Silicon Aluminum Oxynitride Research Articles

Sialon and Alumina Modified UV-Curable Coatings with Improved Mechanical Properties and Hydrophobicity

This article describes the modification of UV-curable coatings with silicon aluminum oxynitride (Sialon) and aluminum oxide (Alu C), which improve the hydrophobicity of the coating surface and the scratch hardness. The contact angle is greater due to surface roughness being enhanced with inorganic fillers. Improved scratch resistance results from the formation of a sliding layer triggered by the diffusion of Sialon or alumina on the coating surface. One can observed an increase in the surface hydrophobicity as well as in the scratch hardness (up to 100%) when small amounts (5 wt.%) of the inorganic compounds are added. Imaging microscopies, i.e., SEM, OM, and AFM (with nanoscopic Young’s modulus determination), revealed the good distribution of both types of fillers in the studied matrix.

Spark plasma-sintered MoSi2-reinforced Y−α−SiAlON ceramics: mechanical and high temperature tribological properties

Silicon aluminum oxy-nitride commonly abbreviated as SiAlON is indeed one of the most promising non-oxide engineering ceramics due to its ease of formation compared to extremely covalent silicon nitride ceramic and scope of tailoring the material properties as per application demand. To make it more suitable for high-performance applications, composites containing various secondary phases have been attempted so far to improve the mechanical performance over its monolithic counterpart. In the present work, reinforcement of particulate molybdenum disilicide (MoSi2) in Y-α-SiAlON matrix has been undertaken under spark plasma sintering (1750 °C, 10 min, 50 MPa die pressure) followed by Vickers hardness (HV), fracture toughness (KIC), and high-temperature tribological properties under different conditions of the formed composites containing 10 and 20 wt.% secondary phase were investigated. Sintered specimens were ≥ 97.5% dense with negligible porosity. Addition of MoSi2 in Y−α−SiAlON resulted in reduced HV. The 20 wt.% MoSi2/Y−α−SiAlON composite showed ~ 11% lower HV1 compared to the monolith. On the contrary, KIC of the 20 wt.% MoSi2/Y−α−SiAlON composite was found to be around 24% higher compared to pure Y−α−SiAlON (KIC ≈ 3.9 MPa-m0.5). Additional fracture energy dissipation through crack deflection and bridging by the dispersed MoSi2 particulates was the primary reason behind obtaining higher fracture toughness for the composites. Unlubricated reciprocating ball-on-disc experiments against dense silicon nitride ball revealed a significant effect of MoSi2 oxidation in achieving improved wear resistance of the composites over the monolith, especially, beyond 300 °C in ambient air under applied normal loads up to 90 N and sliding distance up to 70 m.

Effect of secondary phases' structure on the dielectric properties of β-SiAlON

This study investigates the effect of secondary phases microstructure on mechanical and dielectric properties in beta silicon aluminum oxy-nitride (β-SiAlON) produced by pressureless sintering at temperatures of 1650 °C–1750 °C from a mixture of silicon nitride (Si3N4), alumina (Al2O3), aluminum nitride (AlN) and yttrium oxide (Y2O3). β-SiAlON obtained at and above 1700 °C sintering temperature, which showed a fully dense structure with a good combination of hardness and strength of 21–23 GPa and 870–890 MPa, respectively. Surprisingly, the dielectric constant of β-SiAlON was found to increase significantly from 6.4 to 8.2 with an increase in the sintering temperature from 1700 to 1750 °C. Transmission electron microscopy revealed the presence of secondary crystalline phase yttrium rich silicon aluminum oxynitride in the specimen sintered at 1750 °C , whereas yttrium rich amorphous interface structure was found in the specimen sintered at 1700 °C, suggesting the importance of controlling grain boundary structures in β-SiAlON.

Mechanically Robust SiAlON Ceramics with Engineered Porosity via Two-step Sintering for Applications in Extreme Environments

ABSTRACTPorous ceramics have been widely used under extreme environments due to their high strength, good thermal shock resistance, and excellent corrosion resistance. Recently, silicon aluminum oxynitride (SiAlON) ceramic, a solid solution of Si3N4 with AlN, SiO2, and Al2O3, attracted our interest because of its superior mechanical and physical properties for applications under extreme environments (i.e., high temperature, high pressure, excellent mechanical wear, and low PH). However, in spite of its many unique properties, porous SiAlON production has not been scaled up sufficiently to meet industrially relevant quantities due to its high synthesis cost and the difficulty of manufacturing articles. Here, we report on a scalable two-step carbothermal reduction and nitridation (CRN) method to synthesize mechanically robust SiAlON ceramic materials with controlled porosity levels. The morphologies and chemical composition of the synthesized porous SiAlON ceramics were characterized using SEM, XRD, EDAX, and microprobe analysis. In addition, the flexural strength of the engineered porous SiAlON ceramics is also reported in this paper.

