Post on 23-Apr-2023
ISSN:1369 7021 © Elsevier Ltd 2011OCTOBER 2011 | VOLUME 14 | NUMBER 10496
Carbon nanofillers for machining insulating ceramics
The implementation of ceramics in emerging applications is principally
limited by the final machining process necessary for producing
microcomponents with complex geometries. The addition of carbon
nanotubes greatly enhances the electrical properties of insulating
ceramics allowing electrical discharge machining to be used to
manufacture intricate parts. Meanwhile other properties of the ceramic
may be either preserved or even improved. For the first time, a silicon
nitride/carbon nanotubes microgear is electrically discharge machined
with a remarkably high material removal rate, low surface roughness,
and low tool wear. This offers unprecedented opportunities for the
manufacture of complicated ceramic parts by adding carbon nanotubes
for new engineering and biomedical applications.
Olivier Maleka,b, Jesús González-Juliánc, Jef Vleugelsb, Wouter Vanderauweraa, Bert Lauwersa, and Manuel Belmontec*aK. U. Leuven, Department of Mechanical Engineering, Celestijnenlaan 300B, B-3001 Leuven, BelgiumbK. U. Leuven, Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, B-3001 Leuven, BelgiumcInstitute of Ceramics and Glass (ICV-CSIC), Campus de Cantoblanco, Kelsen 5, 28049 Madrid, Spain
*E-mail: mbelmonte@icv.csic.es
Diamond grinding is commonly used for machining hard ceramic
materials, but the development of ceramic microcomponents with
complex geometries requires alternative machining processes.
Through a thermoelectric process, electrical discharge machining
(EDM) enables the low cost manufacture of intricate parts
that require a high degree of flexibility of the shape1. The EDM
process requires sufficient electrical conductivity of the materials
(> 0.3 – 1 Sm-1)2, which are eroded by melting, evaporation,
chemical interaction, and/or spalling due to discrete sparks at a
very high frequency between the material (workpiece) and the tool
electrode, which are both submerged in a dielectric liquid.
The potential use of silicon nitride, Si3N4, (the most promising high-
temperature structural ceramic, commonly employed in engineering
parts3-5) in new, emerging applications, depends on our ability to
machine them. Undoped Si3N4 cannot be electrical discharge machined
due to its insulating properties. Muttamara et al.6 proposed the use of
an assisting electrode in the form of an electrically conductive coating,
applied by means of chemical vapor deposition or colloidal processing.
However, a very low material removal rate and extremely high tool wear
were reported. The addition of large amounts of electrically conductive
secondary phases, typically 30 – 40 vol.% of TiN7,8, to enhance the
electrical conductivity (σ) of the composite through the development
Carbon nanofillers for machining insulating ceramics CURRENT RESEARCH
OCTOBER 2011 | VOLUME 14 | NUMBER 10 497
of a conductive percolating network within the matrix is the most
common method of making EDM possible. Nevertheless, the material
removal mechanism leads to a foamy surface structure with a high
roughness.
Carbon nanotubes (CNTs) have attracted a great deal of attention
since their discovery9 due to their exceptional properties10-12; this
attention includes their use as nanofillers in ceramic materials in
order to develop tougher composites13. While the contribution of
CNTs as a reinforcement phase in ceramics is still under debate14,
other characteristics have been considerably enhanced, such as the
tribological15,16 and the electrical properties17,18. In addition, an
increase of the grindability of Si3N4/CNT composites compared to the
monolithic material has also been demonstrated19. However, to our
knowledge, the successful electrical discharge machining of CNT ceramic
composites has not been reported before. Such a demonstration would
be of importance, as EDM is a unique method for the machining of
intricate microcomponents from ceramics, if they have sufficiently high
electrical conductivity.
