Fabrication and modelling of Si3N4 ceramics with radial grain alignment generated through centripetal sinter-forging

Silicon nitride (Si3N4) based ceramics are one of the most attractive advanced engineering materials, which have been widely used under high-speed rotational operation or for mechanical contacts across a curved surface. In the present study, rotationally symmetric texturing of Si3N4, with radial grain alignment, was obtained by centripetal sinter-forging (CSF) of a partially sintered sample. The average values of the included angles between the c-axis of the local Si3N4 grain and radial direction were approximately 16.4° and 11.0°, on the section plane perpendicular to the pressing direction, and parallel to both the pressing and radial directions, respectively. The compressive strain in the pressing direction forced the ceramic body to flow towards the central axis, resulting in compressive strain in the tangential direction and tensile strain in the radial direction. A fundamental physical model was created to simulate the grain rotation during the 3-dimentional strain reorientation, which revealed the rod-like grain would preferentially rotate toward the center of the sample under the CSF process. In addition, due to the friction between the sample surface and the pressing punch, the increased shear strain could enhance the Si3N4 grain alignment. Consequently, ceramics with rod-like grains perpendicular to the curved side surface could be anticipated by applying the centripetal forming concept in a controlled manner.