Composites: Part B 43 (2012) 1743–1748

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Composites: Part B

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Enhancement of mechanical properties of aluminium/epoxy compositeswith silane functionalization of aluminium powder

H.J. Kim a, D.H. Jung a, I.H. Jung b, J.I. Cifuentes b, K.Y. Rhee b,⇑, D. Hui b,c

a Ocean Development System Laboratory, Korea Research Institute of Ships and Ocean Engineering, 305-600 Taejon, South Koreab Department of Mechanical Engineering, Kyunghee University, 446-701 Yongin, Republic of Koreac Department of Mechanical Engineering, University of New Orleans, LA 70148, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 August 2011Accepted 28 December 2011Available online 6 January 2012

Keywords:A. Polymer-matrix compositesB. Fracture toughnessB. WearD. Mechanical testing

1359-8368/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.compositesb.2011.12.010

⇑ Corresponding author. Tel.: +82 31 201 2565; faxE-mail address: rheeky@khu.ac.kr (K.Y. Rhee).

In this study, the effect of surface modification of aluminum powders on the tensile, fracture, and tribo-logical behaviors of aluminum/epoxy composites was investigated. Aluminum powders were surface-modified with 3-aminopropyltriethoxysilane. Aluminium/epoxy composites were fabricated by castmolding method using 10 wt.% untreated and silane-treated aluminium powders. Tensile, mode I frac-ture, and tribological tests were performed on both composites. The results showed that the tensile mod-ulus and strength of silane-treated aluminum/epoxy composites were �9% and �12% greater,respectively than those of untreated aluminum/epoxy composites. The results also showed that the frac-ture toughness and wear resistance of silane-treated aluminum/epoxy composites were �32% and �56%greater than that of untreated aluminum/epoxy composites. Scanning electron microscope (SEM) exam-ination showed the improvement of tensile and fracture properties of silane-treated aluminum/epoxycomposites was attributed to the improved dispersion and bonding of aluminum particles in the epoxy,due to the silanization of aluminium powders.

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1. Introduction

Aluminum/polymer composites are attractive materials for awide range of industrial applications due to the combination of prop-erties such as low density, corrosion resistance, thermal stability,and ease of fabrication. Accordingly, many studies have been madeto investigate the mechanical, thermal, and electrical properties ofaluminum powder reinforced polymer composites [1–11]. One ofthe main issues in the fabrication of aluminum/polymer compositesis to achieve the good dispersion and interfacial strength betweenaluminium powders and the polymer. To address this issue, somestudies have been performed for the surface treatment of aluminumpowders. Jallo et al. [12] reported the effect of surface modificationon aluminium powder on its flowability, combustion and reactivity.Silanization provides lower surface energy values and such materi-als are expected to flow better after surface treatment. Chen et al.[13] investigated modification of aluminium powders applyingmethyltrichlorosilane. The basic procedure involved mixing of alu-minium powder with silane solution in hexane. Crouse et al. [14]demonstrated functionalization of oxide-passivated aluminiumnanoparticles using 3-methacryloxypropyltrimethosilane (MPS)to provide chemical compatibility within various solvent and

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polymeric systems. However, not much work has been made toinvestigate the surface modification of aluminum powder to im-prove fracture and tribological properties of aluminum reinforcedepoxy composites.

In this study, the effect of silane treatment of aluminum powderson the tensile, fracture, and tribological behaviors of aluminum/epoxy composites was investigated. Aluminium/epoxy compositeswere fabricated by cast molding method using 10 wt.% unmodifiedand silanized aluminium powders. FTIR analysis was made on theuntreated and silanized aluminium powders to determine thechemical change on the surface of aluminum powders due to thesilane treatment. Tensile, mode I fracture, tribological tests wereperformed on both composites. After fracture and tribological tests,scanning electron microscopy (SEM) examination was made todetermine fracture and wear mechanisms of aluminum/epoxy com-posites, depending on the silane treatment of aluminum powders.

2. Experimental

2.1. Materials

The reinforcing material used in this study was aluminium pow-ders of �45 lm size (Strem Chemicals, USA). The epoxy used wasdiglycidil ether or Bisphenol A (YD-115, Kukdo Chemical, Korea),and the curing agent was polyamidoamine (G-A0533, Kukdo

1744 H.J. Kim et al. / Composites: Part B 43 (2012) 1743–1748

Chemicals, Korea). 3-Aminopropyiltriethoxysilane with a purity of99% (Aldrich, USA) was used as a silane functionalization agent.

