Fabrication and Properties of Pressureless Sintered Si3N4/BN Composite Ceramics with La2O3-Al2O3-Y2O3 as Sintering Additives

With commercial alpha-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, alpha-beta 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 beta-Si3N4 and h-BN existed in all the specimens. No alpha-Si3N4 was detected, implying that alpha- to beta-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.