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

With the rapid development of hypersonic vehicles and broadband wave-transparent radome, Si 3 N 4 porous ceramics (Si 3 N 4 -PC) have attracted attention due to their excellent intrinsic properties of Si 3 N 4 and high porosity. However, its high porosity results in low strength and toughness, which are fundamental properties for radome. Reaction bonded (RB) Si 3 N 4 -PC has advantages of dielectric properties and cost over general phase transformation sintering (PTS) and sintered reaction bonded (SRB) Si 3 N 4 -PC while it has been neglected in recent years. In this study, RB and SRB Si 3 N 4 -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 Si 3 N 4 -PC in wave-transparent materials. The results show that RB Si 3 N 4 -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 Si 3 N 4 -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 σ F and γ wof . As a result, the RB and SRB Si 3 N 4 -PC of dual-granularity of 5 μm & 45 μm obtain the maximum σ F of 109.94 MPa and 119.56 MPa as well as maximum γ wof of 990.74 J m -2 and 1167.88 J m -2 , respectively. Furthermore, the ε′ and tanδ of RB and SRB Si 3 N 4 -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 Si 3 N 4 -PC of dual-granularity has good mechanical and dielectric properties, which are favorable for radome.