Microstructure and properties of porous si₃N₄ Ceramics with a dense surface

Porous Si₃N₄ ceramics with a dense surface are fabricated by a joint process of pressureless sintering and chemical vapor deposition (CVD). Before CVD, the Si₃N₄ ceramic shows a uniform microstructure with well-distributed pores. During CVD, the depositions of Si₃N₄ 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 Si₃N ₄ ceramic increase by >30%, the surface hardness increases more than five times, and the dielectric constant and loss increase slightly.