Fabrication and properties of borazine derived boron nitride bonded porous silicon aluminum oxynitride wave-transparent composite

Boron nitride bonded porous silicon aluminum oxynitride composite was fabricated by gel-casting, precursor infiltration and pyrolysis process, and the composition, microstructure, mechanical and dialectical properties of the composite were characterized. The results show that the composite is comprised of β-SiAlON (z = 3) synthesized from mixed ceramic powders, and continuous h-BN pyrolyzed from borazine, with a relatively high porosity of 24.21%. The flexural strength, elastic modulus and fracture toughness of the composite are 178.58 MPa, 75.51 GPa and 4.54 MPa m1/2, respectively. The sintering shrinkage of SiAlON ceramics can be greatly decreased by the borazine infiltration and pyrolysis process. The existence of h-BN phase and the high porosity reduce the values of dielectric constant and loss tangent of the composite, which are 3.51–3.69 and 0.9–3.1 × 10−3 at the frequency from 7 to 18 GHz with the elevating temperature from 25 to 1200 °C.

Edge on Impact Simulations and Experiments

In the quest to understand damage and failure of ceramics in ballistic events, simplified experiments have been developed to benchmark behavior. One such experiment is known as edge on impact (EOI). In this experiment, an impactor strikes the edge of a thin square plate, and damage and cracking that occur on the free surface are captured in real time with high speed photography. If the material of interest is transparent, additional information regarding damage and wave mechanics within the sample can be discerned. Polarizers can be used to monitor stress wave propagation, and photography can record internal damage. This information serves as an excellent benchmark for validation of ceramic and glass constitutive models implemented in dynamic simulation codes. In this paper, recent progress towards predictive modeling of EOI is discussed. Time-dependent crack propagation and damage front evolution in silicon carbide (SiC) and aluminum oxynitride (AlON) ceramics are predicted using the Kayenta macroscopic constitutive model. Aspects regarding modeling material failure, variability, and volume scaling are noted. Mesoscale simulations of dynamic failure of anisotropic ceramic crystals facilitate determination of limit surfaces entering the macroscopic constitutive model, offsetting limited available experimental data.

Mesoscale modeling of nonlinear elasticity and fracture in ceramic polycrystals under dynamic shear and compression

Dynamic deformation and failure mechanisms in polycrystalline ceramics are investigated through constitutive modeling and numerical simulation. Two ceramics are studied: silicon carbide (SiC, hexagonal crystal structure) and aluminum oxynitride (AlON, cubic crystal structure). Three dimensional finite element simulations incorporate nonlinear anisotropic elasticity for behavior of single crystals within polycrystalline aggregates, cohesive zone models for intergranular fracture, and contact interactions among fractured interfaces. Boundary conditions considered include uniaxial strain compression, uniaxial stress compression, and shear with varying confinement, all at high loading rates. Results for both materials demonstrate shear-induced dilatation and increasing shear strength with increasing confining pressure. Failure statistics for unconfined loading exhibit a smaller Weibull modulus (corresponding to greater scatter in peak failure strength) in AlON than in SiC, likely a result of lower prescribed cohesive fracture strength and greater elastic anisotropy in the former. In both materials, the predicted Weibull modulus tends to decrease with an increasing number of grains contained in the simulated microstructure.

Synthesis, crystal structure and photoluminescence of a new Eu-doped Sr containing sialon (Sr0.94Eu0.06)(Al0.3Si0.7)4(N0.8O0.2)6

Colorless transparent platelet single crystals of a novel Eu2+-doped strontium silicon aluminum oxynitride, (Sr0.94Eu0.06)(Al0.3Si0.7)4(N0.8O0.2)6, were prepared at 1800°C and 0.92MPa of N2. Fundamental reflections of electron and X-ray diffraction of the crystals were indexed with a face-centered orthorhombic unit cell (a=5.8061(5)Å, b=37.762(3)Å, c=9.5936(9)Å). Diffuse streaks elongated in the b-axis direction were observed around the fundamental reflections hkl with h=2n+1 of the electron and X-ray diffraction, indicating stacking faults of (010)[100]/2. A crystal structure model without the stacking faults was obtained using the X-ray diffraction data of the fundamental reflections with the space group Fdd2. A SiN4-tetrahedron double layer of [SiN2]2 and a Sr/Eu double layer of [(Sr0.94Eu0.06)Al1.2Si0.8N0.8 O1.2]2 are stacked alternately along the b-axis direction. The title compound showed an emission with a peak wavelength of 490nm under 334nm excitation at room temperature.

