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

ZrB2–SiC composites doped with 0, 1, 3 and 5wt.% SiAlON were prepared by hot pressing (under a low pressure of 10MPa) and pressureless sintering processes at 1900°C for 2h. A fractographical approach was employed to investigate the effects of SiAlON addition on the densification behavior in such ceramic composites. Several reported chemical reactions were investigated to indicate the probable mechanisms in the progression of the sintering process and also the potential unfavorable effects of SiAlON in ZrB2–SiC composites. On one hand, results show that the SiAlON addition promotes the densification in the hot pressed samples by the liquid phase formation, as a fully dense composite can be obtained by hot pressing of 5wt.% SiAlON doped ZrB2–SiC composite. On the other hand, in the pressureless sintering process, more SiAlON content intensifies the formation of gasses which leads to more porosity. In both processes, investigation of fracture surfaces demonstrates the presence of thin layers of glassy phases which act as the sintering aid. The steps of densification process in both hot pressing and pressureless sintering processes were presented by a graphical model. •The effect of SiAlON addition on densification of ZrB2–SiC composites was studied.•SiAlON promotes the densification in hot pressed samples by liquid phase formation.•A fully dense sample was obtained by hot pressing of 5wt.% SiAlON doped composite.•In pressureless sintering, more SiAlON content intensifies the formation of pores.