Nanocomposites mechanical failure

Crucially, structure of CNTs and polymers plays a key role on mechanical properties and load-transfer of nanocomposites. Efficient load-transfer is only possible when adequate interfacial bonding strength is available. Interfacial failure may compromise the reinforcement effect and then the full potential of CNTs may not be realized (11). Therefore, it is of great importance to understand the effect of molecular structure, interfacial structure and morphology characteristics on the tensile properties of nanocomposite materials. 

To increase the wear resistance of surfaces, silicon and metals are often coated with a hard nitride, carbide, boride, or oxide film. Nanoindentation and fracture simulations have been used extensively to elucidate failure mechanisms of these typically more brittle surfaces, which include crack propagation and film delamination. Considerable attention has also focused on nanocomposite materials, which possess nanocrystalline inclusions in an otherwise amorphous matrix. The nanocrystalline component is sufficiently small to preclude the formation of stable dislocations, and thus provide a higher hardness. 

As it is known [13, 14], the scale effects are often found at the study of different materials mechanical properties. The dependence of failure stress on grain size for metals (Holl-Petsch formula) [15] or of effective filling degree on filler particles size in case of polymer composites [16] are examples of such effect. The strong dependence of elasticity modulus on nanofiller particles diameter is observed for particulate-filled elastomeric nanocomposites [5], Therefore, it is necessary to elucidate the physical grounds of nano- and micromechanical behavior scale effect for polymer nanocomposites. 

Nanocomposites with carbon nanotubes have been an area of considerable R D ever since the excellent electrical and mechanical properties of carbon nanotubes were demonstrated. However, attempts to prepare carbon nanotube RPs often result in phase separation of the CNT and polymer phases causing premature material failure. Researchers at Nomadic Inc. and Oklahoma State University developed a layer-by-layer (LBL) assembly process that permits preparing polyelectrolyte/CNT RP with a CNT loading greater than 50 wt%. The excellent mechanical properties of these materials can be improved further by additional chemical action crosslinking of the CNT and polymer phases and by parallel alignment of the CNTs. The LBL method has been used to prepare various types of RPs. 

However, the tensile strength of in-situ polymerized nanocomposites is higher than other systems. The improvement in tensile property of nanocomposites occurs at the expense of strain to failure. The clay platelets dispersion and interaction with polyamide matrix are important factors that influence the mechanical properties. The high aspect ratio of the clay platelets and exfoliated clay platelets in nanocomposites are responsible for the improvement in mechanical properties. 

Previous studies on nanocomposites made with highly conductive nanoparticles and amorphous polymers have been reported (Jimenez and Jana, 2007 Mathur et al., 2008) such nanocomposites possessed a strain-to-failure of less than 5%. In a recent study (Vdlacorta et al., 2012), we have investigated the EM SE and electrical properties of heat-treated CNFs dispersed in a flexible linear low-density polyethylene (semiciystalhne) matrix. This chapter explores the effect of two other carbon-based modifiers on the EM SE of composites prepared by multiple melt-mixing routes with LLDPE for potential use in ductile/flexible EMC apphca-tions. Attention is also directed to the electrical and mechanical properties of such composites in relation with their electromagnetic shielding performance. 

Fig. 9 Effect of interfacial interactions on the strength and deformation mechanism of PA nanocomposites. Reinforcement and failure initiation. The symbols are the same as in Fig. 4...

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