Micro Electrical Discharge Machining of Si3N4-based Ceramic Composites K. Liu, J. Peirs, E. Ferraris, B. Lauwers, D. Reynaerts Afd. PMA, Department of Mechanical Engineering, Katholieke Universiteit Leuven, Leuven, BE-3001, Belgium

Abstract The Electrical Discharge Machining (EDM) behaviour and machining properties of advanced engineering Si3N4-based ceramic composites Si3N4-TiN are investigated and discussed in this paper. Two types of EDM machining configurations, micro-EDM milling and die-sinking EDM, are employed in the investigation. Relaxation type of pulse is used, and the performances of EDM process in the form of material removal rate, tool wear and surface quality are studied. These tests result in a performance comparison and a discussion on the ceramic composites material removal mechanism. The feature of material removal mechanism is characterised as chemical decomposition of Si3N4 and TiN at elevated temperature rather than melting/evaporation. The generation of nitrogen gas bubbles leads to a porous and foamy top surface structure. Due to the ideal mechanical and physical property of Si3N4-TiN ceramic composites, an application example - a turbine impeller - as a crucial component in a micro power generation system is manufactured with obtained knowledge in both machining configurations.

Keywords: Micro Electrical Discharge Machining, Ceramic composites, Si3N4-TiN

1 Introduction

Advanced engineering ceramics are attracting more and more attentions in the last decades and employed as critical components in the modern mechanical systems because of their excellent mechanical, physical and chemical properties. Among those various ceramic materials (Al2O3, ZrO2, B4C…), Si3N4 has being regarded as one of the best ceramic material for structural application, owing to its low density, high hardness, high strength at high temperature, oxidation and thermal shock resistance. However, these promising properties also bring difficulties in machining and structuring this material, the wider applications in the industry thus are limited. In recent years, a successful approach by incorporating electrically conductive reinforcements such as TiN, TiC, TiCN, and TiB2 etc. into the silicon nitride matrix is developed [1]-[3]. Among all these mentioned secondary phases materials, the introduction of 30-40 vol. % TiN not just dramatically increase the electrical conductivity of the composite, but also enhance the mechanical properties for instance fracture toughness, strength and wear resistance [4]. On the other hand, the composite is difficult to be machined efficiently by conventional manufacturing methods. Nevertheless, the significantly lowered electrical resistivity provides the possibility of structuring the composite by using other methods such as Electrical Discharge Machining (EDM). It attracts more and more attention in the last few years, major ceramic suppliers and merchants have already commercialised this composite especially for the EDM application.

Researches on electrical discharge machining of this composite have already been performed in the past on the machining behaviours. Liu and Huang [5] studied the effects of wire-EDM conditions on properties, reliability and microstructure of hot pressed Si3N4-TiN composites; furthermore, Liu also examined the electrode wear during the sinking EDM of this material [6]. In [7], Lauwers et. al. investigated

the machining properties of various EDM configurations and with different dielectric, and proposed the variable material removal mechanisms. However, it still lacks information on the micro/meso-scopic EDM of the Si3N4-TiN ceramic composites.

In this paper, the machining properties of Si3N4-TiN composite are evaluated on both micro milling- and sinking-EDM. The miniaturized input pulse energy is the main difference comparing to macro-scale machining. Surface quality, microstructure, material removal behaviour regarding to the various condition are investigated and discussed. As an application example, the micro-manufacturing of the Si3N4-TiN composite into a miniature turbine impeller, which is a crucial component in a micro power generation system, with obtained the knowledge are demonstrated. 2 Experimental Investigation 2.1 Material properties and microstructure

The Si3N4-TiN ceramic composite employed in this research is obtained from a commercial ceramic supplier Saint-Gobain. Manufacturer’s grade is Kersit 601. The measured mechanical and physical properties of the composite are listed in Table 1.

