Graphite tubes having a length-to-bore-diameter ratio of 430 were coated on the inside with uniform adherent layers of niobium carbide in thicknesses of 0.002 and 0.004 in. Intimate conformity to the graphite surface and improved uniformity were obtained by reacting NbCl/sub 5/ vapor with the graphite under pressuretemperature conditions thermodynamically unfavorable for the deposition of niobium by thermal dissociation of the chloride, but favorable for the formation of NbC. Thus, the rate of coating fermation depended primarily on the rate of diffusion of carbon through the carbide, which is a function of temperature and independent of vapor-flow considerations over a wide range, Although adherent coatings were obtained on a porous uraniumloaded graphite in the as-received condition, it was necessary to etch a more dense graphite to promote adherence. (auth)