Int J Fract (2011) 170:95–100 DOI 10.1007/s10704-011-9600-3
© Springer Science+Business Media B.V. 2011
LETTERS IN FRACTURE AND MICROMECHANICS
MECHANICAL BEHAVIOR OF NANODIAMOND/EPOXY NANOCOMPOSITES M. R. Ayatollahi, E. Alishahi, S. Shadlou Fatigue and Fracture Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology, Narmak, Tehran, 16846, Iran. E-mail:
[email protected] Abstract. The mechanical properties of epoxy-based nanocomposites reinforced by nanodiamond (ND) particles were investigated. The results showed that while the addition of 0.1 wt% of ND improved the Young's modulus and tensile strength compared with those of the pure epoxy, the mode I fracture toughness did not show any improvement. Furthermore, in order to study the effect of shear deformation on fracture properties of nanocomposites, mixed mode fracture resistance of nanocomposites was investigated. It was found that as the share of shear deformation in mixed mode loading increases, the positive effect of ND particles enhances. Keywords: mechanical behavior, fracture toughness, nanodiamond, nanocomposites 1. Introduction. Nanodiamonds (ND) have attracted considerable attention in several engineering fields such as biological systems, abrasive pastes and suspensions for high precision polishing (Artemov, 2004), polymer-based nanocomposites (Kuznetsov and Lipa, 2007), wear-resistant surface coatings, cooling fluids, lubricants, and electroplating baths (Dolmatov, 2001). However, very few research studies have been performed to investigate the mechanical properties of nanocomposites reinforced by ND. For instance, Ekimov and Gromnitskaya (2002) reported 4.6% and 11.6% improvements in the tensile strength and Young's modulus of nanocomposites reinforced with 3 wt% of ND, respectively. Although the fracture behavior of nanocomposites has been examined by some researchers, only the pure mode I fracture toughness has been reported. (e.g. Li et al. 2008). However, there are numerous situations where a crack is subjected to mixed mode loading conditions. Therefore, it is important to study the mixed mode fracture resistance of different materials including nanocomposites. Among different specimens available for measuring mixed mode fracture properties of materials, the semi circular bending (SCB) specimen has frequently been used for exploring the fracture behavior of rocks, asphalts and polymers. For instance, the SCB specimen was employed by Ayatollahi et al. (2006) for investigating brittle fracture in polymers. This paper deals with the mechanical properties of ND/epoxy nanocomposites, including the Young's modulus, the tensile strength, pure mode I fracture
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toughness and mixed mode fracture resistance. Some of the possible mechanisms that affect their mechanical properties are also discussed. 2. Experiment. The ND particles used in this study had diameters between 4 and 8 nm according to the supplier. The ND particles usually form micro-size agglomerations. Therefore, in order to functionalize the fillers, first the ND particles were oxidized in a furnace at 430°C for five hours and then they were dispersed in Tetrahydrofuran (THF) as solvent. Similar to Chia-Chen and Chun-Lung (2010), the dispersion of ND in THF was carried out by dispersing 1 wt% ND powder in the THF containing 30 wt% (based on the weight of ND) of Cetyl trimethylammonium bromide or CTAB (an Amino group surfactant). Then, the mixture was sonicated for 30 min and the solvent was removed by using a rotary device. The epoxy resin ML-520 (Bisphenol A) was selected as matrix because of its low viscosity and extensive industrial applications. The low viscosity of the matrix makes the dispersion of additives easier. The curing agent was HA-11, triethylene-tetramine. First, the epoxy was mixed with desired amounts (i.e. 0.1, 0.3 and 0.5 wt %) of previously functionalized ND particles and stirred for 10 min at 2000 rpm. Then, the mixture was sonicated for 30 min. Afterwards, the hardener was added gradually (i.e. drop by drop) while the mixture was being stirred at 100 rpm. Then, the mixture was degassed in vacuum for 10 min. The dog-bone shaped specimens were used to perform tensile tests and the notched three point-bend specimens were employed to determine pure mode I fracture toughness. Young's modulus and tensile strength were measured according to ASTM-D-638-99 standard and the fracture toughness was determined based on the ASTM-D5045-99 standard from the experimentally obtained fracture loads. Additionally, mixed mode fracture resistance of nanocomposite was measured using the SCB specimen, schematically shown in Fig. 1a. The fracture tests were performed using a Santam universal apparatus with a 10 kN load-cell. According to Ayatollahi and Aliha (2007), pure mode I, mixed mode (with KI=KII) and pure mode II loading conditions in the SCB specimen can be achieved by taking Į=0Û, 30Û and 40Û, respectively. Crack tip
Į
(a)
Crack growth path Initial crack
(b) Figure 1. a) Schematic of the SCB specimen b) A typical broken SCB specimen
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3. Tensile behavior. The Young's modulus and ultimate tensile strength of nanocomposite for different contents of ND are depicted in Figs. 2a and 2b. The values given on the vertical axes correspond to the pure epoxy. It is seen that the maximum increase in the Young's modulus and tensile strength of nanocomposite occurs both in 0.1 wt% of ND particles and these properties decrease by increasing the filler contents. In 0.1 wt % of ND, the Young's modulus and the tensile strength increase 15.2% and 6.2 % compared with the pure epoxy, respectively.
