Coatings hafnium carbides

Hafnium carbide [12069-85-1] can be used as surface coating on cemented-carbide cutting tools, shows promise as a stable field emission cathode... 

Hafnium carbide has a melting point of 3890°C and exhibits extreme hardness and good electrical conductivity. It is used as an oxidation-resistant coating for composites and as a coating for superalloys [10]. The material is prepared by the CVD of a mixture of hafnium tetrachloride, methane and hydrogen [193, 194]. 

The most simple methods for preparation of hafnium carbide and its composites are the follows. The powder of HfO was thermally treated with Mg in molar ratio 5 4 under a CH flow ranging from 800 to 950 °C [4]. The effective high temperature coating for carbon fiber reinforced carbon and carbon fibre reinforced silicon carbide was prepared with use of HfC [5]. For this purpose hafnium carbide layers were obtained in a thermally simulated chemical vapor deposition (CVD) reactor on nonporous substrates by reaction of hafnium tetrachloride, methane and addition of hydrogen (Eq. 10.1) ... 

Hafiiium carbide powder is prepared by the reaction of Hf02 with carbon at 1800-2200°C in hydrogen by the carburization of hafoium sponge by the carburization of hafiiium hydride at 1600-1700°C, or by plasma CVD. Hafnium carbide coatings are deposited by CVD, evaporation or sputtering (see Chs. 14 and 15). 

Nitrides are used for wear-resistant applications, most notably surface engineering of cemented carbide cutting tools and tool steels. Titanium nitride (TiN) is the most frequently employed coating, but titanium car-bonitride (TiCN), hafnium carbide (HfC), titanium aluminum nitride (TiAlN), titanium zirconium nitride (TiZrN), and chromium nitride (CrN) have also been used commercially. These coatings are applied by vapor deposition techniques (Fig. 2). 

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

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