2.2. Silanization of aluminium powders

60 g of unmodified aluminium powders were dispersed in250 ml of ethanol via mechanical stirring for 30 min. Then thereaction was conducted with 3-aminopropyltrietoxysilane andthe solution was stirred at 60 �C for 6 h. After completion of reac-tion, the aluminum powders were purified by repeated washingwith distilled water followed by acetone. The resulting silanizedaluminium powders were separated by filtration and dried undervacuum oven at 80 �C for 14 h.

2.3. Fabrication of aluminum/epoxy composites

Untreated aluminum powders (U-aluminum) and silane-trea-ted aluminum powders (S-aluminum) were mixed with the etha-nol at 10 wt.%. Ultrasonication was performed for 5 min forbetter dispersion. After sonication, the U-aluminum and S-alumi-num were mixed with the epoxy resin and then stirred for approx-imately 12 h at 80 �C to completely evaporate the ethanol. Afterthe ethanol was removed, hardener was added (at an epoxy tohardener ratio of 2:1) and then poured into a Teflon mold withthe cast molding method. The mixture was de-gassed in a vacuumoven for 30 min and cured at 100 �C for 15 h. Then, then it was postcured for 2 h at 125 �C. The fabricated composites plates were ma-chined as tensile, fracture, and wear specimens. Thickness andgage length of the tensile specimens were 5 mm and 80 mm,respectively. Single edge notched specimens (SENT) of 4 mmthick � 90 mm long � 18 mm wide with a 6 mm initial crack wereprepared for fracture tests. Disk type specimens of 30 mm andheight of 8 mm were prepared for wear tests.

2.4. Characterization

FTIR spectral analysis was made on the untreated and silane-treated aluminum powders, and SEM examination was made tostudy fracture surface morphology. Tensile and fracture tests wereperformed using a universal testing machine at a cross-head speedof 0.5 mm/min and 0.2 mm/min, respectively. Fracture tests wereperformed at room temperature according to the ASTM E399-90procedure [15]. The tribological test was performed by applyingsliding speed of 0.12 m/s, sliding distance of 144.5 m, load of29.5 N. The counter-wear material used was zirconia ball

Fig. 1. FTIR spectra of untreated and s

(�12 mm in diameter). At least, three specimens of each case weretested to ensure reliability of tests results.

3. Results and discussion

FTIR spectral analysis was made to determine the chemicalchange on the surface of aluminum powder due to the silane treat-ment. Fig. 1 shows the FTIR spectra of untreated and silane-treatedaluminium powders. For untreated aluminium powder, the peaksat 3440 cm�1 (Fig. 1a) and 1048 cm�1 (Fig. 1b) were caused by hy-droxyl groups (Al–OH) on the surface of aluminium powder due tothe presence of atmospheric moisture on the surface of aluminumpowder. For the silane-treated aluminum powder, the characteris-tic absorption peak at �1070 cm�1 (Fig. 1b) is attributed to the for-mation of networked system of (Si–O–AL)n groups followingextensive cross-linking through silanization of Al–OH group. Thisconfirms the reaction of 3-aminopropyltriethoxysilane organiccoating film on aluminium powder surface.

Fig. 2 shows the typical tensile stress versus strain curves of un-treated and silane-treated aluminum/epoxy composites. As shownin the figure, the tensile stress increased linearly with strain at anearly stage and nonlinear behavior occurred for both composites.The figure also shows that the tensile strength and elastic modulusof aluminum/epoxy composites were affected by the silane treat-ment of aluminum powders.

Table 1 shows the comparison of tensile strength between un-treated and silane-treated aluminum/epoxy composites. The ten-sile strength of was determined by measuring the maximumstress in the stress–strain curve. As shown in the figure, the tensilestrength of silane-treated aluminum/epoxy composites is �12%higher than that of untreated aluminum/epoxy composites. Specif-ically, the average tensile strengths of untreated and silane-treatedaluminum/epoxy composites were 45.1 MPa and 50.5 MPa, respec-tively. The table also shows the comparison of elastic modulus be-tween untreated and silane-treated aluminum/epoxy composites.The elastic modulus was determined by measuring the slope inthe beginning linear region of the stress–strain curve. As shownin the figure, the elastic modulus of silane-treated aluminum/epoxy composites is �9% greater than that of untreated alumi-num/epoxy composites. Specifically, the average tensile moduliof untreated and silane-treated aluminum/epoxy composites were2.1 GPa and 2.3 GPa, respectively.

Fig. 3 shows typical load–displacement curves of both compos-ites. As shown in the figure, both composites exhibited brittle frac-ture behavior. That is, the load increased almost linearly withdisplacement up to fracture. The figure also shows that fracture

ilane-treated aluminium powders.

tensile strain (mm/mm)0.25 0.26 0.27 0.28 0.29

tens

ile s

tress

(MPa

)

untreated

silane treated

-10

0

10

20

30

40

50

60

Fig. 2. Typical tensile stress versus strain curves of untreated and silane-treatedaluminum/epoxy composites.

