Phase Transformation Of Si3N4 Research Articles

Effect of ZrB2 on promoting the phase transformation mechanism of Si3N4-based ceramics at low temperature

In this study, the mechanism of the effect of ZrB2 on phase transformation of Si3N4 at a low temperature and the influence of its content on Si3N4-based ceramics were investigated. Previous study has shown that oxide impurities, i.e., B2O3 and ZrO2 on ZrB2 particles, alone cannot contribute to phase transformation of Si3N4 at a low temperature. But, the introduction of 0.5 vol% ZrB2 into Si3N4 ceramics can promote the α-β phase transformation of Si3N4, which is confirmed to be the role of boron by comparison of the experimental results obtained from the addition of 0.5 vol% Zr and 0.5 vol% B. Increasing the ZrB2 content from 0 vol% to 2.5 vol% intensifies the α-β phase transformation while decreasing the α phase content of Si3N4-based ceramics, accompanied by a slight grain growth, leading to a decrease in hardness. At the same time, aspect ratio and the quantities of elongated grains per square micron increase, and thus the fracture toughness increases significantly. However, when the content of ZrB2 increases to 5 vol%, the Si3N4-based ceramics not only have a substantial decrease in hardness, but also the fracture toughness fails to be effectively improved due to high porosity and the decrease in aspect ratio and the quantity of elongated grains per square micron. The current study demonstrates that the dense Si3N4-based ceramics with high hardness and toughness (hardness ∼19.9 ± 0.2 GPa, toughness ∼6.27 ± 0.19 MPa m1/2) can be prepared successfully at 1600 °C by introducing 0.5 vol% ZrB2.

In situ fabrication of continuously graded Si3N4 ceramics via DC field-assisted hot pressing

The effects of various DC electric fields (0–167 V/cm) on liquid-phase hot pressing of Si3N4 ceramics were investigated. The application of a DC field promoted the α→β phase transformation of Si3N4 near the cathode while inhibiting it near the anode. As a result, the Si3N4 ceramics with continuously graded microstructures and mechanical properties were in situ fabricated from a homogeneous raw powder. Although the DC field-driven migration of Mg2+ and Y3+ ions was recorded, the graded phase transformation phenomenon was linked to the Peltier effect, which led to additional heating at the cathode and cooling at the anode.

Sintering of Si3N4 with Li-exchanged zeolite additive

Abstract This paper deals with the densification and phase transformation of Si3N4 with additives of Li-exchanged zeolite during pressureless sintering at significantly reduced temperatures. Dilatometric shrinkage data show that the first liquid forms as low as 1080 °C. Upon sintering at 1500 °C the bulk density increases to more than 95% of the theoretical density without phase transformation from α-S3N4 to β-Si3N4, i. e. the phase transformation lags behind the densification process. Above 1500 °C the secondary phase is completely converted into a glass and the α-to-β transformation takes place. Under these conditions the grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. The results show that a very effective low-temperature sintering additive for silicon nitride can be obtained from Li-exchanged zeolite.

The effect of silicon particle size on the characteristics of porous sintered reaction bonded silicon nitride

The effect of silicon particle size on the β-phase ratio, microstructure and the flexural strength of porous sintered reaction-bonded silicon nitride(SRBSN) was systematically investigated. Six kinds of raw powders containing Yb2O3 addtive (5, 10 wt%), prepared with different particle size distribution, were prepared by planetary ball-milling duration of 0, 3, 24 h. The behavior of α-to-β phase transformation of Si3N4 sintered at 1650 °C and 1750 °C, was found to be in good relation with SiO2 molar ratio over all the oxide additives. High flexural strength of SRBSN over 400 MPa was obtained when the amount of SiO2 was less than 0.7 owing to having the rod-like β-particles. Otherwise, the flexural strength of SRBSN became lower due to major occurrence of the equiaxed β-particles. This significant difference was considered to be caused from the seeding effect of β-ratio of RBSN samples as well as the different solubility of nitrogen element in the eutectic liquid phase.

