Properties Of Porous Si3N4 Ceramics Research Articles

Improved mechanical properties of porous Si3N4 ceramics strengthened by β-Si3N4 seeds fabricated by vat photopolymerization

Porous Si3N4 ceramics are widely applied in aerospace and mechanical fields owing to their excellent properties. Furthermore, vat photopolymerization (VPP) technology can fabricate Si3N4 components with complicated structures and high precision, but its layer-by-layer printing method leads to poor mechanical properties of ceramics. In this study, porous Si3N4 ceramics with a porosity of 28.41 % strengthened by directional β-Si3N4 were fabricated by combining VPP technology and seeding method. Rheological behavior and curing properties of the slurry were explored, and the influence of β-Si3N4 content on the mechanical properties of printed Si3N4 ceramics was investigated systematically. With the increase of β-Si3N4 content, the orientation degree of β-Si3N4 grains increased gradually, while fracture toughness and flexural strength of the ceramics exhibited a trend of increased first and then decreased and Vickers hardness gradually decreased. As β-Si3N4 content increased to 5 wt%, the fracture toughness and flexural strength of porous Si3N4 ceramics were improved from 4.23 MPa m1/2 and 214.7 MPa–5.65 MPa m1/2 and 272.0 MPa, respectively. Therefore, this work indicates that vat photopolymerization combined with seeding method is a promising approach for the fabrication of porous Si3N4 ceramics with high performance and complex structures.

Microstructure and properties of porous Si3N4 ceramics fabricated by digital light processing combined with spark plasma sintering

Sintering process is an important factor affecting the properties of porous Si3N4 ceramics. However, there are few studies on the properties of porous Si3N4 ceramics fabricated by combining digital light processing (DLP) technology and spark plasma sintering (SPS) technology. In this study, a method to prepare porous Si3N4 ceramics by combining DLP with SPS technology was proposed, and ceramic samples were successfully prepared. The effect of sintering temperature and holding time of SPS technology on the microstructure and properties of porous Si3N4 ceramics prepared by DLP technology was discussed. With the increase of sintering temperature from 1600 °C to 1700 °C, the conversion content of α-Si3N4 phase to β-Si3N4 phase increased from 19.19% to 76.72%. The fracture mode of porous Si3N4 ceramics gradually transformed from intergranular to transgranular. The porosity of Si3N4 ceramics gradually decreased from 40.34 ± 4.9% to 30.54 ± 2.7% with opposite trend in bulk density. And the flexural strength increased from 109.1 ± 7.6 MPa to 249.5 ± 4.0 MPa. When the sintering temperature was 1700 °C, the content of β-Si3N4 increased first and then remained essentially constant with the increase of holding time from 5 min to 15 min. The porosity increased from 30.54 ± 2.7% to 35.15 ± 2.5%, while the variation in bulk density were opposite. And the flexural strength severely decreased from 249.5 ± 4.0 MPa to 126.1 ± 7.2 MPa. This research provides valuable insights into the DLP-forming ceramic materials sintered by SPS technology.

Influence of the ratio of sintering aids on the properties of porous Si3N4 ceramics fabricated by digital light processing

The sintering aids play an important role in affecting the properties of porous Si3N4 ceramics. However, there are few researches on the properties of porous Si3N4 ceramics fabricated by digital light processing (DLP) with different ratios of sintering aids. In this paper, porous Si3N4 ceramics with different ratios of sintering aids (Y2O3-Al2O3) were formed by DLP technology. The influence of Y2O3-Al2O3 ratios on the properties of Si3N4 slurry and porous ceramic was studied systematically. The ratio of Y2O3-Al2O3 had little effect on the rheology and cure depth of Si3N4 slurry due to the low addition of sintering aids. The increase of Y2O3-Al2O3 ratio promoted the anisotropic growth of β-Si3N4. When the ratio of Y2O3-Al2O3 was 9:1, the aspect ratio of β grains reached the maximum. As the ratio of Y2O3-Al2O3 powders increased, the linear shrinkage of porous Si3N4 ceramics showed an increasing and then decreasing trend in three directions. When the Y2O3-Al2O3 ratio was 3:7, the shrinkage rate in the length, width and height direction reached the maximum (27.03%, 30.27% and 40.02%, respectively). The bulk density and flexural strength exhibited an initial increase followed by a subsequent decrease, while the porosity showed the opposite trend. When the Y2O3-Al2O3 ratio was 9:1, the porosity reached a maximum of 28.1%. And the bulk density and flexural strength were 2.42 g/cm3 and 421.58 MPa, respectively. This study is of great significance as it lays the experimental foundation in the performance control of porous Si3N4 ceramics fabricated by DLP.

