Sintering behavior of ZrB2-SiC composites doped with Si3N4: A fractographical approach

ZrB2-SiC composite ceramics were doped with 0, 1, 3 and 5 wt% Si3N4 plus 1.6 wt% carbon (pyrolized phenolic resin) as sintering aids and fabricated by hot pressing process under a relatively low pressure of 10 MPa at 1900 degrees C for 2 h. For a comparative study, similar ceramic compositions were also prepared by pressureless sintering route in the same processing conditions, with no applied external pressure. The effect of silicon nitride dopant on the microstructural evolution and sintering process of such ceramic composites was investigated by a fractographical approach as well as a thermodynamical analysis. The relative density increased by the addition of Si3N4 in hot pressed samples as a fully dense composite was achieved by adding 5 wt% silicon nitride. A reverse trend was observed in pressureless sintered composites and the relative density values decreased by further addition of Si3N4, due to the formation of gaseous products which resulted in the entrapment of more porosities in the final structure. The formation of ZrC phases in pressureless sintered samples and layered BN structures in hot pressed ceramics was detected by HRXRD method and discussed by fractographical SEM-EDS as well as thermodynamical analyses.