Ceramic Green Bodies Research Articles

Application of SiO2-coated SiC powder in stereolithography and sintering densification of SiC ceramic composites

Stereolithography additive manufacturing of SiC ceramic composites has received much attention. However, the forming efficiency and mechanical properties of their products need to be improved. This study aimed to prepare SiC ceramic composites with complex shapes and high flexural strength using a combination of digital light processing (DLP) and reactive solution infiltration process (RMI). A low-absorbance SiO2 cladding layer was formed on the surface of SiC powder through a non-homogeneous precipitation process. With the densification of the cladding layer at high temperatures, SiO2-coated SiC composite powder was used to formulate a photosensitive ceramic slurry with a solid content of 44 vol%. The resulting slurry exhibited a considerable improvement in curing thickness and rate and was used to mold ceramic green body with a single-layer slicing thickness of 100 μm using DLP. The ceramic blanks were then sintered and densified using a carbon thermal reduction combined with liquid silica infiltration (LSI) process, resulting in SiC ceramic composites with a density of 2.87 g/cm3 and an average flexural strength of 267.52 ± 2.5 MPa. Therefore, the proposed approach can reduce the manufacturing cycle and cost of SiC ceramic composites.

Biomimetic high toughness Si3N4 ceramics with inverse-bouligand structure

Inspired by the Bouligand structure in nature, a kind of Inverse-Bouligand structure was designed. Different from the bionic Bouligand structure of a one-dimensional material unit, the inverse-Bouligand structure is obtained by stacking and twisting the ceramic layer with a parallel groove structure. The grooves arranged in parallel are first etched on the ceramic green body and then sintered after stacking and twisting. After sintering, the groove structure can still be maintained, so the resulting laminated twisted groove structure in the bulk forms an inverse-Bouligand structure, which can promote crack deflection and improve the toughness of the material during the fracture process. After testing different twist angles, the results show that when the twist angle is 15°, the toughness of the material can reach 11.22 ± 1.77 MPa m1/2, and high strength can be guaranteed. The synergistic effect of interlayer crack deflection and regional crack twist is the main mechanism for the improvement of material toughness.

Binder removal from ceramic stereolithography green bodies: A neutron imaging and thermal analysis study

AbstractStereolithography of ceramics remains one of the most powerful additive manufacturing routes for the creation of intricate ceramic parts. Despite its utility as a forming tool, ceramic stereolithography requires a challenging debinding stage due to the requisite high polymeric loading. Earlier research has identified both the polymeric resin composition and debinding atmosphere to be crucial factors in improving debinding performance. Here, we use a combination of thermogravimetric analysis and neutron imaging to examine samples of different compositions printed using the same processing and exposure parameters. We quantify the influence of both polyethylene glycol addition and the use of different debinding atmospheres (argon and vacuum) on the debinding behavior of ceramic pellets. Specifically, we demonstrate a method for examining the concentration gradients that develop during thermal debinding with the aid of neutron tomography. We find that at a constant heating rate of 1°C/min up to 500°C, vacuum atmosphere appears to result in a greater number of cracks as compared to the use of argon. The vacuum atmosphere led to the development of lower concentration gradients in the samples on average. The greatest improvement resulted with the addition of polyethylene glycol to the samples. This addition led to significantly less cracking and much lower concentration gradients in samples during debinding. These results prompt us to conclude that while keeping printing and exposure parameters constant, composition modification has a more significant effect on the debinding improvement than heating atmosphere.

The Effect of Nano-Silica Surface Infiltration on Bond Strength of a Phosphate-Monomer-containing Composite Cement to Zirconia.

