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.