Preparation And Properties Of Ceramic Composites With Silicon Carbide Matrix And Carbon Nanoparticles

Ceramic matrix composite (CMC) is one of advanced inorganic materials composed ofceramic substrate and reinforcing body such as ceramic fiber, whisker, chips or particles,which is prepared through developed composite technologies in recent years. In this thesis,carbon nanoparticles/silicon carbide (Cp/SiC) ceramic matrix composites (Cp/SiC-CMC) wereprepared as a promising material of glass fixtures in glass processing. The Cp/SiC-CMC canbe used as glass fixtures and mold materials due to the improved machinability, appropriatemechanical strength/hardness and unbonding with high-temperature glass melts.In Chapter1, recent development on the preparation and application of C/SiC-CMC wasreviewed. The existing preparation methods for carbon fiber reinforced silicon carbide(Cf/SiC) CMC were represented. Besides Cf/SiC-CMC, carbon nanoparticle (Cp) reinforcedSiC CMC can be used as a promising material instead of Cf/SiC-CMC due to the lowpreparation cost. In addition, a mechanochemical method for the preparation of ceramicand/or composite powders was also introduced. The mechanochemical method can be used toprepare some special ceramic materials that were commonly produced under high temperatureand/or high pressure conditions. The corresponding principle and theory of mechanochemicalmethod for the preparation of ceramic particles were described. In this chapter, the researchobjectives and contents of the thesis were given.In Chapter2, a carbon-silicon carbide (Cp/SiC) composite powder was prepared withnano-carbon (Cp) and silicon (Si) powders as starting materials via a mechanochemicaltreatment in Ar atmosphere in a dry stirred ball mill. The composite powder was characterizedby X-ray diffraction (XRD), thermogravimetry/differential scanning calorimetry (TG/DSC),scanning electron microscopy (SEM) and transmission electron microscopy/high resolutiontransmission microscopy (TEM/HRTEM). Effects of parameters (such as grinding time, ratioof ball to powder, grinding mode and atmosphere) on the synthesis of β-SiC particles, and the simultaneous preparation of Cp/SiC composite powder with the synthesized particles of β-SiCand the remaining Cpby the mechanochemical treatment were investigated. The results showthat β-SiC particles can be synthesized and the Cp/SiC composite powder in the presence ofexcessive nano-sized carbon can be simultaneously obtained via the mechanochemical route.In Chapter3, sphaeroid particles of Cp/SiC composite were fabricated with awell-dispersive aqueous slurry of Cp/SiC composite powder by a spray granulation technique.The parameter influences on the spray granulation process were investigated. In addition, theeffects of dispersant type, dispersant amount, stirring rate and carbon nanoparticle content onthe dispersibility of the aqueous slurry of Cp/SiC composite powder before the spraygranulation were also analyzed. The results show that the sphaeroid Cp/SiC compositeparticles can be effectively obtained with the well-dispersive aqueous slurry by the spraygranulation technique.In Chapter4, Cp/SiC-CMC with different contents of carbon nanoparticles (i.e.,5wt%,10wt%,15wt%and25wt%) was prepared by pressureless sintering. The densificationmechanism of Cp/SiC-CMC by pressureless sintering was discussed. The influence of carbonnanoparticles content on the mechanical properties of Cp/SiC-CMC was investigated. Theexperimental results reveal that the densification mechanism of Cp/SiC-CMC with carbonnanoparticles by pressureless sintering is consistent with that of pure SiC ceramic，i.e., theexpansion mechanism is dominant at the initial stage of sintering, the shrinkage mechanismbecomes dominant and offsetsagainst the expansion mechanism at the middle stage and theexpansion mechanism is slightly dominant before the end of sintering. It was also indicatedthat the bending strength and the hardness of Cp/SiC-CMC decrease andthe apparent porosityincreases with increasing carbon nanoparticle content in Cp/SiC-CMC. However, the fracturetoughness firstly increases and then decreases with increasing the nanoparticle content, givingan optimum value of2.58MPa·m1/2at the nanoparticle content of15wt%. When thenanoparticle content is in the range of0～15wt%, the ignition loss of Cp/SiC-CMC is5～6%. The ignition loss increases sharply atthe nanoparticle content of>15wt%, and becomes>18%at the content of25wt%.In Chapter5, the oxidative behavior and thermal shock resistance of Cp/SiC-CMC wereinvestigated. The results show that the oxidation of Cp/SiC-CMC in air atmosphere iscontrolled via the reaction of C-O2at400～700℃, exhibiting a homogeneous oxidation. Theapparent porosity of Cp/SiC-CMC increasesand the bending strength decreaseswith increasingthe oxidation temperature. The bending strength becomes minimum at700℃; At700～1000℃, the oxidation is controlled via the O2diffusion, and O2diffusion through themicrocracksleads to the formation of SiO2phase. The apparent porosity decreases and thebending strength increases with increasing the temperature. The bending strengthis maximumat1000℃; At1000～1100℃, the oxidation is controlled by the O2diffusion through thedefects of SiC. The apparent porosity increases and the bending strength decreases withincreasing the temperature. Also, since there exist massive carbon nanoparticles in the SiCgrain boundaries when the carbon nanoparticle content increases, readily causing the increaseof the crack probability inthe repeated psychro-thermal cycles,the strength andthe thermalshock resistance of the composite material thus reduce.It was indicated that the oxidationtemperature and carbon nanoparticle content both have effects on the strength, oxidationbehavior and thermal shock resistance of Cp/SiC-CMC.In Chapter6, the influence of carbon nanoparticle content on the machinability ofCp/SiC-CMC was investigated. The results show that the machinability of Cp/SiC-CMC canbe improved due to the presence of carbon nanoparticles evenly distributed through the SiCgrain boundary in Cp/SiC-CMC. However, the net-like hole structure in Cp/SiC-CMC canappear at carbon nanoparticle content of>15wt%, resulting in the decrease of the density ofCp/SiC-CMC. According to the evaluation of the machinability by machining speed V,machinability index M，brittleness index B and malleability parameter n, the maximummachinability index (Mmax) is0.921and the minimum brittleness index (Bmin) is1.09, and themaximum malleability parameter (nmax) is+0.342when the content of carbon nanoparticles is15wt%, showing a proper machinability of Cp/SiC-CMC with carbon nanoparticle content of 15wt%.In Chapter7, the preparation process and the practical application of Cp/SiC-CMC usedas glass fixtures were investigated.In addition, the wettability of Cp/SiC-CMC was also examined. The results show thatCp/SiC-CMC does not bond with molten glass at <1000℃when carbon nanoparticle content is≥15wt%, exhibiting a poor wettability. In this case, carbon nanoparticles are oxidized prior toSiC substrate, inhibiting the formation of SiO2film on the surface of substrate and thuspreventing the wetting effect between the substrate and molten glass, which can favor theapplication of Cp/SiC-CMC for glass fixtures.Finally, the conclusions obtained and some prospects for future research work weregiven.