Nano-toughening versus micro-toughening of polymer syntactic foams

COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 2924–2933 www.elsevier.com/locate/compscitech

Nano-toughening versus micro-toughening of polymer syntactic foams E.M. Wouterson a, F.Y.C. Boey a, S.-C. Wong b, L. Chen c, X. Hu a

a,*

School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Republic of Singapore b Department of Mechanical Engineering, University of Akron, OH 44325-3903, USA c Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Republic of Singapore Received 14 December 2006; received in revised form 14 February 2007; accepted 1 March 2007 Available online 18 May 2007

Abstract This paper examines the effect of nanoclay content on tensile and fracture properties of syntactic foam. Results showed that the tensile strength decreased slightly with increasing nanoclay content. The Young’s modulus showed an increase of 17% with the addition of 2 wt% clay. Interestingly, the fracture properties reached a maximum for samples containing 1 wt% of nanoclay. SEM and OM studies were performed to scrutinize the toughening mechanisms in nanoclay-reinforced syntactic foam. A comparison of the tensile and fracture results obtained for nano-reinforced syntactic foam against short-fiber reinforced syntactic foam revealed the superiority of micro-reinforcements over nano-reinforcements in improving the tensile properties. Both short micro-fibers and nanoclay were able to give rise to substantial increase in toughness in polymer syntactic foam. Detailed comparison revealed that micro-fiber toughening was more effective than nano-toughening in this case, except for nanoclay-reinforced syntactic foam containing 1 wt% nanoclay which showed equal or even better fracture properties compared to short fiber reinforced syntactic foam.  2007 Elsevier Ltd. All rights reserved. Keywords: A. Hybrid composites; A. Nanoclays; A. Carbon fibres; Nano- and micro-toughening

1. Introduction Syntactic foam is a composite material synthesized by mechanical mixing of hollow particulates (filler) in a matrix (binder). Due to the presence of the hollow particulates, syntactic foams feature a low density, high strength-toweight ratio, tremendous compressive properties and low absorbance of moisture [1]. Syntactic foam is an attractive material to be used as the core material in sandwich composites for aerospace and marine applications. Despite all these advantages, the number of applications of syntactic foams has been limited due to its brittle behavior under mechanical loading. In order to be able to employ syntactic foam-based composites in high-impact, damage-tolerant conditions, it is necessary to increase the fracture toughness of syntactic foam. In our previous works, we highlighted

*

Corresponding author. Fax: +65 67909081. E-mail address: [email protected] (X. Hu).

0266-3538/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.05.019

two successful approaches leading to the improvement of the fracture toughness of syntactic foam. The first approach involved the change of the foam microstructures [2] whereas the second approach involved the inclusion of short carbon fibers [3]. Another approach commonly used to improve the tensile and fracture properties of polymeric materials involves the use of nanofillers [4]. Especially, polymer-based nanocomposites reinforced with montmorillonite nanoclay particles have attracted wide attention. Substantial improvements in tensile, fracture, thermal and barrier properties have been reported [5–13]. The improved properties for polymer/clay nanocomposites are attributed to the high aspect ratios of the nanoclay layers. Despite the fact that nanoclay has been proven to be an effective reinforcement for polymeric materials and foams [14,15], nanoclay-reinforced syntactic foam has been relatively unexplored [16]. This paper aims to study the effect of nanoclay content on the tensile and fracture properties of syntactic foam. Fracture mechanics and microscopic techniques are employed. The results will be discussed in

