Cast molding is a gel forming method, and ceramic materials made by this method are characterized by homogeneous structure, designability, and high reliability,and have been widely used to prepare thin flat ceramic materials, especially in multilayer composites. In the previous experimental work, carbon-containing porous billets with uniform microstructure and narrow pore size distribution have been prepared by the flow casting method, and dense reaction bonded silicon carbide have been obtained by high-temperature liquid-phase silicon infiltration. The three-point flexural strength of reaction bonded silicon carbide was up to 410±14 MPa at a C/SiC ratio of 3:10 in the billets.However, the residual free Si content in the sintered ceramic obtained by this method is high, and the free Si content after reaction sintering is 37 vol% when the C/SiC ratio in the plain billet is 3:10. The presence of excess free Si not only limits the service temperature of reaction bonded silicon carbide, but also does not help to improve the mechanical properties of the ceramics. Therefore, it is necessary to find a method to effectively reduce the free Si content in the reactive sintered body and further improve the flexural strength of the sintered body.
It has been reported in the literature that the reduction of free silicon content in the sintered body of reaction bonded silicon carbide can be considered in two ways: first, by reducing the open porosity of the billet; second, by increasing the carbon content of the billet. However, no matter which method is chosen, it is important to consider that liquid phase silicon
infiltration is an exothermic and volumetric expansion process, and the increase in carbon content is accompanied by greater thermal stress and volumetric expansion. If the billet does not have a certain strength, cracks can easily form in the sintered body after liquid-phase silicon infiltration, which in turn reduces the strength of the ceramic body. Previous work has shown that when the C:SiC in the plain billet is increased to 5:10, crack-like defects appear in the sintered body and the flexural strength is reduced to 225±79 MPa.
In terms of strengthening porous materials, many researchers have taken the approach of impregnating resins, for example, Furuno et al. (Furuno, T., Imamura, Y., Kajita, Minute . , 2004. Wood Sci. Tec min nol. 37, 349-361.) impregnated phenolic resins into the cell walls and found that this method significantly improved the mechanical strength of wood.Phenolic resins are versatile and compatible with a wide range of organic or inorganic fillers, which are often used as binders and can provide some mechanical strength when cross-linked. In addition, phenolic resins decompose under vacuum or inert gas conditions at about 1000°C leaving about 55 wt% carbon, which is also advantageous of this kind silicon carbide ceramic.In previous work, the inventors of the present application applied the flow casting method to prepare carbon containing porous plain billets with uniform structure and obtained this kind silicon carbide ceramic with good mechanical properties. Based on this, the present invention proposes to further vacuum impregnate the porous billets with phenolic resin solution, which not only effectively improves the strength and carbon content of the billets, but also reduces the open porosity of the billets, ultimately reducing the free silicon content of the sintered body and enhancing the mechanical strength of the sintered body. At present, there are few reports on the process of vacuum impregnation of phenolic resin solution into carbon-containing porous billets followed by reactive silicon infiltration at home and abroad.
