Preparation,Structure And Properties Of RGO-B₄C-（SiC） Composites By Self-assembly Polymerization

As an important advanced structural material,boron carbide（B₄C）ceramic has low density,high hardness,as well as high wear resistance and excellent neutron absorption capacity,making it an ideal candidate for the application of lightweight armor material,wear-resistant material,and atomic reactor control material.However,the low fracture toughness and poor machinability of B₄C ceramics cannot meet some special engineering applications,which seriously restricts its further development.The discovery of graphene has brought new opportunities for the research and development of B₄C ceramics.Graphene is a carbon material with a two-dimensional honeycomb network structure.It has a large specific surface area,excellent mechanical properties,electrical properties and thermal conductivity,etc.,making it an ideal reinforcement and toughening phase for B₄C ceramics.However,due to the existence ofπ–πbonds and strong interlayer van der Waals forces,it is easy to agglomerate in B₄C ceramics and cause local defects,which affect the mechanical properties of the composites.This paper is based on solving the inherent defects of B₄C ceramics and the difficulty of graphene dispersion in B₄C ceramics.On the premise that reduced graphene oxide（rGO）after spark plasma sintering（SPS）has been confirmed to maintain the structural integrity,a self-assembly polymerization method was first proposed,using graphene oxide（GO）as raw material,using acrylamide（AM）cross-linking B₄C powder,as well as the hydrogen bond between amide bond（–NH₂）and oxygen-containing functional groups on the surface of GO,obtaining the uniform dispersion of GO in B₄C powder,and high toughness rGO–B₄C composites.Adding silicon carbide（SiC）to strengthen and toughen rGO–B₄C composites,the B₄C–SiC–rGO composites with the best comprehensive mechanical properties were obtained in the same component,which provides a theory and experimental basis for the industrialized preparation of high-performance graphene-based boron carbide composites.The main research contents of this paper are as follows:（1）Structural evolution of GO under high temperature and high pressureThe sintering and densification of B₄C ceramics often require extremely high temperature and pressure.The structural evolution of GO under high temperature and high pressure and the structural integrity of rGO are crucial for the study of GO toughened B₄C ceramics.In this experiment,GO with stable structure and excellent performance was prepared by the modified Hummers method,and a perfect GO quality evaluation system was established.The as-prepared GO contained a large number of abundant oxygen-containing functional groups（such as–OH,C=O,C–O,C–O–C,etc.）,providing many active sites for chemical reactions.The C–O,C=O,C–O–C groups on the surface of GO gradually decomposed and exfoliated with the increase of thermal reduction temperature,and GO was reduced to rGO after heat treatment at 800°C.The rGO after SPS sintering（1800°C,30/50 MPa）maintained the structural integrity of graphene without obvious degradation of the sheet-like structure,and the pressure effect effectively restored the edge flatness of the consolidated rGO.（2）Preparation and characterization of rGO–B₄C composites by self-assembly polymerization combined with spark plasma sinteringTo solve the problem of graphene dispersion in B₄C ceramics,a self-assembly polymerization method was first proposed.using GO as the graphene source and acrylamide（AM）as the organic monomer,using the cross-linking effect of AM on the B₄C powder,and the hydrogen bonding of amide bond（–NH₂）with the oxygen-containing functional groups of GO,the GO–B₄C composite powders with GO uniformly dispersed were prepared.High-density rGO–B₄C composites were prepared by a tube furnace combined with spark plasma sintering.The effects of rGO addition on the microstructure,mechanical properties and toughening mechanisms of the composites were studied.The results showed that rGO was uniformly distributed in the rGO–B₄C composites,changing the fracture mode of B₄C ceramics from transgranular fracture to a mixed fracture mode of transgranular fracture and intergranular fracture.The optimal flexural strength（643.64 MPa）and fracture toughness（5.56 MPa m1/2）of rGO–B₄C composites were obtained at 1wt.%and 2.5 wt.%rGO,respectively,which were 99.11%and 71.6%higher than those of pure B₄C ceramics.However,the hardness and flexural strength of the high-toughness 2.5 wt.%rGO–B₄C composites decreased to 28.32 GPa and 476.52 MPa,respectively.（3）Research on the compatibility of B₄C powder particle size and graphene sheet diameterUsing three different particle sizes of B₄C powders（W0.8,W3.5,W10）and GO dispersion（5 wt.%）as raw materials,high-density rGO–B₄C composites were prepared by a self-assembly polymerization combined with spark plasma sintering.The effects of B₄C powder particle size on the microstructure and mechanical properties of the rGO–B₄C composites were studied.The compatibility of B₄C powder particle size and rGO sheet size was analyzed,and the toughening mechanism of the rGO–B₄C composites were further verified.The results showed that the rGO diameter size had the best compatibility with the W3.5 B₄C powders.rGO was uniformly dispersed in the W3.5 rGO–B₄C composites and formed an interconnected network structure,making it possess the highest relative density（99.67%）,hardness（31.07 GPa）,flexural strength（496 MPa）and fracture toughness(5.45 MPa·m^1/2).（4）Preparation,strengthening and toughening mechanisms of B₄C–SiC–rGO compositesAlthough the addition of rGO could significantly improve the fracture toughness of rGO–B₄C composites,the relative density,hardness and flexural strength of rGO–B₄C composites have different trends with the addition of rGO than the fracture toughness.In order to obtain the graphene-based boron carbide composite ceramics with the best comprehensive mechanical properties in the same component,the component with the highest fracture toughness（5 wt.%GO）in the rGO–B₄C composites,adding SiC as a reinforcing phase to strengthen and toughen rGO–B₄C composites.The effects of SiC content on the microstructure,mechanical properties and toughening mechanism of rGO–B₄C composites were investigated.SiC inhibits the growth of B4C grains through the pinning effect,which effectively improves the mechanical properties of rGO–B₄C composites.When SiC content was15 wt.%,the BS15G5 composite simultaneously obtains the optimal hardness（34.23GPa）,flexural strength（545.25 MPa）and fracture toughness(5.72 MPa·m^1/2),which were improved by 20%,15%and 3%,respectively,compared to the high-toughness 2.5 wt.% rGO–B₄C composite.