The combined-stage sintering model was used to determine the activation energy, Q, of sintering for selected SiC-based materials. SiC densified with a liquid (1.65 wt% Al) had an activation energy of 842 +/- 79 kJ/mol, a value between those for a silicon carbide densified with 1 wt% C and 0.25 wt% B4C (Q=643 +/- 37 kJ/mol) and one densified with 2.5 wt% AlN (Q=1022 +/- 122 kJ/mol), compositions which have no liquid phase below 1850 degrees C. The SiC with Al additive began densification by 1500 degrees C and the densification curve was offset by approximately 100 degrees C compared with the other two materials below 1850 degrees C. The choice and amount of additives not only affect densification and activation energy, but also influence microstructure and fracture mode, allowing engineering of mechanical properties.