• Synergistic regulation of temperature (950 °C), CH4 partial pressure (1.25%), and time (60 min) yields ultrafine WC-Co powder without η phase.
• The oxygen loss rate of WO2 is positively correlated with temperature and methane partial pressure, while CH4 partial pressure has no significant effect on WC formation rate.
• The obtained composite powder has a particle size of 128 nm, total carbon content of 6.16%, free carbon content of 0.4%, and residual oxygen content of 0.05%.
• A growth rate equation for tungsten carbide was established: d(t) = 1.21 × 10^-13 exp(-12809.72/T) √t, providing a predictive tool for process optimization.