Materials and Design 46 (2013) 802–808

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Materials and Design

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Synthetic staggered architecture composites

Abhishek Dutta, Srinivasan Arjun Tekalur ⇑Department of Mechanical Engineering, Michigan State University, East Lansing, MI 48823, USA

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

Article history:Received 17 August 2012Accepted 2 November 2012Available online 23 November 2012

Keywords:Staggered microstructureSynthesisToughness

0261-3069/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.matdes.2012.11.018

⇑ Corresponding author. Tel.: +1 517 884 1608; faxE-mail address: tsarjun@egr.msu.edu (S.A. Tekalur

Structural biocomposites (for example, nacre in seashells, bone, etc.) are designed according to the func-tional role they are delegated for. For instance, bone is primarily designed for withstanding time-depen-dent loading (for example, withstanding stresses while running, jumping, accidental fall) and hence themicrostructure is designed primarily from enhanced toughness and moderate stiffness point of view. Onthe contrary, seashells (which lie in the abyss of oceans) apart from providing defense to the organism (itis hosting) against predatory attacks, are subjected to static loading (for example, enormous hydrostaticpressure). Hence, emphasis on the shell structure evolution is directed primarily towards providingenhanced stiffness. In order to conform between stiffness and toughness, nature precisely employs a stag-gered arrangement of inorganic bricks in a biopolymer matrix (at its most elementary level of architec-ture). Aspect ratio and content of ceramic bricks are meticulously used by nature to synthesizecomposites having varying degrees of stiffness, strength and toughness. Such an amazing capability ofstructure–property correlationship has rarely been demonstrated in synthetic composites. Therefore, inorder to better understand the mechanical behavior of synthetic staggered composites, the problembecomes two-pronged: (a) synthesize composites with varying brick size and contents and (b) experi-mental investigation of the material response. In this article, an attempt has been made to synthesizeand characterize staggered ceramic–polymer composites having varying aspect ratio and ceramic contentusing freeze-casting technique. This will in-turn help us in custom-design manufacture of hybridbio-inspired composite materials.

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1. Introduction

Biological materials are composites structures synthesized bynature via bottom-up route and exhibit complex hierarchicaldesign over several length scales. Nacre, or the mother-of-pearl,is one of the most widely studied biocomposites since it is inher-ently light weight and strong (high stiffness and specifically, supe-rior toughness) [1–5]. Nacre is composed of polygonal mineraltablets (width = 5–10 lm and thickness = 0.4–0.5 lm) of highweight fraction (U = 0.95) which are cemented together with verysmall amount of polymeric matrix (thickness = 20–30 nm) which iscomposed of proteins and polysaccharides. Irrespective of the de-gree of hierarchical complexity exhibited by the biocomposites,at the most elementary level, they exhibit a generic microstructurecomprising of nanometer sized inorganic crystals embedded in asoft organic matrix (biopolymers) in the form of a staggered archi-tecture [6], otherwise known as a brick-and-mortar type structuralarrangement (Fig. 1).

Nature cleverly uses aspect ratio and ceramic content for finetuning properties for functions. For instance, bone is primarily

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: +1 517 884 1601.).

designed for withstanding time-dependent or, impact loading(for example, withstanding stresses while running, jumping, acci-dental fall) and hence the microstructure is designed primarilyfrom enhanced toughness point of view and moderate stiffness.On the contrary, seashells (which lie in the abyss of oceans) apartfrom providing defense to the organism (it is hosting) against pred-atory attacks, is also subjected to quasi-static loading (for example,enormous hydrostatic pressure). Hence, emphasis on the shellstructure evolution is directed primarily towards providingenhanced stiffness. Such an amazing capability of tuning proper-ties based on a function has rarely been seen in synthetic compos-ites. Although bone and nacre differ from each other in varyingaspects, deformation and toughening mechanisms in both thesematerials are similar. Thus, in order to synthesize compositeshaving brick and mortar architecture, freeze-casting techniquecan be used to first synthesize the ceramic relic. It consists of fourprincipal steps: (1) ceramic slurry preparation, (2) freezing theceramic–solvent system, (3) sublimation of the frozen solvent,and (4) sintering of the porous ceramic relic. A short summary ofthe recent work in the literature of freeze-casting is given in thefollowing paragraph.