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light of known toughening mechanisms operative in nanoclay- and microsphere-reinforced epoxy. The results will be compared against results previously obtained for short fiber-reinforced syntactic foam (SFRSF) in order to establish the effectiveness of both approaches. 2. Experimental work 2.1. Materials and equipment For all the specimens Epicote 1006 epoxy resin was used as the binder. Epicote 1006 is a combination of liquid bisphenol-A, epichlorohydrin epoxide resin, amine and polymeric additives. Phenoset BJO-093 hollow phenolic microspheres, supplied by Asia Pacific Microspheres, were used for the filler. The hollow phenolic microspheres had an average diameter of 71.5 lm and an average wall thickness of 1.84 lm. The nanoclay used for this work was Nanomer I.30TC supplied by Nanocor, Inc. Nanomer I.30TC is a surface modified montmorillonite mineral. The clay was organically modified with octadecylammonium ion and methyl tallow bis-2-hydroethyl quaternary ammonium ion as the swelling agent. According to Meng and Hu [17], the interlayer spacing of the organically modified clay calculated from the wide angle X-ray diffraction (WAXD) d00l-reflection was 2.4 nm. To study the effect of the nanoclay content on the tensile and fracture properties of nanoclay-reinforced syntactic foam, the amount of nanoclay added to the epoxy resin ranged from 0 to 2.0 wt%. The amount of nanoclay was kept to a minimum as the authors’ work focuses on lightweight materials. In addition, it has been reported by several researchers that optimum fracture properties were obtained for 1–2 wt% of nanoclay content [18]. It is well understood that the method of processing epoxy/clay nanocomposites affects the morphology of the epoxy/clay nanocomposites [7,9,10]. Depending on the processing conditions, the morphology can be described as ‘particulate’, ‘intercalated’, or ‘exfoliated’ [4]. Disordered exfoliated nanoclay layers are preferred as this morphology elucidates the highest material properties [4]. For the dispersion of the nanoclay into the epoxy-based syntactic foam, a mixing strategy reported by Wang et al. [18] was adopted. According to [18], the mixing process also referred to as ‘slurry-compounding’ process results in improved exfoliation morphology for epoxy/clay nanocomposites. The mixing strategy started with the dispersion of nanoclay in deionized water to form a suspension. The suspension was then stirred for 24 h and sonicated for 30 min. The suspension was then poured in ethanol and stirred vigorously until a white precipitate was formed. After washing the precipitate, the precipitate was mixed again with ethanol to form slurry. The slurry was stirred for 10 h and sonicated for 30 min at room temperature. A specific quantity of epoxy resin was added to the ethanol/clay slurry. The mixture was stirred for 2 h and put in a vacuum oven at 50 C for 48 h to evaporate the

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ethanol from the mixture. Once the ethanol was evaporated, a stoichiometric amount of hardener and 30 vol% of BJO-093 phenolic microspheres were added to the epoxy/nanoclay mixture. The amount of microspheres was fixed at 30 vol% as this amount of microspheres elucidated the highest fracture toughness as presented in our previous work [2]. The mixture was then compression molded in a pre-coated aluminum mold and left under a pressure of 16 MPa for 18–22 h to cure at room temperature. Fig. 1 shows the general structure of nanoclay reinforced syntactic foam (NCRSF). The state of clay exfoliation in the nanocomposites was observed with a transmission electron microscope (TEM). Samples were cut using a Leica Ultracut UCT ultramicrotome. Microtomed thin sections were collected on 200 mesh copper grids and examined by a Philips CM300 TEM at 300 kV in bright field mode. The short fiber reinforced syntactic foam (SFRSF) was created by adding 1, 2, and 3 wt% short carbon fibers (SCFs) to the epoxy resin. The length of the SCF was fixed at 10 mm. The SCF were added to the epoxy resin, followed by microspheres. This mixing order results in a reduction of the air bubbles present in the curing process. The amount of microspheres was again fixed at 30 vol%. The microspheres were added in multiple steps to the epoxy resin to avoid agglomeration. After dispersion, the SFRSF was compression molded using an aluminum mold coated with a silicone release agent. The mixture was left under the press at a pressure of 16 MPa for 18–22 h to cure at room temperature. 2.2. Tensile test NCRSF was machined into standard ‘dog-bone’ specimens by means of a TensilKut I from TensilKut Engineering. The size of the ‘dog-bone’ specimens was in accordance with the Type I specimens as reported by the ASTM Standard D638 [19]. The specimens were uniaxially loaded at the ambient temperature using an Instron Model 5567 at a crosshead speed of 5 mm/min. For each test the tensile strain was recorded with a clip-on strain gauge. The Young’s modulus, Et, was measured from the initial

Fig. 1. Reflective optical micrograph showing the general structure of nanoclay-reinforced syntactic foam at high magnification.

