| Carbon nanotubes (CNTs) have excellent physical and chemical properties with advantages in toughened metallic matrix composites. It was a key issue that how to dispersing light carbon nanotubes into metal matrix. In this thesis, three methods of adding carbon nanotubes into steel matrix was explored. Finally, carbon nanotubes have been incorporated in steel matrix using melting casting method. Effect of Carbon Nanotube on Steel was studied.The CNTs were firstly ball milled with Al powder. These obtained CNT-Al mixture were slowly cold pressed to cake like block. The CNT-Al block was then crushed and sieved to make powders (mean particle size=2mm). These powders were spread in the ladle prepared to be combined with the steel melt. Pour the steel melt into the ladle when temperature is lowered to 1550℃. The CNT-A1 powder was mixed with steel melt there in the ladle for 5s. Pour the steel melt into the casting mold, and then the composite is made.The dispersion of CNTs, the combination between CNTs and steel matrix were investigated and analyzed. By using SEM, HRTEM, tensile test and metallographic analysis techniques, the microstructure, the mechanical properties and the CNT addition parameters of the composites were examined and optimized. In addition, for the first time, fast and thorough graphitization of CNT was discovered, the straight graphitized CNTs were observed and investigated.The mechanical properties of the composite were tested. The results show carbon nanotubes had positive effect in hardness, tensile property and tribological properties on composite:the optimal addition is 0.04% when tensile property is tested; the optimal addition is 0.01% when hardness is tested; the optimal addition is 0.04% when friction coefficient is tested; the optimal addition is 0.1% when wear loss is tested.Metallographic research on the composite show the addition of CNT can bring contribution to grain refinement, the refining effect is increased with the increase of addition of CNTs. The optimal addition is 0.1 wt.%. The refinement is directional, i.e. the grain is refined in one direction, yet in the other direction the refining effect remain inconspicuousness.Morphologies of the samples were observed by means of a field-emission scanning electron microscope (FEI Quanta200F, USA) and a high-resolution transmission electron microscope (HR-TEM, JEM-2100, Japan). Structure of CNT-Steel combination that looks like CNT reinforced steel bar was observed. This structure reveals steel melt can grow around CNT while solidification and form a strong combination which could contribute to the enforcement of the steel matrix mechanical properties. This combination may assist effective load transfer from the matrix to CNT.Perfectly straight CNT was found. The results of HRTEM show the wall of straight CNT have been developed to highly aligned straight layers which implies it experienced fast and complete graphitization in the presence of catalytic reaction of steel melt. The straight CNT may become a key point to the complete dispersion of CNTs.It was first time that fast and thorough graphitization of CNT was found during 10s and below 1550 ℃. The CNT wall layers become straight and well aligned which means most of the defects existed in the pristine CNT were eliminated. The graphitization temperature is no more than 1550 ℃ which is significantly lower than related reports which believed CNT cannot be graphitized below 1800℃ and the thorough graphitization will occurs only when the temperature is raised to 2200 ℃, under pressure 2.5a.t. and it will take 15 mins to complete the graphitization. This discovery is a scientific breakthrough on CNT graphitization temperature and graphitization speed. |
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