Silicified graphite is a kind of composite material which is coated with silicon carbide layer on the surface of graphite material. The hardness of silicified graphite is actually the hardness of SiC, which is only inferior to that of diamond, boron nitride and boron carbide, which is higher than that of tungsten carbide and three aluminium oxide two. The production methods of silicified graphite are 3 methods, such as chemical vapor deposition (CVD), chemical vapor reaction (CVR) and liquid silicon infiltration reaction.
Chemical vapor deposition (CVI) method
The gas containing silicon and carbon is decomposed by high temperature graphite matrix, and SiC is deposited on the surface of graphite matrix. The raw material is three methyl silane (CH3SiC 3), four silicon chloride, hydrogen, silicon vapor and so on. The deposition temperature range is wide, from 1175 to 1775. The SiC layer produced by this method is very dense, uniform in thickness and approximately 0.1 to 0.3mm in thickness. But the combination of SiC and graphite as pure mechanical bonding and weak binding force in the blast temperature when SiC layer is prone to cracking and spalling.
Chemical vapor reaction (CVR)
The raw material is coke powder and excess quartz sand or amorphous silica powder. When heated to 2000 DEG C, a chemical reaction is generated to produce SiO vapor. SiO vapor reacts with the carbon matrix to form SiC. The SiC layer and the carbon substrate two has no obvious interface, are firmly combined, can not drop in temperature change and high load, but CVR method is SiO gas penetrated into the carbon based body reaction, therefore, still retain a porous carbon matrix, are used as sealing material, filling with resin impregnated or CVD method the pore.
Liquid silicon infiltration method
This method is also a kind of CVR. Under the vacuum condition, the carbon matrix is directly immersed in molten silicon liquid when heated to 800 DEG C. The liquid silicon gradually enters the carbon matrix and reacts to form SiC. Raw material is 99.9999% pure silicon. The thickness of SiC layer is up to 3.5mm. After reaction, the carbon matrix contains about 17% of the free silicon, filling the pores of the matrix, so that the matrix becomes dense and impermeable. However, the presence of free silicon reduces the corrosion resistance and high temperature oxidation resistance of silicified graphite.