(a)
(b) Figure 2. Tensile properties of pure epoxy and nanocomposite a) Young's modulus, b) Tensile strength
Due to the fact that nano-particles have a strong tendency to join each other and minimize their surface area (Neitzel et al, 2011), it is difficult to achieve a uniform dispersion that leads to a strong interfacial adhesion between the nano-particles and the matrix. Besides, the mechanical properties are dependent on the strength of interface. When a strong and fully bonded interface forms between the fillers and matrix, the matrix stresses can be easily transferred to the nano-fillers. As a result, the nanocomposites can bear more loads and exhibit higher values of Young's modulus and tensile strength. Uniform dispersion also decreases the free space between the particles which gives rise to the reduction in the flexibility of polymer chains. This can be another reason for increase in the Young's modulus. According to Fig. 2, there is a noticeable reduction in the trend of results for both tensile strength and
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Young's modulus by increasing the nano-filler contents beyond 0.1 wt% of ND. One may suggest that the ND particles form some aggregates and the amount of the aggregation grows for higher ND contents. This could be responsible for reduction in the Young's modulus and tensile strength after 0.1 wt% of ND. On the other hand, the values of Young's modulus and tensile strength for all nanocomposites are above those of the pure epoxy which indicates that the positive effect of ND as an enhancer still overcomes the negative effect of forming agglomeration. This can be due to the functionalization of the ND surface which reduces the tendency of ND particles for joining each other. In other words, the functionalization of particles would result in a better dispersion state or at least reduces the size of agglomerated particles. 4. Fracture behaviour. Fig. 3 shows the results obtained for fracture toughness of the ND/epoxy nanocomposite. It can be observed that in all the contents of ND, the fracture toughness of nanocomposites is below that of pure epoxy. Moreover, with further increase in ND contents, a significant decrease occurs in fracture toughness of nanocomposite which is similar to the trend observed for the tensile behaviour.
Figure 3. Fracture toughness of pure epoxy and ND/epoxy nanocomposite
Several fracture mechanisms can influence fracture toughness in nanocomposites such as crack deflection, plastic deformation and crack front pinning (Watzel et al, 2006). The crack deflection theory suggests that nanoparticles act as obstacles that force the crack to deviate from its initial path of propagation and thus increase the fracture energy. Similar to the crack deflection theory, in the crack pinning mechanism the crack front may be pinned between the nano-fillers and extend in a way that leads to an increase in the crack length. However, it is well known that all these mechanisms are considerably dependent on the interface strength between the filler and matrix. As mentioned earlier, the interface between ND and the epoxy seems to be not very strong and becomes weaker with increasing the nano-particle contents because of increase in the amount and size of agglomerations. In addition, when the interface is weakly bonded, the ND particles can act as local stress
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concentrators which decrease the energy required for crack growth and hence reduce the nanocomposite fracture toughness. The fact that the presence of ND as a filler led to an increase in the tensile properties but slightly decreased the pure mode I fracture toughness, can be attributed to different mechanisms through which ND particles affect the tensile properties and fracture toughness. Considering that there is a pre-crack in the fracture test specimens, a localized damage zone (often called the process zone) takes shape around the sharp crack tip. One may suggest that the weak adhesion between the ND particles and the matrix accelerates the progressive damage initiated from the crack tip when the cracked specimen is subjected to tensile deformation. In other words, the ND particles play mainly the role of stress concentrators rather than strength enhancer when they are approached by the localized damage zone. In contrast, for the conventional tensile experiments where no considerable localized damage exists in the test specimens, the strength enhancement role of ND particles is high enough to improve tensile properties. Several mixed mode fracture tests were also performed, using the SCB specimen. In order to evaluate the influence of ND particles on the mixed mode fracture resistance of nanocomposites, a parameter named the effective fracture toughness Keff
K eff
K I 2 K II 2
(1)
was used. Fig. 4 shows the increase in the effective fracture toughness of ND/epoxy nanocomposites, containing 0.5 wt% of filler. The results are presented for three different mode mixities including pure mode I, pure mode II and mixed mode KI=KII.
