Preparation And Characterization Of Reaction Bonded Silicon Carbide Reinforced By Random Chopped Fiber

The demands for large scale, light weight optical systems are growing graduallyin the field of space and aviation with the development of modern optics. After theemployments of Be, glass, silicon and crystallite glass, reaction bonded siliconcarbide （RBSC） is becoming mirror materials with excellent comprehensiveperformances. However, the intrinsic brittleness of RBSC limits its machine work,and thus reduces the qualified rate. Composite is a significant approach to improveproperties of monolithic ceramics. To solve the above problems of RBSC, choppedfiber was introduced into SiC/C system to obtain homogeneous slurry. Randomchopped fiber reinforced RBSC composites were prepared by slip casting andreaction sintering. In this study, the influences of starting materials, forming andsintering techniques on the properties of RBSC composites were investigated.Taking advantage of random chopped fiber, the weaknesses of high brittleness andlow fracture toughness of RBSC are relieved; at the same time, the merits of highstiffness, low coefficient of thermal expansion and high thermal conductivity of SiCceramics are retained. This provides a necessary condition for the preparation oflarge scale, ultra-light weight and high precision optical mirror.To protect carbon fiber from destructive oxidation, a Zirconia coating wasdeposited on the chopped carbon fiber surface by sol-gel method. The influences ofmaterial constitutes and process route on the microstructure and properties ofZirconia coating were investigated. When the chopped fiber was dipped for sixcycles, a uniform coating was prepared on the axial and fracture surface. Elementalconstitute of the coating was analyzed by EDS method, which confirmed theexistence of Zirconia coating on the fiber surface. Oxidized performance wasdetermined by TG-DTA method. With respect to the carbon fiber with3mol.%YSZcoating, the destructive oxidation started at650°C and the mass loss arrived only20%until800°C.Random chopped fiber reinforced RBSC composites were prepared withbimodal SiC and carbon as matrix, and chopped carbon fiber as reinforcement. Thisroute solved the aggregation of chopped carbon fiber, giving rise to a homogeneousdispersion of chopped fiber in the SiC/C slurry. The volume fraction of choppedfiber is a decisive factor in the microstructure, bulk density and mechanicalproperties of the RBSC composites. The bulk density of the composites increasedwith increasing of carbon fiber which exists in the form of carbon; whereas, whenthe volume fraction exceeded30vol.%, the bridge effect of chopped fiber led to a decrease of bulk density. The introduction of chopped fiber improved both flexuralstrength and fracture toughness of the composites, reaching the peak values of416MPa and5.1MPam^1/2, at the chopped fiber fraction of30vol.%. Severaltoughening mechanisms, such as fiber pullout, fiber debonding, crack deflection,intergranular fracture and residual stress, were found in the composites. An obviousmorphology change was observed for the chopped fiber in the reactionsintering-acid corrosion test. Based on the morphology change, a bilayer model wasproposed to characterize the structure of the siliconized fiber. The formingmechanism of the bilayer model was analyzed.The chopped carbon fiber reacted with liquid silicon during the sintering, andthe strengthening and toughening performances were damaged. Due to the excellentperformance at high temperature, silicon carbide whisker was chosen asreinforcement. Silicon carbide whisker reinforced RBSC composites were preparedby slip casting and reaction sintering with silicon carbide whisker as reinforcement.After sintering, the whisker maintained the burl profile on the surface and thestarting diameter, indicating excellent high temperature stability. Due to the bridgeeffect of whisker, the content and scale of residual silicon were increased in thewhisker reinforced RBSC composites, leading to a lower bulk density comparedwith the chopped fiber reinforced RBSC composites. The silicon carbide whiskerwas characterized with high elastic modulus and stiffness, so the fracture toughnessof the composites reached4.2MPam^1/2at the whisker fraction of20wt.%. Whiskerpullout was observed on the fracture surface, implying an appropriate bondingstrength between the whisker and the matrix. As a result, whisker pullout wasconsidered as the main toughening mechanism. The compressive residual stress,which can restrain the origin and propagation of micro-cracks, was detected on thepolished surface of the material. The diameter of whisker is approximate1.5μm,thus confining the reinforcing effect.A protective film was prepared on the material surface by mild oxidation inambient air. The formation mechanism, morphology and protective behavior of SiO2film were investigated. Because of the mild oxidation, the defects on the materialsurface, such as holes, cracks, pores, was healed by the SiO2film. At the mildtemperature tips of the residual silicon melted, resulting in a reduction of thediameter. The siliconized fiber underwent an obvious expansion, indicating a highreactivity for the oxidation reaction. At900°C the oxidation product was amorphousSiO2. The amorphous SiO2crystallized to cristobalite at1100°C, whose content wasdetermined by the fraction of the reinforcement. For the chopped fiber reinforcedRBSC composites, the flexural strength and fracture toughness were increased by34% and49%by mild oxidation at1100°C. For the whisker reinforced RBSC composites,the fracture toughness reached5.1MPam^1/2at whisker fraction of15wt.%by mildoxidation at1100°C.The reaction sintering of RBSC is on the basis of carbon-silicon reaction, so theeffect of carbon black in the random chopped fiber reinforced RBSC wasinvestigated. The siliconization of carbon black is accompanied by a volumeexpansion of approximate100%; as a result, the bulk density of the RBSCcomposites was increased with increasing of the carbon fraction. The CET（coefficient of thermal expansion） of SiC is higher than that of Si, so the CET of thecomposites was enhanced with the fraction increase of SiC. When the chopped fiberfraction was lower than20vol.%, the flexural strength was improved with increasingof fiber fraction; if further increasing the fiber fraction, the strength was reduced forthe nonuniform structure resulted from the carbon black aggregation. With respect tofracture toughness, the value increased steadily with increasing of carbon black. Theaggregation of carbon black was turned into porous SiC-Si structure during reactionsintering. By mild oxidation, a dense SiO2film formed on the surface of the spherestructure.