Influence of hexagonal boron nitride nanosheets on phase transformation, microstructure evolution and mechanical properties of Si3N4 ceramics

Fully dense Si3N4 materials with 1 wt.% (∼ 1.5 vol.%) and 2 wt.% (∼ 3.0 vol.%) h-BN nanosheets were prepared by spark plasma sintering at 1750 °C with the dwell of 7 min under a pressure of 50 MPa in a vacuum. BN nanosheets with different dimensions were prepared by ultrasound-assisted liquid phase exfoliation of h-BN powder, followed by centrifugation at different speeds (1000 rpm and 3000 rpm). The addition of BN nanosheets hindered the particle rearrangement stage of sintering, which resulted in the delayed α→β phase transformation of Si3N4. To study a direct effect of BN nanosheets on the mechanical properties of Si3N4, the results were compared to the monolithic Si3N4 with similar grain size and α/β-Si3N4 ratio. The addition of 2 wt.% h-BN nanosheets (1000 rpm) increased both the fracture toughness (∼ 26 %) and the strength (∼ 45 %) of Si3N4, when compared to the monolithic Si3N4 with similar microstructure.

Silicon nitride ceramics consolidated by oscillatory pressure sintering

We report the fabrication of silicon nitride (Si3N4) ceramics with high flexural strength by oscillatory pressure sintering (OPS). The effects of the sintering procedure on the grain growth, microstructure evolution, and mechanical properties of Si3N4 were investigated. It was found that, in comparison to the traditional gas pressure sintering technique, the OPS process produced Si3N4 ceramics with higher density with a shorter sintering time and lower sintering temperature, leading to products with higher bending strength and Vickers hardness. Furthermore, the OPS promoted the α-β phase transformation of Si3N4, which facilitated the ion diffusion rate in the liquid phase and increased the aspect ratio of the formed β-Si3N4 grains. The Si3N4 ceramics sintered at 1720 °C by OPS exhibited high bending strength and Vickers hardness of 1432 ± 51 MPa and 16.14 ± 0.38 GPa, respectively, representing their potential for exploring ceramics with high performances.

Effect of metal phases on microstructure and mechanical properties of Si3N4-based ceramic tool materials by microwave sintering

Si3N4-based composite ceramic tool materials were fabricated by microwave sintering. The effects of Ni and Co on the mechanical properties, phase transformation and microstructure of the Si3N4-based composites were studied, and the toughening mechanism was analyzed. The optimum comprehensive mechanical properties with the relative density of 97.15 ± 0.2%, Vickers hardness of 17.28 ± 0.17 GPa and fracture toughness of 6.59 ± 0.12 MPa m1/2 were obtained at 1650 ℃ when the content of Ni was 2 wt%. Ni can promote the formation of grains with high aspect ratio more effectively than Co, but the hardening effect of Co was higher than Ni. Both of two metals would restrain the phase transformation of Si3N4 from α phase to β phase. The main toughening mechanism was the combined action of pulling-out of β-Si3N4 columnar crystals, crack bridging by columnar crystals and crack deflection.

Crystal Growth in the Combustion Synthesis of α‐Si3N4 Using Si with Different Particle Sizes

In this present work, Si3N4 powders with high α‐phase contents and distinct crystal morphologies were prepared via a promising approach of combustion synthesis (CS), using Si powders with different particle sizes as reactants. The influence of Si particle size on phase composition and crystal morphologies in the products was systematically investigated. Two unique crystal morphologies, radial‐spheroidal‐cluster and flowerlike, were observed in the Si3N4 products. The crystal growth mechanisms of Si3N4 granules in the CS system with disparate Si have been proposed based on experiments and thermokinetic calculations. As conclusion, the α‐phase content in the final product was synergetically dominated by the vaporization process of Si particles and the α–β phase transformation of Si3N4 during the after‐burn period. Si3N4 powders with high α‐phase content can be obtained from Si powders with an appropriate particle size.

