A comparative study of the tensile, fatigue, and creep properties of sintered (SNW-1000 & GS-44) and HIPed (PY-6) silicon nitride ceramics

摘要

Significant progress has been made in understanding the mechanical properties of two differently processed, namely hot isostatically pressed (HIPed) (GTE-PY6) and sintered silicon nitrides (SNW- 1000 and GS-44). Experiments showed that as the temperature increased from room to 1000℃ the tensile strengths of sintered SNW- 1000 and GS-44, and HIPed PY-6 dropped by 10%, 15%, and more than 26% respectively. At higher temperatures, strength of SNW-1000 dropped much faster than those of GS-44 and PY-6. This is due to the difference in the amount and types of densification aids and the processing of the sintered and the HIPed materials. S-N curves show that endurance limits of SNW- 1000 is higher than those of GS-44 at temperatures 1000℃ and 1100℃. But at 1200℃ the endurance limit of GS 44 is higher than that of SNW- 1000. From the results of the creep tests a stress exponent 'n' of 9.36 was obtained for PY-6, while SNW-1000 exhibited a stress exponent of 4.32 at 1200℃. The 'n' for GS-44 is found to be unity at lower temperatures such as 1100℃, implying that the creep is diffusional. It increased to 2 and above at temperatures 1 200℃ and 1275℃ indicating a change in the creep mechanism. In contrast to PY-6, the as-received SNW- 1000 and GS-44 revealed amorphous phases at multi-grain junctions and grain boundaries. TEM observations(GS-44) revealed that dislocation density and the number of strain whorls in samples tested at 1000℃ is much less than the specimens tested at 1 275℃ under the same stress level.