Porous alumina (Al2O3) and silicon nitride (Si3N4) ceramicswere produced using water as the carrier where growth of

Fig. 1. Brick and mortar type structural arrangement observed in seashells.

A. Dutta, S.A. Tekalur / Materials and Design 46 (2013) 802–808 803

dendritic shaped ice crystals was observed upon freezing whichproduced elliptical pores (dendritic channel) having a lengths ofmajor and minor axis equal to 500 lm and 30 lm respectively[7]. Simultaneously, camphene (C10H16) was also being used as afreezing vehicle because of its environment friendliness and itsability to cast at room temperatures dense Al2O3 ceramics [8]. Ara-ki and Halloran [9] produced Al2O3 ceramics by using an aminederivative of a fatty acid condensation polymer as a dispersant inaddition to the Al2O3–C10H16 slurry which was cast on polyure-thane molds and subsequently sintered at 1600 �C for 4 h and ob-tained circular cross-sectional channels. Porous silica (SiO2)ceramics were produced using polyvinyl alcohol (PVA) as the bin-der by casting the slurry onto glass tubes which was subsequentlyimmersed in liquid nitrogen at a controlled speed [10]. Poroushydroxyapatite (HAP) scaffold were produced using glycerol asthe carrier, PVA as a binder (1.5 wt%) and Dynol 604 as the disper-sant (0.75 wt%) by casting the solutions on PVC tubes which wascovered with polyurethane foam [11]. Similarly, porous HAP cera-mic scaffold, titanium foam and HAP/tricalcium phosphate (TCP)ceramic scaffold respectively using camphene as the freezing vehi-cle [12–14]. Pore channels generated were greater than 100 lmwhich enabled their use for bone-tissue engineering applications.Porous HAP have been produced which were modified by silica[15], with functionally graded core–shell structure (using camph-ene as the carrier) [16], and using distilled water as the carrier,PVA as a binder and ammonium polyacrylate as the dispersant[17]. Titanium foams [18] and porous Si3N4 ceramics [19] havebeen produced using distilled water as the media and using a vari-ety of compounds as an addition agent to observe subsequent porestructure and geometry (for example, ammonium polymethacry-late anionic dispersant, polyacrylamide dispersion agent, etc.).Macroporous alumina ceramics [20], Al2O3–ZrO2 ceramic (20:80with a 40–80 wt% solid loading) [21], and porous alumina ceramicusing Al2O3 sol [22] as a substitute for water and as a medium formaking ceramic slurry have also been produced. The authors [22]also discussed the use of PVA both as a binder as well as an emul-sifying agent to stabilize air bubbles during magnetic stirring.Following the manufacturing of the ceramic backbone, the cera-mic–polymer composites can be synthesized via infusion of poly-mer into the porous ceramic. Deville and co-authors [23] werethe first to produce Alumina epoxy lamellar composite by fillingthe porous IT scaffolds by epoxy. Al2O3–PMMA brick and mortarand lamellar composites [24] have been synthesized by introduc-ing a polymeric phase (by free radical polymerization of methyl-methacrylate (MMA) initiated by 2,20-azobisisobutyronitrile(AIBN)) into the porous Al2O3 freeze cast component. By graftingthe Al2O3 with another polymeric component [3-(trimethoxysi-lyl)propyl methacrylate (c-MPS)], significant improvement inmechanical properties have been obtained in comparison to thenon-grafted, Al2O3 and PMMA counterparts alone.

All the above mentioned researches have attempted to mimiclamellar/brick and mortar microstructure in its entirety. In our cur-rent investigation, we are not trying to mimic brick and mortarmicrostructure of nacre; rather, what we are interested is in draw-ing inspiration from staggered architecture in biological compositesat their elementary level (such as nacre: 0.90–0.95 ceramic content[1], bone: 0.50 ceramic content [25], and dentin: 0.43 ceramic con-tent [25]), we want to (a) synthesize synthetic ceramic–polymercomposites with varying ceramic content and aspect ratio, and (b)experimentally understand the structure–property correlationshipof these composites when subjected to external loading. Excerptsdrawn from this investigation would thereby help in custom-designmanufacture of hybrid bio-inspired composite materials.

