1

Liquid Metal Synthesis of Two-Dimensional Aluminium Oxide Platelets to Reinforce Epoxy 1

Composites 2

Anil R. Ravindrana, Raj B. Ladania, Ali Zavabetib,c, Torben Daenekeb, Shuying Wud,e, Anthony J. 3

Kinlochf, Chun H. Wange, Kourosh Kalantar-Zadehg and Adrian P. Mouritza* 4

a Sir Lawrence Wackett Aerospace Research Centre, School of Engineering, RMIT University, GPO 5

Box 2476, Melbourne, VIC 3001, Australia. 6

b School of Engineering, RMIT University, GPO Box 2476, Melbourne, VIC 3001, Australia 7

c College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, 8

29 Jiangjun Ave, 211100, Nanjing, China 9

d School of Engineering, Macquarie University, Sydney, NSW 2109, Australia 10

e School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, 11

NSW 2052, Australia 12

f Department of Mechanical Engineering, Imperial College London, South Kensington Campus, 13

London SW7 2AZ, U.K. 14

g School of Chemical Engineering, University of New South Wales, Sydney, NSW 2052, Australia 15

Corresponding author: *adrian.mouritz@rmit.edu.au 16

17

Abstract 18

A liquid metal synthesis process provides a new low energy pathway avenue to synthesise various 19

low-dimensional nanomaterials in order to improve the mechanical properties of polymer 20

composites. This paper presents an investigation of the strengthening and toughening performances 21

of two-dimensional platelets of boehmite (γ-AlO(OH)) and alumina (γ-Al2O3). Using a liquid metal 22

alloy reaction process, two-dimensional metal oxide hydroxide and oxide platelets were synthesised 23

and then used for reinforcing epoxy polymer composites at different weight fractions up to 10%. 24

Both boehmite and alumina platelets increased the tensile modulus, yield stress and fracture 25

toughness of the epoxy composite by up to 40%, 35% and 320%, respectively. Of the two materials, 26

the boehmite platelets were more effective than the alumina platelets in increasing the tensile 27

modulus (up to 27%) and ultimate strength (up to 14%) of the epoxy. In contrast, the alumina 28

platelets promoted a 50% greater improvement in the mode I fracture energy when compared to 29

using boehmite platelets. The primary mechanisms responsible for the measured property 30

improvements are identified. 31

2

Keywords: Particle-reinforced composite; Liquid metal; Oxides; Fracture toughness 1

1. INTRODUCTION 2

Aluminium oxides have a wide range of applications which include uses in pharmaceuticals, medical 3

implants, abrasives and electrical insulators [1-4]. Alumina (Al2O3) is also employed as a filler 4

material to increase the mechanical properties of polymers due to its high stiffness and strength. 5

Spherical or platelet-based Al2O3 nanofillers can increase the tensile properties, fracture toughness 6

and fatigue strength of epoxy polymers [5-9]. Boehmite (AlO(OH)) nanofillers, which is a hydrated 7

precursor in the processing of alumina, can also improve the mechanical properties of epoxies [10, 8

11]. The magnitudes of the property improvements using either alumina or boehmite particles 9

depend on their morphology, size and concentration [5, 8, 11]. Boehmite particulates, which are used 10

for synthesising various types of aluminium oxides (e.g. α-Al2O3, γ-Al2O3), have been produced 11

through several processes [11-13]. These include solid-state decomposition of gibbsite (Al(OH)3) 12

[14], precipitation from an acidic or basic aluminium salt solution via neutralisation and ageing [15], 13

and sol-gel processing [16]. Recently a new technique called liquid metal synthesis has been reported 14

as an effective way of producing two-dimensional metal oxide nanosheets [17-23]. Such liquid 15

metals are made by alloying a low melting temperature metal such as gallium with transition and 16

post-transition metals [13, 17-19, 23]. Galinstan is a eutectic alloy of gallium (68.5 wt%), indium 17

(21.5 wt%) and tin (10 wt%), with a low melting temperature (i.e. 19oC to 11oC) [13, 18, 24]. 18

Zavabeti et al. recently discovered [18] that low-dimensional sub-nanometer thick sheets of metal 19

oxides can be synthesised by alloying various reactive metals, such as hafnium, aluminium and 20

gadolinium, with galinstan. Ultra-thin sheets of metal oxides are then formed at the liquid galinstan 21

surface, which can be exfoliated via a reaction with deionised water [13]. The continuous generation 22

of low-dimensional metal oxide nanoparticles of various shapes (e.g. platelets and rods) can be 23

achieved until eventually all the reactive alloying elements within the galinstan are depleted. 24

Zavabeti et al. [13] reported that high flux membranes containing sub-nanometre sheets and fibres of 25

γ-AlO(OH) and γ-Al2O3 (via annealing) could be produced for water filtration applications using the 26

liquid metal synthesis process. Benefits of this liquid metal approach include the reusability of 27

3

galinstan for multiple synthesis cycles and low input energy at room temperature [13, 18]. 1

