In-situ formation of BN layers by nitriding boron powders in BN/Si3N4-based composite ceramics

Boron nitride/silicon nitride (BN/Si₃N₄) composite ceramics were fabricated via the in-situ nitridation of boron (B) and silicon (Si) powders in forming gas (95%N₂/5%H₂) at 1390?°C. The effect of the B content on the phase composition, microstructure, density/porosity, machinability as well as mechanical properties of nitridized BN/Si₃N₄ composite ceramics was investigated. The addition of B slightly increased the nitridation degree of the Si and B powders mixture, and improved the ratio of the β-Si₃N₄ phase significantly at low B contents. B powders may have acted as a nucleating agent to promote the formation of β-Si₃N₄ crystals. A core-shell Si₃N₄/BN structure was revealed by the TEM technique, and the number of BN layers increased with the increase of the B content. The in-situ BN formed by the nitridation of B played a similar role with the BN directly added in enhancing the machinability of the BN/Si₃N₄ composite ceramics. The method of the in-situ nitridation of B is also effective to prepare SiC fiber-reforced BN/Si₃N₄ ceramic matrix composites.     

Preparation and characterization of porous Si3N4-bonded SiC ceramics and morphology change mechanism of Si3N4 whiskers

Porous Si₃N₄-bonded SiC ceramics with high porosity were prepared by the reaction-sintering method. In this process, Si₃N₄ was synthesized by the nitridation of silicon powder. The X-ray diffraction (XRD) indicated that the main phases of the porous Si₃N₄-bonded SiC ceramics were SiC, α-Si₃N₄, and β-Si₃N₄, respectively. The contents of β-Si₃N₄ were increased following the sintering temperature. The morphology of Si₃N₄ whiskers was investigated by scanning electron microscope (SEM), which was shown that the needle-like (low sintering-temperature) and rod-like (higher sintering-temperature) whiskers were formed, respectively. From low to high synthesized temperature, the highest porosity of the porous Si₃N₄ bonded SiC ceramic was up to 46.7%, and the bending strength was ~11.6?MPa. The α-Si₃N₄ whiskers were derived from the reaction between N₂ and Si powders, the growth mechanism was proved by Vapor–Solid (VS). Meanwhile, the growth mechanism of β-Si₃N₄ was in accordance with Vapor–Solid–Liquid (VSL) growth mechanism. With the increase of sintering temperature, Si powders were melted to liquid silicon and the α-Si₃N₄ was dissolved into the liquid then the β-Si₃N₄ was precipitated successfully.     

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.     

Preparation of high-strength Si3N4 antenna window using selective laser sintering

In this study, a high-strength silicon nitride (Si₃N₄) antenna window was successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding before final sintering. The effects of CIP after debinding and sintering aids on the bulk density, total porosity, bending strength and microstructure of Si₃N₄ ceramics were examined. The results show that the bending strength of SLS Si₃N₄ ceramics can be greatly improved by adding sintering aids between Si₃N₄ granules and by CIP after debinding. Optimal performance of ceramics is obtained by CIP after debinding and the use of inter-granule sintering aids. The porosity, bulk density, and bending strength are 18.7%, 3.11 g/cm³, and 685 MPa, respectively. Eliminating the pores by the CIP after debinding and by inter-granule sintering aids promotes the growth of rod-like β-Si₃N₄, which lock with each other contribute to the strengthening of Si₃N₄ ceramics.     

Combustion synthesis of MoSi2 based composite and selective laser sintering thereof

Additive manufacturing is gaining increasing attention as it provides cost-effective and waste-less production of materials with multi-axis geometries. Selective laser sintering of ceramics is very challenging in terms of poor sinterability caused by low thermal shock resistance and insufficient electron conductivity blocking absorption of laser beam energy.Here, we present a novel strategy for manufacturing dense, hierarchically structured ceramics, particularly, MoSi₂-based composites by selective laser sintering. MoSi₂-Si composite powders were prepared by combustion synthesis technique, where the ceramic grains were covered with different amount of Si. MoSi₂-Si powder was consolidated by selective laser sintering reaching 92% of density. The hardness of the manufactured samples varied with the amount of Si and applied laser current from 7.7–11.4?GPa. The maximum value of the compressive strength was determined to be 636?MPa. The manufactured MoSi₂-Si was subjected to nitridation, which resulted in the growth of Si₃N₄ fibres on the surface and pores of the samples.     