Ceramic/CNT composites can exhibit increases in conductivity,
σ, of ~13 orders of magnitude compared to the monolithic material,
changing from highly insulating to conducting once the CNT network
percolation is achieved17-23, although the final electrical properties can
vary with the CNT type, purity, or composite processing. In the case of
Si3N4/CNTs nanocomposites, σ values from ~1021,23,24 to 9225 Sm-1
were attained for multi-walled and single-walled CNTs, respectively,
for CNTs contents up to 6 vol.%. As the percolation threshold (pc)
depends on the degree of filler dispersion within the matrix, a wide
range of pc values ranging from 0.6 to 6.0 vol.% CNTs were reported
for ceramic based composites20,22,23,25. The present authors analyzed
the electrical properties of Si3N4/multi-walled carbon nanotube
(MWCNT) nanocomposites with various MWCNT contents in dc and
ac conditions for a wide temperature range23. A MWCNT pc < 0.9
vol.% was obtained, providing evidence for the excellent dispersion
of the nanotubes within the matrix, although for this low CNT
content σ was too small (4 × 10-2 Sm-1) for EDM. When the MWCNT
content was increased to 5.3 vol.%, a σ value of 14 Sm-1 was reached,
corresponding to an increase of 13 orders of magnitude over the
monolithic material. According to this value, Si3N4 composites could
potentially be machined by EDM, using MWCNTs as fillers due to their
lower manufacturing costs compared to single-walled CNTs.
In this work, we used dense discs of Si3N4/5.3 vol.% MWCNTs with
an α:β Si3N4 phase ratio of 40:60 and an average matrix grain size of
~240 nm as the workpiece. The nanocomposite presents a homogeneous
dispersion of the nanotubes within the ceramic matrix (Fig. 1) without
signs of MWCNTs degradation16,23. For comparison purposes, dense
Si3N4/40 vol.% TiN composites were tested as a reference material
under the same EDM conditions. Square pockets (5 × 5 mm2) and lines
(7 × 0.5 mm2) were machined (Figs. 2a-c) using micro EDM milling tests
Fig. 1 FESEM (a) and TEM (b) micrographs of the fracture surface of Si3N4/MWCNTs nanocomposite. The arrows in (b) point to the MWCNTs.
(b)(a)
Fig. 2 (a) Optical micrograph of square pockets (5 × 5 mm2) and lines (7 × 0.5 mm2) EDM milled at different voltages and (b,c) FESEM micrographs of the machined lines at 100 V.
(b)
(a)
(c)
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OCTOBER 2011 | VOLUME 14 | NUMBER 10498
at different discharge energy levels, which are expressed in terms of open
circuit voltage26 and varied between 60 and 160 V.
The evolution of the material removal rate (MRR) and the relative
tool wear (RTW) with the applied voltage are collected in Fig. 3. Note
that the MRR (Fig. 3a) for the CNTs nanocomposite continuously
increases with the voltage up to 120 V, where a maximum MRR
value of 1.27 mm3 min-1 is attained, slightly decreasing as discharge
energy increases further. Although the reference Si3N4/TiN material
exhibits a similar MRR trend with voltage, the removal rate values are
half of those of the CNT nanocomposite. In addition, when the tool
acts as an electrode for machining the reference material, the RTW is
approximately three times higher (Fig. 3b) than that measured for the
Si3N4/CNTs workpiece (RTW ~2 – 3 %).
An important parameter in all EDM die sinking and milling
processes is the surface roughness of the machined material. To gain
more information about the topography of the machined surfaces,
measurements of the 2D (Ra) and 3D (Sa) roughness average, as well as
the 3D peak-peak height (St), were performed (Fig. 4). The machined
Si3N4/CNTs specimens show a very smooth surface, with an Ra value of
0.2 µm that was almost independent of the discharge energy (Fig. 4a),
whereas the reference material was much rougher, with Ra up to four
times higher than for the CNTs nanocomposite. These differences in
the quality of the machined surfaces were confirmed after measuring
the 3D characteristics. In this sense, the reference material exhibited Sa
(Fig. 4b) and St (Fig. 4c) values of about three times higher than those
for the CNTs nanocomposite, which had an excellent finished surface
with Sa and St below 0.4 µm and 2.8 µm, respectively. This distinct
surface topography is clear when the 3D images of the machined
surfaces are compared (Fig. 5).
Therefore, it seems clear from these results that Si3N4/CNTs
nanocomposites can be machined by means EDM, and surprisingly, that
the performance is outstanding, and much better than that obtained
for a commonly used reference composition.
