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

ZrB₂-SiC composites doped with 0, 1, 3 and 5 wt.% SiAlON were prepared by hot pressing (under a low pressure of 10 MPa) and pressureless sintering processes at 1900 °C for 2 h. 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 5 wt.% 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.