Sintering Shrinkage Research Articles

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.

Investigation of processing the alumina slurry through an economic vat photopolymerization process

Vat Photopolymerization (VP) is a 3D printing process to fabricate delicate and precise ceramic parts. LCD VP is one of the most economical 3D printing processes and is less explored for ceramic materials. This work investigated the feasibility of consolidating alumina slurry (45 vol %) using an LCD-based resin 3D printer. Results show that a lower exposure time is better for building a multilayer part than a higher exposure time to avoid scattering and delamination. However, higher exposure time is required to bond the initial layers with the build platform. Fine features, such as holes lesser than 3 mm, were not possible to print due to the scattering of the light. A heating rate of 0.1 °C/min is preferable for crack-free debinding. The final sintered part showed sintering shrinkage of 25% in the x-y direction and 30% in the z-direction with a sintered density of 94%.

Slurry flow characteristics control of 3D printed ceramic core layered structure: Experiment and simulation

Vat photopolymerization 3D printing ceramic technology provides a feasible process for the preparation of complex internal cooling channels for aeroengine single crystal superalloy hollow blades. However, the typical layered structure characteristics of 3D printing ceramic technology led to the anisotropy of ceramic core strength and sintering shrinkage, which greatly affects the performance and accuracy of the complex structure core and requires further research and improvement. Herein, the influence of the thickness of the slurry layer on the flow characteristics of the slurry in the process of the vat photopolymerization 3D printing slurry spreading was systematically studied by the method of simulation and experiment. The simulation results show that the positions of the turbulent zone and maximum velocity zone in the scraper front affect the redistribution of powder particles with different sizes. The layered structure was caused by the redistribution of ceramic particles of different sizes in the slurry layer. By controlling the turbulent flow zone and the maximum velocity zone of the scraper leading edge, the phenomenon of laminar flow can be weakened and the particle redistribution can be improved. With the increase of the thickness of the printing layer, the layered structure appears gradually, and the pores at the interface of the layered structure gradually concentrated into the interfacial pore lines from the uniform distribution, and the crack propagation changes from intergranular micro-crack to interlayer macro-crack. The combination of finite element simulation and experiment, through the slurry flow characteristics to control the layered structure of reductive vat photopolymerization ceramic core 3D printing, the control of crack propagation mode, element distribution and pore evolution of the core was accomplished, which lays a foundation for the performance control of ceramic 3D printing technology.

Direct Ink Writing of Tubular Al<sub>2</sub>O<sub>3</sub> Membrane Support Using Agar-Based Ink in 3D-Printing

Direct ink writing was used for 3D printing of tubular Al2O3 membrane support. Agar-based ink mixtures were prepared as a paste with a proper viscoelastic behavior in achieving printing. Using agar only for mixing with Al2O3 slurry in preparing the ink mixtures showed the flow behavior resulting in failure to print the Al2O3 tube due to too low viscosity of the ink mixture—100 Pa at 40°C. However, the introduction of Hydroxyethyl cellulose (HEC) as thickener and Polyethylene glycol 1500 (PEG 1500) as lubricant helped improve the behavior of the ink mixtures to be more proper paste for printing. The amounts of HEC were varied from 0 to 2wt% of solid loading. At 2wt% HEC, the ink mixture was able to be printed highest, compared to the other ink mixtures. However, the 2wt% HEC-using ink mixtures possessed the highest sintering shrinkage at 12%, while its relative density was highest at 70%. The results indicated that it was possible to print the alumina membrane tube if its fidelity was improved further.

Influence of powder mixing processes and WC contents on microstructure, shrinkage behavior, and mechanical properties of WC-Si3N4 composites

WC-Si3N4 composite structural material with excellent mechanical properties is essential for achieving high-strength connections between WC and Si3N4. Proper powder mixing process can significantly improve the mechanical properties of sintered samples. This study investigates the effects of two different ball milling methods: (1) mixing WC powder, Si3N4 powder, and sintering aid powders and then ball milling for 24 h (BM24); and (2) ball milling WC powder, Si3N4 powder, and sintering aid powders separately for 12 h and then ball milling in same jar for 12 h (BM12 + 12), as well as WC content on sintering shrinkage behavior of mixed powders. Furthermore, the bonding between the layers of one-time sintered sample of the WC-Si3N4 composite structure was investigated. The results showed that the microstrains of powders prepared by BM24 mixing process were greater than those of the powders prepared by BM12 + 12 mixing process. Shrinkage displacements and shrinkage rates during sintering of BM12 + 12 powders were greater than those of BM24 powders to varying degrees. Moreover, sintering temperature at which shrinkage rate of composite powders reaches its maximum value gradually increases as WC content decreases, while maximum value of shrinkage rate gradually decreases. Additionally, mechanical properties of all BM12 + 12 samples were better than those of BM24 samples. WC-Si3N4 composite structure sample sintered with BM12 + 12 powders had a smooth transition between the layers without any defects.

