With the development of science and technology, especially the development of energy and spacetechnology, materials are required with characteristics of high temperature resistance, corrosion resistance and abrasive resistance in order to be used in more demanding work environment. Because Pressureless SSiC ceramic material has oxidation resistance, good abrasive resistance,high temperature resistance, excellent thermal stability, high temperature strength, small coefficient of thermal expansion high thermal conductivity thermal shock resistance and chemical corrosion resistance, it has become an important part of advanced science.
SiC ceramics are the new materials that have been developed in the last 20 years, in particular,pressureless SSiC ceramics has become as an advanced ceramics in last 8 or 9 years. As result of high strength and hardness, corrosion resistance and high temperature resistance of SiC ceramics,it develops quickly and is applied in the petrochemical, metallurgical machinery, aerospace,microelectronics, automotive, steel, power generation fields. It increasingly shows the advantages with which other special ceramic cannot be incomparable.
Main characteristics and application
Structure of SiC
Silicon carbide is an artificial material. After the artificial synthesis of silicon carbide, it was confirmed that there occasionally exists silicon carbide in the meteorite and the crust. The molecular formula of silicon carbide is SiC, molecular weight of 40.07, with 70.045% of silicon and 29.955% of carbide. SiC`s density is 3.16-3.21g/m3 in theory.
SiC is a covalent compound based on covalent bond. When carbon and silicon element are formedin SiC crystal, SiC crystal`s basic unit is tetrahedron. All SiC are formed by SiC tetrahedron, but the only difference is the parallel combination and anti-parallel combination, which lead to the formation of SiC with diamond. SiC has 75 variants, such as 3C-SiC.4H-SiC and 15R-SiC.Especially, α-SiC and β-SiC are two common variants. The crystal structure of -SiC is cubic crystal,Si and C are composed of a face centered cubic lattice; α-SiC has more than 100 kinds of polymorph, such as 4H, 15R and 6H, where 6H is the most common one in industrial application.There is a certain thermal stability relationship between SiC polymorphs. When the temperature is below 1600℃, SiC will be in the form of β-SiC. However, when the temperature is above 1600℃, β-SiC will transform slowly into various forms of α-SiC. 4H-SiC is easy to generate at 2000℃, 15R and 6H polymorph can be easy to be produced at above 2100℃. For 6H-SiC, even though the temperature is above 2200℃, it is still stable. The free energy difference between various forms of SiC polymorph is very small, therefore, the solid solution of microimpurity will result in changes in thermal stability relationship between polymorphs.
Chemical properties of SiC
The chemical stability of SiC is closely related to its oxidation characteristic. SiC itself is so easy to be oxidized, but after being oxidized, it will form a thin SiO2 film which will stop the oxidation. In the air, SiC starts to be oxidized at 800℃ in a very slow rate. As the temperature increases, the rate of oxidation is rapidly increasing. Its oxidation rate is 1.6 times faster than in the air, and the rate of oxidation will slow down over time.
Vapor can react strongly with SiC under high temperature. When the temperature is above1100℃, silicon, carbon, or SiO2 will be generated depending on the circumstances. When the
temperature is around 1000℃, SiC and sulfur compounds, like H2S, can generate brown vulcanized silicon.
SiC ceramic has a unique structure that is closely related to its specific structure. SiC is a compound with strong covalent bond, where the iconicity of Si-C covalent bond in SiC is only about 12%. Thus, SiC has high strength, high elastic modulus and excellent abrasion resistance.Pure SiC will be eroded by alkali solution such as NaOH, and acid solution such as HCL, HNO3,H2SO4 and HF.
Application
SiC is used in mechanical seal with high PV value, where PV value is the product of the sealing fluid pressure V and the average sliding velocity of the sealing end P, and presents the limit value of the use of mechanical seal.
1.For high fluid pressure
SiC is a material with high strength and rigidity. Since it has high modulus of elasticity, it will not be easy to become deformed under high pressure. Meanwhile, SiC has high compressionstrength. SiC has strong adhesiveness resistance and wear ability, therefore, it`s very useful as a sealing material in high pressure.
2.For mechanical seal under high temperature
SiC has very high hot strength. At 1400℃, the flexural strength is the same as that in normaltemperature.
3.For high speed
Under high velocity, the major problem for mechanical seal is heat and vibration from frictionpairs, as result of high linear velocity. Pressureless SSiC could totally meet the requirements of mechanical seal with high velocity.
4.For mechanical seal under corrosive condition
Pressureless SSiC has single structure, no free silicon and free carbon. It also has strongcorrosion resistance, excellent chemical stability and can be used for all kinds of strongly corrosive acid and alkali medium.
5. For mechanical seal containing solid particles
Many of the sealing fluids in industrial production contain solid particles which are easilydeposited on the surface of the seal. These particles not only cause serious erosion of the sealing surface from abrasion by particles, but also cause local heat on the sealing cover. SiC has high hardness, abrasion resistance and thermal shock stability. It also has outstanding emergency operation. Thus, adopting SiC self-contrast dense sealing pairs exists great potential for application in granular media.
6.We are working together with nuclear safety researcher from Hefei on the subject of delivery of liquid metal pipe within modern nuclear fusion applications.
7.We put forward that in order to increase the toughness of the SiC ceramic which is used forpump s, haft in production machinery and chemical industry, we add appropriate amount of carbon fiber in the original SiC. We have applied for a national invention patents which has already been accepted, and wait for the publicity.
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