| Due to the outstanding mechanical properties, thermal properties and chemical stability, reaction bonded silicon carbide(RBSC) ceramics have been widely used in aircraft engine, optical mirror, gas turbines and other devices. With the increasingly wide application of RBSC, the demands for homogeneous structure, complex-shaped and low-cost ceramics components are growing gradually, and advanced ceramic forming processes are key techniques to solve the above problems. The gelcasting process is based on a synthesis of ideas stemmed from traditional ceramics and from polymer-chemistry, and the generic principle is that monomers react to form polymers and create a 3D network that freezes the suspended particles in the desired shape. The green bodies prepared with gelcasting process have the properties of homogeneous microstructure, high density and high strength.The commonly used acrylamide monomer is a neurotoxin and the polymerization is inhibited by oxygen, which limit its industrialization. In the present work, a new low-toxicity gelcasting system consisting of phenolic-formaldehyde resin(PF) and furfuryl alcohol(FA) is developed for the casting of reaction bonded silicon carbide ceramics. The chemical processes involved in the reactions between PF and FA have been studied. Polymerization of the premixed solutions is modified by changing FA and benzenesulfonyl chloride content in the suspension and the initial temperature. When the resin mixture consists of only PF and EG, the rate of polycondensation is very slow. When PF is partly substituted by FA, polycondensation of the resin mixtures tends to occur in advance and can be divided into two stages-namely gel reaction and curing reaction, respectively, according to the results of changes in solution temperature and weight. The results also show that, with an increase of furfuryl alcohol and benzenesulfonyl chloride content in the suspension and the initial temperature, the rates of gel and curing reactions both increase, and the gelation time of the gelcasting system significantly reduces.In the present work, porous carbons derived from pyrolysis of the gelcasting system are used as an alternative source of carbon to prepare RBSC. The formation mechanism of the porous carbons has been studied. Pore structure of the porous carbons is modified by changing ingredients of the gelcasting system and the carbonization temperature. For the initial homogeneous solution of phenolic resin and furfuryl alcohol in ethylene glycol, phase separation will occur in the course of polymerization, with only two phases being evident, a polymeric resin-rich phase and an ethylene glycol-rich phase. This is known as polymerization-induced phase separation. Porous carbons will be obtained after curing and subsequent pyrolysis, because the ethylene glycol-rich phase will remove to leave pores in the carbon matrix formed from polymeric resin-rich phase. The results show that continuous ethylene glycol-rich phase, which results in interconnected pores, can be formed during polymerization-induced phase separation, only when the weight of ethylene glycol is no less than that of polymeric resin. Higher FA and benzenesulfonyl chloride content in the resin mixtures result in bigger pore size, thicker carbon skeleton and higher apparent porosity. The morphological and pore structure change of the porous carbons is induced by phase separation dynamics and reaction kinetics change on curing of initial resin compositions, by varying ingredients in the resin-glycol mixtures. More FA and curing catalyst results in chemical reactions between PF and FA occurring at a higher speed and the polymerization degree of the cured bodies increasing. With the carbonization temperature elevated from 800 to 1400 oC, total pore volume and average pore size of the porous carbons both increase, while the BET surface area decreases, and pore structure change of the carbon monoliths is a result of graphitization and collapse of the micropore structure.Preparation of stable high solid-loading ceramic suspension, with a low viscosity, still is a prerequisite for successful application of gelcasting. Two commercial α-Si C powders of 3 μm and 45 μm was chosen as the raw materials, and the bimodal Si C powders were homogeneously dispersed in premixed solutions of phenolic resin/furfuryl alcohol/ethylene glycol by selecting PEG400 as the dispersant and ball milling. The effect of dispersant, blend ratio and solid loading on the viscosity, rheological properties and stability of the suspensions were systematically investigated. When 1.0 wt.% polyethylene glycol 400 was used as dispersant and the volume fraction of course Si C powder was 70 vol.%, a non-aqueous Si C slurry with a solid loading of 55-70 wt.% but low viscosity, which was meeting the demands of the following gelcasting process, was prepared.After degassed and gelcasting, complex-shaped Si C/porous carbon green bodies were prepared, and the microstructure and properties were systematically investigated. Porous polymer network between Si C particles derived from the resin mixtures was related to the strength of green body. With solid loading increasing from 55 wt.% to 70 wt.%, liner shrinkage of the samples during curing and carbonization decreased, thus deformation and cracking of the green bodies would be effectively restrained. At the same time, flexural strength of the cured bodies reduced, but still met the demands of machining. Porosity of the green bodies decreased with the increase of solid loading, while the bulk density increased.After reaction sintering, large-scale complex-shaped RBSC ceramics with dense and homogeneous structure were successfully fabricated. The reaction sintering process of Si C/porous carbon green bodies conformed to dissolution-precipitation mechanism, that is carbon dissolved in the molten silicon diffused to the initial α-Si C interface, where it precipitated as β-Si C particles. With an increase of solid loading, the volume fraction of Si C increased while that of Si decreased, and the bulk density, flexural strength and fracture toughness increased. When the solid loading increased to 70 wt.%, epitaxial growth of the newly-formed β-Si C bonded the initial α-Si C, thus the skeleton structure of the RBSC ceramics was formed, and volume fraction of free Si, bulk density, flexural strength and fracture toughness of the RBSC ceramics reached 17.2 %, 3.06 g/cm3、358±16 MPa and 4.22±0.13 MPam1/2, respectively. |
PDF Full Text Request