Mechanical properties of epoxy composites with high ...

Composites Science and Technology 71 (2011) 710–716

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Mechanical properties of epoxy composites with high contents of nanodiamond I. Neitzel a, V. Mochalin a, I. Knoke c,1, G.R. Palmese b, Y. Gogotsi a,⇑ a

Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA Department of Chemical & Biological Engineering, Drexel University, Philadelphia, PA 19104, USA c Center of Nanoanalysis and Electron Microscopy (CENEM), Department of Material Science and Engineering WW 7, University of Erlangen-Nuernberg, 91058 Erlangen, Germany b

a r t i c l e

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Article history: Received 5 October 2010 Received in revised form 16 December 2010 Accepted 20 January 2011 Available online 1 February 2011 Keywords: A. Nano composites A. Polymer-matrix composites (PMCs) B. Friction/wear B. Mechanical properties Nanodiamond

a b s t r a c t Mechanical properties and thermal conductivity of composites made of nanodiamond with epoxy polymer binder have been studied in a wide range of nanodiamond concentrations (0–25 vol.%). In contrast to composites with a low content of nanodiamond, where only small to moderate improvements in mechanical properties were reported before, the composites with 25 vol.% nanodiamond showed an unprecedented increase in Young’s modulus (up to 470%) and hardness (up to 300%) as compared to neat epoxy. A significant increase in scratch resistance and thermal conductivity of the composites were observed as well. The improved thermal conductivity of the composites with high contents of nanodiamond is explained by direct contacts between single diamond nanoparticles forming an interconnected network held together by a polymer binder. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Nanodiamond (ND) powder produced by detonation is becoming an increasingly important nanomaterial manufactured on an industrial scale in many countries. Having superior mechanical strength and thermal conductivity, unique electrical and optical properties [1], this biocompatible [2] nanomaterial combining in a 5 nm particle an inert diamond core with a large accessible and reactive surface [3], has proven to be an excellent candidate for numerous applications ranging from composites [4–6], to biomedical [7,8] and electrochemical applications [9]. In contrast to many other nanomaterials, ND can be synthesized in large quantities at moderate cost. However, careful characterization and purification of ND is required to ensure quality [3] and achieve a better control of the surface chemistry [10] and particle size [11], prior to its use in any application. Recent studies have shown that an air oxidation process dramatically increases the purity of ND powders [3], allowing to control the ratio of sp2 to sp3 carbon, surface chemistry and crystallite size of ND [12]. Nanocomposites are a promising new class of materials which combine the advantages of the matrix and the nanofillers [13]. Current polymer nanocomposite research includes various fillers such as: CNTs [14], hydroxyapatite nanocrystals [15], clay nanoplatelets [16] and others. However, to date, nanocomposites have not demonstrated anticipated mechanical properties mainly because ⇑ Corresponding author. Tel.: +1 215 895 1934. 1

E-mail address: [email protected] (Y. Gogotsi). Present address.

0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.01.016

of issues related to nanoparticles aggregation and a poor interface between the nanofillers and the matrix. For example in the case of CNTs, their entanglement and bundling mediated by Van-der-Waals interactions [17,18] and a weak interface between them and the matrix leading to pull-out at relatively low strains [19] are commonly mentioned as the reasons for the underperformance of polymer–CNT composites. In contrast to high aspect ratio structures, spherical equiaxial fillers are free of entanglement and can be better dispersed in the matrix [20]. Moreover, the volume of the polymer–nanoparticle interface in proportion to the volume of the nanoparticle itself is maximized for spherical nanoparticles as compared to platelets and rods [21], resulting in a larger volume fraction of the interface. Most research with spherical nanofillers published so far dealt with nanosilica [22] and nanotitania [23]. Several recent studies focused on the incorporation of ND powder into thermoplastic polymers. Small additions of nanodiamond were reported to improve the mechanical properties of poly(vinyl alcohol) and poly(l-lactic acid) [5,24]. In our previous work, using an electrospinning technique, polyamide 11-ND composite fibers and coatings were produced with a 400% increase in Young’s modulus and a 200% increase in hardness in the case of 20 wt.% ND in polyamide 11 [4]. In earlier works by Russian scientists properties of polyisoprene-ND composites were reported [25]. However, the ND composites with thermosetting polymers are far less studied, and only address the effects of low concentrations of ND [26]. In order to fully benefit from nanoparticle properties at low concentrations, a uniform dispersion in the matrix is critical. Thwarting aggregation and achieving uniform distribution of the nanofiller is always problematic because nanoparticles tend to minimize