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.

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.

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.

Microstructure and properties of porous Si3N4 ceramics by gelcasting-self-propagating high-temperature synthesis (SHS)

Porous silicon nitride ceramics have attracted a considerable attention due to their excellent overall performance, but poor porosity homogeneity and structural shrinkage induced by prolonged high temperature sintering limit its further application. Herein, as a three-in-one solution for the above issues, for the first time we develop a novel approach that integrates the merits of gelcasting-SHS (self-propagating high-temperature synthesis) to prepare porous Si3N4 ceramics to simultaneously achieve high porosity, high strength, high toughness, and low thermal conductivity across a wide temperature range. By regulating the solid content, porous Si3N4 ceramics with homogeneous pore structure are obtained, where the pore size falls inbetween 1.61 and 4.41 µm, and the elongated grains are interlaced and interlocked to form micron-sized coherent interconnected pores. At the same time, porous Si3N4 ceramics with porosity of 67.83% to 78.03% are obtained, where the compressive strength reaches 11.79 to 47.75 MPa and fracture toughness reaches 1.20 to 6.71 MPa·m1/2.

Microstructure evolution and high-temperature mechanical properties of porous Si3N4 ceramics prepared by SHS with a small amount of Y2O3 addition

Porous Si3N4 ceramics with well-developed microstructure and high flexural strength were successfully prepared by self-propagating high temperature synthesis (SHS) using a small amount of Y2O3 as sintering additive. Influences of Y2O3 content on the phase composition, microstructure evolution, flexural strength at both room temperature and elevated temperature of ~1400 °C and their fracture behavior of the obtained porous Si3N4 ceramics were investigated. Porous Si3N4 ceramic with high average aspect ratio of 4.03 and excellent flexural strength of 101 MPa was obtained by 0.4 wt% Y2O3 addition. It also shows good strength retention of 76.6% and evidently elastic fracture behavior at 1400 °C because of its rather low amorphous intergranular phase content. With the increase of Y2O3 content, the creep resistance of sample degraded owning to the softening of amorphous intergranular thus resulting in the degradation of high-temperature strength retention.

Effects of N2 pressure and Si particle size on mechanical properties of porous Si3N4 ceramics prepared via SHS

Porous silicon nitride ceramics with high flexural strength and high porosity were directly fabricated by self-propagating high temperature synthesis (SHS). The effects of N2 pressure and Si particle size on the phase composition, microstructure, and mechanical property were investigated. N2 influences not only the thermodynamics but also the kinetics of the SHS as initial reactant. Flexural strength ranged between 67 MPa and 134 MPa with increasing N2 pressure. On the other hand, flexural strength ranged from 213 MPa to 102 MPa with different Si particle sizes. This plays an important role on the final diameter and length of β-Si3N4 grains and the formation mechanism of porous Si3N4 ceramics.

