Carbide diffusion process applications

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). 

The Dilex Process utilises a molten lead bath as transfer medium and is applicable to diffusion coatings of Cr, Al, Ti, Mo, Ni and Co. Finally, a Japanese fused borate bath process produces carbide coatings (Cr, V, Nb or Ta) on carbon and tool steels. The coatings are wear and corrosion resistant. The TD Process uses this technique. 

An example of this behavior occurs during the reaction with H2. All of the carbon-saturated carbides will lose carbon as hydrocarbons at high temperatures. This lowers the carbon concentration, hence its activity, and raises the hydrocarbon concentration in the gas. At some point, depending on the carbide system and the temperature, these processes will come to equilibrium, but at a reduced carbide stoichiometry. If, on the other hand, the H2 is made to flow and the hydrocarbons are swept away, the carbide will continue to lose carbon at a rate which will depend on the diffusion rate of carbon through the carbide. Fortunately for many applications, this rate is small. At low temperatures, H2 can dissolve in the defect lattice forming a carbohydride. 

Based on those observations, we can conclude that the oxidation temperature and process of nonstoichiometric ZrC, especially that with ordered carbon vacancies, are strongly dependent on the concentration and state of carbon vacancies. The ordered vacancies provide the additional fast tracks for the oxygen diffusion, inducing the fast oxidation of non-stoichiometric ZrC. The ordered carbon vacancies are not conducive to the application of carbides at high temperature and under aerobic conditions. However, the fast oxidation of ordered ZrC provides a new route to fabricate the metastable tetragonal zirconia nano-powders. 

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