Field emission scanning electron microscopy (FESEM) images of
the Si3N4/CNTs surfaces machined under different EDM conditions
are shown in Fig. 6. The surface topography of the machined
nanocomposite at 80 V (Figs. 6a,b) is a recast layer with discharge
craters of ~ 5 µm (Fig. 6b). That layer is produced by the decomposition
of Si3N4 into silicon and nitrogen gas at temperatures above 1700 °C7.
When the discharge energy level increases up to 140 V (Figs. 6c,d), the
(a)
Fig. 4 (a) 2D-surface roughness, Ra, (b) 3D-surface roughness, Sa, and (c) 3D peak-peak height, St, versus the applied voltage.
Fig. 3 (a) Material removal rate, MRR, and (b) relative tool wear, RTW, as a function of the applied voltage.
(b)
(a)
(b)
(c)
Carbon nanofillers for machining insulating ceramics CURRENT RESEARCH
OCTOBER 2011 | VOLUME 14 | NUMBER 10 499
Si3N4 decomposition is enhanced and the crater diameters are almost
three times larger than at 80 V, leading to a slight increase in the
S parameters (Figs. 4b,c). The bright white spots in the FESEM images
correspond to tungsten particles originating from the worn electrode, as
confirmed by x-ray energy dispersive spectroscopy (EDX). The mildest
EDM conditions at 80 V allow the survival of numerous MWCNTs
(Fig. 6e), whereas the nanotubes are hardly identified on the machined
surface at 140 V. Finally, the EDX analysis of all machined surfaces
proves the absence of nanocomposite oxidation, even at 140 V, mainly
due to the use of oil instead of a water-based dielectric fluid.
Raman spectroscopy was used to analyze both the integrity of
MWCNTs and the recast layer of the machined surfaces (Fig. 7).
For all discharge energy conditions, the Raman spectra (Fig. 7a)
present the three characteristic peaks of MWCNTs27: (i) the D-band
at about 1350 cm-1 that corresponds to defects in MWCNTs; (ii)
the G-band at about 1580 cm-1 that provides an indication of the
MWCNTs crystallinity; and (iii) the G’-band at about 2700 cm-1 that
is associated with second-order Raman scattering. The intensity ratio
between the D and G bands (ID/IG) is commonly used as a tool to
quantify the crystallinity of the CNTs. An increase of the ID/IG value
is indicative of an increased number of defects in the nanotubes and,
therefore, of higher degradation. Fig. 7b shows the ID/IG values versus
the voltage acquired within the trenches. Up to 100 V, ID/IG values
slightly increase (~0.85) compared to the un-machined surface (0.73),
providing evidence that the nanotubes are somewhat damaged on
the surface; probably due to the high temperature achieved during
electrical discharge machining. However, MWCNTs still maintain
part of their integrity, as can be observed in Fig. 6e. As the voltage
increases, the MWCNTs degradation becomes larger, reaching average
ID/IG values of 1.08 and 1.14 at 120 and 140 V, respectively; both with
higher ID/IG deviation than for the case of lower voltages. Under these
severe EDM conditions the nanotubes are extensively damaged and
graphitized.
The Raman spectra also show a sharp peak at ~520 cm-1, which
increases in intensity with the discharge energy. In addition, a low
intensity broad band at ~960 cm-1 that is more pronounced at higher
voltages is observed. Both peaks correspond to the first-order and the
second-order Raman scattering, respectively, for crystalline silicon28
which is presumably formed after the decomposition of Si3N47. The 2D
surface topography images at 100 and 140 V of an area of 20 × 20 µm2
(Fig. 7c) show the distribution of the D and G bands of MWCNTs
(green and blue zones, respectively) as well as the 520 cm-1 band of
Si (black zones). Light blue areas correspond to a mixture between D
and G bands, typical of CNTs. An increase in the discharge energy from
100 to 140 V produces a clear reduction of blue areas (G band) as a
consequence of the graphitization of the MWCNTs, and an increment of
Si rich area due to a larger decomposition of Si3N4.
Fig. 5 3D surface images of the machined surfaces at 100 V for (a) the CNTs nanocomposite and (b) the reference material.
Fig. 6 FESEM micrographs of Si3N4/MWCNTs nanocomposite surfaces machined at: 80 V (a) and (b), and 140 V (c) and (d). (e) MWCNTs protruding from the tested surface at 80 V are observed.