Sintering of Protonic Ceramics for Tubular Solid Oxide Fuel Cells

The fabrication of proton-conducting solid oxide fuel cells (PC-SOFCs) has been challenging mainly due to the poor sinterability of common proton-conducting electrolyte materials. Extremely high sintering temperatures (up to 1600 °C) and long sintering times (>10 h) are typical for obtaining dense, gas-tight proton-conducting electrolytes. Apart from the high energy cost and long production cycle time, the high sintering temperature leads to several other issues, including chemical stability of the electrolyte and its compatibility with the electrodes. In the present work, we focused on optimizing the co-sintering temperature of tubular PC-SOFCs using a freeze-cast anode support. The densification of the BaZr0.3Ce0.5Y0.2O3 (BZCY) electrolyte was greatly facilitated by the sintering shrinkage of the NiO-BZCY anode support, which was as high as 35%. Preliminary electrochemical testing of the fabricated cells was also conducted.

Investigation of sintering mechanism for novel composite structural thermal barrier coating

AbstractA micro‐agglomerated particle embedded–thermal barrier coating (TBC) structure was prepared by an improved plasma spray process to withstand the sintering‐induced degradation of TBCs during service. In this study, the sintering resistance and thermophysical and mechanical properties of conventional and novel‐structured TBCs were systematically characterized. The results suggested that the thermal conductivity and sintering shrinkage of the novel‐structured TBCs were approximately 30% lower than those of conventional air plasma spraying TBCs. The elastic modulus of the novel‐structured coating is only 32% of that of the conventional structure after thermal exposure at 1300°C for 100 h. The distinct structure of the coating is the main factor that influences its performance. The relationship between the structural evolution and residual strain of the coating was analyzed using electron backscatter diffraction and transmission electron microscopy. Significant differences were observed in the sintering behavior of the dense matrix and embedded particle regions in the coating. Some columnar grains near the intersplat pores in the dense matrix have similar lattice orientations, and they tend to connect and consequently heal the intersplat pores. The large pores between the agglomerated particles and non‐oriented submicron‐sized grains that constitute these particles are responsible for the sintering resistance of the coating.

Analysis of uniaxial viscosity and activation energy of deformation for 3Y-TZP by bending creep test

The uniaxial viscosity is an important material property that has to be determined in order to understand the densification of a material during the sintering process. It depends simultaneously on temperature, relative density and grain size which complicates its determination. To determine its dependence on these three factors, bending creep tests are performed on pre-sintered 3Y-TZP specimens with different relative densities and grain sizes up to the corresponding pre-sintering temperatures. The uniaxial viscosity of 3Y-TZP specimens is calculated after obtaining the deflection rates. It is found that the uniaxial viscosity decreases with temperature according to the Arrhenius equation. In addition, the tetragonal to cubic phase transformation at temperature above 1300 °C leads to a decrease of the activation energy. Apart from that, the contribution of densification and grain growth to the increase of the uniaxial viscosity is quantitatively determined. When grain boundary diffusion dominates, the contribution of grain growth is up to 1.8 times that of densification to the uniaxial viscosity. Furthermore, it is found that Rahaman's model best fits the normalized uniaxial viscosity. At the end, an evolution profile of the uniaxial viscosity for polycrystalline materials during the sintering process is proposed to facilitate the analysis of the sintering shrinkage rate.

Fabrication of particle-stacking porous anode for solid oxide fuel cell using laser powder bed fusion

Interconnected micropores play a critical role as fuel gas flow channels in solid oxide fuel cell (SOFC), which is quite difficult to prepare using conventional methods. In this work, an interconnected microporous structure of Ni-based anode was newly fabricated by laser powder bed fusion (LPBF), controlling the melting behavior of particles and stacking them up to form a high-porosity space net structure with point contacts. The results show that the YSZ sprayed powder initially behaves as nanoparticles, forming an unmanageable sub-microporous structure. After densification, however, it melts as microparticles and forms micropores distributing with a wider manageable range. In addition, densified YSZ powder forms interconnected pores with isolated island-shaped sintered particles finely distributed along the laser scanning direction at XY orientation. Due to a narrow molten pool and large sintering shrinkage generated, an overlapped area forms only at Z orientation. These overlaps can be improved by adding Ni element, behaving as a particle binder in the matrix, but this is easy to form Ni segregations and deteriorate the element homogeneity. In contrast, adding NiO can promote a finer composition distribution.

