Strength and dynamic fatigue of silicon nitride at intermediate temperatures

Abstract

The flexure strength distributions and dynamic fatigue of GS44, NT551, and NT154 silicon nitrides were determined at intermediate temperatures (≤850°C) in ambient air. GS44 and NT551 were fabricated by gas pressure sintering and had a larger volume fraction of secondary phase than the hot-isostatically pressed NT154. The inert characteristic strength for the GS44 and NT551 decreased with temperature while that for the NT154 was statistically independent of temperature. The slow crack growth exponent, N, for the GS44 and NT551 exhibited a 70–80% decrease between 700 and 850°C while the strength of NT154 was comparatively independent of stressing rate at 850°C. Associated with those changes in mechanical performances, the coefficient of thermal expansion (CTE) for the GS44 and NT551 sharply increased between 700 and 850°C while that for the NT154 did not. Although the observed change in CTE (and the temperature range where it occurred) did not have any obvious correlation with a change in inert strength at these intermediate temperatures, its presence was an indicator of a threshold temperature where dynamic fatigue resistance was compromised if the temperature exceeded it. The change in material state (and the associated temperature region for which it occurs) is linked to the equilibrium state of the secondary phase in the silicon nitride.