Reaction Bonded Silicon Carbide

Reaction bonded silicon carbide  is a high-strength material that is used in gas turbine components. It also has excellent wear, impact and chemical resistance.Reaction bonded silicon carbide is a type of infiltrated ceramics that is composed of 85 to 94 {3acecd06353d99efc7e310a3f1da5a7d22fc0f88af6041abe641b496d156e631} SiC and 15 to 6 {3acecd06353d99efc7e310a3f1da5a7d22fc0f88af6041abe641b496d156e631} metallic silicon. It has practically no residual porosity.

Characteristics

Reaction bonded silicon carbide is a silicon metal infiltrated ceramic that has been shown to exhibit unique mechanical, thermal and electrical properties. These characteristics make it an attractive material for many applications.Typically, bulk SiC ceramics consist of polycrystalline materials that may contain a variety of material phases that are obtained through sintering, hot pressing or reaction bonding. These additional phases often include carbon, silicon metal, or silicon nitride. The chemical composition of the starting powders is usually controlled and the resulting ceramics can be made to meet specific specifications for mechanical, thermal or electrical properties.In addition, boron nitride can be added to the sintering melt to produce a B-Si alloy and this can improve the mechanical properties of reaction bonded silicon carbide. However, this can cause contamination of the product surface and can interfere with processing.One way to avoid this is to use fused silicon in the sintering process. This is a high temperature method wherein the molten silicon is infiltrated into a preform made from silicon carbide and carbon. The resulting preform is then cooled in a vacuum sintering furnace at 1500 deg C and reacted with the free silicon present within it to form b-SiC along with the a-SiC particles.The resulting microstructure of reaction bonded silicon carbide consists of elongated, fiber-like b-Si3N4 grains, glassy MgO, or crystalline Y2O3 depending on the composition of the sintering melt. Moreover, the intergranular phase may be amorphous Si or crystalline Al4C3 depending on the chemical composition of the sintering melt and the final temperature at which it is fired.X-ray diffraction, scanning electron microscopy, and transmission electron microscopy have all been used to characterize the structural behavior of RBSC and RBSD. The a-SiC particles form a core-rim structure and the b-SiC grains form an epitaxial layer of SiC over the original a-SiC particles. This epitaxial growth of a-SiC can be followed by amorphous or crystalline Al4C3 overgrowth and is believed to be the primary reason for the faceted grain boundaries that are observed in some specimens.In addition to being a strong and hard material, RBSC is also known for its excellent wear resistance. It is used for bearings, seals and other components. It is also resistant to corrosion and can be used in hydrofluoric acid and strong alkali conditions. It is also a preferred choice for high temperature gas turbine components due to its extreme hardness.

Applications

Reaction bonded silicon carbide (SiC) is an excellent material for high temperature and extreme condition applications due to its high thermal shock resistance, chemical stability, and low coefficient of friction. This material is often used in mechanical seals and bearings, gas turbines, and combustion nozzles.The manufacturing process of reaction bonded silicon carbide is a chemical reaction between porous carbon and molten silicon. After being contacted to a compact body of silicon carbide (preform), a silicon supplying body is heated in the heat processing within a temperature higher than the melting point of the silicon, for example 1410 – 1550degC, fused silicon infiltrates into the silicon carbide preform to manufacture the reaction bonded silicon carbide.In the present invention, a ratio between the silicon powder of the silicon supplying body and the silicon carbide powder of the reaction-bonded preform is increased as possible to fasten the infiltration speed of the fused silicon, thereby to reduce the reaction time greatly. Moreover, the difference between the particles of the silicon powder and the silicon carbide powder is increased as well to prevent the cohesion of the fused silicon by the surface tension.Further, a silicon carbide framework structure is formed by reacting the silicon powder and the bonding agent in the silicon supplying body due to the heat processing. This framework structure has low strength and can be separated easily from the reaction-bonded silicon carbide, and the corresponding material is obtained by simple processing.On cooling down of a siliconised body excess silicon is exuded and typically forms non-uniform nodules which can be removed by leaching with alkali. Nevertheless, when the coating of the invention is used however, the excess silicon combines with the open porous silicon carbide skeleton formed on the body and thus uniform leaching of the exuded silicon is facilitated.Another advantage of the method is that it can be used to produce dense bodies of silicon carbide by adding a plasticizing agent. The plasticizing agent may include a mixture of di-butyl phthalate, benzyl butyl phthalate, polyethylene glycol, di-methyl phthalate, di-octyl phthalate and glycerol. This plasticizing agent is preferably deposited on the surface of the silicon carbide/carbon preform to form the granules.

