Processing and mechanical properties of Si3N4 composites employing polymer-derived SiAlOC as sintering aid

Silicon nitride ceramics have been densified with polymer-derived SiAlOC sintering aid. Dense samples were prepared at relatively mild temperatures (1600 °C) from blends with 30 wt.% of pyrolysed SiAlOC additives. Decreasing the SiAlOC aid content to 15 wt.% resulted in porous Si3N4 samples (∼85% rel. density). The properties of dense samples were influenced by the remaining SiAlOC glass (HV = 15.5 GPa, KIC = 4 MPa m1/2). Increasing the sintering temperature to 1780 °C for 5 min significantly changed the phase composition and properties of the composites. The major phase was O′-sialon in the sintered samples. Additional annealing of the samples at 1530 °C for 16 h further decreased the amount of the residual glassy phase and consequently affected the mechanical properties. The Vickers hardness of dense samples was 18.5 GPa and the fracture resistance ranged between 4.0 and 4.5 MPa m1/2. The compressive creep test (1400 °C/100 MPa/24 h) of the SNA30-A sample sintered at 1600 °C for 30 min without an additional crystallisation step showed a promising low creep rate of 8.6 × 10−8 s−1. Further improvement of creep resistance is expected for the crystallised samples.