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Micropatterning of titanium surfaces using electrochemical micromachining with an ethylene glycol electrolyteTerje Sjöström ⁎,Bo SuSchool of Oral &Dental Sciences,University of Bristol,Lower Maudlin Street,BS12LY,Bristol,UKa b s t r a c ta r t i c l e i n f o Article history:Received 20June 2011Accepted 29July 2011Available online 6August 2011Keywords:TitaniumMicropatterning Biomaterials SurfacesMicropatterning of bulk titanium (Ti)surfaces is of high interest for both biomedical applications and other areas of microengineering.The techniques available for precise patterning of Ti all have intrinsic problems such as slow material removal rate,up-scaling issues or unwanted surface modi fications.Electrochemical micromachining (EMM)is an effective technique for micropatterning of metal surfaces in a direct writing fashion with recent advancements having pushed the resolution into the submicron region.Although most conducting surfaces can be machined using EMM not many studies have been performed on Ti substrates.Here we present a technique utilizing a water-free electrolyte with which EMM of Ti surfaces easily can be performed.Pit and groove structures with approximately 50μm diameter/width were fabricated at high etch rates utilizing a simple tungsten carbide tool.©2011Elsevier B.V.All rights reserved.1.IntroductionTitanium and its alloys are commonly used biomaterials in medical devices and implants and surface micropatterning of Ti has received a lot of interest due to the in fluence of surface topography on biomaterial bone bonding performance and also because of the ability to control speci fic cellular activity [1–3].Surface patterning of bulk Ti is also of interest for micro electromechanical systems (MEMS)with Ti providing superior properties to traditional semiconductor mate-rials due to its fraction toughness and corrosion resistance [4].Electrochemical micromachining (EMM)is a highly attractive technique for surface micropatterning which allows for rapid local machining of surfaces which are often dif ficult to machine with conventional techniques.Basically,a microsized cathode tool is brought in close contact with a metal workpiece and the workpiece is electrochemically dissolved by applying a potential across an electrolyte appropriately selected to dissolve the metal surface [5].By using pulsed potentials and small distances,b 50μm,between the tool and workpiece dissolution of the workpiece is con fined to the region in close vicinity of the tool.EMM can in principle be performed on most conducting surfaces but Ti is more dif ficult to machine than most other metals due to the nature of the passive oxide film which instantly forms on the Ti surface under ambient conditions [6].Previous attempts to electrochemically machine Ti surfaces includes the work by Lu et al.[7]who used a jet-EMM technique for micropatterning of Ti but with resulting dimensions in the region ofseveral hundred micrometers.Jiang et al.[8]machined a Ti alloy surface with a scavenger electrolyte technique but with very long machining times,25min was needed to etch a single pit feature.Madore et al.[9]used both aqueous NaBr electrolytes and sulphuric acid and methanol mixtures to dissolve Ti.The micropatterning in their work did however rely on dissolving the Ti through a photoresist mask,thus limiting the amount of freedom in the patterning.In this work we demonstrate for the first time that rapid,mask-less EMM whereby surface features can be created in a direct writing manner by moving a single-tip tool across the surface can be performed on Ti surfaces.This is also,to the best of our knowledge,the first time that EMM using a microsized tool in conjunction with short voltage pulses has been demonstrated in an ethylene glycol (EG)electrolyte.By utilizing a water-free electrolyte some of the dif ficulties normally associated with EMM,such as heavy gas formation in the gap between the tool and the workpiece are signi ficantly reduced.2.ExperimentalCircular Ti disks were punched from a 0.9mm thick ASTM grade 1Ti sheet (Ti metals Ltd,UK).The Ti disks were polished to a mirror image and mounted in a PTFE electrolytic cell which was placed on the stage of a CNC machine (Roland MDX-650).A pulsed potential was applied using an arbitrary waveform generator (Agilent 33220A)and a multimeter (Keithley 2000)was used to measure the average current over the cell.All experiments were conducted with 20V potential,200ns pulse length and 20%duty cycle.The tool was prepared from a tungsten carbide (WC)rod which was grinded to a conical shape with an apex diameter of approximately 30μm using a succession of SiC paper grades finishing with #2400.The tool wasMaterials Letters 65(2011)3489–3492⁎Corresponding author.Tel.:+441173424180;fax:+441173424313.E-mail address:terje.sjostrom@ (T.Sjöström).0167-577X/$–see front matter ©2011Elsevier B.V.All rights reserved.doi:10.1016/j.matlet.2011.07.103Contents lists available at ScienceDirectMaterials Lettersj o u r na l ho m e p a g e :w w w.e l s ev i e r.c o m /l o c a t e /m a t l e tmounted in the CNC machine and connected to the negative outlet of the waveform generator without any insulation of the side walls.The CNC machine provided control of the tool in three axes with10μm precision.The distance between the tool and the Ti surface,or the inter-electrode gap(IEG)was kept at10μm throughout the experiments.All experiments were performed using a3M NaBr(99+%anhydrous, Acros Organics)in EG(99+%,Acros Organics)electrolyte.All experiments were conducted at room temperature.The surfaces were imaged using scanning electron microscopy(SEM)(JEOL JSM6330F). Cross-sectional data was obtained using a Proscan2000light profil-ometer(Scantron,UK).3.Results and discussionFig.1shows parallel groove structures machined on a bulk Ti surface.To machine the grooves the tool was moved at a rate of 100μm/s along the surface and the tool movement was repeated four times.The total machining time for the grooves was thus40s/mm at the current conditions.The grooves had a width of approximately 50μm along the entire groove length.The cross-section profiles in Fig.2shows that the depth of the grooves was approximately6–7μm. The depth of the grooves was relatively constant along the groove length with some variation as seen in the profile following the length of the groove.The higher magnification images of the grooves shown in Fig.3shows that the bottom of the grooves appeared smooth and importantly no pitting of the surfaces was observed