Formation Of Β-sialon Research Articles

Spark plasma sintering of silicon(oxy-)nitride/boron nitride composites and their subsequent thermo-physical properties

Silicon nitride and boron nitride (Si3N4/h-BN) based ceramic composites were prepared by Spark Plasma Sintering (SPS) using aluminium oxide (Al2O3) and/or yttrium oxide (Y2O3) as sintering additives. The amount of h-BN introduced varied from 0 vol.% to 20 vol.% in order to study its influence on ceramic properties. In the same way, the amount of oxide additives has been controlled and the influence of the additive's nature on the final properties has been underlined. The sintering temperature has been selected in order to obtain equivalent relative densities. The formation of β-SiAlON has been underlined by XRD and TEM analysis. Moreover, due to the fast-sintering process, amorphous secondary phase has been formed. Mechanical properties (hardness, fracture toughness, Young modulus) and thermal diffusivity have been evaluated. It appears that the nature of the oxide additive used plays an important role on the mechanical and thermal behaviour of Si3N4/h-BN composites.

Multifunctional Nanocomposites in B4C-Tic-Tib2-Sic-BN-Al2O3-Sialon-C Carbon Fiber System for Armor Fillets, Turbine Discs and Wings, High-Temperature Wear Resistant Nodes

Goal - to receive on first stage β - SIALON containing nanocomposites by reactive sintering method at 14000C, with nitrogen pro-cess from origin composition in TiC-BN-SiC-B4C-Si-Al-Al2O3 (nanopowder) system. By using this method of synthesis, it became possible to receive nanocomposites with different percentages of β - SIALON. Our task was to study the phase composition of re-ceived consolidated materials in the TiC-TiB2-BN-SiC-B4C-β-SiAlON-Al2O3 (nanopowder-400nm.) system. Method. The obtained mass was grounded in an attritor and the consolidated composite was obtained by hot pressing at 16200C during 40 minutes, with glass perlite (Armenia) dope 2 mass%, delaying at final temperature for 8 min, under 30 MPa pressure and vacuum – 10-3 Pa. Perlite from Aragatc contained 96 mas. % glass. To study the phase composition of the composites, we conducted an X-ray structural analysis on the DRON-3 device. And to study the microstructure, we conducted research on an optical microscope -AC100 and a raster electron microscope “Nanolab 7” of the company "OPTON”. The values of the electrical parameters of the studied composites were calculated on the basis of the ob-tained "lgp- t" dependence. We have studied mechanical properties. Result. In TiC-TiB2-BN-SiC-B4C- β-SiAlON-Al2O3 – C Carbon Fiber system we obtained nanocomposites with high mechanical proper-ties. The advantage of this method is that compounds, which are newly formed thanks to interaction going on at thermal treat-ment: Si3N4, Si, AlN are active, which contributes to β-SiALON formation at relatively low temperature, at 1300-13500C. It is evi-dent that inculcation of ALN in crystal skeleton of ß-Si3N4 is easier since at this temperature interval crystal skeleton of Si3N4 is still in the process of formation. ß-SiAlON was formed at 14500C. Part of boron carbide was transformed into boron nitride in ni-trogen environment and in titanium diboride, which in the case of both composites is in small quantities. Conclusion. The phase composition of the obtained composite provides high physical-technical and performance properties of these nanocomposites. Compression strength-2198 MPa, Bending strength-271 MPa, Thermal expansion coefficient a20-700-3.8 10-6 0C.

Synthesis and phases relationships of Si6-zAlzOzN8-z in a wide range of z

Formation of Si6-zAlzOzN8-z from Si3N4/Al2O3/AlN mixtures was studied in a wide range of z value. Mixtures were annealed under 1 bar nitrogen flow at 1550, 1650 and 1750 °C for 1.5 and 3 h. The annealed samples were characterized with X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that in general formation of β-SiAlON phases with higher z value requires lower temperature but longer annealing time. XRD analysis revealed a high yield of β-SiAlONs (≥75 wt%) in the whole range of z value (1, 2, 3 and 4) at 1550–1650 °C (from 77% (z = 4) up to 93% (z = 3)). For z = 1 and 2 formation of β-SiAlON with z = 2 and 3 respectively was observed at 1550 °C with the yields up to ∼59 and ∼74%. At 1750 °C decomposition and sublimation of Si3N4 leads to formation of silicon poor phases derived from AlN including 12H, 15R and 21R. The only exception is a formation of Si3Al7O3N9-ζ phase with quite a high yield (∼68%) in the system relating to z = 3 at 1750 °C. As a result, some conclusions about formation of SiAlON phases in a broad Si/Al ratio range were made.

