Tailoring of functionally graded zirconia–mullite/alumina ceramics

A new tailored zirconia–mullite/(0–100 vol%) alumina as functionally graded ceramics (FGCs) was designed and synthesized by reaction sintering of zircon and alumina. Zircon and alumina powder mixtures were mixed, stacked, compacted in a cylindrical die and sintered. The sintered samples made of 11 layers and varied gradually in composition by 10 vol% from one layer to the other layer (i.e. from zirconia–mullite layer to alumina layer) resulted in continuous functionally graded ceramics without sharp interfaces. Phase composition and densification behaviors of the samples were investigated. Microstructure, mechanical and thermal properties of FGC and its non-layered composites were studied. Results showed that the tailored FGZM/A gave continuous homogenous structure with highly improved physical, mechanical and thermal properties. The different properties of tailored FGZM/A recorded average values or rather better of its non-layered composites which gave a new way for material design.     

Binder stabilization and rheology optimization for vat-photopolymerization 3D printing of silica-based ceramic mixtures

Enhancing inlet gas temperature in aero/gas turbines to reduce their carbon-footprint, has led to a strive for better performing inlet cooling mechanism of the turbine blades. The internal cooling of the blades is made by ceramic cores in their casting process, but conventional ceramic molding has long reached its maximum possible geometrical complexity, hence shedding light on 3D printing of these cores. The objective of this study is to develop low-viscous, fully stabilized, commercially viable ink for vat-photopolymerization of silica-based ceramics. This paper investigates the best dispersion type and amount for different formulated monomer mixtures, and explains the best correlation between viscosity, solid loading, binders, dispersants, peeling forces and mechanical properties, and offers an optimized mixture to avoid the common ceramic printing issue, namely crack propagation of cores during sintering. Among five dispersant agents, the SOL20, SOL24 and FA4611 exhibited better performance than other dispersion agents, and the optimum concentration level for each binder and dispersant agent was ensured through sedimentation test. Their dispersion capability and long-term stability were further investigated to designate the best dispersion agent for each binder system. Further verification was made by sedimentation study of the samples at 40 °C for 40 days and reducing the superficial area of the used powder mixture. According to the result of the rheology analysis, the best dispersions were achieved using SOL20 for the loaded binder mixtures of M1 and M4, SOL24 for M3and FA4611 for M2. The instability of M1 and M2 with their respective dispersant agent was coordinated through the thixotropic agent of TX/2, and complete stabilization and near-Newtonian behavior were achieved. However, the research showed that the addition of TX/2 to fully stabilized M4 and M2 suspensions negatively impacts the mixtures’ rheological behavior from near-Newtonian to shear-thickening. In the final stage of this study, peeling forces, sintering and three-point bending tests were conducted to determine the final formulated suspension to print ceramic core components. M4 and SOL20 combination was selected for SiO₂-ZrSiO₄ loading and dispersing, respectively. The impact of solid loading between the range of 58 and 65 vol% on the rheological behavior of the final suspension and the mechanical properties of sintered bodies were investigated to assign an optimum solid rate. The adequate strength on sintered and degree of viscosity for ceramic vat-polymerization processing was achieved at 58 vol%. Lastly, a validation study is conducted by printing a complex ceramic core model by a commercial LCD hobby printer. This validation shows the significance of this study to scale up the manufacturing of complex-shaped ceramic cores and to revolutionize the sector, by printing inexpensive and readily available irregular-shaped (non-atomized) ceramic powder, using the most cost-effective LCD printers (non-specialized expensive ceramic printers).     

Yttria stabilized zirconia formed by micro ceramic injection molding: Rheological properties and debinding effects on the sintered part

Micro Ceramic Injection Molding (μCIM) is a near net-shape process to produce smaller and intricate parts at a competitive cost. The application of nano-sized ceramic powder in μCIM has the advantages of fine grain size growth and good surface finish. However, the nano size effect causes agglomeration and low powder loadings, which result in defects during the μCIM process and in the sintered components. This study extensively investigated the debinding and sintering of yttria-stabilized zirconia (YSZ), as well as its rheological properties, using polypropylene (PP) as the primary binder and palm stearin as the secondary binder. 50 nm Yttria stabilized zirconia (YSZ) powders were mixed with palm stearin and PP at a powder loading of 37–43 vol%. The results of rheological studies showed that the feedstock had a dilatant flow characteristic and a viscosity of around 10–40 Pa s. Feedstock with 38 vol% powder loading had the lowest activation energy of 9.48 kJ/mol. The green part of the injected feedstock had flexural strength ranging from 13 to16 MPa, within which the feedstock with 43 vol% powder loading had the highest green density. Solvent debinding was carried out at three temperatures (50, 60, and 70 °C) using heptane. A large porous region was clearly identified at 70 °C compared with 50 °C. A debinding split furnace with argon gas was used to remove PP at 450 °C for 4 h. The debound samples did not shrink when 94%–98% of the binder system was removed. All debound samples sintered at 1350 °C and 41 vol% had the highest mechanical properties with hardness of 900 HV and a flexural strength of 400 MPa.     