(a) (b)
(a) (b)
(c) (d)
(e)
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CURRENT RESEARCH Carbon nanofillers for machining insulating ceramics
Therefore, considering the above results from EDM responses and
the machined surface characterization, 100 V is considered the most
suitable discharge energy for machining the MWCNTs nanocomposite
because it allows combining a high MMR, low RTW, excellent surface
finishing, and limited nanotubes degradation. Based on this, a more
complex structure like a gear wheel is performed by EDM at 100 V
(Fig. 8). The gear dimensions are a diameter of 3.55 mm and a
thickness of 1.95 mm with a circular centre bore diameter of 1.0 mm.
At present, it is difficult to estimate the economical benefits that
EDM of CNT doped hard insulating ceramics can achieve compared
to classical machining processes. Nevertheless, the huge reduction
in the machining time in conjunction with the excellent workpiece
surface finishing attained by EDM would compensate the high CNT
cost, bearing in mind that EDM would allow the creation of intricate
components only achievable through this process. Respecting the safety
considerations linked to the use of CNTs, special care must be taken
in the processing steps until the full densification of the composite is
completed. Then, the nanotubes will be confined within the specimen
and the material removed during EDM will mostly be decomposed.
Finally, it should be mentioned that the addition of small amounts
of CNTs also affects other ceramic properties. In this way, the present
Si3N4/5.3 vol.% MWCNTs nanocomposite exhibited an enhanced
tribological performance compared to the blank material containing
the same α/β-phase ratio16, increasing the wear resistance by more
than 50 % and reducing the friction about 30 %. In contrast, the CNTs
produced a deleterious effect (~35 % decrease) in the mechanical
properties of the nanocomposite; mainly the elastic modulus (E),
hardness (H), and fracture toughness (KIC), with respect to the
monolithic material29, mostly due to a poor load transfer between the
matrix and the nanotubes. However, we have demonstrated that the
mechanical interlocking between the Si3N4 grains and the nanotubes
can be strengthened by functionalizing the pristine MWCNTs by means
of oxidation and boron nitride and silicon oxide coatings29. The new
nanocomposites showed enhanced mechanical properties with E, H,
and KIC values close to or even higher (KIC) than those measured for
undoped Si3N429. Recent studies of the present authors (unpublished
data) have provided evidence for a slight decrease in the thermal
diffusivity from 0.088 to 0.075 cm2s-1 with the addition of CNTs, which
can be due to both the thermal resistance within the nanotubes and at
the CNTs-Si3N4 interface and, also to the lower matrix grain size.
ConclusionsWe show the feasibility of EDM of Si3N4 by introducing small amounts
of CNTs to create an electrical conductive network within the ceramic.
This approach could also be extended to other insulating ceramics
such as alumina or zirconia, and different carbon nanostructures like
graphenes. This work demonstrates that the EDM performance of
Si3N4/CNTs nanocomposites in terms of MRR, surface roughness, and
RTW is significantly enhanced compared to that of Si3N4/TiN materials,
commonly considered the reference ceramics for EDM. Intricate parts
with good mechanical and tribological properties as required for
emerging applications such as microturbines, MEMS, microreactors, and
microbioimplants could be produced by electrical discharge machining
of Si3N4/CNTs.
Materials and methodsMaterials
Si3N4/MWCNTs nanocomposites containing 5.3 vol.% of commercial
nanotubes (30 nm diameter and 1 – 5 mm length, Nanolab Inc., USA)
were prepared as described elsewhere30. In summary, both MWCNTs
and ceramic powders (Si3N4 plus 2 wt.% of Al2O3 and 5 wt.% of Y2O3)
were separately dispersed in ethanol and mixed in an ultrasonic bath
under continuous stirring. Disc shaped samples measuring 3 mm thick
and 20 mm in diameter were fully densified using the spark plasma
sintering technique (SPS) at 1585 ºC for 5 minutes. This temperature
Fig. 8 Optical images of the electrical discharge machined Si3N4/MWCNTs microgear at 100 V.
