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Processing-of-titanium-single-walled-carbon-nanotube-metal-matrix-composites-by-induction-melting

Processing-of-titanium-single-walled-carbon-nanotube-metal-matrix-composites-by-induction-melting
Processing-of-titanium-single-walled-carbon-nanotube-metal-matrix-composites-by-induction-melting

Processing of Titanium Single-Walled

Carbon Nanotube Metal-Matrix

Composites by the Induction

Melting Method

K.W ILSON,E.V.B ARRERA AND Y.B AYAZITOGLU*

Department of Mechanical Engineering and Materials Science

Rice University,Houston,TX77005,USA

ABSTRACT:Single-walled carbon nanotube(SWCNT)reinforced titanium(Ti)

matrix composites were produced by powder metallurgy(PM)and induction heating

methods.It was found that nickel coating and a fast processing time associated with

the induction heating method enabled nanotubes to survive the high-temperature

(above1950K)processing conditions.The nanotubes were first covered with a nickel

coating via the electroless plating method and then dispersed in the powder by ball

milling.The powder was then cold pressed to make a PM preform which was melted

by induction heating in an argon environment.Scanning electron microscopy(SEM)

and transmission electron microscopy(TEM)observations as well as Raman and

X-ray Photoelectron Spectroscopy verification indicate that the nanotubes survived

the high-temperature melting.Testing demonstrated that the hardness of the com-

posites,which is directly related to strength,is improved with increasing weight

percent of SWCNTs.

KEY WORDS:nanotubes,electroless plating,induction heating,titanium.

INTRODUCTION

T HE DISCOVERY OF single-walled carbon nanotubes(SWCNTs)[1]has made these structures with nanometer dimensions the leading research area for composite materi-als.Theoretical calculations and experimental measurements have both demonstrated that due to their unique structure individual SWCNTs display enhanced electrical,thermal,and mechanical properties[2].Because of these enhanced properties,SWCNTs are excellent additives for composites particularly in polymers[3 6].Titanium,on the other hand,has not been successfully explored as a Ti-SWCNT composite even though other Ti alloys are

*Author to whom correspondence should be addressed.E-mail:bayaz@https://www.sodocs.net/doc/fc1193674.html,

Figures2 7appear in color online:https://www.sodocs.net/doc/fc1193674.html,

Journal of C OMPOSITE M ATERIALS,Vol.44,No.9/20101037

0021-9983/10/091037 12$10.00/0DOI:10.1177/0021998310367263

?The Author(s),2010.Reprints and permissions:

https://www.sodocs.net/doc/fc1193674.html,/journalsPermissions.nav

1038K.W ILSON ET AL. very important for their uses in aerospace,high-temperature,high strength-weight,and biomedical applications.Titanium has low density,high-strength(roughly the strength of steel,yet only half as dense),and an outstanding resistance to corrosion.The unique properties of the SWCNTs make them ideal additives to materials such as titanium for creating composites to use in demanding applications that range from aerospace to chem-ical industrial equipment and offshore oil drilling platforms.The addition of nanotubes to titanium to form a lighter and stronger composite can lead to numerous positive advance-ments in any of these industries.

In this investigation,a Ti-SWCNT metal-matrix composite(MMC)was produced by processing SWCNTs with electromagnetic melting in a titanium matrix.This technique of induction heating the titanium allows for melting of the matrix without compromising the nanotube infrastructure.Several imaging and spectroscopic methods were used to show that nanotubes survived the high-temperature melting procedure.SWCNTs that were prepared with a surface nickel coating by electroless plating were added to titanium up to4.5wt%.While grain size was decreased,a corresponding increase in microhardness was observed that led to a188%increase in Vickers hardness.

EXPERIMENTAL PROCEDURE

Production of the Composite

The purified SWCNTs were a commercial material containing4wt%Fe catalyst(pur-chased from Carbon Nanotechnology,Inc.).These SWNTs were nickel(Ni)coated via the electroless plating method[7 12].A preliminary step in the electroless plating process consisted of functionalizing the tubes with Fluorine(creating F-SWCNTs).Then a mixture of Ti powder($144m m)along with0 4.5wt%of these Ni coated tubes was mixed by15h of ball milling[13 16].

