Mehdi Mahmoodie-mail: mahmoom@ucalgary.ca

M. G. Mostofae-mail: mgmostof@ucalgary.ca

University of Calgary,

2500 University Drive, NW, Calgary,

Alberta T2N1N4, Canada

Martin JunUniversity of Victoria,

3800 Finnerty Road, Victoria V8P5C2,

British Columbia, Canada

e-mail: mbgjun@uvic.ca

Simon S. Park1

University of Calgary,

2500 University Drive, NW, Calgary,

Alberta T2N1N4, Canada

e-mail: simon.park@ucalgary.ca

Characterization andMicromilling of Flow InducedAligned Carbon NanotubeNanocompositesCarbon nanotube (CNT) based polymeric composites exhibit high strength and thermalconductivity and can be electrically conductive at a low percolation threshold. CNTnanocomposites with polystyrene (PS) thermoplastic matrix were injection-molded andhigh shear stress in the flow direction enabled partial alignment of the CNTs. Thesamples with different CNT concentrations were prepared to study the effect of CNTconcentration on the cutting behavior of the samples. Characterizations of CNT polymercomposites were studied to relate different characteristics of materials such as thermalconductivity and mechanical properties to micromachining. Micro-end milling wasperformed to understand the material removal behavior of CNT nanocomposites. It wasfound that CNT alignment and concentrations influenced the cutting forces. Themechanistic micromilling force model was used to predict the cutting forces. The forcemodel has been verified with the experimental milling forces. The machinability of theCNT nanocomposites was better than that of pure polymer due to the improved thermalconductivity and mechanical characteristics. [DOI: 10.1115/1.4023290]

Keywords: nanomanufacturing, injection molding, carbon nanotubes, composite, micro-milling, characterization

1 Introduction

CNTs possess exceptional mechanical, thermal, and electricalproperties, making them suitable fillers for polymers [1–3]. Theirlarge surface area per unit of volume compared with other fiberfillers leads to larger filler/polymer interfacial areas than conven-tional fiber-reinforced composites. In addition, since nanotubeshave high aspect ratios, small loadings of CNTs can enhance elec-trical conductivity of the polymeric composite up to several ordersof magnitude [4]. Compared to carbon black or carbon fibers,CNTs show lower percolation threshold, which is defined as thecritical value at which the conductive network begins to forminside the polymer matrix, leads to a sharp decrease in the electri-cal resistivity of the composite.

The mechanical and electrical behaviors of the CNT/polymernanocomposites strongly depend on the dispersion and alignmentof the nanotubes inside the microstructure of the polymer. Highlyordered CNTs are useful for many applications, such as field emis-sion displays and sensors, data storage, and light emitters [5].Accordingly, considerable attention has been devoted to CNTalignment in polymer matrices. Several techniques have been uti-lized to align CNTs in thermoplastics, such as ex situ alignment,force field induced alignment, magnetic field induced alignment,and electrospinning induced alignment [5]. In this study, we uti-lized flow induced alignment by applying an intensive shear forceto the CNT/polymer system through the injection molding process[6]. Moreover, injection molding enables the manufacture of cost-effective polymeric components with a very short production cycle.

In order to fabricate components without disturbing the align-ment of CNTs, we employed the micro-end milling process,where a miniature tool was used to mechanically remove materi-als. Several researchers have investigated the machining of

fiber-reinforced polymer composites (FRPCs) and determined thatthe alignment of fibers can significantly affect the cutting forcesand burr formation [7,8]. While CNT-reinforced nanocompositeshave attracted considerable attention in industry and academia,investigations into the machining characteristics of these materialshave been very limited [9,10].

The objectives of this study are the examination of the charac-teristics and micromachining of flow induced aligned multiwalledcarbon nanotube (MWCNT) nanocomposites with differentMWCNT loadings. In this study, we utilized MWCNTs as fillers,since they are reportedly always electrically conductive, lower incost and more available, making them attractive for fabricatingconductive polymer based nanocomposites. Amorphous PS wasused as the polymer matrix.