Additionally, two dimensional (2D) nanosheets of aluminium oxides produced by liquid metal 2

synthesis [13] have been reported to exhibit a significantly higher Young’s modulus compared to 3

other alumina fillers [25, 26]. However, there are no reports into the effect of reinforcing polymers 4

with low-dimensional alumina or boehmite platelets produced from liquid metals. Their aspect ratio 5

and two-dimensional morphology may result in higher property improvements compared to oxides 6

produced from other processes. Furthermore, it is not clear which of the two types of platelets, i.e. γ-7

AlO(OH) and γ-Al2O3, is more effective in strengthening and toughening epoxy composites. 8

The aim of the present study is to determine the effects of low-dimensional γ-AlO(OH) and 9

γ-Al2O3 platelets synthesised using liquid metal processing on the mechanical and fracture properties 10

of an epoxy composite. Boehmite (γ-AlO(OH)) platelets were grown on the surface of the liquid 11

metal (galinstan alloyed with aluminium), and were then thermally annealed to form alumina (γ-12

Al2O3) platelets. These platelets were then added to an epoxy at different weight fractions up to 10%. 13

Tensile and fracture tests were carried out to assess the improvements in the mechanical properties of 14

the epoxy. The improvements to the tensile and fracture toughness properties derived from using the 15

γ-AlO(OH) or γ-Al2O3 platelets were compared to employing alumina particles of previous reports to 16

show the enhancement that is achieved via our introduced procedure. Detailed microscopic 17

examinations are conducted to identify the strengthening and toughening mechanisms responsible for 18

the property improvements. 19

2. MATERIALS AND EXPERIMENTAL DETAILS 20

2.1 Liquid metal synthesis of boehmite and alumina platelets 21

A liquid metal water-based synthesis process similar to that developed by Zavabeti et al. [13, 18] was 22

used for synthesising platelets of boehmite and (following thermal annealing) alumina. The multi-23

stage synthesis process is shown schematically in Fig.1. A eutectic alloy of galinstan was prepared 24

by melting and mixing 68.5 wt% gallium, 21.5 wt% indium and 10 wt% tin at 250°C (Fig. 1a). 25

These precursor materials had a purity of ~99.99% (Roto Material Inc.). Further details on the 26

4

preparation of galistan are provided in [13, 18]. The galinstan (in 50 g batches) was then 1

mechanically alloyed with Al rods (>99.99% purity, ~2-3 mm length, 0.58 mm diameter) at ~3.3 2

wt% by grinding for 20 minutes using a pestle and mortar at room temperature (25°C). The 3

aluminium embrittles and becomes soluble when ground thoroughly with galinstan [27]. The 4

grinding was performed in an inert atmosphere (N2 gas, <2 ppm O2) at 25°C to stop oxidation of the 5

aluminium before it dissolved in the galinstan. Deionised water was then added to the liquid 6

galinstan-aluminium alloy to synthesise boehmite (see Fig. 1b) from the reaction process: 7

2Al + 6H2O → 2Al(OH)3 + 3H2 (1) 8

The reaction formed an ultra-thin film of α-Al(OH)3 along with H2 gas at the liquid galinstan surface 9

(Fig. 1c). Nucleation of H2 bubbles forced the α-Al(OH)3 to delaminate into thin sheets in the water 10

phase of the solution. The α-Al(OH)3 sheets were extracted from the solution using a pipette (see 11

Fig. 1d) and drop-casted onto a glass petri dish. The α-Al(OH)3 was then converted into γ-AlO(OH) 12

at 170°C (see Fig. 1e) via the following reaction: 13

Al(OH)3 → AlO(OH) + H2O (2) 14

The γ-AlO(OH) platelets were extracted from the petri dish and dried at 200°C for 24 hours. As 15

shown in Fig. 2a, the γ-AlO(OH) platelets extracted consists of multiple sheets, each measuring ~1.2 16

– 2 nm [13]. 17

The γ-Al2O3 platelets were synthesised by annealing γ-AlO(OH) at 550°C for 15 hours. The 18

conversion process is: 19

2AlO(OH) → Al2O3 + H2O (3) 20

The mean diameter and thickness of the γ-AlO(OH) and γ-Al2O3 platelets were measured using a 21

scanning electron microscope (FEI Nova NanoSEM) and digital image correlation software (ImageJ, 22

NIH, USA). The mean thickness of the platelets was measured from scanning electron micrographs 23

of the fracture surface of the tensile test samples. This process of measuring the particle size was 24

adapted from [28, 29]. The distributions of the diameter and thickness of the two platelets are 25

5

presented in Fig. 1f and Fig. 2b. The mean diameter and thickness of the γ-AlO(OH) are both 1

slightly larger than those pertinent to γ-Al2O3. The typical shape and morphology of the platelets and 2

the individual sheets are shown in Figs. 1f1, 2a, 2c and 2d. The crystallinity of the γ-AlO(OH) and γ-3

Al2O3 sheets can be found in [13]. 4

2.2 Manufacturing of epoxy composites 5

The γ-AlO(OH) and γ-Al2O3 platelets were added to a low-temperature cure epoxy at different 6

weight fractions (0% (i.e. unmodified), 1%, 2%, 5%, 10%). The epoxy was a two-part bisphenol-A 7

and bisphenol-F based resin system (Resin-105 from West System®) [30]. The platelets were initially 8

mixed mechanically into the resin for five minutes using a stirring rod and then further dispersed 9

using a three-roll mill (Dermamill 100). This milling process involved passing the epoxy-metal oxide 10

liquid mixture between three rolls rotating at 50 rpm, and with each pass, the gap opening between 11

the rollers was reduced progressively down to 20 μm in the final pass. Following the milling process, 12

the amine hardening agent was added to the mixtures at the weight ratio of 1-to-5 to activate curing. 13

