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Application of Micro/Nano Mechanics on Nanocomposites and Gecko-inspired Nanofibrillar Arrays

Author: WangZhengZhi
Tutor: GuPing; WuXiaoPing
School: University of Science and Technology of China
Course: Solid Mechanics
Keywords: nanocomposites nanoindentaion micro friction and wear nanoporousalumina template gecko-inspired adhesives elasticity of individual nanowire effective modulus of nano-arrays micro adhesion
CLC: TB383
Type: PhD thesis
Year: 2012
Downloads: 245
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Abstract


With the rapid development of nano science and technology, material science encounters numerous opportunities and challenges. For example, combining the peculiar traits of nanomaterials and the synergistic effects of composites, nanocomposites always show more excellent physical and mechanical properties than traditional composites. Nanocomposites have raised lots of attentions and attained many outstanding achievements and practical applications since the80s of last century. For another example, many studies regarding to natural organisms show the unique features are related to their microscopic structures. As a result, micro-and nano-biomimetics is becoming a concerned topic these years. Specially, studies on gecko-inspired adhesives began in the turn of this century and have gained plentiful constructive progresses.Based on the fabrication and mechanical characterization techniques of micro/nano materials, the first part of this study systematically investigates the micro-to nano-scale mechanical and tribological properties of two series of nanocomposites, which fills the gaps of micro/nano mechanical and tribological studies on nanocomposites. The second part of this work focuses on fabricating and testing the gecko-inspired nanofibrillar arrays. This fibrillar structure was fabricated through versatile anodic alumina porous template. The elasticity of the individual nanowire was measured by using atomic force microscope (AFM). The effective modulus of the array was adapted on the basis of the properties of individual fiber. The micro adhesive behaviors of the fabricated nano-arrays were studied through two parallel and independent approaches:experiments and finite element simulations. The detailed research contents and results are summarized as follows:1. The nanocomposites are comprised of epoxy resins (EP) with different structures and colloidal silica particles with average size of25nm. The micro mechanical properties of the nanocomposites with various components were studied through nanoindentations tests. Two different probes, conical-shaped and Berkovich triangular pyramid-shaped, were used to study the geometrical effects on nanoindentations. Based on the finite element simulations, the probe effects and the strengthening mechanisms of nanoparticles were analyzed. Experimental and simulative results show that the addition of silica nanoparticles can significantly improve the hardness and modulus of the polymeric matrix. The test results are in close agreements with the theoretical predictions of mesomechanics. The mechanisms of these improvements are the stiff nanoparticles can effectively distribute and transmit the stress in the matrix and thus enhance the load capacity of the matrix. Different geometries of the probes can cause different stress distributions during the indentations and result in different surface effects (pile-up or sink-in).2. The micro scratch-resistance and the tribological properties of the nanocomposites were studied through the scratch mode in the nanoindenter equipment. The probe geometrical effects were also included. The modification mechanisms of nanoparticles were analyzed based on finite element simulations. Results show the scratch-resistance is largely improved meanwhile the friction is reduced after the incorporation of nanoparticles. The improved scratch-resistance should be ascribed to the enhanced mechanical properties and the anti-friction effect can be due to the lubrication effects and the rolling effect of the smooth nanoparticles. Different dominated contact mechanisms can be used to interpret the different frictional performances for the two probes.3. The micro/nano-wear behaviors of the nanocomposites were tested by using the multiple nano-scratch patterning techniques. The effects of the wear loads and cycles were studied. A novel method to precisely calculate the wear volumes based on image processing techniques was proposed. Experimental results show the wear volume increase with the increasing wear loads and cycles. Under the larger loads and multiple cycles wear conditions, the wear volumes change abruptly, which signifies the wear regime transition from abrasive and adhesive wear to fatigue and peeling wear. Under slight wear conditions, the improved wear-resistance of the nanocomposites can be ascribed to the increased hardness, the decreased frictional coefficient and the rolling effect of the nanoparticles. While under severe wear conditions, the transition thresholds for the fatigue wear are improved due to the decreased plasticity index and the increased strength of the nanocomposites, thus the fatigue wear-resistance is enhanced.4. Nanoporous alumina template was fabricated by a typical two-step anodization process. The polypropylene (pp) nano-wire arrays were fabricated through universal template wetting method. The optimal fabrication conditions were explored with the help of the scanning electron microscope (SEM) and transmission electron microscope (TEM). The morphological characterization results show the diameters of the template pores and the pp fibers are approximate to100nm. The height of the array is close to the thickness of the template, implying the complete wetting and template-dissolving. Within the research scope of this study, the optimal fabrication conditions for the template and the pp nano-arrays are:0.3M oxalic acid;4h for the second anodization;38min for pore-opening and enlarging in5wt%phosphoric acid at30℃; morethan45min for template-dissolving in0.1M sodium hydroxide solution.5. The elastic modulus of single pp nanowire was measured by the typical three-point bending test carried out in AFM. The relationships between the effective modulus of the array and the bulk modulus of the material together with the elasticity of the individual nanowire were discussed. The adhesion was simulated by finite element method on the basis of the traditional adhesion theories. The simulative results were compared with the ones obtained by experiments, which were implemented in the Tribolndenter system with a wedge-shaped probe (1×100μm2). According to the three-point bending test, the elastic modulus of the individual pp nanowire is10.15GPa, which is more than6times larger than its bulk modulus. Experimental results show the adhesion strength reaches12.3N/cm2under the preload of254N/cm2. This adhesion strength is comparable to natural gecko except the ultrahigh preload. The simulative results show quantitative agreements with the tests. The mechanisms of the directional adhesion of the slanted fiber arrays were explored by using finite element simulations and theoretical derivations. The directional adhesion of the slanted structures can be attributed to the additional peeling moment caused by relative deformations of the fiber tip and fiber stalk.

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