2.2 Experimental Set-up

One of the tools applied in this research is a

SARIX SX-100-HPM micro-EDM milling machine. The equipped relaxation type generator and high precision positioning system makes it dedicate for the micro-scale machining [8]. The short discharge duration time (350-400 ns) and low discharge current (< 0.5 A) guarantees the small energy input for each pulse. The usage of hydrocarbon oil as dielectric also assures the smaller sparking gap to attain the machining accuracy. The special designed spindle and clamping head allow the use of thin solid or tubular electrodes with diameters ranging from

© 2008 Cardiff University, Cardiff, UK. Published by Whittles Publishing Ltd. All rights reserved.S. Dimov and W. Menz (Eds.)Multi-Material Micro Manufacture

40 µm to 1.0 mm combined with external or internal flushing. The automatic electrode feeding system can compensate the tool length due to the wear during machining. However, the rotational speed is limited to 600 rpm.

Another tool is a die-sinking machine, Roboform 350γ, from Charmilles. The advanced generator can generate various pulses from iso-energetic static to relaxation type. It also uses hydrocarbon oil as the dielectric.

Table 1 Mechanical and Physical properties of Si3N4-TiN ceramic composite

Supplier Saint-Gobain Grade Kersit 601

Si3N4 64 vol% Chemical composition TiN 36 vol%

Binder Al2O3 Grain size (µm) See Fig. 1

Density (ISO 3369) (g/cm3) 3.97 HV10 1508 ± 33 Hardness (ISO

3878), (kg/mm2) HV30 1465 ± 6 3-point bending strength (MPa) 979 ± 120

Young’s modulus (GPa) 333 ± 3 10 kg 8.7 ± 0.7 Fracture toughness

(MPa.m1/2) 30 kg 5.5 ± 0.4 Resistivity (10-7Ω.m) 160

20°C 28 Thermal conductivity (Wm-1K-1) 800°C 19

2.3 Experimental results

It is known that the contradictory effects of the

tool electrode wear with the material removal rate and the surface quality in the EDM process; and all are very much related to the given technology parameters. Thus it is impossible to study all the combination of parameters on the outcomes of machining abilities. This research is more focusing on achieving a good surface integrity of the ceramic composites in various machining configurations. More process investigation can refer to [9].

2.3.1 Micro-EDM milling

In the micro-EDM milling of Si3N4-TiN composite, tungsten carbide solid rod is used in the experiments. The relationship of the input actual machining parameters with the material removal rate, tool wear and the surface quality are shown in Table 2.

For the finishing regime, the material removal rate and tool wear ratio are difficult to be concluded because of the extremely slow process and unstable machining condition. As can be seen, the micro-EDM milling properties of Si3N4-TiN have the same

variation trend as the machining of steel: lower pulse energy is necessary for achieving better surface quality, but not in favour of the machining speed and tool wear.

To further optimizing the surface quality after the micro-EDM milling process, a series of experiments are conducted with gradually lowed energy input of the pulse. In Fig. 2 the relationship between the actual discharge parameters and the surface roughness Ra is plotted. Apparently the surface quality can only be optimized to a certain value. The smoothest surface quality obtained during the tests is 0.74 µm Ra. Further minimized pulse energy cannot provide any change on the Ra, which is unlike the micro-EDM process of steel.

Accordingly the surface topography is examined. In Fig. 3 a), the topography after the semi-finishing process is shown. The surface roughness Ra is 1.65 µm. No regular formed craters like normal EDMed steel surface are appeared; in contrary, the porous, sponge-like surface is revealed. A same phenomenon is also observed on the finest micro-EDMed surface. The cross-section (Fig. 3 b) exhibits this effected surface layer. The thickness is comparably equals to the 10 times of Ra.

2.3.2 Die-sinking EDM

Though the die-sinking EDM machine is not delicately designed for micro manufacturing purpose as aforementioned, it still has modules or technology settings which are specialized for micro-machining. Parameters investigated in the following experiments are set in this range. Copper infiltrated graphite (POCO® EDM-C3) is applied as the tool electrode material. For obtaining low energy input, relaxation type pulse is used. The machining parameters and the performances are listed in Table 3.