The effects of β-Si3N4 on the formation and oxidation of β-SiAlON

AbstractThe influence of the additive β-Si3N4 on the formation and oxidation of Si4Al2O2N6 during the sintering of Al, Si, and Al2O3 powders under flowing nitrogen atmosphere was examined. An increasing molar percentage of β-Si3N4 was shown to alter the morphology of Si4Al2O2N6 from a fiber-like to a rod-like structure and also shortened the time needed to form a dense, continuous oxide layer, which served as a barrier to the diffusion of O2. An optimal molar percentage of β-Si3N4 of 29.9 mol% was discovered, at which the grain growth was enhanced, and the surface area was, in turn, reduced, yielding superior resistance to oxidation. Our results provided a theoretical basis for the formation of β-SiAlON and demonstrated the potential of its use in high-temperature oxidizing environments.

Morphological regulation and simulation of β-Sialon and its effect on thermo-mechanical properties of Al2O3-C refractories

In this work, β-Sialon of different morphologies was generated using in situ morphological regulation and simulation. Their effects on the thermo-mechanical properties of Al2O3-C refractories were then investigated. The results indicate that columnar, plate-like, and hexagonal prism β-Sialon were formed from specimens without additives, with Fe2O3 as a catalyst, and with Ni as a catalyst, respectively. Transmission electron microscope (TEM) analysis results confirmed that the three morphologies of β-Sialon grow along with (110), (101), and (100), respectively. Density functional theory (DFT) results show that the addition of catalyst atoms could reduce the adsorption energy of gaseous Al2O on different crystal planes, which promotes the formation of β-Sialon. The formation of three different shapes of β-Sialon gave rise to materials with good mechanical strength. The cold modulus of rupture (CMOR) and cold crushing strength (CCS) of the specimen containing plate-like structure was slightly higher than those with a columnar and hexagonal prism, with maximum CMOR and CCS values of 27.2 MPa and 105.2 MPa, respectively. In addition, good thermal shock resistance was obtained due to the formation of β-Sialon; the residual strength ratio of the specimen with columnar β-Sialon was 78% after five thermal shock cycles, and those with plate-like and hexagonal prism reached 93% and 86%, respectively.

Formation mechanism of β-Sialon with different morphologies based on DFT calculation and its effect on the performance of refractory composites

β-Sialon with whisker-like, columnar, and plate-shaped structures was successfully synthesized with Al2O3–C refractory compositions. In the presence of a catalyst, the preferential growth face of columnar β-Sialon was (100), whereas plate-like β-Sialon preferred (101) and (111). DFT calculations demonstrated that the SiO adsorption energy on the (100), (101), and (111) faces of β-Sialon crystal were –34.95, –52.88, and –45.08 eV, respectively. Therefore, the (101) and (111) faces were more stable in Al-rich β-Sialon. With the increase in catalyst content, the Si-rich Si–Al–O–N liquid phases, which contributed to the generation of columnar β-Sialon, were converted into the Al-rich Al–Si–O–N system that was conducive to the formation of plate-like β-Sialon. Compared to the sample without the catalyst, the CMOR and HMOR of the specimen were increased by 45.07% and 60.47%, respectively, with the addition of Fe2O3. This was attributed to the formation of β-Sialon with a one- or two- dimensional shape.

In situ nitriding reaction formation of β-Sialon with fibers using transition metal catalysts

β-Sialon powder containing nanofibers was synthesized at 1300–1450 °C for 5 h in nitrogen by using transition metals (Fe, Co and Ni) as catalysts. Such synthesis conditions obviously reduced the nitriding temperature (~100 °C) of β-Sialon and increased the degree of nitridation. Observations from the weight gain rates of specimens relative to the content of catalysts suggest that the nitridation process was accelerated sharply via the addition of 0–2.5 wt% metallic catalysts. In addition, the amount of nanofibers of β-Sialon found in the specimens increased with the amount of transition metals, which was governed by the VLS formation mechanism with the help of transition metals. It could be concluded that transition metals not only greatly accelerated the nitridation of Si but also facilitated the formation of β-Sialon nanofibers in the specimens, as the formed metal-containing liquid greatly enhanced the amount of vapor generation.

