Carbon nanofillers for machining insulating ceramics

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: [email protected]

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

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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)

CURRENT RESEARCH Carbon nanofillers for machining insulating ceramics

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)



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)

OCTOBER 2011 | VOLUME 14 | NUMBER 10500

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)



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

Carbon nanofillers for machining insulating ceramics

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