Β-sialon Grains Research Articles

Preparation of β-SiAlON/SiC composite ceramic foam filters and their oxidation resistance

In this study, ceramic foam filters with high porosity, strength, and oxidation resistance are produced at low temperatures by preparing porous β-SiAlON/SiC composite ceramic foams using the foam impregnation method with Si, Si, α-Al2O3, AlN, and SiC as the raw materials. The presence of SiC inhibits the growth of β-SiAlON grains, which reduces the amount of β-SiAlON produced. As the proportion of β-SiAlON increases, the porosity of the β-SiAlON/SiC composite ceramic foam decreases, whereas the bulk density and compressive strength increase. Non-isothermal oxidation experiments show that β-SiAlON/SiC composite ceramic foams exhibit high oxidation resistance and their macroscopic morphology does not change significantly after oxidation. As the proportion of β-SiAlON increases, the smooth SiO2 oxide layer on the surface of the ceramic foam gradually changes to acicular and columnar mullite crystals, and the oxidation resistance increases. Therefore, β-SiAlON acts as a high-temperature binder when it is added to SiC. Hence, the prepared β-SiAlON/SiC composite ceramic foams can be used as ceramic filters in the field of molten metal filtration.

Phase transformation kinetics and grain orientation of multi-cation doped (Yb, Sm) α/β-SiAlON ceramic tool materials

The densification behavior, phase transformation kinetics, and grain orientation of multi-cation doped (Yb, Sm) α/β-SiAlON ceramic tool materials prepared via spark plasma sintering (SPS) were studied. The results indicated that the (Yb, Sm) α/β-SiAlON ceramics were rapidly densified at 1540 °C. Various intergranular phases were formed due to the doping of rare earth oxides with different ionic radii. β-SiAlON precipitated ahead of α-SiAlON and elongated rapidly above 1600 °C. The β-SiAlON grains were preferred to orient in the direction perpendicular to the SPS pressure due to the dissolution/precipitation and grain rotation mechanism. The tendency of orientation decreased with the growth of β-SiAlON grains and the increase of liquid phase. The orientation of β-SiAlON grains resulted in anisotropic mechanical properties of (Yb, Sm) α/β-SiAlON ceramics, which fracture toughness measured in the perpendicular plane (⊥) was higher than that in the parallel plane (//).

Preparation of In Situ Growth Multiscale β-Sialon Grain-Reinforced Al2O3-Based Composite Ceramic Tool Materials

A kind of multiscale β-sialon grain-reinforced Al2O3 matrix composite ceramic tool material, named ASN, was prepared and studied. For the ASN, β-sialon (molecular formula: Si4Al2O2N6) was synthesized in situ by a hot-pressing and solid-solution reaction process. A total of six samples were prepared at varying sintering temperatures and holding times under vacuum conditions. The solid solution reaction mechanism of β-sialon, the phase composition, mechanical properties, microstructure, and strengthening and toughening mechanisms of the composite ASN were investigated. As a result, within the experimental parameters, an optimal ASN tool material was obtained under a pressure of 32 MPa and at a temperature of 1550 °C for 20 min. The tested mechanical properties of the optimal sample were as follows: flexural strength 997 ± 59 MPa, fracture toughness 6.4 ± 0.3 MPa·m1/2, Vickers hardness 18.2 ± 0.4 GPa, and relative density 98.1 ± 0.2%. According to crystal defect theory, the solid solution reaction mechanism of in-situ-synthesized β-sialon in an Al2O3 matrix involves a double mechanism of unequivalence (or hetero-valence) and interstitial filling. The multiscale β-sialon grains mainly consisted of four grains, which were elongated β-sialon grains with a diameter of 0.3-0.4 μm and an aspect ratio of 6-9, elongated β-sialon grains with a diameter of 70 nm and an aspect ratio of 10, β-sialon whiskers with a diameter of 0.2 μm and an aspect ratio of 12-15, and intragranular β-sialon whiskers with a diameter of 70 nm. The mechanical properties were improved due to strengthening and toughening mechanisms, such as mixed structure mode (intergranular and transgranular), elongated grain pullout, interface bonding, crack reflection, pinning, and bridging.