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elastic region of the stress–strain curve. The error bars represent the standard deviation for four measured values. 2.3. Fracture toughness The Mode-I critical stress intensity factor, KIc, was measured using single-edge notched bend (SENB) specimens, loaded in a three-point bend (3PB) geometry. The tests were performed at room temperature by an Instron Model 5567 at a crosshead speed of 5 mm/min. The specimen dimensions were 60 · 12.7 · 6.35 mm3. For all the specimens, a constant crack-to-width ratio, a/W, of 0.5 was prepared by a vertical band saw. A sharp crack was introduced by tapping a fresh razor blade into a notch. The critical stress intensity factor, KIc, can be estimated from the following equations [20,21]: pffiffiffi 3PS a K Ic ¼ Y ; ð1Þ 2tW 2 a  a 2  a 3  25:11 þ 14:53 Y ¼ 1:93  3:07 W W W  a 4 þ 25:80 ; ð2Þ W where Y is a shape factor, P is the peak load at the onset of crack growth in a linear elastic fracture, W the width of the specimen, t the thickness of the specimen, S the support span, and a the crack length. The plane strain critical strain energy release rate, GIc, was calculated from the stress intensity values using the following relationship [22]: GIc ¼

 K 2Ic  1  m2 ; Et

plane, perpendicular to the fracture surface by means of a Buehler Petro-thin Thin Sectioning System. The thin section was observed under a transmission optical microscope (Nikon Eclipse E600 POL) with bright and polarized light. 3. Results and discussion 3.1. Morphology The clay morphology within the epoxy resin is shown in Fig. 2a–c. The light, white area is the epoxy matrix, and the black area is made up of nanoclay layers. Due to their relatively large size, microspheres are not shown in Fig. 2a–c. Fig. 2a clearly shows that agglomeration of nanoclay particles is present in NCRSF. The agglomeration increases with increased nanoclay content. Similar observations have been reported by Liu et al. [6]. Fig. 2b shows a part of the aggregate shown in Fig. 2a at higher magnification. It is clear that the nanoclay layers are positioned in their preferred face-to-face alignment. By using the public domain software ImageTool [23], the thickness of the clay platelets and average distance between the platelets are estimated to be 1.0 and 3.9 nm, respectively. It is clear from Fig. 2b that a nanocomposites comprising of epoxy resin and intercalated nanoclay has been synthesized. A similar kind of intercalation of the nanoclay layers is observed for specimens containing 0.75, 1.0, 1.5 and 2.0 wt% of nanoclay content. Besides intercalation of the nanoclay layers, minor exfoliation can also be observed in Fig. 2b. Intensified disorientation and even delamination of the clay platelets are also observed, see Fig. 2c.

ð3Þ

where Et is the Young’s modulus and m the Poisson’s ratio. The value of m was assumed to be 0.3 as the precise value of m was not measured. As indicated in Ref. [21] this assumption is justified as the factor cannot change the major results obtained from GIc conversion in a significant way. The fracture surface of the tensile and SENB-3PB specimens was cut from the specimen and coated with a thin layer of gold. SEM was employed to study the fracture surface. The samples were observed using a Jeol JSM 5410LV, a low vacuum scanning electron microscope (SEM). The accelerating voltage was 10 kV. It is well understood that SEM alone is not sufficient to study the fracture behavior of polymeric materials. For this reason, optical microscopy was employed to study the subsurface damage in the crack process zone. The center section, containing the initiated crack was cut from the SENB specimen using a vertical band saw, followed by encapsulation in epoxy resin. To facilitate the study, the sample was polished through the mid-section using a Struers Rotopol-35 polisher. Grinding and polishing was performed with silicon carbide polishing papers with grade numbers from 320, 500, 800, 1200 to 4000, respectively, followed by diamond paste containing particles of 3 lm and 1 lm. Thin-sections were taken from the specimen’s mid-

