Silicon carbide mechanical/chemical properties

Silicon nitride is prized for its hardness (9 out of 10 on the Mohr scale), its wear resistance, and its mechanical strength at elevated temperatures. It melts and dissociates into the elements at 1,900 °C, and has a maximum use temperature near 1,800 °C in the absence of oxygen and near 1,500 °C under oxidizing conditions.41 It also has a relatively low density (3.185 g/cm3). Unlike silicon carbide, silicon nitride is an electrical insulator. The bulk material has a relatively good stability to aggressive chemicals. This combination of properties underlies its uses in internal combustion engines and jet engines. 

Silicon carbide has attracted considerable interest because of its good mechanical and physical properties and chemical inertness. One of the most important applications of SiC is to produce a matrix reinforced by fibres, forming ceramic-matrix composites. These composite materials exhibit much better fracture toughness than monolithic ceramics. Compared with carbon/carbon composites, fibre-reinforced SiC matrix composites possess superior oxidation resistance and mechanical properties. The Si-C-H-Cl system (e.g. methyltrichlorosilane, CH3SiCl3) has been used for SiC deposition because it is easy to produce stoichiometric SiC deposits. 

Briefly, three points of porous SiC-based catalytic support properties can be emphasized (i) SiC shows very good mechanical properties which gives resistance to erosion and attrition, in addition to a high thermal stability (ii) SiC has a higher thermal conductivity compared with the more conventional supports which could prevent the metal sintering (iii) SiC is particularly inactive with respect to chemical reagents such as acids or bases. Therefore, the active phase can be easily reprocessed after simple acidic or basic treatments. Among refractory materials, the thermal conductivity of silicon carbide, SiC (500 W m-1K-1 for crystalline state, at room temperature) is close to that of metals such as Ag or Cu (400-500 Wm K-1). 

The extraordinary mechanical, thermal and electrical properties of carbon nanotubes (CNT) have prompted intense research into a wide range of applications in structural materials, electronics, and chemical processing.Attempts have been made to develop advanced engineering materials with improved or novel properties through the incorporation of carbon nanotubes in selected matrices (polymers, metals and ceramics). But the use of carbon nanotubes to reinforce ceramic composites has not been very successful. So far, only modest improvements of properties were reported in CNTs reinforced silicon carbide and silicon nitride matrix composites, while a noticeable increase of the fracture toughness and of electrical conductivity has been achieved in CNTs reinforced alumina matrix composites. ... 

Nonoxides such as silicon carbides, nitrides, metal sulfides, and so on, possess unique chemical, thermal, and mechanical properties and can be used as catalysts and catalyst carriers. The properties can be further enhanced by the confined-space effect in nanospace, high surface areas, and large pore volumes. 

Self-bonded silicon carbide materials in molded form and of high SiC content have been available only from the late 1960s onwards. These are being increasingly used for structural components in mechanical engineering, and have proved to be highly successful for use under extreme abrasive, corrosive, and thermal conditions due to their excellent mechanical, chemical, and physical properties. 

Xu Y, Cheng L, Zhang L, Yan D, Mechanical properties and microstructural characteristics of carbon fibre reinforced silicon carbide matrix composites by chemical vapour infiltration, Niihara K, Nakano K, Sekino T, Yasuda E eds.. Ceramic Society of Japan, High Temperature Ceramic Matrix Composites III, Proc 3rd Int Conf, Osaka, Sep 6-9 1998, 73-16, Key Eng Mater, Vol 164-165. 

An example of mechanical aids to cavitation can be found in the field of ultrasonic cell disintegration. These activators are often glass beads or ceramic disks, but sand or plastic chunks can also be employed. Other examples of solids added to improve cavitation effects are silicon carbide, any hard ceramic material or AI2O3 for micronizing species or for surface abrasion, Si02 which favors mechanochemical events, talc for the activation and probably alteration of the chemical properties of surfaces. 

Ceramic fibers. The other fibers shown in Table 4.6 have varying uses, and several are still in development. Silicon carbide continuous fiber is produced in a chemical vapor deposition (CVD) process similar to that for boron, and it has many mechanical properties identical to those of boron. The other fibers show promise in metal matrix composites, as high-temperature polymeric ablative reinforcements, in ceramic-ceramic composites, and in microwave transparent structures (radomes or microwave printed wiring boards). 

The physical and mechanical properties of steel depend on its microstructure, that is, the nature, distribution, and amounts of its metallographic constituents as distinct from its chemical composition. The amount and distribution of iron and iron carbide determine most of the properties, although most plain carbon steels also contain manganese, silicon, phosphorus, sulfur, oxygen, and traces of nitrogen, hydrogen, and other chemical elements such as aluminum and copper. These elements may modify, to a certain extent, the main effects of iron and iron carbide, but the influence of iron carbide always predominates. This is true even of medium alloy steels, which may contain considerable amounts of nickel, chromium, and molybdenum. 

---