Carbon fibres used as electrode tools for micro-electrochemical machining

B. Wielage 1, Th. Mäder 1, B. Fürderer 2, D. Weber 1, H. Reinecke 2

1 Chemnitz University of Technology, Institute of Materials Science and Engineering

Professorship of Composite Materials, D-09107 Chemnitz thomas.maeder@mb.tu-chemnitz.de

2 University of Freiburg, Department of Microsystems Engineering

Georges-Köhler-Allee 103, D-79110 Freiburg bernd.fuerderer@imtek.uni-freiburg.de

ABSTRACT Carbon fibres were used as electrode tools for micro-electrochemical machining. Different fibre types and coated fibres were employed as tools. Small channels and holes could be machined in hard metals with this novel tool material. A characterisation of side effects influencing the tools was carried out and proper types were identified. SEM studies on machined fibre electrodes were carried out in order to identify changes in their general appearance. EDXS analysis was used to quantify material diffused into fibres and material deposited during ECM. The evaluation of changes of the tensile strength of the fibre induced by electrochemical reactions was made by single-fibre tensile tests.

Keywords: carbon fibre electrodes, micro-electrochemical machining, coating, micro-manufacturing, carbon-single-fibre application

1 CARBON FIBRE ELECTRODES AS TOOLS FOR MICRO-ELECTROCHEMICAL MACHINING

Electrochemical machining (ECM) is a method predestinated for machining extremely hard, but conductive materials using the anodic dissolving effects of the electrolysis. The shape of the cathode is negatively copied to the workpiece which bears the anodic potential [1].

Carbon fibre electrodes and preparative treatment Carbon fibres are electrically conductive and, for this reason, used as electrochemical electrodes in rechargeable batteries and fuel cells. Single carbon fibres compared to metal wires show a higher tensile strength and naturally have a smaller diameter of less than 10 µm [2]. This property combination has lead to the assumption that carbon fibres are suitable working electrodes for micro-electrochemical machining (µECM) [3]. In subsequent experiments, plain carbon fibre electrodes of different fibre types and coated fibres were tested. In order to allow electrochemical machining, single fibres were separated from bundles, were desized by heat or acetone treatment and then conductively attached to an electrode holder. In order to test the influence of different

conductivity rates, the fibres were coated with copper in galvanic processes and with nickel by means of wet-chemical deposition. The quality of the coated layers differs.

Micro-electrochemical machining experiments with carbon fibres In the experiments, different kinds of carbon fibres (PAN- and pitch-based carbon fibres) were used in two different arrangements. Carbon fibre electrode tools were used for electrochemical drilling and wire cutting in pulsed electrochemical machining [3] or direct-current electrochemical machining systems [1]. By horizontally clamping a fibre between two bars and moving it vertically into the workpiece, channels can be created (µECM wire cutting). Holes can be achieved by driving a fibre fixed only on one side along its longitudinal axis with the non-fixed and cut part into the workpiece (µECM drilling). Figure 1 shows the working principles of both configurations. Carbon fibres were fixed into a holder by means of conductive glue. The holder was placed within the machine (PEM 1360). This machine includes a mechanism to oscillate the tool to raise machining quality [4]. In the experiments, cold work steel, DIN 1.4301 steel and copper were machined. Sodium nitrate was used as electrolyte.

Results of micro-electrochemical machining experiments with carbon fibres

Having overcome the first difficulties of cracking fibres, it was possible to realise small structures in both configurations. With the first configuration (wire electrode tool), channels of a depth of 2 mm with an average diameter of 60 µm were created in DIN 1.4301 steel with a thickness of 1 mm (see Fig. 2). The smallest structures had a diameter of about 40 µm. Holes with an average diameter of 75 µm and a depth of 100 µm could be achieved in the same material with the second configuration (drill electrode tool). The average fibre electrode diameter was about 8 µm.

Figure 2: Light-microscopical image of a machined channel in DIN 1.4301 steel.

Figure 1: Working principles of µECM with carbon fibre electrode tools – wire electrode tool and drill electrode tool.

2 CHARACTERISATION OF FIBRES AND RESULTS

Characterisation of fibre types The characterisation of carbon fibres included SEM and EDXS analyses. Fibre surfaces were observed to identify the surface condition. The as-received state was compared with surface properties of fibres after ECM. With EDXS measurements, the nature of deposited material which was found on the fibre surface after ECM was determined. The SEM analyses were carried out to measure the fibre diameter and to qualify the cross-sectional shape as well. The fibre diameter is important for the calculation of the specific electrical resistance of fibres after measuring the resistance of a certain fibre section. In first experiments, the fibre diameter was measured only using the lateral view of the surface. Since some fibre types have an irregular cross-sectional shape, laterally measured diameters are not representative to calculate the cross-sectional area. The observation of fibre cross-sections, embedded into copper by means of electrochemical deposition prior to the materialographic preparation, has therefore been used in newer investigations. Another goal of SEM studies has been the investigation of coated carbon fibres to qualify the coating quality and the bonding condition of coated layers, to measure the thicknesses of layers and to analyse diffusion during the coating process.