3.2. Tensile testing of clay-reinforced syntactic foam From the fractured tensile specimens, it is observed that the addition of silicate clay particles to syntactic foam does not change the fracture behavior of syntactic foam. Similar to neat syntactic foam, a brittle fracture behavior is observed for NCRSF. However, more than for neat syntactic foam, it is observed that fracture occurs due to an oversized void [24]. The authors believe that the voids remain in the system, despite the degassing process and application of a pressure during curing, due to highly intercalated and exfoliated clay platelets which may form a barrier for voids to float to the surface. The presence of voids is probably one of the causes for the reduction in the tensile strength, rt, with increasing nanoclay content, see Fig. 3a. A similar trend has been reported elsewhere [9,13,16]. Besides voids, agglomeration of the nanoclay could be another reason for the reduction in rt with increasing nanoclay content. The presence of relatively large micro-aggregates in the nanocomposite plates may act as stress concentrations. These clusters of nanoclay layers easily split up under an applied load, significantly lowering the tensile strength [25]. Fig. 3b shows the results for the Young’s modulus, Et, for NCRSF containing different amounts of nanoclay. Et increases from 1.59 GPa to 1.9 GPa with the addition of

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Fig. 2. TEM images of nanoclay-reinforced syntactic foam with various nanoclay content; (a) 0.5 wt% nanoclay (low magnification), (b) 0.5 wt% nanoclay (high magnification), and (c) 1.0 wt% nanoclay.

40

4.0 3.5

20

NCRSF SFRSF

10

Young's Modulus (GPa)

Tensile Strength (MPa)

3.0 30

2.5 2.0 1.5

NCRSF SFRSF Halpin-Tsai Model

1.0 0.5 0.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

wt%

0.0

0.5

1.0

1.5

2.0

2.5

3.0

wt%

Fig. 3. Tensile properties of (j) nanoclay-reinforced syntactic foam and (d) short fiber-reinforced syntactic foam as a function of reinforcement concentration; (a) tensile strength and (b) Young’s modulus.

2 wt% nano-size clay. Kornemann et al. [25] believe that the increase in Et could correspond to an increase in the effective volume fraction of the reinforcement in the nanocomposite. As the interlamellar spacing is increased, the effective particle volume fraction is also increased. Yasmin et al. [8] suggest that an increasing Young’s modulus can be attributed to a better dispersion and intercalation/exfolia-

tion of the nanoclay particles that restricts the mobility of the polymer chains under loading as well as to the good interfacial adhesion between the particles and the epoxy matrix. Over the years various models have been developed to predict the Young’s modulus of particle-modified polymers. One commonly used model is the Halpin–Tsai

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relationship [26,27]. The Halpin–Tsai model gives the modulus of the composite as a function of the matrix modulus and of the particles, but also as a function of the aspect ratio by the inclusion of a shape factor. According to Halpin–Tsai, the Young’s modulus can be estimated by using the following equation:   1 þ fgV f ENCRSF ¼ ð4Þ Em ; 1  gV f with 