Characterization of zirconia specimens fabricated by ceramic on-demand extrusion

The Ceramic On-Demand Extrusion (CODE) process is a novel additive manufacturing method for fabricating dense (~99% of theoretical density) ceramic components from aqueous, high solids loading pastes (>50?vol%). In this study, 3?mol% Y₂O₃ stabilized zirconia (3YSZ) specimens were fabricated using the CODE process. The specimens were then dried in a humidity-controlled environmental chamber and afterwards sintered under atmospheric conditions. Mechanical properties of the sintered specimens were examined using ASTM standard test techniques, including density, Young’s modulus, flexural strength, Weibull modulus, fracture toughness, and Vickers hardness. The microstructure was analyzed and grain size measured using scanning electron microscopy. The results were compared with those from Direct Inkjet Printing, Selective Laser Sintering, Lithography-based Ceramic Manufacturing (LCM), and other extrusion-based processes, and indicated that zirconia specimens produced by CODE exhibit superior mechanical properties among the additive manufacturing processes. Several sample components were produced to demonstrate CODE’s capability for fabricating geometrically complex ceramic components. The surface roughness of these components was also examined.     

Environmentally-harmless polylactic acid-polyethylene glycol binder for deformable ceramic green body

We report a polylactic acid (PLA)-polyethylene glycol (PEG) mixture as a thermoplastic binder to prepare shape-machinable ceramic green bodies. During the mixing process, the high viscosity of the PLA binder impedes effective mixing with ceramic powders. To address this issue, PEG was introduced as a plasticizer into ceramic/PLA composites to diminish the overall viscosity of the feedstock, which resulted in effective mixing with ceramic powders. After sintering green bodies composed of PLA-PEG binders and ceramic compounds, the mechanical properties (flexural strength and porosity) of the sintered specimens were investigated and compared with those of sintered specimens made from wax-based green bodies. The sintered specimens made from ceramic/PLA-PEG composites showed comparable mechanical properties and porosities with the sintered specimens made from ceramic/wax composites. In addition, the shape-machinable characteristic of the green bodies made from ceramic/PLA-PEG composites was demonstrated by deforming the entire structure of the green bodies via simple heat treatment.     

二次烧结温度对牙科氧化锆陶瓷组织和力学性能的影响

研究二次烧结温度对氧化锆牙科陶瓷微观组织和力学性能的影响。方法：氧化锆亚微米粉经过干压、冷等静压成型后在1050ºC预烧结,然后将预烧结牙科氧化锆瓷块在1300ºC-1600ºC进行二次烧结。对不同二次烧结温度下材料的线收缩率、烧结密度、物相、三点抗弯强度进行测量分析,并通过扫描电镜观察试样的断面形貌。结果：结果表明随着二次烧结温度提高,氧化锆的密度、弯曲强度呈上升趋势。在1350ºC时体积密度达到6.10g/cm³,1500ºC时的机械性能最优,三点弯曲强度为852MPa,主晶相为四方相。结论：亚微米氧化锆粉体烧结活性高,力学性能优良,能够满足口腔全瓷修复材料的要求。     

Processing and mechanical properties of new hierarchical metal-graphene flakes reinforced ceramic matrix composites

Hierarchical tantalum-graphene flakes reinforced zirconia (3Y-TZP) ceramic matrix composites were fabricated by wet processing route and freeze drying followed by spark plasma sintering (SPS). The microstructures and mechanical properties were investigated. The results show that graphene and Ta particles are homogeneously dispersed in the ceramic matrix and the optimum sintering temperature for complete densification of composites and thermal reduction of the graphene oxide is 1500 °C. The addition of dual reinforcements of tantalum microflakes and graphene nanoflakes results in significant improvement in the mechanical properties of the ZrO₂ matrix. Approximately a 30% increase in flexural strength vs the zirconia-Ta composite and a 175% increase in fracture toughness vs the monolithic zirconia have been achieved by introducing 0.5 vol% GO and 20 vol% Ta particles.     

