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Abstract: Fine-grained silicon nitride ceramics were investigated mainly for their high-strain-rate plasticity. The preparation and densification of fine silicon nitride powder were reviewed. Commercial sub-micrometer powder was used as raw powder in the “as-received” state and then used after being ground and undergoing classification operation. Chemical vapor deposition and plasma processes were used for fabricating nanopowder because a further reduction in grain size caused by grinding had limitations. More recently, nanopowder has also been obtained by high-energy milling. This process in principle is the same as conventional planetary milling. For densification, primarily hot pressing was performed, although a similar process known as spark plasma sintering (SPS) has also recently been used. One of the advantages of SPS is its high heating rate. The high heating rate is advantageous because it reduces sintering time, achieving densification without grain growth. We prepared silicon nitride nanopowder by high-energy milling and then obtained nanoceramics by densifying the nanopowder by SPS. [Copyright &y& Elsevier]

We report the preparation of nanosized silicon nitride (Si3N4) ceramics via high-energy mechanical milling and subsequent spark plasma sintering. A starting powder mixture consisting of ultrafine β-Si3N4 and sintering additives of 5-mol% Y2O3 and 2-mol% Al2O3 was prepared by high-energy mechanical milling. After milling, the powder mixture was mostly transformed into a non-equilibrium amorphous phase containing a large quantity of well-dispersed nanocryslalline β-Si3N4 particles. This powder precursor was then consolidated by spark plasma sintering at a temperature as low as 1600°C for 5 min at a heating rate of 300°C/min. The fully densified sample consisted of homogeneous nano-Si3N4 grains with an average diameter of about 70 nm, which led to noticeable high-temperature ductility and elevated hardness. [ABSTRACT FROM AUTHOR]

A feasible doping strategy is introduced to synthesize Eu2+-doped α-Si3N4 nanowires coated with a thin BN film. The nanowires were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and a fluorescence spectrophotometer. The Eu2+-doped α-Si3N4 nanowires emitted strong yellow light, which is related to the 4 f65 d–4 f7 transition of Eu2+, upon a broad excitation wavelength range between 250 and 450 nm. The obtained nanowires provided a potential candidate for application in optical nanodevices, as well as in white LEDs. [ABSTRACT FROM AUTHOR]

The effect of sintering additives on superplastic deformation of nano-sized β-Si3N4 ceramics has been studied by compression tests at 1500°C. The sintering additives were (i) Y2O3+Al2O3; (ii) Y2O3+MgO; and (iii) Y2O3. Nano-sized Si3N4 ceramics with different sintering additives had similar microstructures. For the first two sintering additives, the stress exponents were determined to be ∼2 at a lower stress region and ∼1 at a higher stress region, where the strain rate was dependent on sintering additives only at the higher stress region, and was independent at the lower stress region. Nano-ceramics with Y2O3 additives had only one region, which had a stress exponent of ∼1 within the stress range that we studied. The results could be explained by the different deformation mechanisms at the higher and lower stress regions and the influence of viscosity of liquid phase on the transition stress. [ABSTRACT FROM AUTHOR]

Abstract: The superplastic deformation of nano-sized (∼68nm) Si3N4 ceramics has been studied in compression over a wide range of strain rates and temperatures. The experimental results revealed, for the first time, a transition in the stress exponent (n) at each temperature. The n values decreased from ∼2 to ∼1 with increasing stress. The activation energy was also different for the two regions, decreasing from 858.2kJ/mol in the n ∼2 region to 571.8kJ/mol in the n ∼1 region. Concurrent grain growth occurred during deformation tests; however, grain growth was not accompanied by obvious strain hardening due to the strong texture developed. Assessment of possible rate controlling deformation mechanism indicated that in the n ∼2 region, superplastic behavior was considered to be controlled by the interface-reaction controlled solution-precipitation process with two-dimensional nucleation, whereas deformation in the n ∼1 region was controlled by a grain boundary sliding process accommodated by a diffusion-controlled solution-precipitation. [Copyright &y& Elsevier]

Compressive superplastic deformations of nano-size Si3N4 ceramics with different amounts of sintering additives were investigated at 1500°C. The stress exponents of all specimens were determined to be ∼2 in the lower stress region, and ∼1 in the higher stress region, while the dependency of strain rates on additive contents was observed only in the higher stress region. This indicates that the deformation was controlled by a diffusion process in the higher stress region and by an interface reaction process in the lower stress region. [Copyright &y& Elsevier]