Interface Design And Mechanical Properties Of Carbon Nanotubes Reinforced Aluminum Matrix Composites

For the high strength-to-weight ratio,good anti-wear properties,good corrosion resistance,appropriate thermal conductivity,and excellent electrical conductivity,the aluminum matrix composites（AMCs）are potential lightweight structural materials for aerospace and automobile applications.The carbon nanotubes（CNTs）with high Young’s modulus,high tensile strength,low density,and superior thermal conductivity are considered as one of the most suitable reinforcements for high strength and multifunctional composites.The strength and toughness of aluminum matrix composites can be synergistically improved by using CNTs as reinforment.However,despite the high potential ability to achieve high mechanical properties of AMCs reinforced by CNTs,the desired enhancement effects of CNTs have not been fully developed.The main reasons can be summarized as follows:（1）CNTs are difficult to be evenly dispersed in the Al matrix due to the large length-diameter ratio of CNTs and the large van der Waals forces between CNTs;（2）due to obvious differences in physical properties between Al and CNTs,the interface bonding between them is weak;（3）The structure of CNTs was easily destroyed by the in situ chemical reaction between Al matrix and CNTs.To solve the above bottlenecks,according to the interface structure design of the composite,a novel idea of introducing Ti O₂ or Cu nanoparticles coating with specific structure into CNTs-Al interface was proposed to regulate the interfacial interaction and improve the interfacial wettability and interfacial bonding,so as to give full play the strengthening effect of CNTs.In this paper,the microstructure,wettability,interfacial structure and mechanical properties of AMCs reinforced by Ti O₂@CNTs or Cu@CNTs were systematically studied,and corresponding strengthening mechanisms of composites was revealed.It provides a new research idea for preparing high performance aluminum matrix composites.The main research results of this paper are as follows:（1）Preparation and wettability analysis of Ti O₂@CNTs:novel Ti O₂nanoparticles-coated CNTs（Ti O₂@CNTs）were fabricated by the alcohol-thermal method and its formation mechanism was explored,which realized the controlled preparation of Ti O₂@CNTs reinforcment.The results show that the nanoscale Ti O₂particles are uniformly distributed on the surface of carbon nanotubes and covered almost the whole surface to form Ti O₂ coating.By incorporating Ti O₂nanoparticles on the surface of CNTs,the contact angle was reduced from 140.5°to 119°,indicating a significant improvement in the wettability of CNTs to aluminum melt.However,the wettability of Ti O₂@CNTs to Al melt still shows a non-wetting state.（2）Preparation,interfacial structure,and mechanical properties analysis of Ti O₂@CNTs/Al composites:Ti O₂@CNTs reinfored aluminum matrix composites were synthesized by high-energy ultrasound-assisted casting method and the corresponding strengthening mechanisms were explored,which realized the controlled preparation of Ti O₂@CNTs/Al composites.The results show that the introduction of Ti O₂nanoparticles coating changes the interfacial structure of composites.The semi-coherent interface was formed at the Ti O₂/α-Al interface,and the mismatch of（001）_{Ti O2}//（100）_Al is only 6.52%,indicating a good lattice space matching between Ti O₂ andα-Al matrix.TEM results reveal that Ti O₂ nanoparticles homogenously dispersed on the surface of CNTs act as an interlayer structure between CNTs and matrix,locking the CNTs and improving the CNTs-Al interfacial bonding.The composites with 1.2 wt.%Ti O₂@CNTs addition display high mechanical properties with yield strength and ultimate tensile strength of 196 MPa and 262 MPa,which are 54%and 29%higher than those of the matrix,respectively.The better mechanical properties may be attributed to good interfacial bonding caused by the semi-coherent interface created by the Ti O₂ interlayer,leading to an effective load transfer from matrix to CNTs.（3）Preparation and wettability analysis of Cu@CNTs:novel Cu nanoparticles-coated CNTs（Cu@CNTs）were prepared by liquid-phase reduction method and the effects of different process parameters on the microstructure of Cu@CNTs were investigated in detail,which realized the controlled preparation of Cu@CNTs reinforcment.The results show that Cu nanoparticles were completely reduced by ethylene glycol and N₂H_4·H₂O reducing agents and disperse evenly on the surface of carbon nanotubes to form Cu coating.By incorporating Cu nanoparticles on the surface of CNTs,the contact angle was reduced from 133°to 91°,indicating a significant improvement in the wettability of CNTs to aluminum melt.The better wettability of Cu@CNTs to Al melt shows a wetting state,which attributes to small amount of element Al can react with Cu on Al-Cu@CNTs interface to form Al Cu or Al2Cu.（4）Preparation,interfacial structure,and mechanical properties analysis of Cu@CNTs/Al composites:Cu@CNTs reinfored aluminum matrix composites were synthesized by high-energy ultrasound-assisted casting method and the corresponding strengthening mechanisms were explored,which realized the controlled preparation of Cu@CNTs/Al composites.The results show that Cu nanoparticles act as a bridge connecting the carbon nanotubes and matrix and forming Al₂Cu intermetallic,which is in favour for better dispersion of carbon nanotubes and interfacial bonding.In addition,Cu nanoparticles are preferentially deposited at the defects of carbon nanotubes to repair the defects and inhibit the formation of brittle Al₄C₃phases.As a result,Cu-coated composite displays the superior strength with yield strength and ultimate tensile strength of 203 MPa and 287 MPa,which are significantly higher than those of matrix and uncoated composite,respectively.The introduction of Cu nanoparticles can significantly improve the interfacial bonding strength of Cu-coated composite,ensuring the effective transfer of load from the matrix to carbon nanotubes and exerting its strengthening and toughening effect.（5）The dendritic growth process of aluminum alloy was simulated using COMSOL software and the effect of different ultrasonic power on the dendritic growth process was investigated detial.The results show that when high energy ultrasound is applied,different power of ultrasound will provide different subcooling degrees,the higher the subcooling degree,the faster the growth rate of dendrites and the more developed the growth of secondary dendrites.In summary,the innovative idea of introducing nano-Ti O₂（or nano-Cu）coating at the interface of CNTs/Al was proposed to improve the wettability of carbon nanotubes in aluminum melt and achieve the controlled preparation of composites.The above strategy effectively improves the dispersion and interfacial bonding strength of carbon nanotubes in aluminum matrix,which solves the common bottleneck problem that CNTs are difficult to achieve the strengthening effect in aluminum matrix.and improves the common bottleneck of the strengthening effect of CNTs in aluminum matrix.In addition,the strengthening mechanisms of composites was studied in detial,which provides an experimental basis and theoretical reference for the development and application of lightweight and high-strength AMCs.