Electrophoretic deposition of ceramic coatings on ceramic composite substrates

Abstract

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al₂O₃/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.     

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.     

Investigation of microstructure evolution and properties in silica-based ceramic cores with alumina as a mineralizer

In this work, silica-based ceramic cores with alumina as a mineralizer were prepared via an injection molding method, and the effects of alumina on the microstructural evolution and properties at 1450°C (simulating the process of equiaxed castings) and 1550°C (simulating the process of columnar/single crystal castings) were investigated. It was found that alumina promoted the cristobalite crystallization of fused silica refractory during sintering but inhibited the devitrification rate in the subsequent heating. The flexural strength of silica-based ceramic cores at an ambient temperature and 1450°C improved with an increasing alumina content, whereas the opposite trend appeared at 1550°C. The creep resistances of silica-based cores were improved significantly and then slightly deteriorated with an increasing alumina content from 5% to 20%, depending on the competition effects of alumina hindering the viscous flow of liquid silica (favorable), but suppressing the devitrification rate (unfavorable). The results of this work show that silica-based cores need to follow different compositional design principles for equiaxed and columnar/single-crystal turbine blade castings.     

Improved collapse properties of silica-based ceramic cores via flexibly regulating cristobalite content

Although silica-based ceramic cores have important applications in the precision casting of metallic devices, their high-temperature stability and removal performances are seriously affected by the liquid phase sintered fused silica. Herein, we develop a manufacturing strategy of high-collapse silica-based ceramic core via using cristobalite crystals as the sintering inhibitor, waterglass as the binder, and injection moulding at 100°C and 80 MPa, followed by heat treatment simulating the casting process for sintering at 1200°C and 1500°C. The results demonstrated that the addition of cristobalite crystals could effectively form the core skeleton to ensure high-temperature performance. Meanwhile, it inhibited the liquid flow during sintering and induced the crytsallization from fused SiO₂ glass into cristobalite crystals, and the resulting plenty of micropores and microcracks within the microstructure effectively improve the removal performance. Especially, the porosity was highest up to 35.36% and the flexural strength was only 6.74 MPa when the addition of cristobalite reached 45%, realizing a 100% removing by high-frequency and fast-speed specific mechanical vibration. And, the casting is guaranteed to be flat and free of defects. This work provides a simple and flexible strategy to manufacture high-collapse silica-based ceramic cores, which can be removed by specific mechanical vibration without immersion in acid or alkali solutions after casting.     

Sintering Behavior of Pt Metal Particles Supported on Silica-Coated Alumina Surface

The sintering behavior of Pt metal particles was studied by supporting them on silica-coated alumina. Silica coating was found to be effective for the retention of a large surface area of alumina even after calcination at elevated temperatures. Before sintering, the size of Pt metal particles on all the silica-coated aluminas, including the uncoated alumina, was identical, while the particle size was larger on silica than on alumina. After sintering the Pt catalyst at 1073 K, the particle size increased on uncoated alumina as well as on alumina coated with thicker silica layers, especially on the supports previously calcined at >1473 K. On the other hand, the size of Pt metal particles did not increase much on alumina coated with monolayer silica. The observed suppression of sintering of Pt metal particles resulted from the retention of a large surface area of alumina with a thinner silica layer. In the case of a thicker silica layer, although a large surface area was maintained after calcination at elevated temperatures, the existence of a bulk silica-like property of the support did not favor the suppression of sintering of Pt metal particles.     

Effects of Green Density on Crystallization and Mullitization in the Transiently Sintered Mullite

The effects of green density as an experimental parameter in influencing the crystallization of an amorphous silica layer coated on the surface of α-alumina powder particles and the subsequent reaction of crystalline cristobalite and alumina to form mullite have been investigated. Green compacts consisting of alumina powder particles coated with an amorphous silica layer were fabricated using both a conventional uniaxial die pressing and rotary forging at room temperature, resulting in various green densities from 15% to 68% theoretical density. It has been shown that both the crystallization temperature of the amorphous silica layer coated on the surface of alumina powder particles and the subsequent mullitization temperature are lowered in the highly compacted materials.     