Simulation and experimental investigation of preferred β-Sialon growth and its effects on thermo-mechanical properties of Al2O3–C refractories

Abstract β-Sialon bonded Al2O3–C refractories possess high strength and superior thermal shock performance. In this study, the growth of preferred β-Sialon (Si3Al3O3N5) and its effects on thermo-mechanical properties of Al2O3–C refractories were investigated via simulations and experiments. The results indicate that the additive Fe2O3 contributed to the formation of β-Sialon and helped its column structure become plate-like. Transmission electron microscopy confirmed that the (101) crystal plane was a growth plane of plate-like β-Sialon. The growth mechanism of β-Sialon was suggested by density functional theory; calculation results revealed that the key step for the formation and growth of β-Sialon was the adsorption of the gaseous molecule Al2O on the Si3N4 (101) crystal plane. It was found that the existence of Fe atoms could significantly reduce the adsorption energy. Additionally, Al2O3–C refractories containing plate-like β-Sialon possessed a high cold modulus of rupture and crushing strength, which increased by 40% and 15%, respectively, compared with the specimens containing column β-Sialon. It was also found that the formation of plate-like β-Sialon resulted in significantly better thermal shock resistance for the Al2O3–C refractory specimens, and the residual strength loss ratio of the sintered specimens was only 4% after five thermal shock cycles.

Thermo-mechanical properties of Al2O3-C refractories with in situ synthesized non-oxide bonding phases

A study of non-oxide bonding phases formed in situ by adding La2O3 to Al2O3-C refractories, and their influence on the thermo-mechanical properties, is presented. Calculations of the thermodynamics of the Al-Si-O-C-N system were thoroughly explored. Phase assemblage and microstructure were analyzed by means of X-ray diffraction and scanning electron microscopy. The results were that β-Sialon (Si3Al3O3N5) and 15R-Sialon (SiAl4O2N4) formed during sintering at 1400 °C. In addition, the presence of the rare earth oxide, La2O3, promotes columnar β-Sialon formation in a plate-like grain morphology, and this can improve the in situ synthesis of 15R-Sialon. The non-oxide bonding phases give rise to specimens with excellent strength and toughness. Increasing the La2O3 content from 0 wt% to 0.6 wt% enhanced the cold modulus of rupture and the cold crushing strength of the specimens fired from 31.1 MPa to 34.6 MPa, and from 101.3 MPa to 113.8 MPa, respectively. Meanwhile, the displacement was increased by 68% to 0.32 mm. The generation of plate-like β-Sialon and 15R-Sialon also had a positive effect on the hot strength and thermal-shock resistance. The hot modulus of rupture of the specimens fired improved by 21% compared to specimens without La2O3. After three thermal-shock cycles, the residual strength ratios of the specimens fired were above 88%.

Influence of NH4F additive on the combustion synthesis of β-SiAlON in air

The preparation of β-SiAlON, Si6–zAlzOzN8–z (z = 1–4), was investigated using the combustion synthesis (CS) technique with Si, Al, and CO(NH2)2 as the main raw materials in air. Different amounts of NH4F were used as a low-melt additive to the raw materials to decrease the reaction temperature and promote β-SiAlON formation. Phase and microstructural analyses of the reaction products were performed by X-Ray diffraction (XRD) and FE-SEM/energy dispersive X-ray spectrometer techniques. FT-IR spectroscopy was also used to demonstrate the formation of β-SiAlON in the final product. The phase and microstructural analysis results showed that NH4F addition improved the exothermic reaction of β-SiAlON phase formation and increased its content in the final synthesized powders. Additional heat treatment of the sample at the optimum z-value (z = 4) and different percentages of NH4F revealed the positive effect of the low-melt additive on β-SiAlON phase formation under very low N2 pressure.

Microstructure and Properties of Porous Si3N4/Dense Si3N4 Joints Bonded Using RE–Si–Al–O–N (RE = Y or Yb) Glasses

The joining of porous Si3N4 to dense Si3N4 ceramics has been successfully performed using mixed RE2O3 (RE = Y or Yb), Al2O3, SiO2, and α-Si3N4 powders. The results suggested that the α-Si3N4 powders partly transformed into β-SiAlON and partly dissolved into oxide glass to form oxynitride glass. Thus, composites of glass/β-SiAlON-ceramic formed in the seam of joints. Due to the capillary action of the porous Si3N4 ceramic, the molten glass solder infiltrated into the porous Si3N4 ceramic side during the joining process and formed the “infiltration zone” with a thickness of about 400 μm, which contributed to the heterogeneous distribution of the RE–Si–Al–O–N glasses in the porous Si3N4 substrate. In-situ formation of β-SiAlON in the seam resulted in a high bonding strength. The maximum bending strength of 103 MPa and 88 MPa was reached for the porous Si3N4/dense Si3N4 joints using Y–Si–Al–O–N and Yb–Si–Al–O–N glass solders, respectively.