The hardener (Resin 206 from West System®) was a blend of aliphatic amines and aliphatic amine 14

adducts based on diethylenetriamine (C4H13N3) and triethylenetetramine (C6H18N4). Before curing, 15

the mixtures were degassed in a vacuum chamber for five minutes to remove entrapped air. The 16

liquid mixtures were then cast in moulds to produce coupons for tensile and fracture toughness 17

testing. Before testing, the coupons were cured at room temperature following the manufacturers’ 18

guidelines [30], but without an elevated temperature post-cure. 19

2.3 Tensile and fracture toughness testing 20

The tensile properties of the unmodified epoxy and different types of epoxy composites were 21

measured using the specimen design (ASTM D638 Type IV configuration) shown in Fig. 3a. The 22

tensile tests were conducted using a 10 kN Instron universal testing machine operated at an extension 23

rate of 1 mm/min in accordance with ASTM D638 [31]. The strain was measured using an 24

extensometer (initial gauge length of 25 mm) attached to the gauge section of the tensile sample. A 25

6

minimum of five samples was tested for each material. The yield stress was measured using the 0.2% 1

strain offset method. 2

The mode I fracture energy and toughness of the materials was measured using the double 3

cantilever beam (DCB) test. The DCB specimen consisted of a 2 mm thick layer of the epoxy-based 4

material sandwiched between two bonded composite adherends (Fig. 3b). Each adherend was made 5

of unidirectional T700 carbon fibre/epoxy (VTM264, Lavender Composites, Australia) composite. 6

The pre-crack in a DCB sample was made by initially embedding a polytetrafluoroethylene (PTFE) 7

film measuring 42.5 mm long and 11 μm thick at the mid-plane of the epoxy layer, followed by 8

wedging to sharpen the crack tip and to extend the crack length to 50 ± 2.5 mm. The DCB test 9

involved applying a monotonically increasing crack opening displacement to the pre-cracked end of 10

the specimen at a rate of 1 mm/min using a 10 kN capacity Instron universal test machine (ISO 11

25217 [32]). The crack was forced to grow incrementally along the mid-plane of the epoxy-based 12

layer by loading and unloading. At each increment, the crack opening force, crack opening 13

displacement and crack length were measured, which were then used to calculate the mode I fracture 14

energy and toughness. Five tensile and DCB samples were tested for each type of composites. 15

3. Results and Discussion 16

3.1 Tensile properties 17

The effect of γ-AlO(OH) and γ-Al2O3 platelets on the tensile properties of the epoxy polymer is 18

shown in Fig. 4 and Fig. 5. The Young’s modulus increased with the weight fraction of platelets, 19

with γ-AlO(OH) providing a slightly ~14-33% greater stiffening effect than γ-Al2O3, as shown in 20

Fig. 5a. The higher moduli of the γ-AlO(OH)/epoxy composites are statistically significant with a 21

99.8% confidence based on an unpaired t-test. Zavabeti et al. [13] reported that the DMT (Derjaquin-22

Muller-Toporov) modulus of the γ-AlO(OH) (496 GPa), which is similarly used in the epoxy 23

composites studied here, is approximately five times greater than the γ-Al2O3 (96 GPa) platelets. 24

Using the stiffness values of the boehmite and alumina fillers (Ef) above, a modified Halpin-Tsai 25

7

analytical model [33] was adapted to predict the tensile modulus (Ec) of the epoxy composites 1

containing randomly oriented platelets using the expression below: 2

𝐸𝑐 = 3𝐸𝑚8

⎣⎢⎢⎢⎡2𝑤𝑡 �

𝐸𝑓𝐸𝑚

−1𝐸𝑓𝐸𝑚

+2𝑤𝑡

�𝑉𝑓+1

1−2𝑤𝑡 �

𝐸𝑓𝐸𝑚

−1𝐸𝑓𝐸𝑚

+2𝑤𝑡

�𝑉𝑓⎦⎥⎥⎥⎤

+ 5𝐸𝑚8

⎣⎢⎢⎢⎡4𝑤𝑡 �

𝐸𝑓𝐸𝑚

−1𝐸𝑓𝐸𝑚

+2�𝑉𝑓+1

1−2𝑤𝑡 �

𝐸𝑓𝐸𝑚

−1𝐸𝑓𝐸𝑚

+2�𝑉𝑓

⎦⎥⎥⎥⎤

(4) 3

where the tensile modulus of the epoxy matrix (see Table 1) and the volume fraction of the platelets 4

are denoted by Em and Vf, respectively. The model used considers the diameter (i.e. width) (w) and 5

thickness (t) of the platelets presented in Table 1. The analytically calculated tensile modulus are in 6

good agreement with experimentally measured value at filler contents up to 2 wt% (see Fig. 5a). 7

