Development of Silicon Nitride of Improved Toughness.

The application of reaction sintered Si3N4 energy absorbing surface layers to hot-pressed Si3N4 was investigated. The surface layer was formed by in-place nitridation of silicon powder. It was found that reaction sintered Si3N4 layers of 1 mm thickness, fabricated from either -100, 200, -200, or -325 mesh Si powder and nitrided in 96 N24 H2 so that approximately 20-25 vol unnitrided Si remained in the layer, resulted in a sevenfold iNCrease in ballistic impact resistance of a 0.64 cm thick hot-pressed Si3N4 substrate from RT to 1370 C. Both Nc-132 Si3N4, with mgO additive, and NCX-34 Si3N4, with Y2O3 additive, were evaluated as substrate material. The finer grain size -200 and -325 mesh nitrided Si layers were found to be preferable for their smoothness and relatively high density. It was found that nitriding in N2H2 mixtures, rather than pure N2, resulted in a microstructure that did not substantially degrade the strength of the hot-pressed Si3N4 substrate.