Thesis of Zohre Ahmadi

Effect of nitride sintering aids on densification and microstructure of ZrB2-SiC composites

By zohre ahmadi

Supervisor: Dr. M. Gassemi Kakroudi

Advisor: Dr. M. Rezvani

University of Tabriz

September 2015

Abstract: ZrB2–SiC composites doped with 0, 1, 3 and 5 wt% SiAlON and AlN (Individually) were prepared by hot pressing (under a low pressure of 10 MPa) and pressureless sintering processes at 1900 ̊C for 2 hours. A fractographical approach was employed to investigate the effects of SiAlON and AlN addition on the densification behavior in such ceramic composites.

In the case of SiAlON, 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.

In the case of AlN, 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 metakaolinite spinel. In the pressureless sintering method, adding more AlN can increase the formation of gas products which raised the amount of porosities in the final microstructure. The formation of graphite 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 analysis.