Criterion for self-sustaining coating in metal- or carbide-coated graphite limiter

Abstract

We have studied the conditions for self-sustaining coatings of a nickel and titanium carbide layer deposited on graphite, and a nickel layer deposited on nickel carbide film. Segregated carbon atoms are responsible for the self-sustaining nature of the coating. Both the concentration of carbon atoms at the surface and the sputtered flux of carbon atoms from the nickel and titanium carbide deposited on the graphite substrate are calculated as a function of ion flux. It is shown that the critical ion flux for a self-sustaining coating depends on the rate constant of segregation to the bombarded surface, on the diffusion constant, and on the rate constant of dissolution from graphite into the deposited layer. The experimental results on the ion flux dependence of the sputtered flux of carbon and on the critical ion flux are compared with the calculated results and, the rate constant of segregation of carbon on the Ni surface is determined to be 57 s⁻¹ at 650° C. That on the TiC surface is determined to be 0.1 s⁻¹ at 900° C. It is pointed out that segregation from the grain boundary to the surface of the deposited layer, grain boundary diffusion, and dissolution from graphite into the grain boundary are important factors for self-sustaining coating.