Effect of β–Si3N4 seeds on microstructure and properties of porous Si3N4 ceramics prepared by gelcasting using DMAA system

Porous Si3N4 ceramics fabricated by gelcasting with DMAA demonstrated excellent mechanical and dielectric properties after introducing β–Si3N4 seeds into ceramic matrix. The microstructure and properties of slurries, green and sintered bodies were highly correlated with β–Si3N4 seed size, content and morphology. The slurry viscosity reached the maximum of 0.87 Pa s when introducing 10 wt% elongated seeds, but overall, exhibiting good fluidity. The green body strength dropped remarkably after adding small equiaxial seeds, but improved within 6 wt% elongated seeds and afterwards declined to the minimum of 25.99 MPa, which was quite ideal for processing. The elongated seeds impeded liquid phase flow and particle rearrangement more pronouncedly than equiaxial ones during sintering, thereby causing the highest porosity of 41.77% and the largest pore size of 1.04 μm when employing 10 wt% elongated seeds. The elongated seeds were more helpful in developing the strong self–reinforced bimodal microstructure due to the original hexagonal prism morphology and achieved the highest bending strength of 378.50 MPa and fracture toughness of 8.54 MPa m1/2 at 2 wt% and 6 wt% elongated seeds, respectively. The dielectric constant and loss tangent depended strongly on the porosity and separately varied in 4.70–4.82 and 0.041–0.047 ranges in 8.2–12.4 GHz when using 6 wt% elongated seeds, demonstrating superior microwave–penetrating properties.

Effects of solid loading and calcination temperature on microstructure and properties of porous Si3N4 ceramics by aqueous gelcasting using DMAA system

Abstract Herein, a low–toxic N, N–dimethylacrylamide (DMAA) system was used in preparation of porous Si3N4 ceramics by aqueous gelcasting, and variations in microstructure and properties with solid loading and calcination temperature were systematically investigated. In the considered solid loading range of 28–44 vol%, all the slurries exhibited superior rheological properties (≤145 mPa⋅s at 95.40 s−1 for 44 vol% solid loading) perfectly suitable for casting. With increasing solid loading, a decreased bulk density (1.71–1.69 g/cm3), volume shrinkage (37.73–13.77%) and flexural strength (46.56–26.75 MPa) of green bodies were obtained, exhibiting better mechanical properties than those derived from the conventional acrylamide (AM) system. Regarding Si3N4 ceramics with various solid loadings, the increase in calcination temperature favored the phase transformation α→β–Si3N4 and β–Si3N4 growth, however, the increased solid loading exhibited an inhibiting effect on those since mass transport in gas phase was blocked due to the disruption of pore connectivity. The resulting microstructure changes imparted Si3N4 ceramics increasing flexural strength (110.36–367.88 MPa), fracture toughness (2.54–5.03 MPa⋅m1/2), as well as decreasing porosity (54.21–41.05%) and pore size (0.38–0.33 μm). This work demonstrates the potential research value of DMAA system in preparing high–performance porous Si3N4 ceramics through gelcasting technique.

Fabrication and mechanical properties of porous Si3N4 ceramics prepared via SHS

Abstract Porous silicon nitride ceramics were directly prepared by self-propagating high temperature synthesis (SHS). The effects of Si 3 N 4 diluent and Y 2 O 3 sintering additive on the phase composition and microstructure were investigated. The XRD patterns and SEM microstructures proved the sample with 63 wt% Si 3 N 4 diluent could provide appropriate temperature, reaction time and porosity for the fabrication of elongated β-Si 3 N 4 porous ceramics. A porosity of 40%–47% and a flexural strength of 118–216 MPa were obtained after SHS with different Y 2 O 3 content. Besides, the excellent thermal shock resistance of Si 3 N 4 assured that strength retention keeps stable in the temperature difference ( Δ T) of 800 °C.