A uniformly dispersed powder was the result of ball milling.These well-mixed final samples were slowly cold pressed under a pressure of100MPa for10min[17 19]The resulting sample size was a cylinder0.635cm in diameter and1.9cm in length weighing1.5g. Originally other methods,such as conventional ovens,were used to heat the samples while still following the same powder metallurgy(PM)process.Attempts were also made to use pristine uncoated SWCNTs.From experimental observations,it was noticed that in conventional convection ovens,which take hours to heat to the melting temperature of titanium,the SWCNTs would react chemically with the titanium destroying the struc-tural integrity of the tubes and producing titanium carbide(TiC).It was also observed that no uncoated nanotubes survived in a titanium environment at elevated temperatures.The thin metal coating around the nanotube was essential.

Since the heating time was a concern,the samples were then melted by a Radyne EI-40 model radio frequency generator,characterized by a400-600A current at a single fre-quency https://www.sodocs.net/doc/fc1193674.html,rge currents at sufficient frequency are known to create an electro-magnetic field strong enough to induce a heating eddy current sufficient to melt metals [20 24]The samples were melted(above the melting point of Ti,$1950K)in an argon atmosphere up to30seconds.Also,a tungsten(W)wire was used in this study to hold the sample because the field strengths of the Radyne EI-40generator were insufficient for levitation which is commonly used in the induction melting method.After cooling,the samples were then broken to reveal fractured surfaces and SEM was applied to analyze

the sample.Some samples were cut with a diamond-tip blade and polished to prepare for Vickers hardness testing and to investigate grain structure.

The SWCNTs were removed from the matrix by selective dissolution [25,26]using hydrofluoric (HF)acid to separate the SWCNTs from the Ti matrix.The SWCNTs were next filtered from the Ti using sub-micron filter paper under vacuum.These SWCNTs were subsequently analyzed by a number of methods.It should be noted that the matrix was completely dissolved in this final step and only carbon nanotubes (CNTs)remained.The SWNTs are typically submicron to microns and tens of microns in length with diameters on the order of $1.4nm.Processing using fluorination leads to individual SWNT’s,however it is likely we also had some ropes present in the samples.The ropes (bundled SWNT’s)have a diameter of 10s of nanometers and can be microns to tens of microns long.

Imaging Analysis

Electron microscopy images were taken of the fractured surfaces and dissolved nano-tubes to determine MMC and CNT morphology.The results can be seen in Figure 1[27].These include examination with both scanning electron microscopy (SEM)and transmis-sion electron microscopy (TEM).The tube-like structures which are seen in these images are nanotubes or nanotube strands.In Figure 1(a)a single SWNT strand can be

observed Figure 1.Electron microscopy results for nickel-coated SWCNTs in a SWCNT-Ti MMC [27].(a)and (b)SEM images of the nanotubes after being collected from the processed Ti matrix.Samples observed were seen following specimen fracture.Thin lines that are curved and coiled on the micrographs are the nanotubes.Thicker lines are multiple CNT fibers also called SWCNT strands,bundles,or ropes.(c)and (d)TEM images at higher resolution depicting CNT morphology of the nanotube residue dissolved from the MMC.Reference bars (a)200nm,(b)100nm,(c)20nm and (d)20nm,respectively .

Processing of Titanium SWCNT Metal-Matrix Composites 1039

overlapping three other strands.In Figure 1(b)a SWNT strand is seen that has made a circular or cylindrical loop.It should be noted that the samples were fractured in a brute-force-type method and for some samples liquid nitrogen was used to make the fracture appear more brittle.

After collecting the SWCNTs from the dissolved Ti matrix,these were imaged at higher resolution afforded by TEM analysis (using the JEOL 2010model).Figure 1(c)and (d)give a detailed view of single nanotubes and CNT strands or fibers.The cylindrical geom-etry of the nanotubes is clearly present after processing.

Spectroscopic Analysis

X-ray photoelectron spectroscopy (XPS)was used to confirm the elemental composition of the final material illustrated in Figure 1(c)and (d).From Figure 2,it can be seen that only carbon (C),Ti,fluorine (F)and oxygen (O)are present.More importantly,the inset of Figure 2shows the existence of graphitic carbon with a C peak At 284eV which agrees with XPS handbook standards [29].If TiC were present the C peak would be 281 282eV according to the same standards.Tungsten is not present,so the interaction of the sample with the tungsten wire used to hold the sample is considered negligible.