Effect of Y2O3 content on the microstructural characteristics and the mechanical and thermal properties of Yb-doped SiAlON ceramics

SiAlON ceramics are primarily employed in ceramic cutting tools, which exploit their stable physical properties in high-temperature cutting environments due to their excellent mechanical properties. Here, Yb/Y-co-doped SiAlON ceramics are prepared by adding Yb and Y rare-earth (RE) ions in the RExSi12-(m+n)Alm+nOnN16-n (m = 0.4, n = 1.0) composition. Yb2O3, the RE oxide, is the main sintering additive. For REx composition design with (Yb1-y + Yy)x, Yb2O3 is replaced by Y2O3 (y = 0.00, 0.25, 0.50, 0.75, 1.00). While Yb2O3 has excellent high-temperature stability, it is limited by its microstructural characteristics, that is, the β-SiAlON morphology and limited fracture toughness due to the small cation sizes. Thus, the changes in the above properties are investigated for various Y2O3 additive contents substituting for Yb2O3. The average grain width decreases, and the equiaxed β-SiAlON grains are elongated with increasing Y2O3 content. Regarding the mechanical properties, the hardness and fracture toughness are evaluated using the indentation fracture method. The hardness decreases with increasing Y2O3 content; however, the fracture toughness exhibits a significant increase (∼53.6%) from 4.6 to 7.0 MPa⋅m1/2. Regarding crack propagation, intergranular fracture is mainly observed in the Yb/Y-co-doped SiAlON ceramics, whereas transgranular fracture is primarily observed in the Yb-single-doped SiAlON ceramic. Y2O3 substitution increases the α/β-SiAlON phase ratio, and the grain boundary phase exhibits increasing vitrification with increasing Y2O3 content. Moreover, the thermal properties of the Yb/Y-co-doped compositions are analyzed and discussed regarding intrinsic properties such as phonon scattering. The microstructural characteristics and improved fracture toughness derived from the Yb/Y-co-doped system designed in this study suggest considerable potential for the future composition design of ceramic cutting tools.

Effect of hBN addition on the fabrication, mechanical and tribological properties of Sialon materials

This work aims to investigate the effect of hBN on the friction and wear resistance of Sialon composite. Sialon and its composite with 10 wt% hBN were fabricated by SPS sintering. The effect of hBN additive on the phase composition, microstructure, densification behavior, mechanical and dry sliding tribological properties of Sialon material was studied. Being sintered at 1600 °C for 10 min, compared to monolithic Sialon, Sialon-hBN composite has more refined β-Sialon grains with smaller aspect ratios and slightly declined relative density. The hardness of the Sialon-hBN composite was reduced due to the weak bonding between Sialon and hBN grains. Nevertheless, its fracture toughness increased ascribing to the toughening mechanisms, including crack deflection and crack bridging. hBN had an essential impact on the tribological performances of the composite due to its lower friction coefficient and good lubrication action. Under the same densification level (i.e., with a relative density of around 97.5%), the friction and wear resistance of Sialon-hBN composite were much better than monolithic Sialon. The main wear mechanisms were tribolayer formation, oxidized wear, and abrasive wear.

Fabrication of in situ elongated β-Sialon grains bonded to tungsten carbide via two-step spark plasma sintering

In order to reduce the difficulty of preparing binder-less cemented carbide and further broaden its application prospects, tungsten carbide toughened by in situ elongated β-Sialon grains was developed via sintering ball-milled WC and α-Si3N4 powders using Al2O3–ZrO2 as a sintering aid and transformation additive. The two-step spark plasma sintering of the mixture at 1650 °C with dwelling at 1500 °C for 10 min was conducted under 30 MPa uniaxial pressure, and the densification behaviors, phase transformations, mechanical properties, and microstructures of the produced composites were investigated. The addition of Al2O3–ZrO2 reduced the initial temperature of the densification process by approximately 100 °C and its final temperature by 200 °C (compared with the densification temperatures of pure WC and Si3N4 materials) and fully transformed α-Si3N4 to Sialon (Si–Al–O–N) phases. Microstructural characterization data showed that the WC matrix contained homogeneously distributed equiaxed and elongated β-Si5AlON7 grains. The WC composites containing in situ elongated β-Sialon grains exhibited an optimal hardness of 18.93 ± 0.03 GPa and enhanced fracture toughness of 10.43 ± 0.27 MPa m1/2. The toughening mechanism of the β-Sialon phase involved the pull-out of elongated grains and crack bridging.