Advantages

Reaction bonded silicon carbide is a highly wear resistant material that can be formed into complex shapes. The unique combination of high strength and corrosion resistance makes it a preferred choice for a variety of applications. It is particularly used where the highest wear resistance is required or when shape is too intricate to be produced in other mixtures.The advantages of this material include excellent chemical and refractory properties; low dimensional change; exceptional corrosion resistance, high oxidation resistance and thermal shock resistance; and exceptional resistance to abrasion and impact. It is suitable for use in a wide range of applications such as gas turbines, boilers, mechanical seals and combustion nozzles.This product is available in a range of densities and pore sizes for optimum performance in various applications. It is particularly well suited for high temperature, extreme condition applications, such as compressor blades and heat exchangers.It can also be fabricated into a wide variety of special-shaped structural parts. It is a great alternative to dense silica based ceramics for use in pumps, heat exchangers, burners and other demanding applications that require a strong seal face.In addition, it has excellent abrasion and impact resistance and a low coefficient of friction against mating materials. The abrasion resistance makes it particularly suitable for use in the chemical and refining industries as well as the mining industry.When a coated body of carbon and silicon carbide is heated in the presence of molten silicon to siliconise the body, any surplus silicon on the surface of the body melts and reacts with the carbon skeleton to form an open porous silicon carbide skeleton. The molten silicon is able to climb very rapidly through the carbon skeleton, moving laterally into the body and reacting with the remaining carbon.Reaction bonded silicon carbide can be produced at lower cost than other products in a similar range of hardness and strength. It is also a relatively simple production process, with less equipment and reduced processing time.In addition, the production process of reaction bonded silicon carbide can be economically improved by using a boron carbide infiltration preform instead of free carbon to reduce the proportion of residual silicon. The boron carbide infiltrates into the carbon preform in a similar manner as carbon added free carbon and generates B-C-Si particles, which can decrease residual silicon up to 15 times. Moreover, the addition of boron carbide improves the ductility and fracture resistance of the final product.

Limitations

Reaction bonded silicon carbide (RBSC) is a material which has been used extensively to manufacture heat-resistant components. However, a limitation of RBSC is that it contains free silicon. This free silicon is detrimental to the mechanical properties of RBSC ceramics. There are a number of methods to reduce the free silicon content of RBSC. The most commonly used method is to increase the diamond fraction in RBSC. This is a useful approach since it increases the k value, which can be important for high temperature applications.The fabrication of a reaction bonded silicon carbide preform has also been improved using this technique. This involves supplying a silicon containing body of a predetermined shape including silicon powder and phenol resin, furfuryl alcohol resin or epoxy resin as a bonding agent.This is then contacted to one surface of the preform in a reaction bonding furnace. The molten silicon is infiltrated to the underlying preform, where it reacts with carbon particles existing in the underlying preform to form reaction-bonded silicon carbide. The reaction-bonded silicon carbide is then heated to a temperature higher than melting temperature of the silicon under vacuum or inert atmosphere to infiltrate fused silicon.Although this process is successful, it can be difficult to supply a sufficient amount of fused silicon evenly over the entire test piece. This is a problem for all kinds of Si materials, but it becomes particularly difficult when manufacturing reaction bonded silicon carbide.Despite the limitations, a lot of work has been done to improve the processing conditions for reaction bonded silicon carbide. This includes removing contaminants and improving the purity of the reaction bonding process.In order to achieve this, an effort was made to prevent the formation of the generally observed layer of large porosity adjacent to the as-nitride surfaces of RBSN during processing. To accomplish this, isostatically pressed bars of various densities were prepared from wet vibratory milled Si powder, then sintering and nitriding was performed under three different conditions: direct exposure to the furnance atmosphere; packing in either Si or Si3N4 powder; or packing in either Si or Si3N4 and then sintering and nitriding.