Fabrication of silicon carbide ceramics by combination of slip casting and spark plasma sintering

In this work, first, a systematic study has been done to optimize the rheological behavior of silicon carbide (SiC) slurries to produce 20 mm disc-shaped green bodies with the highest relative density by adding sintering aids and changing the most effective parameters of the slip casting process. In the second step, slip-casted SiC green bodies were sintered via spark plasma sintering (SPS) under the same conditions: at a heating rate of 200 °C/min and under a vacuum atmosphere (10−2 Torr), the temperature reached 1900 °C and was maintained for 15 min, then, a pressure of 50 MPa was applied, and sintering was performed for 15 min. Initial powders, as well as green bodies, were well characterized via different methods. Also, the microstructure, physical, and mechanical properties of sintered samples were investigated. Results showed that the optimal concentration of tetramethylammonium hydroxide (TMAH) as a dispersant agent for slurry with 60 wt% of SiC was 2 wt%. In addition, the optimal milling time for slurries containing sintering aids was 24 h. The sintered SiC samples containing a mixture of Al2O3 and Y2O3 powder as a liquid phase sintering aid in a range of 5–10 wt% showed low sinterability and as a result, their microstructures contained pores. In contrast, the use of B4C powder as a solid phase sintering aid in a range of 5–10 wt% resulted in forming of sintered SiC samples with completely uniform and dense structures without visible pores. The optimal percentage of B4C was 7 wt%, and the relative density and the hardness of the corresponding sintered sample were 99.5% and 3300 ± 20 Hv, respectively.