Novel Two‐Step Sintering Process to Obtain a Bimodal Microstructure in Silicon Nitride

A two‐step sintering process is described in which the first step suppresses densification while allowing the α‐to‐β phase transformation to proceed, and the second step, at higher temperatures, promotes densification and grain growth. This process allows one to obtain a bimodal microstructure in Si 3 N 4 without using β‐Si 3 N 4 seed crystals. A carbothermal reduction process was used in the first step to modify the densification and transformation rates of the compacts consisting of Si 3 N 4 , Y 2 O 3 , Al 2 O 3 , and a carbon mixture. The carbothermal reduction process reduces the oxygen:nitrogen ratio of the Y‐Si‐Al‐O‐N glass that forms, which leads to the precipitation of crystalline oxynitride phases, in particular, the apatite phase. Precipitation of the apatite phase reduces the amount of liquid phase and retards the densification process up to 1750°C; however, the α‐to‐β phase transformation is not hindered. This results in the distribution of large β‐nuclei in a porous fine‐grained β‐Si 3 N 4 matrix. Above 1750°C, liquid formed by the melting of apatite resulted in a rapid increase in densification rates, and the larger β‐nuclei also grew rapidly, which promoted the development of a bimodal microstructure.