Theoretical and experimental investigations on the mechanism of carbothermal reduction of zirconia

Zirconium carbide (ZrC) is an important ultra-high temperature ceramic due to its refractory properties. It is commonly synthesized via carbothermal reduction of zirconia above 1657 °C according to the reaction ZrO2 (s)+3C (s)→ZrC (s)+2CO (g). Contrary to this reaction, prior research indicates that carbon monoxide (CO) is the responsible species for carburizing ZrO2 to form ZrC. To explore this reaction pathway, investigations were performed by making two mixed phase pellets with 3 mol% yttria-stabilized zirconia (YSZ) and graphite. Both had an upper half made of YSZ. The lower half of one sample consisted of finely mixed YSZ and graphite powder whereas the other was pure graphite. Similar experiments were conducted with sintered YSZ pellets on top. After heat treatment at 1800 °C, X-ray diffraction analysis revealed higher ZrC conversion for the YSZ pellet face in direct contact with pure graphite. This contradicts the previous work as one would assume higher ZrC yield for YSZ pellet in direct contact with YSZ/graphite mix as they produce more CO upon reaction. Lastly, diffusional experiments showed conversion to be highly localized to a depth of ∼25 μm. This is in close agreement with calculations for carbon diffusion in YSZ based on a diffusion coefficient (D)=3×10−14 m2/s, which confirms solid–solid reaction rather than solid–gas reaction.