Fused Silica Powders Research Articles

Boosting the collapse performance of Wax-binder-free silica based ceramic core via adding nano-alumina sol modified fused silica powder

Boosting the collapse performance of Wax-binder-free silica based ceramic core via adding nano-alumina sol modified fused silica powder

Reinforcement of silica-based ceramic cores based on amorphous and polycrystalline mullite fibers

In the investment casting of turbine blades, ceramic cores are key components to form complex hollow structures. Superior mechanical property and leaching rate are demanded for ceramic cores. Herein, ceramic cores were fabricated using fused silica powders as the matrix, and amorphous and polycrystalline mullite fibers as the reinforcement phases, respectively. The microstructure and property evolution of ceramic cores rely on the crystallization degree of mullite fibers are explored. Both of the mullite fibers lead to improved crystallization of cristobalite, reduced sintering shrinkage, increased apparent porosity, and benefited bending strength, creep resistance, and leaching rate of the cores. Compared to the polycrystalline mullite fibers, the amorphous fibers are metastable with large quantities of structural defects, promoting the diffusion mass transfer and forming strong interface between fibers and matrix. Therefore, the amorphous fibers have larger promotion on the bending strength and resistance to creep deformation of ceramic cores. Moreover, the structural defects of amorphous fibers ensures the high chemical activity in alkaline solutions and exhibits excellent leaching rate. The ceramic core with 4.5 wt% of amorphous mullite fibers exhibits excellent comprehensive performance with bending strengths of 28.9 MPa and 23.8 MPa at room temperature and 1550 °C, creep deformation of 0.3 mm, and leaching rate of 1.4 g/min, well meeting the casting requirements of hollow blades.

Analysis of the slurry and ceramic properties for investment casting obtained with domestic colloidal silica binders

The quality of cast parts produced by investment casting is largely determined by the quality of the ceramic molds. Currently, aircraft and engine building enterprises are switching to an environmentally friendly colloidal silica binders for the manufacture of ceramic molds. In this work, the dynamic and relative viscosity of slurries prepared using fused silica powder and colloidal silica binders of the VT13-02U (Vakuumteh LLC), Stavroform VS (Polymet LLC), UltraCast One + and UltraCast Prime (both Technopark LLC) manufacturers were determined. It is shown that the slurries prepared on the considered binders have similar viscosity values, and in their rheological properties they are close to Newtonian liquids. The values of dynamic and relative viscosity at a binder content of 400 mL per 1 kg of fused silica powder were ~732 mPa·s and ~380 s, respectively. With an increase in the binder content to 600 mL per 1 kg of fused silica powder, the dynamic and relative viscosity decreased to ~70 mPa·s and ~16 s, respectively. An equation was also found that relates the dynamic viscosity determined using a rotational viscometer and the relative viscosity determined using the VZ-4 viscosimeter. The mechanical properties were determined during three-point bending tests on ceramic samples obtained using slurries on the above-mentioned colloidal silica binders and fused silica stucco. Samples obtained on binders VT13-02U, Stavroform VS and UltraCast One+ showed very similar bending strength values, namely 3.5–4.3 MPa after drying and 5.8–6.1 MPa after firing. Due to the presence of a polymer addition in the binder, the ceramic samples obtained on the UltraCast Prime binder had higher values of bending strength after drying and after firing – 6.4 and 7.2 MPa, respectively. It was also shown that with an increase in the viscosity of the slurry and a decrease in the fraction of fused silica stucco, the strength of the samples increases. The lowest surface roughness was observed for samples obtained with UltraCast grade binders.

High emissivity scale structure MoSi2-silicon glass coating on aluminum silicate fiber cloths with excellent flexibility and oxidation resistance below 1000 °C

In order to maintain the strength of flexible substrate while applying the oxidation resistance dense glass coating, a scale structure coating on aluminum silicate fiber cloth was prepared with fused silica powder and silica sol as the binder and MoSi2 as the emissive agent. The scale structure of the coating with large cracks along the braided grain and small random cracks on the fiber bundles allowed the fiber cloth to retain its flexible properties. Due to the block effect of dense SiO2 glass phase on oxygen diffusion, the mass loss was only 0.35% after oxidation at 800 °C for 24 h while the average normal emissivity (2–14 μm) remained about 0.88. The MoSi2-silicon glass coating could avoid the generation of excessively strong interface with the substrate and improve the tensile strength of the sample to 140.52 MPa.