Kinetic mechanism of anisotropic sintering shrinkage in triaxial porcelain

The kinetic mechanism of sintering shrinkage at 900–1240 °C was investigated to optimize the sintering process of triaxial porcelain. The respective reactions partially overlapped in differential thermodilatometric curves, namely Al-Si-spinel formation, mullite nucleation and mullite growth, were separated by mathematical deconvolution analysis. Then the kinetic mechanisms (m) were calculated using nonlinear regression analysis on experimental master plots of the generalized reaction rate (dα/dθ) versus the reacted fraction (α). The sintering shrinkage exhibits strong anisotropy which origins from approximately parallel arrangement of clay plates and is dominated by the mullite growth during densification, leading to anisotropic preexponential factors (A) and activation energies (E). The values of m systematically change with heating rates, in which the mullite growth is induced by the development of the chemical compositions and microstructure of secondary mullite nanotubes. Nevertheless, preexponential factors are independent on the heating rates.

Phenomenon and mechanism of ion diffusion at Co-fired joint interface of NiZn Ferrite/MCT ceramic heterostructure

Nickel-zinc ferrite (NZF) discs and magnesium calcium titanite (MCT) dielectric ceramic discs were laminated and co-fired to prepare NZF/MCT bi-layer laminated heterostructure samples. The sintering shrinkage of NZF and MCT was matched well via adjusting the sintering temperature. Ion diffusion curves at the co-fired joint interface were derived by electron-probe microanalysis (EPMA) and fitted basing on infinite diffusion coupling model. Ion diffusion mechanism at the co-fired joint interface was investigated in detail. The results show that Zn2+ ions have the highest diffusion coefficient because of the smaller ion radius which is benefit for interstitial diffusion, while Ca2+ ions have the lowest diffusion coefficient and segregate along the interface of MCT side. Fe3+ ions have the biggest diffusion width because of the interstitial diffusion in the incompletely densified MCT. Sintered at 1260 °C, the maximum interdiffusion width is narrow to be about 102 μm. With increasing temperature, ion diffusion coefficient and diffusion distance both go up obviously.

SiC Nanoparticles Strengthened Alumina Ceramics Prepared by Extrusion Printing

Extrusion-free-form printing of alumina ceramics has the advantages of low cost, short cycle time, and high customization. However, some problems exist, such as the low solid content of ceramic paste and the unsatisfactory mechanical properties of pure alumina ceramics. In this study, SiC nanoparticles were used as a reinforcement phase added to the alumina ceramic matrix. Methylcellulose is used as the binder in the raw material system. Ammonium polyacrylate is used as a dispersant to change the rheological properties of the slurry and increase the solid content of ceramics. SiC nanoparticle-strengthened alumina ceramics were successfully prepared by the extrusion process. The relative settling height and viscosity of ceramic slurries were characterized. The sintering shrinkage of composite ceramics was tested. The flexural strength, fracture toughness, and hardness of the ceramics were characterized. The strengthening and toughening mechanisms of the composite ceramics were further explained by microscopic morphology analysis. Experimental results show that when the content of the dispersant is 1 wt.%, the rheological properties of the slurry are the best. Maximum measured bending strength (227 MPa) and fracture toughness (5.35 MPa·m1/2) were reached by adding 8 wt% SiC nanoparticles; compared with pure alumina ceramics, flexural strength and fracture toughness increased by 42% and 41%, respectively. This study provides a low-cost and effective method for preparing ceramic composite parts.

Effect of sintering aids on mechanical properties and microstructure of alumina ceramic via stereolithography

In this study, the effects of sintering aids (CaO, TiO2, La2O3 and ZrO2) on the physical and mechanical properties and microstructure of alumina ceramics made by stereolithography were systematically investigated. The addition of 3% CaO promoted internal porous structure at high temperatures, increasing the possibility of interlayer microcracking along the inner pores. 0.5% TiO2 had the best bending strength enhancement of alumina, increasing from 110.4 MPa to 201.1 MPa at a sintering temperature of 1650 °C. La2O3 took effect after 1550 °C and basically had no effect on refining grains. 1% ZrO2 increased 15% apparent porosity and 88% interlayer shear strength and reduced sintering shrinkage by about 30%, while maintaining bending strength. Optimised alumina powder ingredients and sintering conditions that could meet the performance index requirements of the ceramic core were obtained: particle size distribution 10 μm/2 μm (the mass fraction ratio of coarse to fine particles was 6:4), solid content 55%, sintering aid 1% ZrO2, sintering temperature 1450 °C.

