Cure behavior of colorful ZrO2 suspensions during Digital light processing (DLP) based stereolithography process

In this study, the light absorption of the pure ZrO₂ and three types of colorful ZrO₂ mixtures were investigated. It was found that the absorbance of colored ZrO₂ powder increases with the colorant content, and the yellow-colored powder has the strongest absorbance at the wavelength of 405 nm compared with the other two types of colored ZrO₂ powder. The cure behavior investigation on the ceramic suspensions of the above four types of ZrO₂ powders during DLP process shows that as the colorant content increases, the cure depth and excess cure width both decrease due to the ceramic absorbance. The cure depth of colored suspensions is linear with logarithm of incident energy, consistent with Beer-Lambert model, while the further research into the cure width shows that the excess cure width increases nonlinearly with logarithm of incident energy, which is inconsistent with Quasi-Beer-Lambert model. Additionally, colorful ZrO₂ accessories were successfully fabricated.     

Depth and width of cured lines in photopolymerizable ceramic suspensions

When a photopolymerizable ceramic suspension is illuminated, the cured region is characterized by the cure width and cure depth. The cure depth follows a semilogarithmic behavior with increasing energy dose, as expected for Beer–Lambert absorption, and is described by the depth sensitivity (S_d) and depth critical energy dose (E_d). The excess cure width, which is the cured width beyond the incident illumination width, is also found to increase with the logarithm of energy dose. This quasi-Beer–Lambert behavior can be described by a width sensitivity (S_w) and width critical energy dose (E_w). The semilogarithmic dose dependence is demonstrated for ceramic suspensions containing silica, mullite, alumina, and zircon powders. Broadening can be quantified by the broadening depth (D_b), which is the cure depth at which broadening begins to occur. It is shown that the broadening depth decreases with the logarithm of the normalized refractive index contrast between the powder and monomer solution.     

Effect of polystyrene addition on properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics with high strength and high transmittance have been widely used in the field of defense and military. Additive manufacturing (AM) technology is one of the effective means to fabricate porous Si₃N₄ ceramics. Nevertheless, it is difficult to prepare porous Si₃N₄ ceramics by using digital light processing (DLP) because of the large refractive index difference between Si₃N₄ powders and photosensitive resin. In this study, the effects of the amount of polystyrene (PS) powders on the properties of Si₃N₄ ceramic slurries and sintered ceramics were systematically discussed. The addition of PS reduced the overall refractive index of powders and increased the average particle size of powders, thus improving the cure depth of Si₃N₄ ceramic slurries from 11.0 ± 2.0 μm to 55.7 ± 1.8 μm. With the increase of PS content, the shrinkage and porosity of Si₃N₄ ceramics gradually increased, and the bulk density and flexural strength showed the opposite trend. The slurry with low viscosity (2.38 Pa٠s at a shear rate of 30 s⁻¹) and high cure depth (51.2 ± 4.6 μm) was obtained when the content of PS was 15 wt%, which met the thickness requirements for printing. The total porosity of Si₃N₄ ceramics reached the maximum values at 28.21 ± 2.58%. The addition of PS solved the problem of low cure depth of slurries, and PS as a pore-forming agent could help Si₃N₄ ceramics form porous structure. This research provides valuable insights into the fabrication of non-oxide ceramics with high refractive index using DLP technology.     

Influence of the content of polymethyl methacrylate on the properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics are highly regarded as ideal materials for radomes due to their unique characteristics. However, the slurry used for the preparation of porous Si₃N₄ ceramics suffers from a low cure depth, making it challenging to fabricate ceramic components using DLP technology. In this study, porous Si₃N₄ ceramics were prepared by combining DLP technology with pore-forming agent method. The addition of polymethyl methacrylate (PMMA) powders with lower refractive index than that of Si₃N₄ powders can improve the penetration depth of ultraviolet light in the Si₃N₄ slurry. A systematic study was conducted to investigate the influence of the addition of PMMA powders on the properties of Si₃N₄ slurries and porous Si₃N₄ ceramics. When PMMA powders were added at 10 wt%, the slurry with a lowest viscosity of 0.13 Pa s (the shear rate is 30 s⁻¹) and cure depth of 40.0 μm (the exposure energy is 600 mJ/cm²) was obtained. With the increase of PMMA content, porous Si₃N₄ ceramics experienced a gradual decrease in both the flexural strength and bulk density, while the porosity increased from 14.41% to 27.62%. Specifically, when 20 wt% PMMA was added, the resulting porous Si₃N₄ ceramics had a lowest bulk density (2.41 g/cm³), a maximum porosity (27.62%), and a flexural strength (435.87 MPa). The study is of great significance in establishing an experimental foundation for fabricating porous Si₃N₄ ceramics by using DLP technology.     