Effect of the molecular weight and concentration of PEG solution on microstructure and mechanical properties of Si3N4 ceramics fabricated by gel-casting

Gel-casting is a promising preparation technology of Si₃N₄ structural ceramics. The process involves drying of the “green” gel-cast parts before densification. And the drying of green gel-cast bodies is an important step in the gel-casting manufacturing process. In this work, the Si₃N₄ gel-cast green bodies were dried in polyethylene glycol (PEG) solution with the purpose of obtaining Si₃N₄ ceramics with good mechanical properties. The effect of the molecular weight and concentration of PEG solution on drying rate, microstructure and mechanical properties of Si₃N₄ ceramics was studied. The results indicated that with the increase of molecular weight of PEG, the drying rate increased obviously and the structure became more uniform and dense when the concentration of solution was 20?wt%. The Si₃N₄ ceramics after sintering have the excellent flexural strength (662.6?MPa) under PEG600 drying condition. Furthermore, the concentration of PEG600 solution had a positive effect on drying and sintering of the green body. Therefore, the bending strength reached 871.1?MPa under 65?wt% PEG 600 solution drying condition. Overall, the drying process (drying in 65?wt% PEG600 solution) promotes the efficiency and quality of drying of Si₃N₄ gel-cast green bodies, which is beneficial for the subsequent drying and sintering process.     

Enhanced high-temperature dielectric properties and microwave absorption of SiC nanofibers modified Si3N4 ceramics within the gigahertz range

Si₃N₄ ceramics modified with SiC nanofibers were prepared by gel casting aiming to enhance the dielectric and microwave absorption properties at temperatures ranging from 25?°C to 800?°C within X-band (8.2–12.4?GHz). The results indicate that the complex permittivity and dielectric loss are significantly increased with increased weight fraction of SiC nanofibers in the Si₃N₄ ceramics. Meanwhile, both complex permittivity and dielectric loss of SiC nanofibers modified Si₃N₄ ceramics are obviously temperature-dependent, and increase with the higher test temperatures. Increased charges mobility along conducting paths made of self-interconnected SiC nanofibers together with multi-scale net-shaped structure composed of SiC nanofibers, Si₃N₄ grains and micro-pores are the main reason for these enhancements in dielectric properties. Moreover, the calculated microwave absorption demonstrates that much enhanced microwave attenuation abilities can be achieved in the SiC nanofibers modified Si₃N₄ ceramics, and temperature has positive effects on the microwave absorption performance. The SiC nanofibers modified Si₃N₄ ceramics will be promising candidates as microwave absorbing materials for high-temperature applications.     

Low-temperature preparation of Si3N4/SiC porous ceramics via foam-gelcasting and microwave-assisted catalytic nitridation

Si₃N₄/SiC porous ceramics were fabricated by a novel foam-gelcasting and microwave-assisted catalytic nitridation method at a temperature as low as 1273?K for 60?min or after only 10?min at 1373?K utilizing commercial Si and SiC with trace of impurity Fe (0.33?wt%) as starting materials. The Si₃N₄/SiC porous ceramics containing porosity of 68.54?±?0.73% which were fabricated at 1373?K for 10?min had flexural and compressive strengths of 5.28?±?0.17?MPa and 12.86?±?1.55?MPa.     

Laser surface nanostructuring for reliable Si3N4/Si3N4 and Si3N4/Invar joined components

IR pulsed laser radiation in air was applied to Si₃N₄ and Invar to obtain reliable Si₃N₄/Si₃N₄ and Si₃N₄/Invar adhesive bonded components. The laser pre-treatment produced a homogeneous nanostructured oxide layer on the surfaces, which effectively increased the adhesion at the adhesive/adherends interface and led to cohesive failure in the joining material. The mechanical strength of Si₃N₄/ Si₃N₄ and Si₃N₄/Invar joined components was measured, with and without laser nanostructuring, before and after thermal cycling from room temperature to 50?K, and it resulted that the exposure to extremely low temperatures did not affect the mechanical integrity of the joints. It was also demonstrated that this laser pre-treatment did not alter the mechanical properties of the ceramic substrate.     

Preparation and properties of silicon nitride-phosphate composites for application in microwave furnace

Si₃N₄ ceramic is one of the most promising microwave metallurgy furnace materials because of the outstanding mechanical, relatively low dielectric properties and excellent thermal shock resistance. However, the difficult sintering of Si₃N₄ ceramics extremely restrict their large-scale application in the field of refractories for microwave metallurgy. In this work, silicon nitride-phosphate ceramics were fabricated by introducing aluminum phosphate or chromium phosphate aluminum into Si₃N₄ ceramics at 1500 °C. The effect of the amount of aluminum phosphate and chromium phosphate aluminum on sintering performance and dielectric properties was investigated. The results showed that the addition of aluminum phosphate or chromium phosphate aluminum could promote sintering, and the mechanical and dielectric properties of Si₃N₄ ceramics were efficiently improved. The Si₃N₄-aluminum phosphate composites exhibited better sintering performance (higher density and mechanical property) than that of Si₃N₄-chromium phosphate aluminum composites. Meanwhile, the dielectric constant and dielectric loss of Si₃N₄-chromium phosphate aluminum composites were better than Si₃N₄-chromium phosphate aluminum composites.     