Improving the properties of binder jetted ceramics via nanoparticle dispersion infiltration

To improve the dimensional accuracy, surface quality, and mechanical properties of binder jet additive manufactured ceramic parts, in this work, nanozirconia dispersion and binder were used to prepare a jet solution to investigate the effect of nanozirconia content of the jet solution on the properties of zirconia ceramic green and sintered bodies. When the nanozirconia content of the jet solution increased from 0 to 20 wt%, the forming precision of the zirconia ceramic green bodies improved, the surface roughness decreased, and the density increased. After sintering at 1400 °C for 2 h, linear shrinkage along the height, width, and length of the zirconia ceramics decreased by 6.27%, 10.20%, and 5.45%, respectively, while the surface roughness decreased by 42.87%, and the bending strength increased by 145.60%. With increasing nanozirconia content of the jet solution, the spreading and infiltration distance of the jetted solution in the powder layer decreased, the thickness of the deposition layer of nanozirconia on the surface of the zirconia particles in the powder layer increased, and the pore size of the powder layer decreased significantly. This increased the density of the green bodies; thus, improving the sintering properties of the zirconia ceramics.     

Rheological Behavior and Slip Casting of Al2O3–Ni Aqueous Suspensions

A strong effort has been devoted recently toward processing of metal–ceramic composites with tailored microstructure by colloidal methods. The aim of this work is to optimize the rheological behavior of concentrated Al₂O₃–nickel (Ni) aqueous suspensions and further slip casting in order to obtain dense green composites. Compositions with Ni relative contents ranging from 5 to 75 vol% were prepared from suspensions with high solids loadings (50 vol%) by adjusting the colloidal stability of each component in terms of pH, mobility, dissolution conditions, and influence of polyelectrolytes. The rheological properties were measured under controlled rate and controlled stress conditions at different basic pH conditions and contents of polyelectrolyte. Better rheological conditions of the mixtures were found for pH 10 and 1.0 wt% polyelectrolyte. Minimum viscosity was obtained for suspensions containing 15 vol% of Ni. The analysis of flow curves demonstrates that the suspensions form a structure at very low shear, hindering sedimentation. Homogeneous slip cast bodies with green densities up to 70% of theoretical and up to 75 vol% Ni were sintered in Ar to achieve dense biphasic composites.     

Accurate printing of a zirconia molar crown bridge using three-part auxiliary supports and ceramic mask projection stereolithography

Zirconia ceramic is a widely used material for dental restoration. Stabilized zirconia all-ceramic teeth have excellent mechanical properties, biocompatibility, and aesthetic properties. At present, the CAD/CAM technique for zirconia all-ceramic dental prosthesis leads to low material efficiency and high tool wear. Although restorations fabricated using additive manufacturing are gaining attention, it is still very challenging to obtain accurate shapes and proper mechanical properties in zirconia restorations. In this investigation, a type of three-part auxiliary support was adopted and added to the occlusal surface to fabricate a typical molar crown bridge. A ceramic solid content of 40 vol% acrylic-based slurry was prepared, and a molar crown bridge was fabricated using mask projection stereolithography. The experimental results showed that the average dimensional error of the printed green body was ±150 μm. The density of the sintered ceramic parts was 6.026 g/cm³, and the three-point bending strength was 541 ± 160 MPa, which is higher than that of human dentin (160 MPa), but lower than that of CAD/CAM zirconia (900–1200 MPa). Although the proposed process still needs to be optimized to improve the mechanical properties and reliability of the crown bridge, the mask projection process combined with the adopted three-part auxiliary supports are capable of individualized manufacturing of complex zirconia crown bridges.     

