Microstructure Of Si3N4 Ceramics Research Articles

Preparation of high-strength Si3N4 antenna window using selective laser sintering

In this study, a high-strength silicon nitride (Si3N4) antenna window was successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding before final sintering. The effects of CIP after debinding and sintering aids on the bulk density, total porosity, bending strength and microstructure of Si3N4 ceramics were examined. The results show that the bending strength of SLS Si3N4 ceramics can be greatly improved by adding sintering aids between Si3N4 granules and by CIP after debinding. Optimal performance of ceramics is obtained by CIP after debinding and the use of inter-granule sintering aids. The porosity, bulk density, and bending strength are 18.7%, 3.11 g/cm3, and 685 MPa, respectively. Eliminating the pores by the CIP after debinding and by inter-granule sintering aids promotes the growth of rod-like β-Si3N4, which lock with each other contribute to the strengthening of Si3N4 ceramics.

Effects of sintering additives on mechanical properties and microstructure of Si3N4 ceramics by microwave sintering

Si3N4 ceramics with excellent mechanical properties and microstructure were fabricated by microwave sintering. The effects of sintering additives on the densification, phase transformation and grain growth of Si3N4 ceramics were investigated. The results showed that Y2O3 can promote the phase transformation, MgO can decrease the sintering temperature, Al2O3 can increase the Vickers hardness of the samples and the combination of Y2O3 and MgO was more effective in enhancing the densification and phase transformation in the microwave sintering. The samples with 5wt%Y2O3, 5wt%MgO and 2wt%Al2O3 sintering additives had the optimum mechanical properties when sintered at 1700℃ for 10min. Its relative density, fracture toughness and Vickers hardness were 98.52±0.13%, 6.44±0.02MPam1/2 and 14.92±0.20GPa, respectively. Compared to other microwave sintered Si3N4, the hardness and fracture toughness were enhanced by 6.6–24.3% and 5.6–11%, respectively, while the sintering holding time was reduced by 33.3–83.3%.

Preparation and Properties of Si<sub>3</sub>N<sub>4</sub>-SiC Radar Wave-Absorbing Nanocomposites

The vacuum-sintering technology was adopted to prepared Si3N4-SiC composite ceramic, a type of Radar Wave-Absorbing materials (RAW). The mechanical performances of the composite materials were tested and the influences of SiC powers on the mechanics performances and microstructure of Si3N4 ceramics were investigated. The results show that the sintering density, the content of size ofβ-Si3N4 and the strength of Si3N4-SiC all decrease along with the increase of SiC powers in sample, compared with the pure matrix Si3N4 .Meanwhile, Relationship between SiC content and relative permittivity of Si3N4-SiC within the frequency range of 8.2–12.4 GHz (X-band) was studied. The average of relative permittivityεof Si3N4-SiC increased from 7.878 to 13.88 and the average of dielectric loss tanδincreased from 5.6×10-3 to 0.7869 when the content of SiC increased from 0 to 15 wt.%. The excellent microwave absorbing abilities of Si3N4–SiC ceramic were attributed to the interfacial polarization at interface between Si3N4 and SiC and at grain boundary between SiC nanocrystals.

Effect of pure β-Si3N4 powder on microstructure and mechanical properties of porous Si3N4 ceramics

Porous Si3N4 ceramics were produced by the initial Si3N4 powder added different amounts of β-Si3N4 powder. The density, porosity and flexural strength were tested by the methods of Archimedes method and three-point bending measurements, respectively. Phase composition and microstructure of Si3N4 ceramics were studied by X-ray diffraction and scanning electron microscopy. Fibrous β-Si3N4 grains developed in the porous microstructure. The grain morphology and size were affected by adding different amounts of β-Si3N4 powder. Fibrous β-Si3N4 grains dispersed in the porous Si3N4 ceramics uniformly with the length of 4∼10μm, the average diameter of 0.6μm and average aspect ratio of 7 were obtained. Adding 3 mass% β-Si3N4 powder, porous Si3N4 ceramics with excellent properties were prepared.

