Secondary Phase Evolution in Silicon Nitride During Creep and Heat Treatment
F Lofaj, H Gu, A Okada, Sheldon M. Wiederhorn, H Kawamoto
The microstructure of a ytterbia- and yttria-containing grade of silicon nitride, SN 88, was investigated after it had been annealed in nitrogen, in air and crept in air. We were particularly interested in phase transformation within the secondary silicate phases at the grain boundaries. X-ray diffraction and electron microscopy studies showed a gradual transformation of Y5(SiO4)3N and Yb4Si2N2O7 present in the as-received material into (Y/Yb)2SiO5 and epitaxially deposited Si3N4. In air, the transformation started at the surface and progressed into the interior, while in nitrogen the transformation occurred partially throughout the body. We suggest that Y5(SiO4)3N and Yb4Si2N2O7 reacted with silica to form (Y/Yb)2SiO5 and Si3N4. Changes in the composition due to these reactions and to an accompanying reduction in the amount of silica in the microstructure account for an increase of creep resistance after annealing.
, Gu, H.
, Okada, A.
, Wiederhorn, S.
and Kawamoto, H.
Secondary Phase Evolution in Silicon Nitride During Creep and Heat Treatment, Journal of the American Ceramic Society
(Accessed March 3, 2024)