Characterization and high-temperature degradation mechanism of continuous silicon nitride fibers

Amorphous silicon nitride fibers were characterized with respect to their chemical and phase compositions. The high-temperature behavior of the fibers was investigated in N₂ and Ar at temperatures ranging from 1350 to 1600 °C through tensile tests, pore volume analysis, elemental analysis, weight loss, X-ray diffraction, and scanning electron microscopy to elucidate the effects of the atmosphere on the fiber degradation mechanism. The fibers demonstrated better high-temperature stability in N₂ instead of Ar. In particular, the fibers heat-treated in N₂ at 1450 °C retained 43% of their original strength (0.64 GPa), while the fiber strength was completely lost in Ar at the same temperature. The primary reason for the strength degradation of the amorphous fibers was the expanding nanopores before the formation of the large Si₃N₄ grains. However, a catastrophic decrease in strength was due to the drastic thermal decomposition of SiN_xO_y and crystallization of Si₃N₄ grains, influenced by N₂ pressure. Thermodynamic considerations were additionally applied to estimate the decomposition and carbothermal decomposition of crystalline Si₃N₄, thereby explaining the significant discrepancy of crystallization transformation at 1600 °C under different atmospheres.