2. Materials and methods

Sodium dodecyl sulfate (C12H25NaO4S) was obtained from Sig-ma Aldrich (St. Louis, MO, USA). Poly (vinyl alcohol) [–CH2-

CH(OH)–]n 98% hydrolyzed having an average molecular weightMw 13,000–23,000 was obtained from Aldrich Chemical Company,Inc. (Milwaukee, WI, USA). Sucrose (C12H22O11) crystals wereobtained from Roche Diagnostics Corporation (IN, USA). Two typesof ceramic powders were purchased: Silica (SiO2) spheres of 8 lmand 1 lm diameter were obtained from Fiber Optic Center Inc(New Bedford, MA, USA). SC-15 epoxy resin (toughened two phase)was obtained from Applied Poleramic Inc. (Benicia, CA, USA).

The ceramic scaffolds were prepared following the sequence ofsteps as have been carried out by authors [23,24] and for brevitypurposes not described in detail. Suspension of micrometer sizedceramic powders were prepared by dispersing ceramic powdersof different concentrations (30 wt%, 40 wt%, and 50 wt%) into asolution composed of 2 wt% polyvinyl alcohol (PVA), 4 wt% sucroseand 5 wt% sodium dodecyl sulfate (SDS) in order to produce cera-mic scaffolds having varying porosity. The colloidal solutions weresonicated for 10 min duration (Fisherbrand, FB 11021) at 35 W. Theceramic suspensions were casted in Teflon (PTFE) molds on top of ametal plate which was cooled by liquid nitrogen. The frozen sam-ples were subsequently transferred to a freeze dryer (Freeze Dryer1.0, Labconco, Kansas City, MO) for a period of 48 h which in turnpromotes sublimation of ice and thereby, we are left behind withthe ceramic green body. Densification of the ceramic was by ob-tained by sintering at 1550 �C for 2 h. The epoxy is prepared bymixing 100 wt% SC-15 Part A (resin) and 30 wt% of SC-15 Part B(hardener). The mixture is stirred gently for approximately10 min followed by degassing for another 10 min. Finally, the scaf-folds were infiltrated with the two-phase epoxy followed by curingof the epoxy at 60 �C for 2 h and 94 �C for 4 h.

Morphological features of the samples were acquired using aJEOL JSM-6400 V scanning electron microscope (JEOL Ltd., Tokyo,

804 A. Dutta, S.A. Tekalur / Materials and Design 46 (2013) 802–808

Japan). Samples thermogravimetric analysis (TGA Q500 V20.10Build 36, TA Instruments) were conducted at a heating rate of20 �C min�1 up at temperatures up to 550 �C. ASTM D695 was fol-lowed for the determination of compressive strength of these cera-mic–polymer composites under quasi-static rates of loading. Thespecimen dimension chosen for testing was 5 mm � 5 mm �10 mm and was performed in a MTS machine (10 kN load cell) ata crosshead speed of 1 mm/min. ASTM C1421 was followed forthe determination of fracture toughness of the synthesized com-posites. The specimen dimension chosen for testing was4 mm � 4 mm � 16 mm (with an initial crack length of 1.40 mm),and was performed in a MTS machine (10 kN load cell) at a cross-head speed of 0.01 mm/s.