Effects of AlN inorganic binder on the properties of porous Si3N4 ceramics prepared by selective laser sintering

Porous Si₃N₄ ceramics are widely used in the aerospace field due to its lightweight, high-strength, and high wave transmission. Traditional manufacturing methods are difficult to fabricate complex structural and functional ceramic parts. In this paper, selective laser sintering (SLS) technology was applied to prepare porous Si₃N₄ ceramics using AlN as an inorganic binder. And the effects of AlN content on the properties of the obtained ceramic samples were explored. As the AlN content increased, nano-Al₂O₃ and nano-SiO₂ formed the eutectic liquid phase, enhancing the sintering densification and phase transformation of Si₃N₄ poly-hollow microspheres (PHMs). The island-like partial densification structures in Si₃N₄ green bodies increased. During the high-temperature sintering, the eutectic liquid phase partially transformed into the mullite phase or reacted with AlN and Si₃N₄ to form the Sialon phase. With the increase of AlN content, the fracture mode of Si₃N₄ ceramics changed from fracturing along PHMs to fracturing across PHMs. The bonding depth between PHMs increased and the connection between the grains was tighter, so the Si₃N₄ ceramics became denser. With the increase of AlN addition, the total porosity of the porous Si₃N₄ ceramics tended to decrease and the flexural strength gradually increased. When AlN content was 20 wt%, the total porosity and the flexural strength were 33.6% and 23.9 MPa, respectively. The addition of AlN inorganic binder was carried out to develop a novel way to prepare high-performance porous Si₃N₄ ceramics by SLS.     

Additive manufacturing of complex-shaped and porous silicon nitride-based components for bionic bones

Si₃N₄ is a novel implant material with promising applications in the replacement of human hard tissues. The biomimetic human bone structure used in this study was created using digital light-processing technology. The effect of pore-forming agent content on the curing and mechanical properties of Si₃N₄ ceramics was studied. The obtained results indicated that with an increase in the pore-forming agent content, the cure depth of the ceramic suspension first increased and then decreased, while the excess cure width decreased. Furthermore, as the pore-forming agent content increased, the porosity of the sample increased, whereas the compressive strength and Young's modulus decreased. The maximum porosity of the sample at the optimal mass ratio (pore-forming agent: Si₃N₄ = 5:10) is 58.48 ± 0.49%, and the compressive strength and Young's modulus are 79.01 ± 6.78 MPa and 18.18 ± 0.26 GPa, respectively.     

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     

Cure behaviour and mechanical properties of Si3N4 ceramics with bimodal particle size distribution prepared using digital light processing

Digital light processing is a vital additive manufacturing technology used for manufacturing ceramic parts. The particle size distribution of ceramic suspensions significantly affects the cure behaviour and mechanical properties of ceramics. In this study, the cure behaviour and mechanical properties of Si₃N₄ ceramics with a bimodal particle size distribution were studied. The results indicated that the suspension with coarse particles had a higher cure depth for a lower absorbance but poor mechanical properties. The bending strength of the samples with the optimal ratio (coarse:fine particles = 3:7) reached a maximum of 728.7 ± 10.33 MPa, which is 16.5% higher than that of the samples prepared using only fine particles.     

Effect of graphite particles as additive on the curing behaviour of β-tricalcium phosphate suspensions and scaffold fabrication by digital light processing

Reducing the viscosity of high solid-loading ceramic suspensions and controlling the resolution of ceramic green parts produced by digital light processing (DLP) 3D printing are two important concerns in the ceramic additive manufacturing industry. The presence of ceramic particles within a photopolymerizable system leads to light scattering that reduces the resolution of the ceramic green scaffolds. This study introduced a graphite additive to solve these problems and focused on the effects of graphite concentration on the viscosity, curing behaviour and scaffold fabrication of β-TCP ceramic suspensions. As a result, it was found that an appropriate addition of graphite reduced the viscosity of the ceramic suspensions, and the light scattering decreased with the increase of graphite concentration. Both the cure depth (C_d) and excess width (C_ex) also decreased with the increase of graphite concentration. But, graphite has a larger effect on the width curing than depth curing.     