Fabrication and properties of porous Si3N4 ceramics by aqueous gelcasting using low-toxic DMAA gelling agent

A low-toxic and water-soluble monomer N, N-dimethylacrylamide (DMAA) was employed as a gelling agent in the gelcasting of porous Si3N4 ceramics. The process conditions and composition for slurry preparation (with a solid loading of 36 vol%), the consolidation and sintering of green bodies were investigated and optimized. The effects of various factors such as zeta potential, pH value of the premix solution, dispersant dosage and ball milling time on the rheological properties of the slurries were investigated. The results suggest that the best rheological properties (66.5 mPa.s at a shear rate of 96.3 s−1) of the slurries were obtained when pH value ranged between 9 and 11, dispersant dosage reached 1 wt%, and ball milling time was 6 h. All the as-prepared green bodies showed a homogeneous microstructure and high flexural strength ≥ 26 MPa with a maximum up to 46.3 MPa when the ratio of DMAA to MBAM, initiator dosage, polymerization temperature and time were 14, 1 wt%, 70 °C and 90 min, respectively. The sintered bodies had a homogeneous microstructure, excellent and regulatable properties, a flexural strength of 216.3–327.3 MPa, and a porosity of 39.6–29.1% by varying the sintering temperature from 1710 °C to 1810 °C and the holding time from 1 h to 3 h. The superior comprehensive effect makes DMAA a promising candidate for an environmentally friendly gelling agent in gelcasting of porous Si3N4 ceramics.

Effect of Si3N4 solid contents on mechanical and dielectric properties of porous Si3N4 ceramics through freeze-drying

Porous Si3N4 ceramics with uniform pore distributions were fabricated by freeze drying with liquid N2 as refrigerant, and then sintered at 1700 °C under an N2 pressure of 0.3 MPa. The Si3N4 dispersant slurries were prepared by ball milling for 12 h and casting into a mold using liquid nitrogen as a freezing medium with the top surface of the sample exposed to air at room temperature. The temperature gradient contributed to bimodal peaks of pore size distribution. Effects of solid concentration on the porosity, mechanical, and dielectric properties of the Si3N4 sample were systematically investigated. As the solid content increased from 15 vol.% to 40 vol.%, the porosity decreased from 83.9% to 40.58%. Therefore, flexural strength and compressive strength increased from 0.1 MPa to 94.7 MPa, and 1.3 MPa–314 MPa, respectively. Moreover, the dielectric constant increased from 1.4 to 4.0. This study indicated that the freeze-drying process is a useful method for fabricating novel porous Si3N4 ceramics with unidirectional channels.

Electromagnetic properties of porous Si3N4 ceramics with gradient distributions of SiC and pores fabricated by directional in–situ nitridation reaction

Porous Si3N4 ceramics with gradient distributions of SiC and pores are fabricated by directional nitridation sintering diatomite preforms with phenolic resin as a carbon source and pore forming agent. The nitrogen flow rate during sintering and the phenolic resin content in the preforms greatly impact the distribution of SiC and pores on the ceramics. Trace SiC and high porosity make the ceramics show weak surface impedance mismatch at their upper surface, so electromagnetic waves can enter the ceramics from their upper surfaces with little reflection. The enhancement of interfacial polarization with increasing SiC in the downward direction is the reason for the excellent electromagnetic wave absorption by the ceramics. The ceramics are excellent electromagnetic wave absorbing materials with a mean reflectivity far lower than −10dB in the frequency range of 8–18GHz.

The effect of oxidation on the mechanical properties and dielectric properties of porous Si3N4 ceramics

The effect of oxidation temperature and time on the microstructures, phase compositions, mechanical properties, and dielectric properties of porous Si3N4 ceramics was investigated in the temperature range from 900°C to 1300°C for 1h, 5h, and 24h. The weight gain measured either at lower temperature (900°C) for long time (24h) or at higher temperature (1300°C) for 1h demonstrated that the porous Si3N4 ceramics were easily oxidized under the current test conditions. Results showed that the amount of open pores, flexural strength, compressive strength, and dielectric constant all decreased with the increase of oxidation temperature independent upon the oxidation time. The oxidation product SiO2 was low-temperature quartz in mild condition (low temperature, short time) and cristobalite in severe condition (high temperature, long time). The existence of cracks on the oxide scale was due to the phase transformation of SiO2 and thermal expansion coefficient mismatch between SiO2 and Si3N4.