High mechanical properties of β-SiAlON/TiC0.3N0.7 ceramic composite prepared by pressureless spark plasma sintering

ABSTRACT β-SiAlON/TiC0.3N0.7 composites containing 2.0-7.5 wt.% TiC0.3N0.7 are prepared by pressureless spark plasma sintering (SPS) with Al2O3-AlN-Y2O3 additives. The TiC0.3N0.7 phase particles are homogeneously dispersed in the β-SiAlON matrix. The reaction between the sample and the graphite mold can be avoided during the pressureless SPS. The β-SiAlON/TiC0.3N0.7 composite with 5 wt.% TiC0.3N0.7, fabricated at 1550°C, exhibits a high density of 3.28 g cm−3 (98% relative density), high Vickers hardness of 14.53 GPa, and high fracture toughness of 7.20 MPa m1/2. By further SPS under a pressure of 24 MPa at 1400 oC, these properties are enhanced to a higher density of 3.32 g cm−3 (full density), higher Vickers hardness of 15.87 GPa, and higher fracture toughness of 7.44 MPa m1/2. Obviously, the fracture toughness of the as-prepared β-SiAlON/TiC0.3N0.7 composites is apparently higher than those of the reported β-SiAlON-based ceramic composites prepared by pressurized SPS, which is mainly attributed to the highly dense microstructures of the interlocking lath β-SiAlON grains and the uniformly distributed TiC0.3N0.7 particles, and the disappearance of the oriented ununiform stress caused by uniaxial pressure in normal pressurized SPS. Accordingly, the pressureless SPS could be a potential technique for the fabrication of high-performance β-SiAlON matrix composites.

Joining of Si3N4/Si3N4 with in-situ formed Si-Al-Yb oxynitride glasses interlayer

Si3N4 ceramic was successfully joined to itself with in-situ formed Yb-Si-Al oxynitride glass interlayer. The joints were composed of three parts: (I) Si3N4 matrix, (II) oxynitride glass interlayer in which hexagonal or fine elongated β-sialon grains and a few ball-like β-Si3N4 grains exist, and (III) diffusion zone in Si3N4 matrix containing a thin dark layer and a ~ 25 µm thick bright layer. The seam owned similar microstructure to matrix and was inosculated with the matrix as a whole. The strength of the joint tended to increase with the increase of bonding temperature and reached the value of 225 MPa, when the joints were prepared at 1600 °C for 30 min under a pressure of 1.5 MPa. The high-temperature strength remained 94.7% and 75.2% of R.T. strength when the joints were tested at 1000 °C and 1200 °C, respectively. It may be contributed to the high softening temperature of the Yb-Si-Al oxynitride glass phase formed in the seam. Even suffered to the air exposure for 10 h at 1200 °C, the residual strength of the joints was still 143 MPa, attributed to the existence of YbAG phase.

Effect of La2O3 addition on densification behavior and properties of Mg-doped sialon

Mg-doped sialon ceramics with three compositions were hot pressed at 1850°C for 1h, using La2O3 as a sintering additive. Full density was achieved in these samples. The influence of La2O3 on densification behavior, phase assemblage, microstructure, and mechanical and optical properties were investigated. La2O3 addition could enhance the densification by supplying more low-viscosity liquid phase. Moreover, the development of elongated β-sialon grains was also facilitated mainly due to the segregation of La to prismatic plane. The La2O3-added samples exhibited high hardness (20GPa) and fracture toughness up to 4.9MPa·m1/2. Besides, high transmittance of ~ 48% was also achieved even if a certain amount of glassy phase remained. This was explained by the increased refractive index of La-containing glassy phase, which reduced light scattering.

Effect of composition on phase assemblage, microstructure, mechanical and optical properties of Mg-doped sialon

Mg-doped sialon (Mgm/2Si12−m−nAlm+nOnN16−n) ceramics with different compositions of m=2n=0.6, 0.84, 1.0, 1.2, 1.6 were hot pressed at 1850°C for 1h. Phase assemblage, microstructure, mechanical and optical properties of these samples were investigated. All samples achieved/approached full densification. However, the densification of Mg-doped sialon ceramics with higher MgO/AlN content becomes more difficult. Additionally, the anisotropic growth of β-sialon grains was significantly inhibited. The unique characteristics of Mg-doped sialon ceramics intrinsically derive from the formation of Mg-containing AlN polytypoids, which consumed most of the high-temperature liquid. Furthermore, their high stability at high temperatures accounts for the difficulty in preparing single-phase Mg-α-sialon., The hardness of these samples gradually increases while indentation fracture toughness gradually decreases with increasing m=2n value. Due to little residual glassy phase, high infrared transparency/translucency was more readily achieved in Mg-doped sialon. The m=1.2 sample possesses the maximum transmittance of ∼50% at ∼2μm.

