β-Si3N4 Phase Transformation Research Articles

Impurity Oxygen-Triggered α- → β-Si3N4 Phase Transformation at 1900 °C

Oxide additive-free α- → β-Si3N4 phase transformation of a high-purity commercial α-Si3N4 powder was investigated at 1600 to 1900 °C under a nitrogen pressure of 980 kPa. The XRD analysis revealed that the α- → β-Si3N4 phase transformation proceeded mainly at 1900 °C, and was completed by the extensive 1900 °C heat treatment for 20 h. This phase transformation temperature was 33 °C lower than the theoretical α-Si3N4 dissociation temperature and was confirmed as completely different from that often discussed for the liquid-phase sintering of α-Si3N4 powder by direct comparison with the phase transformation behavior of a reference powder, α-Si3N4 powder doped with 1 mol% Y2O3. The unique α- → β-Si3N4 phase transformation was further studied by a set of characterization techniques including elemental analysis, HAADF-STEM and STEM-EDS analyses. The results strongly suggested that the oxide additive-free α- → β-Si3N4 phase transformation was governed by the formation of a metastable solid solution between α-Si3N4 and impurity oxygen of approximately 0.6 wt%, which promoted the dissociation below the theoretical α-Si3N4 dissociation temperature to afford thermodynamically favorable β-Si3N4. Along with the β-Si3N4 formation, the impurity oxygen concentrated at the grain boundaries was released from the sample via the grain boundary diffusion to afford high-purity β-Si3N4.

Porous SiC-Si2N2O-Si3N4 composite ceramics with excellent EMW absorption properties prepared by gelcasting and carbonthermal reduction

Porous SiC-Si2N2O-Si3N4 composite ceramics with excellent electromagnetic wave absorption properties were prepared by the combination of gelcasting and carbonthermal reduction, with the in-situ generated SiC by the reaction of PyC with SiO2 and Si3N4 as microwave absorbent and the Si2N2O fibers by the reaction of SiO2 with Si3N4 as reinforcing agent. The increase in monomer content within 5.94–20.79 wt% resulted in PyC and SiC formation, which seriously inhibited the α→β-Si3N4 phase transformation and β-Si3N4 growth. Nevertheless, the bulk density, flexural strength and fracture toughness of porous SiC-Si2N2O-Si3N4 composite ceramics were significantly increased with the values of 1.84–2.48 g/cm3, 115.66–220.50 MPa and 1.88–3.37 MPa m1/2, respectively. The in-situ generated SiC effectively improved the electromagnetic wave absorption properties of porous SiC-Si2N2O-Si3N4 ceramics with reflection loss of −41.01 dB at 12.34 GHz and 2.0 mm thickness, accompanied with the effective absorption bandwidth of 10.55–12.40 GHz, demonstrating broad application prospects on electromagnetic wave absorption fields.

Effect of ZrB2 and its oxide impurities (ZrO2 and B2O3) on hot-pressed Si3N4 ceramics at low temperature

Based on the previous work on Si3N4-ZrB2, the addition of 2.5 vol.% ZrB2 promoted α- to β-Si3N4 phase transformation, bimodal microstructure, and toughness and strength improvement after hot-pressing at 1500 °C. However, the mechanism needed to be further explored. In the present work, the effect of ZrB2 and its oxide impurities (ZrO2 and B2O3) on phase composition, microstructure and mechanical properties of Si3N4 ceramics with MgO-Yb2O3 additives were studied. Results showed that the addition of B2O3 had no influence on Si3N4 ceramics, whereas the addition of ZrO2 inhibited the α- to β-Si3N4 phase transformation, formed an uniform equiaxed microstructure, and deceased the toughness and strength. The positive effect of oxide impurities can be eliminated. Based on the STEM analysis, the possible reason was that the addition of ZrB2 led to the formation of Si-Mg-O-N-Yb-Zr-B liquid phase, and then promoted α- to β-Si3N4 phase transformation.