Carbonitridation of Fly Ash. 3. Effect of Indecomposable Additives

A series of indecomposable additives was tested for its effect on the nitridation of fly ash. The carbonitridation reaction was carried out in a tubular high-temperature furnace. SiAlON (silicon aluminum oxynitride)-based powders were prepared from fly ash/carbon mixture in a nitrogen atmosphere. Fly ash from the Huntley and Hatfield power plant was examined in this paper. The additives, in the amount of 4 wt %, are represented by several inorganic salts. Systematic analysis made it possible to select proper elements or groups having a strong effect on the nitridation reaction. The results revealed the possibility of manufacturing of expensive SiAlON whiskers at a low cost. This group of additives was never tested in the literature. A nitrogen/oxygen analyzer measured nitrogen content in the final product. Weight losses were also measured to indicate the extent of the evaporation of the solid materials. Scanning electron microscopy and an energy-dispersive X-ray spectrometer were used to characterize the ...

Carbonitridation of Fly Ash. II. Effect of Decomposable Additives and Whisker Formation

A series of additives releasing ammonia after thermal decomposition were examined to evaluate their effect on the carbothermal reduction and nitridation of fly ash in the preparation of SiAlON (silicon aluminum oxynitride) powders. The reactions were studied at 1400 °C and times of 1-4 h. The addition of certain decomposable additives can accelerate the nitridation. Therefore, a reduction of the nitridation temperature and/or a shorter nitridation time can be achieved. Ca-SiAlON whiskers were observed in the reaction products by scanning electron microscope, X-ray diffractometer, and energy-dispersive X-ray spectrometer measurements. The nitrogen content in the final product was measured by nitrogen/oxygen analyzer as an index of the extent of the nitridation reaction. Fly ash powders from the Huntley and Hatfield power plants were investigated in this work.

Carbonitridation of Fly Ash. I. Synthesis of SiAlON-Based Materials

The purpose of this paper is to examine the effect of reaction parameters on the preparation of SiAlON powder. The conditions to obtain maximum concentration of nitrogen in the product were also examined and discussed. SiAlON (silicon aluminum oxynitride) powders have been prepared from Huntley fly ash in a tubular laboratory high-temperature furnace through carbothermal reduction and nitridation reactions. Almost pure β-SiAlON can be produced from fly ash at 1300 °C in a nitrogen atmosphere. Depending on the reaction conditions, the nitrogen content could reach as high as 30%. Nitrogen in the SiAlON products was measured by a nitrogen/oxygen analyzer. X-ray diffraction and Brunauer−Emett−Teller surface area analysis have been used to further characterize the products.

Lubrication of High Rolling Speed Ceramic Contact with Two Percent Slip at 815°C (1500°F)

A selection of liquid lubricants was vaporized and deposited on a hot, 705°C (1300°F), ceramic surface. Friction was then evaluated in low speed, 7 MPa (1000 psi), sliding contact. The ceramic materials included aluminum oxide, silicon aluminum oxynitride (SiAlON), silicon nitride, and silicon carbide. Each specimen was mated with an equally hot specimen of the same material. Deposits reduced friction significantly compared to the unlubricated contact. Two superior lubricants were then evaluated in continuous lubrication of silicon nitride using a high speed ball-on-disk contact. Rolling speed was 2.5 m/sec at a Hertzian stress of 2 GPa (300 ksi). Operating temperatures were varied between 621° C (1150°F) and 815°C (1500°F). With the slip level maintained at 2%, the traction coefficient recorded at 815°C was 0.05, comparable to current metallic bearings in aerospace applications. Wear volume was reduced by an order of magnitude compared to the unlubricated case. Temporarily stopping the lubricant flow resulted in a steady rise of the traction coefficient, while resuming the lubricant flow reduced the traction to the original level. Presented at the 51st Annual Meeting in Cincinnati, Ohio May 19–23, 1996

Thermal conductivities of thin, sputtered optical films

The normal component of thin-film thermal conductivity has been measured for the first time, to the best of our knowledge, for several advanced sputtered optical materials. Included are data for single layers of boron nitride, silicon aluminum nitride, silicon aluminum oxynitride, silicon carbide, and for dielectricenhanced metal reflectors of the form Al(SiO(2)/Si(3)N(4))(n) and Al(Al(2)O(3)/AlN)(n). Sputtered films of more conventional materials such as SiO(2), Al(2)O(3), Ta(2)O(5), Ti, and Si have also been measured. The data show that thin-film thermal conductivities are typically 10 to 100 times lower than conductivities for the same materials in bulk form. Structural disorder in the amorphous or fine-grained films appears to account for most of the conductivity difference. Conclusive evidence for a film-substrate interface contribution is presented.