3. Results and discussion

The first task in the current investigation involves synthesizingcomposites with varying ceramic content and aspect ratio. As men-tioned earlier, freeze-casting technique followed by infusion ofpolymer (into the porous ceramic) has been employed for synthe-sis of ceramic–polymer composites. The most critical stage of theentire freeze-casting process is the controlled solidification of theslurry. Formation of the structure and future porosity are deter-mined in this step. The cooling rates, for freezing the water toice, typically will range from 0.1 �C min�1 to 10 �C min�1. Freezingfront velocity, which is directly proportional to cooling rate, playsan important role in the final microstructure development. Withreference to the basic crystallography of ice, water solidifies intoanisotropic hexagonal ice crystals during freeze casting. Empiri-cally, the inter-ceramic spacing w has been found to vary withfreezing-front velocity v [26] as follows: w / 1

vn. In order to controlthe freezing rate and thereby, the porosity of the ceramic, we em-ploy a simple and fundamental approach. The ceramic suspensionsare casted in PTFE molds on top of a metal plate which in-turn iscooled by liquid nitrogen. The freezing velocity or the solidificationrate of the solution can be tuned by changing the base metal plate.In current experiments, we have employed two types of metalplate: aluminum and copper. Since, thermal conductivity of copper(401 W/m K) is greater than the thermal conductivity of aluminum(167 W/m K), for the same ceramic concentration in solution,

Fig. 2. Microstructure of the synthesized composites (c and d) exhibiting staggered arranvarious samples. q and Uw indicate the aspect ratio and ceramic content respectively in

denser ceramic backbone (or, lower porosity ceramics) will beobtained which in-turn implies, higher weight fraction of ceramiccontent in synthesized ceramic–polymer composites. Additionally,the starting particle size is seen to have an effect on the solidifica-tion kinetics, and subsequently on the structural wavelength (orrather, the final thickness of the ceramic bricks). Hence, weemployed two different sized powders (particle diameters of8 lm and 1 lm) to control the structural wavelength and thereby,the aspect ratio of the ceramic blocks in the synthesized composite.Thus following this simple approach, both ceramic content andaspect ratio were controlled in the synthesized silica-SC 15 com-posites. Figs. 2a and c and 3b shows the staggered microstructureof a representative sample.

Table 1 lists the details of the above set of processes for differ-ent starting ceramic concentrations and powder size. Fig. 3a dem-onstrates the TGA curves of different samples for subsequentevaluation of ceramic content in the samples, the values of whichare tabulated in Table 1. Aspect ratio of the ceramic platelets isdefined as the ratio of length to the width of the ceramic platelet.Corresponding to each sample ID, SEM was done to obtain theaspect ratio of the ceramic platelets. Fig. 2a–d shows the micro-structure of the synthesized samples which helps in identifyingthe aspect ratio of the ceramic platelets in the composite. For agiven particle size as a precursor, as the ceramic content in thecomposite increases, the aspect ratio of the platelets decreaseswhich is intuitive as the thickness of the platelets remainsconstant; however, the length of the platelets decreases owing tohigher particle distribution in the interdendritic space [26]. Never-theless, the aspect ratio of the ceramic platelets varied from 6 to 10and 15 to16 for 8 lm and 1 lm ceramic spheres respectively. Cera-mic concentration in solution greater than 30 wt% and 50 wt% for1 lm and 8 lm were not probed into as it was difficult to maintainsuspension of ceramic particles in solution under those conditions.

An integral part of understanding predator–prey interactions isto first understand the mechanisms operating in food web dynam-ics. For instance in a marine food chain, otters and crabs can beconsidered as the predators who use their molar tooth and clawsrespectively to prey the protein-rich flesh of the organism (actingas the prey) hosted by seashells. Similarly, bone has to be durableenough to withstand high stresses associated with biting of tooth,

gement (a and b) shown as blown-out view to obtain aspect ratios associated withthe synthesized composites.

Fig. 3. TGA curves of the samples identifying varying ceramic content (Uw) in the synthesized composites (a); blown-out view another sample to obtain aspect ratiosassociated with it. q indicates aspect ratio.

Table 1Influence of starting powder size & metal (base) plate on ceramic content and aspectratios of the synthesized silica-SC 15 composites.

