Preparation And Properties Of Graphene Nanosheets Composite Silicon Carbide Ceramics

Silicon carbide ceramics are widely applied in heat exchangers,radiant tubes,and galvanic protection tubes because of its excellent thermal stability,corrosion resistance,high thermal conductivity and high wear resistance in harsh environments,Especially in the field of heat exchanger,high thermal conductivity isa key which achieve the aim of improving heat transfer efficiency.These application examples are inseparable from the excellent thermal conductivity of SiC ceramics.In addition,SiC is widely used as an abrasion resistant structural member in the industrial field.Therefore,high thermal conductivity and good wear resistance are the basis for the wide application of SiC ceramics,and it is of great significance to increase the intensity of its research.In this paper,Al₂O₃ and Y₂O₃ were used as sintering aidsby adjusting the ratio of Al₂O₃to Y₂O₃ to form liquid phase yttrium aluminum garnet(YAG:Y₃Al₅O₁₂)at high temperature.By ulitilizing the principle of liquid phase sintering,graphene nanosheets（GNPs）are introduced to the SiC material,and finally the GNPs/SiC ceramic matrix composite is prepared by hot-pressed sintering.The dispersion behavior of GNPs in SiC ceramic matrix was investigated.The effect of GNPs content on the density,microstructure,mechanical properties,thermal conductivity and friction and wear properties of composites were investigated.The thermal conductivity and friction and wear properties of ceramics by introducing GNPs was enhanced.The results show as follow:（1）GNPs/SiC composites containing 0-20vol.%GNPs was treated by planetary high energy ball milling firstly.The results illustrate that GNPs were approximately parallel in the microstructure（the vertical axial direction of the xy plane）,due to axial pressure,which meeaned high orientation of the composites.As thecontent of GNPs was increased from zero to 20vol.%,the bulk density of composite decreased,the relative density decreased from99.10%to 88.61%,and the porosity increased;the Vickers hardness decreased from28.82GPa to 9.68GPa.When the content of GNPs was 5vol.%,the fracture toughness was as high as 5.66MPa·m^1/2,which was about 29.2%higher than that of neat SiC ceramics.The main toughening mechanism of composites is GNPs.Pull-out and bridged and deflection of the crack.When the content of GNPs was 2.5vol.%,the thermal conductivity of composites at 25°C and 2.5vol.%GNPs was as high as 95.32Wm^-1K^-1,which was about 9.56%more than that of SiC ceramics without addition of GNPs.（2）GNPs/SiC ceramic matrix composites containing 0⁵vol.%GNPs was mixed by ordinary mechanical ball milling before hot-pressed sintering.The results also show that GNPs were approximately parallel in the microstructure（xy The plane perpendicular to the axial pressure direction）show high orientation.When the content of GNPs was increased from 0vol.%to 5vol.%,the bulk density of the composite decreased,the density decreased from 99.89%to 97.89%,and the porosity increased;the Vickers hardness declined from28.94GPa to 21.96GPa.With the increasing of the content of GNPs,the fracture toughness of composites increases gradually.When the content of GNPs was 5vol.%,the fracture toughness was as high as 5.72MPa·m^1/2,which was higher than that of SiC ceramics without GNPs.The improvment wasabout 29.4%;The main toughening mechanism of composite materials was the extraction and bridged of GNPs and the deflection of cracks.From 25°C to300°C,the thermal conductivity of the composite decreased with the increasing temperature.At 25°C,when the GNPs content was 2.5vol%,the thermal conductivity of the composite is as high was 99.03Wm^-1K^-1,improvement of about 11.39%compared to SiC ceramics without GNPs.At last,thewear and abrasion between GNPs/SiC ceramic matrix composites with0⁵vol.%GNPs mixed by ordinary mechanical ball milling and four kinds of Al₂O₃,SiC,Si₃N₄ and GCr15 steels were investigated by pin-disk friction and wear experiments..The results show that with the increasing of GNPs content to 5vol%,the dry friction coefficients of different friction media were reduced by 20%³0%,,and the wear amount were reduced by 80⁹0%;Raman spectroscopy analysis of worn surface show lubricating films containing GNPs were formed on the surface of the GNPs/SiC ceramic.As the content of GNPs increasing,the I_D/I_G value increased,and the nano-lubric film was formed.More continuous and complete the film layer was tightly attached to the surface of the composite material,benefiting from the higher flexibility of the GNPs,and some of them were filled pores in the surface and promoted the film layer to be flat.The GNPs embedded in the matrix was the key factor,which lead to reducing the friction stress,decreaseing the friction coefficient of the SiC ceramic,and reducing the amount of wear.The wear mechanism of composite materials was mainly brittle fracture,abrasive wear and delamination wear.For different friction media such as Al₂O₃,SiC,Si₃N₄ and GCr15,the friction coefficient with GCr15 were the highest,followed by Al₂O₃,and the friction coefficient of Si₃N₄ and SiC were similar.