Fabrication and Properties of Pressureless Sintered Si_3N_4/BN Composite Ceramics with La_2O_3-Al_2O_3-Y_2O_3 as Sintering Additives

摘要

With commercial α-Si_3N_4 and h-BN powders as starting materials, La_2O_3, Al_2O_3, Y_2O_3 as sintering additives, Si_3N_4/BN composite ceramics with 25vol% h-BN were fabricated by pressureless sintering. Various amounts of La_2O_3 (0, 2, 4, 8, 15wt%) were added, with constant Y_2O_3/Al_2O_3 weight ratio and additives (Y_2O_3/Al_2O_3/La_2O_3) amount. The densification behaviors, α-β transformation and room-temperature strength of Si_3N_4/BN composite were investigated. The porosity of samples decreased with La_2O_3 content increasing, and the lowest porosity of 20.83% was observed for samples containing 4wt% La_2O_3, 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% La_2O_3 was added. Results of XRD patterns revealed that β-Si_3N_4 and h-BN existed in all the specimens. No α-Si_3N_4 was detected, implying that a- to β-Si_3N_4 transformation has been completed during the pressureless sintering process. These results show that the La_2O_3-Al_2O_3-Y_2O_3 system can act as effective sintering additives for pressureless sintered Si_3N_4/BN composite.