Effects of Gd2O3 and MgSiN2 sintering additives on the thermal conductivity and mechanical properties of Si3N4 ceramics

Si₃N₄ ceramics were prepared by gas pressure sintering at 1900°C for 12 h under a nitrogen pressure of 1 MPa using Gd₂O₃ and MgSiN₂ as sintering additives. The effects of the Gd₂O₃/MgSiN₂ ratio on the densification, microstructure, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were systematically investigated. It was found that a low Gd₂O₃/MgSiN₂ ratio facilitated the thermal diffusivity of Si₃N₄ ceramics while a high Gd₂O₃/MgSiN₂ ratio benefited the densification and mechanical properties. When the Gd₂O₃/MgSiN₂ ratio was 1:1, Si₃N₄ ceramics obtained an obvious exaggerated bimodal microstructure and the optimal properties. The thermal conductivity, flexural strength, and fracture toughness were 124 W·m⁻¹·k⁻¹, 648 MPa, and 9.12 MPa·m^1/2, respectively. Comparing with the results in the literature, it was shown that Gd₂O₃-MgSiN₂ was an effective additives system for obtaining Si₃N₄ ceramics with high thermal conductivity and superior mechanical properties.     

Effects of Y2O3/MgO ratio on mechanical properties and thermal conductivity of silicon nitride ceramics

In this work, the effects of Y₂O₃/MgO ratio on the densification behavior, phase transformation, microstructure evolution, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were investigated. Densified samples with bimodal microstructure could be obtained by adjusting the ratio of Y₂O₃/MgO. It was found that a low Y₂O₃/MgO ratio facilitated the densification of Si₃N₄ ceramics while a high Y₂O₃/MgO ratio benefited the phase transformation of Si₃N₄ ceramics. Best mechanical properties (flexural strength of 875 MPa, and fracture toughness of 8.25 MPa·m^1/2, respectively) and optimal thermal conductivity of 98.04W/(m·K) were achieved in the sample fabricated with Y₂O₃/MgO ratio of 3:4 by sintering at 1900°C for 4 h.     

Cost effective preparation of Si3N4 ceramics with improved thermal conductivity and mechanical properties

High-purity silicon powder is used as the starting material for cost-effective preparation of silicon nitride ceramics with both high thermal conductivity and excellent mechanical properties using RE₂O₃ (RE=Y, La or Er) and MgO as sintering additives. Nitridation is a key procedure that would affect the properties of green bodies and the sintered samples. The β: (α+β) ratio can be increased as the samples nitrided at 1450ºC and a large amount of long rod-like β-Si₃N₄ grains were developed in the samples. It was found that the addition of Er₂O₃-MgO could help to improve the mechanical properties of the sintered Si₃N₄ ceramics, the thermal conductivity, flexural strength and fracture toughness of the sample were 90 W/(m∙K), 953±28.3 MPa and 10.64±0.61 MPa·m^1/2, respectively. The RE³⁺ species with larger ionic radius tended to increase the oxygen of nitrided samples and decrease N/O ratio (triangle grain boundary) of sintered samples.     

Improved oxidation resistance properties of Si3N4/O′–SiAlON composite ceramics by a repeated sintering method

Si₃N₄/O′–SiAlON composite ceramics with superior oxidation resistance properties were fabricated by a repeated sintering method. The effects of sintering time on the phase evolution, microstructure, and oxidation resistance properties of the Si₃N₄/O′–SiAlON composite ceramics were investigated. The results indicated that the content of the O′–SiAlON phase and the densification of Si₃N₄/O′–SiAlON composite ceramics increased after two-time sintering. Furthermore, the thickness of the oxide layer of the Si₃N₄/O′–SiAlON composite ceramics after oxidation at 1100–1500°C for 30 h was not significant. Compared to the oxidation weight gain after the one-time sintering process, the oxidation weight gain of Si₃N₄/O′–SiAlON composite ceramics was 0.432 mg/cm² after two-time sintering when oxidized at 1500 C for 30 h, which was reduced by 43.3%. The mechanism of the improved oxidation resistance properties was ascribed to the formation of more O′–SiAlON and the enhancement of the densification.     

Non-additive sintering of Si2N2O ceramic with enhanced high-temperature strength,oxidation resistance,and dielectric properties

The high-temperature mechanical and dielectric properties of Si₂N₂O ceramics are often limited by the introduction of a sintering aid. Herein, dense Si₂N₂O was prepared at 1700 °C by hot-pressing oxidized amorphous Si₃N₄ powder without sintering additives. A homogeneous network with short-range order and a SiN₃O structure was formed in the oxidized amorphous Si₃N₄ powder during the hot-pressing process. Si₂N₂O crystals preferentially nucleated at positions within the SiN₃O structure and grew into rod-like and plate-like grains. Fully dense ceramics with mainly crystalline Si₂N₂O and some residual amorphous phases were obtained. The as-prepared Si₂N₂O possessed a good flexural strength of 311 ± 14.9 MPa at 1400 °C, oxidation resistance at 1500 °C, and a low dielectric loss tangent of less than 5 × 10⁻³ at 1000 °C.     