Effects of particle size and color on photocuring performance of Si3N4 ceramic slurry by stereolithography

Due to high absorbance of UV light and low solid loading, the stereolithography-based additive manufacturing of gray-colored and dense Si₃N₄ ceramic is of significant difficulty and challenge. The effects of geometric properties of ceramic powders on the curing performance were investigated by studying the absorption difference of the Si₃N₄ ceramic particles with different colors and particle sizes and ultraviolet light. The results show that the transmission of ultraviolet light and curing performance of the darker Si₃N₄ ceramic slurry are much poor. Under the same particle size, the Si₃N₄ ceramic slurry using lighter particles presents the smaller scattering coefficient. The scattering coefficient (～202) of the gray powder with ～0.8 μm average particle size is the smallest. Under the same color, the larger the particle size, the smaller the scattering coefficient. The smallest scattering coefficient of the white powder with ～2.0 μm average particle size is ～110.     

Phase formation and microstructural evolution of Ca α-sialon using different Si3N4 starting powders

Silicon nitride has two polymorphous structures, α-Si₃N₄ and β-Si₃N₄. In this study three different Si₃N₄ starting powders (∼100%α, 40%α+60%β, ∼100%β) were used to prepare Ca α-sialon with the composition Ca_1.8Si_6.6Al_5.4O_1.8N_14.2 by pressureless sintering. Comparison was made concerning the densification process, reaction sequence and microstructure of the corresponding materials. The sluggish reactivity of β-Si₃N₄ resulted in poorer densification during sintering. All the three starting powders produced a similar final phase assembly, namely α-sialon together with a small amount of AlN and AlN polytypoid except that traces of unreacted β-Si₃N₄ remained until 1800° in samples prepared with ∼100%β-Si₃N₄ powders. Elongated α-sialon grain morphology has been identified in the samples prepared using all the three different Si₃N₄ starting powders. Coarser elongated α-sialon grains with lower aspect ratio were found in samples using higher β phase starting powders.     

The influence of Y2O3-containing sintering additives on the oxidation of Si3N4-based ceramics and the interfacial interactions with liquid Al-alloys

Sintering additives containing Y₂O₃ influence the microstructure and the crystalline-state of Si₃N₄-ceramics produced via pressureless sintering, and determine their response towards oxidation. Y₂SiO₅ and Y₂Si₂O₇ were formed after sintering and oxidation, respectively. The superficial layers formed after oxidation are thinner and formed faster on the surface of the compositions 90Si₃N₄–5Y₂O₃–5Al₂O₃ and 90Si₃N₄–5Y₂O₃–5AlN than on 90Si₃N₄–5Y₂O₃–2.5Al₂O₃–2.5AlN (in wt.%). The 90Si₃N₄–5Y₂O₃–5Al₂O₃/liquid Al interface features strong interfacial adhesion while mild diffusion should govern the interfacial interactions. Compounds, whose formation results from the yttria-containing sintering aids, such as yttrium aluminates, should act as diffusion barriers at the ceramic/liquid metal interface. The experimental results indicate attractive features for applications in both Al-foundry industry and production of Si₃N₄–Al composites.     

Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering

Photocurable gray-colored Si₃N₄ ceramic slurry with high solid loading, suitable viscosity and high curing depth is critical to fabricate dense ceramic parts with complex shape and high surface precision by stereolithography technology. In the present study, Si₃N₄ ceramic slurry with suitable viscosity, high solid loading (45 vol %) and curing depth of 50 μm was prepared successfully when surface modifier KH560 (1 wt%) and dispersant Darvan (1 wt%) were used. The slurry exhibits the shear thinning behavior. Based on the Beer-Lambert formula, D_p (the attenuation length) and E_c (the critical energy dose) of Si₃N₄ ceramic slurry with solid loading of 45 vol % were derived as 0.032 mm and 0.177 mJ/mm², respectively. Si₃N₄ ceramic green parts with complex shape and high surface precision were successfully fabricated by stereolithography technology. After optimizing the debinding and sintering process for green parts, dense Si₃N₄ ceramics with 3.28 g/cm³ sintering density were fabricated. The microhardness and fracture toughness of as-sintered Si₃N₄ ceramics are ~14.63 GPa and ~5.82 MPa m^1/2, respectively, which are comparable to those of the samples by traditional dry-pressed and pressureless sintering technology. These results show that ceramic stereolithography technology could be promising to fabricate high performance ceramics, especially for gray-colored monolithic Si₃N₄ ceramics.     

