Fabrication And Properties Of Carbon Nanotubes Reinforced Aluminum Composites By Pressureless Infiltration Technology

Some progress in fabrication of carbon nanotubes(CNTs) reinforced aluminum composites has been achieved during the past decade.However,several essential problems such as the chemical stability of CNTs in fabricating process,dispersion of CNTs in metal matrix. interfacial bonding between CNTs and aluminum,strengthening mechanism of CNTs,have not yet got over up to now.In literatures,some studies on the chemical stability of CNTs in Al matrix have been done.but the conclusions are not in complete agreement with each other. Chemical reaction of CNTs with Al-matrix involving the wetting and interracial bonding between CNTs and Al,is also closely related to the dispersion and strengthening mechanism of CNTs in the matrix.In this thesis,mixed powders of CNTs,Al and Mg were calcined at 700～850℃to sudy the chemical reactions between CNTs and Al.The results demonstrated that the oxide films on the surface of Al powders can be removed by reaction with Mg element at higher temperature and then the molten Al wonld react with CNTs.The molten Al was spreaded and coated on the surfaces of CNTs via a reaction-wetting mechanism.As a matter of fact,the moten Al reacted with the amorphous carbon and defects on the surface of CNTs.This kind of reactions was restricted by the size and quantity of amorphous carbon and defects on the surfaces of CNTs.So the resulted carbides were limited and were in nano-scale.The production of carbides would not increase when the reaction time was markedly extended at 850℃.The carbides tightly bonded on the surface of CNTs,which may enhance the combination of CNTs and Al matrix.The CNTs-reinforced Al composites were fabricated by a pressureless infiltration method. The wetting between CNTs and molten Al was controlled by the chemical reactions between Al and amorphous carbon and defects on the surfaces of CNTs.As the reactions conducted,the contact angle of Al on the surfaces of CNTs declined gradually.Once the contact angle became steady,the wettability of Al on CNTs achieved and the infiltration process occurred.Accordingly, the pressureless infiltration process contains an incubation period.The duration of incubation period was influenced by the temperature and the structure of preform.The primary graphite structure of CNTs was kept almost intact even when the CNTs were annealed in molten Al for 24 hours at 800℃.Therefore.the pressureless infiltration process is appropriate to synthesize the Al composites reinforced by CNTs.The wetting of Al on CNTs is the key factor for dispersing CNTs in the metal matrix.As a result of the wetting oral on CNTs,the molten Al can spread on the surfaces of CNTs and be suffused in the space between CNTs.The wetting and coating of molten Al on CNTs would break the van der Waals force between CNTs,thus the CNTs were uniformly embedded into the Al-matrix,which avoided the agglomerations of CNTs in grain boundaries,and the grain growth would also be restrained.CNTs were not agglomerated together even if the content of CNTs in composites was up to 2.1 wt%,which indicates that the pressureless infiltration process is efficient to disperse CNTs in Al-matrix.The tentative investigations on the thermal mismatch between CNTs and Al matrix showed that both the volume fraction of CNTs and colling speed have significant influences on the release of thermal mismatch stress.Either the volume fraction of CNTs was high enough or the coiling speed was too fast,the thermal stress could not be released in time by the proliferation of dislocations.Consequently,the micro-cracks would initiate and propagate.Nevertheless,the bridging and debonding of CNTs in the matrix would retard and divert the development of the cracks.The tensile strength of the composites declined due to the small quatity of micro-flaws.The Young's modulus and hardness of the composites observably increased in virtue of the uniform dispersion of CNTs in Al-matrix.The tribology properties of the composites were markedly improved based on the self-lubricating and strengthening of CNTs on the Al-matrix.The morphology of wear surface validates the influence of micro-flaws on the wear process.As the volume fraction of CNTs increased,which would induce more micro-flaws,the delamination process became a dominant wear mechanism.The preliminary study on thermal performance has shown that the coefficient of the composites' thermal conductivity decreased with the content of CNTs.On account of random array of CNTs and the grain refinement of Al,the interfacial thermal resistance increased.On the other hand,the micro-flaws,derived from thermal mismatch between CNTs and Al matrix, would baffle the thermal transfer,resulting in the decrease of the thermal conductivity of the composites.