A thin-walled injection mold was designed and fabricated toinduce large shear flow, and the aligned MWCNT/PS nanocompo-sites were prepared by adjusting the injection molding processingconditions. Transmission electron microscopy (TEM), scanningelectron microscopy (SEM), thermal conductivity, mechanicaltensile, and electrical resistivity tests were performed to character-ize the nanocomposites.

We examined the effect of CNT alignment and concentrationsin the cutting behavior through the micromilling process. We mod-eled micromilling forces using a mechanistic approach, where weidentified the mechanistic cutting parameters with different CNTorientations. As the CNTs are partially aligned in the polymer ma-trix, the cutting forces will be different in the cross-flow and in-flow directions. Therefore, the mechanistic force model can help topredict the cutting forces which are invaluable in determining theoptimal machining parameters to maintain good surface finishesand tool longevity. With the aid of the injection molding andmicromilling processes, CNT-based nanocomposite componentscan be manufactured with unique functional properties.

The paper is organized as follows: In Sec. 2, experimental set-ups are described for sample preparation and micromilling opera-tions. Section 3 describes the characterization of CNT compositesamples. Section 4 discusses the results obtained from micromil-ling operation and mechanistic modeling.

1Corresponding author.Contributed by the Manufacturing Engineering Division of ASME for publication

in the JOURNAL OF MICRO AND NANO-MANUFACTURING. Manuscript received July 23,2012; final manuscript received December 11, 2012; published online March 22,2013. Assoc. Editor: J. Rhett Mayor.

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2 Experimental Setup

A masterbatch of 20 wt. % MWCNT/PS (Hyperion Catalysis)was diluted to concentrations of 0.5, 2, and 5 wt. % MWCNT/PS.The used MWCNTs typically had an outer diameter of 10–15 nmwrapped around a hollow core; and, their lengths ranged between1 and 10 lm, while their density was approximately 1.75 g/cm3.The pristine PS (Styron 610, Americas Styrenics) had a meltflow index and density of 11 g/10 min (200 �C/5 kg) and0.94–0.96 g/cm3, respectively, and was used to dilute the CNTconcentration using a corotating twin-screw extruder.

2.1. Injection Molding of the CNT-Filled Nanocomposites.The diluted composite pellets were injection molded using amicro-injection molding machine (Boy 12 A) with a screw diame-ter of 18 mm and an aspect ratio of 20 (shown in Fig. 1(a)).A mold equipped with cartridge heaters was designed and manu-factured with a two-degree draft angle. The cavity was rectangularin shape with dimensions of 10� 25� 0.7 mm. The mold wasfed through a trapezoidal runner and an edge gate as shown inFig. 1(b).

In a previous study, we showed that greater alignment ofMWCNTs can be obtained in thermoplastics through edge gatesrather than fan gates [11]. To obtain a high degree of alignment inthe injection-molded samples, high injection speed and pressurecoupled with a low melt temperature should be applied to themelt. The injection/holding pressure was set to 100 bars, the injec-tion speed was 240 mm/s, and the melt temperature was set at215 �C. The holding and cooling times were each set to 8 s.

2.2 Micro-End Milling of the Molded Samples. Micro-milling experiments were conducted on a vertical CNC millingmachine (Kern Micro 2255), as depicted in Fig. 2. This machinewas equipped with a hybrid ball bearing spindle that rotates up to160,000 rev/min. The runout of the end mill was measured to beapproximately 1.8 lm using a capacitance sensor (Lion PrecisionDMT20).

To measure the cutting forces during the microcutting experi-ments, a miniature piezoelectric table dynamometer (Kistler9256C2) was used. A force gauge (Omega DFG51-2) was used toverify the static force measurement of the table dynamometer.The machine base was polymer concrete, which dampens outexternal vibrations. Both the measured forces from the dynamom-eter and acoustic emission (AE) signals were passed through anti-aliasing filters (Krohn-Hite 3364) and acquired through a dataacquisition system (NI cDAQ-9172) at 50 kHz sampling rate.