However, the analytical model overpredicts the stiffness value of the epoxy composites containing 8

higher contents (i.e. greater than 2 wt%) of γ-AlO(OH) or γ-Al2O3 platelets. This may be attributed 9

to the agglomeration of the platelets caused by inadequate dispersion, which is a common problem 10

with dispersing high concentration of particles [34]. Such agglomeration results in a plateau in the 11

stiffness enhancement observed (see Fig. 5a) with filler content. 12

The yield stress of the epoxy increases with the weight percentage of the filler, with the γ-13

AlO(OH) giving a ~10-13% greater enhancement than the γ-Al2O3 (Fig. 5b). The ultimate strength 14

of the epoxy increased to ~14% with the γ-AlO(OH) content of up to 5 wt%, but there was a slight 15

loss in the strengthening effect at the highest content of 10 wt% (see Fig. 5c). This is also attributed 16

to poor dispersion and agglomeration of the platelets at higher contents [34]. The agglomerated 17

platelets act as localised stress raisers within the polymer matrix [10, 35], causing a reduction in the 18

ultimate strength as well as minimising the stiffening efficacy with the filler content. The γ-Al2O3 19

had no statistically significant effect on the ultimate strength of the composite, even at the highest 20

content. The difference in the strengthening effect between the two types of platelet is attributed to 21

differences in their bonding to the epoxy matrix. Fractographic examination of the broken tensile 22

specimens revealed that a large proportion of γ-AlO(OH) platelets ruptured (see Fig. 6a) whereas the 23

γ-Al2O3 platelets debonded and pulled-out (see Fig. 6b) from the epoxy matrix. Fracture of the γ-24

8

AlO(OH) is indicative of its strong bonding to the epoxy, which is promoted by the hydroxyl groups 1

covalently bonding to amine and tertiary hydroxide groups of the epoxy during the curing process 2

[10, 36-38]. The covalent bonding promotes stress transfer across the platelet-matrix interface and 3

thereby increased the yield and ultimate tensile strengths of the composites. 4

By contrast, interfacial debonding followed by pull-out (without fracture) of the γ-Al2O3 5

platelets is indicative of its weaker bonding and consequently disbonding before rupture. These 6

platelets do not covalently bond to epoxy, and instead bonded to the matrix via van der Waals forces. 7

Opelt et al. [9] reported that weak bonding exists between alumina particles and epoxy polymer due 8

to the absence of functional groups to promote covalent bonding. For this reason, the γ-Al2O3 does 9

not significantly increase the ultimate tensile strength of the epoxy when compared to using γ-10

AlO(OH). The ultimate tensile strength (𝜎𝑐) of the epoxy composites containing γ-Al2O3 was 11

predicted using the rule-of-mixtures expression: 12

𝜎𝑐 = 𝜎𝑓𝑉𝑓 + 𝜎𝑚(1 − 𝑉𝑓) (5) 13

based on the ultimate tensile strength of the alumina fillers (σf = 300 MPa [39]) and of the 14

unmodified epoxy (σm) presented in Table 1. The rule-of-mixtures model assumes that the γ-Al2O3 15

platelets are well bonded (e.g. covalently) with the epoxy matrix up to the point of failure, when both 16

the platelets and the epoxy matrix fail simultaneously. Due to this incorrect assumption, the predicted 17

ultimate tensile strengths of the γ-Al2O3/epoxy composites from the model (see Fig. 5c) significantly 18

exceeds the experimentally measured values. The analytical predictions for the epoxy composites 19

containing γ-AlO(OH) could not be conducted as no published reports on the ultimate tensile 20

strength of boehmite could be found. 21

Table 1 and Fig. 7 present a comparison of the values and relative improvements to the 22

Young’s modulus and ultimate tensile strength from the present study with data reported in the 23

literature with AlO(OH) or Al2O3 fillers at various sizes, morphologies and contents [5, 6, 8, 10]. 24

The tensile property values for the composites measured herein are compared with published values 25

for other epoxy polymers containing AlO(OH) or Al2O3 with either a spherical (0D) or platelets (2D) 26

9

morphology. It is important to note the types of epoxy used from the present study and other reports 1

from the literature are not the same (i.e. different type of epoxy and curing conditions). Therefore, 2

the percentage improvements in the tensile property values presented in Fig. 7 are normalised based 3

on the type of unmodified epoxy used within the present work and reported in the literature [5, 6, 8, 4

10]. With the addition of the aluminium oxide particles, there is a general improvement in the 5

Young’s modulus of the composite by up to 40%. However, the greatest relative improvements in 6

the tensile properties correspond to when the AlO(OH) platelets are employed, which were obtained 7

using the liquid metal synthesis process. However, the ultimate tensile strengths measured in the 8

present work when using either the γ-AlO(OH) or the γ-Al2O3 platelets in the epoxy composites are 9

relatively modest compared to the values reported by Wu et al. [10], Lim et al. [5] and Zhang et al. 10

[6]. A major reason for this is clearly that their type of unmodified epoxy polymers had a 11

significantly greater ultimate tensile strength than the one employed in the present study. 12