Low sintering shrinkage porous mullite ceramics with high strength and low thermal conductivity via foam‐gelcasting

AbstractExcessive sintering shrinkage leads to severe deformation and cracking, affecting the microstructure and properties of porous ceramics. Therefore, reducing sintering shrinkage and achieving near‐net‐size forming is one of the effective ways to prepare high‐performance porous ceramics. Herein, low‐shrinkage porous mullite ceramics were prepared by foam‐gelcasting using kyanite as raw material and aluminum fluoride (AlF3) as additive, through volume expansion from phase transition and gas generated from the reaction. The effects of AlF3 content on the shrinkage, porosity, compressive strength, and thermal conductivity of mullite‐based porous ceramics were investigated. The results showed that with the increase of content, the sintering shrinkage decreased, the porosity increased, and mullite whiskers were produced. Porous mullite ceramics with 30 wt% AlF3 content exhibited a whisker structure with the lowest shrinkage of 3.5%, porosity of 85.2%, compressive strength of 3.06 ± 0.51 MPa, and thermal conductivity of 0.23 W/(m·K) at room temperature. The temperature difference between the front and back sides of the sample reached 710°C under high temperature fire resistance test. The low sintering shrinkage preparation process effectively reduces the subsequent processing cost, which is significant for the preparation of high‐performance porous ceramics.

Hydrolysis‐induced simultaneous foaming and coagulation casting of secondary aluminum dross aqueous suspension

AbstractThe reactive AlN, heavy metal ions, and salts in secondary aluminum dross make them harmful to the environment and difficult to handle, and therefore categorized as hazardous solid waste. A novel, simple, and versatile technique has been proposed here to prepare hierarchically porous ceramics using secondary aluminum dross as the sole raw material, namely, the hydrolysis‐induced simultaneous foaming and coagulation casting method. As the main component, Al and AlN could be hydrolyzed to generate aluminum hydroxide sol, which can form three‐dimensional network and lead to the outstanding compressive strength of green body (7.03 MPa). Gas release during hydrolysis, high‐temperature oxidation of AlN, and metallic Al based on Kirkendall effect could account for the final multilevel pores. The utilization of harmful components in secondary aluminum dross, supplys an alternative colloidal forming way for porous green body with complex shape when combined with other ceramic powders or inorganic solid waste. Low sintering shrinkage of 1.38%–3.60% for hierarchically porous ceramics offers this highly effective and low‐cost method to be easily promoted for industrial production.

Manufacturing of ceramic cores: From hot injection to 3D printing

With the improvement of aero-engine performance, the preparation of hollow blades of single-crystal superalloys with complex inner cavity cooling structures is becoming increasingly urgent. The ceramic core is the key intermediate part of the preparation and has attracted wide attention. To meet this challenge, new technologies that can make up for the defects of long periods and high costs of fabricating complex structural cores by traditional hot injection technology are needed. Vat photopolymerization 3D printing ceramic technology has been applied to the core field to realize the rapid preparation of complex structural cores. However, the industrial application of this technology still needs further research and improvement. Herein, ceramic cores were prepared using traditional hot injection and vat photopolymerization 3D printing techniques using fused silica, nano-ZrO2, and Al2O3 powders as starting materials. The 3D printed ceramic core has a typical layered structure with a small pore size and low porosity. Because of the layered structure, the pore area is larger than that of the hot injection ceramic core, the leaching performance has little effect (0.0277 g/min for 3D printing cores, 0.298 g/min for hot injection cores). In the X and Y directions, the sintering shrinkage is low (2.7%), but in the Z direction, the shrinkage is large (4.7%). The fracture occurs when the inner layer crack expands and connects with the interlayer crack, forming a stepped fracture in the 3D-printed cores. The bending strength of the 3D printed core at high temperature (1500 °C) is 17.3 MPa. These analyses show that the performance of vat photopolymerization 3D-printed ceramic cores can meet the casting requirements of single crystal superalloy blades, which is a potential technology for the preparation of complex structure ceramic cores. The research mode of 3D printing core technology based on the traditional hot injection process provides an effective new idea for promoting the industrial application of 3D printing core technology.