To evaluate the bonding receptiveness of zirconia treated with nano-silica surface infiltration and the bond strength of composite cement after aging. Zirconia ceramic green bodies (Ceramill zolid, Amann Girbach) with dimensions of 10 x 10 x 4 mm were divided into three groups (n = 4): group C (control: no treatment after sintering), group S (sandblasted: 50-μm alumina airborne particle abrasion after sintering) and group N (nanosintered: infiltrated with nano-silica colloid, sintered, and then etched with hydrofluoric acid). Phase transformations were examined through X-ray diffraction (XRD). Composite resin (Filtek Z250, 3M Oral Care) was bonded to zirconia using the 10-MDP-containing composite cement Panavia F (Kuraray Noritake). The composite-cement/zirconia bond strength was immediately measured using the microtensile bond strength test (µTBS) as well as after three months of artificial aging in water (n = 20 microstick specimens/group). Failure mode patterns were examined using SEM. The specimens of groups C and S, as tested by XRD, exhibited almost full tetragonal phases, while a small extent of tetragonal-monoclinic phase transformation (t→m) was observed for group N. Group N achieved the highest bond strengths (41.5 ± 8.6 MPa), which was significantly higher than that measured for groups C and S (p < 0.05). There was a significant drop in µTBS after 90 days of water storage for groups C and S. SEM revealed a decrease in the percentage of cohesive failure in groups N and S after water storage. Infiltrating zirconia with nano-silica is a reliable method to establish a strong and stable bond to zirconia. The combination of surface infiltration with nano-silica and application of a phosphate monomer-containing composite cement can significantly improve the composite-cement/zirconia bond strength.

In-situ investigation on crack-initiation and deformation of Al2O3 green bodies during dewaxing process by combined OCT-TG-FTIR and TMA

The dewaxing process is used to remove an organic binder from the ceramic green bodies before sintering, which occasionally generates cracks. The crack formation behavior depends on various factors including softening and decomposition of the organic binder, generation of gases, and strength degradation of the green body thereby. Herein, this correlation was investigated to elucidate the crack formation behavior during the dewaxing process using two types of Al2O3 green bodies; one is added with polyvinyl butyral (PVB) and stearic acid (SA) and the other is with paraffin. The internal structures of Al2O3 green bodies during dewaxing were observed using optical coherence tomography (OCT), and the generated gases were analyzed simultaneously using a thermogravimetric (TG) analyzer and Fourier transform infrared (FTIR) spectroscopy (TG-FTIR). The mechanical properties of the green bodies were investigated at RT–600 °C using a thermomechanical analyzer (TMA). The weight change occurred in both the green bodies with formation of gases depending on the type of the binder. In the OCT studies, cracks were observed with substantial deformation in the PVB/SA-added green body during the dewaxing, whereas no cracks were seen in the paraffin-added one. The TMA investigation showed that the paraffin-added sample possessed higher strength and better structural stability than the PVB/SA-added one throughout the dewaxing, leading to the crack-free green body of the former. Therefore, the crack-initiation and deformation behaviors of the green bodies were significantly affected by the type of the binder used. The combination of the in-situ observations using the combined OCT-TG-FTIR system and the mechanical properties measurement using TMA was found to be effective in verifying the structural stability of the green bodies during the dewaxing.

A model based study of the drying and shrinkage behavior of a ceramic green body

This work presents a 1D diffusion model to characterize the drying procedure of a ceramic slab with respect to shrinkage. The proposed mathematical framework is comprised of a set of partial–differential and algebraic equations along with moving boundary conditions. The model predictions are compared to experimental data from the literature and a good agreement is accomplished. Afterwards, the parameters of a desorption isotherm equation are estimated based on experimental data from three drying tests, to characterize an industrially used clay material. A parametric analysis is conducted to study the impact of the drying-induced shrinkage on the drying kinetics of the ceramic body. The results indicate that the scale of shrinkage affects the porosity change which in turn, has a great impact on the drying kinetics and the properties distribution along the material. Finally, the drying dynamics of the green ceramic material is studied for a realistic shrinkage scenario, followed by the discussion of the results.