 1 ; g¼ Ef þ f Em Ef Em

ð5Þ

where f is a shape factor, Ef is the Young’s modulus of the nanoclay particles, Em is the matrix modulus. Values for the nanoclay aspect ratio, Ef, and f are similar to those presented by Kinloch and Taylor [4]. It is clear from Fig. 3b that the estimation of the Young’s modulus of NCRSF by using the Halpin–Tsai model represents the trend and values for the experimentally measured Young’s modulus vs. weight fraction of nanoclay quite closely. Fig. 4a–c shows the SEM fractographs of NCRSF specimens after being subjected to tensile loading. It is clear that the fracture surface of specimens containing 1.5 wt% nanoclay, see Fig. 4b, is rougher compared to the fracture surface of specimens containing 0.75 wt% nanoclay, see Fig. 4a. According to [8], the rough fracture surface is caused by the intercalated clay platelets. The intercalated platelets cause the crack to follow a tortuous fracture path. Fig. 4b also shows some agglomerated particles, which is in

line with the previous reported findings on the reduction in tensile strength with increasing nanoclay content. A closeup of an agglomerate is shown in Fig. 4c. It is clear that cracks initiate from the clay agglomerates, leading to a reduction in the tensile strength. 3.3. Fracture toughness of nano reinforced syntactic foam Fig. 5a and b shows the fracture toughness, KIc, and critical energy release rate, GIc, of NCRSF, respectively. Initially, a decrease in KIc is observed for specimens containing 0–0.75 wt% of nanoclay. At the moment, it is not fully clear why NCRSF shows a reduction in KIc for samples containing 0–0.75 wt% nanoclay. A similar drop in KIc has not been reported before. It is suggested that a critical amount of nanoclay is required in order to increase KIc. Indeed for NCRSF containing 1–2 wt% nanoclay particles, an increase in KIc is observed compared to pristine syntactic foam. KIc of NCRSF increases from 1.15 MPa m0.5 for 0 wt% nanoclay to 1.63 MPa m0.5 for 1 wt% nanoclay, an increase of about 42%. The KIc value slightly drops for specimens containing more than 1 wt% nanoclay. The drop is attributed to the increased agglomeration of the nanoclay layers. The critical energy release rate, GIc, for syntactic foam containing various amounts of nanoclay is shown in Fig. 5b. GIc is calculated from the KIc and the respective Young’s modulus data. The trend in GIc vs. wt% nanoclay is similar to the trend observed for KIc as shown in Fig. 5a. Introduction of up to 0.75 wt% nanoclay causes GIc to decrease from 0.66 kJ/m2 to 0.41 kJ/m2, a reduction of

Fig. 4. Fracture surface of nanoclay-reinforced syntactic foam subjected to tensile loading; (a) 0.75 wt% nanoclay content, (b) 1.5 wt% nanoclay content (low magnification), and (c) 1.5 wt% nanoclay content (high magnification).

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2.0

2.5

NCRSF SFRSF Wang et al.

2.0

1.5 2

G1c(kJ/m )

0.5

K1c(MPa.m )

1.5

1.0

1.0

0.5

NCRSF SFRSF Wang et al.

0.5

0.0

0.0 0.0

0.5

1.0

1.5

2.0

2.5

0.0

3.0

0.5

1.0

1.5

2.0

2.5

3.0

wt%

wt%

Fig. 5. Fracture properties of (j) nanoclay-reinforced syntactic foam and (d) short fiber-reinforced syntactic foam as a function of reinforcement concentration; (a) KIc and (b) GIc.

41%. Compared to pristine syntactic foam, GIc shows a significant increase of 104% for specimens containing l wt% nanoclay. The increase in both KIc and GIc for syntactic foam containing 1 wt% nanoclay indicates the excellent toughening potential of nanoclay in syntactic foams. A comparison has been made against fracture data for nanoclay-toughened epoxy available in the literature. Wang et al. [18] report about the toughening behavior of two different kinds of nanoclay, namely Cloisite 93 A and S-clay, in epoxy resin. Comparing the results of nanotoughened syntactic foam against the results of S-clay toughened epoxy resin, see Figs. 5a and b, it is obvious that NCRSF features a superior fracture behavior compared to nanoclay-toughened epoxy. The result suggests that an exfoliated nanoclay system might not result in the optimal fracture behavior. Agglomeration of nanoclay layers is preferred as the agglomerates are able to divert the path of crack growth and induce the formation of micro-cracking, increasing the fracture toughness. The improved fracture behavior of NCRSF over nanoclay-toughened epoxy suggests a synergistic behavior between the hollow microspheres and the nanoclay layers. Besides superior absolute KIc and GIc values, Table 1 shows that nanoclay-toughened syntactic foam also shows the largest relative increase in (specific) KIc and GIc compared to nanoclay-toughened epoxy resin. In order to elucidate the toughening mechanisms responsible for the fracture behavior, the fracture surfaces