Ceramicconc. insolution(%)

Plate type Initialparticlesize(lm)

Ceramiccontent incomposite (Uw)⁄From TGA (%)

Aspectratio (q)⁄From

SEM

ID

30 Aluminum 1 35 15 Uw = 0.35/q = 15

30 Copper 1 40 15 Uw = 0.40/q = 15

40 Aluminum 8 55 10 Uw = 0.55/q = 10

40 Copper 8 58 10 Uw = 0.58/q = 10

45 Aluminum 8 60 10 Uw = 0.60/q = 10

50 Aluminum 8 65 7 Uw = 0.65/q = 7

50 Copper 8 70 6 Uw = 0.70/q = 6

A. Dutta, S.A. Tekalur / Materials and Design 46 (2013) 802–808 805

sudden fall, etc. Predatory attacks involve application of compres-sive loads to the structural component associated with the preythat has to be not only durable but fracture-resistant as well. How-ever, it can be pointed out that mechanical behavior of structuralbiocomposites is tuned predominantly for the function that it isspecifically designed for. For instance, bone is primarily designedfor withstanding time-dependent or, impact loading (for example,withstanding stresses while running, jumping, accidental fall) andhence the microstructure is designed primarily from enhancedtoughness point of view and moderate stiffness. On the contrary,seashells (which lie in the abyss of oceans) apart from providingdefense to the organism it is hosting against predatory attacks, isalso subjected to quasi-static loading (for example, enormoushydrostatic pressure). Hence, emphasis on the shell structure evo-lution is directed primarily towards providing enhanced stiffness.In order to observe whether the synthesized composites also exhi-bit this variance in mechanical behavior for varying ceramic con-tent and aspect ratio, quasi-static compression and fractureexperiments were performed to better understand this aspect.

3.1. Effect on composite stiffness and strength

Fig. 4a shows the representative stress–strain plots obtainedupon quasi-static compressive loading of the synthesized compos-ites. It has been shown theoretically, as shown in Fig. 5a and b, that

the stiffness of a staggered architecture composite can be tuned viacontrolling its aspect ratio for a given ceramic content under bothstatic [6] and dynamic rates [27] of loading. Indeed similar obser-vations have been observed from experimentally obtained data asshown in Fig. 4b and c where as the aspect ratio increases, the stiff-ness of the composite increases as well even if high polymeric con-tent is present in the composite. For a ceramic–polymer compositehaving a staggered microstructure, the ceramic forms the load-bearing component and load transfer from one platelet to anotheris accomplished via shear-deformation of the polymer. Larger isthe aspect ratio, larger will be the force that would be transmittedfrom one ceramic brick to another via the polymer matrix. Failureof the interfacial matrix is initiated at the extremities of the poly-meric layer joint via nucleation and growth of plastic zone fromeither ends of the overlap length towards the center [28–31].Indeed failure of the synthesized composite has been observedalong the interfaces as shown in Fig. 4e. Hence, aspect ratio ofthe ceramic platelets plays a superior role in its contribution tothe composite’s stiffness.

It might be speculated that the shear model might not be appli-cable under compressive loading scenario for staggered architec-ture composites, because microbuckling has been observed as thefailure mode operational under quasi-static and dynamic rates ofloading in seashells [2]. This aspect, however, can be addressedas follows [32]:

The ceramic backbone in a ceramic–polymer composite is brit-tle in nature. Hence, in these composites failure initiates at themicro-scales in the form of interfacial matrix failure starting atthe joint extremities. The nacreous layer in seashells is a exhibitsa hierarchical architecture (2–3 levels) where the staggered archi-tecture is exhibited only at the level of micro-scales; however, themeso-scale is representative of a lamellar architecture (equivalentto a fiber-reinforced composite structure) comprising of 300 lmlayers between nearly 20 lm layers of viscoplastic material [2].Plastic microbuckling is the dominant failure mechanism infiber-reinforced composites under the action of compressive loads,and thereby based on this analogy, microbuckling was henceobserved in seashells under compressive loading conditions. Simi-lar inferences have been laid down [2] where it has been observedin the lamellar structures of seashells that cracks initiates first inthe viscoplastic layers separating the mesolayers. Additionally,with reference to the microstructure of the synthesized compos-ites, ceramic bridges present amidst the ceramic layers (an attri-bute associated with nacre at micro-scales) also cripples Eulerbuckling of the ceramic layers at microscopic scales [23]. Similarly,

Fig. 4. Compressive stress–strain plots (a) for different sample ID’s as indicated in Table 1; variation of Young’s modulus of elasticity (b) and maximum compressive strength(c) of silica-SC 15 epoxy staggered composites for varying ceramic content and aspect ratio; (d) sample used for compressive experiments (specimen dimensions mentionedin Section 2); failure of the specimens along the interface (e).