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.     

On the fabrication and mechanical properties of Si3N4 ceramics with low content sintering additives

Si₃N₄ ceramics were prepared by hot pressing (HP) and spark plasma sintering (SPS) methods using low content (5 mol%) Al₂O₃–RE₂O₃(RE = Y, Yb, and La)–SiO₂/TiN as sintering additives/secondary additives. The effects of sintering additives and sintering methods on the composition, microstructures, and mechanical properties (hardness and fracture toughness) were investigated. The results show that fully density Si₃N₄ ceramics could be fabricated by rational tailoring of sintering additives and sintering method, and TiN secondary additive could promote the density during HP and SPS. Besides, SN-AYS-SPS possesses the most competitive mechanical properties among all the as-prepared ceramics with the Vickers hardness as 17.31 ± .43 GPa and fracture toughness as 11.07 ± .48 MPa m^1/2.     

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.     

Effects of Al2O3–Y2O3/Yb2O3 additives on microstructures and mechanical properties of silicon nitride ceramics prepared by hot-pressing sintering

Silicon nitride (Si₃N₄) ceramics doped with two different sintering additive systems (Al₂O₃–Y₂O₃ and Al₂O₃–Yb₂O₃) were prepared by hot-pressing sintering at 1800℃ for 2 h and 30 MPa. The microstructures, nano-indentation test, and mechanical properties of the as-prepared Si₃N₄ ceramics were systematically investigated. The X-ray diffraction analyses of the as-prepared Si₃N₄ ceramics doped with the two sintering additives showed a large number of phase transformations of α-Si₃N₄ to β-Si₃N₄. Grain size distributions and aspect ratios as well as their effects on mechanical properties are presented in this study. The specimen doped with the Al₂O₃–Yb₂O₃ sintering additive has a larger aspect ratio and higher fracture toughness, while the Vickers hardness is relatively lower. It can be seen from the nano-indentation tests that the stronger the elastic deformation ability of the specimens, the higher the fracture toughness. At the same time, the mechanical properties are greatly enhanced by specific interlocking microstructures formed by the high aspect ratio β-Si₃N₄ grains. In addition, the density, relative density, and flexural strength of the as-prepared Si₃N₄ ceramics doped with Al₂O₃–Y₂O₃ were 3.25 g/cm³, 99.9%, and 1053 ± 53 MPa, respectively. When Al₂O₃–Yb₂O₃ additives were introduced, the above properties reached 3.33 g/cm³, 99.9%, and 1150 ± 106 MPa, respectively. It reveals that microstructure control and mechanical property optimization for Si₃N₄ ceramics are feasible by tailoring sintering additives.     

Silicon Oxynitride Ceramics Prepared by Plasma Activated Sintering of Nanosized Amorphous Silicon Nitride Powder without Additives

Si₂N₂O ceramics were prepared by plasma activated sintering using nanosized amorphous Si₃N₄ powder without sintering additives within a temperature range of 1400°C–1600°C in vacuum. A mixed Si–N_4?n–O_n (n = 0, 1…4) amorphous structure was formed in the process of sintering, and Si₂N₂O crystals were nucleated where the local structure was similar with Si₂N₂O. After sintering at 1600°C, the Si₂N₂O ceramic was composed of elongated plate‐like Si₂N₂O grains and amorphous phase. The Si₂N₂O grains showed a width of less than 100 nm and a very high aspect ratio.     

Effect of rare earth oxides addition on the mechanical properties and coloration of silicon nitride ceramics

The aim of this study was to evaluate the mechanical properties and coloration of silicon nitride ceramics in the presence of RE₂O₃ (RE = Nd, Eu or Dy). Dense Si₃N₄ ceramics were prepared by gas pressure sintering at 1800 °C for 2 h. XRD analysis confirmed the complete transformation of α-Si₃N4 to β-Si₃N₄. The fracture toughness and flexure strengths were 11.93 ± 0.56 MPa·m^1/2, 667 ± 40.98 MPa with the addition of Eu₂O₃ (SE). Base on the SEM image, the pull-out, bridging and deflection of large grains were observed and contributed to the increase in mechanical properties. The chromaticity of sintered bodies was measured using a spectrophotometer. The color difference of the ceramics is due to the formation of different color developing compounds according to the EDS. Results showed that high-toughness and colorful Si₃N₄ ceramics can be prepared using YAG:Ce³⁺ as sintering additive and RE₂O₃ as the colorant.     

