Functionally Graded Nylon 11silica Nanocomposites Produced by Selective Laser Sintering

8/10/2019 Functionally Graded Nylon 11silica Nanocomposites Produced by Selective Laser Sintering

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Materials Science and Engineering A 487 (2008) 251257

Functionally graded Nylon-11/silica nanocompositesproduced by selective laser sintering

Haseung Chung a, Suman Das b,

a Department of Mechanical and System Design Engineering, Hongik University, Seoul, Republic of Koreab Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, United States

Received 4 September 2007; received in revised form 4 October 2007; accepted 5 October 2007

Abstract

Selective laser sintering (SLS), a layered manufacturing-based freeform fabrication approach was explored for constructing three-dimensionalstructures in functionally graded polymer nanocomposites. Here, we report on the processing and properties of functionally graded polymer

nanocomposites of Nylon-11 filled with 010% by volume of 15 nm fumed silica nanoparticles. SLS processing parameters for the different

compositions were developed by design of experiments (DOE). The densities and micro/nanostructures of the nanocomposites were examined by

optical microscopy and transmission electron microscopy (TEM). The tensile and compressive properties for each composition were then tested.

These properties exhibit a nonlinear variation as a function of filler volume fraction. Finally, two component designs exhibiting a one-dimensional

polymer nanocomposite material gradient were fabricated. The results indicate that particulate-filled functionally graded polymer nanocomposites

exhibiting a one-dimensional composition gradient can be successfully processed by SLS to produce three-dimensional components with spatially

varying mechanical properties.

2007 Elsevier B.V. All rights reserved.

Keywords: Selective laser sintering; Polymer nanocomposites; Functionally graded materials

1. Introduction

Polymer systems are widely used due to their unique

attributes including ease of production, light weight, and often

ductile nature. However, polymers have lower modulus and

strength as compared to metals and ceramics[1].Fillers includ-

ing fibers, whiskers,platelets,or particles are important additives

for altering and enhancing the properties of polymers. Using

this approach, polymer properties have been improved while

maintaining their light weight and ductile nature [211].Parti-

cle fillers are frequently employed to improve the mechanical

performance of polymers for engineering applications in which

stiffness and toughness are the most important parameters to

be taken into account. Both stiffness and toughness can be

improved by the addition of inorganic particles, with the parti-

cle ranging in size from the micro- to nanoscale. Nanoparticles

have some unique features compared to microparticles. The

higher surface area can promote stress transfer from the matrix

Corresponding author. Tel.: +1 404 385 6027; fax: +1 404 894 9342.

E-mail address:[email protected](S. Das).

to the nanoparticles, improving the Youngs modulus of poly-

mers more dramatically than with microscale filler particles. The

required loadings of nanoparticles in polymer matrices are usu-

ally much lower than those of microscale filler particles which

are typically in the 1040vol.% range[12].

Functionally graded materials (FGMs) are materials that

incorporate deliberately designed transitions in materials com-

position and properties within a component in preferred

directions to optimize the functional value of that component

[13,14].The FGM concept is applicable to many fields of engi-

neering, for example, aerospace, nuclear energy, chemical plant,

energy conversion, electronics, optics, bio-systems, and com-

modities[15].Various processing methods for FGMs have been

discussed elsewhere[1520]but most of these efforts are lim-

ited to creating one-dimensional FGMs in simple shapes. While

the ability to manufacture complex components using FGMs

is highly desirable, at present, efficient automated techniques

to build such components with material gradations realized

as per design are limited. In this respect, layered manufac-

turing techniques such as selective laser sintering (SLS) [21],

have the potential to be ideal techniques to automatically build

such components[1].SLS creates objects directly from CAD

0921-5093/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2007.10.082
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252 H. Chung, S. Das / Materials Science and Engineering A 487 (2008) 251257

models using a layer-by-layer material deposition and con-

solidation approach. Thin layers of powders are successively

deposited and selectively fused using a computer-controlled

scanning laser beam that scans patterns corresponding to slices

of the CAD model. At present, fabrication of components with

true three-dimensional compositional heterogeneity via SLS

requires significant modification or redesign of powder delivery

mechanisms. Chung and Das[1]successfully demonstrated the

fabrication of one-dimensional microcomposite FGMs by SLS

in a glass-bead particulate-filled polymer. In this paper, the fab-

rication of one-dimensional nanocomposite FGMs by SLS of a

nanoparticle-filled polymer is demonstrated in the same manner,

showing improved properties over the microcomposite FGM.