The molded nanocomposite samples were glued on top of aninterface plate screwed to the table dynamometer. The top surfa-ces of the samples were machined flat. The microtools used in thisstudy were coated tungsten carbide (WC) microflat end mills(SECO JM 905) with a diameter of 475 lm and a helix of 30 deg.SEM images were taken from the tools to measure the cuttingedge radius. Using IMAGEJ

TM software, edge radius was found tobe approximately 2 lm. The tool overhang length was 15 mmfrom the collet and was kept constant during all the experiments,in order to keep the tool dynamics the same. A capacitance sensor(Lion Precision DMT20) was used to identify the flute locationwith respect to the rotational angle.

The cutting forces were measured in two different directions(in-flow and cross-flow) of the nanocomposite samples at differentfeed rates. The spindle speed was kept constant for 60,000 rpmand 200 lm axial depth of cut was applied. To detect the zeropoint in Z direction, an AE sensor (Physical Acoustics Nano30)was used. The zero point was found by moving the rotating tooldown very slowly and observing changes in the AE signals [12].The rotating speed of 60,000 rpm has been selected due to thebandwidth of the force dynamometer. The axial depth of cut of200 lm has been selected due to the inherent design of cuttingtools and also to obtain sufficient cutting forces. All the tests wereperformed at room temperature without the use of any coolant.

3 Characterization of CNT/PS Composites

The alignment of the CNTs in the polymer matrix is importantfor a variety of applications. Exerting shear force to a melt in theinjection molding process can partially align CNTs parallel tothe flow direction [6,11]. However, the degree of alignment in theinjection-molded parts strongly depends on the injection moldingconditions, mold geometry and base polymer.

Figure 3 illustrates the TEM image from the microtomed sam-ple. As this figure shows, MWCNTs were more aligned in theflow direction along their length axis, due to the effect of the shearflow in injection molding. However, considering the high aspectratio and entanglement of the carbon nanotubes, some of themremained transverse to the flow direction.

The alignment of MWCNTs in a polymer matrix influences theelectrical conductivity of the composite. The chance of CNT–CNTcontact decreases when shear flow partially aligns nanotubes in-flow direction, while random distribution of CNTs in polymer ma-trix provides a better conducting path for the electrons to passthrough the nanocomposites, results to increase in electrical con-ductivity [11]. Therefore, the volume resistivity of the MWCNT/PS

Fig. 1 (a) Injection molding setup and (b) schematic of thedesigned mold

Fig. 2 Micro-end milling setup

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composites was measured using a high-resistance electrometer(Keithley 6517 A) connected to a test fixture. Results from volumeresistivity test are shown in Fig. 4 for both the injection-moldedsamples, featuring partial alignment and compression molded sam-ples featuring randomly aligned CNTs in PS matrix. To betterunderstand the effect of CNT loading on the volume resistivity ofthe nanocomposites, the results are shown for 0.1, 0.5, 2, 5, 10, and20 wt. % MWCNT. The electrical resistivity of the molded nano-composites decreased when the MWCNT loading increased. Inter-estingly, a significant difference between the volume resistivity ofthe injection and compression molded samples can be observedwhich as it was mentioned before, it could be related to the increasein the CNT–CNT distance in injection-molded samples due tonanotubes alignment parallel to the flow direction [6,11].

Adding CNTs to polymeric samples can improve their thermalconductivity, and significantly influence their machinability dueto the minimization of melting of the polymer at high machiningspeeds. The thermal conductivities of the molded samples weremeasured in both parallel and perpendicular to the flow directionaccording to ASTM D5470 and results are shown in Fig. 5.Details of the thermal conductivity measurements can be foundelsewhere [13]. Increases in CNT loadings led to increase in thethermal conductivity of the molded samples in both the paralleland perpendicular to the flow directions. Interestingly, the meas-urements show higher values of thermal conductivity in the direc-tion of flow which could be attributed to the effective contributionof MWCNTs in transferring the heat along their axis. Anisotropicthermal conductivity in aligned CNT nanocomposites has beenreported by the previous researchers [14–16]. For 5 wt. % CNTloading, thermal conductivity increased by three and two times inthe parallel and perpendicular to the flow direction, respectively.

