Carbon self-diffusion in tungsten carbide

Dense, hot pressed tungsten carbide specimens were used to study the self-diffusion of ¹⁴C into WC in the temperature range 2238 to 2643 ° K. The necessity for extended diffusion anneal times was eliminated by using a submicron sectioning technique, and the diffusion penetration depth was determined by spectrophotometric W^V thiocyanate analysis of the sample sections. The existence of two clearly delineated diffusion mechanisms was demonstrated from the shape of the activity versus penetration curves. The first obeyed a bulk diffusion law, originated at the specimen surface, demonstrated anomalously low diffusion coefficients, dominated to a depth of about 0.5 m, and could be represented by the expression: D_vol=1.90×10^–6 exp–(88,000/RT).Autoradiography demonstrated that the second mechanism was grain-boundary diffusion which dominated at depths greater than 1 m. The Fisher grain-boundary diffusion analysis and the Suzuoka analysis gave apparent grain-boundary diffusion activation energies of 74 Kcal/mole and 71 Kcal/mole respectively. By using an estimated value of the bulk diffusion coefficient, the Suzuoka analysis permitted direct calculation of the grainboundary diffusion coefficient, and can be represented by: D_g.b.=4.57×10² exp–(71,000/RT).