Microstructure and Properties of Porous Si 3 N 4 Ceramics with a Dense Surface

Porous Si 3 N 4 ceramics with a dense surface are fabricated by a joint process of pressureless sintering and chemical vapor deposition (CVD). Before CVD, the Si 3 N 4 ceramic shows a uniform microstructure with well‐distributed pores. During CVD, the depositions of Si 3 N 4 in the inner surface and on the external surface of the ceramics lead to a decrease of porosities and pore sizes. As a result, both the flexural strength and fracture toughness of the Si 3 N 4 ceramic increase by >30%, the surface hardness increases more than five times, and the dielectric constant and loss increase slightly.