However, the increased thermal conductivity values of the nano-composites caused by adding nanotubes, are below the predictedtheoretical values due to the large interfacial thermal resistancebetween the MWCNTs and PS matrix [2,17].

Micromechanical tensile testing was performed on the moldedspecimens using a tensile tester (Bose Electroforce 3330). As thealignment of the MWCNTs in the polymer matrix stronglydepends on the injection molding conditions and mold geometry[11], the samples produced with the same processing conditionsand thickness were used for tensile testing according to ASTMD638. The samples were machined to obtain dog-bone-shapedspecimens prior to tensile testing (Fig. 6). Strain–stress behaviorof the prepared specimens under tensile test is shown in Fig. 6.Results showed that the strain at the failure occurred at 5.74%,5.77%, 4.60%, and 3.41% for plain, 0.5, 2, and 5 wt. % MWCNT/PS samples, respectively. Ultimate tensile strength was observedto be 53.3, 57.4, 48.83, and 37.66 MPa for plain PS, 0.5, 2, and5 wt. % CNT loading, respectively. Samples with higher concen-tration of MWCNT/PS showed a slight increase in the elasticmodulus but became very brittle. This could be due to inhomoge-neous distribution of MWCNTs in the polymer matrix giving riseto high strain localization in the composite. In addition, extrememodulus mismatch between the MWCNTs and polymer matrixcan lead to high strain localizations, resulting to crack propagationthrough aggregates of CNTs in the polymer matrix [18].

Characterization results of injection-molded nanocompositesshow that carbon nanotubes are partially aligned in the flow direc-tion. Thermal conductivity of the molded nanocomposites shows

Fig. 3 TEM image of the 2 wt. % MWCNT/PS composite

Fig. 4 Volume resistivity of the injection-molded nanocompo-sites and comparison with compression molded samples

Fig. 5 Thermal conductivity of the injection-molded samplesmeasured in parallel and perpendicular to the flow direction

Fig. 6 Tensile stress–strain curves of the samples under strainrate of 5 mm/min

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anisotropic properties. While elastic modulus of the nanocompo-sites has not changed noticeably, brittleness of the nanocompo-sites is changed significantly by adding MWCNTs to the PSmatrix. The next section of this work explores the micromechani-cal machining of the molded composites by considering the orien-tation of MWCNTs in the PS matrix. The machining tests willhelp understanding the implications of the presence of MWCNTson the micromilling of polymeric composites.

4 Micro-End Milling of CNT-Filled Nanocomposites

Micromilling experiments were performed both perpendicularand parallel to the flow direction, and the measured radial and tan-gential forces were compared. The fiber orientation angle, h, isnot constant and changes continuously depending on the rotationalposition of the flute for milling. In the milling process, chip thick-ness also varies with respect to the rotational angle and feed rate.Considering Fig. 7, the radial (Fr) and tangential (Ft) forcesexerted on the tool are defined based on the following equations:

Fr ¼ �FX sin /� FY cos / (1)

Ft ¼ �FX cos /þ FY sin / (2)

where FX are the forces acting in the feed direction, FY are theforces acting in the normal to the feed direction, and / is the rota-tion angle. The fiber orientation angle, h, can be defined as

h ¼ w� / for / � w; h ¼ pþ ðw� /Þ for / > w (3)

where w is the orientation angle of the MWCNTs, with respect tothe feed direction.

In order to predict the micromilling forces for the CNT nano-composites, we utilized the mechanistic microforce model basedon the previous work [19]. However, there are several challengesfor the modeling of CNT-based composites, since the material isnot isotropic and forces are highly dependent on the orientation ofthe CNTs. The CNT concentration also affects the overall forceprofiles, due to thermal conductivity and material brittleness. Fur-thermore, the elastic recovery is high compared to metallic alloys.The cutting coefficients can be identified, with respect to the ori-entation and concentration of CNTs.