3.2 Fracture toughness properties 13

The mode I fracture energy (GIc) measured for the unmodified epoxy and the epoxy composites are 14

given in Fig. 8. The GIc values of the epoxy increase with the γ-AlO(OH) content, and at the highest 15

weight fraction (i.e. 10%) the toughness is improved by ~270%. The GIc values also increase with 16

the weight fraction of γ-Al2O3 platelets, although the highest toughness (with an improvement of up 17

to ~320%) corresponds to a content of 5 wt%. Up to this platelet content, the γ-Al2O3 fillers is ~50% 18

more effective than the γ-AlO(OH) at increasing the GIc value. However, the toughening induced by 19

γ-Al2O3 platelets decreases at the highest content and the reasons for this are described below. The 20

platelets not only improved the GIc value but also improved the stability of the crack growth process 21

in some of the epoxy composites. Typical load versus crack opening displacement (P-δ) curves 22

measured during DCB testing of the epoxy composites containing different weight fractions of γ-23

AlO(OH) or γ-Al2O3 platelets are shown in Fig. 9. The P-δ curve for the unmodified epoxy polymer 24

shows a series of large and abrupt drops in the crack opening force with increasing displacement, 25

which is indicative of unstable (slip-stick) crack growth. Unstable cracking is commonly observed in 26

unmodified epoxies due to their intrinsically low toughness. Unstable cracking also occurred in the 27

10

epoxy composites reinforced with γ-AlO(OH), as characterised by the slip-stick behaviour of the 1

crack growth process. More stable crack growth, indicated by the absence of large and sudden load 2

drops in the P-δ curve can be observed in the epoxy reinforced with γ-Al2O3, except at the lowest 3

content. The crack growth process transitioned from unstable to stable when the γ-Al2O3 content was 4

above 1 wt%. 5

The γ-AlO(OH) or γ-Al2O3 platelets induced intrinsic toughening mechanisms in the process 6

zone at the crack tip and extrinsic toughening mechanisms in the crack wake, as illustrated 7

schematically in Fig. 10. Both γ-AlO(OH) and γ-Al2O3 platelets induced localised plastic shear 8

yielding the matrix, contributing to the intrinsic toughening. Localised plastic yielding of the epoxy 9

matrix occurred adjacent to the γ-AlO(OH) and γ-Al2O3 platelets due to the stress concentrations 10

induced by their high stiffness (Figs. 11a & 11b), and this contributed to the increased GIc value for 11

both types of composites. The γ-Al2O3 platelets tended to fully debond from the epoxy matrix under 12

the triaxial (i.e. hydrostatic) stress field concentrated within the crack tip process zone (Fig. 11b) 13

because these platelets were only weakly bonded to the epoxy. Particle-matrix debonding is an 14

effective toughening process in some composite materials [40, 41]. The γ-AlO(OH) platelets, which 15

were more strongly bonded to the matrix, showed very limited debonding (Figs. 11a and 12a) and 16

therefore were less effective than γ-Al2O3 at increasing the fracture energy. The plastic shear 17

yielding of the epoxy adjacent to the debonded γ-Al2O3 platelets led to the initiation and growth of 18

microvoids (Fig. 11c). However, γ-AlO(OH) platelets did not generate microvoiding (or cavitation) 19

because their strong bonding to the matrix inhibited complete debonding. Void formation is an 20

effective intrinsic toughening process in polymers [41-43], and its absence in the γ-AlO(OH) 21

composites accounts, in part, for their lower GIc values compared to the γ-Al2O3 composites. The 22

density of microvoids within the process zone increased with γ-Al2O3 content, which correlates to 23

the rapid increase in the GIc value up to a content of 5 wt%. The maximum size of the voids (~15-25 24

µm) was measured using scanning electron microscopy to be 3 to 5 times the size of the γ-Al2O3 25

platelets. At a given filler content, optimal toughening occurs in composites when the spacing 26

between stiff and strong inclusions/particles is sufficiently large, and the fillers are evenly dispersed 27

11

so that they do not impede the plastic flow of the epoxy matrix needed for void growth to occur [44, 1

45]. The growth of microvoids was impeded at the highest γ-Al2O3 content (i.e. 10 wt%) due to (i) 2

the relatively close spacing and increased the packing density of the platelets and (ii) the tendency of 3

the platelets to form agglomerates at this relatively high content (Fig. 11d). 4

Multiple extrinsic toughening mechanisms were operative over a very short distance (~35 5

µm) behind the main crack front in the composites, including rupture of the γ-AlO(OH) platelets and 6

pull-out of the γ-Al2O3 platelets. Platelets that were orientated in a near-normal direction to the 7

direction of crack growth were able to bridge the crack. This bridging action generates a traction load 8

that reduces the stress exerted on the crack tip, thereby increasing the fracture energy. Under 9

increasing crack opening, the γ-AlO(OH) platelets bridging the crack wake ruptured along the crack 10

propagation plane (Figs. 12a and 12b) whereas the bridging γ-Al2O3 platelets were pulled-out 11

(usually without breaking) (Figs. 11b and 12c). The strong bonding of the γ-AlO(OH) platelets to the 12

epoxy restricted their pull-out, so they ruptured instead. With a weaker bonding to the matrix, the γ-13

