Carbon-based Fillers Reinforced Si₃N₄ Composites With Enhanced Thermal Conductivity And Mechanical Properties

The existence of human beings has always depended on the availability of materials.This dependency on materials is still sustain today,not only because of our physical existence clothing,buildings,vehicles,and countless other inventions that mark human progress but also because of our cultural and intellectual life and literature,art,music.In fact,without materials,human efforts are almost impossible.Ceramics as an inorganic nonmetallic materials are known for more than a thousand centuries and are made from naturally existing raw?powdered?materials.Ceramic materials have a variety of interesting properties,including high stiffness,high mechanical strength,and high temperature stability,making them suitable for cutting tools,biomedical implants,electronics,defense,space,and automotive applications.Despite the various interesting properties of monolithic ceramics,they are still mechanically brittle and unreliable.For glass,this behavior becomes more serious because their amorphous structure does not cause any hindrance to crack propagation,resulting in a very low fracture toughness value.Ceramic nanocomposites have been developed to solve these problems.Ceramic nanocomposites can be defined as materials with more than one Gibbs solid phase,where at least one phase has a size in the nanometer range,and the solid phase can exist in an amorphous,semi-crystalline,or crystalline state.In the case of ceramic nanocomposites,the matrix phase can be a ceramic?SiC,Si₃N₄?or glass material,and the nanophase can be nanoparticles such as nanotubes,nano fibers,nano-sheets?graphene?,or the mixture of these materials.1)Si₃N₄ based composites have been extensively studied and widely used in industrial applications.Herein,SiC/Si₃N₄ composites were prepared through hot press?HP?sintering at 1700? under N2 atmosphere.Fluoride additives?AIF3 and MgF2?were used to reduce lattice oxygen content,which can affect the bulk density and a source to increase the porosity of the composite.Experimental results revealed that the chemical compatibility between silicon SiC and Si₃N₄ matrix under high temperature sintering could be effectively improved by integrating SiC/Si₃N₄ ceramics interphase.The XRD and SEM results indicate that a complete transformation of a to ?-Si₃N₄ and intergranular behavior of material occurred,which revealed that the phase transformation was unaffected by the addition of SiC.Furthermore,the enhanced crystallization degree and a strong interfacial effect of the SiC/Si₃N₄ composites enabled to obtain a significant increase in thermal conductivity?90.67 to 145.66 W/m.K?,fracture toughness?8.52 to 10.3 MPa.m0.5?and Vickers hardness?1849 to 2125 HV?.2)Graphene nanocomposites can significantly enhance the thermal conductivity and mechanical properties of ceramics at relatively low nano-filler addition.Herein,graphene nano-platelets reinforced Si₃N₄?GNP/Si₃N₄?composites were prepared by HP sintering?at 1650? for 2h?using fluorides?AlF3,MgF2?sintering-additives.The microstructural properties revealed the enhanced crystallization degree and density of the GNP/Si₃N₄ composites with different concentrations of graphene nano-platelets?GNP?.These properties facilitate to achieve a significant improved thermal conductivity?from 82.42 to 137.47 W/m.K?of GNP/Si₃N₄ composites.The morphology of composites shows uniform distribution of GNP,whereas overlapping of GNP?2 to 4 platelets?at the grain boundaries of Si₃N₄ were observed.The fracture toughness and Vickers hardness of the composites were also increased with the increasing content of GNP.The toughening mechanism was similar in all composites with GNP addition in respect of pull out,crack deflection,crack branching and crack bridging.3)The addition of carbon nanotubes?CNT?in the ceramic composites has stimulated a substantial interest due to their high mechanical,thermal and electrical properties.This approach uses fluoride additives?AIF3 and MgF2?to prepare multi-walled CNT reinforced Si₃N₄?MWCNT/Si₃N₄?composite densified at 1700? for 1h by HP sintering.The microstructural analysis of MWCNT/Si₃N₄ composites indicates that fluoride additives may substantially improve densification as well as the transformation of phase ?-Si₃N₄-?-Si₃N₄.As observed the mechanical properties i.e.flexural strength and fracture toughness of MWCNT/Si₃N₄ composites are improved with an increasing concentration of MWCNT.It is because of the highly-dense composites,strong interfacial interaction and the pull-out mechanism of MWCNT and ?-Si₃N₄.In the present work,the maximum values of fracture toughness flexural strength,Young's modulus,and hardness are reported as 12.76±1.15 MPa.m0.5,883±46 MPa,255±8 GPa,and 26.4±0.7 GPa,respectively.The improved mechanical properties also ascribed to the synergistic strengthening and toughening influence of MWCNT and ?-Si₃N₄.4)Carbon nano-structures?CNS?as a kind of reinforcement material can remarkably enhance the mechanical and thermal properties of ceramics.This research presents an analysis of the influence of CNS on thermal conductivity and mechanical properties of silicon carbide whisker reinforced Si₃N₄?SiCw/Si₃N₄?composite.The SiCw/Si₃N₄ composites containing various types of CNS e.g.carbon nano fiber?CNF?,MWCNT and GNP were fabricated by the hot-press sintering.The XRD analysis confirmed a complete transformation of ?-Si₃N₄ to ?-Si₃N₄ and the microstructural analysis shows a uniform distribution,as well as the pullout and bridging mechanism of CNS.The results revealed that the thermal conductivity and mechanical properties of SiCw/Si₃N₄ composites increased with the addition of CNS.The maximum values of fracture toughness?9.70±0.8 MPa.m1/2?and flexural strength?765±58 MPa?have been achieved for MWCNT containing SiCw/Si₃N₄ composite,whereas the maximum values of Young's modulus?250±3.8 GPa?and hardness?27.2±0.9 GPa?have been achieved for CNF containing SiCw/Si₃N₄ composite.Moreover,thermal conductivity also improved with the addition of CNS and reached to a maximum value of 110.6 W/m.K for CNF containing SiCw/Si₃N₄ composite.This work provides a useful approach for the fabrication of high-performance multifunctional composites for emerging engineering applications.