Improved toughness of spark-plasma-sintered Si3N4 ceramics by adding HfB2

High toughness and high hardness Si3N4 ceramics with fine and bimodal microstructure was prepared by combination of HfB2 additive and spark plasma sintering (SPS). HfB2-free SPSed Si3N4 ceramics at 1600 °C exhibited major α-Si3N4 phase, a fine and equiaxed grain microstructure, high Vickers hardness (21.3 ± 0.2 GPa), and low fracture toughness (3.7 ± 0.3 MPa m1/2). The introduction of 2.5 vol% HfB2 into Si3N4 ceramics at 1600 °C promoted α-to β-Si3N4 phase transformation, produced a fine and bimodal microstructure, significantly improved the toughness (7.7 ± 0.6 MPa m1/2) but slightly decreased the hardness (20.4 ± 0.6 GPa) which was higher than that of traditionally hot-pressed or pressurless sintered Si3N4 ceramics (~14–18 GPa).

Influence of MXene (Ti3C2) Phase Addition on the Microstructure and Mechanical Properties of Silicon Nitride Ceramics.

This paper discusses the influence of Ti3C2 (MXene) addition on silicon nitride and its impact on the microstructure and mechanical properties of the latter. Composites were prepared through powder processing and sintered using the spark plasma sintering (SPS) technic. Relative density, hardness and fracture toughness, were analyzed. The highest fracture toughness at 5.3 MPa·m1/2 and the highest hardness at HV5 2217 were achieved for 0.7 and 2 wt.% Ti3C2, respectively. Moreover, the formation of the Si2N2O phase was observed as a result of both the MXene addition and the preservation of the α-Si3N4→β-Si3N4 phase transformation during the sintering process.

Densification of silicon nitride powder by spark plasma extrusion

The simultaneous extrusion and sintering of ceramic materials using the spark plasma extrusion (SPE) technique is an alternative densification route for the powders examined in this study. SPE with Y2O3 and Al2O3 as the additives was employed for the concurrent densification and extrusion of pure α-Si3N4 commercial powder. Three compositions of α-Si3N4 powders, namely SN-4, SN-12, and SN-20, were studied to understand the effect of additives on the sinter-extrusion process. The number in the mixture name corresponded to the weight percentage of additives in the samples. A mechanical shaker was used to thoroughly mix the selected powder compositions in isopropanol for 24 h at 720 rpm. It was then dried in a furnace at 100 °C for 3 h. The sinter-extrusion treatment was conducted for a holding time of 5 min at a heating rate of 300 °C/min and a sintering temperature of 1500 °C with a pressure in the range of 12–63 MPa. The maximum degree of extrusion was attained when the total amount of additives was at 20 wt% (SN-20 series). As the total amount of additives increased, the capillary pressure gradients led to an inhomogeneous liquid distribution throughout the specimen, which in turn promoted further densification of the Si3N4 ceramics. Owing to the conical geometry of the graphite die used for the extrusion process in this study, the largest densification values observed in the extruded ceramics corresponded to the base and the middle sections of the SN-20 series (relative density ~92%). The latter could only take place once the α-to β-Si3N4 phase transformation occurred, leading to significant shrinkage of the material.

Effect of temperature and pre-sintering on phase transformation, texture and mechanical properties of silicon nitride ceramics

Silicon nitride composite ceramics with excellent mechanical properties were prepared by hot pressing. The effects of temperature on the sintering process, β-Si3N4 phase transformation ratio (β%) and mechanical properties were investigated. By calculating the diffraction intensity (I), the β-Si3N4 phase transformation ratios of the three samples considered in this study were 57.2% (pre-sintered samples, PSN), 63.5% (urea homogeneous precipitation method, USN) and 66.6% (mechanical milling method, MSN) at 1550 °C, respectively. These results showed that the Si3N4 composites coated by using Y-Al precursor (USN) have a lower ratio of the phase transformation from α to β. Meanwhile, from the analysis of SEM images of the etched surface, the grain sizes were 0.47 µm (PSN), 0.39 µm (USN), and 0.43 µm (MSN) at 1650 °C, respectively. Compared to the other samples, the USN sample offered superior sintering behaviour and mechanical properties. The optimal flexural strength, hardness and fracture toughness of the Si3N4 composites coated by using the Y-Al precursor were 911 ± 69 MPa, 16.23 ± 0.24 GPa and 4.9 ± 0.35 MPa/m2 at 1650 °C, respectively.