Study on the Effect of Binders on the Properties of Mullite Porous Ceramics for Flue Gas Filtration

Fused mullite particles are used to prepare mullite porous ceramics by gel-casting method, using polysilicon waste and ρ-Al2O3 powder as the binder, and starch as the pore forming agent. The content of the pore forming agent was 20%, and the ratio of the mass of fused mullite particles to the mass of the binder was 9:1, 8:2, 7:3, and 6:4, respectively. The effects of binder content on the properties of mullite ceramics in terms of phase composition, microstructure, apparent porosity, compressive strength, bulk density, pore size distribution, and filtration pressure drop were investigated. It was found that prepared porous mullite possessed relatively high apparent porosity (56.64-58.19%), and low bulk density (1.28-1.31g/cm3). With increasing binder content, the sintering shrinkage rate increased from 2.07 to 5.92%, the compressive strength increased from 1.01MPa to 6.08MPa. Therefore, the preparation of mullite ceramics was a near net size preparation, and the prepared porous ceramics meet the requirements of flue gas filtration.

Research on Manufacturing Technology of Complex Modeling Chinaware Integrated with 3D Printing Technology

Study the manufacturing process of making complex chinaware by combining 3D printing technology with traditional technology. In this process, the ceramic artwork is divided into several component, The manufacturing process of each part of the green body is selected according to the shape, and then all the green bodies fit together by the assembly structure and interpenetrating network binder, then biscuit firing (debinding), glazing and glaze firing. Adjust the composition ratio of ceramic slurry to make its sintering shrinkage the same as that of traditional ceramic slurry. The technological process and parameters are given through experiments. The structure were analyzed by field emission scanning electron microscope. The influence factors of glue discharge process, microstructure and pore distribution of printing body after glue discharge were studied. This provides a basis for the popularization and application of the process.

Fabrication of Ti-6Al-4V alloy powder by a novel sintering-deoxygenation process

Given the high cost of fabricating the Ti-6Al-4V alloy powder by the traditional hydriding-dehydriding and atomization methods, a novel sintering-deoxygenation process was proposed and studied in this research. With the help of the developed deoxygenation treatment, value-added raw materials such as Ti sponge can be substituted by low-cost and low-grade materials with high oxygen content. In addition, sintering in a solid state saves a significant amount of processing energy compared to alloying above the melting point of Ti in the conventional alloy production process. This paper prepared six sintering precursors with different initial compositions and oxygen contents using the low-valued raw materials of crude titanium, crude vanadium, V2O3, Al powder, and Al2O3 powder. These precursors' sintering performances on shrinkage, phase composition, microstructure, and porosity were investigated. The results show that the sintering shrinkage is positively correlated with the sintering temperature and holding time and is also affected by the raw materials. The pellet made by mixed crude titanium and AlV master alloy showed the largest shrinkage and the densest bulk. The various combinations of the raw materials led to the changes in the oxygen contents in the precursors, the alpha-beta phase ratio, and the shift of the diffraction peaks of the α-phase. The dissolving of the AlV master alloy and the formation of the Ti3Al intermediate phase took place during sintering due to diffusion. Sintering at 900–1000 °C is critical for elemental diffusion and densification, and sintering at higher temperatures is decisive for microstructure homogenization and further densification. Using the hydrogen-assisted Mg deoxygenation method, all the sintered samples could be treated to an oxygen level of around 0.4 wt% regardless of oxygen content in precursors. The contents of Ti, Al, V, and Fe in the obtained Ti-6Al-4V powders met the requirements of GB/T 34486-2017. This research has demonstrated the feasibility of fabricating Ti-6Al-4V alloy powder by the novel sintering-deoxygenation process.

Microstructure and mechanical properties of high strength porous ceramics with high sewage sludge content

Sewage sludge is a by-product produced in the process of sewage treatment, which has characteristics of large yield, wide source, high pollution and strong harmfulness. The resource and harmless treatment of sewage sludge has attracted extensive attention all over the world. This research successfully produced environmentally clean porous ceramics using sewage sludge (SS) and kaoline (KE) as the main raw materials. The effects of sintering temperature on the crystalline phase, microstructure and mechanical properties of porous ceramics were also discussed. The results showed that all sintered porous ceramics contained various crystalline phases, such as mullite (3Al2O3·2SiO2), sillimanite (Al2SiO5) and hematite (Fe2O3), which laid a foundation for the development of mechanical properties. When sintered at 1250 °C–1400 °C, porous ceramics presented smooth surface, typical void structure and excellent mechanical properties due to the formation of liquid phase. And the porous ceramics gradually shrank and densified with the raising of sintering temperature, resulting in smoother surface and larger internal pores, so the density and shrinkage presented a continuous increasing trend, while the porosity continued to decrease. The maximum bulk density, sintering shrinkage and apparent porosity were 2.02 g/cm3, 20.34% and 34.74%, respectively. Also, the mechanical strength of porous ceramics reached the maximum value when the sintering temperature was 1350 °C, the compressive strength and flexural strength were 90 and 53 MPa, respectively.