Fabrication and Property of Porous Silicon Nitride Ceramics with Y<sub>2</sub>O<sub>3</sub>-Yb<sub>2</sub>O<sub>3</sub> Additions

Wave-transparent porous Si3N4 ceramics were fabricated using 9wt% multiple sintering additives of Y2O3 and Yb2O3. The influence of sintering additive composition on the porosity, mechanical properties and microstructure of porous Si3N4 ceramics was investigated. The results showed that the porous Si3N4 ceramics with porosity ranging from 32% to 43% were prepared. For the porous Si3N4 ceramics sintered at 1690°C, flexural strength, fracture toughness and the β-Si3N4 phase content increased as the Yb2O3 content of multiple sintering additives increased from 0 to 9 wt%. In most specimens, microstructures with interlocked fibrous β-Si3N4 grains and uniformly distributed fine pores were obtained with employing scanning electron microscopic examination and X ray diffraction analysis. Sinterability and phase transformation of Si3N4 with more Yb2O3 content were more pronounced than others.

Silicon nitride foams with uniform pore structure prepared by using protein foaming method with a planetary ball-milling foaming route

A planetary ball-milling foaming route was employed by the protein foaming method for preparing silicon nitride (Si3N4) foams. In comparison with the commonly used tumbling foaming route of protein foaming method, pores of the Si3N4 foams fabricated by this new foaming route are more uniform in size distribution and the pores pack more compact. Furthermore, effects of the rotating speed on the structure and the property of the Si3N4 foams fabricated by this new foaming route were investigated. It was indicated that the average pore size decreased with the rotating speed, whereas the count of windows on the walls increased with the rotating speed. Moreover, the windows were found to have a negative influence on the phase transformation of Si3N4 from α phase to β phase.

Chemical reactivity of hot-pressed Si3N4–ZrB2 ceramics at 1500–1700 °C

Chemical reactivity of hot-pressed Si3N4–ZrB2 ceramics at 1500–1700°C under N2 and Ar atmosphere is investigated by X-ray diffraction, thermodynamic calculation, and phase diagram. Results show that there are chemical reactions in Si3N4–ZrB2 ceramics. The chemical reactions accelerate the decomposition and phase transformation of Si3N4. The reaction products include ZrN, BN, ZrSi2, SiC or Si, which strongly depend on the atmosphere or temperature. The source of N2 for ZrN and BN products is both atmosphere and decomposition of Si3N4 at 1600°C or above, whereas that is only atmosphere at 1500°C. There is no reaction in Si3N4–ZrB2 ceramics at 1500°C under Ar. The MgO–Lu2O3 sintering adds lead to the significant increase of relative density of Si3N4–ZrB2 from 48% to 94% at 1500°C in Ar. Therefore, it is possible that dense Si3N4–ZrB2 ceramics with no reaction could be achieved at 1500°C or below in Ar.

Fabrication of Si3N4-based composite containing needle-like TiN synthesized using NH3 nitridation of TiO2 nanofiber

Abstract A Si3N4 composite containing needle-like TiN particles (7 vol%) was fabricated. Needle-like TiN particles several micrometers long were synthesized using NH3 nitridation of TiO2 nanofiber, which was obtained using hydrothermal treatment. A mixed powder of α-Si3N4 and the needle-like TiN particles with additives was hot pressed at 24 MPa and 1850 °C for 1 h in N2 atmosphere. Mechanical properties of the composite were compared with those of a composite containing rounded TiN particles and a monolithic β-Si3N4 ceramic. The Si3N4 matrix of the composites containing TiN was mainly a-phase, suggesting that the α–β phase transformation of Si3N4 was inhibited by the presence of TiN. Although fracture strength of the composites was lower, fracture toughness was comparable to that of monolithic β-Si3N4 ceramics. Hardness of the composites was about 19 GPa and was greater than that of the monolithic β-Si3N4 ceramic.