I. Neitzel et al. / Composites Science and Technology 71 (2011) 710–716

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Fig. 1. (a and b) Models of a low and high concentration ND composite. (c and d) TEM micrographs showing agglomeration at low concentrations of ND due to strong particle interactions and the formation of an interconnected network for the 25 vol.% ND sample.

their large surface area via aggregation. To achieve uniform dispersion is especially difficult in the case of ND powders, which are known for their abnormal tendency to aggregation when compared to other nanomaterials [27,28]. As a result, without taking special measures, ND forms large agglomerates which act as micrometer-sized defects in a matrix, severely compromising the benefits expected of well dispersed single ND particles. Common dispersion techniques such as shear mixing, bath or high power sonication etc. only led to limited improvements in ND dispersion, and correspondingly, resulted in no or little increase of mechanical properties. Conversely, sometimes degradation in the properties was observed. For example, Spitalsky et al. reported a reduction in loss modulus due to the addition of ND into an epoxy matrix [26]. These issues of dispersion become less important when nanoparticles are added in high concentrations. In this situation, the use of the term ‘‘dispersion’’ to name such a system is arguable. At higher concentrations the interparticle distance becomes shorter and eventually the particles come into direct contact with each other forming a nanoparticle network infiltrated by the polymer, where the polymer acts as a binder. High concentrations of ND may not only result in improved Young’s modulus, hardness and scratch resistance, but direct contacts between the particles are expected to improve the thermal conductivity as well. The reinforcement of epoxy matrices with high concentrations of nanofillers has been studied before with silica nanoparticles at concentrations of up to 25 wt.% and resulted in a 40% [29] and 200% [30] increase in Young’s modulus. While using high concentrations of nanofillers in large bulk components is in most cases

prohibitively expensive, it can be reasonable for coatings and small parts, especially with ND, due to the existing inexpensive largescale production by detonation. In this study, the potential of ND as a reinforcing filler is investigated by producing and testing epoxy samples with ND concentrations of up to 25 vol.%. The samples with high concentrations were manufactured using a combination of mixing and hot pressing. 2. Materials and methods 2.1. Materials ND powder UD90 (NanoBlox, Inc. USA) was used without any modification. UD90 was extensively characterized elsewhere [3]. The impurities mainly consist of non-diamond (amorphous and graphitic) carbon and traces of metals embedded into the amorphous carbon shells [12]. The epoxy system Epon828 (diglycidyl ether of bisphenol A) – PACM20 (bis-p-aminocyclohexyl methane) was selected (Hexicon and Air Products, Inc.). The stoichiometry of this system is well known and described elsewhere [31]. Both Epon828 and a predetermined amount of ND (wt.% ND per weight of epoxy resin + ND) were mixed in Tetrahydrofuran (Fisher Scientific, 99.9%, stabilized). After 5 min of bath sonication the solutions were stirred in closed vials on a hot plate for 2 days at 50 °C, followed by opening the vials and solvent evaporation for 48 h at 50 °C. The resulting wet Epon828-ND paste was transferred into a hot pressing mold and the curing agent was added. For the epoxy Epon828, 28 pph (parts per hundred, i.e. 28 g of curing agent per

