Fabrication And Properties Of Uniformly Dispersed Carbon Nanotube Reinforced Al Composites

Due to the low density and excellent mechanical and physical properties, carbonnanotubes (CNTs) have been regarded as the ideal reinforcement for metal matrixcomposites (MMCs). However, the difficulty in achieving uniform dispersion ofCNTs in matrix hampers the development of CNT-reinforced MMCs. To solve thisissue, a novel approach is developed in this dissertation to fabricate CNT/Alcomposites, which consists of in-situ synthesis of CNTs, short-time ball-milling, andpowder metallurgy. The influence of fabrication conditions on the properties ofCNT/Al composites were investigated systematically. Furthermore, the structure,properties, interfacial bonding and strengthening mechanism of CNT-reinforce Alcomposites were discussed.The Ni, Co, and Cu catalyst precursors on Al powder were prepared byimpregnation route, and then CNTs were synthesized by in-situ chemical vapordeposition. Thus, CNT/Al composite powders were obtained. It was found that themuti-walled CNTs can be synthesized on Al matrix by using Ni, Co, and Cu catalysts.But the CNTs synthesized using Cu catalyst show a relatively low crystallization.Using0.5wt.%Co and Ni catalysts, the homogeneously dispersed CNTs on the Alpowder surface with relatively high purity, uniform diameters, and crystallization canbe obtained at600℃and underCH4and C2H2as carbon sources, respectively. Theobtained CNT/Al composite powders are suited to further preparation of composite.CNT/Al composites were prepared by direct powder metallurgy of the in-situsynthesized composite powders. It was found that the maximum content of CNTsuniformly dispersed in Al matrix is2.5wt.%, and the mechanical properties of thecomposites decrease rapidly when the content of CNTs is more than2.5wt.%. Theoptimized CNT content is1.5wt.%, at which the composites show best compressiveproperty, and the compressive yield strength and compressive elastic modulus is2.2and3.0times as large as that of pure Al, respectively. Moreover, the corrosionresistance of Al matrix decreases with the increase of CNTs content.In order to further improve the maximally uniform dispersion content in Alpowder matrix, ball-milling was introduced into the fabrication process of CNT/Alcomposites. The execution of ball milling for a short time can further improve the dispersion of CNTs fabricated by in-situ CVD. Meanwhile, CNTs are implanteddeeply into the matrix. The effects of ball milling parameters on the structure anddispersion of CNTs and the properties of CNT/Al composites were investigated. Itwas found that under the optimized milling conditions (ball-milling speed of500rpm,no process control agent added), with the increasing of milling time (≤120min), thedensity, hardness and tensile strength of2.5wt.%-CNT/Al composites increase. Thecomposites through90min of ball milling show well-balanced strength and ductility,whose hardness and tensile strength are2.4and2.7times as large as that of pure Al,and1.6and1.5times as large as that of the pure Al through the same milling process,respectively. Moreover, the elongation is larger than15%. With the increase of CNTcontent, the hardness and tensile strength of CNT/Al composites increase. The4.5wt.%-CNT/Al composites show largest hardness (HV131.2) and tensile strength (420MPa), which is2.3and2.4times more than that of pure Al, respectively. Furthermore,the coefficient of thermal expansion of CNT/Al composites is reduced significantlycompared with Al matrix, which decreases with the increase of the CNT content.The effects of ball milling process and sintering temperature on the evolution ofCNT structure and the interface between CNTs and Al were studied. Thestrengthening mechanisms of CNT/Al composites were discussed. The results showedthat the short-time ball milling causes relatively small damage to CNT structure, andthe CNTs implanted into Al matrix possess well structure. The tensile strength andelongation of CNT/Al composite both reach maximum as630℃sinteringtemperature. The tensile properties of composites decrease when the sinteringtemperature is above Al melting point. There are three different forms of interfacialreaction between CNTs and Al matrix in the composites obtained at630℃sinteringtemperature: no distinct interface reaction, forming CNT@Al4C3structure, andcompletely forming Al4C3phase. The nano-sized Al4C3phase with small content canincrease the interface bonding between CNTs and Al, and is beneficial for theimprovement of the composite properties. The remarkable improvement of themechanical properties of CNT/Al composites results from the cooperative action ofseveral reinforcement mechanisms, including effective load transmission, dislocationstrengthening and grain refining strengthening.