Effect of TiO2 additives on nitridation of Si powders

Effect of TiO2 additives on nitridation of Si powders at 1300°C and 1400°C was studied by combining X-ray diffraction, thermogravimetric analysis, and thermodynamic calculations. Weak Si3N4 phase was detected in pure Si powders after nitridation at 1300°C, whereas no Si3N4 phase was found in TiO2-doped Si powders and only secondary TiN phase was observed in addition to main Si phase. The TiO2 demonstrated an inhibiting effect on nitridation of Si powders at 1300°C. However, the TiO2 led to the rapid nitridation of Si powders at 1400°C by circulating reaction mechanism between TiO2 and TiN, and promoted α-to β-Si3N4 phase transformation. In addition to TiN phase, secondary Si2N2O phase was observed in the TiO2-doped Si powders. The present study suggested that TiO2 additives exhibited effectively nitridation enhancing effect on the formation of reaction-bonded Si3N4 at 1400°C.

Synthesis and properties of SiC/Si3N4 co-reinforced La-containing glass ceramic matrix composites

In-situ grown β-Si3N4 and α-SiC co-reinforced La-containing glass ceramic matrix composites using ZrO2 as addition were obtained by a two-step sintering process. These composites were pressureless sintered under a nitrogen atmosphere at different sintering conditions for comparison of phase transformation and mechanical properties. Densities of the samples were measured and analyses of resultant products were carried out using X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that additions of La-containing glass can effectively promote α-Si3N4 to β-Si3N4 phase transformation. However, β-Si3N4 grain growth was hindered by the existent α-SiC. The La-containing glass-ceramic matrix composites exhibit high mechanical properties.

Ordered coalescence of nanocrystallites contributing to the rapid anisotropic grain growth in silicon nitride ceramics

Microstructural characterization is performed on two dense Si3N4 ceramic samples consolidated by spark plasma sintering (SPS): one fabricated using α-Si3N4 and the other using β-Si3N4 as the starting powder. A novel mechanism is revealed where ordered coalescence of nanoβ-crystallites accelerate the rapid β-Si3N4 anisotropic grain growth. The rapid α- to β-Si3N4 phase transformation via a high supersaturation of dissolved Si3N4 in the melt favors this mechanism. The high heating rate by SPS is essential for achieving such supersaturation.

Silicon nitride nanoceramics densified by dynamic grain sliding

The densification behaviors of two silicon nitride nanopowder mixtures based respectively on α-Si3N4 and β-Si3N4 as the major phase constituent were studied by spark plasma sintering. Sintering conditions were established where a low viscous liquid not in equilibrium with the main crystalline constituent(s) stimulated the grain sliding yet did not activate the reprecipitation mechanism that unavoidably yields grain growth. By this way of dynamic grain sliding full densification of silicon nitride nanoceramics was achieved with no noticeable involvement of α- to β-Si3N4 phase transformation and grain growth. This processing principle opens the way toward flexible and precise tailoring of the microstructures and properties of Si3N4 ceramics. The obtained silicon nitride nanoceramics showed improved wear resistance, particularly under higher Hertzian stresses.

Effect of solid content on pore structure and mechanical properties of porous silicon nitride ceramics produced by freeze casting

Porous Si3N4 ceramics were prepared by freeze casting using liquid N2 as refrigerant. The pore structure, porosity, α→β-Si3N4 transformation and mechanical properties of the obtained materials were strongly affected by the solid contents of the slurries. Increasing the solid content would reduce the porosity, decrease the pore size and change the pore structure from the aligned channels with dendrites to the round pores with decreased pore size. The formation of this round pores impeded the α→β-Si3N4 phase transformation, but was beneficial to the mechanical properties of the obtained porous Si3N4 due to its unique pore structure.

Effect of agarose content on microstructures and mechanical properties of porous silicon nitride ceramics produced by gelcasting

Porous Si3N4 self-reinforce ceramics were prepared by gelcasting using agarose solutions. By changing the agarose content in the slurries, the porous silicon nitride ceramics with different porosities, α→β-Si3N4 phase transformation, and mechanical properties were obtained. When the agarose content changed from 0.2% to 0.8% (w/w, based on powder), the porosities increased from 10.3% to 21.4%, while the fracture strength decreased from 455 to 316 MPa and the fracture toughness decreased from 6.6 to 5.5 MPa·m1/2. Many fibrous β-Si3N4 grains grown from the internal wall of the round pores is the typical microstructure of the gelcasting porous silicon nitride ceramic. Both elongated β-Si3N4 grains and suitable interfacial bonding strength contributes to high fracture toughness by favoring crack deflection and bridging. The growth mechanisms of fibrous grains resulted from the synergy of solution-diffusion-reprecipitation and vapor-liquid-solid (VLS).