The radial and tangential forces that act on the stagnant point ofthe cutting tool during the shearing and ploughing regimes areshown in Eq. (4), where the cutting edge and ploughing coeffi-cients are functions of the orientation angle (w), with respect tothe feed direction [19].

dFt ¼ðKtcðwÞhþ KteðwÞÞdz

ðKtpðwÞAp þ KteðwÞÞdz

(when h � hcðshearingÞwhen h < hcðploughingÞ

dFr ¼ðKrcðwÞhþ KreðwÞÞdz

ðKrpðwÞAp þ KreðwÞÞdz

(when h � hcðshearingÞwhen h < hcðploughingÞ

(4)

where Ktc is the tangential cutting coefficient, Kte is the tangentialedge coefficient, Ktp is the tangential ploughing coefficient, Krc isthe radial cutting coefficient, Kre is the radial edge coefficient, Krp

is the radial ploughing coefficient, h is the chip thickness, and Ap isthe ploughed area. Ktc and Krc are the shearing coefficients, and Kte

and Kre are the edge coefficients. The mechanistic model is de-pendent on the elastic recovery and the minimum uncut chip thick-ness (MUCT), hc. The pile-up and thermal effects have not beenconsidered in the model and they are subjects for future study.

In order to identify the elastic recovery, a conical scratchingtool with an apex angle of 90 deg and edge radius of 15 lm wasused [20]. The elastic recovery ratio, her, has been found to varydepending on CNT concentrations. It was observed that, when thefeed rates and depth of cuts increased, the elastic recovery gradu-ally decreased, which may have been due to the strain hardeningand large plastic deformation. At a higher CNT concentration theelastic recovery decreased, due to the material becoming morebrittle. The elastic recovery ratio was approximately 29%, 26%,24%, and 9.9% for 0, 0.5, 2.0, and 5.0 wt. % MWCNT/PS. Adecrease in elastic recovery caused by an increase in CNT loadinghas also been reported in the previous work [21].

The MUCT, hc, was approximated by the minimum energymethod [22] to determine the transition between the ploughingand shearing dominant cutting regimes. Orthogonal cutting testswere performed by using an orthogonal cutting tool with a rakeangle of 0 deg to determine the friction angle (bs), which equatesapproximately to the stagnant angle [20]. Equation (5) was usedto approximate the MUCT, and the stagnant angle (vm) was foundto be approximately 42 deg.

hc ¼ reð1� cos vmÞ where vm � bs (5)

The cutting coefficients were identified by optimizing the instanta-neous forces [19]. The cutting coefficients found for the plain PSand CNT-loaded nanocomposites are shown in Table 1 for both 0deg and 90 deg orientation angles (w).

The cutting coefficients for pure PS are higher than the 0.5 wt.% and 90 deg for 2.0 wt. % CNT loading. One reason for thiscould be the effective role of MWCNTs in transferring heat fromthe machining area and easier formation of chips by addingMWCNTs to the PS matrix. It should be mentioned that the mol-ten PS adhered to the cutting tool during machining of pure PSand this may also lead to an increase in the cutting force. For themolded nanocomposites, no molten PS could be observed on thecutting edges of the tool, mainly due to the higher thermal con-ductivity obtained by addition of the MWCNTs and the increasein brittleness of the material.

By increasing the CNT loading up to 5 wt. %, the interaction ofthe cutting tool with CNTs increases and therefore higher cuttingcoefficients can be observed. Also, the difference in the cuttingcoefficients for 0 deg and 90 deg is due to the orientation ofMWCNTs in the nanocomposite. This has been discussed furtherin this section.

Fig. 7 Schematic of the micromilling of CNT nanocomposites

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The experimental and simulated forces achieved by using theobtained cutting coefficients are shown in Fig. 8 for the in-flowand cross-flow directions. From this figure, a slightly higher tan-gential force was observed for the in-flow cutting (w¼ 0 deg)compared to the cross-flow cutting (w¼ 90 deg). For cutting per-pendicular to the nanotubes’ orientation (i.e., h¼ 90 deg), the tan-gential force reflected the bending and possibly the shearing of theMWCNTs. If the adhesion between CNTs and polymer matrix isinsufficient, CNTs may also be pulled out from the matrix. How-ever, when CNTs are parallel to the cutting (i.e., h¼ 0 deg), thecutting edge causes buckling of the nanotubes, which can result instress concentration at the end of the nanotubes and cracks form-ing ahead of the tool’s cutting edge [9]. The radial forces for thecross-flow and in-flow directions did not show much difference.To verify the cutting coefficients, half immersion (single flute cut-ting) milling operation was conducted and the experimentalresults were compared with the simulation results. The compari-son between the simulation and experimental results are shown inFig. 9. The deviation observed was minimal and this may comefrom the misalignment of CNTs inside the composite matrix.

