Synthesis In-situ Of Carbon Nanotubes Over Al Matrix And The Structure And Property Of Their Composite

With wide application potential in various industrial fields, aluminium matrix composite is always the focus of metal matrix composite development and research work, due to its distinct advantages including low density, size stability, high specific strength, corrosion resistance, excellent high temperature performance, and so on. Carbon nanotubes (CNTs) have been introduced as an ideal reinforcement phase of composites by their virtues of small diameter, high slenderness ratio, and outstanding physical and chemical properties. However, there are some inevitable problems, such as the agglomeration and breakage of CNTs and the interface pollution between CNTs and Al when using the traditional methods including powder metallurgy and casting. Thus, the development of new preparation process and the investigation about structure and property of CNTs/Al composite are essential to promote its application.The emphasis of this study was to prepare aluminum matrix composite reinforced by CNTs by the method of synthesis in-situ. On the basis of successful synthesis in-situ of CNTs over aluminium matrix, the crystallization and oxidative stability of CNTs in the composite powders, and the heterogeneous interfaces between different structural CNTs and Al were investigated. Further, the preparation process, structure, property and strengthening mechanism of aluminum matrix composite reinforced by CNTs in-situ were discussed.In this paper, the feasibility and effectiveness of CNTs synthesized using Fe, Co, Ni as catalysts over Al matrix at low temperature by chemical vapor deposition (CVD) were researched systematically for the first time. In addition to the optimization of catalyst type, a comparative analysis for CNTs synthesized over metal Al and the traditional Al2O3 ceramic matrix was investigated, and the influences of synthesis parameters on the morphology, structure and crystallization of CNTs were also studied. The results showed that, CNTs with high yield, good shape and high crystallization could be always obtained using different Ni contents by CVD, while Fe/Al and Co/Al catalysts did not work well, that is, Ni is the fittest catalyst for the synthesis in-situ of CNTs over aluminum matrix. Compared with Al2O3 matrix, Al matrix realized the high yield of CNTs at low temperature by preventing Ni catalyst particles from agglomerating. Under our experimental conditions, CNTs synthesized by Ni catalyst at 600℃always have ideal structure and morphology, which is the fittest temperature for the preparation of CNTs/Al composite powders.The oxidative stability, crystallization and interface reaction of CNTs synthesized at different conditions were compared. It was found that herringbone and tubular CNTs could be synthesized under different temperatures and the heterogeneous interface characteristics between herringbone and tubular CNTs with Al were investigated. And the formation mechanism of Al4C3 and the difference in interface reaction between different structural CNTs with Al were analyzed by both solid-state reaction thermodynamics and kinetics. The study on oxidative stability showed that, Ni content could affect its thermal stability by controlling CNTs diameter, structural defect and oxidative stability, namely, the higher Ni content, the poorer of CNTs stability. So CNTs/Al composite powders should be prepared by low Ni content. The results also showed lower crystallization, more structural defects and poorer chemical stability of herringbone CNTs synthesized at low temperature resulted in obvious formation of Al4C3, the brittle interface reaction layer, along CNTs-Al interface. In reverse, the graphite basis plane and high crystallization of tubular CNTs obtained at high temperature led them outstanding chemical stability, and the direct bonding interface was formed without detected interface reaction.Using composite powders synthesized in-situ, CNTs/Al composite was prepared by powder metallurgy. The influences of two different processes, multiple compression or hot extrusion after cold molding and sintering, on the physical and mechanical properties of the composite were systematically studied, as well as the microstructures and strengthening mechanisms. To the physical properties, CNTs/Al composite gains the advantages over pure aluminum in density, coefficient of thermal expansion and resistive softening temperature, but too high content of CNTs will negatively affect the tightness and high temperature stability of the composite. To the mechanical properties, the strengthening effect of CNTs on aluminum matrix went up firstly and then down with the content of CNTs rising. The composite with 0.5wt%CNTs showed the best performance on hardness, elastic modulus, tensile strength, and so on, which is the result of combined effects of several reinforcement mechanisms, including effective load transmission, dislocation strengthening, dispersion strengthening, etc. But the plasticity of the composite decreased with its strength rising. Furthermore, the process of hot extrusion deformation could improve the physical and mechanical properties of CNTs/Al composite by reducing the porosity, changing grain orientation of aluminum and dispersing the CNTs, so that, it is a positive technology for the preparation of this new composite.