Fractographical characterization of hot pressed and pressureless sintered AlN-doped ZrB2–SiC composites

• The effect of AlN addition on densification of ZrB2–SiC composites was studied.
• AlN promotes the densification in hot pressed samples by liquid phase formation.
• A fully dense composite was obtained by adding 1 wt.% AlN in hot pressing process.
• In pressureless sintering, more AlN content intensifies the formation of porosities.

In this paper, ZrB2–SiC composites doped with 0–5 wt.% AlN were prepared by a low pressure hot pressing as well as a pressureless sintering methods at 1900 °C for 2 h. The influence of aluminum nitride addition on the sinterability and microstructure development of such ceramic composites was studied by a fractographical approach. The results revealed that only 1 wt.% AlN can aid the densification process of the hot pressed ceramic composite via the liquid phase sintering mechanism due to the formation of nano-scale metakaolinite spinel layers. In the pressureless sintering method, adding more AlN can increase the formation of gaseous products which raised the amount of porosities in the final microstructure. The formation of nano-graphite phase in the hot pressing process, the formation of Al2OC in the pressureless sintering process, and the formation of BN in the both processes were disclosed by X-ray diffraction, SEM and TEM analyses.