Al2O3 platelets debonded from the epoxy in the crack tip process zone and underwent pull-out under 14

increasing crack opening. The interfacial shear friction traction stress generated during particle pull-15

out promotes a stronger toughening effect than when the particles fracture [46]. Therefore, the 16

additional toughening mechanism of particle pull-out, together with the other processes of interfacial 17

debonding and microvoid formation, are responsible for the γ-Al2O3 platelets being more effective 18

than the γ-AlO(OH) platelets for increasing the GIc value of the epoxy polymer. It is likely that these 19

additional mechanisms, and the resultant increase in fracture energy, are also responsible for the 20

more stable (i.e. less slip-stick) crack growth process in the epoxy polymers reinforced with γ-Al2O3. 21

The platelets obtained herein using the liquid metal reaction method are more effective at 22

increasing the mode I fracture toughness or critical stress intensity factor (KIc) of epoxy when 23

compared to other types of aluminium oxide based reinforced epoxy. A comparison of the toughness 24

values and relative improvements of epoxy containing aluminium oxide particles and platelets is 25

presented in Table 1 and Fig. 13. It is shown that the γ-AlO(OH) and γ-Al2O3 platelets synthesised 26

using the liquid metal processing technique promote a much greater relative percentage improvement 27

12

(see Fig. 13) in the fracture toughness values of the epoxies than the aluminium oxides reported in 1

[5, 6, 8, 10]. However, it is important to note that the magnitude of the toughening effect induced by 2

particles and platelets depends on several factors, with the dominant factors being the filler aspect 3

ratio [8], size or length [5], shape [8], surface chemical functionality and type of epoxy used [7, 9]. 4

The γ-AlO(OH) platelets prepared in this study are much larger (i.e. ~6.1 µm) in diameter than those 5

reported in other studies (i.e. with an average size ~20-200 nm). The relative large size of the γ-6

AlO(OH) aids both the intrinsic (e.g. void growth) and extrinsic (e.g. crack bridging) processes in 7

toughening the epoxy polymer. Similarly, the γ-Al2O3 platelets used in this study were also in the 8

form of platelets with a diameter of ~5.30 µm. In contrast, other studies have employed alumina 9

particles in the shape of spherical particulates or platelets at smaller dimensions (see Table 1). 10

4. Conclusions 11

The tensile modulus, yield stress and fracture toughness of an epoxy polymer have been greatly 12

improved by incorporating low-dimensional boehmite γ-AlO(OH) and alumina γ-Al2O3 platelets. 13

The boehmite platelets were synthesised via a liquid metal reaction technique and the alumina γ-14

Al2O3 platelets were obtained by annealing the boehmite γ-AlO(OH) platelets. The Young’s modulus 15

and yield stress of the epoxy increased with increasing boehmite or alumina content, with the 16

boehmite platelets providing a slightly stronger reinforcing effect due to their higher elastic modulus. 17

The ultimate tensile strength of the epoxy composites also increased, at least initially, with increasing 18

boehmite content, while the alumina platelets resulted in no signficant increase. The reason for the 19

higher effectiveness of the boehmite was that this type of platelet covalently bonds with the epoxy 20

matrix to give a relatively strong interface between the γ-AlO(OH) platelets and the epoxy matrix. 21

The mode I fracture energy and toughness increased with the content of the boehmite γ-AlO(OH) or 22

alumina γ-Al2O3 platelets, with the alumina providing a higher toughness up to a limiting 23

concentration of 5 wt%. The toughness of the epoxy composite was increased by the boehmite due to 24

this type of platelet (i) inducing more localised plastic flow at the crack tip and (ii) bridging across 25

the crack faces in the wake of the advancing crack. The alumina, due to its weaker bonding to the 26

epoxy, increased the toughness via (i) interfacial debonding, plastic yielding and microvoid 27

13

formation at the crack tip and (ii) bridging and pull-out in the crack wake. These additional intrinsic 1

and extrinsic toughening mechanisms initiated by the presence of the alumina platelets are 2

responsible for the alumina being more effective than boehmite at increasing the toughness. 3

In adopting the liquid metal synthesis process to produce low-dimensional aluminium oxides 4

to improve the mechanical properties of polymer composites, this work has demonstrated the 5

efficacy of boehmite and alumina platelets. Indeed, most importantly, the relative improvements in 6

some of the mechanical and fracture toughness properties by the boehmite and alumina platelets, 7

obtained by the liquid metal technique, were significantly higher than those previously reported for 8

alumina reinforced epoxy composites. It is feasible that these platelets could also be beneficial in 9

enhancing other properties of polymers such as fire resistance, dielectric properties, thermal 10

conductivity and thermo-mechanical properties [11], and these aspects are currently under 11

investigation. 12

Acknowledgements 13

The authors are thankful for the financial support received from the Australian Research Council’s 14

Discovery Grant Program (DP140100778). 15

References 16

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28

29

30

31

32

33

34

16

1

Fig. 1. Schematic depicting the liquid metal synthesis process of γ-AlO(OH) where (a-b) deionised 2

water is added to a vial containing the galinstan-aluminium liquid metal alloy resulting in the 3

formation of a thin bayerite (α-Al(OH)3) layer at the alloy surface. (c) Delamination of bayerite 4

sheets due to nucleation of hydrogen gas bubbles between the liquid metal alloy and water interface. 5