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locations following this protocol. CSM allows for conversion of the load–displacement curves into stress–strain curves as well as calculation of Young’s modulus and Meyers hardness, as described by Kalidindi and Pathak [32]. Additionally, tests were performed to gain information on the scratch resistance of the composite. Scratch tests were performed using a 1.4 lm radius spherical indenter tip at a constant load of 10 mN over a distance of 100 lm. The cross-sections of the scratches were analyzed using an optical profilometer Zygo NewView 6000 at the beginning, middle, and the end of the scratch. A similar measurement was performed to gain information on the frictional behavior of the composite: sliding tests with a 100 lm radius spherical indenter tip were performed to minimize plastic deformation. The lateral and horizontal forces were recorded at various loads, starting the test at 0 mN and increasing the load up to 10 mN over a sliding distance of 250 lm. The obtained force values were used to calculate friction coefficients. Micro-hardness was measured using a Vickers indenter M400, Leco Corp. with a testing force of 300 mN. 2.4. Thermal conductivity The thermal conductivity of epoxy–ND samples has been measured using a Differential Scanning Calorimeter (DSC) Q2000, TA instruments. About 10 mg of reference metal (Ga 99.99%, Sigma Aldrich) were placed atop a cylindrical composite sample to mark the moment when the temperature on top of the sample reaches the Ga melting point. From the temperature at the bottom of the sample and the heat flow into the sample, thermal conductivity was calculated [33]. 3. Results and discussion

100 g of Epon828) of the curing agent PACM20 is recommended [31]. After hand mixing in the mold, the paste was hot-pressed at a load of 2 metric tons for 2 h at 80 °C followed by 2 h at 165 °C, – a standard curing procedure for the Epon828–PACM20 system. The produced Epon828-ND pellets of 5 mm in diameter were polished for subsequent mechanical tests.

At low concentrations, ND particles aggregate and form large sparse clumps in the polymer matrix (Fig. 1a). To eliminate dispersion issues and achieve maximum reinforcement, samples with concentrations of up to 25 vol.% ND were produced. At such high ND concentrations, the composite material can be thought of as a nanodiamond network infiltrated by a polymer acting as a binder (Fig. 1b). Thus, the composite consists of an interpenetrating diamond skeleton and polymer filling voids in this skeleton. Therefore, this technique provides a way to manufacture a variety of shapes made essentially of diamond particles, to some extent addressing the problem of ‘‘the first shape being the final shape’’ that limited industrial applications of diamond for many years [34].

2.2. TEM

3.1. TEM

The cured composite samples were sectioned by a diamond knife with an Ultramicrotome Leica Ultracut EM UC6 at room temperature. The sections showed a golden color, which indicates their thickness to be of about 100 nm. These thin films were transferred from water onto 200-mesh Cu grids. Bright-field TEM images were taken from several spots on the films with a JEOL JEM 2100 microscope at 200 kV.

TEM images of the samples are shown in Fig. 1c and d. In the 2 vol.% ND sample (Fig. 1c) large agglomerates are formed whereas in the 25 vol.% ND sample (Fig. 1d) particles form an interconnected network. It should be mentioned that this photograph is a 2-D projection of a 3-D structure and the real distribution of ND in the 3-D space is less dense than could be concluded from the image.

2.3. Mechanical characterization

3.2. Nanoindentation

Mechanical tests were performed using a NanoIndenter XP (MTS Corp.) equipped with the continuous stiffness measurement (CSM) attachment. All indents were made with a 5 lm radius spherical indenter tip. The tests were performed at a constant strain rate of 0.03 s 1 until a maximum load of 20 mN was reached. Upon achieving the maximum load, a 30 s hold segment was applied to measure creep. Each sample was tested at 10 random

Nanoindentation is a depth-sensing indentation testing technique allowing to obtain Young’s modulus and hardness of materials [35]. Additionally, for soft materials, creep measurements give valuable information about visco-elastic properties. Fig. 2a shows the measured load–displacement curves for epoxy–ND samples with concentrations of ND 0–25 vol.%. The maximum displacement decreased from 2434 nm for neat epoxy to 890 nm for the 25 vol.%

Fig. 2. Load–displacement curves of epoxy–ND samples with varying ND concentrations from 0 to 25 vol.% (a). Besides a clear decrease in the maximum indentation depth, a reduction of creep from 222 nm to 91 nm is observed for the 25 vol.% ND composite. (b) Stress–strain curves show an improved Young’s modulus (increased slope of the initial elastic portion of the stress–strain curves).