Table 1Comparison of tensile strength and elastic modulus between untreated and silane-treated aluminum/epoxy composites.

Tensile strength (MPa) Elastic modulus (GPa)

Untreated 45.1 ± 2.0 2.1 ± 0.2Silane treated 50.5 ± 2.0 2.3 ± 0.2

displacement (mm)0 1 2 3 4 5 6

fract

ure

load

(N)

untreated

silane treated

0

100

200

300

400

500

600

Fig. 3. Typical load–displacement curves of untreated and silane-treated alumi-num/epoxy composites.

untreated silane treated

fract

ure

toug

hnes

s, K

IC [M

Pa·

m1/

2 ]

Fig. 4. Comparison of fracture toughness between untreated and silane-treatedaluminum/epoxy composites.

H.J. Kim et al. / Composites: Part B 43 (2012) 1743–1748 1745

displacement of silane-treated of aluminum/epoxy composites isgreater than that of untreated aluminum/epoxy composites, inwhich the fracture displacement are defined as the displacementat which fractured occurred. It can be also seen in the figure thatsimilar to tensile strength results, the fracture load of the silane-treated of aluminum/epoxy composites is greater than that of un-treated aluminum/epoxy composites. The effect of silane treat-ment on the fracture toughness of aluminum/epoxy compositeswas investigated by comparing the fracture toughness of silane-treated of aluminum/epoxy composites with that of untreated alu-minum/epoxy composites. Generally, fracture toughness becomesequal to the critical stress intensity factor when the materialexhibits a linear elastic fracture behavior. In this study, the fracturetoughness, KIC was determined as follows [15]:

KIC ¼ YPcrBW

ffiffiffi

ap

ð1Þ

where Pcr is the fracture load, B is the thickness of the specimen, Wis the width of the specimen, a is the crack length, and Y is the shape

function, which is a function of the shape of specimen. For the pres-ent study, the shape function was determined by:

Y ¼ 1:99� 0:41a

Wþ 18:7

a

ðWÞ2� 38:48ða=WÞ3 þ 53:85

a

ðWÞ4ð2Þ

Fig. 4 shows the comparison of fracture toughness between un-treated and silane-treated aluminum/epoxy composites. As shownin the figure, the fracture toughness of aluminum/epoxy compos-ites was improved by the silane treatment of aluminum powder.Specifically, the average fracture toughness of silane-treated alu-minum/epoxy composites was �32% greater than that of untreatedaluminum/epoxy composites.

FESEM analysis was performed on the fracture surfaces of theuntreated and silane-treated aluminum/epoxy composites toinvestigate the fracture mechanism of aluminum/epoxy compos-ites due to the silane treatment. Fig. 5 shows the fracture surfacesof untreated and silane-treated aluminium/epoxy composites. Forthe untreated aluminium/epoxy composites, as shown in Fig. 5a,debonding occurs at many interfaces between aluminum powdersand epoxy matrix. It also shows that crack propagated from thedebonded interface due to the weaker interfacial adhesion be-tween the aluminium and polymer. For the silane-treated alumin-ium/epoxy composites, as shown in Fig. 5b, better dispersion ofaluminum powders in the epoxy matrix was observed. It can beseen in the figure that several microcracks but less interfacial deb-onding occurred, which indicates the improved interfacial adhe-sion. Thus, more energy is needed to break this interlink andstart the fracture.

The tribological property of composite materials is character-ized by the friction coefficient and wear rate. The effect of silanetreatment on the tribological behavior of aluminum/epoxy com-posites was investigated by determining the change in frictioncoefficient as a function of wear distance. Fig. 6 shows the changein friction coefficient as a function of the wear time for untreatedand silane-treated aluminum/epoxy composites. As shown in thefigure, vibrational sliding friction behavior occurred in the un-treated aluminum/epoxy composites while stable tribologicalbehavior occurred for silane-treated aluminum/epoxy composites,which indicates that silane-treated aluminum/epoxy composite ismore compact and uniform. It can be seen in the figure that theaverage friction coefficient of silane-treated aluminum/epoxycomposites is far lower than that of untreated aluminum/epoxycomposites. Specifically, the friction coefficient of untreated andsilane-treated aluminum/epoxy composites are in the ranges of0.34–0.42 and 0.04–0.06, respectively.