Observation of mixed aluminum oxynitride (Al(O,N)4) structural units by aluminum-27 magic angle spinning NMR

It is shown for the first time that the application of high-speed magic angle spinning (MAS) (up to 16 kHz) and high applied magnetic fields (up to 11.7 T) allow unequivocal identification of specific mixed aluminum local coordinations AlO y N 4-y (y=1, 2, or 3) in a bulk solid using 27 Al NMR spectroscopy. In combination with 29 Si MAS NMR the structures of three silicon aluminum oxynitride ceramics (sialon), 15R and 21R AIN-polytypoid and β'-sialon, are refined, confirming strict bonding preferences of Si-N and Al-O in these materials

Ceramic coatings on ceramic for improved oxidation-corrosion resistance

Thin films of covalent ceramics were deposited onto a variety of hot-pressed and pressureless sintered silicon nitride substrates to reduce oxidation-hot corrosion of the underlying material and to minimize pitting and localized attack of the substrate. The extent of oxidation-corrosion of the hot-pressed and pressureless sintered nitride ceramics in the presence of thin films of sodium sulfate at 1000 °C was dependent on the out-diffusion of cations from the intergranular phase into the predominantly silica corrosion product. Silicon nitride, silicon oxynitride, silicon carbide and silicon aluminum oxynitride films were deposited by reactive r.f. sputtering to form an effective diffusion barrier to cation migration. The amount of protection provided by these covalent ceramic coatings was dependent on the composition, microstructure and phase distribution within the coating. Crystallization of the as-deposited films was performed by conventional furnace heat treatment. Corrosion rates of the silicon nitride ceramics protected with covalent ceramic coatings were significantly reduced. Pitting of the ceramic substrates was also significantly reduced when protective coatings were applied.

ChemInform Abstract: THE INTERACTION OF MOLTEN SILICON WITH SILICON ALUMINUM OXYNITRIDES

AbstractMischkristalle zwischen β‐Si3 N4 und Al2O3 N (I) (ß′‐Sialon) und zwischen Si, N3 Ound Al2O3 (II) (O′‐Sialon) verhalten sich im Kontakt mit geschmolzenem Si unterschiedlich.

The Interaction of Molten Silicon with Silicon Aluminum Oxynitrides

Silicon aluminum oxynitride solid solutions (sialons) based on β‐Si3N4 and Si2N2O behave differently in contact with molten sih'con. The Si2N2O‐based sialons convert to almost pure Si3N4, apparently through a two‐step decomposition and solution‐precipitation reaction, whereas the β‐Si3N4 sialons are preferentially attacked at the grain boundaries. The composition of the grain‐boundary phase appears to control the rate of reaction.

Effect of Si3N4 Powder Reactivity on the Preparation of the Si2N2O‐Al2O3 Silicon Aluminum Oxynitride Solid Solution

Dense high‐purity Si2N2O‐Al2O3 silicon aluminum oxynitride (O'‐sialon) was prepared by reactive hot‐pressing of an Si3N4‐Al2O3‐SiO2 mixture. The formation of a single‐phase material was found to be critically dependent on the Si3N4 powder in the Si3N4‐Al2O3‐SiO2 starting mixture. It is suggested that evolution of a chlorine‐ and nitrogen‐containing species may enhance the reactivity of Si3N4 in this reaction. Densities of O'‐sialons are very similar to that of Si2N2O (2.82 g/cm3), the widely quoted value in the ceramics literature of 3.1 g/cm3 for the density of Si2N2O being incorrect.

ChemInform Abstract: REACTION OF SI3N4 WITH AL2O3 AND Y2O3

AbstractDie genannte Reaktion wird hinsichtlich des Verdichtungseinflusses der Oxide Al2O3 und Y2O3 auf Si3N4 untersucht und die gebildeten Phasen werden röntgenographisch ermittelt.