Comparing to the EDM milling process, the machining speed of roughing regime for die-sinking EDM is higher, as well as the tool wear. However, the surface quality is relatively worse. As for the finishing regime, even with very low energy input of the pulse, the roughness is somewhat higher than the result from milling process semi-finishing regime; furthermore, it also suffers from the little material removal and elevated tool wear.

The SEM micrographs of Si3N4-TiN ceramic composite materials obtained form the die-sinking EDM process at different machining regimes are illustrated in Fig. 4. Similarly, a porous and foamy topography is revealed. Apparently there are no dramatic changes on the microstructure with the varied pulse energy. Moreover, no visible subsurface micro-crack is examined even at high magnification SEM views.

Table 2 Machining properties of micro-EDM milling with WC tool electrode

Machining Regime

Actual open gap voltage

(V)

Actual discharge current (A)

Material Removal rate

(mm3/min)

Tool wear ratio (%)

Roughness Ra (µm)

Sparking gap (µm)

Rough -100 10 0.305 0.05 2.37 15 Semi-finishing -100 5 0.173 0.92 1.26 10 Finishing -70 0.5 - - 0.75 4

a) b) Fig. 1. Scanning microscopic pictures of the Si3N4-TiN composites: a) magnification 2000x;

b) magnification 10000x. Phases: Grey = TiN; Black = Si3N4; White = WC milling ball contamination.

Surf

ace

Rou

ghne

ss R

a (u

m)

Sample6Sample5Sample4Sample3Sample2Sample1

1.75

1.50

1.25

1.00

0.75

0.50

Fig. 2. Surface quality optimization in relation with

the actual discharge parameters

2.4 Discussion

As shown in the experimental results, the continuously decreased pulse energy is not giving any benefit for improving the surface quality of Si3N4-TiN. These improvements are limited due to its intrinsic material removal mechanism. For the reasons of high material removal and much less tool wear comparing to the machining of steel, the material removal mechanism is not just the melting and evaporation, but also the involvement of chemical reaction, as the proposed in [7]: the decomposition of Si3N4 and TiN at elevated high temperature above 1700 °C.

↑+→

↑+→

2

243

N2Ti2TiN2N3SiNSi

This reaction generates enormous amount of

gas nitrogen gas bubbles which prohibits the formation of inerratic craters and leads to the creation of the voids, resulting in a foamy and porous top surface structure as shown in the SEM figures aforementioned. Furthermore, it also can be seen that the energy differences in the pulses cannot change the primary microstructure of the surface.

As a proof of the recognized material removal mechanisms, elemental analysis EDAX (Energy Dispersive X-ray Spectroscopy) of the ceramic matrix, specimens after micro-EDM milling and die-sinking EDM are conducted. The spectrums of the collected elemental emissions for each specimen are demonstrated in Fig. 5.

Table 4 lists the quantification of the detected elements. As expected, the content of N is dramatically decreased comparing to the ceramic matrix because of the decomposition. There is also a trace of carbon on both EDMed surface, indicates that there might be a transfer of the tool electrode material on the workpiece surface. However it cannot be confirmed because the machining environment is in the hydrocarbon oil.

