Low-temperature densification of nano Si-C powder containing Al-C additives prepared by high-energy ball-milling

This study investigates the low-temperature sintering of nano Si-C powder containing Al-C additives prepared by high-energy ball-milling. The synthesized powder was composed of ultra-fine beta-SiC crystallites (similar to 5 nm) and amorphous Si-C matrix. TEM-EDS analysis showed a relatively homogeneous distribution of Al in the synthesized powder (d(50): 170 nm). The relative density of SiC containing 6.5 wt% additives was 98.1% after sintering at 1650 degrees C for 30 min at a pressure of 20 MPa. The SiC could be densified at 1800 degrees C when the additive content decreased to 3.3 wt%. The Al content in the Si-C powders changed, e.g., from 4.11 to 2.6 wt%, after sintering at 1650 degrees C, which value was much higher than the solubility limit (0.26 wt% at 1800 degrees C) due to the homogeneous distribution of Al within the powder and the low sintering temperature. Al was segregated at the grain boundary while liquid phase formation was not identified by TEM analysis, indicating that grain boundary diffusion was the main densification mechanism. The mechanical properties of the sintered specimens were similar to those of a SiC-A1(4)SiC(4) system, which has the same chemical components but containing large amount of sintering additives (5.6 vs. 13 wt%), sintered using higher pressure (20 vs. 60 MPa) and temperature (1650 vs. 1800 degrees C).