Effects of carbonyl nickel powder addition on the mechanical properties,microstructure, and yttrium segregation of sintered 3YSZ composites

In this study, the effects of the addition of carbonyl nickel powder on the density, microstructure, and mechanical properties of sintered yttria-stabilized zirconia (3YSZ) were investigated. Sintering at 1300 °C resulted in the optimum comprehensive mechanical properties. The addition of 5 vol% carbonyl Ni increased the fracture toughness and flexural strength from 9.51 MPa m^1/2 to 14.5 MPa m^1/2 and from 747 MPa to 873 MPa, respectively. The addition of carbonyl nickel showed greater improvement than did the addition of spherical Ni powder. The dendritic morphology improved the interface bonding between the ceramic and the metal, enabling a bridging mechanism of the ductile phase. However, further Ni addition decreased the mechanical properties. X-ray diffraction results showed that the amounts of the monoclinic phase (M) and cubic phase (C) of 3YSZ increased, whereas the amount of the tetragonal phase (T) decreased. The Y segregation near the Ni particles, which was confirmed by an energy dispersive spectrometer (EDS), caused the phase changes. The segregation of Y occurred during the cooling stage, rather than the holding stage, of sintering. During the cooling stage, the heat mismatch between Ni and ZrO₂ resulted in strong elastic strain energy, which promoted Y segregation.     

Zirconia ceramics from coprecipitated powders

The possibility was explored of making dense and strong partly stabilized zirconia (PSZ)-based ceramic materials from coprecipitated zirconium and yttrium hydroxide powders of both factory and laboratory preparation. The effect of dry and wet grinding, powder burning, cold isostatic pressure (CIP) at≤0.8 GPa, and sintering at ≤1600°C on the physicochemical properties of the material was investigated. It was found that the properties (a density of 5.7–5.8 g/cm³, a bending strength of 600 to 800 MPa, and a crack resistance of 7–9 MPa·m^1/2) of the resulting ceramic material would not be reproduced unless the tendency of the PSZ powder to agglomerate spontaneously in storage is overcome or avoided. It is shown that in contrast to other similar materials the ceramic material from a deagglomerated powder has a higher optimal CIP pressure (0.6 GPa), which implies that the material has an improved thermal endurance and a better mechanical stability.     

Digital light processing stereolithography of zirconia ceramics: Slurry elaboration and orientation-reliant mechanical properties

Digital Light Processing (DLP) is a promising technique for the preparation of ceramic parts with complex shapes and high accuracy. In this study, 3 mol% yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) UV-curable slurries were prepared and printed via DLP. Two different solid loadings (40.5 and 43.6 vol%, respectively) and printing directions were investigated to assess the influence of these parameters on physical and mechanical properties of the sintered parts. Zirconia samples were sintered at 1550 °C for 1 h, achieving a very high relative density (99.2%TD), regardless of solid loading and printing direction. FE-SEM micrographs shown a homogeneous and defect-free cross section with an average grains size of 0.56 ± 0.19 µm. Finally, mechanical properties were influenced by printing direction and zirconia vol%. Indeed, the composition with the higher solid loading (i.e. 43.6 vol%) had the highest three-point flexural strength (751 ± 83 MPa) when tested perpendicular to the printing plane.     

Impact of powder morphology on quality of low-pressure injection moulded reaction-bonded net shape oxide ceramics

Low-pressure injection moulding is a very efficient process for net shape manufacturing of ceramic micro parts. In order to obtain sintered ceramic specimens without shape distortion or damages, density gradients in the green bodies have to be avoided. Especially feedstocks with a solid loading near the critical powder volume content often tend to segregate the binder while flowing. However, the value of critical powder content can be significantly influenced by particle size, particle size distribution and particle morphology. This paper compares two powder mixtures of identical chemical compositions with different specific surfaces and morphology and evaluates their workability for low-pressure injection moulding. The aim of this paper is to identify the influence of morphology on feedstock rheology as well as on accuracy, mechanical properties and microstructure of net shape manufactured reaction-bonded zircon ceramics.     

Synthesis of Sintered Diamond with High Electrical Resistivity and Hardness

Diamond and diamond-ultrafine Co powder mixtures were sintered at 7.7 GPa and 1800° to 2000°C. A well-sintered body with a fine-grained homogeneous microstructure, high hardness, and high electrical resistivity was produced when a diamond-5 vol% Co powder mixture was used as the starting material.     

A novel strategy for fabricating zirconia implants with a trabecular biomimetic porous surface

For successful osseointegration of load-bearing implants, an improved bone–implant contact area through a trabecular porous surface resulting in minimized stress shielding effect is highly desirable. We propose a novel strategy of green net shaping a ceramic dough, combined with a reticular foam replica method and gradient coating, to fabricate biomimetic porosity in a customizable ceramic dental implant for the first time. About 85 vol% porosity and 300–600-μm pore size were evident in microCT and electron microscopy of the sintered samples, suitable for bone ingrowth. Excellent integrity at the interface along with homogeneous distribution of secondary alumina phase in zirconia matrix was achieved, despite the difference in the green state powder loading between the dough and the slurry.     

