Preparation Of B₄C-CNTs Composite Ceramics And Study On Its Toughening Mechanism

Boron carbide?B₄C?is considered to be one of the most promising advanced structural ceramic materials with low density,high hardness,high wear resistance and good neutron absorption ability.In order to solve the problem of brittle fracture of B₄C ceramics,Carbon nanotubes?CNTs?with excellent properties were selected to prepare composite ceramics of B₄C-CNTs.When preparing B₄C-CNTs composite ceramics by ultra-high pressure sintering method,the growth of B4C grains can be effectively inhibited,and higher compactness can be achieved at a lower sintering temperature.Due to the obvious reduction of sintering temperature,it is expected to solve the destructive problem of CNTs in the sintering process of ceramic matrix composites in recent years,as well as the bonding strength between CNTs and matrix materials at the interface,so as to give more effective play to the strengthening effect of CNTs in ceramic matrix composites.Based on this,B4C ceramics and B4C-CNTs composite ceramics were prepared by ultra-high pressure sintering in this paper.The mechanism of several important process parameters of ultra-high pressure sintering on the structure and properties of B₄C ceramics was systematically studied.On this basis,the damage degree and existing state of MWCNTs,which were introduced into B₄C ceramics as toughening and reinforcing phase,were further studied under ultra-high pressure sintering,and the properties of B₄C-MWCNTs composite ceramics prepared by this method were analyzed.Finally,the effects of multilayer and hydroxylation of CNTs on the structure and properties of B₄C-CNTs composite ceramics were further studied.The main contents and conclusions of this paper are as follows:?1?The effects of several important technological parameters of ultra-high pressure sintering on the structure and properties of pure B₄C ceramics:Pure B₄C ceramics with high compactness were prepared by hexahedral top press under ultra-high pressure.The results show that the grain growth of B₄C ceramics is almost not obvious under this sintering method,and the original micron size is maintained.When the sintering pressure is 4.5 GPa,the temperature is 1300°C and the residence time is 5 min,the B₄C ceramics with 99.65%relative density can be obtained.The sintered B₄C ceramics have good mechanical properties with Vickers hardness of38.04 GPa,Young's modulus of 487.7 GPa and Fracture toughness of 3.87 MPa?m^1/2.?2?The effect of ultra-high pressure sintering on the damage degree of MWCNTs and the properties of B₄C-MWCNTs composite ceramics:the B₄C-MWCNTs composite was prepared at 4.5 GPa,1300?and 10 min,and the MWCNTs was not destroyed greatly.A good interfacial bonding force was formed between B₄C and B₄C.With the increase of the content of MWCNTs,the fracture toughness and electrical conductivity of the composites were improved,and the fracture toughness and conductivity of the composites reached 7.15 MPa?m^1/2 and2674 S/m respectively with the addition of 5 vol%MWCNTs.The microstructure observation shows that the strengthening mechanisms of the composites are crack bifurcation,crack deflection,MWCNTs bridging and pullout.?3?The effect of multi-layer and hydroxylation of CNTs on the structure and properties of B₄C-CNTs composite ceramics:because of the rotation-slip action between CNTs layers,the load transfer is promoted.The dispersion of CNTs in alcohol solution and the interfacial adhesion between CNTs and B₄C were improved,so that the properties of B₄C-CNTs composite ceramics were also improved at the end of the paper.The properties of CNTs composite ceramics were improved by the addition of hydroxyl groups on the outer tube wall of CNTs and the interfacial adhesion of CNTs with B₄C in alcohol solution.