Preparation and Evaluation Mechanic Damping Properties of Fused Silica Powder@Polyurethane Urea/Cement Composites.

In this paper, cement based on fused silica powder @ polyurethane urea (FSP@PUU) with a micro constrained damping structure was studied. Firstly, FSP@PUU core-shell particles were prepared by heterogeneous stepwise addition polymerization method and added into cement paste as damping filler to form a micro-constrained damping structure inside cement paste. The mechanical property and damping performance of cement-based composites were characterized by compressive strength, dynamic mechanical analysis (DMA) test and modal vibration test. The results showed that the damping performance of FSP @ PUU cement-based composites was affected by temperature, and the loss tangent of cement with 6wt% FSP@PUU increased to about 0.057 at −35 °C to 35 °C, which was 1.5 times cement paste within the glass transition temperature. With 6 wt% FSP@PUU, the damping ratio of cement-based composites increased by 58% compared with cement paste in the frequency range of 175–300 Hz, while the compressive strength decreased by only 5%. The cement with suitable FSP@PUU possesses excellent damping performance.

Anisotropy management on microstructure and mechanical property in 3D printing of silica-based ceramic cores

Ceramic core is an essential component in the precise casting of hollow turbine blades, and the investigation on 3D printing of silica-based ceramic cores is crucial to the development of aviation industry; however, they are suffered from difficulty in high-temperature strength and structural anisotropy. In present work, silica-based ceramic cores were prepared via DLP stereolithography 3D printing, and the anisotropy management on microstructures and properties were explored based on the particle size of fused silica powders. In 3D printed ceramic cores with coarse powders, significant anisotropy was displayed exhibiting multilayer structure with large gaps in horizontal printing and uniform porous microstructure in the vertical direction, which was further explained by the particle deposition in printing. With finer silica powders, the uniformity in the microstructures was highly improved, attributed to the enhanced particle dispersion in ceramic slurries and promoted interlayer particle rearrangement during sintering. To evaluate the anisotropy in mechanical property, the ratio of vertical strength to horizontal strength (σV/σH) was proposed, which rose from 0.48 to 0.86 as the particle size decreased from 35 µm to 5 µm, suggesting enhanced mechanical uniformity. While the average particle size of silica powders was 5 µm, the flexure strengths of ceramic cores in different directions were up to 18.5 MPa and 16.3 MPa at 1540 °C with σV/σH ratio of 0.88, which well satisfied the demands for the casting of turbine blades. This work inspires new guidance on the anisotropy management in ceramic cores prepared by 3D printing, and provides new technology for fabrication of silica-based ceramic cores with superior high temperature mechanical properties.

Experimental study on high temperature performances of silica-based ceramic core for single crystal turbine blades

Silica-based ceramic cores are widely utilized for shaping the internal cooling canals of single crystal superalloy turbine blades. The thermal expansion behavior, creep resistance, and high temperature flexural strength are critical for the quality of turbine blades. In this study, the influence of zircon, particle size distribution, and sintering temperature on the high-temperature performance of silica-based ceramic cores were investigated. The results show that zircon is beneficial for narrowing the contraction temperature range and reducing the shrinkage, improving the creep resistance and high-temperature flexural strength significantly. Mixing coarse, medium and fine fused silica powders in a ratio of 5:3:2, not only reduced high temperature contraction, but effectively improved the creep resistance. Properly increasing the sintering temperature can slightly reduce the thermal deformation and improve the high-temperature flexural strength of the silica-based core, but excessively high sintering temperature negatively impacts the creep resistance and high-temperature flexural strength.

Effect of Cristobalite Content on Physical, Dielectric Constant, and Bending Strength of Fused Silica Ceramics Formed by Slip Casting Method

Fused silica ceramics are widely used in electronics and aerospace industries. In the present study, 70 µm of fused silica powder was milled to 10 µm through fast milling. The appropriate slurry was prepared for slip casting with the powder-to-water ratio of 80:20. After drying the specimens, the samples were sintered at different temperatures of 1100 °C to 1400 °C. The density increased upon increasing the temperature from 1.79 to 1.98 g/cm3. The phase transformation of the samples was investigated using XRD. The structure of the samples was analyzed using FTIR, and their microstructure was examined using a Field Emission Scanning Electron Microscope (FESEM). The bending strength of the samples was measured using the three-point method. According to the results, the cristobalite phase increased upon increasing the sintering temperature. The best flexural strength value (48.7 MPa) was obtained for the sample sintered at 1300 °C. The dielectric constants of the fused silica ceramics were about 3-3.8 in the frequency range of 8 to 12 GHz.

