Study On Microstructure And Properties Of Nanocarbon Reinforced Magnesium Layered Composites With Integrated Mechanical Bearing And Electromagnetic Interference Shielding

In order to meet the urgent needs of military electronic equipment for the development of new materials with integrated light weight,mechanics and shielding function,in this paper,graphene（GNS）,carbon nanotube（CNT）and hybrid graphene-carbon nanotube（GNS-CNT）were used as the nanocarbon layer,respectively,and Mg matrix was treated as the micro layer to prepare the nanocarbon/Mg composites with micro-nano layered structure.The microstructure,mechanical properties and electromagnetic interference shielding effectiveness（EMI SE）of the Mg matrix composites reinforced by the above three kinds of nanocarbon were systematically characterized and analyzed.Moreover,compared with GNS and CNT,the advantages of GNS-CNT in the synergistic enhancement of strengthening-toughening and EMI SE for Mg matrix were analyzed and discussed in detail.The nanocarbon was pickling treated by the mixture of H₂SO₄ and HNO₃to introduce certain oxygen-containing functional groups to overcome the tendency of agglomeration.Meanwhile,the ultrasonic oscillation was applied to further improve the dispersibility of nanocarbon in ethanol.The uniformly deposited nanocarbon/Mg layered units were obtained by spray deposition.Among them,the GNS-CNT reinforcement was prepared by successively depositing CNT-GNS-CNT on the Mg foil surface,and the deposition content of CNT at both ends maintained the same.The sandwiched GNS could effectively hinder the stacked agglomeration of CNT,while the CNT could connect adjacent GNS like an extended antenna,forming a unique three-dimensional（3D）network structure.In the GNS-CNT reinforcement,three ratios of GNS:CNT=1:1,GNS:CNT=1:2 and GNS:CNT=1:4 were designed and recorded as GNS₁-CNT₁,GNS₁-CNT₂ and GNS₁-CNT₄,respectively.The effective densification and bonding of layered units were realized by hot pressing sintering and hot extrusion process,which was conducive to obtaining higher strengthening and toughening efficiency.Under the condition of the same nanocarbon content,the GNS/Mg and CNT/Mg layered composites exhibited similar mechanical properties and EMI SE.Compared with GNS and CNT,GNS-CNT showed significant synergistic enhancement advantages in strengthening-toughening and EMI SE for Mg matrix.And among the GNS-CNT reinforcements with different hybrid proportions,GNS₁-CNT₂ was the most prominent for the above synergistic strengthening advantages.Finally,GNS₁-CNT₂/Mg layered composites with the nanocarbon content of 0.5 wt.%simultaneously achieved the ultra-high mechanical properties and EMI SE.Compared with pure Mg,the yield strength of GNS₁-CNT₂/Mg layered composite was increased from 89 MPa to 165 MPa,the elongation was increased from 6.4%to 11.5%,and the EMI SE was improved from 20 d B to 70 d B,meeting the military application standard.Combined with the theoretical calculation and experimental results,the main strengthening mechanism in the GNS/Mg and CNT/Mg composites were dislocation strengthening induced by the high thermal mismatch at the interface.Compared with GNS and CNT,the 3D networked GNS-CNT exhibited larger effective size,which significantly improved the load transfer efficiency of GNS-CNT,making the load transfer become the main strengthening mechanism in the GNS-CNT/Mg layered composites.The local strain evolution of the composites during in-situ tension process was tracked by DIC.The results showed that the nanocarbon layer could effectively alleviate the local strain concentration and make the deformation of composites more uniform in the macroscopic point of view.From the microscopic point of view,the local stress induced by the nanocarbon layer led to the strain difference between adjacent layered units.The plastic deformation ability of Mg matrix was effectively improved through this strain state that macroscopically uniform but microscopically inhomogeneous.Compared with CNT and GNS,the larger effective size of 3D networked GNS-CNT increased the contact area between GNS-CNT and Mg matrix,which promoted good interface bonding and endowed GNS-CNT with outstanding load transfer efficiency.Thus GNS-CNT showed more efficient ability to adjust strain localization.The geometric necessary dislocation（GND）densities of pure Mg and nanocarbon/Mg layered composites were characterized by EBSD,which proved that the nanocarbon layer induced the high densities of GND distribution.And the EBSD-based crystal slip trace analysis confirmed that the introduction of nanocarbon layer played a key role in promoting the activation of<c+a>dislocations.Based on the above DIC analysis results,the larger effective size of the 3D networked GNS-CNT caused greater local internal stress,which could induce higher densities of GND distribution and further promote the more activation of<c+a>dislocations.The micro-nano layered structure in the nanocarbon/Mg layered composite extended the multiple reflection paths of electromagnetic waves（EMWs）in the Mg matrix,thus effectively improving the absorption loss of incident EMWs;Compared with GNS and CNT,the GNS-CNT reinforcement,due to its unique3D network structure,generated additional multi-level reflection loss,ohmic loss,heterogeneous interface polarization loss and dipole polarization loss of the incident EMWs in the GNS-CNT layer,making the GNS-CNT/Mg layered composites show higher EMI SE.