Reusing Waste Coffee Grounds in the Preparation of Porous Alumina Ceramics

Porous ceramics can be used in various industrial applications, such as thermal insulation, orthopedic implants, high-temperature filtration, lightweight structural components, and catalyst supports, etc., and can be obtained using various methods. In this study, the sacrificial fugitive method was used to prepare a porous alumina ceramic. The appropriate amount of sacrificial fugitive was combined with raw ceramic powder as a pore-forming agent, and was then evaporated or burned out either before or during the sintering process to create the desired pores. Various materials can be used as pore-forming agents; in this work, eco-friendly waste coffee grounds (WCG) were utilized. First, alumina ceramic green bodies were prepared via slip casting of 60 wt. % alumina suspensions with five different amounts of WCG (0 wt. %, 1 wt. %, 5 wt. %, 10 wt. % and 15 wt. %) and the dispersant Dolapix (0.2 wt. %), and using PVA (0.5 wt. %) as a binder for all solutions. The effect of the various amounts of WCG on the alumina ceramic green bodies, and subsequently on the obtained sintered ceramics, was tracked and validated through different analyses. Suspension viscosity was determined through a rotational viscometer. Simultaneous differential thermal and thermogravimetric (DTA/TGA) analyses were used to observe the thermal decomposition of WCG and to determine the sintering regime. After sintering, the density, porosity, and shrinkage of the samples were examined and calculated. In addition, the phase composition and crystallite size of all sintered samples were determined by powder X-ray diffraction (PXRD) analysis, as well as their morphology and composition using Scanning Electron Microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). The results show that density decreased from 3.743 to 2.172 g/cm3 and porosity increased from 6.12% to 45.52%, both with the increasing amount of WCG (from 0 wt. % to 15 wt. %).

Mesoscale Models for Describing the Formation of Anisotropic Porosity and Strain-Stress Distributions during the Pressing Step in Electroceramics.

Porous ceramics are often produced by using pyrolisable additives to generate porosity during the sintering step. The examination of the experimental microstructures of the resulted porous ceramics revealed certain levels of anisotropy, even if the original soft additives used as pore forming agents were spherical. The paper shows that anisotropic porosity may result in ceramics when using equiaxed soft polymeric additives for generating porosity, due to the deformation of soft inclusions during the pressing step. It has been found, by means of analytical and numerical calculations, that uniaxial pressing of a mixture of solid particles with contrasting mechanical properties (hard/soft) generates modifications in the shape of the soft phase. As a result, anisotropic shape distribution of the soft inclusions in the green ceramic body and elongated porosity in the final ceramic product are obtained. The elongated pores are statistically oriented with the major axes perpendicular to the pressing direction and will generate anisotropy-related functional properties. Analytical calculations indicate the deformation of a single soft inclusion inside a continuum solid. Further, by finite element simulations performed in 2D planes along the transversal and radial directions of the pressing axis, a bimodal angular distribution of the long axes of the soft inclusions has been found.

3D printing and in situ transformation of SiCnw/SiC structures

Successful preparation of photopolymerizable ceramic slurries using non-oxide ceramics and their 3D printing is highly challenging as the ceramic powders often possess high UV-light absorbance. Even though pre-oxidation of the powders could improve the slurry printability, the residual oxygen element in the material would eventually deteriorate the material purity and performance. In this study, a separated redox route (pre-oxidation, 3D printing and carbothermal reduction) was proposed to overcome these challenges. SiC ceramic powder was pre-oxidized into SiC@SiO 2 core-shell form to lower the UV-light absorbance. It was found that the ultraviolet (UV) light absorbance value of SiC powder at 405 nm decreased 20% from 0.357 to 0.284 after pre-oxidation process. The UV-light penetration depth of SiC@SiO 2 ceramic slurries at 40 ~ 55 vol% solid loading ranging from 14.81 to 12.82 µm, which surpassed two times that for 40 vol% raw SiC ceramic slurry. Different-shaped structures of SiC@SiO 2 /resin green ceramic bodies were successfully fabricated by vat photopolymerization 3D printing. The oxygen element resided was then eliminated by carbothermal reduction to avoid detriment of SiO 2 to the mechanical performance of the SiC ceramics. Phenolic epoxy acrylate resin with 14 wt% pyrolytic carbon (PyC) yield was chosen as the carbon source and thus the SiC@SiO 2 /resin green body pyrolyzed into SiC@SiO 2 /PyC ceramics after 1200 °C. It was found that after the heating temperature was further raised to 1600 °C, most of the pre-introduced SiO 2 shells on the surface of SiC particles were in situ transformed into SiC nanowires through carbothermal reduction reaction between SiO 2 and PyC, and the content of oxygen element in the ceramic matrix sharply dropped from 20.21% to 2.08%. The results demonstrated that through the proposed redox route, the photopolymerization-related issues including high UV-light absorbance and pre-oxidation-induced impurity for 3D printing of SiC ceramic slurries can be tackled. Finally, high purity porous SiC nw /SiC ceramic components with different structures were produced. This study provides a promising route for the preparation and 3D printing of photopolymerization ceramic slurries using non-oxide ceramics possessing high UV-light absorbance. • UV-light absorbance of SiC powder at 405 nm decreased 20% after pre-oxidation. • UV-light penetration depth of SiC@SiO 2 slurries (40 ~ 55 vol%) surpassed two times that for 40 vol% raw SiC slurry. • Pre-introduced SiO 2 shells on SiC particles in situ transformed into SiC nanowires through carbothermal reduction. • SiC nw /SiC components with complex structures successfully fabricated by photopolymerization 3D printing.