Fig. 5. Dependence of stiffness of a staggered architecture composite as a function of aspect ratio and ceramic content as investigated theoretically under quasi-static [6] anddynamic rates of loading [27] (plot adapted from [27]).

806 A. Dutta, S.A. Tekalur / Materials and Design 46 (2013) 802–808

buckling is restrained at micro-scales, under rapid-compressionballistic conditions, via synchronized deformation twinning ofthe nano-scale particles [33].

Similarly, for a staggered ceramic–polymer composite, thestrength [6] can be expressed as follows:

S ¼ minq/Sp

2;/Sm

2

� �ð1Þ

where Sp is the strength of the polymer (or polymer–ceramic inter-face) and Sm is the compressive strength of the ceramic bricks.

A. Dutta, S.A. Tekalur / Materials and Design 46 (2013) 802–808 807

Based on the experimentally obtained data and the theoreticalfoundation [6,34], it can be clearly seen from Fig. 4c that aspectratio plays a superior role than ceramic content in its contributiontowards the strength of the staggered ceramic–polymer compositeand confirms that fine tuning of properties can be obtained in syn-thetic composites by varying ceramic content and aspect ratio.Thus, from the point-of-view of stiffness and strength, the aspectratio of mineral crystals significantly affects the mechanical proper-ties of the composite materials in a number of aspects (for instance,microstructural load transfer) as is evident from established theo-retical models [34] and current experimental observations.

3.2. Effect on composite toughness

What makes bioinspired staggered architecture very promisingfrom structural point of view is its unique structural arrangementwhich confers it superior toughness in comparison to its ceramicand polymer counterparts alone. Whether similar attributes existin the synthesized composites, fracture toughness of the compos-ites were measured to observe this effect and the results are shownin Fig. 6a and b.

ASTM C1421 was followed for the determination of fracturetoughness of the synthesized composites. Fig. 6a shows the repre-sentative load–displacement curves obtained during fracture test-ing of the samples in a three-point bending set up. As per ASTMC1421, determination of KIc, as shown in Fig. 6b is as follows:

KIc ¼ gPmaxS

BW3=2

� �3½a=W�1=2

2½1�½a=W��3=2

" #

where;

g¼ ga

W

� �¼1:99�½a=W�½1�a=W�½2:15�3:93½a=W�þ2:7½a=W�2�

1þ2½a=W�ð2Þ

As it can be seen from Fig. 6b, the measured fracture toughnessof all the synthesized composites is significantly greater than thefracture toughness of the ceramic (silica: 0.64 MPa m0.5) and thepolymer (SC 15: 1.54 MPa m0.5) [35]. Once again, the effect of bothaspect ratio and ceramic content of the ceramic can be seen to con-tribute a superior role to the composite’s fracture toughness even ifhigh polymeric content is present in the composite. This can beexplained as follows.

Fig. 6. Representative load–displacement curves obtained upon fracture testing of samplebrick and mortar composites with varying ceramic content and aspect ratio and their com(b).

Dugdale’s cohesive strip model [6,34] is given by the followingequation:

J ¼ ð1�UÞLZ

Cpdep ¼ ð1�UÞLHp minðSp; Sint;CpÞ ð3Þ

where J is the fracture energy, Hp denotes the effective strain towhich the polymer can deform before failure, Sp denotes the yieldstrength of the polymer, Sint denotes the ceramic–polymer interfacestrength, and sp denotes shear strength of the polymer. With refer-ence to Eq. (3), the expression under the integral is representative ofthe dissipation energy by the polymer per unit volume and thus, Hp

is a key parameter which contributes to fracture energy. The poly-meric matrix is thereby essential for enhancing toughness via en-ergy dissipation due to its viscoelastic nature that contributes tolarge deformation. Also it is discernible that inelasticity is a keymaterial parameter that renders a material notch insensitive byenabling a material to eliminate stress concentration at strain con-centration locales. However, the aspect ratio also plays an impor-tant role which is explained as follows. The staggeredmicrostructure provides a strain amplification mechanism [34]and contributes to full utilization of large deformation capabilityof the polymer.