Effects of Si3N4 and WC on the oxidation resistance of ZrB2/SiC ceramic tool materials

ZrB₂/SiC, ZrB₂/SiC/Si₃N₄ and ZrB₂/SiC/WC ceramic tool materials were prepared by spark plasma sintering technology, and their oxidation resistance was tested at different oxidation temperatures. When the oxidation temperature is 1300 °C, the oxide layer thickness, oxidation weight gain and flexural strength of ZrB₂/SiC/Si₃N₄ ceramic tool material after oxidation are 8.476 μm, 1.436 mg cm^?2 and 891.0 MPa, respectively. Compared with ZrB₂/SiC ceramic tool materials, the oxide layer thickness and oxidation weight gain are reduced by 8.2% and 11.8%, respectively, and the flexural strength after oxidation is increased by 116.1%. However, the addition of WC significantly reduces the oxidation resistance of the ceramic tool material. A dense oxide film is formed on the surface of ZrB₂/SiC/Si₃N₄ ceramic tool material during oxidation, which effectively prevents oxygen from entering the inside of the material, thereby improving the oxidation resistance of the ceramic tool material.     

Effects of amino groups on dispersibility of silicon nitride powder in aqueous media

Silicon nitride (Si₃N₄) powders were subjected to amination modification by grafting γ-aminopropyltriethoxysilane (APTES) via a direct blending method in solution. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses indicated that the hydroxyl groups present on the surface of Si₃N₄ powder particles interacted with the silanols groups of APTES to combine through covalent bonding. Thermogravimetric analysis (TGA) suggested that the grafting of APTES on Si₃N₄ powder surface was successful with grafting content reaching up to 7%. Compared to native Si₃N₄, the surface hydrophilicity of amino Si₃N₄ powder was enhanced and dispersibility was improved. Overall, these findings indicated the promising aspects of amination modification and future potential use in environmental protection by using water instead of organic solvents during Si₃N₄ ceramic formation process.     

Iron-bearing silicon nitride composite materials made from powders milled in a planetary mill

The effect of milling in an AGO-3 centrifugal planetary activator with steel milling balls on the properties of Si₃N₄ powders, the conditions of sintering, and the phase composition, structure, and properties of composite materials based on them is described. The possibility of and the process conditions for obtaining dense shock-resistant composite materials based on preliminarily milled β-Si₃N₄ (SHS) powder and additives that activate the sintering and consist of AlN, Al₂O₃, and ZrO₂ powders are studied. It is established that in the sintered materials the presence of FeSi formed due to the interaction of Si₃N₄ and Fe that appears in the milling process does not worsen appreciably the thermomechanical properties of the composite materials.     

Mechanism of active and passive oxidation of reaction-bonded Si3N4-SiC refractories

The mechanism of the active and passive oxidation of reaction-bonded Si₃N₄-SiC refractories used for six-month in silicon nitridation shuttle has been analyzed. The change of morphology and component of reaction-bonded Si₃N₄-SiC block of saggar have been analyzed. The results show that in Si₃N₄-SiC, Si₃N₄ has a higher activity and is more easily oxidized than SiC in both active and passive oxidation modes. All the fibrous α-Si₃N₄ which exists in a substantial amount in Si₃N₄-SiC before use has transformed to β-Si₃N₄ or was oxidized. The burning side of Si₃N₄-SiC block, which operates in high oxygen partial pressure, is oxidized in passive mode generating SiO₂, Si₂N₂O and carbon. The working side of Si₃N₄-SiC block, who works in low oxygen partial pressure, is oxidized in the active mode generating gaseous SiO. In another stage of service, SiO generated by silicon nitridation in the saggar infiltrates into the block and reacts with nitrogen forming small rods of Si₃N₄ that fill pores, which leads to lower apparent porosity. In the middle section of the block, two different oxidation modes coexist, since there is a gradually variation of oxygen partial pressure.     