The electrical conductivity of both plain and coated carbon fibres was measured using the measurement setup displayed in Figure 3. By means of light exposure, conductive paths were copied from a mask to a standard printed board. After developing of the exposed surface, the board was contacted. Using a light microscope equipped with an exact x-y-stage, the width of the paths and their mutual distance was measured. For the determination of the electrical resistance of certain fibres, the fibres were laid across the conducting paths and electrically connected with conductive silver. After measuring the resistance, the length of the conductive areas was measured using a light microscope (compare Fig. 3).

Figure 3: Measurement setup to determine the electrical resistance of fibres and determination of the fibre length between two conducting paths

In order to judge the influence of the electrochemical process to the mechanical properties of carbon fibres in dependence on the fibre type, single-fibre tensile tests were carried out using the test equipment provided by the company Kammrath and Weiß. Coated carbon fibres were tested as well.

Results of the electrical characterisation The results of the investigation of the electrical conductivity of carbon fibres with the average value of the specific electrical resistance, the average fibre diameter and the minimum and maximum values are summarised in Table 1. Figure 4 shows the relative frequency of the diameter of different carbon fibres determined in experimental series. As expected, both pitch-based carbon fibre types (YS90A, YSH50A – produced by NGF) show a relatively low electrical resistance. The lowest measured specific electrical resistance of YS90A is still 10 times higher than that of steel (0.1-0.2 Ωmm2m-1) [5]. Both PAN-based fibre types (T300, T800 – produced by Toray Industries Inc.) show an electrical resistance 100 times higher compared to steel.

The comparison of the measured resistance of pitch-based carbon fibres with the resistance of graphite (8.00 Ωmm2m-1) as given in literature [6] is interesting.

Table 1: Electrical resistance and diameter of different fibres and wires.

Fibre type

Mean spec. elect. resist. [Ωmm2m-1]

Min. spec. elect. resist. [Ωmm2m-1]

Max. spec. elect. resist. [Ωmm2m-1]

Mean fibre diameter

[µm]

Min. fibre diam. [µm]

Max. fibre diam. [µm]

YS90A 3.00 ± 0.01 2.84 5.92 7.37 ± 0.14 6.25 8.16YSH50 7.44 ± 0.01 7.05 10.57 7.97 ± 0.16 6.94 9.06T800 16.33 ± 0.05 13.02 18.14 5.34 ± 0.11 4.52 6.02T300 20.33 ± 0.08 16.64 22.98 7.48 ± 0.15 6.32 8.41EDM20 0.13 ± 0.01 0.12 0.13 20.59 ± 0.41 20.40 20.74T800Cu 4.64 ± 0.02 0.06 11.74 11.65 ± 0.23 7.05 15.95T800Ni 1.29 ± 0.01 1.25 1.34 15.90 ± 0.32 14.94 16.48

Figure 4: Diameter of different carbon fibre types

The order of lattice planes in pitch-based carbon fibres seems to be a strongly influencing property to increase the electrical conductivity. Supposing that the electrical resistance is a direct measure for the order and size of lattice planes in graphite, YS90A shows the highest order of all tested fibre types. In comparison with the theoretical resistance of ideal graphite (0.5 Ωmm2m-1) found in [7], pitch-based carbon fibres cannot be classified as ideal graphite with regard to the electrical resistance.

In order to compare EC machining results of conventional wire electrode processing to the results achieved with carbon fibre electrodes, tungsten wires with a diameter of 20 µm were tested. The electrical properties of coated carbon fibres were tested as well (T800 coated with Cu or Ni by chemical or electrochemical deposition).

The measurement of the low resistances of these materials was more difficult. The varying contact resistance, weighing stronger in relation to the wire resistance compared to pure fibres, influences the measurement. Therefore the determined resistance of tungsten (0.13 Ωmm2m-1) is twice as high as the resistance reported in literature (0.055 Ωmm2m-1) [6]. Coated carbon fibres, especially Cu-coated fibres, did not show a homogeneous surface covering. This relates to a strong variation in the measured resistance within the experimental series (Tab. 1) and to much higher resistances compared to pure copper or pure nickel.

Results of SEM studies Due to the EC processing, carbon fibres showed different changes in their appearance. These changes have an influence on the properties of fibres and therefore on the EC process. In the end, these changes indicate the eligibility of fibre types for ECM. After all ECM experiments carried out with carbon fibres, deposition layers were found on the fibre surface. The grade of deposition and layer thickness varied depending on the fibre type, the process parameters and the processing time. In areas close to the working zone of electrochemical reactions, the deposited material is of dark brown or red brown colour. In the outer fibre electrode sections, relatively far from electrochemical reactions, the deposition is semitransparent. The different colours indicate a different

Figure 5: EDXS analysis of deposited material found on the fibre surface after ECM

element mixture. Brown depositions are caused by metal oxide or metal hydride compounds. Transparent layers suggest that the ions of the electrolyte are the source of this material deposition. EDX analyses have proved the assumptions (Fig. 5). The brown material consists of the elements oxygen (51 %), sodium (34 %), iron (12 %) and low contents of nickel and chrome. Iron oxide or iron hydroxide was deposited on the fibre surface. The analysis of transparent layers shows a composition of 13 % of

Figure 6: Condition of carbon fibre surfaces before and after ECM, comparison of different fibre types

nitrogen, 59 % of oxygen and 26 % of sodium. This relates to the elemental composition of sodium nitrate used as electrolyte.