Figure 4. Increase in fracture toughness for different mode mixities.
It is seen from Fig. 4 that in pure mode I, the fracture toughness of nanocomposite decreases slightly in comparison with the pure epoxy. On the contrary, under pure mode II and mixed mode loading the enhancements in the effective fracture toughness are considerable. In order to justify the trend of these results, two important fracture mechanisms including the initiation of local micro cracks and the deviation of crack path can be suggested here.
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When the shear stresses reach a critical value, some local micro cracks are generated close to the crack tip (Bhattacharjee and Knott, 1995). In the presence of shear deformation, the ND particles can act as obstacles which resist against the growth of these micro cracks and hence increase the fracture energy. Another point that should be considered is the crack path. Unlike the mode I specimen in which the fracture trajectory was generally straight and in the same direction as the pre-crack, mixed mode crack growth took place in a curved path (see Fig. 1b). One may suggest that the crack growth path is curved because the plane of maximum tangential stress around the crack tip changes as the crack advances. In other words, the crack is forced to change its direction in order to achieve the maximum tangential stress plane. A longer crack path provides more opportunity for the ND particles to dissipate more energy. 5. Conclusion. According to the experimental results presented in this paper, the addition of ND particles to the epoxy improved its Young's modulus, tensile strength and mixed mode fracture resistance. However, the pure mode I fracture toughness did not show any improvement. The trend of results was discussed based on different mechanisms which are likely to occur in the mechanical behavior of nanocomposites. References Artemov, A.S., (2004). Polishing Nanodiamonds. Physics of the Solid State 46, 687–695. Ayatollahi, M.R. , Aliha, M.R.M., (2007). Wide range data for crack tip parameters in two disc-type specimens under mixed mode loading. Computational Materials Science 38, 660–670. Ayatollahi, M.R., Aliha, M.R.M., Hassani, M.M., (2006). Mixed mode brittle fracture in PMMA—An experimental study using SCB specimens. Materials Science and Engineering 417, 348–356. Watzel, B., Rasso, P., Haupert, F., and Friedrich, K., (2006). Epoxy nanocomposites- fracture and toughening mechanisms. Engineering fracture mechanics 73, 2375-2398. Bhattacharjee, D., Knott, J.F., (1995). Effect of mixed mode I and II loading on the fracture surface of polymethyl methacrylate (PMMA). International Journal of Fracture. 72, 359-81. Chia-Chen, L., Chun-Lung, H., (2010). Preparation of clear colloidal solutions of detonation nanodiamond in organic solvents, Colloids and Surfaces. Physicochemical and Engineering Aspects 353, 52–56. Dolmatov, V.Y., (2001). Detonation synthesis ultradispersed diamond: properties and applications. Russian Chemical Reviews 70, 607–626. Ekimov, E., Gromnistkaya, E.L., (2002). Mechanical behavior and microstructure of nanodiamond-based composite materials. Journal of materials science letters 21, 16991702. Kuznetsov, V., Lipa, S., (2007), Nanodiamond and onion-like carbon polymer nanocomposites. Diamond and Related Materials 16, 1213–1217. Li X. F., Lau, K.T., and Yin, Y.S., (2008), Mechanical properties of epoxy-based composites using coiled carbon nanotubes. Composites science and technology 68, 2876-81. Neitzel, I., Mochalin, V., Knoke, I., Palmese, G.R., Gogotsi, Y., (2011). Mechanical properties of epoxy composites with high contents of nanodiamond. Composites science and technology 71, 710-716.
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