The Investigation of Ultrahigh Pressuer Sintering of Cubic Boron Nitride with the Binding Agent of Si<sub>3</sub>N<sub>4</sub>

Selecting Si3N4 as binding agent and Al2O3–Al–Y2O3 as sintering additives, the sintered compact of polycrystalline cubic boron nitride (PcBN) was studied . By modifying the ratio of each component and the parameters of the sintering process, the compact sintered at 4.5 GPa and 1650 °C showed good mechanical properties.The phase constitution, microstructure of the sintered compact were characterized by XRD and SEM respectively. The degree of phase transformation of Si3N4 , the chemical reaction during the sintering process, and the sintering mechanism of the additives were discussed . The best relative density is more than 97 % , the flexural strength is more than 650 MPa, the Vickers hardness Hv is more than 8.7 GPa.

Alumina-based hetero-modulus ceramic composites with extreme dynamic strength – phase transformation of Si3N4 during high speed collisions with metallic bodies

Till today several kinds of ceramics and ceramic matrix composites are developed for extreme environmental conditions. Most of these ceramics have microstructures with relatively „big” crystals, having high rigidness and strong inclination to nick, pitting and rigid fractures, so they are not usable for collision with metallic or other bodies under high speeds like 800 m/sec or more. On the basis of several years experiments in development and testing of ceramic materials and corundum matrix composites the authors successfully developed new alumina-matrix composite materials reinforced with Si 2 ON 2 , SiAlON, AlN and Si 3 N 4 . These new alumina based ceramic matrix composites were tested under collisions with different metallic bodies having high densities and speeds higher than 800 m/sec. During the collisions the kinetic energy of flying metallic objects distributing to fracture energies, heatings and recrystallizations both of ceramic and metallic bodies. In the centres of collisions, where oxygen was absent, the authors have found new, high density “diamond-like Si 3 N 4 “ materials with cubic crystals, where nitrogen atoms distributed in the centres of the cubes. These new crystal structures of Si 3 N 4 in the alumina matrix have extreme dynamic strength and hardness, like diamond. Having surplus of oxygene in the centres of collisions this new “diamond-like Si 3 N 4 “ was not observed, when a very strong oxydation of metallic bodies was taken place. Using the energy conception of collision, the authors mathematically described the energy engorgements of destruction of ceramic materials and heating of participating bodies as well as energy engorgement used for the phase transformations of ceramic and metallic particles during their collision.

알루미나 첨가에 의한 질화규소의 방전 플라즈마 소결 거동과 상전이 특성 및 기게적 특성에 미치는 영향

Silicon nitride(Si₃N₄) is one of the most widely used structural ceramic materials. However silicon nitride is difficult to sinter because of its strong covalent bonding characteristics. In this study, Si₃N₄ ceramics were fabricated by spark plasma sintering process with Y₂O₃ and Al₂O₃ addition to improve the sinterability and the mechanical properties and their phase transformation behavior, microstructure and mechanical properties were evaluated. Fully densified Si₃N₄ ceramics could be obtained by spark plasma sintering process at a lower temperature than conventional sintering method. The formation of network microstructure was affected by the addition of Al₂O₃ because it could accelerate α to β phase transformation of Si₃N₄. As a result, the mechanical properties depended on amounts of Al₂O₃ addition. The hardness value increased with increasing α-phase fraction, but fracture toughness value increase with increasing β-phase fraction.

