Silicon nitride ceramics for applications in demanding environments require high toughness and adequate hardness. A well known route to making tough silicon nitride compositions is to control the grain size distribution. For beta silicon nitrides, the grain shapes in the form of their acicularity is known to be very important too. In this paper, we report on the use of multiple strategies to achieve increased toughness and toughening in silicon nitrides. These strategies include the use of a blend of nano-scale and micron-scale silicon nitride powders, the use of nano-scale sintering aids, and the addition of carbon nanotubes. Microstructures and mechanical properties are determined for these hot-pressed ceramics and are compared with a baseline silicon nitride prepared with conventional micron-scale powders. Hardness and fracture toughness are determined at room temperature using hardness indents produced by a macro Vickers hardness indenter. The toughening ability of these ceramics are compared by R-curve measurements. Grain boundary debonding and crack path deviation are identified as toughening mechanisms.

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