Crack silicon carbides

Sintered silicon carbide retains its strength at elevated temperatures and shows excellent time-dependent properties such as creep and slow crack growth resistance. Reaction-bonded SiC, because of the presence of free silicon in its microstructure, exhibits slightly inferior elevated temperature properties as compared to sintered silicon carbide. Table 2 (11,43) and Table 3 (44) show selected mechanical properties of silicon carbide at room and elevated temperatures. 

When a composite is subjected to external forces, the energy of the matrix is only transferred to the fibres when there is question of a proper attachment. For that reason fibres are some-times provided with a layer of another material. An example boron fibres in an aluminium matrix are provided with a silicon carbide coating and as a result the fibres are called borsic fibres. The thermal expansion coefficient of a fibre and its matrix must correspond. Figure 14.9 is a representation of what takes place when a crack in a fibre-reinforced matrix grows. 

Hoffman, M., Rodel, J., Stemitzke, M. et al., Fracture toughness and subcritical crack growth in alumina/silicon carbide nanocomposites , Fracture Mechanics of Ceramics, 1996, 12 179. 

Multiple matrix cracking perpendicular to the fibre axis due to thermal shock in UD Nicalon /CAS (reprinted from Journal of Materials Science 32(2) 1997, Thermal shock behaviour of unidirectional silicon carbide reinforced calcium aluminosilicate Blissett, Smith and Yeomans, Figure 2, with kind permission of Springer Science and Business Media). 

The bulk densities of all the materials were determined using Archimedes method (AS 1774.5, 1979). The Vickers indentation technique was used to measure the hardness in each case. The applied load in the Vickers hardness tests was 10 kg for silicon nitrides and sialons. However, using the same load produced severe lateral cracking in silicon carbides around indents, which prevented the accurate measurement of the diagonals of indents. Therefore the load was reduced to 0.3 kg for silicon carbide samples. 

L. X. Han and S. Suresh, High Temperature Failure of an Alumina-Silicon Carbide Composite under Cyclic Loads Mechanisms of Fatigue Crack-Tip Damage, J. Am. Ceram. Soc., 72[7], 1233-1238 (1989). 

Other forms of carbon-carbon composites have been or are being developed for space shuttle leading edges, nuclear fuel containers for satellites, aircraft engine adjustable exhaust nozzles, and the main structure for the proposed National Aerospace plane (34). For reusable applications, a silicon carbide [409-21-2] based coating is added to retard oxidation (35,36), with a boron [7440-42-8] based sublayer to seal any cracks that may form in the coating. 

In, the central zone, the regular distribution of transverse cracks shows that the induced strain is rather homogeneous. Consequently, many analytical models can be applied in order to determine the intrinsic parameters of the coatings. The critical cracking energy and the mode I fracture toughness of the deposited silicon carbide film were assessed by means of the model presented previously. It should be remembered that it was first established and developed for composite materials based on research by Kelly and subsequently by Hu ", that when the stress normal to the coating reaches a critical value... 

Specihcally with regard to the pyrolysis of plastics, new patents have been filed recently containing variable degrees of process description and equipment detail. For example, a process is described for the microwave pyrolysis of polymers to their constituent monomers with particular emphasis on the decomposition of poly (methylmethacrylate) (PMMA). A comprehensive list is presented of possible microwave-absorbents, including carbon black, silicon carbide, ferrites, barium titanate and sodium oxide. Furthermore, detailed descriptions of apparatus to perform the process at different scales are presented [120]. Similarly, Patent US 6,184,427 presents a process for the microwave cracking of plastics with detailed descriptions of equipment. However, as with some earlier patents, this document claims that the process is initiated by the direct action of microwaves initiating free-radical reactions on the surface of catalysts or sensitizers (i.e. microwave-absorbents) [121]. Even though the catalytic pyrolysis of plastics does involve free-radical chain reaction on the surface of catalysts, it is unlikely that the microwaves on their own are responsible for their initiation. 

There is the possibility to make substrates in various materials Alumina is an obvious possibility, but monoliths formed from alumina are particularly susceptible to thermal shock problems, and they readily crack during rapid temperature excursions. Silicon carbide and boron nitride are other possible materials having good properties, but they are expensive. 

Figure 15 Silicon carbide heat exchange/reactor design for steam/naphtha cracking process. (From Ref. 28, reprinted with permission of the American Institute of Chemical Engineers.)...

Silicon carbide refractories are employed in considerable quantities in kiln furniture and in muffles in the ceramic industry, where use is made of their thermal conductivity. The life depends on conditions it is impaired by access of air and water vapour. Oxidation results in volume expansion, embrittlement and cracking of the product. 

---