Papers by Author: Yong Feng Li

Abstract: Porous silicon nitride ceramics with various amounts (25, 35, and 45 vol %) of hexagonal boron nitride (h-BN) were fabricated at 1800°C for 2h by the pressureless sintering process. With FESEM and TEM, the effects of h-BN on the microstructure and mechanical properties of Si3N4 ceramics were investigated. Results of the microstructure and mechanical properties of Si3N4/BN composites showed that the growth of the elongated β-Si3N4 were hindered by h-BN additive, which resulted in the decrease of fracture toughness of Si3N4/BN ceramics with increasing h-BN content. The morphologies of the fracture surfaces by FESEM revealed the fracture mode for Si3N4/BN composites to be intergranular. However, phase analysis by XRD indicated that the effect of h-BN on the α- to β- Si3N4 phase transformation of Si3N4/BN composites was negligible.

Abstract: Si3N4/BN composite ceramics with 25vol% h-BN were prepared by pressure-less sintering process with Nd2O3/Al2O3/Y2O3 as sintering additives. The effects of these ternary additives on the densification behaviors and mechanical properties were investigated. XRD and FESEM were used to investigate the α-β phase transformation and microstructure. The XRD results showed that α-Si3N4 has transformed to β-Si3N4 completely in all the samples during the pressureless sintering process. The line shrinkage increased with the Nd2O3 contents increasing, and the highest line shrinkage (7.75%) was observed when 4wt% Nd2O3 was added, then decreased. The same trends were observed in flexural strength and fracture toughness testing. The ternary additives of Y2O3-Al2O3-Nd2O3 could improve the density, strength and fracture toughness of the material effectively.

Abstract: With commercial α-Si3N4 and h-BN powders as starting materials, La2O3, Al2O3, Y2O3 as sintering additives, Si3N4/BN composite ceramics with 25vol% h-BN were fabricated by pressureless sintering. Various amounts of La2O3 (0, 2, 4, 8, 15wt%) were added, with constant Y2O3/Al2O3 weight ratio and additives (Y2O3/Al2O3/La2O3) amount. The densification behaviors, α-βtransformation and room-temperature strength of Si3N4/BN composite were investigated. The porosity of samples decreased with La2O3 content increasing, and the lowest porosity of 20.83% was observed for samples containing 4wt% La2O3, then leaded to an increase. The flexural strength of all the specimens increased with the addition up to 4wt% and changed greatly thereafter. The highest room-temperature flexural strength, 272.4MPa, was obtained when 4wt% La2O3 was added. Results of XRD patterns revealed that β-Si3N4 and h-BN existed in all the specimens. No α-Si3N4 was detected, implying thatα- toβ-Si3N4 transformation has been completed during the pressureless sintering process. These results show that the La2O3-Al2O3-Y2O3 system can act as effective sintering additives for pressureless sintered Si3N4/BN composite.