Porous Si3N4/SiC Ceramics Prepared via Nitridation of Si Powder with SiC Addition

Porous Si₃N₄/SiC ceramics with high porosity were prepared via nitridation of Si powder, using SiC as the second phase and Y₂O₃ as sintering additive. With increasing SiC addition, porous Si₃N₄/SiC ceramics showed high porosity, low flexural strength, and decreased grain size. However, the sample with 20wt% SiC addition showed highest flexural strength and lowest porosity. Porous Si₃N₄/SiC ceramics with a porosity of 36–45% and a flexural strength of 107‐46MPa were obtained. The linear shrinkage of all porous Si₃N₄/SiC ceramics is below 0.42%. This study reveals that the nitridation route is a promising way to prepare porous Si₃N₄/SiC ceramics with favorable flexural strength, high porosity, and low linear shrinkage.     

Dielectric and mechanical properties of porous Si3N4 ceramics prepared via low temperature sintering

Borophosphosilicate bonded porous silicon nitride (Si₃N₄) ceramics were fabricated in air using a conventional ceramic process. The porous Si₃N₄ ceramics sintered at 1000–1200 °C shows a relatively high flexural strength and good dielectric properties. The influence of the sintering temperature and contents of additives on the flexural strength and dielectric properties of porous Si₃N₄ ceramics were investigated. Porous Si₃N₄ ceramics with a porosity of 30–55%, flexural strength of 40–130 MPa, as well as low dielectric constant of 3.5–4.6 were obtained.     

The fabrication of tungsten reinforced silicon nitride ceramics by altering nitrogen pressure

Due to high ductility, high-temperature melting, low thermal expansion coefficient, etc., tungsten (W) might be considered to be an ideal reinforcement in toughening or strengthening Si₃N₄ ceramics. However, it is difficult to fabricate W/Si₃N₄ composites due to the possible reactions between W and Si₃N₄ during sintering process at the high temperature. In this work, a novel way to avoid the reactions and fabricate the W/Si₃N₄ composites was proposed by thermodynamic analysis and verified by experiment. Firstly, the phase equilibrium between W and Si₃N₄ as a function of temperature and nitrogen pressure was thermodynamically calculated, which indicates that one critical nitrogen pressure exists for reactions between W and Si₃N₄ at a certain temperature. As the nitrogen pressure is higher than the critical value, the reactions would be inhibited or adversely proceeded. Based on the results, W was innovatively in-situ introduced in the form of WSi₂ after sintering at 1750?°C under 50?bar nitrogen pressure. Moreover, the fracture toughness of Si₃N₄ ceramics was enhanced from 7.1?±?0.2 to 8.0?±?0.4?MPa?m^1/2, which proposes a new reinforcement or method in toughening Si₃N₄ ceramics.     

Preparation mullite/Si3N4 composites by reaction spark plasma sintering and their characterization

In the present study, in-situ mullite/Si₃N₄ composites were prepared successfully by reaction spark plasma sintering. For this purpose, 5, 10 and 15?wt% of Si₃N₄ were added to stoichiometric mullite made of mechanically milled mixture of alumina and kaolin clay to investigate the effect of reinforcement content on the final properties of the prepared composites. The sintering processes were performed at 1400?°C under the initial and final applied pressures of 10 and 30?MPa and the vacuum condition of 17?Pa. The XRD patterns revealed the mullite and Si₃N₄ peaks as the dominant crystalline phases. Microstructural investigations demonstrated a uniform distribution of Si₃N₄ inside mullite matrix for the composites containing 5 and 10?wt% of the reinforcement particles. Meanwhile, some agglomerates of Si₃N₄ were observed in the microstructure of the mullite-15?wt%Si₃N₄ composite. Moreover, no evidence of reaction between the starting materials was detected through XRD and FESEM analyses. The highest values of hardness, bending strength, and fracture toughness obtained for the composite containing 15?wt% of Si₃N₄ were 19.14?GPa, 481?MPa and 3.85?MPa?m^?1/2, respectively. The fracture toughness mechanisms were detected as crack branching, breaking and deflection, as well as particles pulling-out, all of which were observed in the mullite-15?wt%Si₃N₄ composite.     

Microstructure control and mechanical properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with a fibrous interlocking microstructure were synthesized by carbothermal nitridation of silicon dioxide. The influences of different starting powders on microstructure and mechanical properties of the samples were studied. The results showed that the microstructure and mechanical properties of porous silicon nitride ceramics depended mostly on the size of starting powders. The formation of single-phase β-Si₃N₄ and the microstructure of the samples were demonstrated by XRD and SEM, respectively. The resultant porous Si₃N₄ ceramics with a porosity of 71% showed a relative higher flexural strength of 24 MPa.     

