The two-tier tissue reinforcing and toughening mechanism caused by dual-granularity Si powders ingredient, and dielectric properties in reaction bonded and sintered reaction bonded Si3N4 porous ceramics

Abstract

With the rapid development of hypersonic vehicles and broadband wave-transparent radome, Si3N4 porous ceramics (Si3N4-PC) have attracted attention due to their excellent intrinsic properties of Si3N4 and high porosity. However, its high porosity results in low strength and toughness, which are fundamental properties for radome. Reaction bonded (RB) Si3N4-PC has advantages of dielectric properties and cost over general phase transformation sintering (PTS) and sintered reaction bonded (SRB) Si3N4-PC while it has been neglected in recent years. In this study, RB and SRB Si3N4-PC prepared by non-aqueous gelcasting and the influence of Si powders ingredient on their properties are discussed in order to illuminate the potential of RB Si3N4-PC in wave-transparent materials. The results show that RB Si3N4-PC with dual-granularity ingredients of 5 μm & 45 μm produces a two-tier tissue of framework of coarse whiskers enhanced by a network of tiny whiskers. SRB Si3N4-PC evolves into a two-tier tissue of framework of columnar and rod-like grains joining together to brace each other. The two-tier microscopic tissue strongly reinforces and toughens the structure and results in higher σF and γwof. As a result, the RB and SRB Si3N4-PC of dual-granularity of 5 μm & 45 μm obtain the maximum σF of 109.94 MPa and 119.56 MPa as well as maximum γwof of 990.74 J m-2 and 1167.88 J m-2, respectively. Furthermore, the ε′ and tanδ of RB and SRB Si3N4-PC of dual-granularity of 5 μm & 45 μm are about 4.20 and 4.52 as well as 7.01 × 10-3 and 22.90 × 10-3, respectively. It is concluded that RB Si3N4-PC of dual-granularity has good mechanical and dielectric properties, which are favorable for radome.