Microstructure Control And Mechanical Properties Optimization Of Silicon Nitride Based Ceramics

Amorphous BaAl₂Si₂O₈ (BAS) and Ba_0.75Sr_0.25Al₂Si₂O₈ (BSAS) powders were synthesized by a sol-gel process. The effect of Sr on the non-isothermal crystallization kinetics and hexacelsian to celsian phase transformation were studied by DSC and heating at 1450℃for 30min to 210min. Si₃N₄ reinforced BAS/BSAS composites were hot-pressed, using the sol-gel BAS, BSAS, andα-Si₃N₄ as starting powders. Phase compatibility research results suggested that hexacelsian was stabilized by the presence ofβ-Si₃N₄, and consequently the desired celsian phase was unobtainable in the Si₃N₄/BAS system.Si₃N₄ reinforced BAS/BSAS glass ceramic matrix composites of optimal compositions were synthesized by spark plasma sintering (SPS, 1800℃/5min), pressureless sintering (PLS, 1900℃/60min), and hot pressing (HP1, 1800℃/80min, and HP2, 1700℃/40min+1800℃/120min), using oxides as the BAS/BSAS precursors. Post heat-treatment at 1800℃for 60 min in 2.0MPa nitrogen was held. The effects of the transient liquid, sintering techniques, and heat-treatment, on the microstructure and thereof the mechanical performances of the composites was elucidated. The results indicated that (1) SPS and HP increased theα→β-Si₃N₄ phase transformation rate because of the promoted wetibility by the external pressure in comparison to PLS; (2) In case of SPS, Si₃N₄/BAS and Si₃N₄/BSAS composites were densified rapidly within 5min during sintering, butα→β-Si₃N₄ phase transformation was completed only in the Si₃N₄/BAS system; (3) By manipulating the grain growth driving force through a two-step hot-pressing or through the combination of SPS with heat-treatment, the abnormal growth of theβ-Si₃N₄ nuclei in the starting powders resulted in a bimodal microstructure, hence enhancement of mechanical properties; (4) When microstructure optimization and thermal mismatch stress release were achieved by controlled heat-treatment, high flexure strength and fracture toughness could reach 968MPa and 8.9MPam1/2 respectively. Load transfer, bridging, and crack deflection were the main reinforcing mechanisms revealed by fracture SEM observation.Dual-phase (α+β)-SiAlON with 10wt% and 40wt%α-SiAlON were obtained using 10wt% BAS as additives via hot pressing. Elongatedβ-SiAlON grains with large aspect ratio over 10 dispersing homogeneously in the BAS andα-SiAlON matrix was the microstructure feature of the composites. BAS intergranular phase benefited the interfacial debonding, crack deflection, pull-out, and bridging of the largeβ-SiAlON grains, imparting high toughness to the materials.Self-reinforcedα-SiAlON ceramics of RE_1/3Si₁₀Al₂ON₁₅ composition with 5wt% BAS additive were fabricated using hot-pressing. The effects of sintering parameters, post heat-treatment, types of rare-earths and dual-rare-earth dopants on the phase assemblage development were systemically manifested. The presence of metastableβ-SiAlON and its elimination by means of sintering process control and post sintering heat-treatment were established. The successful application of the three-step sintering of 1200℃/2h-1500℃/1h-1800℃/1h to elude the dissolve of rare earth, and AlN into the BAS liquid yieldedα-SiAlON/BAS(5wt%) ceramics of ideal phase constituents. High aspect ratio ofα-SiAlON grains from the elongated growth which was secured by the steady BAS liquid during sintering, guaranteed the self-reinforcing of the composites. Microstructure and mechanical properties could be further improved by tailoring the rare-earth dopants and the sintering-heat treatment procedures. Small amount of rare-earth dissolving in the BAS accelerated its crystallization and the hexacelsian to celsian phase transformation, which should have positive effects on the high temperature strength of the SiAlON.