Mechanical Properties Of Carbon Nanotubes Reinforced Pure Aluminum Matix Composites

Due to their lightweight,high strength,high modulus,and relatively low costs,Al matrix composites(AMCs)had been widely used in aerospace,automobiles,ships and other industrial fields.Carbon nanotubes(CNTs)should be an ideal reinforcement phase of composite materials because of their excellent mechanical,electrical and thermal properties.Hence,it was usually believed that the Al-based composites reinforced with CNTs(CNTs/Al)would have a bright future of research and application.But many studies showed that weak interfaces between Al and CNTs were obtained due to the poor dispersion and wettability with Al of CNTs in Al matrix.In addition,amorphous carbon and catalytic impurities were usually found in the CNTs prepared by chemical vapor deposition(CVD)method,which was the main method for the massive production of CNTs.All these problems would result in the decreasing performance of CNTs/Al.Accordingly,how to improve the mechanical properties of CNTs/Al with a better interface is one of the key problems in this field.In view of the above problems,the method of acid cleaning+carboxylation treatment of CNTs was adopted to improve the dispersion of CNTs.The sintering condition of CNTs/Al was optimized by density and hardness tests.As sintered CNTs/Al materials with two kinds of length to diameter ratio(R)were hot extruded with a self-designed die.By measuring the hardness,tensile property and dynamical mechanical analyzer spectrum(DMA),the effects of both CNTs content(0-2.0 wt.%,weight percentage,the same as the follows)and length to diameter ratio R(large R LR=4200,small R SR=110)on the mechanical properties of CNTs/Al were obtained.Combined with the corresponding microstructure observation,the strengthening mechanisms of CNTs/Al were analyzed.The microstructure observation on CNTs and Al powders showed that CNTs uniformly distributed on the surface of Al powders for CNT content lower than 1.0wt.%,while some CNT agglomerates were observed as CNT content continuously increased to 2.0 wt.%.The optimization on sintering temperature indicated that a sound sintering result was obtained at 580? under in situ pressure of 60 MPa.After hot extruding with an extrusion ratio of 20,the density of all LR-CNTs/Al is99.79~99.86%,which showed good consistency.For all SR-CNTs/Al,the density decreased slightly to 99.41~99.79%;furthermore,the consistency of density was also reduced somewhat.Tensile test results at room temperature of CNTs/Al sintered at followed by hot extrusion at 580? indicated the strength and hardness increased rapidly with the increasing CNTs content for CNTs smaller than 1.0 wt.%,and remained almost constant for further increasing the CNTs content to 2.0 wt.%.The maximum tensile strength is up to 158 MPa for CNT content of 1.0 wt.%,which is 40%greater than that(113MPa)of pure Al prepared by the similar condition.Compared with the influence of CNTs content,the effect of length to diameter ratio R on the strength of CNTs/Al was apparently smaller.At the same CNT content,the R ratio seemed to have little influence on tensile strength,but it had a certain effect on the yield strength.Slight higher yield strength was obtained for SR-CNTs/Al.The detail analysis shows that the increments of hardness,yield strength and tensile strength of CNTs/Al with uniform distribution of CNTs are proportional to the square root of CNTs content,which indicates that the main strengthening mechanism of the present CNTs/Al should be dispersion strengthening,not the load transfer and other strengthening mechanisms mentioned by many other works.DMA results revealed that the traditional grain boundary internal friction peak observed in polycrystalline pure Al was absent for all CNTs/Al,which suggests that the grain boundary structure of CNTs/Al should be different from that of pure Al.As a whole,only the modulus of Young's modulus|E|showed apparent regularity.As the CNT content increased,|E|initially increased to the maximum,and then remain almost constant.Moreover,the CNT content for the maximum|E|decreased with the increasing temperature.The length to diameter ratio R also had some effect on|E|.At the same CNT content,a higher|E|was obtained for LR-CNTs/Al.It is noteworthy that only the|E|at 100? and lower of LR-CNTs/Al with 1.0 wt.%CNTs was slight larger than that of commercial pure Al at the same temperature.For all other CNT contents,including all LR-CNTs/Al,a smaller|E|than that of pure Al was observed at the same temperatures.Furthermore,the decreasing amount of|E|of all CNTs/Al increased with increasing temperature.It maight be associated with that the interface between CNTs and Al became worse with increasing temperature.In conclusion,based on the purification and carboxylation of CNTs,two kinds of CNTs/Al containing 0~2.0 wt.%CNTs were prepared by sinter and hot extrusion.The effects of the content and length to diameter ratios of CNTs on the mechanical properties of CNTs/Al were obtained by hardness,tensile,and DMA testing.As a whole,the mechanical properties of CNTs/Al increased with the increasing CNT content to 1.0 wt.%,and remained almost constant for further increasing CNT content to 2.0 wt.%.