Enhanced thermal properties of silica-based ceramic cores prepared by coating alumina/mullite on the surface of fused silica powders

In this study, an alumina/mullite coating was synthesized on the surface of fused silica powders to form an alumina/mullite-silica core-shell structure. The effects of the alumina/mullite coating on the cristobalite crystallization, thermal properties, and leachability of the silica-based ceramic cores were investigated using the simulated casting process. The X-ray diffraction results indicated that the crystallization of cristobalite was significant at the simulated casting temperature of approximately 1400 °C. An increase in the cristobalite content during this stage resulted in a large thermal expansion because of its higher coefficient of thermal expansion compared with that for fused silica. The addition of optimum amounts of the alumina/mullite powders resulted in an increase in the initial shrinkage temperature and a decrease in the shrinkage of the specimens. When the coating powders were added at 43 wt%, the initial shrinkage temperature increased from 1092 °C to 1200 °C and the shrinkage decreased sharply. Leaching tests showed that the silica-based ceramic cores were removed in the form of stripped layers. The washing and shaking process accelerated the disintegration of the ceramic core and improved its leachability.

Porous fused silica ceramics prepared by gelcasting using multigrade fused silica powders

Porous fused silica ceramics were successfully prepared by gelcasting using N, N–dimethylacrylamide (DMAA) as gelling agent and multigrade powders as starting materials. The results suggest that the ratio of fine–grained to coarse–grained mixed powders, solid loading and sintering temperature were highly correlated with microstructure and properties of fused silica ceramics. When the ratio was 0.8 and solid loading was 80 vol%, the resulting slurries had a relatively high viscosity, which ensured the excellent homogeneity and stability of slurries, ultimately endowing fused silica ceramics desirable microstructure and properties. As sintering temperature increased from 1225 °C to 1300 °C, the bulk density (1.78–2.05 g/cm3) and linear shrinkage (1.98–4.23%) were increased, but the flexural strength was only enhanced within 1275 °C with a maximum of 20.69 MPa, since excess fused silica crystallized into cristobalite at 1300 °C, which deteriorated mechanical properties.

Study on Molding of Fused-Silica Material by Pressure Slip-Casting from Alcohol-Based Slurry

In this manuscript a kind of alcohol-based slurry was prepared, in which the fused-silica powder, alcohol and thermosetting phenolic resin were the main components. Then, the fused-silica material was prepared by the process of pressure slip-casting, drying, curing and high temperature sintering. And at last, the properties of fused-silica material were analyzed. The slurry used for molding consisted of 82.5% fused-silica powder in which the 6 µm and 30 µm powders were at the ratio of 8:1, 17% alcohol and 0.5% phenolic resin, and settlement of the slurry was 3% after standing still for 2 hours. By pressure slip-casting at 0.5MPa pressure for 30 minutes, the green body with 12 mm thickness was obtained, and the strength of which could reach 2.3MPa after being cured at 90°C for 4 hours. After sintering at 1200°C for 2 hours, the strength of the fused-silica material reached 78MPa and the elastic modulus was 37.7GPa, and quartz and other crystalline phases did not appear in the test of XRD. Scanning electron microscopy showed that silica particles were uniformly distributed in the material.

High strength silica-based ceramics material for investment casting applications: Effects of adding nanosized alumina coatings

A nanosized alumina coating was synthesized on the surface of fused silica particles by electrostatic attraction. The effects of the coated fused silica particles on the cristobalite crystallization behavior, microstructure evolution, and flexural strength of silica-based ceramic cores were investigated. X-ray diffraction (XRD) was used to characterize phase transformations in the specimens, and the results indicated that the formed nanosized alumina coatings could retard cristobalite formation by inducing compressive stress on the fused silica particle surface. A mullite phase was also found due to the reaction of the nanosized alumina coating and the surface of the fused silica when the sintering temperature was increased to 1300 °C. Analysis using scanning electron microscopy equipped with energy dispersive spectrometry (SEM/EDS) suggested that alumina nanoparticles in the coated layer dispersed into a liquid phase and formed a barrier layer to impede the movement of the liquid phase, preventing the pore-filling process and increasing the open porosity of the ceramic specimens. Flexural strengths at room temperature were tested, indicating that increases in the sintering temperature of the specimens without coated fused silica powders had little effect on flexural strength. However, the flexural strength of the specimens with coated fused silica powders increased with increases in sintering temperature. The improvement in flexural strength was related to the reinforcement by sintering necks between particles and the improvement in the strength of the coated fused silica powder.

