Hot‐Pressed Silicon Nitride with Lu 2 O 3 Additives: Oxidation and Its Effect on Strength

The oxidation behavior and effect of oxidation on room‐temperature flexural strength were investigated for hot‐pressed Si 3 N 4 ceramics, with 3.33 and 12.51 wt% Lu 2 O 3 additives, exposed to air at 1400° and 1500°C for up to 200 h. Parabolic oxidation behavior was observed for both compositions. The oxidation products consisted of Lu 2 Si 2 O 7 and SiO 2 . The Lu 2 Si 2 O 7 grew out of the surface silicate in preferred orientations. The morphology of oxidized surfaces was dependent on the amount of additive; Lu 2 Si 2 O 7 grains in the 3.33 wt% composition appeared partially in a needlelike type, compared with a more equiaxed type exhibited in the 12.51 wt% case. The high resistance to oxidation shown for both compositions was attributed to the extensive amounts of crystalline, refractory secondary phases formed during the sintering process. Moreover, after 200 h of oxidation at 1400° and 1500°C, the strength retention displayed by the two compositions was 93%–95% and 85%–87%, respectively. The strength decrease was associated with the formation of new defects at the interface between the oxide layer and the Si 3 N 4 bulk.