ep ¼U

2ð1�UÞq e� Dm

L

� �/ qe ð4Þ

As it can be seen from Eq. (4), the capability of the polymeric ma-trix to be strained is magnified over composite strain by the aspectratio of the ceramic platelets, thereby allowing the polymer todeform and dissipate energy at the microscopic level without givingrise to large deformation on the composite level. For compositeshaving high ceramic content, fracture is often triggered before plas-tic instability inception via abrupt percolation of damage across thematerial [36]. This explains, that why toughness decreases in com-posites with high ceramic content and lower aspect ratios. As is evi-dent from Figs. 4c and 6b, higher toughness comes at a cost ofdecreasing strength; this is in agreement with analytical foundation[37] where this behavior can be attributed to increasing difficulty indriving the polymeric matrix deformation unless smaller aspectratios are used. Another way to enhance toughness of the compos-ites is by grafting the ceramic platelets with polymeric chains. Thisin turn will lead to increase in Sint which in current non-grafted syn-thesized composites is one of the most vulnerable entities undercontemporary loading conditions.

s in accordance with ASTM C1421 (a), variation of fracture toughness of silica-SC 15parison against the fracture toughness of silica (ceramic) and SC 15 (polymer) alone

808 A. Dutta, S.A. Tekalur / Materials and Design 46 (2013) 802–808

From structural engineering application view point, a knowl-edge and an understanding of the synthesis–structure–propertyrelationship in these composites is vital for the development ofadvanced ceramic–polymer composites with enhanced mechanicalstiffness, strength and toughness. Based on the informationobtained from experimental data, we can summarize our observa-tions as follows. If high stiffness and strength is desired, it wouldbe advantageous to have composites synthesized having lowaspect ratios and high ceramic content. Similarly, if high toughnessis desired, it would be advantageous to have composites synthe-sized having high aspect ratios and low ceramic content. This isin agreement with Wilbrink and co-authors [38] where they dem-onstrated, via development of a unit-cell model, that as the aspectratio of the bricks increases, a trade-off exists between increasingstrength and decreasing ductility. Indeed, the same set of phenom-ena can be observed in biological composites as well. As mentionedearlier, bone comprises of plate-like crystals (having an aspectratio varying from 25 to 50) embedded in a collagen-rich proteinmatrix (�0.40) resulting in variation of elastic modulus from �2to 25 GPa and a strength varying from �150 to 200 MPa. Similarly,nacre is made of enormously high ceramic content (�0.90–0.95) ofplate-like inorganic tablets (having an aspect ratio from 10 to 20)resulting in variation of elastic modulus from �40 to 70 GPa anda strength varying from �20 to 120 MPa [39]. From literature, itcan be recalled that toughening rate of bone is higher than thatof nacre; corresponding to a crack extension of 0.6 mm, resistanceincreases to 30 kJ m�2 in bone but only to 1.5 kJ m�2 in nacre[3,40]. Similarly, elk antler falls in the realm of tougher side ofthe bone family materials, exhibits a toughness increase of60 kJ m�2 at a crack extension of 0.6 mm [40].

4. Conclusion

‘Mother nature’ cleverly uses the aspect ratio and content ofinorganic matter to synthesize composites having varying degreesof stiffness, strength and toughness. Such an amazing capability offine-tuning the properties for functions has rarely been demon-strated in synthetic composites. In this article, an attempt has beenmade to synthesize staggered ceramic–polymer composites havingvarying ceramic content and aspect ratio using freeze-casting tech-nique. Inferences obtained from experimental investigation pro-vides us useful information as to their effects on the mechanicalproperties of synthesized composites. This will in-turn help us incustom-design manufacture of hybrid bio-inspired compositematerials.

Acknowledgement

Research was sponsored by the Army Research Laboratory andwas accomplished under Cooperative Agreement NumberW911NF-08-2-0059.

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