Diffusion coefficients of graphite foil carbon in Mo during spark plasma sintering process

Abstract

This study investigated the influence of electric current on carbon diffusivity in molybdenum (Mo) during spark plasma sintering (SPS). The samples comprised 0.2-mm-thick graphite foils, 3-mm Mo discs annealed at 1000 °C for 1–8 h, and a Ti-Mo alloy annealed at 1200 °C for 4 h. Elemental concentration profiles were analyzed using electron probe microanalysis (EPMA), while microstructure evolution was examined via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) in various modes. Diffusion coefficients were determined using the Den Broeder method and compared with literature values to assess possible enhancement. Contrary to expectations, the results indicated no enhancement of diffusion coefficients under electric current; instead, they decreased with increasing carbon concentration. This reduction was attributed to the formation of a molybdenum carbide layer, which hinders further carbon diffusion into the Mo matrix. The study also observed that diffusion of carbon into the sample occurs predominantly via solid-state diffusion at the graphite-metal interface, rather than vapor-phase transport.