In-situ synthesis and reaction mechanism of β-SiAlON in the Al-Si3N4-Al2O3 composite material

An Al-Si3N4-Al2O3 composite material has been successfully prepared by sintering at 1300°C under nitrogen atmosphere, and X-ray diffraction, scanning electronic microscopy, and electronic differential system were used for its characterization. β-SiAlON with Z=1 (Si5AlON7) appears in the as-synthesized composite material as a reinforcing phase leading to an improvement of the overall physical properties of the Al-Si3N4-Al2O3 composite. The thermal dynamic analysis of the reaction indicates that Al reacts to Al4C3 and Al4O4C. Our thermal dynamic analysis reveals that the addition of Si3N4 to Al4O4C leads to the formation of β-SiAlON.

Preparation and artificial neural networks analysis of ultrafine β-Sialon powders by microwave-assisted carbothermal reduction nitridation of sol–gel derived powder precursors

β-Sialon powders were synthesized by microwave-assisted carbothermal reduction of powder materials resultant from a sol–gel process using Si(OC2H5)4, Al(NO3)3·9H2O and sucrose (C12H22O11) as the main starting materials and artificial neural networks (ANNs) was applied to model and predict relative contents of β-Sialon in the final product samples. β-Sialon was formed at as low as 1250°C by using the technique developed with the present work. Furthermore, addition of Fe2O3 promoted the β-Sialon formation. As-prepared β-Sialon ultrafine powders were granular with primary size of about 69nm. A back propagation (BP) ANNs was used to establish a model to predict the reaction extents (relative contents of β-Sialon) under various processing conditions. The results indicated that the BP ANNs could be effectively used to establish the nonlinear relationships between the relative contents of β-Sialon and the processing conditions.

In-Situ Synthesis of (O′+β)-Sialon/Mullite Composite Materials from Coal Gangue

(O′+β)-sialon/mullite composite materials were synthesized by an in-situ carbothermal reduction nitridation method, with coal gangue and carbon black as raw materials. The influences of temperature and carbon content on the phase composition and microstructure of the pre-synthesized materials were investigated by XRD and SEM. The effects of variation in the phase composition, the microstructure on the bulk density, the bending strength of the pre-synthesized materials were investigated in detail. The results indicated that increasing temperature and carbon content benefit the generation of O′-sialon and β-sialon. The (O′+β)-sialon/mullite composite material can be synthesized at 1400°C (6 h) ∼ 1500°C (6 h). The generative process including the formation of β-sialon, O′-sialon and the conversion process from β-sialon to O′-sialon. The O′-sialon and β-sialon in the synthesized materials have needle-like and pillar structures, respectively. Given all that, the (O′+β)-sialon/mullite composite material prepared by coal gangue and carbon black contributes to solving the problem of the recycling of coal gangue, and high-performance composite materials can be obtained.

Inorganic polymers (geopolymers) as precursors for carbothermal reduction and nitridation (CRN) synthesis of SiAlON ceramics

Inorganic polymers (geopolymers) containing carbon are shown to be viable precursors for CRN synthesis of SiAlONs at 1400°C in nitrogen. The reaction mechanism and rate depends on the form and fineness of the carbon, the formation of β-SiAlON being faster and more complete with fine carbon black. The presence of the charge-compensating alkali cations in the geopolymer leads to the formation of a mixture of β-SiAlONs of low and high z-value, the former being formed via O-SiAlON resulting from the presence of silica-rich eutectics, and the latter forming by CRN reaction from an alumina-rich amorphous component. In ammonium-exchanged geopolymers lacking the alkali, SiAlON formation occurs primarily by conventional CRN of crystalline mullite and cristobalite intermediates. The potential advantages of geopolymer CRN precursors include their ability to be tailored to specific SiAlON compositions and their formation and curing at ambient temperatures to required shapes prior to the CRN reaction.