Full immersion micromilling tests were also performed for dif-ferent CNT concentrations, in order to examine their resultant

cutting forces, as shown in Fig. 10. By increasing the feed rate, theresultant cutting forces increased due to increase in the chip load.At low feed rates (0.2, 1, and 1.5 lm/flute), slightly higher cuttingforces could be observed for the composites, compared to the purePS. Since ploughing-dominant cutting occurs in micromilling atlow feed rates [19], thermal softening of the pure polymer samplemay occur, due to poor thermal conductivity. For the CNT-loadedcomposites in a ploughing-dominant cutting regime (i.e., low feedrates), excessive heat can be transferred from the machining area tochips preventing formation of polymer gumming [9].

When the feed rate increased, higher resultant cutting forceswere observed for plain PS compared to the 0.5 and 2 wt. %, whileresultant cutting forces are still highest for 5 wt. % CNT/PS nano-composite. This could be due to the increase in thermal conductiv-ity and brittleness of the nanocomposites, which can help in theformation of cracks in front of the tool. However, when the CNTconcentrations were increased to 5 wt. %, the cutting forcesincreased slightly due to the increase in the interactions betweenthe cutting edge and CNTs [23]. High forces occurring for purepolymer at very high feed rates may be due to melting of the poly-mer as shown in Fig. 11(a). Whereas, for the 5 wt. % MWCNT/PScomposites, the machined floor surfaces and edges were free from

Table 1 Different cutting coefficients (cutting, edge, and ploughing) obtained from the mechanistic force model for differentMWCNT wt. % in PS

CNT wt. % w (deg) Ktc (N/mm2) Krc (N/mm2) Kte (N/mm) Kre (N/mm) Ktp (N/mm3) Krp (N/mm3)

0% 0 359.3 290 0.2 0.56 11800 801290 360 290.9 0.21 0.52 11800 8012

0.5% 0 351 281.9 0.19 0.54 11422 760890 332 266 0.13 0.36 10874 7189

2% 0 378 302.0 0.31 0.46 12100 790090 334 259.4 0.12 0.45 10527 6715

5% 0 434.8 337.7 0.31 0.65 13249 882490 415.9 323.9 0.28 0.61 12612 8362

Fig. 8 Experimental and simulated tangential (Ft) and radial forces (Fr) of full immersioncutting at feed rate of 4 lm/flute for 2% CNT loading

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melting as shown in Fig. 11(b). Tool wear was also observed incutting of high CNT loading nanocomposites (i.e., 5 wt. %), whichmight be due to the excessive interaction between the nanotubesand the cutting edge. High tool wear has also been reported inmachining of carbon fiber-reinforced composites due to the abra-sive nature of fibers [24].

Chips were carefully collected using double sided tapes duringthe micro-end milling operations, and the chip morphology wasinvestigated using an SEM. Figure 12 illustrates the SEM imagesof the chips at different CNT concentrations. Wrinkled chips wereproduced during machining of pure PS sample which couldbe due to the excessive heat and poor thermal conductivity(Fig. 12(a)); whereas, even with a concentration of 0.5 wt. %MWCNT/PS (Fig. 11(b)), this issue was significantly improved.No crack could be seen on the plain PS chips, but for the CNT-filled nanocomposites, cracks were formed. For higher CNT con-centrations, the increase in the brittleness of the material causedthe brittle fracture of chips.