Optimal Composition of Zircon–Fused Silica Ceramic Cores for Casting Superalloys

The dependence of flexural strength on crystalline phase content, residual porosity, and sintering temperature has been investigated for ceramic cores based on a mixture of fused silica and zircon. The rule of binary particle mixture, adopted for three particle sizes of fused silica, predicts a critical weight fraction of fine (zircon) particles of 46.9 wt% for the best packing. However, the optimal composition of a 37.8 wt% zircon–fused silica mixture for the flexural strength of ceramic cores found experimentally is lower in zircon content. The content of α-cristobalite +α-quartz crystallized from infilling silica sols exceeds ∼3 wt% when the strength decreases from ∼10 MPa to ∼6 MPa in samples sintered at 899–927°C. The degradation is ascribed to microcracking and the loss of coherency between the fused silica grains and the infilled silica due to β- →α-phase transformation on cooling and differential densification during sintering.     

Influence of Zirconia Interfacial Coating on Alumina Fiber‐reinforced Alumina Matrix Composites

The present study demonstrates an approach for fabricating fiber‐reinforced ceramic matrix composites (CMCs) involving the coating of 2‐dimensional woven alumina fibers with zirconia layer by sol gel, followed by impregnation of these coated fibers with alumina matrix and pressureless sintering. To emphasize the benefits of the zirconia coating on these CMCs, a reference sample without interfacial coating layer was prepared. The zirconia‐coated CMCs showed superior flexural strength and thermal shock resistance compared with their uncoated counterparts. Foreign object damage tests carried out on the ZrO₂ coated CMCs at high impact speed showed localized damage without any shattering.     

Improving the flexural strength of porcelain by residual stress in anorthite coating

Flexural strength is an important parameter of domestic ceramics to meet the criteria for mechanized washing, filling, and sealing process in the automatic production line. In this work, the anorthite coating was prepared using calcite, silica, and alumina as raw materials. Taking the temperature with the highest matrix strength as the optimal temperature, the influences of chemical formulation on the phase composition and the coefficient of thermal expansion (CTE) of coatings were investigated. The enhancement effect of the coating with different formulations and surface number and thickness was compared. As a result, the flexural strength of the ordinary domestic ceramic body was improved due to the residual compressive stress in the coating caused by a mismatch of CTE between the coating and the matrix. At the optimal sintering temperature (1280°C), the coating with 20 mass% alumina addition has the best strengthening effect on the body, in which the thickness of the anorthite coating is approximately 50 µm and the flexural strength has increased by 64.6%, from 88 ± 4 to 144 ± 6 MPa.     

Effects of micro-sized mullite on cristobalite crystallization and properties of silica-based ceramic cores

Silica-based ceramic cores are extensively used in investment casting process, during which they must exhibit sufficient flexural strength and deformation resistance. In this study, micro-sized mullite was used as an additive to silica-based ceramic cores to optimize their high temperature properties. To investigate the effects of micro-sized mullite on cristobalite crystallization, mechanical and thermal properties of silica-based ceramic cores, ceramic cores with different amounts of micro-sized mullite were fabricated. The XRD results showed that additional micro-sized mullite diminished the crystallization of cristobalite at high temperatures, primarily caused by the mullite related compressive stresses on the surface regions of fused silica particles. Three-point bending tests and SEM results showed that micro-sized mullite had a more significant effect on the flexural strength of ceramic cores compared with conventional additives. Particularly, the fracture mechanism of silica-based ceramic cores had been changed from intergranular fracture into a mixed fracture consisting of both intergranular and transgranular fracture. The mechanical and thermal properties of ceramic cores were all reduced slightly as the mullite content exceed 4.6 wt%. Hence, to optimize the properties of silica-based ceramic cores, the micro-sized mullite content should not exceed 4.6 wt%.     

Effect of silica sol on performance and surface precision of alumina ceramic shell prepared by binder jetting

Using 3D printing technology to prepare ceramic shell used for precision investment casting can realize short process and efficient preparation of the ceramic shell, which has a great application potential in the casting field. However, the 3D printed ceramic shells often have the problems of low strength and accuracy. In this paper, a silica sol room temperature dip coating treatment combined with high temperature sintering method was proposed to improve the strength and surface precision of the ceramic shell prepared by the binder jetting. The effects of silica sol concentration and dip coating time on performance and surface precision of the alumina ceramic shell were studied. The mechanical properties and surface precision of the alumina ceramic shell prepared by the binder jetting were improved significantly with the increases of the sol concentration and dip coating time. With the dip coating time of 90 s and sol concentration of 30%, the maximum bending strength of the alumina ceramic reached 44.8 MPa, which was 18.9 times higher than that of the untreated alumina ceramic. The top surface roughness and side roughness of the alumina ceramic decreased from 6.87 μm to 5.70 μm and 7.55 μm–6.46 μm, respectively, compared to those of the untreated alumina ceramic.     