Preparation of porous Si3N4 ceramics via tailoring solid loading of Si3N4 slurry and Si3N4 poly-hollow microsphere content

Porous Si3N4 ceramics were prepared by aqueous gelcasting using Si3N4 poly-hollow microspheres (PHMs) as pore-forming agent. The effects of solid loading of Si3N4 slurry and Si3N4 PHM content on the properties of porous Si3N4 ceramics were investigated. Only β-Si3N4 phase is observed in porous Si3N4 ceramics, and Si3N4 PHMs distribute uniformly both in Si3N4 green samples and porous Si3N4 ceramics. Results show that solid loading of Si3N4 slurry and Si3N4 PHM content could considerably influence the properties of porous Si3N4 ceramics. With the increase of solid loading of Si3N4 slurry (decrease of Si3N4 PHM content), the distributing state of Si3N4 PHMs changes from contacting with each other to just embedding in the matrix, thus their porosity decreases, while their shrinkage, flexural strength, and fracture toughness increase.

Influence of Yb<sub>2</sub>O<sub>3</sub> and Y<sub>2</sub>O<sub>3</sub> on the Mechanical Properties of Porous Si<sub>3</sub>N<sub>4</sub> Ceramics

Using Yb2O3 or Y2O3 as s single sintering additive, the porous Si3N4 ceramics with excellent mechanical properties were successfully fabricated by pressureless sintering. The influence of Y2O3 and Yb2O3 on the microstructure and mechanical properties of porous Si3N4 ceramics was investigated. The results show that both sintering additives can effectively promote α to β phase transformation, and microstructures with elongated β-Si3N4 grains interlocking and superimposing on each other can be obtained. However, sinterability, phase transformation and mechanical properties of the porous Si3N4 ceramics with Yb2O3 were more excellent than the ceramics with Y2O3. The specimens with Yb2O3 had high aspect ratio β-Si3N4 grains, and formed the high melting crystalline Yb4Si2N2O7.

Porous Si3N4 Ceramics Prepared by TBA-based Gel-casting

In this work, the effects of ratio of monomer to cross-linker (AM/MBAM) and solid loading on the microstructure and mechanical properties of porous Si3N4 ceramics prepared by tert-butyl alcohol (TBA)-based gel-casting process were investigated. It was found that, when the ratio of monomer to cross-linker was 8, and the solid loading was 50 wt%, the mechanical properties of sintered samples were the most excellent, which resulted from the uniform pore size distribution and well-grown rod-like β-Si3N4 grains. In that case, the porosity and flexural strength of sintered samples were 50% and 125 MPa, respectively.

Preparation of Porous Si3N4 Ceramics with Controlled Porosity and Microstructure by Fibrous α‐Si3N4 Addition

Porous silicon nitride ceramics with various porosities were fabricated by liquid phase sintering of mixtures containing fibrous and equiaxed α‐Si3N4 powder with a various content ratios. The effects of the contents of the fibrous α‐Si3N4 powder (0%–100%) on the microstructure and mechanical properties of porous Si3N4 ceramics were studied. As the increase of the fibrous α‐Si3N4 powder content, both the density of green bodies and the linear shrinkage decreased, resulting in increased porosity due to the inhibited densification by the fibrous Si3N4 particle. XRD analysis proved the complete formation of single β‐Si3N4 phase. SEM analysis revealed that the microstructure of the low content of fibrous α‐Si3N4 porous ceramics was almost composed of fine elongated β‐Si3N4 grains with high aspect ratio while numerous coarse elongated β‐Si3N4 grains with low aspect ratio surrounding fine grains were formed as the content of the fibrous α‐Si3N4 powder increased. With the increase in content of the fibrous α‐Si3N4 powder from 0% to 100%, the porosity changed from 47.8% to 56.6%, and the flexural strength decreased from 146 to 62 MPa correspondingly, indicating a flexural adjustment of the porosity and mechanical properties.