Effect of grain-boundary crystallization on the high-temperature strength of silicon nitride

Si₃N₄ specimens having the composition 88.7 wt% Si₃N₄-4.9wt% SiO₂-6.4wt% Y₂O₃ (85.1 mol% Si₃N₄-11.1 mol% SiO₂-3.8mol% Y₂O₃) were sintered at 2140° C under 25 atm N₂ for 1 h and then subjected to a 5 h anneal at 1500° C. Crystallization of an amorphous grainboundary phase resulted in the formation of Y₂Si₂O₇. The short-time 1370° C strength of this material was compared with that of material of the same composition having no annealing treatment. No change in strength was noted. This is attributed to the refractory nature of the yttrium-rich grain-boundary phase (apparently identical in both glassy and crystalline phases) and the subsequent domination of the failure process by common processing flaws.Chemical analysis of the medium indicated 5.25 wt% O₂, 0.46 wt% C, 0.8 wt% Al, and expressed in p.p.m. 670 Ca, 30 Cu, 2000 Fe, <2 Ti, 370 Cr, 130 Mg, 90 Mn, <10 V, <20 Zr, 2000 Mo, 240 Ni, 130 Zn, <30 Pb, <60 Sn.