Raman spectroscopy was also used as a means of confirmation that a Ti-SWCNT MMC was produced and the CNTs were not compromised in this study.Characterization and comparison of the nanotubes before and after each stage of the production of the MMC and its post-processing to dissolve away the Ti were carried out.This was to establish that the MMC’s CNT components still survived.All data were taken with a Renishaw Raman RM1000Spectrometer (780nm wavelength).

The results can be seen in Figure 3.The data comparison consisted of three stages illustrated in Figure 3(a):pure SWCNTs,Ni coated F-SWCNTs before processing in

I n t e n s i t y (a r b i t r a r y u n i t s )1

0.9

0.8

0.70.60.50.40.3

0.2

0.1

0140012001000800600Binding energy (eV)

4002000286285284283

Figure 2.Full range XPS scan (0 1200eV)of the collected CNT material after processing dissolved Ti from the MMC [28].The inset is a scan in the region of the primary carbon peak (282 286eV)with a peak value at 284eV .Intensity is normalized to the highest peak.

1040K.W ILSON ET AL .

melted Ti,and the recovered SWCNTs including CNT strands after dissolution of the Ti from the MMC as seen in Figure 1(c)and (d).It is important to remember that the latter stage included CNT material that once existed in a melted Ti environment and was not destroyed.

No distinct change is evident in Figure 3(a)between the three stages of comparison that is,the spectra of the Ni-coated SWCNTs before processing in melted Ti and that of

Comparison of SWNT Raman data before and after processing in Ti

500

1000

1500

20002500

3000

3500

0Raman shift (cm –1)

Raman shift (cm –1)

I n t e n s i t y (a)Comparison of SWCNTs after processing

in Ti vs. TiC and Rutile

500100015002000

2500100I n t e n s i t y (a r b i t r a r y u n i t s )(b)3500

30002500200015001000500900

700500

300Figure 3.(a)Raman spectra of the SWCNTs before and after processing the SWCNTs in melted titanium.(b)Raman comparison of commercial TiC,TiO 2with the SWCNTs used in this investigation.

Processing of Titanium SWCNT Metal-Matrix Composites 1041

1042K.W ILSON ET AL. the collected SWCNTs after dissolution from the Ti composite matrix.The existence of a peak below500cmà1(labeled by206.07,207.16,and208.275cmà1)in the Raman spec-tra is indicative of radial breathing modes(RBMs)[30]that characterize resonance excita-tions and vibrational modes of the cylindrical structure of SWCNTs.This in itself demonstrates the existence and presence of SWCNTs.After melting in the titanium, the character of the cylindrical carbon-carbon bond is still in the radial structure which distinguishes it from graphene.

Figure3(b)is a detailed view of the Pure SWCNTS curve in Figure3(a)in the region 100 900cmà1.The SWCNT peak is the same as that in Figure3(a)although the relative intensity has been changed in order to make a comparison with other candidates that materials scientists might argue are possible.Raman spectra were determined for TiC and TiO2using commercially available samples and the results compared with the Pure SWCNTs in Figure3(b).They are obviously not present in the three-stage study shown in Figure3(a).

It can therefore be said that the nanotube geometry is not altered during the processing of SWCNTs in the Ti matrix reported here.When SWCNTs are placed in the Ti powder without a coating or without fast induction heating,and the same procedure is followed, the Raman spectroscopy results reveal only TiC as shown in Figure3(b).This demon-strates that the nanotube structure is destroyed in the heating process without a coating, and in the slow oven heating process.

Grain Structure

The primary motivation of this study was to create a Ti-SWCNT composite and to explore its implications as a new MMC with increased hardness and strength.With that in mind,evaluation of grain structure was necessary.Grain structure evaluations have been studied in PM using titanium as the matrix[31]but not for the material presented here. Hence grain measurements were performed to explore the promise of the composite man-ufactured in this work.