Table 3 Machining properties of die-sinking EDM with copper infiltrated graphite tool electrode (Poco EDM-C3)

Machining Regime

Open voltage (V)

Charging Current (A)

Charging time (µs)

Capacitance (nF)

Material Removal rate (mm3/min)

Tool wear ratio (%)

Roughness Ra (µm)

Sparking gap (µm)

Roughing -200 6 6.4 67 13.78 3.29 2.91 57 Finishing -120 1 25 1.0 0.032 25.5 1.54 23

Table 4 EDAX quantification (wt%) of each element at Si3N4-TiN matrix and the surface after EDM

Elements C N O Al Si Ti Matrix - 24.19 5.00 1.69 39.79 29.33

EDM Milling 24.00 13.06 4.06 1.21 30.37 27.29 EDM Sinking 11.13 16.92 10.79 1.40 27.86 31.90

Sample 1 ui=100V ie=5.2A Sample 2 ui=90V ie=1.0A Sample 3 ui=90V ie=0.4A Sample 4 ui=90V ie=0.7A Sample 5 ui=70V ie=0.5A Sample 6 ui=50V ie=0.4A

a) b) Fig. 3. SEM pictures of topography and cross-section of micro-EDM milled Si3N4-TiN surface. a) topography with

Ra 1.65 µm; b) cross-section with Ra 0.75 µm

Fig. 4. SEM pictures of topography die-sinking EDM Si3N4-TiN surface. a) roughing regime with Ra 2.91 µm; b)

finishing regime with Ra 1.54 µm

a) Si3N4-TiN matrix b) After micro-milling EDM

O

N

Al

Si

Ti

Ti

C O N

Al

Si

Ti

c) After die-sinking EDM

Ti

O AlN

Si

Ti

TiC

Fig. 5. EDAX spectrum of the Si3N4-TiN matrix and the machined surface after the EDM process

3 Application

One of the most important application of the

Si3N4-TiN ceramic composite is as a core component - a gas turbine impeller - of a portable fuel-based power units:. The miniaturised unit system has an

overall size less than 1 dm3 and electrical power output is intended to achieve 1kw with more than 20% efficiency. Thus the turbine impeller is expected to endure inlet temperature up to 1200 K and rotational speed of more than 500,000 rpm. The Si3N4-TiN is thus to be an ideal material. The turbine

has a mixed axial-radial design and 8 blades with three-dimensional geometry (Fig. 6).

Both micro-EDM milling and die-sinking are employed in the manufacturing of the prototype turbine impellers. The schematic views of the die-sinking process and the milling process are illustrated in Fig. 7 and Fig. 9, respectively.

Fig. 6. Dimension of the miniature turbine impeller

In die-sinking process, the graphite electrode,

which has a negative shape of a cavity, is produced on a 5-axis micro-milling machine (Kern MMP) with 3-axis machining configuration [10]. In total 10 electrodes are used for manufacturing a turbine. One of the electrodes and the finished product are shown in Fig. 7. Since the electrode milling and die-sinking process can run parallel, the total production time is approximately 15 hours. The dimensional control of the turbine is conducted on a Mitutoyo FN 905 CMM by using a Ø 0.7 mm styli. 600 points on each cavity are measured and results are compared with the CAD model. The deviation map is illustrated in Fig. 8. All the cavities are machined very consistently. However, the tip at the outlet has an overcut about 50 µm, and an unexpected single point undercut (around 36 µm) is observed at all side surfaces.

A close view of the micro-EDM milling process for the turbine manufacturing is presented in Fig. 9. The milling of each cavity starts with Ø1.0 mm WC tool for pocketing and Ø 0.7 µm tool for wall finishing, with layer-by-layer milling process. And the layer thickness is 8 µm and 3 µm, respectively. Due to the low machining speed of finishing regime for this method, only roughing setting is applied. Even with relatively high material removal rate and no need for electrode preparation, it still takes 20 hours for machining one cavity. Thus total machining time for manufacturing a turbine impeller is about 160 hours.

Furthermore, due to the lack of an accurate clamping system, the dimensional control on this finished part is unable to conduct. The improvement is under investigation.