Numerous works on nanocomposites including nanoparticle-

loaded composites exist in the literature. Brechet et al. [23]

proposed a model of interactions between the surface of the

dispersed nanoparticles and their interfacial surface with poly-

mer chains in order to analyze and predict the behavior of

nanocomposite materials, and to better understand the origin of

the high reinforcing effect generally observed. The influence offiller size on elastic properties of nanoparticle-reinforced poly-

mer composites was investigated by Adnan et al. [24] using

molecular dynamics (MD) simulations. The mechanical prop-

erties for neat polymer and nanocomposites were evaluated by

simulating a series of unidirectional and hydrostatic tests, both

in tension and compression. MD simulations by Smith et al. [25]

showed that the properties of polymer-nanoparticle composites

(PNPCs) were strongly influenced by nanoparticle size and filler

fraction, nanoparticle shape, nanoparticle distribution, polymer

molecular weight and the nature of the interactions between

the nanoparticle and polymer matrix. Zhang et al.[12]showed

that smaller particles, especially nanoparticles, may be moreeffective for polymer toughening. Recent work by Shah et al.

[26]has led to the development of nanohybrid materials, more

specifically poly-vinylidene fluoride (PVDF) nanocomposites,

which exhibit a simultaneous increase in stiffness and tough-

ness. Beyond these, researches on the mechanical properties of

nanocomposites have beenreported elsewhere[2729]. Kimand

Creasy[30]investigated selective laser sintering (SLS) charac-

teristics of clay nanoparticle/Nylon 6 composite for applications

in rapid prototyping and manufacturing (RP&M). Differences

in sintering behaviors between the nanocomposites and neat

polymers were investigated in this work.

2. Experimental

A commercially available SLS machine (SinterstationTM

2000, 3D Systems, Valencia, CA) was used in our experiments.

The selection of materials suitable for the fabrication of FGM by

SLS was based on the following criteria. First, materials must be

available in powder form. Forgood spreadingon thepowder bed,

there are limitations on the powder particle size. Powders should

flow freely, even at elevated temperatures, since good powder

flow and spreading are required to form each new layer in SLS

processing. Materialsmade with diameters less than 10mwere

found to exhibit poor bulk flow at high temperatures, presumably

due to the higher interparticle friction found in extremely fine

Fig. 1. Particle size distribution of Nylon-11 (RilsanD80) by sieve analysis.

powders[31].On the other hand, a typical build layer thickness

in the SinterstationTM 2000 machine is 100200m. Therefore,

materials with particle sizes in the 10150m range are pre-

ferred. Semi-crystalline polymers with relatively low melting

point and low melting viscosity are also preferred[32].

For our experiments Nylon-11 (Rilsan D80, Arkema Inc.)

and fumed silica nanoparticles (SigmaAldrich Co.) were cho-

sen.Fig. 1shows the particle size distribution of Nylon-11 by

sieve analysis. As shown, the majority of particles are in the

106150m range, desirable for SLS processing. On the otherhand, the fumed silica nanoparticles have 15 nm particle size as

reported by the vendor.

To produce one-dimensional FGMs, particulate loaded

Nylon-11 composites with different loadings of silica nanopar-

ticles were first generated and their tensile and compressive

mechanical properties were tested. The goal was to examine

the effects of different processing parameters on the quality of

the resulting product and to find optimized processing parame-

ters for each mixture composition. Pure Nylon-11 was blended

with different volume fractions of silica nanoparticles over the

26% range in 2% increments, as well as 10% using a rotary

tumbler (784 AVM, U.S. Stoneware, OH) for 24 h.

