Effect of temperature and pre-sintering on phase transformation, texture and mechanical properties of silicon nitride ceramics

Silicon nitride composite ceramics with excellent mechanical properties were prepared by hot pressing. The effects of temperature on the sintering process, β-Si3N4 phase transformation ratio (β%) and mechanical properties were investigated. By calculating the diffraction intensity (I), the β-Si3N4 phase transformation ratios of the three samples considered in this study were 57.2% (pre-sintered samples, PSN), 63.5% (urea homogeneous precipitation method, USN) and 66.6% (mechanical milling method, MSN) at 1550 °C, respectively. These results showed that the Si3N4 composites coated by using Y-Al precursor (USN) have a lower ratio of the phase transformation from α to β. Meanwhile, from the analysis of SEM images of the etched surface, the grain sizes were 0.47 µm (PSN), 0.39 µm (USN), and 0.43 µm (MSN) at 1650 °C, respectively. Compared to the other samples, the USN sample offered superior sintering behaviour and mechanical properties. The optimal flexural strength, hardness and fracture toughness of the Si3N4 composites coated by using the Y-Al precursor were 911 ± 69 MPa, 16.23 ± 0.24 GPa and 4.9 ± 0.35 MPa/m2 at 1650 °C, respectively.