Silica-based ceramic core for aviation applications: Facile pore filling and flexural strength improvement

The effect of alumina addition on the pore filling process, crystallization behavior and mechanical properties of silica-based ceramic cores was investigated. The sintered samples at 1250°C were treated at 1550°C for 0.5 hour in order to simulate a casting process condition. The microstructure analysis indicated that an appropriate alumina addition enhanced the pore filling process by supplying a substantial liquid viscous flow. With an increase in the liquid viscous flow, smaller pores were filled first, and larger pores were filled later. The results also indicated that the pore filling process could be enhanced by adding small powder additives to decrease the pore size between the particles in the ceramic material. However, an excessive alumina addition in the silica-based ceramic cores had a negative effect on the flexural strength and leaching rate. As the alumina content increased from 8 to 12 wt%, the flexural strength of the ceramic core decreased from 16.37 to 4.60 MPa, respectively, and the leaching rate also decreased obviously. These results were explained by an acceleration in crystallization trend of the fused silica particle surface and the merging of connected pores in the sintered body.     

Influence of silicon carbide as a mineralizer on mechanical and thermal properties of silica-based ceramic cores

Ceramic cores have been designed with compounds based on fused silica due to its excellent thermal stability and chemical inertness against molten metals. To endure the high temperatures present during investment casting, mineralizers have been widely used to enhance the flexural strength and shrinkage of ceramic cores. In this study, we demonstrated a silica-based ceramic core with silicon carbide as a mineralizer for improving the mechanical and thermal properties. The SiC in the silica-based ceramic cores can enhance the mechanical properties (i.e., flexural strength and linear shrinkage) by playing a role as a seed for the crystallization of fused silica to cristobalite. The SiC also improves the thermal conductivity due to its higher value compared with fused silica. The results suggest that using the optimal amount of silicon carbide in silica-based ceramic cores can provide excellent mechanical properties of flexural strength and linear shrinkage and improved thermal conductivity.     

Novel fabrication route for porous silicon carbide ceramics through the combination of in situ polymerization and reaction bonding techniques

For the first time, an in situ polymerization technique was applied to produce mullite‐bonded porous SiC ceramics via a reaction bonding technique. In this study, SiC microsized particles and alumina nanopowders were successfully coated by polyethylene (PE), which was synthesized from the particle surface in a slurry phase reactor with a Ziegler–Natta catalyst system. The thermal studies of the resulting samples were performed with differential scanning calorimetry and thermogravimetric analysis. The morphology analysis obtained by transmission electron microscopy and scanning electron microscopy (SEM) confirmed that PE was successfully grafted onto the particle surface. Furthermore, the obtained porous ceramics were characterized in terms of their morphologies, phase composition, open porosity, pore size distribution, and mechanical strength. SEM observations and mercury porosimtery analysis revealed that the quality of the dispersion of nanosized alumina powder into the microsized SiC particles was strongly enhanced when the particles were coated by polymers with in situ polymerization. This resulted in a higher strength and porosity of the formed ceramic porous materials with respect to the traditional process. In addition, the X‐ray diffraction results reveal that the amount of mullite as the binder increased significantly for the samples fabricated by this novel method. The effects of the sintering temperature, forming pressure, and polymer content on the physical and mechanical properties of the final porous ceramic were also evaluated in this study. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40425.     

Reliability studies of gelcast fused silica between organic and inorganic binder systems

Fused silica ceramics was prepared by using conventional organic binder, mathacrylamide‐N,N′‐methylenebisacrylamide (MAM‐MBAM) system by gelcasting process. Mechanical properties of green bodies were studied as a function of solid loading varying from 60 to 72 vol%. After evaluating the green body mechanical properties, the samples were densified at different sintering temperature from 1200 to 1450°C with definite intervals of 50°C and subjected to flexural strength analysis. Variation in flexural strength with sintering temperature was observed and correlated with the quantity of devitrification of fused silica during sintering. Quantification of devitrified cristobalite was carried out by using 20 wt% rutile (TiO₂) as an internal standard by X‐ray diffraction. It was found that, as the cristobalite content increased, flexural strength decreased. Reliability studies were carried out for the samples having maximum flexural strength with and without crystalline content. Reliability studies have shown that for this organic binder system the sample sintered at 1300°C is crystalline free and most reliable product. The mechanical properties and reliability of this product processed with organic binder are compared with inorganic binder system. Results indicate that the sample fabricated using inorganic binder system is exhibiting high Weibull modulus and thus better reliability.     