Fig. 3. Young’s modulus as a function of volume fraction of ND. For comparison, the rule of mixture and Lewis–Nielsen model calculations have been included into the plot.

ND sample. To obtain hardness values, the area of contact at a specific load must be calculated. In this study, we calculated the area of contact according to the procedure described by Kalidindi and Pathak [32] for spherical indenters. Meyers hardness obtained from these calculations showed an increase of 300% from 0.06 GPa for neat epoxy to 0.2 GPa for the 25 vol.% ND composite. The unloading behavior of samples with low and high ND contents changes at concentrations starting from 12 vol.% ND. Fig. 2a shows a reduced recovery for the 12 vol.% ND sample, which can be explained by a reduced mobility of polymer chains confined between the ND particles in the ND network. Reduced elastic recovery is observed for 18 and 25 vol.% ND samples as well. Young’s modulus was derived from the initial elastic portion of indentation curves (first 50 nm) as described by Pathak et al. [36] after zero point correction [32]. There is a 350% improvement in Young’s modulus from 3.4 ± 0.5 GPa for neat epoxy to 12 ± 2 GPa for the 25 vol.% ND composite. It should be mentioned that the experimental values of Young’s modulus to some extent depend on the measurement and data analysis technique. For example, the modulus for neat epoxy obtained from Dynamic Mechanical Analysis is 2.5 ± 0.2 GPa [31]. Our highest measured modulus for the 25 vol.% ND sample was 16.2 GPa, an increase of 470% compared to neat epoxy, showing the potential of this composite. As of now, this is the highest increase in Young’s modulus reported for a nanofiller-reinforced polymer system [4,37,38]. To our best knowledge, the previous highest reported increase in Young’s modulus due to the addition of nanofiller to a Bisphenol-A based

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polymer, reported by Devaprakasam et al. with nanosilica, was 300% [39], whereas for a polyamide-11, a thermoplastic polymer, the maximum reinforcement due to the addition of ND was 400% [4]. In addition to increased Young’s modulus and hardness, a two times decrease in creep from 222 nm for epoxy to 91 nm for 25 vol.% ND composite was observed (Fig. 2a). The decrease in creep indicates that contacts between ND particles lead to the formation of a diamond skeleton in the composite, reducing the viscous response of the composite. Stress–strain curves have been calculated from raw nanoindentation data as described in [32] (Fig. 2b). The increase in slope for the 18 and 25 vol.% ND samples corresponds to an increase in Young’s modulus derived from nanoindentation curves in Fig. 2a. The averaged Young’s moduli for all samples are plotted in Fig. 3. At low concentrations of ND, no change or even a slight decrease in Young’s modulus is observed. This can be explained by poor dispersion of ND in the Epoxy matrix (Fig. 1a and c) as mentioned above. At 12 vol.% ND, the modulus increases (Fig. 3). But the most remarkable conclusion form nanoindentation tests is a significant improvement in Young’s modulus and hardness for the 18 and 25 vol.% ND samples, which follows the trend based on the Lewis–Nielsen model [40] (Fig. 3). For comparison the upper and lower bounds of the rule of mixture are plotted in Fig. 3. As can be seen, the rule of mixture (even the lower bound) is not applicable in this case since it does not consider nanofiller–polymer interactions. In contrast, the Lewis–Nielsen model, which takes into account the interactions between the filler and polymer, shows a better agreement with the experimental data. According to this model a strong interface due to attractive interactions between the filler and polymer results in higher mechanical properties (Lewis–Nielsen, no slippage in Fig. 3) than an interface that is dominated by repulsive interactions between the polymer and filler (Lewis–Nielsen, slippage in Fig. 3) [41]. We therefore conclude that no slippage between the ND particles and the polymer matrix occurs, i.e. the epoxy–ND interface is mediated by attractive interactions.