Effects of FeMo alloy on nitridation and mechanical properties of reaction bonded β-Sialon/FeMo ceramic composites

β-Sialon/FeMo materials with higher strength and toughness were fabricated by a reaction bonding method using raw materials of silicon, Al2O3, AlN, FeMo and Sm2O3 at 1500 and 1600°C. The effects of FeMo alloy on nitridation and mechanical properties of the materials were investigated. Thermal analysis up to 1400°C and nitridation experiment at 1200 and 1300°C were performed. The density, bending strength and toughness of β-Sialon were determined. XRD analysis was performed to analyze the phase composition of the materials, and SEM was employed to analyze their microstructure. The results showed that, due to the addition of the FeMo alloy, the starting temperature of the reaction between silicon and nitrogen could be decreased, and the nitridation velocity could also be promoted. Therefore, the degree of nitridation could be increased at the same reaction temperature and time. SEM images showed that the addition of the FeMo alloy could promote the crystallization of β-Sialon and result in fine β-Sialon grains with a high aspect ratio. For reaction bonded β-Sialon/FeMo, the bending strength was improved and the fracture toughness was also increased.

A novel approach on the synthesis of β-SiAlON:Eu2+ phosphors from kaolin through carbothermal reduction and nitridation (CRN) route

Doping β-SiAlON crystal structure with rare earths leads to a superior tuneable luminescence properties for light emitting diodes (LEDs). However, the widely used conventional methods require high purity starting powders and time consuming processes. Here, a new approach to synthesise Eu2+ activated β-SiAlON phosphors from kaolin through carbothermal reduction and nitridation route was introduced. The Rietveld refinement on powder X-ray diffraction method and scanning electron microscopy with energy dispersive X-ray spectroscopy analyses showed that 78·28 wt-%Eu2+ doped β-Si6−zAlzOzN8−z (z = 2·1) was successfully obtained. Photoluminescence examinations under the 267 nm excitation revealed two distinct broad emission peaks: maxima at around 415 nm (violet) and 540 nm (green). Transmission electron microscopy observations demonstrated the incorporation of Eu2+ ions into β-SiAlON grains giving the observed violet and green emissions. It is anticipated that this study paves a way for cost effective synthesis of rare earth doped single phase β-SiAlON phosphors in LED applications.

Microstructure and Strength of Si<sub>3</sub>N<sub>4</sub> Joints Bonded with Sialon-Oxynitride Glass

Si5.4Al0.6O0.6N7.4(β-Sialon)/ Y1.75Si2.625Al1.0O7.5N1.25(oxynitride glass) composite solders with different ratio were designed using SiO2-Al2O3-AlN-Y2O3-Si3N4mixture powders to join Si3N4ceramic to itself. It was found that both solder composition and bonding temperature have strong influence on the microstructure and strength of the joints. As far as using a pure oxynitride glass solder was concerned, the thickness of the seam turned narrow with the increase of temperature, and the strength had a peak value at 1550 °C. When increasing the designed content of β-Sialon, serious composition separation occurred in the seam at low bonding temperatures, and the strength was also lower than as using pure oxynitride glass. Increasing bonding temperature, the strength of the joints was improved and reached the maximum value of near 80 MPa at 1600 °C for the designed 20%β-Sialon. In this instance, large amounts of fine elongated β-Sialon grains were uniformly distributed in the seam. However, for the 60%β-Sialon containing solder, some pores appeared in the seam and its strength was very low at bonding temperatures.

Electric Conductive Si<sub>3</sub>N<sub>4</sub>/TiN Composite Ceramic Manufactured by Liquid-Qhase Sintering

Si3N4 based electric conductive ceramics were fabricated by pressureless liquid phase sintering with La2O3-AlN and TiN as sintering and conductive additives. The Si3N4 powder was synthesized by SHS (self-propagation high-temperature synthesis) method. The bulk density, mechanical properties and electrical conductivity of the sintered samples were studied. Microstructure and phase composition were analyzed by SEM and XRD. The results show that the density of the samples is above 96%. The sintered samples consist of β-Sialon and the conductive phase. β-Sialon grains shows elongated and the conductive phase granular which formed electric conductive network. The sintered samples have good mechanical properties and its electric resistivity is from 1012 Ω•cm above drop to 1 Ω•cm below.