Thermal and mechanical properties of Si3N4 ceramics obtained via two-step sintering

Si₃N₄ ceramics were prepared using novel two-step sintering method by mixing α-Si₃N₄ as raw material with nanoscale Y₂O₃–MgO via Y(NO₃)₃ and Mg(NO₃)₂ solutions. Si₃N₄ composite powders with in situ uniformly distributed Y₂O₃–MgO were obtained through solid–liquid (SL) mixing route. Two-step sintering method consisted of pre-deoxidization at low temperature via volatilization of in situ-formed MgSiO₃ and densification at high temperature. Variations in O, Y, and Mg contents in Si₃N₄–Y₂O₃–MgO during first sintering step are discussed. O and Mg contents decreased with increasing temperature because SiO₂ on Si₃N₄ surface reacted with MgO to form low-melting-point MgSiO₃ compound, which is prone to volatilize at high temperature. By contrast, Y content hardly changed due to high-temperature stability of Y–Si–O–N quaternary compound. In the second sintering step, skeleton body was densified, and the formation of Y₂Si₃O₃N₄ secondary phase occurred simultaneously. Two-step sintered Si₃N₄ ceramics had lower total oxygen content (1.85 wt%) than one-step sintered Si₃N₄ ceramics (2.51 wt%). Therefore, flexural strength (812 MPa), thermal conductivity (92.1 W/m·K), and fracture toughness (7.6 MPa?m^1/2) of Si₃N₄ ceramics prepared via two-step sintering increased by 28.7%, 16.9%, and 31.6%, respectively, compared with those of one-step sintered Si₃N₄ ceramics.     

Fabrication of in situ elongated β-Sialon grains bonded to tungsten carbide via two-step spark plasma sintering

In order to reduce the difficulty of preparing binder-less cemented carbide and further broaden its application prospects, tungsten carbide toughened by in situ elongated β-Sialon grains was developed via sintering ball-milled WC and α-Si₃N₄ powders using Al₂O₃–ZrO₂ as a sintering aid and transformation additive. The two-step spark plasma sintering of the mixture at 1650 °C with dwelling at 1500 °C for 10 min was conducted under 30 MPa uniaxial pressure, and the densification behaviors, phase transformations, mechanical properties, and microstructures of the produced composites were investigated. The addition of Al₂O₃–ZrO₂ reduced the initial temperature of the densification process by approximately 100 °C and its final temperature by 200 °C (compared with the densification temperatures of pure WC and Si₃N₄ materials) and fully transformed α-Si₃N₄ to Sialon (Si–Al–O–N) phases. Microstructural characterization data showed that the WC matrix contained homogeneously distributed equiaxed and elongated β-Si₅AlON₇ grains. The WC composites containing in situ elongated β-Sialon grains exhibited an optimal hardness of 18.93 ± 0.03 GPa and enhanced fracture toughness of 10.43 ± 0.27 MPa m^1/2. The toughening mechanism of the β-Sialon phase involved the pull-out of elongated grains and crack bridging.     

Influence of α-Si3N4 coarse powder on densification,microstructure, mechanical properties,and thermal behavior of silicon nitride ceramics

Silicon nitride (Si₃N₄) ceramics, with different ratios of fine and coarse α-Si₃N₄ powders, were prepared by spark plasma sintering (SPS) and heat treatment. Further, the influence of coarse α-Si₃N₄ powder on densification, microstructure, mechanical properties, and thermal behavior of Si₃N₄ ceramics was systematically investigated. Compared with fine particles, coarse particles exhibit a slower phase transition rate and remain intact until the end of SPS. The remaining large-sized grains of coarse α-Si₃N₄ induce extensive growth of neighboring β-Si₃N₄ grains and promote the development of large elongated grains. Noteworthy, an appropriate number of large elongated grains distributed among fine-grained matrix forms bimodal microstructural distribution, which is conducive to superior flexural strength. Herein, Si₃N₄ ceramics with flexural strength of 861.34 MPa and thermal conductivity of 65.76 W m⁻¹ K⁻¹ were obtained after the addition of 40 wt% coarse α-Si₃N₄ powder.     

Low-temperature densification by plasma activated sintering of Mg2Si-added Si3N4

In this study, highly dense Si₃N₄ ceramics with excellent mechanical properties were fabricated using Mg₂Si as a sintering additive by plasma-activated sintering at 1400–1500 °C. The effects of the sintering temperature and content of Mg₂Si on the densification, microstructures, and mechanical properties of the Si₃N₄ ceramics were investigated. The mechanism responsible for the effect of Mg₂Si in the promotion of the sinterability of Si₃N₄ is discussed. The results showed that the addition of Mg₂Si could effectively remove the oxide layers on the Si₃N₄ particles and form a liquid phase during the sintering, promoting the densification and phase transition of the Si₃N₄ ceramics. The Si₃N₄ ceramic sintered at 1450 °C with 6.0 wt% of Mg₂Si exhibited the maximum strength of 1050 MPa.     

