Peculiarities of SPS nanostructured ceramics based on Si3N4 in the presence of varying liquid phase amounts

DOI: 10.62564/M4-MZ1126

Maryna Zamula, Valerii Kolesnichenko, Nadiya Tyschenko, Oleksandr Shyrokov, Artur Stepanenko, Hanna Borodianska, Andriy Ragulya

Frantsevich Institute for Problems of Materials Science National Academy of Science of Ukraine


Si3N4 displays excellent tribological and mechanical properties, making it promising for high-temperature applications [1]. To achieve high strength and fracture toughness, the formation of anisometric grains of β-Si3N4 due to polymorphic α→β-phase transformation during sintering of Si3N4 is crucial. This study aims to investigate the phase transformation in Si3N4 and the consolidation of materials using the spark plasma sintering (SPS) method in the presence of varying amounts of liquid phase. The starting mixtures of Si3N4 with varying ratios of oxide additives (1-7wt.% Y2O3 and ~10wt.% SiO2) were prepared by ball milling for 4 hours at 100 rpm [2]. At a SPS temperature of 1800°C, the addition of 1.5wt.%Y2O3 to Si3N4 requires more than 30 min for complete shrinkage. The sample has a remaining open porosity of 6.4%, meaning there is not enough liquid phase for complete densification. Adding 3wt.%Y2O3 allows for consolidation in about 20 min, resulting in almost pore-free ceramics. Adding 7wt.% of Y2O3 reduces the holding time to reach 0% open porosity to 15 min. The analysis of the ceramic microstructure indicates that the average grain size in sintered Si3N4 ceramics increases from 300 to about 500 nm when the quantity of Y2O3 is raised from 1.5 to 7wt.%. This increase in grain size is likely due to a longer exposure time at consolidation temperature. All ceramics have anisometric grains with a length of 1–2μm, corresponding to β-Si3N4 phase. According to the XRD results, no α-Si3N4 phase was detected in any of the sintered samples. However, a significant content (~50vol.%) of the Si2N2O phase was observed. Thus, there is a correlation between the exposure time and the amount of liquid phase formed during sintering, which changes with the amount of Y2O3 additive at a fixed amount of SiO2. Therefore, the significant volume of the liquid phase promotes densification under pressure via the mechanism of rearrangement of Si3N4 nanoparticles to obtain a dense ceramic.

Keywords
Silicon nitride, SPS, nanostructured ceramic, phase transformation, Si2N2O

Acknowledgments
Not provided

References
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