Carbon Nanotubes Reinforced Al Matrix Composites Fabricated By In-situ Chemical Vapor Deposition

Carbon nanotubes (CNTs) have attracted much attention because of their excellent mechanical, electrical, and thermal properties. Of particular interest is the use of CNTs in the reinforcement of metal matrix composites as a means of overcoming the performance limits of conventional materials. The most important issues for preparing CNTs/metal composites with high performances are the homogenous molecular-level mixing between CNTs and metal powders, the retention of perfect structure of CNTs during preparation of composites, and the strong interfacial bonding between CNTs and matrix to ensure load translation. However, these issues are very hard to be solved by means of traditional powder mixing process (high-energy ball milling). Therefore, it is very urgent to search a novel method to improve the traditional route and develop CNTs/metal composites with high performances.This thesis reports for the first time the synthesis of Al matrix composites reinforced with CNTs with a low Ni content, produced by a novel fabrication process, which involved preparation of a Ni(OH)2/Al precursor by a deposition-precipitation route, calcinations, and reduction of the precursor to yield active Ni nanoparticles spread evenly on the Al powder surfaces, and in situ synthesis of CNTs through chemical vapor deposition (CVD) followed by pressing and sintering of the CNTs(Ni)/Al composite powders. This process produced CNTs(Ni)/Al composites in which the in situ synthesized CNTs were very homogeneously dispersed within the Al powders, unlike conventional CNT-reinforced Al matrix composites. This unique in situ synthesis of CNTs in a matrix is particularly suitable for the fabrication of CNTs-reinforced metal matrix composites, because the desired dispersion of CNTs can be rather easily achieved.Ni/Al catalysts and CNTs(Ni)/Al composite powders were prepared using deposition-precipitation route and CVD method, respectively. The influence of reduction temperature on the diameters of Ni nanoparticles, and the influences of CVD technics on the yield, morphology and structure of CNTs were investigated. The results showed that the aluminum powder as a catalyst carrier could prevent the transition metal particles from agglomerating and promote the dispersion of nano-scale Ni particles after reduction, which are frequently responsible for CNT growth; the morphology and structure of CNTs were dependent on the in-situ synthesis technics, when the Ni content in the catalyst was relatively low, reduction temperature was 400 oC, reduction time was 2 h, synthesis temperature was 630 oC, and the ratio of reaction gases was VN2: VH2: VCH4 = 480:120:60 (ml/min), the CNTs synthesized were with better morphology, higher purity, and more homogenous dispersion in Al powder; the interaction between Ni catalyst and Al during CNT growth was feeble, which led to a tip growth mechanism for CNTs.The temperature ranges of phase change of CNTs(Ni)/Al composite powders and composite bulk, the influences of annealing on the CNTs-Al interfacial structure, the chemical stability of CNTs in the composite system, and the interfacial wetting theory were explored by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. The results showed that the CNTs were thermodynamically stable in Al below 800 oC, and only the locations containing an amorphous carbon coating of CNTs have reacted with Al to form Al4C3 thin film; the composite bulk annealed at 850 oC has good interfacial bonding between CNTs and Al, and the CNTs in the composite presented perfect structure; the interfacial reactive wetting kinetics in the composite system was developed by lateral growth of Al4C3 on the CNT surface. The carbide formed on the surface of the CNTs could improve the interfacial interaction between CNTs and Al. This also contributed to the enhancement of the mechanical properties of the composites.In-situ CNTs(Ni)/Al composites were fabricated by pressing and sintering of CNT(Ni)/Al composite powders. The optimal powder metallurgy technical parameters were obtained by analyzing the influences of initial pressure, sintering temperature, sintering time and repressing pressure on the microstructure and performances of composites. Meanwhile, the effects of CNT content on the mechanical properties and microstructure of composites, and the strengthening mechanism of composites were discussed. The results showed that the CNTs in-situ synthesized could remarkably improve the hardness, tensile strength and elastic modulus of the composites. The hardness and tensile strength of the Al-5%CNT-1%Ni composite were 4.3 and 2.8 times that of neat Al, respectively, and 2.0 and 1.8 times that of the same composition composites obtained by traditional methods; the strengthening mechanisms of CNTs(Ni)/Al composites were the work hardening of matrix resulted from the significant coefficient of thermal expansion mismatch between the matrix and the CNTs, Orowan and grain strengthening due to the inhibition of dislocation motion and matrix distortion.Carbon nano-onions (CNOs) were synthesized over Ni/Al catalyst with relatively high Ni content by CVD. The influences of CVD technics on the CNO yield, morphology and structure, and the purification, magnetic and friction properties of CNOs were investigated. The results showed that the Ni@CNOs with high purity and superparamagnetic behavior were obtained when using hydrogen as a carrier gas, and mixture of Ni@CNOs and CNOs with a hollow core were produced in the case of using nitrogen as a carrier gas; after addition of Ni@CNOs, the ability of resistance to wear as well as friction coefficient of the lubricant were improved remarkably.