as was the case for electrochemical etching of Ti in aqueous NaBr electrolytes[9].Some crack-like features could be seen on the etched surfaces,presumably caused by enhanced etching along the grain boundaries.At high magnification(10000×)there was evidence of a slightly rough surface finish as seen in Fig.3b.We expect that it is possible to improve the surfacefinish further by optimizing the conditions during the machining.Machining at a lower potential may give a betterfinish, however at the cost of lower machining rate.Since a single-tip tool was used as cathode the surface patterns could be controlled with complete freedom.Fig.4shows an array of pit structures and a curved groove structure,demonstrating the capability of creating arbitrary surface patterns on the Ti surfaces.The pits were machined at the same conditions as the groove structures with a machining time of2s per pit without any movement of the tool.The curved groove was machined in the same manner as the parallel grooves with the tool following a pre-programmed curve repeated four times at100μm/s.Non-aqueous EG based were recently used by Fushimi et al.[10,11] for electropolishing of Ti surfaces.The mechanism of dissolution of Ti in an EG electrolyte was discussed in detail in[11]and is different to aqueous systems due to the formation of a salt layer on the Ti surface. Using EG for EMM has several advantages over aqueous electrolytes. Firstly,any unwanted anodic oxide formation on the Ti surface which can interfere with the dissolution process is significantly reduced. Secondly,the higher viscosity of EG results in a slower diffusion rate in the electrolyte and therefore a slower etch rate[12].If the dissolution rate of the metal surface is too high during the EMM process it becomes difficult to control the surface feature shape evolution,which was the case when using a5M aqueous NaBr electrolyte[9].The main benefit of using the EG electrolyte for EMM is the reduced gas formation compared to aqueous systems.During polarization in an aqueous electrolyte oxygen gas forms at the anode and hydrogen gas forms at the cathode[13].This gas bubble formation leads to increased stray currents and spark generation in the IEG gap which leads to overcut,poor feature resolution and tool wear[14].With no or low water content in the electrolyte,both oxygen and hydrogen gas formation are reduced in the space between the Ti workpiece and the tool[15].We noted that some gas formation still occurred during machining in the EG electrolyte which was evident by thegeneration Fig.1.SEM image of two parallel grooves machined on the bulk Tisurface.Fig.2.3D surface plot of the two parallel grooves exported from light profilometry data.The line A–A shows a cross-section plot of the two grooves and B–B shows a cross-section along one of the grooves.3490T.Sjöström,B.Su/Materials Letters65(2011)3489–3492of small bubbles at the machining area,likely caused by oxygen being released from the native titanium oxide layer and due to trace amounts of water in the electrolyte.This gas formation did not signi ficantly in fluence the machining although it is possible that the generation of bubbles may have contributed to the small variations in feature size and depth that were seen along the grooves.Without any heavy gas formation there was no need to flush away gas bubbles with a high flow rate pump thus simplifying the set-up needed for EMM.The dissolution mechanism during EMM with short voltage pulses in the EG electrolyte is the same as when aqueous electrolytes are used.When a pulsed potential is applied between the tool and the workpiece the double layer that exists in the IEG acts as a capacitor which is charged during the pulse on time [16].The pulse on time must therefore be suf ficiently long to charge the double layer before etching of the surface occurs.The pulse length can thus be chosen so that only the region in close vicinity of the tool is etched whereas for longer distances between tool and workpiece no etching takes place.With increasing pulse length the distance between tool and workpiece for which etching occurs is increased and a larger region on the workpiece is etched.In this work the pulse length was kept at 200ns,which was the minimum pulse length to achieve etching of the surface with the current tool design and IEG distance.For pulse lengths longer than 200ns a larger region on the Ti surface was etched thus reducing the surface feature resolution.All the machinings in this study were performed at 20V and with 20%duty cycle which was found to give high machining rates with relatively good feature resolution.It should be noted that no dissolution of the Ti surfaces took place for potentials below 10V which is in agreement withprevious studies which showed that potentials above 10V was needed to break through the passive Ti oxide layer in EG electrolyte [10].We acknowledge that further optimizing of the machining conditions with the technique presented in this work is possible to achieve even better feature resolution,possibly at the cost of lower material removal rates.4.ConclusionsIn summary,we have for the first time demonstrated rapid micropatterning of Ti surfaces via EMM by utilizing an EG electrolyte.The setup used is simple without any need for a high flow rate of electrolyte and the technique allows for well-de fined micropatterning of Ti with high degree of freedom at fast machining rates.A single-tip tool was used which means that complex surface patterns can be fabricated without the need for etch masks.For biomaterial applications it is desirable to mimic the osteoclast resorption pits that are present on the surface of bone during remodeling.These pit and trail structures have a depth of approximately 1–5μm [17]making the EMM technique presented here ideal for fast fabrication of such surface features on Ti biomaterial surfaces.AcknowledgmentsThe authors would like to acknowledge the EPSRC for funding the presented work under grant numberEP/G049076/1.Fig.3.High magni fication SEM images of machined grooves.a)1400×magni fication.The groove bottom appears smooth with only a few cracks present at the groove bottom.b)At 10000×magni fication the surface had a typical etched appearance in comparison to the smooth non-machined Tisurface.Fig.4.a)An array of pit structures machined for 2s/pit.b)A curved groove structure.To machine the curved groove the tool was moved at 100μm/s and repeated four times along the curve.3491T.Sjöström,B.Su /Materials Letters 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