Experimental additive manufacturing of green body of SiC/Graphite functionally graded materials by stereolithography

AbstractCeramic stereolithography (CSL)‐additive manufacturing (AM) technology is used to create a functionally graded ceramic (FGC) green body made of silicon carbide (SiC) and graphite. For the SiC/graphite FGC, the mixing parameters of ceramics powders and ultraviolet (UV) curing resin are improved, and correlations of the resultant slurry curing depth with integrated light intensity are discovered. Therefore, the SiC/graphite FGC‐produced green body has no flaws, pores, or cracks on its surfaces. According to the association between cure depth and integrated light density for each slurry's composition, several interfacial collapses discovered in a cracked cross‐section might be decreased.

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.

Efficient cleaning of ceramic green bodies with complex architectures fabricated by stereolithography-based additive manufacturing via high viscoelastic paste

Complete removal of uncured ceramic paste from the surface of a green body with complex profiles fabricated by stereolithography-based additive manufacturing is critical to the ultimate achievement of the designed 3D part with high definition and ideal surface roughness. The present work provided a novel cleaning strategy for the design of cleaning agents on the basis of competitive absorption of viscosity-reducing dispersants on the surface of ceramic particles within viscoelastic pastes. In combination with cleaning agent design and auxiliary cleaning equipment, distinct dispersants were selected to enhance the flowability of the uncured paste during the ultrasonic cleaning and spraying processes. The morphology and reconstruction characteristics of specific structures illustrated the advantages and disadvantages of the two cleaning methods. The cleaning effect for small holes by ultrasonic cleaning was superior, but the structural damage of the green body was more serious. After spray cleaning, the surface roughness was lower with less damage to the green body, and there was less residual paste after cleaning. The results demonstrated that the selection of a cleaning strategy was beneficial to posttreatment of stereolithography-fabricated ceramic green bodies, achieving subsequent superior surface quality and accuracy of the internal feature size.

Progress on Materials Reinforcement using Mechanically Defibrillated Cellulose Nanofibers

Cellulose nanofibers (CNFs) are highly crystalline, fibrous materials with a high aspect ratio of long cellulose molecules linked together with strong hydrogen bonds. These long cellulose molecules are incorporated into hemicellulose and lignin, the cell walls of higher plants. The modulus of elasticity of CNF remains constant between −200 °C and +200 °C. However, the linear coefficient of thermal expansion of cellulose fibers was 0.17 ppm/K, which is comparable to that of quartz glass. Further, CNFs have a thermal conductivity in the same order of magnitude as glass. Therefore, they are promising next-generation fiber owing to the excellent mechanical and thermal properties, sustainability, and biodegradability of CNF. Most studies on using CNFs to increase strength have focused on polymer matrix composites, particularly biodegradable polymers. However, it is difficult to increase the strength of materials using CNFs due to the agglomeration of CNFs. Compared to other composite materials, the uniform dispersion of nanosized CNFs is crucial. This article reports that CNFs may behave not as reinforcing fibers but as cross-linking agents to strengthen the polymer when the amount of CNFs is very small in polymer matrix composites, and the addition of CNFs is also effective in strengthening silk yarn and Ti. The mechanical properties of the ceramic green bodies can be increased using CNFs as an auxiliary agent in ceramic slurry. Moreover, the fabrication of composite materials using CNFs is essential for expanding the CNFs applications.

