Carbide composites

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

See Carbides, CEMENTBD carbides Composite maseriads, ceramic matrix Glassymetads. 

It is known that Cr23Q can contain up to 25% iron on the metal atom sites, and Cr Cj up to about 60% iron Therefore the minimum activities of CrjjQ and Cr, have been calculated for these phases of maximum iron content using the ideal laws to calculate activities from carbide composition. The free-energy lines which were thus obtained are shown as A and... 

Tantalum and niobium are added, in the form of carbides, to cemented carbide compositions used in the production of cutting tools. Pure oxides are widely used in the optical industiy as additives and deposits, and in organic synthesis processes as catalysts and promoters [12, 13]. Binary and more complex oxide compounds based on tantalum and niobium form a huge family of ferroelectric materials that have high Curie temperatures, high dielectric permittivity, and piezoelectric, pyroelectric and non-linear optical properties [14-17]. Compounds of this class are used in the production of energy transformers, quantum electronics, piezoelectrics, acoustics, and so on. Two of... 

Isothermal Infiltration. Several infiltration procedures have been developed, which are shown schematically in Fig. 5.15.P3] In isothermal infiltration (5.15a), the gases surround the porous substrate and enter by diffusion. The concentration of reactants is higher toward the outside of the porous substrate, and deposition occurs preferentially in the outer portions forming a skin which impedes further infiltration. It is often necessary to interrupt the process and remove the skin by machining so that the interior of the substrate may be densified. In spite of this limitation, isothermal infiltration is used widely because it lends itself well to simultaneous processing of a great number of parts in large furnaces. It is used for the fabrication of carbon-carbon composites for aircraft brakes and silicon carbide composites for aerospace applications (see Ch. 19). 

Other ceramic cutting-tool materials include alumina, Si-Al-0-N, alumina-carbide composites and, more recently, a composite of silicon nitride reinforced with silicon carbide whiskers. This last material can be produced by chemical-vapor infiltration (CVI) and has high strength and toughness as shown in Table 18.3.Cl... 

What is the relationship between carbide composition and catalytic activity Figure 5.9 (right) compares the two properties. The catalytic activity starts low, increases rapidly to a maximum and decreases slowly thereafter. The increase in activity occurs simultaneously with the carburization, but, without any direct correlation, the maximum in rate occurs when there is still metallic iron present. The interpretation given to the results in Fig. 5.9 is the following [22,24],... 

Singh, R.N. and Gaddipati. A.R. (1991). A uniaxially reinforced zirconia-silicon carbide composite. J. Mater. Sci. 26, 957. 

Meschke, F., Alves-Riccardo, P., Schneider, G.A. and Claussen, N. Failure behavior of alumina and alumina/silicon carbide composites with natural and artificial flaws , J. Mat. Res. 12 (1997) 3307-3315. 

SEM images of the SiC-coated diamond-dispersed cemented carbide composite (a) polished surface, (b) crack propagation. 

MWCNTs have been tested to reinforce various matrices because they have many unique mechanical and physical properties.14,15 However, these nanotubes become corroded with metals (such as iron, cobalt, and aluminum) at temperatures above 850°C. These shortcomings limit the applications of MWCNTs as nano-reinforcements. The SiC coating can effectively protect the diamond from molten cobalt, thus allowing dense SiC-coated diamond-dispersed cemented carbide composites to be successfully fabricated at lower pressures. If MWCNTs can be coated with the same SiC layer, more stable MWCNTs would be produced and expected to be used as nano-reinforcements for various matrices. The development of SiC-coated MWCNTsAVC-Co composites has potential to extend functions of both MWCNTs and WC-Co. 

Turan, S. and Knowles, K.M., (1995), A comparison of the microstructure of silicon nitride-silicon carbide composites made with and without deoxidized starting material , J. Microscopy, YU (3), 287-304. 

Yu, Z.B., Krstic, Z. and Krstic, V.D., (2005), Laminated silicon nitride/silicon carbide composites with self-sealed structure , Key Engineering Materials 280-283, 1873-1876. 

R. N. Singh, High-Temperature Mechanical Properties of a Uniaxially Reinforced Zicon-Silicon Carbide Composite, J. Am. Ceram. Soc., 73[8], 2399-2406 (1990). 

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). 

Many nonoxide ceramics form gaseous reaction products when oxidized. For example, when an alumina/silicon carbide composite is exposed to an oxidizing environment, SiC will oxidize, forming carbon monoxide via the following reaction ... 

Precursors for Silicon Nitride/Silicon Carbide Composites.455... 

V. PRECURSORS FOR SILICON NITRIDE/SILICON CARBIDE COMPOSITES... 

Based on this approach, families of silicon carbide fiber and silicon ceramic composites are now being routinely produced based on polycarbosilane precursors [22] These new materials are finding a wide range of new applications, for example, as a hot zone component in the next generation turbojets where silicon carbide composite components now routinely service at operating temperatures well in excess of 1000°C under high static thrusts (up to 50,000 psi) and high sonic pressure (up to 800 DB). No metallic components survive under these conditions. 

B. Zirconium Diboride-Reinforced Zirconium Carbide Composites. 93... 

Fig. 5. The microstructure of zirconium diboride-reinforced zirconium carbide composites produced by directed metal oxidation, (a) A composite prepared with 22 vol % metal, (b) A composite produced with less than 2 vol % metal. 

Fig. 18. The Hf-B-C isothermal ternary cross sections at three different temperatures. Points A and B represent two possible stoichiometries for processing hafnium boride/hafnium carbide composites. After Rudy [57],...

Snead LL, Jones RH, Kohyama A, Fenici P (1996) Statue of silicon carbide composites for fusion. J Nucl Mater 233-237 26-36... 

Figure 6.6. Columnar SiC in carbon-fibre-reinforced silicon carbide composites [18]...

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