Effects of Additives on the Pressure-Assisted Densification and Properties of Silicon Carbide
The densification of silicon carbide (SiC) was studied using a variety of additives (Al, AlN, Al2O3, B4C, C, Si3N4, and Y2O3). The onset of densification of SiC with small amounts of additives occurred at temperatures between 1500° and 1900°C with 28 MPa applied pressure. Al, B4C, and C promoted densification, while N (added as AlN or Si3N4) retarded sintering. A 96.75 wt% SiC–2 wt% Al–1 wt% C–0.25 wt% B4C starting composition yielded the same percent of theoretical density (in the range of 70%–90% theoretical density) 400°C lower than a 95 wt% SiC–5 wt% AlN material. Yttria additions promoted intergranular fracture, which increased the single-edged precracked beam fracture toughness. The appropriate selection and amount of additives allowed for the tailoring of grain size and intergranular fracture, thus controlling the mechanical properties. While oxygen was present in all materials containing aluminum, the incorporation of additional oxygen as alumina resulted in reduced sintering activity compared with Al metal. Corrosion resistance decreased in both HF and NaOH solutions at 80°C for materials containing a grain boundary phase.
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