Effects of processing parameters on in situ synthesis of β-Sialon/ZrN/ZrO2 from fly ash and zircon

β-Sialon(Si4Al2O2N6)/ZrN/ZrO2 ceramic powder was synthesised by an in situ carbothermal reduction–nitridation (CRN) process using fly ash, zircon and active carbon as raw materials. The effects of processing parameters on the phase composition and microstructure of the products were investigated by X-ray diffraction and scanning electronic microscope. The formation process of β-Sialon/ZrN/ZrO2 ceramic powder was also discussed. Increasing carbon content in a sample, reaction temperature and soaking time all could promote the CRN reaction and benefit the formation of β-Sialon and ZrN. The β-Sialon/ZrN/ZrO2 ceramic powder could be fabricated successfully at 1450°C for 2 h, then continually at 1550°C for 4 h while heating the sample with the zircon/fly ash/active carbon mass ratio of 49∶100∶100. The as received β-Sialon particles existed as flaky and their average grain sizes were about 2–3 μm, and the average grain size of ZrN and ZrO2 reached 1–2 μm. The formation process of β-Sialon/ZrN/ZrO2 ceramic powder included the decomposition of zircon and mullite, the formation processes of O’-Sialon (Si1·83Al0·17O1·17N1·83), X-Sialon (Si3Al6O12N2), β-Sialon and ZrN as well as the conversion processes of X-Sialon and O’-Sialon to β-Sialon.

Fabrication of β-Sialon/ZrN/ZrON composites using fly ash and zircon

β-Sialon/ZrN/ZrON composites were successfully fabricated by an in-situ carbothermal reduction-nitridation process with fly ash, zircon and active carbon as raw materials. The effects of raw materials composition and holding time on synthesis process were investigated, and the formation process of the composites was also discussed. The phase composition and microstructure of the composites were characterized by means of XRD and SEM. It was found that increasing carbon content in a sample and holding time could promote the formation of β-Sialon, ZrN and ZrON. The proper processing parameters to synthesize β-Sialon/ZrN/ZrON composites were mass ratio of zircon to fly ash to active carbon of 49:100:100, synthesis temperature of 1550 °C and holding time of 15 h. The average grain size of β-Sialon and ZrN(ZrON) synthesized at 1550 °C for 15 h reached about 2 and 1 μm, respectively. The fabrication process of β-Sialon/ZrN/ZrON composites included the formation of β-Sialon and ZrO2 as well as the conversion of ZrO2 to ZrN and ZrON.

Reduction Nitridation Synthesis of Bauxite Based β-SiAlON by Aluminum / Silicon Compound

Study of aluminum / silicon compound reduction nitridation synthesis of bauxite-based β -Sialon reaction process, using mass change ratio of the specimens and XRD, SEM, EDS analysis. The results were compared with the specimens separately using Al and Si reducing agent.The results show that:at 1100 only Al reacts with N2 and SiO2 to form AlN and Si; At 1200~1350°C, Si takes part in reduction-nitridation with subsequent formation of SiAlON；At 1450~1550°C，there is noticeable increase in β-SiAlON formation, the appropriate temperature is 1500°C.The amount of β-SiAlON formed is higher and crystale growth is better than that of single reducing agent.

Influence of nitrogen overpressure on the nitridation, densification and formation of β-SiAlONs produced by silicothermal reduction

Abstract β-SiAlON materials (with z = 1) with 2 mol% DyAG have been synthesised by silicothermal reduction under different nitrogen pressures. The possibility of rapidly nitriding silicon and forming the SiAlON phase has been investigated through its reaction sequence of formation under different pressures. β-SiAlONs were fully nitrided (degree of nitridation greater than 93%) and formed after a 1-h hold at 1400 °C, or after a straight ramp to 1500 °C under 0.7 MPa of nitrogen. This was not achievable under static nitrogen at atmospheric pressure where the degree of nitridation was only 43% and β-SiAlON phase represented only about 60% of the crystalline phase assemblage at the same temperature. The formation of β-SiAlON depended on the formation of Si 3 N 4 whose reaction rate was enhanced by nitrogen overpressures.

Effect of high energy milling on the formation and properties of sialon ceramics prepared from silicon nitride-aluminium nitride precursors

Abstract Nanostructured silicon nitride and aluminium nitride powder mixtures have been successfully produced by high-energy mechanical activation. Subsequent high sintering activity and β-sialon formation at low temperature resulted from activation milling at high acceleration. Densification of the samples prepared from the activated powders commenced at temperatures by about 200 °C lower, as compared with the samples prepared from the non-activated powder mixture. 27 Al MAS NMR spectra clearly showed the effect of mechanical activation on the structural changes and phase evolution of the sialon precursor during heat treatment. After pressureless sintering at 1600 °C for 60 min, β-sialon samples had a Young’s modulus of 281 GPa, microhardness of 12.8 GPa, fracture toughness of 4.6 MPa m 0.5 and flexural strength of 632±129 MPa. The unique structure and bi-modal grain size distribution that is retained after conventional sintering at 1600 °C opens up new possibilities for tailoring the microstructure and related properties of these materials.