Two-level factorial design of experiments (DOE) methods[33]were then used to determine the optimized SLS processing

parameters for achieving high quality parts. DOE is a systematic

method to determine the influence of a set of processing param-

eters on overall part quality. Here, the optimized SLS processing

parameters aredefined as those that resultin parts which arefully

dense or near-fully dense in regions where material is present

in the parts design, dimensionally accurate, and easily removed

from its support powder. Procedures previously developed by

Partee et al. [34]for determining the optimized SLS process-

ing parameters for CAPA 6501 polycaprolactone powder were

adopted for the present study. In the context of this study involv-

ing polymer nanocomposites, additional criteria were applied


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H. Chung, S. Das / Materials Science and Engineering A 487 (2008) 251257 253

so that the optimized SLS processing parameters resulted in

smoothly deposited powder layers in the shortest possible time,

and yielded SLS processed specimens exhibiting good bond-

ing of the polymer matrix to the silica nanoparticles with no

inter-layer delamination.

Once optimized processing parameters for each material

composition were achieved by DOE, the tensile and compres-

sive mechanical specimens based on these parameters were

produced and mechanical property tests with these speci-

mens were conducted. The microstructure and nanostructure of

these samples was examined through optical microscopy (Leica

DMLM Microscope) and transmission electron microscopy

(JEOL 2010F Transmission Electron Microscope), respectively.

An Instron machine (4502) and a MTS machine (Alliance

RT/30) were used for tensile tests and compression tests (ASTM

standard D-695), respectively.

FGM composites with these material compositions were fab-

ricated based on theoptimized processing parameters. Dueto the

nature of the powder delivery subsystem in the SinterstationTM

2000machine, laser sintering of 3D functionally graded multiplematerials is not possible. Therefore, a vertical FGM in discrete

layers in the powder supply cylinders was created manually. As

the SLS process proceeded, the roller deposited material of a

gradually changing composition on the part side piston as the

stacked powders of different compositions were incrementally

consumed. The processing parameters for each composition

were changed synchronously. This technique created multilayer

FGM samples in which the material gradient was oriented along

the build direction[1,22]. Using these techniques, two differ-

ent components were also fabricated to demonstrate the FGM

approach for complex three-dimensional shapes.

3. Results and discussion

Optimal processing parameters for each composition were

determined by fabricating 10 mm test cubes using different com-

binations of laser power, scan speed, substrate temperature,

and roller speed by DOE. The fabricated parts were sectioned

and examined by optical microscopy and image-J software

(http://rsb.info.nih.gov/ij/). Similar procedures were previously

developed for glass beads filled composites elsewhere [1,22].

Initially, the same processing parameters for 30% vol-

ume fraction of glass beads loaded Nylon-11 composite were

attempted for 2% volume fraction of silica nanoparticles loadedNylon-11 composite and the resulting parts had 99.8% density

Fig. 2. Cross-sectional optical micrographs of Nylon-11+ 2% silica nanoparti-

cles.

[2,10].This meant that the same processing parameters could

be used to optimize 2% volume fraction of silica nanoparticles

loaded Nylon-11 composite. Fig. 2 shows the cross-sectional

optical micrograph of the fabricated part for Nylon-11 filled with

2% volume fraction of silica nanoparticles. The fabricated part

was sectioned, dyed using a black marker pen and then wiped

using a smooth cloth. Areas of porosity were indicated by those

regions that remain black when black ink was wiped from the

section face because pores retain some ink while the fully dense

regions do not. Thus, the black regions shown in Fig. 2indicate

porosity.

However, as the volume fraction of silica nanoparticles was

increased by 2% from 2% to 4%, the process could not be

continued with these parameters. Therefore, DOE was applied

here as well to determine the optimized processing parameters.

Although DOE with large range of each processing parameter

was executed, none of the processing parameters were suc-

cessful. Therefore, an alternate approach was attempted. Once

pure Nylon-11 bases were generated using the optimized pro-

cessing parameters developed previously [1,22], 46% silica

nanoparticle-filled composites were fabricated on top of the

bases. DOE was applied to these silica nanoparticle-filled com-

posites. This approach was successful and led to fabrication of

several parts. Table 1 shows the optimizedprocessingparameters

and densities attained by this approach for silica nanoparticlesloaded Nylon-11 composites.

