Reaction Bonded Silicon Carbide
Reaction bonded silicon carbide is a material with excellent properties such as high strength, good abrasion resistance, high rigidity and low thermal expansion. It can be used as structural components for heat-resistant applications.The process involves a chemical reaction between porous carbon or graphite and molten silicon. This produces RBSiC with a proportion of carbon content of 8% to 30% by weight, with boron carbide having an additional proportion of 5% to 90% by weight.
Process
The present invention relates to a method of making Reaction bonded silicon carbide, which is especially suitable for use as an infiltration material in a shaped article. It is also characterized by the fact that it enables a geometrical freedom in the manufacture of the shaped articles.To make the shaped articles, a preliminary body of silica carbide and carbon is impregnated several times with an aqueous carbon black suspension with surface-active agent. After a drying time, the preform is subjected to a gas-phase separation in which the molten silica and carbon are separated at an exothermic temperature.After the separation, the preform is immersed in a crucible in which the fused silicon has been filled. The fused silicon is supplied from the crucible to the entire preform through capillary tubes. It is therefore a simple process to produce the shaped articles and can be carried out economically.As a result, the area of the shaped articles which is infiltrated by the silicon can be kept as maximum. This is important in order to prevent cohering of the silicon due to the surface tension when the shaped articles are inserted into the application.The method of making reaction bonded silicon carbide is thus characterized by preventing the cohering of the fused silicon in the shaped article by melting the silicon evenly and supplying it to the entire preform at an even spatial distribution. This enables the maximum infiltration area to be maintained, and reduces processing time as a result.Compared to the conventional technique, the manufacturing process of the Reaction bonded silicon carbide in the present invention is simpler and requires fewer equipment. As mentioned above, the heating device for melting silicon, a crucible in which the fused silicone is contained and the supplying device from the furnace to the entire preform are not required.This can significantly reduce the cost. Moreover, a shaped article can be produced with an improved geometry by allowing the infiltrated silicon to be positioned in the pore space of the preliminary body. This thereby ensures the stabilising effect of the infiltrated silicon and, as a result, the shaped article has high strength.
Materials
Silicon carbide is one of the hardest synthetic materials and retains hardness and strength at elevated temperatures. It is also resistant to a variety of acids and alkalis and has high thermal conductivity, making it an excellent choice for a wide range of refractory applications. It is used for many types of machinery and equipment, including abrasive wheels, ceramic discs and grinding papers.The most common method for manufacturing silicon carbide is the Acheson process, in which a mixture of pure silica sand and carbon in the form of finely ground coke is built up around a carbon conductor within a brick electrical resistance-type furnace. The process produces a dense, solid material with a high degree of purity.Another technique is called reaction bonding, which typifies liquid-solid reactions for the synthesis of refractory ceramic composites. It provides advantages over conventional sintering and hot-pressing techniques in processing times, near-net shape fabrication capabilities and mechanical properties.It is possible to obtain a high proportion of boron carbide by adding boron to the melt in order to reduce the sintering temperature and improve the bending strength. It is preferable to add boron to the melt at a concentration of about 5% to 90% by weight.This makes it possible to produce a silicon carbide ceramic member that has a higher density than that of a member which does not contain boron carbide. A boron carbide RBSiC member can be obtained which has excellent mechanical properties and which is free from cracks and pores.Furthermore, it is possible to suppress the occurrence of problems such as cracking in reaction sintering by modifying the amount of silicon that is infiltrated into the green body. The amount of silicon that is infiltrated can be modified by configuring the porosity of the green body in a range of 30 vol % to 68 vol %. This configuration cancels out the influence of expansion in silicon, thereby suppressing the occurrence of problems such as cracking during reaction sintering.The amount of silicon that is infiltrated is preferably more preferably 1.2 times to 1.5 times the calculated silicon amount. This increases the proportion of boron carbide and thus makes it possible to achieve an silicon carbide ceramic member that has a high density and which is free from cracks and pores.
Properties
Reaction bonded silicon carbide is a durable material that is well suited for use as a high power laser mirror substrate. this kind of silicon carbide ceramic is also used for a wide variety of applications in the electronics industry.this kind of silicon carbide ceramic has a low mass density, a high Young’s Modulus, good thermal conductivity, and a very low coefficient of thermal expansion. Reaction bonded silicon carbide is also a relatively lightweight material, which makes it very easy to fabricate.It is a hard and tough crystalline compound of silicon and carbon, and it retains its hardness and strength at elevated temperatures. Its high temperature properties make it useful in many applications, including refractory linings and heating elements for industrial furnaces.In addition, SiC has a number of other properties that make it an interesting material for applications in the electronics industry. It has a wide bandgap and high electrical conductivity, which makes it a potential semiconductor for electronic devices.However, it has not been widely utilized as a semiconductor because of its poor chemical stability and its lack of reproducible crystal structures. Consequently, there is still much work to be done in order to improve its electronic properties.To address this issue, a new process for preparing ceramics with a high SiC content by two-step reaction bonding was developed. In this technique, a carbon source composed of fine amorphous carbon black and coarse micro-spherical carbon was infiltrated with molten silicon first. This carbon source was then consumed in a reaction with residual silicon at high temperature, resulting in a preform consisting of SiC particles.The results showed that a high level of mechanical properties could be achieved with this method, as demonstrated by bending strengths of 58.6 and 317.0 MPa at 1530 and 1570 degC, respectively. In addition, the microstructure of the samples was examined by SEM.Reaction bonded silicon carbide can be manufactured economically by minimizing the required equipment and by using an innovative processing method. This procedure can eliminate the need for a melting pot to contain the fused silicon, which will save energy and reduce manufacturing costs.
Applications
Silicon carbide is a versatile material used in a wide variety of applications due to its high strength, wear resistance and chemical stability.this kind of silicon carbide ceramic is especially suited to high-temperature applications in gas turbines, heat exchangers and combustion nozzles. It is also used in nuclear reactors because of its low neutron cross section and radiation resistance.Reaction bonded silicon carbide is a high-temperature material with excellent wear, impact, and chemical resistance. It is used for components such as pipe liners and flow control chokes that are exposed to extreme conditions in mining, energy, and other industries.The material is available in various forms, including powder and pellets, and is commonly used for forming engineered pieces in many different types of applications. For example, it is used as a coating on pipes to help control the abrasive flow.It can also be formed into a variety of shapes, such as cone and sleeve, or engineered pieces to help with the processing of raw materials in a number of industries. Its strong covalent bonding allows it to resist corrosion and oxidation in both acid and alkaline environments.SiC is also a good choice for chemically resistant seals and bearings because it is extremely hard, with a hardness that can be almost twice as hard as diamond. It is also highly abrasion resistant and has very low thermal expansion.Reaction bonded silicon carbide has been developed to reduce costs for fabricating many ceramics, intermetallics, and composites by using reactive infiltration of cast microporous carbon preforms with an infiltrating melt. This process is less expensive than other sintering methods, and the resulting aluminized or sintered products have improved mechanical properties.To produce a reaction-bonded body, a green body formed from a coherent mixture of finely divided silicon and carbon powders is coated with a paste comprising the powders suspended in a viscous medium that can be heated and decomposed to leave an open cellular structure of silicon and carbon extending over the surface of the body. The body is then heated in the presence of molten silicon to convert the carbon layer to a silicon carbide skeleton with open pores through which the molten silicon is drawn.