Enhanced mechanical properties of machinable Si3N4/BN composites by spark plasma sintering

Abstract Si3N4/BN composites were fabricated by spark plasma sintering (SPS) through traditional powder mixing process and by a chemical route to Si3N4/BN composite powder, respectively. The SPS-processed Si3N4/BN composites maintain a considerably high bending strength, exhibiting some gradual decrease though with increasing BN content. Given the BN content, the bending strength is enhanced for the composites prepared by powder mixing relative to the chemical route counterparts. Differences in the α → β phase transformation of Si3N4 and grain growth during the SPS process play a key role in the variation of strength. Regardless of the preparation method, the Si3N4/BN composites with a BN content up to 30 vol.% can be well machined with WC tools.

Sintering of Si3N4 with Li-exchanged zeolite additive

Abstract This paper deals with the densification and phase transformation of Si3N4 with additives of Li-exchanged zeolite during pressureless sintering at significantly reduced temperatures. Dilatometric shrinkage data show that the first liquid forms as low as 1080°C. Upon sintering at 1500°C the bulk density increases to more than 95% of the theoretical density without phase transformation from -S3N4 to -Si3N4, i.e. the phase transformation lags behind the densification process. Above 1500°C the secondary phase is completely converted into a glass and the -to- transformation takes place. Under these conditions the grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. The results show that a very effective low-temperature sintering additive for silicon nitride can be obtained from Li-exchanged zeolite.

Influence of Additive Type on Densification and Phase Transformation of Seeded Si<sub>3</sub>N<sub>4</sub>

This paper deals with densification and a®b phase transformation of Si3N4, with a constant b-Si3N4 seeds concentration regarding sintering temperature and different additive types. The seeds of b-Si3N4 were obtained by sintering of silicon nitride in a-form and additive mixture consisting of Y2O3 and SiO2. Two different Si3N4/additive mixtures (Y2O3 + Al2O3 and CeO2) containing constant amount of seeds were prepared and tested. Characterization of sintered samples involved phase analysis by X−ray diffraction and density measurements. The results indicated that in the presence of yttria-alumina mixture both the phase transformation and the densification were enhanced as compared to samples containing only CeO2 as a sintering additive. The reasons for observed behavior are discussed in detail.

Effects of additive amount on microstructure and mechanical properties of silicon carbide-silicon nitride composites

Powder mixtures of β-SiC-α-Si3N4 in a weight ratio of 1 : 1 containing 5–20 wt% Y-Mg-Si-Al-O-N oxynitride as a sintering additive were liquid-phase sintered at 1800°C for 3 h by hot-pressing. These materials had a microstructure of “in situ-toughened composites” as a result of the α → β phase transformation of Si3N4 during sintering. The introduction of larger amount of additives accelerated the grain growth of elongated β-Si3N4 grains with higher aspect ratio, resulting in the improved fracture toughness and strength. Typical flexural strength and fracture toughness of SiC-Si3N4 composites containing 15 wt% oxynitride glass were 860 MPa and 5.7 MPa · m1/2, respectively.

Synthesis of Poly-Titanosilazanes and Conversion into Si3N4-TiN Ceramics.

(1) Poly-titanosilazanes were synthesized by chemical modification with Ti(X)4 [X=N(CH3)2, OCH(CH3)2]. The chemical structure of the synthesized precursors was studied using IR and 1H NMR techniques. The polytitanosilazanes were found to contain some N-Ti bonds.(2) [Si-Ti-O-C-N] multicomponent amorphous powders were synthesized by pyrolysis of the poly-titanosilazanes at 1000°C in NH3.(3) Amorphous Si3N4 in the [Si-Ti-O-C-N] was rather stable below 1400°C and titanium was found to be effective to suppress crystallization of Si3N4. Above 1600°C, the Si-Ti-O-N liquid phase enhanced the α-/β- phase transformation of Si3N4.(4) The poly-titanosilazane derived from PHPS and Ti(N(CH3)2)4 yielded Si3N4-TiN ceramics by heat treatment at 1800°C in N2. TiN was observed as particles with diameter of smaller than 100nm. The poly-titanosilazane was found to be suitable for the synthesis of TiN nanopartitle-dispersed Si3N4-based ceramics.