化学气相沉积法原位合成碳纳米管增强铝基复合材料(2)

the annealing treatment on the wetting behavior, and the interfacial structures between aluminum and CNTs were investigated by differential scanning calorimetry, X-ray diffraction and electron microscopy, etc. The results showed that: (1) the effect of the annealing treatment on the CNTs-Al interfacial structure was obvious. A very thin Al4C3 layer appeared and its thickness was increased with the increasing annealing temperature while the structure of the CNTs was gradually changed with the temperature; (2) the interfacial wetting kinetics in the composite was developed by the growth of Al4C3 along the CNT surface, and the growth of Al4C3 perpendicular to the CNT surface easily reached to the ultimate thickness; (3) the aluminum carbide formed on the surface of the CNTs could greatly decrease the wetting angle and thus improve the interfacial interaction between CNTs and Al due to their covalent bonding. The strengthening boundary also contributes to the enhancement of the load transfer efficiency and the mechanical properties of the composites. In addition, according to above interfacial wetting theory, a large quantity of aluminum carbide nanowires was achieved by the heat treatment of the in-situ synthesized nanotube/Al powder. This novel strategy provides a new method for fabricating aluminum carbide nanowires.

Finally, the aluminum matrix composite bulks reinforced by the in-situ synthesized CNTs were fabricated by the powder metallurgy process. The optimal technical parameters were obtained by exploring the influences of molding, sintering temperature and time on the microstructure and performances of composites. Meanwhile, the effects of CNT content on the mechanical properties and microstructure of the composites were investigated. Moreover, a model of the strengthening mechanism of the composites was proposed. The results showed that: (1) the in-situ synthesized CNTs could remarkably improve the hardness and tensile strength of the composites. The hardness and tensile strength of the Al-5%CNT-1%Ni composite were 3.0 and 1.8 times higher than the hardness and tensile strength obtained using pure aluminum, respectively. The hardness and tensile strength of the Al-5%CNT-1%Ni composite were 1.0 and 0.86 times higher than the hardness and tensile strength produced by using the same composition composites which were obtained by traditional mechanical mixing process, respectively; (2) the CNTs with less than 5wt% could disperse homogeneously on the matrix with few agglomeration, resulting in the fine grain size and high dislocation density in the composites; (3) the fracture surface of the composites showed the characteristics of a coexistence of the toughness fracture and the brittleness fracture. The main mechanism of the fracture of the composite resulted from the CNTs broken by a tensile force; (4) the strengthening of the composites originated from the following aspects: the work hardening of

matrix resulting from the significant coefficient of thermal expansion mismatch between the matrix and the CNTs, grain boundary and dislocation strengthening due to the inhibition of dislocation motion and matrix distortion. Based on above discussions and the geometry and physical properties of nanotubes, three strengthening mechanisms were considered for the in-situ CNTs(Ni)/Al composites which are thermal mismatch, Orowan looping and shear lag models. Comparing the calculated values from the three theoretical models with the experimental values, the enhancement tendency to the composites was consistent and could explain the strengthening mechanism of the in-situ CNTs(Ni)/Al composites.

Besides the synthesis of CNTs over the Al matrix, a new method for preparing the CNOs was also developed. The influences of CVD technical parameters on the CNO yield, morphology and structure were explored. The purification, magnetic and friction properties of the CNOs were investigated. The results showed that: (1) two kinds of nano carbon particles, the hollow CNOs and the CNOs with Ni core (Ni@CNOs) could be synthesized by controlling the CVD technical parameters; (2) the Ni@CNOs with high purity and superparamagnetic behavior were obtained when using hydrogen as a carrier gas, and mixture of Ni@CNOs and hollow CNOs were produced in the case of using nitrogen as a carrier gas; (3) In some cases, the Ni core could escape from the inside of the CNOs during the graphite layer growing, resulting in a hollow CNO, while some others may be trapped inside forming a Ni@CNO; (4) the ability of the wear resistance and the friction coefficient of the lubricant were improved remarkably with the addition of a few amount of Ni@CNOs.

The method of in-situ synthesis of carbon nanostructures within aluminum matrix reported in the thesis may develop a novel approach for fabricating CNTs/metal composites with high performances. In the future work, we would pay more attention to the research on the formation technology of the composites for improving the properties. Furthermore, it is necessary to thoroughly investigate the structure evolution of the CNTs during high temperature sintering, deforming and servicing, which may provide theoretical and experimental guidance for the reliable applications of the CNTs in the structural materials.

Key words: Aluminum matrix composite; Carbon nanotubes; Chemical vapor

deposition; In-situ synthesis; Catalyst

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