Silicon carbide (SiC) comes in several different forms or polytypes; alpha SiC is most popular and features hexagonal crystal structure similar to that of wurtzite.
Cubic b-SiC, on the other hand, boasts excellent electrical properties and is suitable for machining processes. Furthermore, it boasts superior qualities when applied for high precision grinding operations.
Hardness
Silicon carbide is one of the three hardest substances on Earth, trailing only diamond and boron carbide for hardness. This makes it extremely resistant to compression as well as erosion from abrasion and high temperatures; making it suitable for abrasive blasting and grinding applications.
SiC is known for its superior mechanical and electrical conductivity. Beta silicon carbide features a cubic microstructure that allows electrons to pass quickly through it, making it suitable for certain electrical applications. Furthermore, this material can be formed into different shapes with less energy use compared to alpha silicon carbide for manufacturing applications.
Hardness of SiC ceramics depends on many variables, including density and grain size of their matrix. Agglomeration of reinforcing particles also can significantly change its mechanical properties; for this study we investigated what effect adding graphene together with nano b-SiC had on Young’s modulus, hardness, fracture toughness (three point and biaxial), Young modulus modulus of sintered alpha sic ceramics.
Results revealed that both Young’s modulus and hardness of the sample increased with increasing nano b-SiC content up to 5 weight percent; conversely, graphene content caused its hardness to decrease, possibly attributable to crack bridging mechanisms during indentation by Vickers diamond. This increase was attributed to crack bridging mechanisms during Vickers diamond indentation.
Electrical Conductivity
Although alpha sic’s density is well-documented, its electrical conductivity remains obscure to many people. Porous silicon carbide offers many unique electrical properties; its resistivity can even be tailored specifically for specific applications.
Silicon carbide occurs in various polymorphs or crystal structures. Two of the more popular ones are alpha and beta silicon carbides; with alpha usually formed at temperatures exceeding 1,700 degC with hexagonal crystals similar to Wurtzite while beta is typically formed at lower temperatures with zinc blende crystal structures similar to diamond.
b-SiC’s less dense structure makes it less dense than alpha silicon carbide and its unique properties make it unsuitable for certain applications. It features low fracture toughness and high brittleness; however, its superior hardness and resistance to chemical attack make it a popular choice for wafer tray supports and paddles in semiconductor furnaces.
Porous silicon carbide ceramics’ electrical resistivity changes with changing porosity, but its resistance can also be altered by secondary phases within it, such as graphene or carbon nanotubes, that change its electrical resistivity. Another way of modulating its electrical resistance is incorporating different nitrides into its system.
Density
Silicon Carbide (a-SiC) features a hexagonal crystal structure similar to Wurtzite. However, beta modification (b-SiC), with its zinc blende crystal structure has few commercial applications compared to its alpha cousin; nevertheless its zinc blende structure provides increased surface area and less porosity, making it suitable as support for heterogeneous catalysts. Ram-compressed a-SiC bodies and hot isostatically pressed (HIPed) b-SiC feature near-theoretical densities with ultrafine grain sizes for superior strength as well as very good oxidation resistance as high temperature creep resistance while both provide excellent corrosion resistance properties HIPed b-SiC offers excellent resistance as support for heterogeneous catalysts.
Hexoloy SA SiC boasts a theoretical density of 98%. The fine and engineered particle sizes present low porosity levels and provide exceptional protection from rotational and sliding forces that may cause erosion.
Sintering
Sintering is a physical process in which alpha sic powder particles move to areas with lower chemical potential, leading them to pack closer together and create denser aggregates. Atoms may move by diffusion or material transport mechanisms based on plastic deformation and dislocation motion, causing more density among their constituents.
Sintering is typically performed at high temperatures and the schedule may differ depending on the volume and composition of a batch of powder mixture being sinterd. Sintering temperatures must be high enough to promote densification of ceramic bodies without creating excessive grain growth that reduces strength and durability.
Sintering alpha sic can be done using various techniques, including electric current sintering and resistance sintering. Both processes use electric current to drive sintering; they differ only in their application methods: resistance sintering does not use pressure while electric current sintering utilizes both electrical and thermal stresses to achieve this process.
Liquid phase sintering is becoming an increasingly popular method for sintering alpha sic. In this process, liquid additives are added to sintering powder and drawn into pores in the ceramic by capillary forces; once in contact with these pores, these liquid additives melt and diffuse between grains, rearranging their packing patterns into more dense packing arrangements that reduce porosity while simultaneously increasing strength.
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