Carbide-forming materials

Diamond can be deposited as a coating on refractory metals, oxides, nitrides, and carbides. For maximum adhesion, the surface should be a carbide-forming material with a low TCE. Diamond has an extremely high thermal conductivity, several times that of the next highest material. The primary application is, obviously, in packaging power devices. Diamond has a low specific heat, however, and works best as a heat spreader in conjunction with a heat sink. For maximum effectiveness, ... 

Diamondlike Carbides. SiUcon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be iacluded. These materials have electrical resistivity ia the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical stmeture, although its conductivity is low. 

Composites. Another type of electro deposit in commercial use is the composite form, in which insoluble materials are codeposited along with the electro-deposited metal or alloy to produce particular desirable properties. Polytetrafluoroethylene (PTFE) particles are codeposited with nickel to improve lubricity (see Lubrication and lubricants). SiHcon carbide and other hard particles including diamond are co-deposited with nickel to improve wear properties or to make cutting and grinding tools (see Carbides Tool materials). 

The occasional grains of cohenite in a few of the samples of fines are almost certainly meteoritic in origin, so some carbide analysed as CD4 must have been contributed in this form . Although discrete grains are extremely rare, add dissolution shows that carbide (or material behaving as carbide) is ubiquitous in the fines. The carbide, therefore, appears to be finely disseminated. 

Of course, oxygen is not the only impurity that will react with beryllium. Another material that is important in forming mixed-material layers with beryllium is carbon. The saturated value of retention that has been found in beryllium surfaces exposed to a large deuterium ion fluence could easily be overshadowed if a carbon rich layer forms on the beryllium surface due to impurity carbon ions in the incident plasma flux. The hydrogen retention properties of plasma deposited carbon films has been shown to dominate the total retention in beryllium samples exposed to the plasma at lower temperature. Once the sample temperature during exposure approaches 500°C there is little difference between the retention in Be/C mixed-material layers compared to clean beryllium samples [48]. The temperature dependence of the retention of carbon containing mixed material layers, as well as that of clean beryllium surfaces is shown in Fig. 14.10. There are two possible explanations for the reduced retention in the mixed-material layers formed at elevated temperature. The first is that beryllium carbide forms more readily at elevated temperature and less retention is expected in beryllium carbide [11]. The second is that carbon films deposited at elevated temperature also tend to retain less hydrogen isotopes [49]. 

The oxide, MoOj, reacts with C to give a lower oxide, MoOj at 420-640°C, followed by the metal above 820°C. Finally the carbide forms after the oxide is eliminated. Because the Mo oxides are so volatile, T must be kept low during the initial phase of this reaction. The reaction is accelerated by the presence of H2 or HCl. Although the Mo carbides are easily freed of oxygen, a final high-T (Ca. 2000°C) vacuum anneal should be used if the material is not previously melted. 

The carbon atoms of most binary metal carbides have hypercoordinated environments like those shown in Figure 1.5. In particular, octahedral carbon coordination is common in the interstitial carbides formed by many transition metals, materials of variable composition in which carbon atoms... 

Reactions of carbon in alkali metals with carbide forming metallic elements are the driving processes of the carburization of stainless steels. The direction of the carbon exchange between the molten metals and the solid metallic materials depends on the carbon potential in the liquid metals and on the free energy of formation of the metal carbides and the chemical activity of the metallic element in the solid phase. 

How does it react to oxygen amd carbon Both oxides and carbides form a large class of materials in and of themselves. 

Cemented carbides form one of the most important groups of hard materials [41,42], The carbides WC, TiC, and TaC are the technically most important ones. They are produced in amounts of several thousand tons per year. VC, NbC, ZrC, HfC, M02C, and the chromium carbides are somewhat less important. The extreme hardness and high melting points of many transition metal carbides were already recognized in the 1890s by Moissan [43]. 

---