Fabrication of practically pure Si2N2O ceramic with high performance from amorphous BN surface modified nano-sized Si3N4 powders

In this study, amorphous nano-sized Si₃N₄ powders were surface modified by BN. Then a stable and dense Si₂N₂O ceramic was fabricated using the BN surface modified powders, rather than Si₂N₂O-Si₃N₄ composites usually prepared from nano-sized Si₃N₄ powders without surface modification. The effect of BN surface modification on phase transformation, microstructure and mechanical properties were also investigated. Si₂N₂O ceramics obtained by means of the present method have no residual Si, crystal SiO₂ and other oxide additives, which are usually produced by other methods and may seriously influence high-temperature structural and functional applications of Si₂N₂O ceramics.     

3D printing ceramic cores for investment casting of turbine blades,using LCD screen printers: The mixture design and characterisation

The primary objective of this study is to demonstrate the possibility of developing silica, alumina, and zircon-based photocurable ceramic suspensions that can be used for visible light photopolymerization (> 450 nm) and to optimise the binder formulations for the purpose of LCD-based ceramic 3D printing applications. Reference ceramic components for this work are ceramic cores employed in the investment casting of high-pressure turbine blades and vanes. Arguably, one of the most critical steps in photoinduced ceramic 3D printing is developing suitable ceramic suspensions, having high ceramic loading, low viscosity, and short curing times. Ceramic suspensions with four different novel binder formulations and commercial ceramic powders used in core manufacturing (SiO₂, Al₂O₃ and ZrSiO₄) were investigated to achieve the best trade-off between: (1) their curing performance (cure depth and curing speed), (2) rheological properties of the binder mixtures at the solid loadings of 60 vol.% for SiO₂, 55 vol.% for ZrSiO₄, and 45 vol.% for Al₂O₃; and (3) the green body mechanical properties of the mixtures after printing. The effect of ceramic particles on the selected binders was examined individually, and the correlation between cure depth (C_d), volumetric loading, and curing speed are evaluated. The results show all binders designed in this study provide an adequate cure depth, even at high ceramic loadings. When the curing behaviour of all unloaded binder mixtures from the previous study [1] compared with the 10 vol.% SiO₂ loaded mixtures, the cure depth of all formulated binder mixtures increased 50–55 % and the curing thickness of 60 vol.% SiO₂ loaded suspensions were still slightly higher than their unloaded counterparts. The rheology outcomes indicate that lower viscosity binders always result in lower viscosity of the ceramic loaded inks, even without taking the effect of dispersants into account. Besides, the addition of N-Vinyl-2-Pyrrolidone (NVP) monofunctional monomer to the binder mixtures significantly reduces the viscosity and changes the normally linear relationship of the mix viscosity and its silica loading content. Among the binder formulations loaded with 60 vol.% of SiO₂, the formulation providing the lowest viscosity and highest mechanical property consists of 5 wt.% of NVP, 45 wt.% of HDDA and 50 wt.% of Photocentric 34 resin. Although this binder mixture showed the highest green flexural strength when loaded by 55 vol.% ZrSiO₄, all other mixtures loaded with zircon flour also demonstrated a near-fluid behaviour, below 200 s^?1. In Al₂O₃ loaded mixtures, the HDDA di-functional binder formulations present lowest viscosity and the di- and multifunctional monomer blends (HDDA-Photocentric27) showed the highest mechanical properties when used in a 50/50 ratio. This work summarises the best binder choices for silica, alumina and zircon based ceramic suspensions used in core printing for investment casting applications through LCD screen printing.     

Silicon nitride/boron nitride ceramic composites fabricated by reactive pressureless sintering

Using Si and BN powders as raw materials, silicon nitride/hexagonal boron nitride (Si₃N₄/BN) ceramic composites were fabricated at a relatively low temperature of 1450 °C by using the reaction bonding technology. The density and the nitridation rate, as well as the dimensional changes of the specimens before and after nitridation were discussed based on weight and dimension measurements. Phase analysis by X-ray diffraction (XRD) indicated that BN could promote the nitridation process of silicon powder. Morphologies of the fracture surfaces observed by scanning electron microscopy (SEM) revealed the fracture mode for Si₃N₄/BN ceramic composites to be intergranular. The flexural strength and Young's modulus decreased with the increasing BN content. The reaction-bonded Si₃N₄/BN ceramic composites showed better machinability compared with RBSN ceramics without BN addition.