Fig. 7 (a) Micro-Raman spectra and (b) intensity ratio between D and G bands, ID/IG, of Si3N4/MWCNTs nanocomposite surfaces machined using different applied voltages; (c) 2D surface topography at 100 V and (d) 140 V of the corresponding D (green), G (blue) and Si (black) Raman bands. The arrows in (c) point to the G rich areas.
(a) (b)
(c) (d)
(a) (b)
Carbon nanofillers for machining insulating ceramics CURRENT RESEARCH
OCTOBER 2011 | VOLUME 14 | NUMBER 10 501
was selected to provide a similar α:β Si3N4 phase ratio (40:60) as the
monolithic material, which was typically sintered at 1600 ºC. SPS
tests were carried out using a heating rate of 133 ºC min-1, a vacuum
atmosphere of 6 Pa, and applying a uniaxial pressure of 50 MPa. The
average matrix grain size of the nanocomposite was ~240 nm, slightly
lower than the monolithic (300 nm), confirming that CNTs produced
some matrix refinement during the liquid phase sintering process31.
The Si3N4/40 vol.% TiN reference material was produced by mixing
56.4 vol.% Si3N4, 40 vol.% TiN, 2.4 vol.% Y2O3 , and 1.2 vol.% Al2O3
in ethanol using low energy multidirectional mixing. After mixing, the
suspension was dried in a rotating evaporator at 65 °C. Disc shaped
samples measuring 5 mm thick and 40 mm in diameter were fully
densified using SPS at 1800 °C for 1.5 min, with a heating rate of
100 ºC min-1, under a vacuum atmosphere of 4 Pa, and applying a
uniaxial pressure of 60 MPa. More information on the experimental
SPS procedure is provided elsewhere32. The dense composite was
constituted by the following α-Si3N4:β-Si3N4:TiN crystalline phases
ratio in wt.%: 34.6:8.0:57.3, with an average Si3N4 and TiN grain size of
~2 µm and ~1 µm, respectively.
EDM tests
A four-axis tool was used for EDM tests. The electrical settings for
the machining process were: frequency of 120 kHz, gap of 60 µm,
pulse width of 2 µs, gain of 150, negative polarity, an increment
of 5 µm, discharge current index of 50, spark gap/side of 10 µm,
and a voltage range from 60 to 160 V. The equipment used a
relaxation type generator, able to produce low energy discharge
pulses (te = 20 ns, ie < 0.5 A), aimed at micro-scale manufacturing.
Hydrocarbon oil (HEDMA 111) with a viscosity of 2.4 cSt at room
temperature was used as the dielectric fluid. The tool was a WC-Co
cylindrical rod (SE02, Sarix S.A., Switzerland) with a diameter of 0.5 mm.
Materials characterization
Roughness measurements of the electrical discharge machined surfaces
were performed using both a surface profiler over a length of 7 mm
(cutoff lengths Lc = 0.7 mm, Ls = 0.0025 mm) and a white light
interferometer. The microstructure was investigated by FESEM and
transmission electron microscopy (TEM). Point and area microanalysis
were performed using EDX. Micro-Raman spectra were taken to
study both the possible degradation of MWCNTs and the recast layer
composition with a laser wavelength of 514 nm.
AcknowledgementsThanks are given to C. Ramírez (ICV, Spain) for help with micro-
Raman tests and Annabel Braem for assistance with the white light
interferometer. We gratefully acknowledge the financial support of the
Spanish Ministry of Science and Innovation (MICINN) and the Fund for
Scientific Research Flanders (FWO) through projects MAT2009-09600
and G.0539.08. J. González-Julián thanks the financial support of the JAE
(CSIC) fellowship program.
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Instrument citation
Spark plasma sintering technique, SPS-510CE, Syntex and HP D25/1, FCT
Turbula, T2A, WAB
EDM tool, SARIX SX-100-HPM, SARIX S. A.
Profilometer, Form Talysurf-120L, Taylor-Hobson
White light interferometer, NT3300, Wyko
Field emission scanning electron microscope, S-4700, Hitachi
Transmission electron microscope, JEM-4000 EX, Jeol
X-ray energy dispersive spectroscope, Thermo Noran, System SIX
Micro-Raman spectroscope, inVia and System 1000, Renishaw