Though it seems there is no benefit for micro-EDM milling on the machining hours, the appearance of the surface quality on the final product are better than the one machined by die-sinking EDM. The reason might be the more open flushing condition for milling process. 4 Conclusion

Micro-scale electrical discharge machining of commercially available Si3N4-TiN conductive ceramic composites has been performed and the results reveal the attractive machinability of EDMing this hard, brittle material. The influences of pulse energy on the material removal rate, tool wear, the surface integrity and sparking gap are investigated. With the elevated pulse energy, the machining speed increases, tool wear reduces as well as the surface quality. However the improvement of the surface roughness is limited due to the chemical decomposition of the Si3N4 and TiN as a material removal mechanism. The surface also reveals porous, foamy structure.

Precision manufacturing of a miniature turbine with Si3N4-TiN ceramic composites by either die-sinking or micro-milling process has also been conducted. Though the micro-milling EDM seems relatively low in machining efficiency, the better flushing condition provides more homogeneous surface quality comparing to die-sinking EDM. To achieve more accurate machining, a clamping system and the machining strategies are still under investigation.

Acknowledgements

This research is sponsored by the Institute for the Promotion of Innovation by Science and Technology in Flanders, Belgium, project SBO 030288, and by the Belgian programme on Interuniversity Poles of Attraction (IAP5/06: AMS). This work is also carried out within the framework of the EC Network of Excellence "Multi-Material Micro Manufacture: Technologies and Applications (4M)".

Fig. 7. A schematic view of the machining process and an enlarged view of the tool electrode, as well as the

finished turbine impeller

Fig. 8. Comparison results of the sinking turbine with CAD model in top and isometric view

Fig. 9. A close view of the Micro-EDM milling process and a finished turbine impeller

References [1] Chu CY, Singh JP, Routbort JL, High-

temperature failure mechanisms of hot-pressed Si3N4 and Si3N4/Si3N4-whisker-reinforced composites, Journal of the American Ceramic Society, vol. 76 (1993) pp 1349-1353.

[2] Xu HHK, Ostertag CP, Braun LM, Effects of fiber volume fraction on mechanical properties of SiC-fiber/Si3N4-matrix composites, Journal of the American Ceramic Society, vol. 77 (1994) pp 1897-1900.

[3] Shin DW, Tanaka H, Low-temperature process-ing of ceramic woven fabric/ceramic matrix composites, Journal of the American Ceramic Society, vol. 77 (1994) pp 97-104

[4] Herrmann M, Balzer B, Schuberrt C, Hermel W, Densification, microstructure and properties of Si3N4-Ti(C,N) composites, Journal of the European Ceramic Society. Vol.12 (1993) pp 287-296

[5] Liu CC, Huang JL, Effect of the electrical discharge machining on strength and reliability of TiN/ Si3N4 composites, Ceramics Inter-national, vol 29 (2003) pp 679-687

[6] Liu CC, Microstructure and tool electrode erosion in EDMed of TiN/ Si3N4 composites,

Materials Science and engineering A363 (2003) pp 221-227

[7] Lauwers B, Kruth JP, Liu W, Schacht B, Bleys P, Investigation of the material removal mechanisms in EDM of composite ceramic materials, Journal of Materials Processing Technology, Vol 49 (2004) pp 347-352

[8] Liu K, Ferraris F, Peirs J, Lauwers B, Reynaerts D, Process capabilities of micro-EDM and its application, Proceedings of the 3rd International Conference on Multi-Material Micro Manu-facture, 3-5 October 2007, Borovets, Bulgaria, pp.267-270

[9] Liu K, Ferraris E, Peirs J, Lauwers B, Reynaerts D, Process investigation of precision micro-machining of Si3N4-TiN ceramic composites by electrical discharge machining (EDM), Proceedings of the 15th International Sympo-sium on Electromachining (ISEM), 23-27 April 2007, Pittsburgh, PA, USA, pp 221-226

[10] Ferraris E, Liu K, Peirs J, Bleys B, Reynaerts D, Production of a miniature Si3N4-TiN ceramic turbine impeller by die-sinking EDM, Technical Digest, The 7th International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications, 28-29th Nove-mber, 2007, Freiburg, Germany, pp 229-232