Fabrication And Mechanical Properties Of Carbon Nanotubes/Copper Matrix Composites With Micro-nano "Brick And Mortar" Structure

Copper matrix composites reinforced with carbon nanotubes(CNTs)have attracted considerable attention due to their excellent mechanical properties,excellent thermal and electrical properties,and wide application prospects in aerospace,military defense,transportation and electronic industries.However,the new nano-carbon reinforced metal matrix composites still cannot overcome the trade-off between strength and ductility.In recent years,to surmount this dilemma,architecture design in composite is widely accepted by tailoring the orientation and/or distribution of reinforcements in matrix but without change in constituents.Among various architectures,biomimetic “brick and mortar” laminated structure which inspired by nacre can effectively balance strength and ductility.At the same time,demands on MMCs with multifunctional and higher performance are increasing by the applications in severe environments,among which thermal cycling is one of the most important service conditions.For example,the components of artificial satellites have to suffer extreme thermal cycling between –160 ? in the earth's shadow and 130 ? when it is exposed to direct sunlight.Such severe thermal cycling must generate large interfacial stresses between reinforcement and matrix in a composite because of their different coefficient of thermal expansion,which may induce micro-damage in the composites or even their catastrophic failure.In this paper,the carbon nanotubes/copper matrix composite is studied to explore the intrinsic relationship between mechanical properties and biomimetic “brick and mortar” laminated structure.At the same time,the temperature(high and low temperature,thermal cycle)of micro-nano laminated CNTs/copper matrix composites was systematically studied,aiming at providing research ideas and technical approaches for new functional materials working in harsh environments.The main results of this paper are as follows:(1)Carbon nanotubes/copper(CNTs/Cu)matrix composites with a biomimetic “brick and mortar” laminated structure have been successfully prepared via flake powder metallurgy.The CNTs/Cu composite provides a cleaner system than other chemically active metal matrices where it is difficult to exclude the influence of oxides.This fabrication method has the advantages of realizing a laminated structure as well as uniform dispersion of CNTs in metal matrix with as low as possible structural damage.The surface of pristine CNTs is hydrophobic and is difficult to be absorbed on the surface of Cu flakes.In order to improve its affinity for copper,CNTs were negatively charged by modifying with carboxylic groups,while Cu flakes were positively charged by coating with dodecyl trimethylammonium bromide(DTAB).As a result,the CNTs can be evenly adsorbed on the surface of Cu flakes through electrostatic interactions between them.Carbon nanotubes/copper matrix composites with biomimetic “brick and mortar” laminated structure were prepared by hot pressing sintering at 950 ? and hot rolling at 850 ?.The evolution from spherical copper powders to bulk materials was characterized by means of SEM,Raman and TEM.The results show that the flake powder metallurgy is an effective way to prepare metal matrix composites with biomimetic “brick and mortar” laminated structure.The parameters of uniform dispersion and densification of carbon nanotubes is the key to the preparation of high quality bulk materials.Appropriate increase of sintering temperature of densification process is conducive to ensuring the interfacial bonding strength between carbon nanotubes and copper matrix.(2)CNTs/copper matrix composites with biomimetic “brick and mortar” laminated structure present a good balance in strength,ductility and electrical conductivity.The ultimate tensile strength of l.0 vol.% CNTs/Cu composite reaches 395 MPa,with an electric conductivity of 90% IACS and an elongation rate of more than 20%.The excellent comprehensive performance could be contributed to uniform dispersion of CNTs in Cu matrix,slight structural damage of CNTs and strong interfacial bonding between CNTs and Cu.The biomimetic “brick and mortar” laminated structure consists of alternating layers of Cu and CNTs,and no obvious agglomeration of CNTs can be seen.Moreover,the biomimetic “brick and mortar” laminated structure resemble of the intra-lamellar interlocking in nacre.Because of such interlocking,the Cu matrix is continuous in the whole composite,which is beneficial for taking the advantages of the intrinsic plastic properties of metal matrix.Meanwhile,TEM images show high density of dislocations in the Cu matrix next to the CNTs at interface.Both bright field and weak beam dark field images indicate that the CNTs could effectively impede the movement of dislocations,which is of importance for strengthening composite.Characterizations by in-situ digital image correlation and X-ray tomography indicate efficient stress transferring during loading as well as laminated structure guiding crack propagation,contributing to the good strength-elongation balance.Additionally,the electrical conductivity of the composites is anisotropic because of the laminated structure.Our work provides a strategy for fabricating Cu-based materials with enhanced mechanical properties and high electrical conductivity.(3)The effect of thermal cycling on mechanical properties of biomimetic “brick and mortar” laminated CNTs/copper matrix composites are investigated in a temperature range of –196/+200 ?.The thermal cycling results in reductions in strength and elongation and modulus,but the strengthening role of CNTs remains in some extents as compared to the pure Cu counterpart.Tensile tests at room temperature show that high strain hardening ability can be regained even after a sharp stress drop in the thermally cycled composites.The observations on transmission electron microscope(TEM)and scanning electron microscope(SEM)show that the amount of crack increases gradually in the composites upon thermal cycling.Nevertheless,CNT bridges are often found bestriding the cracks and impeding their propagating.The TEM results indicate high density of dislocation in the Cu matrix after thermal cycling,but it decreases after enough thermal cycles,which might be attributed to the stress releasing caused by crack propagation.The failure mode is dominated by the inter-carbon-layer fracture of the multi-walled CNTs in the composites subjected to thermal cycling and tensile loading.(4)The mechanical properties and failure behavior at high temperatures are investigated for biomimetic “brick and mortar” laminated CNTs/copper matrix composites with well-balanced strength and ductility.Tensile strength of the composite decreased considerably as temperature increased because of matrix softening and Cu grain coarsening.CNTs still play role in strengthening matrix even tensile temperature is elevated up to 773 K.However,the yield strength of the CNTs/Cu composite was even lower than that of the pure Cu matrix at 923 K,and the reason may be that the CNTs restrict grain growth and promote Cu matrix softening.Deformation mechanism of the CNTs/Cu composites is controlled by pipe diffusion mechanism at 573 K,while it is changed to dislocation climb controlled by lattice diffusion at 773 K.CNT's pulling-out is observed in tensile failures composite at all the temperatures investigated,but with different failure mechanisms,i.e.interface debonding at room temperature while matrix rupture at high temperature.