Microstructure and Performance of the Hot-Pressed Silicon Nitride Ceramics with Lu<sub>2</sub>O<sub>3</sub> Additives

The influence of Lu2O3 on phase transformation and seeds morphology was investigated. The result showed that the β-Si3N4 seeds with up to 95% β phase content could be obtained with 2wt% Lu2O3 as the additive content under 1750°C for two hours. The microstructure and mechanical properties of hot-pressed Si3N4 ceramics, using 9wt.% of Lu2O3• additives were investigated by the means of MTS measurements and Vickers indentation crack size measurements, as well as XRD and SEM. It was known that the high fracture toughness of Si3N4 ceramics was attributed to the rodlike morphology of β-Si3N4 grains. And the reinforcement effect and mechanism of β-Si3N4 seed were studied. It was found that the grain size and its distribution influence the property and microstructure of Si3N4 ceramics, namely, the relative narrow distribution of grain diameter in some extent and relative wide range of bimodal distribution of grain aspect ratio could improve the property of Si3N4 ceramics. The improvement in the fracture toughness with the amount of additive was mainly attributed to elongated grain growth during the sintering process.The high temperature properties of self-reinforced Si3N4 with different additives were studied. By this method, self-reinforced Si3N4 ceramics with an increment of 10~20 percent of fracture toughness was successfully fabricated.

The Role of Nanodispersed Nitrides in the Sintering of Silicon Nitride Ceramics

The synthesis of ceramics based on silicon nitride using nanopowders of TiN and Si3N4 as additives was studied. The ceramic compositions were pressurelessly sintered under nitrogen atmosphere at different temperatures (1550°C, 1650°C and 1750°C) with a heating rate of 10°C/min and a holding time of 2 h. The nanodispersed nitrides (NDN) were produced by electric-arc plasma synthesis and characterized. The ceramic composites obtained with nanoparticles of 1 wt% to 5 wt% TiN and 20 wt% Si3N4 were characterized by scanning electron microscopy (SEM), atom force microscopy (AFM) and energy-dispersive spectrometry (EDX). The effect of the addition of nanodispersed powders on the mechanical properties and microstructure of Si3N4 ceramics was investigated.

Influence of Yttria–Alumina Content on Sintering Behavior and Microstructure of Silicon Nitride Ceramics

The present study investigates the influence of the content of Y2O3–Al2O3 sintering additive on the sintering behavior and microstructure of Si3N4 ceramics. The Y2O3:Al2O3 ratio was fixed at 5:2, and sintering was conducted at temperatures of 1300°–1900°C. Increased sintering‐additive content enhanced densification via particle rearrangement; however, phase transformation and grain growth were unaffected by additive content. After phase transformation was almost complete, a substantial decrease in density was identified, which resulted from the impingement of rodlike β‐Si3N4 grain growth. Phase transformation and grain growth were concluded to occur through a solution–reprecipitation mechanism that was controlled by the interfacial reaction.

硬質材料 高強度窒化ケイ素セラミックスの開発と製品化

Due to its superior resistance to thermal shock, wear and oxidation, and high toughness, Si3N4 ceramics are expected to be used in industrial applications. However, before these Si3N4 ceramics can become better established on the market, their fracture strength, fracture toughness characteristics and reliabilty must be improved.In this study, the formation mechanisms of the inside pores at the pressureless sintering conditions that yield the thick body of uniform structure were invesigated.The creep properties of sintered body were studied by analyzing the results of internal friction measurements.Hugoniot-compression curve for Si3N4 ceramics with controlled microstructure were measured to study the yeilding properties under shock compression. And the correlations with Hugoniot-elastic limits and microstructure of Si3N4 ceramics were considered.The degradation characteristics of bending strength of specimens after processing under various grindingg condition were evaluated.

Effect of Seed Crystal Properties on the Microstructure of Si<sub>3</sub>N<sub>4</sub> Ceramics

The effect of seed crystal properties on the microstructure of Si3N4 ceramics was investigated. Three β-Si3N4 powders with average grain size of 0.74, 5.84 and 21.66μm and β-Si3N4 whiskers were used as seeds. Seeds (2mass%) were added to the mixture of α-Si3N4 and sintering aids, and the mixture was hot-pressed at 1850°C for 8h. Growth of large rodlike-grains was enhanced by the micron-sized single-crystalline β-Si3N4 particle. Crushed β-Si3N4 whiskers were similarly effective in growing rodlike-grains. On the contrary, polycrystalline β-Si3N4 particles and submicron-sized β-Si3N4 particls did not affect greatly the final microstructure.