a sample enriched Ti6Al4V. 4.

The uniform layer showed high hardness Knoop values when taking the matrix

The proposed mechanism for the formation of titanium perovskite layer is mainly

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5.

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(bulk) as a reference and its hardness value decreases with increasing depth.

re-solidification of the melted mixture comprising of calcium, oxygen and titanium. A secondary role was assigned to an ion implantation mechanism.

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The generated layer on the Ti6Al4V using the sink EDM process has all the

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desirable characteristics needed to

guarantee success

when applied to

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medical/dental implants and this is a new method for obtaining this kind of oxide

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layer with a strong possibility of production on a large scale, thus reducing costs.

Acknowledgments

The authors would like to thanks the FAPEMIG for providing the needed funds to develop this project, number TEC APQ 01481/09. One of the authors is also grateful to CAPES (Project number 002659/2015-08 PDS)

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ACCEPTED MANUSCRIPT 5. References

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Bleys, P.; Knuth, J.P.; Lauwers, B.; Schacht, B.; Balasubramanian, V.; Froyen, L. and

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Engineering Materials, V.8, pages 15–25, February, 2006

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Van Humbeeck, J. Surface and Sub-Surface Quality of Steel after EDM, Advanced

Cheng, S., Wei, D., Zhou, Y., 2012, The effect of oxidation time on the micro-arc titanium dioxide surface coating containing Si, Ca and Na, Procedia Engineering 27,

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713 – 717.

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Fredriksson, G. and Hogmark, S. Influence of Dielectric Temperature in Electrical Discharge Machining of hot Work Tool Steel. Surface Engineering, Volume 11, Issue 4,

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pp. 324-330, January 1995.

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Furutani, K. and Shimizu, Y. Experimental Analysis of Deposition Process of Lubricant Surface by Electrical Discharge Machining with Molybdenum Disulfide Powder

530. 2003.

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Suspended in Working Oil. American Society for Precision Engineering. V. 30 p. 547-

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Furutani, K., Saneto, A.,Takezawa, Mohri, H.N. and Miyake, H.. Accretion of titanium carbide by electrical discharge machining with powder suspended in working fluid. Precision Engineering, 25 (2001), pp. 138–144 Gilla, A.S. and Kumarb, S.. Surface Alloying by Powder Metallurgy Tool Electrode Using EDM Process. Materials Today: Proceedings 2 ( 2015 ) 1723 – 1730 Han, Y., Hong, S.-H., Xu, K.W., 2003, Structure and in vitro Bioactivity of TitaniaBased Film by Micro-Arc Oxidation, Surface & Coatings Technology, 168, 249-258. Kumar, S.; Singh, R.; Singh, T. P.; Sethi, B. L. Surface Modification By Electrical Discharge Machining: a Review. Journal of Materials Processing Technology. V. 209, 2009, p. 3675-3687.

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ACCEPTED MANUSCRIPT Lee, H.G., Simao, J., Aspinwall, D.K., Dewes, R.C. and Voice, W. Electrical discharge surface alloying. Journal of Materials Processing Technology, V. 149, 2004, pp. 334-

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340.

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McGeough, J. A., Advanced Methods of Machining. London: Chapman and Hall, 1988.

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p. 128-152.

Santos, R.F.; Silva, E.R.; Costa, H.L.; Oliveira, A.R.F. and Raslan, A.A. Nitriding of the AISI 4140 Steel by EDM. Proceedings of the World Tribology Congress (WTC),

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Torino – Italy, September 2013, CD-Room, ID 75, pp. 01 – 04.

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Tamura, K., Fujita. F., Morisaki, Y., 2013, Vacuum-sintered body of a novel apatite for artificial bone, Cent. Eur. J. Eng., 3(4) 700-706 DOI: 10.2478/s13531-013-0127-4.

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Wei, D., Zhou, Y., Jia, D., Wang, Y., 2007, Structure of calcium titanate/titania

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bioceramic composite coatings on titanium alloy and apatite deposition on their surfaces in a simulated body fluid, Surface & Coatings Technology 201, 8715–8722.

1003.

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Yang, B.C., Uchida, M., Kim, H.-M., Zhang, X.D., Kokubo, T., Biomaterials 25 (2004)

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Yerokhin, A.L., Nie, X., Leyland, A., Matthews, A., Dowey, S.J., 1999, Surface & Coatings Technology, 122, 73.

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ACCEPTED MANUSCRIPT Highlights CaTiO3 formation in Ti6Al4V using sink EDM process was evaluated.

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Potential biomedical material to medical and dental implants was produced.

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As a dielectric fluid, an aqueous solution of calcium chloride was used.

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Porous and enriched CaTiO3 surface was found in this project.

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Enriched layer with a depth of approximately 200 μm was produced.

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