Preparation And Properties Of SiC/GNPs Composites

Silicon carbide（SiC）ceramics have been widely used in petroleum,metallurgy,chemical industry as an important structural material.SiC ceramics also have potential application in the electronic industry due to excellent properties such as low density,stable thermal performance,low thermal expansion coefficient and high thermal conductivity.However,the disadvantage of low toughness,which affects the machinability and reliability of materials,limited the application of SiC.The thermal conductivity of SiC ceramics is much lower than the single crystal SiC due to the influence of impurities,second phase,grain boundary and porosity in the polycrystall SiC ceramics.Graphene owing excellent mechanical and thermal properties due to its unique crystal structure has been used as reinforcement in ceramic matrix.In this paper,graphene nanoplates（GNPs）were introduced to SiC as reinforcements by liquid phase sintering and solid state sintering.The effect of GNPs content on the mechanical and thermal properties of SiC ceramics was studied.The agglomerated GNPs were dispersed by ultrasound,the effect of ultrasonic time on the microstruecture structure and damage degree of GNPs was studied.The optimum dispersion time was determined to be 12 h by TEM image observation and Raman spectrum analysis.The effects of ball milling,magnetic stirring and ultrasonic-magnetic stirring on the dispersion of composite powders were studied,and ultrasonic-magnetic stirring was determined as the best dispersion method.SiC ceramics were prepared by spark plasma sintering（SPS）using liquid phase sintering mechanism（LPS）.The effects of rare earth oxides on the densification,mechanical and thermal properties of SiC ceramics were investigated.After optimizing the sintering process,the relative density is above 98%,the hardness is above 20 GPa,the fracture toughness is between 3.6±0.2 MPa·m^1/2and 4.0±0.2 MPa·m^1/2,and the thermal conductivity is between 97 W·（m·K）^-1and 108 W·（m·K）^-1.GNPs（0.5 wt.%,1.0 wt.%and1.5 wt.%）were introduced into Y-Gd samples to investigate the effect of GNPs addition on the phase component,mechanical properties,thermal properties and microstructure of LPS SiC.GNPs reacted with Y（Gd）-Si-O to form Y-Si and Y-C compounds and gases,resulting in an increase in the open porosity of the composite.The hardness of samples decreased with the increase of GNPs content due to increase porosity.The pull-out,crack bridging and deflection of GNPs increased the fracture toughness from 3.9 MPa·m^1/2to 4.8MPa·m^1/2.The thermal conductivity decreases with the increase of GNPs,and the main factor affecting the thermal conductivity is porosity increasing.SPS and pressureless sintering were used to prepare SiC/GNPs composites with B₄C and C as sintering additives solid state sintering machanism.The Vickers hardness of samples which were prepared by SPS increased when the addition of GNPs was less than 2wt.%,and when the addition of GNPs was higher than 2 wt.%,the Vickers hardness decreased.The fracture toughness increases with the increase of GNPs.When the GNPs content was added to 1 wt.%,the thermal conductivity increased from 117 W·（m·K）^-1to121 W·（m·K）^-1.With the further addition of GNPs,the thermal conductivity decreases rapidly.In the study of composites prepared by pressureless solid-state sintering,when the addition amount of GNPs increased from 1 wt.%to 4 wt.%,the relative density of SiC ceramics decreased slightly and the porosity was low.The hardness changed little and the fracture toughness increased by 47%.With the increase of GNPs content,the thermal conductivity increased.When GNPs increased to 4 wt.%,the thermal conductivity increased by about22.4%.The increase of grain size leads to the decrease of grain boundary number,which is the main reason for the increase of thermal conductivity.