Mechanical and kinetic studies on the refractory fused silica of integrally cored ceramic mold fabricated by additive manufacturing

A refractory fused silica based integrally cored ceramic mold, the ceramic core with a ceramic mold shell in a single patternless construction, is fabricated by ceramic stereolithography of additive manufacturing. Refractory ceramic molds should satisfy the following restrictions such as similar strength to that of cast metal during solidification, thermal stability for dimensional accuracy, and easy removal of core after casting. Here, we report mechanical and transformation kinetic studies on the refractory fused silica of integrally cored ceramic mold. The flexural strength of sintered silica continually increases with higher density of better densification up to 11.4 MPa at 1300 °C, while it decreases from 11.3 MPa at 1350 °C to 4.6 MPa at 1500 °C. The degradation of the flexural strength is related to the larger amount of the cristobalite and microcracks generated by the abrupt contraction induced during the transformation of beta to alpha cristobalite. Given the quantitative x-ray diffraction study on transformation kinetics, an apparent activation energy Q is 674 ± 53 kJ/mol and the average time exponent 1.85, suggesting that the transformation kinetic is controlled by 1-dimensional interfacial growth.     

Effect of the addition of silica sol on layered extrusion forming of Al2O3-based cores

Layered extrusion forming of ceramic cores with a nanoceramic suspension as a binder was conducted to explore a novel method to produce complex-shaped ceramic cores. Green bodies were prepared using Al₂O₃ particles as precursor materials and silica sol combined with aqueous polyvinyl alcohol solution as a binder. Increasing the silica sol content increased the viscosity of the slurry, enhanced the green bending strength, and decreased the green linear shrinkage. The green microstructure showed the nanosized silica particles were deposited on the surface of the Al₂O₃ particles and among the pores formed by Al₂O₃ particles irregular packing. In addition, increasing the silica sol content increased the bending strength, however, decreased linear shrinkage and open porosity of the sintered bodies. During sintering, the nanosized silica particles converted to the melting phase and reacted with Al₂O₃ and the microstructure of sintered bodies indicated the existence of sintering neck with silica sol addition.     

Investigation of microstructure and properties in short carbon fiber reinforced silica-based ceramic cores via atmosphere sintering

Short carbon fiber (C_sf) reinforced silica-based ceramic cores for investment casting were prepared by an injection molding approach and sintered in air and N₂ atmospheres, respectively. SEM and XRD results present that there are some in-situ formed silicon carbides (SiC) in sintered samples. Moreover, as for the ceramic cores sintered in N₂ atmosphere, the peaks in XRD patterns related to the cristobalite increase with an increment in C_sf content, which may be attributed to the adhesion interface provided by the C_sf and the decreased crystallization free energy. Interestingly, the sample sintered in N₂ exhibits a higher flexural strength about 16.2 MPa, which is 155 % times than that of the samples sintered in air. This is originated from an obvious composite coating consisting of fused silica, SiC and cristobalite on the C_sf. In addition, the sintering necks can further enhance the interfacial bonding strength between the fibers and ceramic cores matrix.     

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.     

Tuning microstructures and separation behaviors of pure silicon carbide membranes

This study reports the preparation of silicon carbide ceramic membranes with pure silicon carbide particles without sintering aids. The effects of sintering temperature on the microstructure, mechanical and filtration properties were investigated. The porosity of the substrate layer increased from 37% to 41% when the sintering temperature ranged from 2150 to 2300 °C, whereas the flexural strength increased from 14.5 to 18.2 MPa. The separation layer was coated on the substrate layer using a spray process. When sintered at 1850 °C, a smooth and defect-free layer was formed with an average pore size and layer thickness of 1.2 and 60 μm, respectively. With the increase of average pore size, the filtration flux increased from 2650 to 2800 L/(m² h bar). Such ceramic membranes can be used to separate corrosive wastewater and high-temperature wastewaters owing to the exclusion of sintering aids, unlike the conventional ceramic membranes.