Middle Temperature Densification Of Si₃N₄/LiAlSiO₄ Composite Material By SPS Process

Spark plasma sintering (SPS) of a-Si3N4 through the melt of lithium aluminosilicate (LiAlSiO4) glass and the simple sol-gel mechanism of synthesis of lithium yttrium oxide (LiYO?) were investigated in this study. Despite the excellent thermomechanical properties of silicon nitride, such as high elevated-temperature strength, low coefficient of thermal expansion, and high resistance to thermal shock and chemical attack; densification of Si3N4 is still a valuable challenge because of the strongly covalent character of the Si-N bond, which results in an extremely low self diffusivity. Hence, density near to its theoretical value was recently obtained by liquid-phase sintering, which enhances the mass transport of Si3N4. However, the high cost of sintering additives as well as high sintering temperatures yield to limit the application of Si3N4 based ceramics. Therefore, the continuous search for higher efficiency at a low-cost for manufacturing processes of both, sintering additives and final product was our main objective.Sol-gel synthesis of lithium yttrium oxide (LiYO2) and solid-state preparation of lithium aluminosilicate (LiAlSiO4) which are promising additives for middle temperatures sintering of ceramics materials were investigated. In fact, LiYO2 is an environmental material having carbon absorbent ability and insolating coating applications. The synthesize of this component was performed with higher lithium precursor content, which provided purer LiYO2 powders by calcining the amorphous powders from the sol-gel process at lower temperature, compared to those synthesized by solid-state reaction. The optimum experimental conditions for sol-gel preparation process is 1:3 and 1:2 molar ratio of [Li(CH3COO)·2H2O]/[Y(NO3)3·6H2O] at 950℃and 1000℃respectively, with 6 hours of holding time. The reaction and synthesis mechanism for LiYO2 was analyzed and proposed. It was found that the calcination of dried gel generated exothermic reactions and synthesize of LiYO2 was performed by simple reaction of Li2O and Y2O3. Moreover, the preparation of nanocrystalline LiAlSiO4 was performed under high stress by planetary milling for 16 h using alumina ball after calcination of precursors made by lithium carbonate, alumina and silicon oxide at 1100℃for 2 h with a right composition of 1:1.38:1.63. Final powder with average size of about 400 nm was obtained. The powder showed suitable distribution of particle with different shapes.Furthermore, the processing of densification of Si3N4/LiAlSiO4 by SPS process at middle temperature up to 1500℃gave significant results at above 1350℃with the increase of bulk density to more than 95% of the theoretical density with only minorα/β-Si3N4 phase transformation. Results also showed that linear shrinkage increased with the increase of temperature and additive content and the liquid phase was not dominant during heat treatment and insufficiency volume of inhibit theα/βphase transformation involving incomplete densification for most of samples. Therefore, densification was mostly completed by the plasma formation under the effect of dielectric properties of LiAlSiO4 at above 1300℃which generate spark plasma. About 0.2% of pores remained in the compact bodies after annealing time up to 30 min for samples having 20 and 30 wt.% of additive. The sample 30 wt.%LAS/Si3N4 sintered at 1450℃for 15 min gave almost fully material (relative density of 98.5%) with Vickers hardness, Hv=13.34±0.50 GPa and toughness, KIC=6.34MPam1/2 after fast processing by SPS.