Silicon nitride with the aligned reinforcing grains was prepared by tape casting with addition of the silicon nitride whiskers and gas-pressure sintering at 2148 K. The microstructure and the mechanical properties of the sintered sample, including the fracture toughness and the three-point flexural strength, were highly anisotropic. Both the fracture toughness and the flexural strength were the highest when the crack-propagation direction was normal to the alignment direction. This result was interpreted from the laminate composite materials’ point of view. Although the large elongated grains were as long as 44.4 ± 12 μm and as wide as 5.1 ± 0.67 μm, they were not the fracture origins.

No abstract

A two-step sintering process is described in which the first step suppresses densification while allowing the ␣-to-␤ 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 ␤-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 ␣-to-␤ phase transformation is not hindered. This results in the distribution of large ␤-nuclei in a porous fine-grained ␤-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 ␤-nuclei also grew rapidly, which promoted the development of a bimodal microstructure.

No abstract

No abstract