Development and mechanical characterization of novel ceramic foams fabricated by gel-casting

Porous ceramic materials are of considerable interest for a variety of chemical and industrial applications in extremely harsh conditions, particularly at very high temperatures for long time periods. A combined gel-casting-fugitive phase process employing agar as a natural gelling agent and polyethylene spheres as pore formers was exploited to produce porous ceramic bodies. Alumina and alumina–zirconia powders were used to prepare samples having a porosity of about 65–70–75 vol%. The composite powder was produced by a surface modification route, i.e. by coating a well-dispersed alpha-alumina powder with a zirconium chloride aqueous solution. On thermal treatment, ultra-fine tetragonal zirconia grains were formed on the surface of the alumina particles. SEM observations and image analysis were used to characterize the microstructure of porous samples and uniaxial compressive tests were carried out to measure their mechanical behavior.     

Direct Coagulation Casting of Silicon Carbide Components

Direct coagulation casting is a novel near-net-shape method for forming ceramic green bodies from homogenous high-solids-loaded particle suspensions. It is based on the principle of the in situ coagulation of a powder suspension via a reaction-rate-controlled internal-enzyme(urease)-catalyzed reaction after casting. Low-viscosity (<3 Pas) suspensions with a high solids loading (>62 vol%) of SiC, boron, and carbon powder mixtures with a high surface area (>7-10 m²/g) have been prepared at pH = 10. Salt ions (up to 1-2 mol/L) are created by the urease-catalyzed decomposition of urea, to destabilize the suspensions. The coagulation kinetics and the strength of the wet green bodies have been investigated. The reaction rate is strongly dependent on the temperature (in the range of 5°-30°C) and the enzyme concentration (for the range of 4-16 units/g SiC) and is independent of the substrate (urea) concentration for urea concentrations of <2 wt%, based on the powder content. The resulting green bodies show no shrinkage during coagulation and 1%-2% linear shrinkage during drying. The compressive strengths of the wet green bodies are as high as 60 kPa and increase as the coagulation time increases. The wet green strength of the coagulated suspensions scales with the solids content, according to a power law with an exponent of 11, in the range of 56-61 vol% solids content. The possibilities of fabricating high-solids-containing complex SiC green and sintered components with homogenous microstructures and high sintered densities are demonstrated.     

Graphene nanoplatelets for electrically conductive 3YTZP composites densified by pressureless sintering

3 mol% yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composites with 2.5, 5 and 10 vol% graphene nanoplatelets (GNP) were pressureless sintered in argon atmosphere between 1350 and 1450 °C. The effects of the GNP content and the sintering temperature on the densification, microstructure and electrical properties of the composites were investigated. An isotropic distribution of GNP surrounding ceramic regions was exhibited regardless the GNP content and sintering temperature used. Electrical conductivity values comparable to the ones of fully dense composites prepared by more complex techniques were obtained, even though full densification was not achieved. While the composite with 5 vol% GNP exhibited electrical anisotropy with a semiconductor-type behaviour, the composite with 10 vol% GNP showed an electrically isotropic metallic-type behaviour.     

Optimization of the ceramic ink used in Direct Ink Writing through rheological properties characterization of zirconia-based ceramic materials

Understanding and optimization the rheological properties characterization of zirconia (ZrO₂) based ceramics inks is critical for optimizing the production of Direct Ink Writing (DIW) components to achieve complex structures with similar properties as those obtained by using the traditional processing routes. In this work, ZrO₂ based ceramic materials with different yttrium contents (3 and 8 mol %) were designed and produced by DIW to determine the most suitable ceramic ink composition in terms of the rheological properties (e.g. flow curves, viscosity, loss modulus G′, storage modulus G″, etc.) to design new components. Different ceramic inks with charges up to 75 wt % were prepared and characterized. A systematic study of the feedstock, as well as the different ceramic inks, was performed to determine the optimal ceramic charge. This characterization evidences that rheological properties of zirconia based ceramic inks are influenced by the particle size and amount of ceramic content. Furthermore, the rheological study highlights that the ZrO₂ inks present a Non-Newtonian behavior depending on the ceramic content. Results revealed that the yttrium content affects the flow properties of ZrO₂ suspensions in such a way that, higher shear rate was required to make the suspensions flow at increasing the amount of powder. It was also found that the best rheological properties corresponded to 73 and 70% for the 3Y- and 8Y–ZrO₂ of ceramic charge, respectively.