Fabrication and Characterization of Barium Aluminum Silicate-Silicon Nitride Composites

The barium aluminum silicate-silicon nitride (BAS-Si3N4) matrix-ceramic composite was fabricated using pressureless sintering, at temperatures ranging from 1720°C, which is below the melting point of BAS, to 1850°C. The effect of processing conditions on sinterability, crystalline structure, microstructure and mechanical properties was evaluated. It was demonstrated the BAS glass-ceramic served as an effective liquid-phase-sintering aid, to attain high densities and completed the α-Si3N4–β-Si3N4 phase transformation, and remained as a structural matrix that was reinforced by the rod-like β-Si3N4 grains. Si3N4 grains nucleated and grow in random directions in an almost completely crystallized matrix of hexacelsian BAS. High flexural strength (665±40 MPa) and fracture toughness (7.74 MPa•m1/2) could be obtained from 30wt%BAS-70wt%Si3N4 samples that have been sintered at 1800°C for 120 min with a fine-grained microstructure.

Microstructure and property enhancement of silicon nitride-barium aluminum silicate composites with β-Si3N4 seed addition

Si3N4-barium aluminum silicate (BAS) self-reinforced composites have been prepared by pressureless sintering at 1800 °C for 2 h. The β-Si3N4 seeds incorporated in the starting α-Si3N4 powders encouraged the α- to β-Si3N4 phase transformation, and the final bimodal microstructure with large grains, consequently, led to the improvement of the fracture toughness, from 7.74 to 8.34 MPa m1/2. The almost-complete crystallized BAS benefited the high-temperature mechanical properties. The residual stress, crack deflection, grain bridging, and pullout were considered as the major toughening mechanisms in this composite.

Celsian formation in Si 3N 4–Ba 0.75Sr 0.25Si 2Al 2O 8 composites

The formation of monoclinic celsian in Ba0.75Sr0.25Al2Si2O8 (BSAS) sol–gel glass powder and in 50 wt.% Si3N4 particle-reinforced BSAS composites fabricated by a low-temperature hot pressing was investigated. Results indicated that celsian formation depended on the polymorphs of the Si3N4. Hexacelsian to celsian transformation was impeded when the α- to β-Si3N4 phase transformation occurred.

Crystallization and Microstructure Development of Si3N4-Ti(C,N)-Y2O3 Ceramics Derived from Chemically Modified Perhydropolysilazane.

[Si-Y-Ti-O-C-N] multicomponent amorphous powders were synthesized by pyrolysis at 1000°C, in NH3 flow, of chemically modified PHPS using Y(OCH(CH3)2)3 and Ti(N(CH3)2)4. The crystallization and microstructure de velopment were studied by comparison with [Si-Y-O-C-N] amorphous powders derived from PHPS chemically modified with Y(OCH(CH3)2)3.(1) Amorphous Si3N4 in the [Si-Y-Ti-O-C-N] powders was rather stable below 1400°C in N2. Above 1600°C, Si-Ti-Y-O-N liquid phase accelerated the α-/β- phase transformation of Si3N4 and the [Si-Y-Ti-O-C-N] powders yielded β-Si3N4-Ti(C, N)-Y2O3 ceramics at 1800°C.(2) Fully dense β-Si3N4-Ti(C, N)-Y2O3 ceramics were also synthesized by hot pressing at 1800°C in N2. The resulting ceramics exhibited a uniform and fine-grained micro structure.(3) Ti(C, N) phase was dispersed as particles having a size range of 10nm to 1.6μm and the large Ti(C, N) particles with diameters larger than 0.5μm were located at the β-Si3N4 matrix boundaries.(4) Titanium in the [Si-Y-Ti-O-C-N] powders could act as a catalyst to accelerate α-/β-Si3N4 phase transformation as well as an in-situ source for nano/micro Ti(C, N) particles, which lead to microstructure uniformity and refinement of β-Si3N4-Ti(C, N)-Y2O3 ceramics.