Table 1

Optimized processing parameters and densities for silica nanoparticle-loaded nanocomposites

Processing parameter Nylon-11 base Nylon-11+ 2% silica

nanoparticles

Nylon-11+ 4% silica

nanoparticles

Nylon-11+ 6% silica

nanoparticles

Nylon-11+ 10% silica

nanoparticles

Part substrate temperature

set point (C)

184 184 184 184 184

Roller speed (m/s) 0.076 (3 in./s) 0.076 (3 in./s) 0.076 (3 in./s) 0.076 (3 in./s) 0.076 (3 in./s)

Laser power (W) 4.5 4.5 4.5 4.5 4.5

Scan speed (m/s) 1.257 (49.5 in./s) 1.257 (49.5 in./s) 0.889 (35.0 in./s) 0.889 (35.0 in./s) 0.635 (25.0 in./s)

Resulting density (%) 99.80.1 99.80.1 99.70.1 99.80.1 99.00.1
http://rsb.info.nih.gov/ij/http://rsb.info.nih.gov/ij/


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However, the disadvantage of this approach is that post-

processing steps such as turning and milling are necessary to

eliminate the pure Nylon-11 base. To eliminate this procedure,

a second alternate approach was experimented. Ten additional

unsintered layers were added between pure Nylon-11 base and

silica nanoparticle composite so that two parts were separated

without any post-processing. This approach was also successful

up to 6% volume fraction of silica nanoparticles.

Fabrication of the Nylon-11 composite with 10% volume

fraction of silica nanoparticles was also attempted using the

same technique as 46% silica nanoparticles loaded Nylon-11

composites. However, this approach was not successful for the

10% silica nanoparticles loaded Nylon-11 composites. There-

fore, the unsintered additional layers were eliminated so that

silica nanoparticle composite was attached directly to pure

Nylon-11 base and DOE was applied to this process. When the

scan speed was decreased from 35 to 25 in./s, while keeping

the other parameters the same, the fabricated part had 99.0%

density.

Cross-sectional optical micrographs can provide the macro-scopicdensity but do notreveal any information of thedispersion

of silica nanoparticles in Nylon-11 matrix. The nanostructure

of the Nylon-11 nanocomposite with 10% volume fraction

of silica nanoparticles was examined by transmission electron

microscopy. Samples were prepared for TEM by using micro-

tome (ULTRACUT E, Reichert-Jung) resulting in samples that

had 100 nm thickness.

Fig. 3 shows the cross-sectional high-angle-annular dark-

field (HAADF) micrographs from two different regions of the

fabricated part, acquired in STEM mode with two different mag-

nifications. Silica nanoparticles are distributed homogeneously

as shown in both micrographs. Fig. 3 confirmed that silicananoparticles had particle size of about 15 nm as given by ven-

dor.

Tensile and compression specimens were generated using

these optimized processing parameters and mechanical tests

were conducted. Five specimens were generated for each

mechanical test for each composition. While no post-processing

procedure was necessary for 26% silica nanoparticles loaded

Nylon-11, the parts could not be fabricated without post-

processing procedures for 10% silica nanoparticles loaded

Nylon-11. Therefore, pure Nylon-11 bases for specimens with

10% volume fraction of silica nanoparticles were removed by

turning for compression specimens and milling for tensile spec-

imens, respectively.Fig. 4shows the mean tensile modulus and the mean strain

at break as a function of the silica nanoparticle volume fraction.

The tensile modulus is a decreasing function of silica nanoparti-

cle composition up to 4% volume fraction of silica nanoparticles

but an increasing function of silica nanoparticle composition

from 4% volume fraction upwards. On the other hand, as the vol-

ume fraction of silica nanoparticles increases, the tensile strain

at break increases up to 2% volume fraction but decreases from

2% to 10% volume fraction of silica nanoparticles. We can con-

clude that as volume fraction of silica nanoparticles increases,

the modulus increasesand strain at break decreasesmeaning that

the parts become stiffer but more brittle. However, there exists

Fig.3. Cross-sectional HAADFmicrographs of Nylon-11+ 10%silica nanopar-

ticles.

a critical composition at which these characteristics undergo an

inversion in their trends.