The worn surfaces of the untreated and silane-treated alumi-num/epoxy composites were examined using surface profiler todetermine the wear loss. Fig. 7 shows the comparison of worn

time (sec)

frict

ion

coef

ficie

nt

Fig. 6. Comparison of friction coefficient change as a function of the wear time foruntreated and silane-treated aluminum/epoxy composites.

(a) (b)

Fig. 5. SEM images of fracture surfaces of (a) untreated and (b) silane-treated aluminium/epoxy composites.

x (mm)0 2.5 5 7.5 10

y 0 2.5 5 7.5 10

y

x (mm)

-220

-140

-60

20

100

-220

-140

-60

20

100(a) (b)

( µm

)

( µm

)

Fig. 8. Comparison of depth profile between (a) untreated and (b) silane-treatedaluminum/epoxy composites.

wea

r rat

e [m

m3 /N

m]

1746 H.J. Kim et al. / Composites: Part B 43 (2012) 1743–1748

surfaces between untreated and silane-treated aluminium/epoxycomposites. As shown in Fig. 7a and b, the wear track of untreatedaluminum/epoxy composites was wider and deeper compared tothat of silane-treated aluminum/epoxy composites. Fig. 8 showsthe depth profile of the untreated and silane-treated aluminum/epoxy composites, respectively. The mean depth profile was ob-tained according to several points taken from the cross-section inthe worn area. Fig. 9 shows the comparison of specific wear ratefrom sliding distance for both composites. The specific wear rate,K was determined by the following equation [16]:

K ¼ VN � l ð3Þ

untreated silane treated

Fig. 9. Comparison of wear rate between untreated and silane-treated aluminum/epoxy composites.

V ¼ 2prA ð4Þ

where K is the specific wear rate, N is the applied load, and l is thesliding distance, V is the wear volume loss, A is the cross-sectionalarea of wear track obtained from the depth profile using a surfaceprofilometer, and r is the radius of the wear track. As shown inthe figure, the wear rate of aluminum/epoxy composites decreasedby the silane treatment of aluminum powders. Specifically, the

Fig. 7. Comparison of worn surfaces between (a) untreate

wear rate of silane-treated aluminum/epoxy composites was�56% less than that of untreated aluminum/epoxy composites. Thisagrees with previous results for MMT/epoxy composites [17].

d and (b) silane-treated aluminum/epoxy composites.

(a)

(b)

Fig. 10. SEM images of worn surfaces of (a) untreated and (b) silane-treated aluminium/epoxy composites.

H.J. Kim et al. / Composites: Part B 43 (2012) 1743–1748 1747

The worn surfaces of the untreated and silane-treated alumi-num/epoxy composites was analyzed using FESEM to investigatethe effect of silane treatment of aluminum on the wear mechanismof aluminum/epoxy composites Fig. 10 shows the comparison ofworn surfaces between untreated and silane-treated aluminium/epoxy composites. SEM image of the untreated aluminum/epoxycomposites (Fig. 10a) shows that typical wear mechanisms suchas matrix cracking and exforlation of matrix occurred. The figurealso shows that local plastic deformation process and welding be-tween contacting asperities and zirconium counter material oc-curred, indicating the adhesive wear behavior occurred. For thesilane-modified aluminum/epoxy composites (Fig. 10b), matrixcracking and exforlation also occurred. However, the matrix crack-ing was less compared to the untreated case. Particularly, thefigure shows the ploughing deformation and micro parallelscratching on the worn surface, which indicates that abrasive wearoccurred. It can be also seen in the figure that the dispersion ofaluminum of silane-treated composites was better than that ofuntreated case. The abrasive wear behavior of silane-treated caseoccurs because the hardness of aluminum/epoxy composites wasincreased by the improved dispersion and interfacial characteristicof aluminum powders due to the silane treatment.

4. Conclusions

This study demonstrated the enhanced mechanical propertiesof aluminium/epoxy composites by silane treatment of aluminiumpowders. The conclusions obtained from this study are as follows:First, the tensile modulus and strength of aluminum/epoxy com-posites are improved �9% and �12% by the silane treatment ofaluminum powders. Second, the fracture toughness and wearresistance of aluminum/epoxy composites are also improved�32% and �56% with silane treatment of aluminum powders.Third, the wear mechanism shows abrasive and mild wear behav-ior in silane surface modified aluminum powder/epoxy compositeand adhesive and severe wear in unmodified aluminum powder/epoxy composite. Fourth, the improved dispersion and interfacialcharacteristics of aluminum powders in the epoxy is the reason

of the improved mechanical properties and performance of alu-minium/epoxy composite.

Acknowledgements

This work was financially supported by the National R&D Pro-ject of ‘‘Development of Energy Utilization of Deep Ocean Water’’supported by the Korean Ministry of Land, Traffic and MaritimeAffairs.

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