Fig. 9 Comparison of experimental and simulated tangential (Ft) and radial forces (Fr) forhalf immersion at feed rate of 3 lm/flute for 2% CNT loading

Fig. 10 Root-mean square (RMS) values of resultant forceswith respect to the concentration of MWCNT/PS in the in-flowcutting at different feed per tooth (FPT)

Fig. 11 SEM pictures of the machined slots on (a) plain PS and (b) 5 wt. %MWCNT/PS composites at feed rate of 4 lm/tooth

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The machined surfaces of the plain PS and CNT loaded com-posite samples were also investigated using an atomic forcemicroscope (AFM, PSI XE-100) and phase images are shown inFig. 13 for plain PS and 2 wt. % CNT loading composite at FPTof 2 lm/flute. The phase imaging is obtained during scanning ofthe sample in the tapping mode while the set-point ratio was set at0.6. Set-point ratio is defined as the ratio of the set point tappingamplitude, ASP, and amplitude of the cantilever oscillation, A0

[25]. In the tapping mode scanning, the AFM probe is excitedclose to the resonance frequency near the surface and the spring

constant of the cantilever changes due to the electrostatic or Vander Waal’s force [26]. The change in the phase of the cantileverfrequency occurs relative to the driving signal due to the variationof material properties such as material composition and hardness.The phase change (phase leading or phase lagging) information ismapped during the scanning and presented as image information[25]. In the studied composite (CNT–PS), CNT is a hard materialcompared to PS. Harder surfaces result in positive phase shiftsduring the noncontact scanning of AFM compared to softer surfa-ces [25] as shown in Fig. 13. CNTs can be observed in Fig. 13(b)from the phase image, where CNTs (white lines) are mainlyaligned in the flow direction. It was observed that there wereCNTs pulled out from the machined surfaces. The surface finisheswere also measured to obtain Ra (average surface roughness). Theaverage surface roughness for pure PS and 2 wt. % MWCNT com-posite were measured to be 41 and 155 nm, respectively. Thehigher surface roughness for 2 wt. % MWCNT was due to the pro-truded CNTs (could be seen in Fig. 13(b)) on the machined sur-face of the composite.

5 Conclusions

Polymeric MWCNT nanocomposites have attracted a great dealof attention, due to unique material properties, reduction in CNTcosts and the ease of processing. Micromachining of fine featuredmolded nanocomposites is especially vital without disrupting theCNT alignment. In this study, MWCNT/PS composites have beencharacterized and found that high shear flow in the injection mold-ing can induce partial alignment of CNTs [11]. Furthermore,higher CNT content exhibited higher thermal and electrical con-ductivity, but also increased brittleness. The mechanistic micro-milling model was used to predict milling forces. Highertangential forces were observed for the in-flow direction com-pared to those of the cross-flow direction, due to higher interac-tions between the tool and CNTs. The relationship between theCNT concentrations and the cutting coefficients were also studied.The cutting coefficients were higher for the in-flow direction com-pared to the cross-flow direction, which shows consistency withthe measured cutting forces. The effect of the CNT loading on theresultant cutting forces was investigated at different feeds pertooth. Experimental results showed that an increase in feed pertooth can significantly increase the cutting forces in micromilling.Furthermore, high CNT loading can result in a significant increasein cutting forces and tool wear, due to both the interactionbetween the CNTs and cutting edge and the abrasive nature of thenanotubes. The machined surface and chip morphology were stud-ied for pure polystyrene and CNT based composite. The CNTcomposites showed distinctly different characteristics from purepolymers. The machining of CNT composites produced better sur-face quality and smaller sized chips compared to pure polystyrene.From the results obtained in this study, one can optimally comeup with processing parameters for CNT nanocomposites to

Fig. 12 SEM pictures of the chips at a feed rate of 1 lm/flute

Fig. 13 Comparison of AFM scanned phase images (a) purePS and (b) 2 wt. % CNT nanocomposite at feed rate of 2 lm/flute

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achieve the desired surface finish and productivity, which can beused for a variety of novel applications.

Acknowledgment

This research was supported by the Natural Sciences and Engi-neering Research Council of Canada (NSERC). The authorsacknowledge Nova Chemicals and Americas Styrenics who haveprovided the twin-screw extrusion mixing and resins. The authorswould also thank Dr. Michael Schoel for assistance with the SEMimaging.

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