of the SENB specimens were examined using SEM. Figs. 6a–c and 7a and b show the SEM micrographs of the fracture surfaces of NCRSF SENB specimens containing 0.75 wt% and 1 wt% nanoclay, respectively. These two amounts of nanoclay content were chosen in order to find a likely explanation for the large difference in fracture properties between these two nanoclay contents. The three characteristic fracture regions, namely pre-crack (a), process zone (b), and the fast fracture region (c), are clearly visible in Fig. 6a. The pre-crack region is produced by tapping of a fresh razor blade into the pre-notch. According to Lee and Yee [21], the process zones are usually of interest in order to understand the toughening mechanisms because the materials’ resistance against crack propagation in this region actually reflects the fracture toughness measured. The importance of the process zone compared to the other regions is reflected in its surface roughness and size. The fracture surface of the process zone as seen in Fig. 6b is rough whereas the fracture surface of the fast fracture region, see Fig. 6c, is relatively smooth. A smooth surface is associated with brittle behavior and does not contribute significantly to the overall composite toughness. According to Zerda and Lesser [13], the rough surface exhibits evidence for crack branching along the crack path length. As the clay content increases, the distance between regions of intercalated clay is reduced. The reduced distance between the intercalated clay particles causes the crack to follow a more tortuous path, either around or between

Table 1 Relative increase in (specific) KIc and GIc for different fillers added to pure epoxy resin Amount of filler

Increase in KIc (%)

Increase in KIc/q (%)

Increase in GIc (%)

Increase in GIc/q (%)

30 vol% BJO-093 30 vol% BJO-093 + 1 wt% nanoclay 30 vol% BJO-093 + 2 wt% nanoclay 1 wt% nanoclaya 2 wt% nanoclaya

37.4 107.7 93.6 21.4 64.3

84.8 170.0 143.6 20.0 60.4

211.9 449.9 352.4 39.5 193.0

319.5 614.8 469.1 37.9 186.0

a

Data taken from [18].

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Fig. 6. SEM images of the fracture surface of an SENB specimen of nanoclay reinforced syntactic foam containing 0.75 wt% nanoclay; (a) overall fracture surface, (b) crack initiation zone, and (c) fast fracture zone. The black arrow indicates the direction of crack propagation.

Fig. 7. SEM images of the fracture surface of an SENB specimen of nanoclay reinforced syntactic foam containing 1.0 wt% nanoclay; (a) crack initiation zone (low magnification) and (b) crack initiation zone (high magnification).

regions of high clay content. Increased clay content leads to increased crack path deflection, causing increased surface roughness. Indeed, the fracture surface of specimens containing 0.75 wt%, as shown in Fig. 6a, is less rough compared to the fracture surface of specimens containing 1.0 wt% nanoclay, see Fig. 7a. Besides the difference in roughness of the fracture surfaces, the size of the process zone also plays a significant role in contributing to the fracture property. It can be seen that the process zone for samples containing 0.75 wt% nanoclay, see Fig. 6a, is much smaller than the process zone observed on the fracture surface of NCRSF specimens containing 1.0 wt% nanoclay. The larger the process zone, the more toughening events take place, the higher the

fracture toughness of the composite. Indeed, the fracture toughness of specimens containing 1.0 wt% nanoclay is higher than specimens containing 0.75 wt% nanoclay. Figs. 6b and 7b show the process zones of NCRSF specimens containing 0.75 and 1.0 wt% nanoclay, respectively. In the process zones many potential toughening mechanisms are observed, i.e. step structures, matrix deformation, microcracks, clusters of nanoclay particles, and fractured and debonded microspheres. The step structures behind the microspheres and clusters of nanoclay are a result of the crack front bowing mechanism, first described by Lange [28]. An approaching crack front is pinned by rigid particles. Secondary crack fronts will be formed and bend/bow between the rigid particles. At a certain point