Average grain size measurements were made using the mean lineal intercept method according to the American Society for Testing and Materials(ASTM)standard E112-88. SEM images are required for this method,and accordingly were taken.Figure4shows this information.Figure4(a) (d)indicate that the addition of SWCNTs causes grain growth refinement,which results in a smaller average grain size visible in the SEM images.

The processing of the Ti with SWCNTs occurs in the melt.Nanotubes are positioned such that their distribution is associated with grain growth.Fewer nanotubes mean grains can grow larger.As nanotube concentration increases they pin grains from growing to larger sizes and the grin size is reduced(grain size refinement).This can also occur with solid state powder metallurgy.So even if the Ti was not melted,grain growth can still be stabilized with increasing nanotube concentration.The change in average grain size as a function of concentration of SWCNTs seen in Figure4is quantified in Figure5which was determined using the previously mentioned ASTM standard.As can be seen,the grain size is decreased by the addition of nanotubes[32 34]for the cases examined here.

The line of best fit is shown with the experimental measurements given as a polynomial spline-function interpolation only and does not represent actual data.The error bars in Figure5indicate the variation in the measurements taken to one standard deviation above and below the average value.

Vickers Hardness Analysis

It is well known that hardness is not an intrinsic property of a material,but is an empirical parameter determined by a certain experimental procedure and method whose value is directly related to strength through a scaling transformation.Hence hardness data

(a)(b)(c)(d)

Figure 4.Grain structure of the composite material with increasing wt%nanotubes:(a)0%,(b)0.5%,(c)1.5%,and (d)2.5%SWCNTs respectively .The scale bar is 200 m and is the same for all four figures.Grain refinement can be seen progressively from (a)to

(d).

Grain size 0

20

40

6080100120140160

180

0wt.% Addition of Ni coated F-SWNTs S i z e o f T i g r a i n s (m m )5

4321Figure 5.Titanium grain size vs.weight-percent addition of Ni-coated SWCNTs with a spline-function fit.Processing of Titanium SWCNT Metal-Matrix Composites

1043

is often seen used interchangeably with stress and strength data as a function of grain size in the Hall Petch law [35 42].The nanotubes effect the final grain size of the Ti.Nanotubes are distributed in the grain boundaries.Since strengthening is a function of grain size (Hall-Petch),smaller grains lead to a higher hardness (strength)of the material.The nanotubes effect the final grain size and therefore effect the hardness of the material.Dislocations can move within the grains but can not cross the grain boundaries.The smaller the grain size the less motion of the dislocations before they get pinned at the boundaries.This strengthens the material.

Hardness data for the MMC examined here were obtained as follows.The starting powder size in this analysis was $140m m as can be seen using Figure 5.After cutting samples of the Ti composite with a diamond-tip saw blade and polishing the resultant surfaces,measurements using a 10g weight were made on those surfaces.In most micro-hardness tests 10g might be considered small,but it was used here because it made an indentation of approximately 5 12m m.As seen in Figure 5,the smallest grain size obtained was 39.7m m.Similar results to those in Figure 6were obtained with a 25g weight.The addition of SWCNTs beyond 4.5%wt.was not considered because of difficulty found with preform expansion during heating.As grain size is reduced through the nano-metric regime (<100nm),hardness typically increases with decreasing grain size and can be factors of two to seven times harder for pure nanocrystalline metals (10nm grain size)than for large-grained (>1m m)metals [37].A 2.88times increase in hardness (188%increase)was found here in the conventional Hall Petch regime.

The Vickers measurements are summarized in Figure 6.The data values as a function of weight-percent are shown,and it can be seen that Vickers hardness value (H V or HV)improved with increasing addition of SWCNTs.The spline-function fit and error bars are similar to Figure 5.

Figure 7depicts the HV hardness data as a Hall Petch plot in units of kilogram-force per square millimeter plotted against inverse square-root of grain size (d à0.5)rather than against weight-percent as in Figure 6.

Titanium vickers hardness value 885535

404

356.2

307

200

400

600

800

1000

1200

0wt% Addition of Ni-coated F-SWNTs

V i c k e r s h a r d n e s s (k g /m m 2)12345

Figure 6.Vickers hardness vs.weight-percent addition of Ni-coated SWCNTs [28]with a spline-function data fit.1044K.W ILSON ET AL .