Preparation and properties of Si3N4 ceramics via digital light processing using Si3N4 powder coated with Al2O3-Y2O3 sintering additives

It is difficult to form Si 3 N 4 ceramics by DLP technology owing to the high refractive index and high ultraviolet (UV) absorption of Si 3 N 4 powder. In this paper, the Si 3 N 4 ceramics were successfully fabricated by DLP technology using Si 3 N 4 powder coated with Al 2 O 3 -Y 2 O 3 sintering additives. The effects of the coating amount of sintering additives on the properties of Si 3 N 4 ceramic slurry, green bodies and parts after sintering were systematically studied. When Al(NO 3 ) 3 and Y(NO 3 ) 3 solutions were added at a content of 20 mL/100 g Si 3 N 4 , the slurry with lowest viscosity (0.042 Pa·s at a shear rate of 30 s −1 ) and a higher monolayer curing depth (47.9 ± 1.2 µm) was obtained, the relative density and flexural strength of the Si 3 N 4 ceramics reached maximum values at 85.4% ± 0.3% and 149.2 ± 1.9 MPa. The shrinkage of the Si 3 N 4 ceramics remained almost unchanged after coating Al 2 O 3 -Y 2 O 3 sintering additives. Finally, the Si 3 N 4 ceramics with excellent quality and clear outline were well prepared. This research has laid a certain experimental basis for DLP forming Si 3 N 4 and other non-oxide ceramics, which is of great researching significance.

Preparation, microstructure and performance of interference microwave absorbing cermet composites with frequency–selective screens

Interference microwave absorbing cermet composites with frequency–selective screens (AC/FSS) were successfully prepared by sintering a ceramic green body embedded with stainless–steel wire mesh screens via a rapid heating method. The addition of the screens made the AC/FSS composites increase slightly in density and open porosity and remain unchanged in surface Vickers hardness and compressive strength. Both the number and structure parameters of the screens had a significant effect on the microwave absorbing performance of the AC/FSS composites. The weak microwave absorption by the small amount of zinc oxide and poor microwave interference by the single–layer screen made the AC/FSS composites possess modest microwave absorbing performance. Moreover, the AC/FSS composites showed an improved microwave absorbing performance due to the significantly enhanced microwave interference by the double–layer screens. By optimizing the structure parameters of the upper and lower screens, the AC/FSS composites with double–layer screens possessed surprisingly good microwave absorbing performance with the whole reflectivity–versus–frequency curve below −10 dB, a mean reflectivity as low as −12.9 dB and the lowest reflectivity of −15.2 dB at 12.4 GHz in the 8.0–18.0 GHz frequency range.

Fabrication of YAG ceramic tube by UV-assisted direct ink writing

Yttrium aluminium garnet (YAG) ceramic slurries with well-posed printability for ultraviolet(UV) curing-assisted, direct ink writing (DIW) were successfully prepared in this study by optimising the types and contents of dispersants, as well as solids loading. The rheological test results showed that Disperbyk-103 had the best dispersion effect, and the ceramic slurry with the solids loading of 76 wt% was obtained by adding 3.0 wt% Disperbyk-103. In addition, the effect of the coupling relationship between the printing parameters on the shape control of the sample was studied. After debinding, the printed ceramic green body was sintered at 1750 °C for 6 h, and the in-line transmittance of the sintered ceramic was 84.0% at 1064 nm. Using the developed method, a high-aspect-ratio ceramic tube with a wall thickness of approximately 1 mm, without obvious deformation, was successfully fabricated.