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the crack front breaks away from the particle, causing the secondary cracks to come together, forming a step structure as both secondary cracks propagate at a different level. The crack front bowing mechanisms is often regarded as the main toughening mechanism in particulate composites. For higher nanoclay content, the size of the agglomerates tends to grow. If the size of the agglomerate is in the order of microns, and the stacking order of the clay layers is perpendicular to the crack growth direction, the agglomerate is able to resist and deflect the crack front. This behavior is clearly visible in Fig. 8b, where significant step structures are visible behind the nanoclay agglomerates. Besides step structures, multiple microcracks are observed, see Fig. 7b (indicated by white arrows). Observations of these microcracks are supported by the report by Wang et al. in an epoxy-nanoclay system without no microspheres [18]. The presence of the microcracks implies that the clay layers act as stress concentrators, promoting the formation of a large number of microcracks upon loading of the sample, causing an increase in the size of the process zone. Optical microscopy (OM) was performed to study the sub-surface damage in the crack process zone. The study

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allows for evaluation of the possible presence of microshear banding as reported by Lee and Yee [29]. Microshear banding has been reported as an important toughening mechanism in glass sphere reinforced polymers. Despite observing multiple specimens containing different amounts of nanoclay under the optical microscope, no micro-shear banding can be seen under polarized light. This is similar to what was observed in an epoxy/clay nanocomposites without the microspheres [18]. The authors believe that the absence of micro-shear banding could be attributed to the relatively low content of nano-clay layers in combination with their random orientation. Further it is suggested by the authors that the system might behave in an elastic manner causing ‘healing’ of the crack. Further investigations on these suggestions are required. Transmission optical microscopy revealed another toughening mechanism, see Fig. 8. The image, taken under crossed polarized light, clearly shows the presence of diffuse matrix shear yielded regions due to the debonding of the microspheres. Diffuse shear yielding is always found around the debonded matrix. Once a microsphere debonds from the matrix, a free surface will be generated, which is more vulnerable to plastic shear deformation. 3.4. Nano vs. micro-reinforcement

Fig. 8. Transmission optical micrograph showing the subsurface damage near the initiation crack area in nanoclay reinforced syntactic foam. The black arrow indicates the direction of crack propagation.

Two of our previous papers involve successful microtoughening of syntactic foam [2,3]. In order to determine the effectiveness of nano-toughening against micro-toughening, a comparison between these two types of inclusions has been made. From Fig. 3a as well as Fig. 3b, it is clear that a hybrid reinforcement comprising of short carbon fibers and hollow microspheres is more effective in increasing the tensile strength and Young’s modulus of syntactic foam than a hybrid reinforcement of nanoclay and hollow microspheres. Inclusion of 2 wt% of SCF with a diameter of 7 lm increases the tensile strength and Young’s modulus by 21.5% and 84%, respectively, whereas 2 wt% of nanoreinforcement reduces the tensile strength by 14% and increases the Young’s modulus by 17%. The difference in strengthening and stiffening effect between SFRSF and

Fig. 9. (a) SEM image of the fracture surface of an SENB specimen of short fiber reinforced syntactic foam; (b) Transmission optical micrograph showing the subsurface damage near the sharp and initiation crack area in short carbon fiber reinforced syntactic foam.