DISCUSSION

In order to put the hardness results from this study into better perspective with respect to current ideas and understanding of hardness and strength,the experimental results in Figure 7need further discussion.Hardness and strength are a function of grain size for conventional crystalline materials (d >1m m).The Hall Petch law [35]expresses the grain-size dependence of flow stress for plastic strain out to ductile failure.That is,it states that grain-size dependence of macroscopic plastic yield stress r y of a polycrystal is given by r y ?r 1tkd à1/2where d is the grain size,while k and r 1are constants.Sometimes r 1is called friction stress.In the literature r y is often referred to as yield strength rather than yield stress.Similar empirical results have been found for hardness

[37,42]with H V ?H O tkd à1/2where H O is the functional hardness equivalent of friction stress when scaled to hardness units.Experimental results from hardness measurements have found different behavior for dependence on grain size at the smallest nanocrystalline grains (<20nm).These include (i)a positive slope or ‘normal’Hall-Petch behavior),(ii)essentially no dependence or zero slope,and (iii)a negative slope.Under such circum-stances,the Hall Petch law has broken down [37].This information is included here because of its relation to nanocrystals of Ni around the SWCNTs.From Figure 7it is clear that the data obtained is related to the Hall-Petch equation and that Hall-Petch breakdown did not occur.Future research can be performed by other groups starting with commercially available nanocrystalline Ti powder to study the effects in the nanoscale area (d à0.5<0.09).

Vicker’s hardenss vs. grain size diameter parameter

100

200

300

400500600700800

900

1000

01/sqrt (d) (m m –0.5)V i c k e r s h a r d n e s s (k g /m m 2)0.30.250.20.15

0.10.05Figure 7.Hall-Petch plot of Vickers hardness vs.inverse square-root grain size (d à0.5).The data fall in the conventional Hall Petch regime (s ).Hall Petch breakdown in the nanocrystal regime (N )depends upon slope and is of three types:(i)positive;(ii)zero;and (iii)negative slope.Only (iii)negative slope is depicted.In the single crystals regime (?),positive curvature is evident and the power of d changes to d à0.8(e.g.,Shulson et al.[38]and Farhat et al.[42]).

Processing of Titanium SWCNT Metal-Matrix Composites

1045

1046K.W ILSON ET AL. The Louchet et al.[35]examination of the breakdown of Hall Petch has given new insight into the effect of grain size on plastic yield stress.In Figure2of their paper,the schematic regime for Hall Petch breakdown is identified.The expected behaviors in Figure7are identical to Figure2of Louchet and the obtained data coincide with the Hall Petch regime.Yield stress and hardness behavior for nanocrystals(dà0.5>0.16) deviate from the Hall Petch equation.Only the expected case for negative Hall Petch behavior(negative slope)is depicted.For larger grain size(single crystals,dà0.5<0.09),the figure has been extended as shown in Shulson et al.[38],Farhat et al.[42],and Louchet et al.[35].In this region the power of grain-size dependence n has been shown by Shulson to change to kdàn with n?0.5as the yield stress r y approaches the friction stress r1and the hardness approaches H O.Although the Shulson result may only be a special case,the method of regression analysis remains relevant.

CONCLUSIONS

SWCNTs have been incorporated in a titaniium metal matrix to create a composite material using a rapid heat transfer technique made available by electromagnetic heating technology.These SWCNTs were processed in fully melted titanium to form the composite matrix.The matrix was then dissolved and the SWCNTs were recovered and analyzed. Raman spectroscopy data verified that the nanotubes were not compromised and survived the melting process.In addition,SEM,TEM,XPS,and Vickers hardness data all confirm the existence of nanotubes in the matrix and provide strong evidence that they serve to harden and strengthen the resulting composite.

ACKNOWLEDGMENTS

The authors are grateful for helpful comments in metallurgy from Professor Rex B. McLellan at Rice University.The manuscript has also benefited from the recommenda-tions of two anonymous referees.This research is supported by NASA Ames grant No. NNA04CK63A,NASA URETI cooperative agreement No.NCC-1-02038,and The Welch Foundation grant No.C-1494.

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