Effect of AlN and ZrO₂ on the microstructure and property of the 2Si-B-3C-N ceramic

The mechanically-alloyed amorphous 2Si-B-3C-N powder was hot pressed at 1900 °C for 30 mins in argon. A small amount (5 mol.%) of AlN and ZrO₂ was added to investigate their effect on the sintering process, microstructure and properties of the 2Si-B-3C-N ceramic. XRD, SEM, TEM and property testing results show that, the addition of AlN or ZrO₂ promotes the sintering process of the 2Si-B-3C-N composite powder, accelerates the crystallization and grain growth, and improves the density and mechanical properties of the prepared ceramic. Under an axial pressure of 80 MPa and a gas pressure of 8 bar, significant densification occurs to the 2Si-B-3C-N ceramic free from any additives just when the temperature is higher than 1830 °C. The addition of AlN and ZrO₂ decreases this critical temperature to about 1760 °C and 1730 °C, respectively. As a result, for the prepared bulk ceramic, the average grain size is increased from 80 nm to 200 nm and 250 nm, the relative density is enhanced from 88.7% to 96.9% and 97.7%, and the bending strength is improved from 331 MPa to 416 MPa and 575 MPa, respectively. Careful investigation reveals that in the prepared bulk ceramic, AlN forms solid solution with SiC, and ZrO₂ exists in the form of t-ZrO₂ and m-ZrO₂ nanocrystals. Thus, the addition of AlN and ZrO₂ may facilitate the atomic diffusion, the grain growth and the densification process of the 2Si-B-3C-N ceramic in difficult ways.