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Debonded Clay layers

microsphere

Fiber bridging

Short fibers

Microsphere

Crack path Debonded Microsphere

Crack front Microsphere

Crack tip

Fig. 10. Schematic illustration of crack propagation in (a) short fiber-reinforced syntactic foam and (b) nanoclay-reinforced syntactic foam showing possible mechanisms of.

NCRSF is mainly attributed to the strength, Young’s modulus and length of the short carbon fibers which are all far superior compared to the nanoclay particles. Comparing micro-toughening to nano-toughening, it is clear from Figs. 5a and b that, in general, for similar weight fractions, micro-toughening is more effective than nanotoughening. Fig. 9a shows an SEM fractograph with various toughening mechanisms, including fiber pull-out, fiber breakage, step structures, debonding of microspheres and fractured microspheres, present in SFRSF. The broken fibers as shown in Fig. 9a is an evidence of the fiber bridging. This is confirmed in Fig. 9b which shows fibers bridging the crack. A sketch of the toughening mechanisms in SFRSF is shown in Fig. 10a. As we concluded in our previous work [3], the main contribution to the overall fracture toughness of SFRSF comes from the formation of new surfaces. In general micro-toughening is superior to nano-toughening, tough an exception is observed for samples containing 1 wt% of nano-reinforcement. NCRSF containing 1 wt% of nanoclay elucidates a similar KIc value and an even higher GIc compared to syntactic foam reinforced with 1 wt% SCF. An image of the toughening mechanisms in NCRSF is shown in Fig. 10b. Although the mechanisms of nanotoughening are complex, it is believed that the nanoclay oriented in different direction may play different roles in the toughening behavior. Nanoclay layers aligned in the direction of crack propagation are expected to split under the applied load and contribute significantly to the toughening because of the possibility of crack deflection and multiple micro-cracking generated ahead of crack tip. Nanoclay layers orientated perpendiculars to the crack growth direction are also expected to contribute to the overall toughness due to their ability pin the crack front and deflect the crack path leading to the formation of step structures. 4. Conclusions Hybrid reinforced syntactic foam was prepared by dispersing nanoclay particles into a syntactic foam. TEM studies revealed the coexistence of intercalated and exfoli-

ated clay morphology. Tensile test results showed that an introduction of 2 wt% of nanoclay led to a 13% reduction in the tensile strength of nanoclay-reinforced syntactic foam and increased the Young’s modulus by 19.5%. The reduction in the tensile strength was attributed to the presence of voids and agglomeration of the nanoclay layers. The tensile properties of nanoclay reinforced syntactic foam were not as good as similar properties observed for short fiber reinforced syntactic foam. For the fracture behavior, a maximum in the fracture toughness, KIc, and critical energy release rate, GIc, was observed for specimens containing 1 wt% nanoclay. The presence of either nanoclay or short carbon fibers in the microsphere-filled syntactic foam resulted in substantial toughening effect. This toughening effect was attributed to toughening mechanisms previously observed for pristine syntactic foam, i.e. crack pinning, matrix deformation, and debonded microspheres. It is believed that the nanoclay layers may introduce further microcracking in the matrix and crack deflection. These are possible even when the nanoclay is not fully exfoliated. On the other hand, the role of the micro-fiber is believed to be mainly in providing crack bridging. Acknowledgements The authors thank Nanyang Technological University and DSO National Laboratories for supporting this work. One of us (E.M.W.) thank Mr. Laurent Lasour for his help with some of the experimental work. References [1] Shutov F. A syntactic polymer foams. In: Klempner D, Frisch KC, editors. Handbook of polymeric foams and foam technology. Munich: Hanser Publishers; 1991. p. 355–74. [2] Wouterson EM, Boey FYC, Hu X, Wong SC. Specific properties and fracture toughness of syntactic foam: effect of foam microstructures. Compos Sci Technol 2005;65:1840–50. [3] Wouterson EM, Boey FYC, Hu X, Wong S-C. Effect of fiber reinforcement on the tensile, fracture and thermal properties of syntactic foam. Polymer 2007;48:3183–91.

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