The effect of BaTiO3 addition on the dielectric constant of Si3N4 ceramics

Si₃N₄ ceramics with different BaTiO₃ contents have been fabricated by pressureless sintering in a N₂ atmosphere at 1680°C for 2 h. Al₂O₃ and Nd₂O₃ were used as sintering additives to promote the densification of Si₃N₄ ceramics. The effect of BaTiO₃ addition on the densification, mechanical properties, phase compositions, microstructure, and dielectric properties of Si₃N₄ ceramics was investigated. The relative density and flexural strength of Si₃N₄ ceramics increased with the addition of BaTiO₃ up to 15 wt% and then decreased, while the dielectric constant increased continuously as the BaTiO₃ contents increased. The dielectric constant of Si₃N₄ ceramics can be tailored in the range from 8.42 to 12.96 by the addition of 5 wt%‐20 wt% BaTiO₃. Meanwhile, these Si₃N₄ ceramics all had flexural strength higher than 500 MPa.     

Preceramic Polymer‐Derived SiAlON as Sintering Aid for Silicon Nitride

Two kinds of sintering additives based on the polysiloxanes or polysilazanes filled with nano‐sized powders as SiAlON precursors were tested for the densification of Si₃N₄‐based ceramics. The results showed that both systems can be successfully used as additives for the preparation of Si₃N₄ ceramics with favorable mechanical characteristics. The ceramics were sintered with 18 wt% of preceramic polymer‐based mixture, and good fracture resistance and high hardness values were obtained after sintering in optimized conditions (temperature, dwell time, nitrogen pressure). Higher densification temperatures and longer holding times were required for sintering of samples with polysilazane‐based precursors. The best toughness values were approximately 5 MPa·m^0.5, while the highest hardness was about 19 GPa. The differences in mechanical properties of the prepared composites can be related to the phase composition, microstructure and different chemical bonds present in the ceramic residue generated upon pyrolysis and final densification.     

Effect of MgF2 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Dense silicon nitride (Si₃N₄) ceramics were prepared using Y₂O₃ and MgF₂ as sintering aids by spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering annealing at 1900 °C for 4 h. Effects of MgF₂ contents on densification, phase transformation, microstructure, mechanical properties, and thermal conductivity of the Si₃N₄ ceramics before and after heat treatment were investigated. Results indicated that the initial temperature of liquid phase was effectively decreased, whereas phase transformation was improved as increasing the content of MgF₂. For optimized mechanical properties and thermal conductivity of Si₃N₄, optimum value for MgF₂ content existed. Sample with 3 mol.% Y₂O₃ and 2 mol.% MgF₂ obtained optimum flexural strength, fracture toughness and thermal conductivity (857 MPa, 7.4 MPa m^1/2 and 76 W m⁻¹ K⁻¹, respectively). It was observed that excessive MgF₂ reduced the performance of the ceramic, which was caused by the presence of excessive volatiles.     

Microstructure and properties of mullite–ZrO2 ceramics with silicon nitride additive prepared by spark plasma sintering

The process of densification and development of the microstructure of mullite–ZrO₂/Y₂O₃ ceramics from mixture of Al₂O₃, SiO₂, ZrO₂ and Y₂O₃ by gradually adding of α–β Si₃N₄ nanopowder from 1 to 5 wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400 °C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si₃N₄ additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33 g/cm³, while for traditionally sintered samples, the value is 2.55 g/cm³, and the compressive strength for SPS grows with Si₃N₄ additives, reaching 600 N/mm². In the case of traditional sintering, it decreases to approximately 100 N/mm². The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO₂ grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si₃N₄ additive, are filled with mullite crystalline formations.     

Fabrication and characterisation of porous silicon nitride ceramics using Yb2O3 as sintering additive

Porous Si₃N₄ ceramics were fabricated by liquid-phase sintering with a Yb₂O₃ sintering additive, and the microstructure and mechanical properties of the ceramics were investigated, as a function of porosity. Low densification was achieved using a lower Yb₂O₃ additive content. Fibrous β-Si₃N₄ grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. A high porosity, ∼40–60%, with β-Si₃N₄ grain development, was obtained by adjusting the additive content. Superior mechanical properties, as well as strain tolerance, were obtained for porous ceramics with a microstructure of fine, fibrous grains of a bimodal size distribution.