Effect of sintering temperature on hardness gradient of Si3N4 ceramics fabricated by spark plasma sintering

The SPS technique has broad prospects for application in the design of Si3N4 materials with customized microstructures and properties. The continuously symmetrical graded silicon nitride ceramics with hard edge and tough center were prepared by spark plasma sintering at different sintering temperatures. By means of experiment and COMSOL simulation, it was revealed that different sintering temperatures created different gradient temperature fields of the samples, which further affected the transformation amount of the internal α-Si3N4 phase. The hardness and fracture toughness values were distributed in gradient, and the gradient between the center and the edge increased first and then stabilized with the temperature rising. The hardness value of silicon nitride changed between 16.93 ± 0.11 GPa and 21.16 ± 0.13 GPa, and the fracture toughness value varied between 3.92 ± 0.12 MPa∙m1/2 and 6.94 ± 0.11 MPa∙m1/2.When the sintering temperature exceeded 1800 °C, the hardness gradient of the sample was reduced, possibly due to the oxidation of silicon nitride. Finally, the mechanism of the influence of sintering temperature on the preparation of gradient silicon nitride ceramic materials was proposed.