The oxidation behavior of NBD 200 Si3N4 containing 1 wt% MgO sintering aid
was investigated in oxygen at 900 degrees-1300 degrees C. The oxide growth
followed a parabolic rate law with an apparent activation energy of 260 kJ/
mol. The oxide layers were enriched with sodium and magnesium because of ou
tward diffusion of intergranular Na+ and Mg2+ cations in the ceramics. The
2-4 orders of magnitude higher oxidation rate for NBD 200 Si3N4 than for ot
her Si3N4 ceramics with a similar amount of MgO could be attributed to the
presence of sodium. The oxidation process was most likely rate limited by g
rain-boundary diffusion of Mg2+.