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.     

A non-sintering fabrication method for porous Si3N4 ceramics via sol hydrothermal process

A non-sintering fabrication method for porous Si₃N₄ ceramics with high porosity and high mechanical strength was proposed. Strength of the porous ceramics can be obtained by silica sol mass transfer process in hydrothermal conditions rather than a traditionally controlled high temperature sintering process. Under hydrothermal circumstances, silica sol is continuously transferred to the necks of Si₄N₃ powder compact, depositing there and thus consolidating the ceramic skeleton. The key of the method to obtain homogeneous microstructure and mechanical strength is how to keep the silica sol from gelatin during hydrothermal procedure. The stabilization of silica sol and its affecting factors were studied. The results indicated that ultrasonic treatment makes alkali-catalyzed silica sol remain stable even in 200?℃ hydrothermal condition, which insures consecutive silica transportation. The effect of hydrothermal time on open porosity/mechanical strength of the porous Si₄N₃ ceramics were also thoroughly investigated. The porous Si₄N₃ ceramics with open porosity above 42% and flexural strength of 45?MPa were obtained without any high temperature sintering process. This method can be widely employed for the preparation of other porous ceramics as well.     

Effects of different types of sintering additives and post-heat treatment (PHT) on the mechanical properties of SHS-fabricated Si3N4 ceramics

Porous silicon nitride (Si₃N₄) ceramics were fabricated by self-propagating high temperature synthesis (SHS) using Si, Si₃N₄ and sintering additive as raw materials. Effects of different types of sintering additives with varied ionic radius (La₂O₃, Sm₂O₃, Y₂O₃, and Lu₂O₃) on the phase compositions, development of Si₃N₄ grains and flexural strength (especially high-temperature flexural strength) were researched. Si₃N₄ ceramics doped with sintering additive of higher ionic radius had higher average aspect ratio, improved room-temperature flexural strength but degraded high-temperature flexural strength. Besides, post-heat treatment (PHT) was conducted to crystallize amorphous grain boundary phase thus improving the creep resistance and high-temperature flexural strength of SHS-fabricated Si₃N₄ ceramics. Excellent high-temperature flexural strength of 140 MPa~159 MPa and improved strength retention were achieved after PHT at 1400 °C.     

Paper derived SiC–Si3N4 ceramics for high temperature applications

Biomorphic Si₃N₄–SiC ceramics have been produced by chemical vapour infiltration and reaction technique (CVI-R) using paper preforms as template. The paper consisting mainly of cellulose fibres was first carbonized by pyrolysis in inert atmosphere to obtain carbon bio-template, which was infiltrated with methyltrichlorosilane (MTS) in excess of hydrogen depositing a silicon rich silicon carbide (Si/SiC) layer onto the carbon fibres. Finally, after thermal treatment of this Si/SiC precursor ceramic in nitrogen-containing atmosphere (N₂ or N₂/H₂), in the temperature range of 1300–1450 °C SiC–Si₃N₄ ceramics were obtained by reaction bonding silicon nitride (RBSN) process. They were mainly composed of SiC containing α-Si₃N₄ and/or β-Si₃N₄ phases depending on the nitridation conditions. The SiC–Si₃N₄ ceramics have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and Raman spectroscopy. Thermal gravimetric analysis (TGA) was applied for the determination of the residual carbon as well as for the evaluation of the oxidation behaviour of the ceramics under cyclic conditions. The bending strength of the biomorphic ceramics was related to their different microstructures depending on the nitridation conditions.     

High-strength and wave-transmitting Si3N4–Si2N2O-BN composites prepared using selective laser sintering

In this study, high-strength and wave-transmission silicon nitride (Si₃N₄) composites were successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding and before final sintering, and the optimal moulding process parameters for the SLS Si₃N₄ ceramics were determined. The effects of the sintering aids and secondary CIP on the bulk density, porosity, flexural strength, fracture toughness, and wave-transmitting properties of the Si₃N₄ composites were studied. The results showed that the increased CIP pressure was beneficial to the densification of SLS Si₃N₄ ceramics and improved their mechanical properties. However, the wave-transmitting performance decreased as the CIP pressure increased. The Si₃N₄ ceramics prepared by the moulding of sample S₁₁ were more in line with the performance requirements of the radomes. To obtain good comprehensive performance, an additional 3% of interparticle Y₂O₃ was added to the pre-printed mixed powder of granulated Si₃N₄ particles and resin and the secondary CIP pressure was adjusted to 280 MPa. After sintering, the bending strength, fracture toughness, and dielectric constant of the Si₃N₄ ceramics were 651 MPa, 6.0 MPa m^1/2, and 3.48 respectively. This study provides an important method for preparing of Si₃N₄ composite radomes using SLS process.