Direct ink writing of 3D piezoelectric ceramics with complex unsupported structures

Piezoelectric ceramic, as a typical smart material, shows great potential in applications of sensing & actuation, smart structure and energy harvesting. However, the use of traditional methods to fabricate complex and high-precision piezoelectric ceramic devices faces huge technical difficulties and high molding costs. Direct ink writing is a typical additive manufacturing technology, but has limited capabilities when preparing ceramic products with complex unsupported structures. In this work, a combined process of direct ink writing (DIW) and secondary shaping of flexible ceramic green body has been developed and proved to produce complex piezoelectric ceramics. The PZT ink shows shear thinning behavior and appropriate viscoelasticity such as moderate viscosity and high storage modulus. The printed green body can be flexibly deformed and the samples after polarization show good piezoelectric properties, with an average d33 up to 265 pC/N. This work demonstrates an attractive method for geometrically complex piezoceramic with unique macro structure due to its simplicity and low cost, and provides a solution to the key problems in the existing manufacturing technology of 3D ceramics.

UV-assisted direct ink writing of Si3N4/SiC preceramic polymer suspensions

Direct ink writing (DIW) offers a flexible and readily available processing route for achieving ceramic components with complex shapes and geometries. The successful printing of ceramic green bodies using DIW typically requires the formulation of particle-loaded inks having a narrow window of rheological properties that enable both flow through the nozzle and support the weight of additional layers. Herein, we present a method for DIW that employs UV-curing to enable printing of otherwise unprintable inks. The inks used in this study are suspensions consisting of a commercially available polycarbosilane precursor and silicon nitride, Si3N4, powders. A diacrylate cross-linker and photointiator were employed to enable UV-curing. The effect of cross-linker content on UV-rheology and cure depth as they pertain to printing, and slump in self-supported lattice structures, are discussed. UV-assisted DIW produced components of a high degree of complexity, capable of supporting over-hanging structures, low shrinkage, and relatively high degree of ceramic conversion

Lattice-structured SiC ceramics obtained via 3D printing, gel casting, and gaseous silicon infiltration sintering

In view of technical difficulties in preparing ceramics with complex shapes, gel casting combined with 3D printing was here adopted to prepare silicon carbide ceramic green body, and gaseous silicon infiltration sintering was used to prepare 3D lattice-structured ceramics. The preparation of the slurry, gel curing, and ceramic molding was investigated. Results demonstrate that the ratio of components affects the fluidity and stability of slurry. However, when volume fraction of the solid phase of the slurry reaches 56%, the viscosity of slurry is only 300 mPa s, and drying shrinkage rate of green body is 6.6%; these characteristics make slurry suitable for 3D complex model injection molding. Furthermore, both the temperature and the initiator affect gel curing speed. As the temperature and initiator content increase, the induction and gel time are rapidly shortened. When demolding at 300 °C and when gaseous silicon infiltration sintering is carried out at 1550 °C, a 3D lattice-structured ceramic with relative density of 87% and average compressive strength of 433 MPa can be obtained.

Three-dimensional printing of transparent ceramics by lithography-based digital projection

Scalable, massive fabrication of complex three-dimensional (3D) structured highly transparent Nd: YAG (Y 3 Al 5 O 12 ) ceramics was achieved by a lithography-based (photopolymerization) digital light processing or digital light projection (DLP) 3D printing method. 3D-printed ceramic green bodies with a printing layer height of 25 µm and an xy resolution of 40 µm were fabricated by using a photocurable slurry with 39 vol% solid loading. The developed slurry exhibited a stable high viscosity of 80–340 Pa s over a large shear rate range. A specially designed pre-sintering treatment based on YAM (Y 4 Al 2 O 9 ) phase formation was applied to prevent cracking and bubbling formation in the DLP 3D printing approach for fabricating highly transparent Nd: YAG ceramics. This thermal treatment also reduced the average grain size, leading to a higher optical quality. Following a series of thermal treatments, the sintered transparent ceramics reached a relative density of 99.9% with a maximum in-line transmittance of 80% in the visible wavelength range. Moreover, the fabricated Nd-doped YAG exhibited 1064-nm emission under 808-nm laser excitation.