Preparation of silica ceramic cores by the preceramic pyrolysis technology using silicone resin as precursor and binder

Abstract A facile and effective preceramic pyrolysis technology was used to prepare the fused silica ceramic cores, among which fused silica powders were used as the raw materials and the silicone resin with -Si-O-Si- as the main chains was used as the precursor. Effects of the silicone resin content and sintering temperature on the properties of fused silica ceramic cores were researched. Results showed that the silicone resin powders had the good cohesiveness, which was beneficial for directly shaping the silica ceramic cores. The organic to inorganic transformation of silicone resin occurred with the increase of temperature, forming the amorphous SiO2 at the temperature less than 1250 °C and the cristobalite by the crystallization process of amorphous SiO2 at the temperature more than 1250 °C. Based on TG analysis, it was found that the ceramic yield (SiO2) of silicone resin had 83.55wt%. In green bodies, the silicone resin was mainly coated on the surface of fused silica particles via the function of pressing force. After the pyrolysis process was completed, the silicone resin formed some small amorphous silica particles, which was coated on the surface of raw fused silica and filled into the gaps between fused silica particles. With the increase of silicone resin content, the apparent porosity gradually increased, and bending strength showed a decreasing trend. The increasing temperature from 1200 °C to 1300 °C promoted the densification of silica ceramic cores when the silicone resin content was the same. However, the highest bending strength was obtained at 1250 °C. Further increasing temperature was detrimental to the bending strength of silica ceramic cores owing to the microcracks caused by the surface crystallization of fused silica to cristobalite.

Microstructure and mechanical properties of boron nitride nanosheets-reinforced fused silica composites

In order to overcome intrinsic brittleness and poor mechanical properties of fused silica (FS), boron nitride nanosheets (BNNSs) as a novel reinforcement were employed for fabrication of BNNSs/fused silica composites. BNNSs with micron lateral size were homogeneously dispersed with FS powder using a surfactant-free flocculation method and then consolidated by hot pressing. The flexural strength and fracture toughness of the composite with the addition of only 0.5wt.% BNNSs increased by 53% and 32%, respectively, compared with those of pure FS. However, for higher BNNSs contents the improvement in mechanical properties was limited. Microstructural analyzes have shown that the toughening mechanisms are combinations of the pull-out, crack bridging, and crack deflection mechanisms.

Mechanical Properties of Epoxy Resin Composite in Hot and Humid Environment

The mechanical properties of epoxy resin will be influenced significantly in hot and humid environment. So silica powder and other fillers are often used to improve the mechanical properties of epoxy resin in real productions. In this paper, the mechanical parameters of epoxy resin and fused silica powder under different temperatures and moisture contents are investigated through the approaches of molecular dynamics simulation and generalized self-consistent method. There-after, the representative volume elements of epoxy resin and silica powder composite materials are constructed. The mechanical parameters of the composite materials are obtained through finite element simulations. Through the comparison of the different approaches, it can be found that the mechanical properties of epoxy resin composite are dwindled in hot-humid environment, which can be improved significantly by filling with silica powder.

The importance of surface hydration and particle shape on the rheological property of silica-based suspensions

Two types of ball milling methods, wet planetary ball milling and dry tumbling ball milling, were used to grind fused silica powders for the preparation of silica-based suspensions in this experiment. The effect of surface hydration and particle shape caused by the two milling methods on the slurry rheology was investigated. The results showed that, with similar particle size distribution, the viscosity of the suspensions prepared from the powders milled by wet ball milling ranged from 275mPas to 311mPas within 20min and the suspension exhibited a continuous shear thickening behaviour whereas the viscosity of the suspensions prepared from the powders milled by dry ball milling ranged from 69mPas to 74mPas and the suspension exhibited a shear thinning behaviour first, and then exhibited a slightly shear thickening behaviour. The reasons were attributed to the differences in surface hydration and particle shape. It was postulated that the two factors affected the slurry rheology through the modification of particle interactions during the flow of high concentration suspensions.