3.3. Vickers hardness To further support nanoindentation data, Vickers micro-hardness tests were performed at a fixed load of 300 mN. The concave edge of the imprint in the pure epoxy (Fig. 4a) indicates significant elastic recovery, whereas the straight edges of the indent in the 25 vol.% ND sample (Fig. 4b) are characteristic for a more plastic deformation behavior. Comparison of the imprint sizes shows an increase in hardness for the 25 vol.% ND sample compared to the neat epoxy sample (Fig. 4a and b). Vickers hardness values were

Fig. 4. Optical images of Vickers indents produced with a load of 300 mN for a neat epoxy sample (a) and a 25 vol.% ND reinforced sample (b). A clear reduction in imprint size for the 25 vol.% ND sample as well as a change in the imprint shape (straight imprint edge) have been observed indicating a drastic increase in hardness.

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calculated by measuring the length of the diagonals of the residual imprints. The obtained Vickers hardness values are 20 HV or 0.2 GPa for neat epoxy and 55.2 HV or 0.54 GPa for the 25 vol.% ND sample. A 270% increase in Vickers hardness is in good agreement with the 300% increase in Meyers hardness derived from nanoindentation. Furthermore, no cracks have been found at the corners of the indents at large loads. This suggests that notwithstanding the high concentrations of ND, the composite still shows no signs of embrittlement. 3.4. Scratch resistance Taking into account the increased hardness of the composites, it is expected that they will demonstrate improved scratch resistance. Additionally, scratch tests complement the nanoindentation data, since the load is applied and the displacement is recorded over a larger area of the sample. Fig. 5 shows profiles of the scratches. The average penetration depth decreases from 2.5 lm (0 vol.% ND) to 1 lm (25 vol.% ND). The low fluctuations in penetration depth indicate consistent material properties over a large area. Upon unloading, the material experiences elastic recovery. The difference between the penetration upon loading and the residual scratch depths shown in Fig. 5 suggests a larger elastic recovery of the neat epoxy compared to the 25 vol.% ND sample in agreement with both, percentage of the reduced elastic recovery shown by nanoindentation of 12, 18 and 25 vol.% ND samples in Fig. 2a, and the difference in shape of the Vickers indents shown in Fig. 4. A reduced scratch depth is observed for the 25 vol.% ND sample in comparison to the neat epoxy sample over the entire scratch distance. To quantify the improved scratch resistance, cross-sections in the beginning, middle and the end of the scratch have been measured using an optical profilometer. The profiles are shown in Fig. 6 as well as the areas of the groove and pile up material calculated using image processing software (ImageJ). An average 43% reduction in groove area from 1448 lm2 to 819 lm2 and 60% reduction in pile-up area from 1652 lm2 to 653 lm2 for the 25 vol.% ND sample have been calculated from the three cross-sectional profiles. Over the entire length of the scratch, the amount of removed and piled up material is consistently less for the 25 vol.% ND sample compared to neat epoxy sample. These results demonstrate that the addition of ND in high concentrations significantly improves scratch resistance of the composite. One of the existing applications of nanodiamond is in lubricants [42]. Therefore tribological properties of ND-reinforced composites

Fig. 5. Scratch tests have been performed over a distance of 100 lm with a constant load of 10 mN. Uniform behavior over a large sample area can be observed. A clear improvement in scratch resistance for the 25 vol.% ND sample is observed. The residual deformation has been reduced significantly.