In SEM studies, different changes of the fibre surface were found after cleaning with oxalic acid (Fig. 6). Defects appeared resulting from material removal. The grade of destruction varied depending on the fibre type. Some fibres had holes and some fibres were lacking big parts of their structure. The material removal strongly influenced the mechanical fibre properties. Strong defects relate to an intense decrease of the tensile strength. Fibre breakage occurred for fibres with a high grade of destruction. There are two possible reasons causing the defects. Firstly, the deposited material induces forces to the fibre surface. A closed layer inhibits the removal of reaction products appearing while the electrochemical reaction runs. The left space is smaller than the required space needed by the reaction products. Some are formed in pores of the fibre surface. This expansion leads to the destruction. A second possible reason is the formation of hydrogen during ECM at the cathode side. Water, diffused into pores, splits under electrochemical conditions and the formation and expansion of hydrogen destroys the fibre. Both pitch-based carbon fibres show the highest number of surface defects. Bigger parts of the fibre surface were removed. Knowing the microstructure of pitch-based carbon fibres [2], it can be deduced that these removed parts are single graphite crystallites. Strong covalent bonding of carbon in axial fibre direction, a minimum bonding of the crystallites in radial direction and the strong axial orientation of lattice planes are reasons for the removal of big single parts of the fibre. The higher the orientation, the higher is this effect. For YS90A, the fibre with the highest elastic modulus and therefore with the highest orientation, the effect was observed strongest. Removed parts of the YSH50A fibres are smaller and fewer in number. Pitch-based

Figure 7: Condition of electrode surfaces (Ni coated T800 fibre and tungsten wire) before and after ECM

carbon fibres usually contain more pores in the outer surface than PAN-based carbon fibres. Despite the good electrical conductivity, pitch-based carbon fibres are not suitable to meet the need of high tensional strength necessary for µECM. In further experiments, the cross-section of destroyed pitch-based fibres will be observed to get a better understanding for the nature of the destructive effect.

Both PAN-based carbon fibre types only showed small defects on the surface (Fig. 6 – 3rd and 4th line). Despite the rougher surface compared to pitch-based fibres, the destructive effect is smaller. The surface size does not seem to influence the defect rate, but, as already discussed, the microstructure rather has a strong influence on the destructive effect. Crystallites in PAN-based carbon fibres are smaller and distributed homogenously. Due to this fact, the fibre strength in radial direction is higher. PAN-based fibres contain fewer pores. Therefore, no electrolyte can diffuse into the fibre area. PAN-based carbon fibres meet the best properties to be used as electrode tools. Further experiments are necessary to test other PAN-based fibre types.

The nickel-coated T800 fibre did not show any changes at the surface after the ECM. Compared to the unused fibre, no differences were found (Fig. 7 – 1st line). The coating does not seem to be influenced by the electrochemical processing. No blistering or flaking of parts occurred, no material dissolved. It can be assumed that nickel coatings are a good surface protection for carbon fibres to be used for µECM. A reduction of the layer thickness is necessary to fit the need of a small electrode tool diameter. Subsequent work will concentrate on this layer thickness reduction.

3 CONCLUSION AND OUTLOOK Altogether, carbon fibres can be qualified suitable as electrodes for the ECM process. The deposition of electrochemical reaction products and electrolyte ions was observed. Defects of the fibre surface could be found after ECM. These defects influence the mechanical properties of carbon fibres. The grade of surface destruction depends on the fibre type. Pitch-based carbon fibres are strongly influenced by the ECM process. PAN-based carbon fibres show a minimum defect rate. PAN-based fibre types are therefore most suitable to be used as µECM electrode tools. Surface defects can be omitted by coatings as well. Especially nickel coatings seem to meet the needs of ECM demands. Higher aspect ratios, smaller overcuts and smaller cut geometries than in former investigations could be achieved by using carbon fibres as electrode tools. Applying carbon fibres allows further progress in µECM and introduces new opportunities of this technique. In consequent examinations, the µECM process will be optimized using carbon fibre electrode tools with modified surface properties targeted to raise the ECM suitability. Different surface treatment and coating technologies will be applied to improve the carbon fibre. The design of holders and the joining process for the fibre fixation will also have to undergo a further optimization. These investigations are pointed towards the goal of achieving smaller structure sizes and higher aspect ratios in the future.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft (German Research Foundation) which has allowed the experimental and theoretical work presented in this paper.

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