Development and characterization of polymer–ceramic continuous fiber reinforced functionally graded composites for aerospace application

Functionally graded materials (FGMs) have continuous change in the composition across the thickness which results in gradual change of the properties. The ability of the FGMs to perform more than one contrasting functions as an integral structure makes them an attractive material for aerospace applications. So far the research focus has been on the functionally graded coatings and particulate reinforced functionally graded composites. In this study, continuous fiber reinforced functionally graded composites (FGCs) have been prepared using quartz fabric reinforcement for thermo-structural aerospace application. Silicone resin and fused silica powder have been used to obtain graded matrix. The FGC laminates have been characterized for mechanical and thermal properties. The laminates were also evaluated for thermal shock resistance and thermal insulation properties by exposing to infra-red heating lamps under high heating rate of about 800°C/s and high heat flux of 0.6 Wmm−2 for 70 s. FGC laminates did not de-laminate or charred. Temperature drop across the laminate of 8 mm thickness was found to be 600°C. The concept of FGC fabrication process is demonstrated to fabricate scaled down typical airframe sections also.

Effect of Preforming Process and Starting Fused SiO2 Particle Size on Microstructure and Mechanical Properties of Pressurelessly Sintered BNp/SiO2 Ceramic Composites

Hexagonal BN particles (BNp) reinforced fused SiO2 matrix composite (BNp/SiO2) was regarded as a very important candidate material for potential applications on wave transparent thermal protection parts. In this study, pressurelessly sintered 15 vol% BNp/SiO2 composites were prepared by cold isostatic pressing (CIP) and gel‐casting routes, respectively, using two kinds of fused silica powders with different particle sizes (5.82 and 3.24 μm in d50, respectively) as starting SiO2. It was demonstrated that the microstructure and mechanical properties of the composites were well dependent on the preforming techniques of green compacts. Gel‐casting preforming route has an overwhelming superiority over CIP route on the thermal stability of fused SiO2 matrix against α‐cristobalite crystallization, sinterability, and thus the mechanical properties. Using the finer fused silica (3.24 μm in d50) as the starting SiO2, the 15 vol% BNp/SiO2 composite sintered at 1375°C get a density of 2.05 g/cm3, the silica matrix primarily keep amorphous phase with uniformly dispersed nanosized α‐cristobalite crystallites (<5 nm in diameter). The bending strength, fracture toughness and Young's modulus all reach the highest values, 101.5 ± 4.3 MPa, 1.57 ± 0.04 MPa·m1/2 and 61.3 ± 2.4 GPa, respectively. Microstructure characteristics, chemical bond information and mechanical properties of the as‐prepared composites are correlated with their preforming routes, starting fused SiO2 particle size, sintering temperature, etc. based on X‐ray diffractometry, scanning electron microscope, transmission electron microscope, high resolution transmission electron microscope, and FT‐IR analysis.

Rational design in suspension rheology of cellular silica

Slurry based processing route involving the use of partially hydrophobized fused silica particles, binders, long chain surfactants and drying control additives have been optimized for the fabrication of cellular silica. The major accomplishments of this study are the optimization of the fused silica powder based suspension composition, foaming duration and foam stabilization to prepare cellular silica-based components. Rheological studies have been carried out on the suspensions to examine the effect of solids loading, and volume fractions of individual additives on the viscosity, with the objective of optimizing the slurry composition and viscosity for maximum air entrainment and foam stability. The rheological studies on silica-based suspensions have exhibited that the final properties of the foams depend on the slurry composition. The suspensions prepared in the study have been found to be non-Newtonian, and have shown shear thinning behavior at higher shear rates. The role of viscosity and surface tension has shown a significant effect on the flow behavior, foaming characteristics and stability of the air bubbles in a suspension, which in turn affects the overall pore volume fraction and porous architecture of the finished foams.

Structure–property correlation in cellular silica processed through hydrophobized fused silica powder for aerospace application

Attempts have been made towards a novel process for the preparation of cellular silica with interconnected cells, tailored porosity and pore size distribution using a combination of hydrophobized fused silica powder and direct foaming methodology. The process has resulted into cellular silica tiles as large as 100 × 100 × 25 mm dimensions. The resulting ceramic foams sintered at 1100 °C, consisted of a highly interconnected network of spherical cells with densities as low as 10% of theoretical. The pore size distributions and cell size have been found in the range of 50–250 μm and ∼6–16 ppi, respectively. The microstructure has shown an open and interconnected porosity with an average permeability occurring in the region of ∼10 −8 m 2. The creation of highly densified cell walls and tortuous struts between the cells has led to foams with a comparatively high compressive strength and Young's modulus and with an excellent thermal shock properties.