Fig. 5shows the mean compressive modulus and the mean

strain at yield as a function of silica nanoparticle volume frac-

tion. The compressive modulus is a decreasing function of silica

nanoparticle volume fraction up to 2% volume fraction but is

an increasing function from 2% to 10% volume fraction. On the

other hand, as volume fraction of silica nanoparticles increases,

the compressive strain at yield decreases up to 4% volume frac-


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Fig. 4. (a)Tensile modulus and(b) strainat breakdue todifferent silicananopar-

ticle composition.

tion but increases from 4% to 10% volume fraction. As the

volume fraction of silica nanoparticles increases, the compres-

sive modulus increases and strain at yield also increases. Similar

to the tensile modulus, there exists a critical volume fraction ofsilica nanoparticles at which these characteristics undergo an

inversion in their trends.

Finally, the fabrication of two different components was

attempted to demonstrate the FGM approach in this paper.

As mentioned earlier, the roller deposited material of a grad-

ually changing composition on the part build platform as the

stacked powders of different compositions were incrementally

consumed. The processing parameters for each composition

were changed synchronously with transitions in supply powder

composition. This technique created multilayer FGM demon-

stration component in which the material composition gradient

was oriented along the build direction.

Fig. 5. (a) Compressive modulus and (b) strain at yield as a function of silica

nanoparticle composition.

The first component selected for FGM demonstration is a

compliant gripper shown inFig. 6.The upper part of this com-

pliant gripper has to be hard and stiff to grasp the object properly

while the lower part has to be flexible to efficiently generate the

motions under the action of applied forces. Therefore, this com-

ponent represents the characteristics of a compliant FGM verywell. A 38.10 mm long component with Nylon-11 filled with

five different volume fractions of silica nanoparticles, each of

7.62 mm length was fabricated.

Thesecond componentselected to demonstrate theFGM con-

cept is a rotator cuff scaffold. The rotator cuff is composed of

four muscles and their tendons, and helps to lift and rotate the

arm and to stabilize the ball of the shoulder within the joint. The

goal of rotator cuff repair is to reattach good quality tendonto the

location on the arm bone from which it was torn. The design of

this scaffold and the corresponding fabricated part are shown in

Fig. 7 along with material composition and corresponding prop-

erty gradations in the investigated material systems. An actual,


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Fig. 6. Schematic description, and fabricated part of compliant gripper.

Fig. 7. Schematic description (courtesy Eiji Saito), and fabricated part of rotator cuff scaffold.

implantable scaffold will likely involve a FGM incorporating

different composition blends of a bioresorbable polymer (e.g.

polycaprolactone) and a biocompatible ceramic (e.g. calcium

phosphate)[1].

4. Conclusions

Selective lasersintering technique was investigated on blendsof Nylon-11 with different volume fractions of silica nanopar-

ticles (210%). Optimized processing parameters for selective

laser sinteringof the above compositionswere developed by sys-

tematic design of experiments (DOE). Optical microscopy and

transmission electron microscopy were used for characterizing

the micro/nanostructures of the resulting parts.

In order to design and fabricate 1D functionally graded

material components, knowledge of mechanical properties as

a function of material composition is necessary. To address this

need, the tensile and compressive mechanical properties for

each composition processed to near full density by SLS were

evaluated.

Finally, the fabrication of 1D functionally graded poly-

mer composite components based on the optimized processing

parameters determined by DOE was successfully demonstrated.

For demonstration,two different parts, namely, a compliantgrip-

per anda rotator cuff scaffold designs were fabricated. This work

demonstrates the fabrication of macroscopic three-dimensional

parts with a one-dimensional material gradient by SLS in a

single, uninterrupted process run.

Acknowledgement

This material is based upon work supported by the National

Science Foundation under Grant No. DMI 0115205.

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Documents

Functionally Graded Nylon 11silica Nanocomposites Produced by Selective Laser Sintering