(d) Bayerite solution extracted via pipette and (e) drop-casted onto petri dish heated at 170˚C to 6

convert bayerite to boehmite. (f) Probability density plots of γ-AlO(OH) and γ-Al2O3 platelet 7

diameters. (f1) SEM image of γ-AlO(OH) platelet. (Note: the probability density for the diameter 8

measurements are in increments of ±0.1 µm). 9

17

1

Fig. 2. (a) SEM image of γ-AlO(OH) platelet cross-section. (b) Probability density plots of γ-2

AlO(OH) and γ-Al2O3 platelet thickness (Note: the probability density for the thickness 3

measurements are in increments of ± 0.1 µm). Transmission electron microscope images of the (c) γ-4

AlO(OH) and (d) γ-Al2O3 platelets along the diameter plane. 5

6

Fig. 3. Schematic of (a) the tensile test specimen and (b) double cantilever beam (DCB) specimen. 7

(b) (a)

18

0.0 0.5 1.0 1.5 2.0 2.5 3.00

10

20

30

40

50

60

Tens

ile S

tres

s (M

Pa)

Strain (%)

Unmodified 1 wt% γ-AlO(OH) 2 wt% γ-AlO(OH) 5 wt% γ-AlO(OH) 10 wt% γ-AlO(OH)

0.0 0.5 1.0 1.5 2.0 2.5 3.00

10

20

30

40

50

60

Tens

ile S

tres

s (M

Pa)

Strain (%)

Unmodified 1 wt% γ-Al2O3

2 wt% γ- Al2O3

5 wt% γ-Al2O3

10 wt% γ-Al2O3

1

Fig. 4. Tensile stress-strain curves for the epoxy with different weight contents of (a) γ-AlO(OH) and 2

(b) γ-Al2O3 platelets. (Note: each curve is from one characteristic sample for each composite type 3

tested). 4

0 1 2 3 4 5 6 7 8 9 102

3

4

5

6

7

Tens

ile M

odul

us (G

Pa)

Filler Content (wt%)

γ-AlO(OH) - Exp. γ-Al2O3 - Exp. γ-AlO(OH) - Model γ-Al2O3 - Model

5

0 1 2 3 4 5 6 7 8 9 1030

35

40

45

50

0.2%

Offs

et Y

ield

Str

ess

(MPa

)

Filler Content (wt%)

γ-AlO(OH) - Exp. γ-Al2O3 - Exp.

6

0 1 2 3 4 5 6 7 8 9 1045

50

55

60

65

Ulti

mat

e Te

nsile

Str

engt

h (M

Pa)

Filler Content (wt%)

γ-AlO(OH) - Exp. γ-Al2O3 - Exp. γ-Al2O3 - Model

7

Fig. 5. Effect of increasing the concentration of γ-AlO(OH) and γ-Al2O3 on the (a) Young’s 8

modulus, (b) 0.2% offset yield stress and (c) ultimate tensile strength of the epoxy composite. (Note: 9

error bars represent the standard deviations.) 10

(c)

(a) (b)

(a)

(b)

19

1

2

Fig. 6. SEM micrographs from the fracture surface of the tensile test specimens for an epoxy 3

composite containing (a) 5 wt% γ-AlO(OH) and (b) 5 wt% γ-Al2O3. 4

0 2 4 6 8 10

-10

0

10

20

30

40

50

60

Rel

ativ

e

AlO(OH) [Present Study] Al2O3 [Present Study] Al2O3 [5] Al2O3 [6] Al2O3 [8]

Youn

g's

Mod

ulus

Impr

ovem

ent (

%)

Filler Content (wt%)

Present study

0 2 4 6 8 10

-15

0

15

30

45

60R

elat

ive

AlO(OH) [Present Study] AlO(OH) [10] Al2O3 [Present Study] Al2O3 [5] Al2O3 [6] Al2O3 [8]

Ulti

mat

e Te

nsile

Str

engt

h Im

prov

emen

t (%

)

Filler Content (wt%)

Present study

5

Fig. 7. Relative percentage improvement (compared to the unmodified epoxy) for the (a) Young’s 6

modulus and (b) ultimate tensile strength of the composites studied in the present work and other 7

types of epoxy composites containing AlO(OH) or Al2O3 nanoparticles reported in [5,6,8,10]. (Note: 8

the type of epoxies reported from the present work and other studies are different [5,6,8,10]. 9

Therefore, the relative percentage improvement values were calculated and normalised using the 10

respective tensile property values from Table 1 for the epoxy composites reported within the present 11

study and in other sources [5,6,8,10].) 12

13

(b) (a)

(a) (b)

20

0 2 4 6 8 100

100

200

300

400

500

600

700

GIc (J

/m2 )

Filler Content (wt%)

γ-AlO(OH) γ-Al2O3

1

Fig. 8. Effect of γ-AlO(OH) and γ-Al2O3 contents on the mode I fracture energy of the epoxy 2

polymer. (Note: error bars represent the standard deviations.) 3

4

0 2 4 6 8 10 120

20

40

60

80

100

120

140

Load

(N)

Crack Opening Displacement (mm)

Unmodified 1 wt% γ-AlO(OH) 2 wt% γ-AlO(OH) 5 wt% γ-AlO(OH) 10 wt% γ-AlO(OH)

stick-slip crack propagation

0 2 4 6 8 10 120

20

40

60

80

100

120

140

Load

(N)

