Threshold Stress Intensity for Crack Growth at Elevated Temperatures in a Silicon Nitride Ceramic

Abstract

The flaw behavior in an as-machined and oxidized hot isostatically pressed silicon nitride (HIPSN) under static load was investigated at various stress intensities, times, and temperatures. Flaw origins were of the pore/cavity type and iron-based inclusion type. There was no strength increase of degradation in as-machined and oxidized specimens static loaded for 10 h at 1,100{degree}C in air below the threshold stress intensity. The threshold stress intensities for crack growth at 1,100{degree} were determined to be 1.75 and 2.00 MPa {center dot} m{sup 1/2} for the as-machined and oxidized HIPSN, respectively. A strength decrease with increased applied stress was observed in as-machined specimens static loaded at 1,200{degree} and 1,300{degree}C in air. This effect is due to the creation of a new flaw population with more severe flaws. Threshold stresses of 160 and 90 MPa were observed at 1,200{degree} and 1,300{degree}C, respectively. Oxidized HIPSN does not experience any strength degradation when static loaded at 1,200{degree}C below threshold stress intensity of 1.50 MPa {center dot} m{sup 1/2}. The oxidation treatment causes microstructural changes in the ceramic, resulting in a material with improved creep and crack growth resistance.