Identification of Ti incorporation into β-SiAlON crystal structure through transmission electron microscopy techniques

In this paper, we report the transmission electron microscopy (TEM) observations on the incorporation of Ti transition metal element into β-SiAlON crystal structure in a bulk β-SiAlON–TiN composite material. Considering our energy dispersive X-ray (EDX) and electron energy loss (EEL) spectroscopy results acquired by using nanometre-scale focused electron probe in scanning transmission electron microscopy (STEM) mode, the Ti-K characteristic X-ray lines and Ti-L3,2 edges were detected in the chemical composition of the β-SiAlON grains. These results clearly reveal that Ti can enter into the β-SiAlON crystal structure. It is anticipated that this data will provide the new engineering insights on the production of transition metal element doped SiAlON-based materials for different applications.

Microstructure observation of β-sialon-15R ceramics synthesized from aluminum dross

Fully dense β-sialon-15R multiphase ceramics were synthesized by hot pressing sintering using aluminum dross as raw material. Transmission electron microscopy (TEM) was used to study the microstructure, notably for the impurity and glass phase. The results show that β-sialon grains are generally equiaxed, and 15R AlN polytypoid grains show a fibre-like morphology. The main impurity phase is Fe5Si3. High resolution electron microscopy (HREM) results confirm the interface between β-sialon grains and/or 15R grains to be clean in the sample synthesized at 1750°C, and the glassy phase only exist in triple junctions and pockets. AlN polytypoids in the multiphase ceramics provide a path to reduce the glass phase formed from the oxide impurity in aluminum dross.

Optical and mechanical properties of Mg-doped sialon composite with La2O3 as additive

Abstract Using 0.5 wt.% La 2 O 3 as a sintering additive, Mg-doped sialon composite with the maximum infrared transmittance of 50% was fabricated by hot pressing at 1800 °C. The addition of La 2 O 3 significantly promotes the densification process of Mg-doped sialon and the anisotropic growth of β-sialon grains. As a result, the sintered material exhibits high hardness (20.2 GPa), fracture toughness (4.8 MPa m 1/2 ) and flexural strength (664 MPa). Furthermore, the nano-sized glassy phases concentrated at triple junctions have no obviously negative impact on infrared translucency of Mg-doped sialon.

Microwave sintering of β-SiAlON–ZrO 2 composites

Densification, phase transformation, microstructure evolution and hardness of microwave sintered β-SiAlON–ZrO 2 composites were investigated and compared with conventionally sintered samples. Sintering trials were performed by a high vacuum capable 2.45 GHz microwave furnace without decomposition. Microwave sintered samples showed better densification behavior than conventional sintered samples. The higher density observed in the case of microwave sintered samples was attributed to volumetric fast heating. X-ray diffraction results of conventionally sintered samples showed β-SiAlON, tetragonal ZrO 2 and ZrN phases, while, ZrO 2 reacted with nitrogen and completely transformed to ZrN in the case of microwave sintered samples. The aspect ratios of microwave sintered β-SiAlON grains were higher than conventional sintered samples whereas, hardness remained lower.

Synthesis of β-SiAlON-AlN-Polytypoid Ceramics from Aluminum Dross

Aluminum dross is one of the main secondary wastes during aluminum recycling procedure. It is utilized to synthesize β-SiAlON-AlN-polytypoid ceramics by the reduction-nitridation method in this paper. The effects of the reaction parameters, such as the Si/Al ratio, reaction temperature and holding time are studied in detail. The results indicate that the synthesized products are mainly composed of β-SiAlON and Mg-AlN-polytypoid at 1450°C to 1650°C for 6 h. As the Si/Al ratio changes from 1.0 to 2.5, the amount of β-SiAlON increases while the amount of Mg-AlN-polytypoid decreases. At 1750°C, the samples consist mainly of AlN and AlN-polytypoid because SiO gas is significantly evaporated. With the increment of synthesis temperature, the growth of elongated Mg-AlN-polytypoid grains is significantly accelerated. SEM and EDS analysis results for the samples formed at 1650°C for 6 h show that the β-SiAlON grains display a short platelet morphology, and Mg-AlN- polytypoid grains possess needle or lath morphology. This study suggests that it is possible to synthesize the low cost SiAlON ceramics with high toughness from aluminum dross.

Novel mixed α/β-SiAlONs with both elongated α and β grains

Yb-, Y- and Dy-doped mixed α/β-SiAlON ceramics were fabricated by hot-pressing using 10 wt.% BaAl 2 Si 2 O 8 (BAS) glass ceramic as an additive. The results showed that BAS could effectively facilitate the anisotropic growth of both α- and β-SiAlON grains. The α-SiAlON grains as well as the β-SiAlON grains were highly elongated. The obtained mixed α/β-SiAlONs possess much better mechanical properties than those of conventional mixed α/β-SiAlONs containing equiaxed α-SiAlON grains in their microstructure.