Crack Opening Displacement (mm)

Unmodified 1 wt% γ-Al2O3

2 wt% γ-Al2O3

5 wt% γ-Al2O3

10 wt% γ-Al2O3

stable crack propagation

5

Fig. 9. Applied load-crack opening displacement curves measured for the unmodified epoxy and the 6

composites containing different weight fractions of (a) γ-AlO(OH) and (b) γ-Al2O3. 7

(a) (b)

21

1

Fig. 10. Schematic representations of the fracture toughening mechanisms exhibited by the 2

composites containing (a) γ-AlO(OH) and (b) γ-Al2O3 platelets. 3

4

Fig. 11. SEM images of the fracture surface of the epoxy composites containing (a) 5 wt% AlO(OH), 5

(b-c) 5 wt% Al2O3, (d) 10 wt% Al2O3. 6

(b) (a)

22

1

Fig. 12. SEM images of the fracture surface behind the crack tip of the epoxy composite containing 2

(a & b) γ-AlO(OH) and (c) γ-Al2O3. 3

4

0 2 4 6 8 100

50

100

150

200 AlO(OH) [Present Study] AlO(OH) [10] Al2O3 [Present Study] Al2O3 [5] Al2O3 [6] Al2O3 [8]

Rel

ativ

e K

Ic Im

prov

emen

t (%

)

Filler Content (wt%)

Present study

5

Fig. 13. Relative percentage improvement (compared to the unmodified epoxy) in the mode I 6

fracture toughness (KIc) of the composites studied in the present work and other types of epoxy 7

composites containing AlO(OH) or Al2O3 nanoparticles reported in [5,6,8,10]. (Note: the type of 8

epoxies reported from the present work and other studies are different [5,6,8,10]. Therefore, the 9

relative percentage improvement values were calculated and normalised using the respective fracture 10

toughness values from Table 1 for the epoxy composites reported within the present study and in 11

other sources [5,6,8,10].) 12

13

14

23

Table 1: Comparisons of tensile and fracture toughness properties for the aluminium oxide 1

reinforced epoxy composites in the present study with reported values from the literature. (Note: The 2

legend contains information presented in the order: (i) morphology (i.e. 0D and 2D is a spherical 3

particulate and a platelet, respectively), and (ii) key dimensions of the filler. Percentage 4

improvements are presented in Figs. 7 and 13) 5

Source Filler content and type

Young’s modulus, E, (GPa)

Ultimate tensile strength (MPa)

Fracture energy, GIc, (J/m2)

Fracture toughness, KIc, (MPa.m1/2)

Present study Unmodified Epoxy 2.67 48.5 136 0.49* (2D, ~6.10 μm dia, 1.17 μm thickness and density of 3050 kg/m3)

1.0 wt% AlO(OH) 3.11 52.0 215 0.67* 2.0 wt% AlO(OH) 3.32 54.5 286 0.79* 5.0 wt% AlO(OH) 3.49 53.4 384 0.94* 10.0 wt% AlO(OH) 3.73 51.0 501 1.12*

(2D, ~5.30 μm dia, 0.99 μm thickness and density of 3650 kg/m3)

1.0 wt% Al2O3 3.00 48.0 324 0.80* 2.0 wt% Al2O3 3.18 48.0 445 0.97* 5.0 wt% Al2O3 3.38 48.7 574 1.13* 10.0 wt% Al2O3 3.61 48.6 462 1.05*

Wu et al.[10] Unmodified Epoxy - 62.9 - 3.07 (2D, 22.2 nm thickness)

1.0 wt% AlO(OH) - 77.0 - 3.61 2.0 wt% AlO(OH) - 82.7 - 3.83 3.0 wt% AlO(OH) - 80.5 - 3.84 4.0 wt% AlO(OH) - 78.2 - 3.94

Lim et al. [5] Unmodified Epoxy 2.47 80.6 - 0.62 (2D, ~40 nm dia and >40nm thickness)

1.0 wt% Al2O3 2.80 80.1 - 0.71 2.5 wt% Al2O3 2.90 81.6 - 0.72 5.0 wt% Al2O3 3.06 84.1 - 0.75

Zhang et al. [6] Unmodified Epoxy 2.61 65.2 124 0.61 (0D and ~30-200 μm dia)

3.6 wt% Al2O3 2.78 65.4 143 0.67 7.0 wt% Al2O3 2.92 76.3 160 0.73

Duan et al. [8] Unmodified Epoxy 2.02 44.2 - 0.82 (0D and ~200 nm dia)

1 wt% Al2O3 2.19 42.6 - 0.94 3 wt% Al2O3 2.39 46.0 - 0.95 5 wt% Al2O3 2.44 46.5 - 1.10 7 wt% Al2O3 2.38 46.0 - 1.12

* Assuming a Poisson ratio (ν) of ~0.35, the KIc values for the present study were calculated using, 6

𝐾𝐼𝑐 = �𝐺𝐼𝑐 × 𝐸 × (1 − 𝑣) , where E is the Young’s modulus and GIc, is the mode I fracture energy 7

(critical strain energy release rate) for the corresponding epoxy composite samples. 8