Fig. 6. The scanned cross-sections at the beginning, middle and end of the scratch. The calculated areas of removed material and piled up material in lm2 have been calculated for neat and 25 vol.% epoxy–ND samples using an imaging software (ImageJ). An improvement in scratch resistance and reduction in pile-up for the 25 vol.% ND sample is observed.

are of great interest. Having demonstrated improvements in scratch resistance of the composites, we also expected better tribological properties, such as a reduced friction coefficient. Indeed, our results from sliding tests indicate that friction can be reduced over a wide range of loads. An average friction coefficient of 0.4 for the neat epoxy and 0.24 for the 25 vol.% epoxy–ND composite have been calculated from the recorded lateral and horizontal forces during sliding of the indenter. Thus an up to 40% reduced friction coefficient can be achieved for high concentration epoxy–ND composites.

3.5. Thermal conductivity Fig. 7 shows the DSC curves for 0–25 vol.% epoxy–ND composites. The decrease in slope between 29 and 31 °C corresponds to an increase in thermal conductivity of about 25% from 0.25 W m 1 K 1 for the neat epoxy sample to 0.32 W m 1 K 1 for the 25 vol.% ND sample. The changes in thermal conductivity follow the same trend shown by the mechanical properties in

Fig. 7. DSC curves for epoxy–ND composites with 0–25 vol.% ND. A more negative slope of the recorded DSC curves relates to an increased thermal conductivity.


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data. This work was partially supported by NSF Grant CMMI0927963. Centralized Research Facilities at Drexel University provided access to the NanoIndenter XP, the optical profilometer and TEM used in this work.

References

Fig. 8. Relationship between the volume fraction of the filler and the interparticle distance. In comparison to nanosilica, the interparticle distance of nanodiamond composites is smaller at lower concentrations and saturates earlier.

Figs. 2–6: while there is no or little change from 0 to 12 vol.% ND, further increases in ND concentration result in a sharp increase of thermal conductivity. Thus both the thermal conductivity and mechanical properties of the composites sharply change at ND concentrations above 12 vol.%. We ascribe this effect to a decrease in interparticle distance (Fig. 8). At 12 vol.% ND, the interparticle distance is 3 nm (i.e., less than the diameter of a single ND particle), suggesting that the particles are coming into direct contact with each other. Therefore, a sharp increase in mechanical properties and thermal conductivity of the epoxy composites at ND content >12 vol.% are probably related to close contacts between the ND particles, and the formation of an interconnected nanodiamond network at these high concentrations. 4. Conclusion Hot pressed epoxy–ND composites with ND content of 12, 18 and 25 vol.% demonstrate an up to 470% increase in Young’s modulus and up to 300% increase in hardness as compared to the neat epoxy. These high values have never been achieved before for any other epoxy with a nanoparticle filler and could be potentially further improved by increasing the packing density of ND (via higher compressive forces for example). Scratch and sliding tests show an increased scratch resistance, with 35% less removed material, a 48% reduction of piled up material and a 40% decreased coefficient of friction for 25 vol.% ND composites as compared to neat epoxy samples. We also show that the thermal conductivity of high concentration epoxy–ND composites is improved, thus adding more value to this material. Due to the small size of only 5 nm, the distance between ND particles is significantly smaller (particle density is higher) at the same concentration when compared to other nanofillers, such as nanosilica having a particle size of 10 nm. Thermal conductivity measurements suggest that the 5 nm ND particles come into close contact at concentrations >12 vol.%, whereas with larger particles, i.e. 10 nm nanosilica, similar reduction in interparticle distance is achieved only at concentrations higher than 18 vol.%. These remarkable results show that it is possible to take full advantage of the mechanical and thermal properties of ND in polymer-ND nanocomposites. Acknowledgments We are thankful to Marry Sullivan for valuable advice on the selection of the epoxy matrix, as well as to Shraddha Vachhani and Prof. S. Kalidindi for their support in the analysis of nanoindentation

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