Fabrication and mechanical properties of boron nitride nanotube reinforced silicon nitride ceramics

In this study, silicon nitride (Si₃N₄) ceramics added with and without boron nitride nanotubes (BNNTs) were fabricated by hot-pressing method. The influence of sintering temperature and BNNTs content on the microstructures and mechanical properties of Si₃N₄ ceramics were investigated. It was found that both flexural strength and fracture toughness of Si₃N₄ were improved when sintering temperature increases. Moreover, α-Si₃N₄ phase could transform into β-Si₃N₄ phase completely when sintering temperature rises to 1800 °C and above. BNNTs can enhance the fracture toughness of Si₃N₄ dramatically, which increases from 7.2 MPa m^1/2 (no BNNTs) to 10.4 MPa m^1/2 (0.8 wt% BNNTs). However, excessive addition of BNNTs would reduce the fracture toughness of Si₃N₄. Meanwhile, the flexural strength and relative density of Si₃N₄ decreased slightly when BNNTs were added. The related toughening mechanism was also discussed.     

A novel strategy for c-axis textured silicon nitride ceramics by hot extrusion

Highly c-axis textured β-silicon nitride (β-Si₃N₄) ceramic with fine grains was prepared by a new method of hot extrusion for the first time. The (002) pole figure on the section plane vertical to extruding direction showed a characteristic of center rotational symmetry. The average cline angle between elongated β-Si₃N₄ grains and hot extruding direction was about 14.4°. The degree of c-axis texturing by hot extrusion was comparable to that achieved by rotating magnetic field. The hardness and toughness anisotropy in different direction was apparent and relatively higher hardness was achieved in the present work mainly due to the finer grain size. Therefore, many different compositions of c-axis aligned Si₃N₄-based ceramics with tailored mechanical properties could be achieved by the strategy of hot extrusion.     

Influence of hexagonal boron nitride nanosheets on phase transformation,microstructure evolution and mechanical properties of Si3N4 ceramics

Fully dense Si₃N₄ materials with 1 wt.% (～ 1.5 vol.%) and 2 wt.% (～ 3.0 vol.%) h-BN nanosheets were prepared by spark plasma sintering at 1750 °C with the dwell of 7 min under a pressure of 50 MPa in a vacuum. BN nanosheets with different dimensions were prepared by ultrasound-assisted liquid phase exfoliation of h-BN powder, followed by centrifugation at different speeds (1000 rpm and 3000 rpm). The addition of BN nanosheets hindered the particle rearrangement stage of sintering, which resulted in the delayed α→β phase transformation of Si₃N₄. To study a direct effect of BN nanosheets on the mechanical properties of Si₃N₄, the results were compared to the monolithic Si₃N₄ with similar grain size and α/β-Si₃N₄ ratio. The addition of 2 wt.% h-BN nanosheets (1000 rpm) increased both the fracture toughness (～ 26 %) and the strength (～ 45 %) of Si₃N₄, when compared to the monolithic Si₃N₄ with similar microstructure.     

Microstructure and mechanical properties of tantalum carbide ceramics: Effects of Si3N4 as sintering aid

Stoichiometric Tantalum carbide (TaC) ceramics were prepared by reaction spark plasma sintering using 0.333–2.50 mol% Si₃N₄ as sintering aid. Effects of the Si₃N₄ addition on densification, microstructure and mechanical properties of the TaC ceramics were investigated. Si₃N₄ reacted with TaC and tantalum oxides such as Ta₂O₅ to form a small concentration of tantalum silicides, SiC and SiO₂, with significant decrease in oxygen content in the consolidated TaC ceramics. Dense TaC ceramics having relative densities >97% could be obtained at 0.667% Si₃N₄ addition and above. Average grain size in the consolidated TaC ceramics decreased from 11 µm at 0.333 mol% Si₃N₄ to 4 µm at 2.50 mol% Si₃N₄ addition. The Young's modulus, Vickers hardness and flexural strength at room temperature of the TaC ceramic with 2.50 mol% Si₃N₄ addition was 508 GPa, 15.5 GPa and 605 MPa, respectively. A slight decrease in bending strength was observed at 1200 °C due to oxidation of the samples.     

Fabrication and mechanical properties of nacre-like alumina with addition of silicon nitride

How to deal with the brittleness of ceramic materials is always one of the hot topics in the field of materials science. Design of layered ceramics with textured structure is one of the effective methods to improve their fracture toughness. The introduction of additives as interlayer phases can balance fracture toughness and flexural strength. However, the research about addition of interlayer phases and their mechanisms in the layered ceramics is still limited. In this work, nacre-like alumina ceramics were successfully fabricated by freeze casting followed by hot pressing. Silicon nitride was incorporated as the interlayer phase, and the effect on the mechanical properties of the nacre-like alumina was investigated. The addition of silicon nitride was beneficial to improvement of interlayer bonding between the alumina layers due to formation of sialon phase, leading to increase of flexural strength but decease of fracture toughness. When the content of silicon nitride was 3.3 wt%, flexural strength and fracture toughness of the sample was 468 MPa and 6.2 MPa m^1/2, respectively. Compared with the sample without silicon nitride, the flexural strength was enhanced significantly. Additionally, both flexural strength and fracture toughness were improved as compared the sample without any additives. This work can provide available references for design and fabrication of high-strength and high-toughness ceramics by properly tuning the layer structure and interlayer phase composition.     

Mechanical and dielectric properties of functionalized boron nitride nanosheets/silicon nitride composites

Uniformly dispersed boron nitride nanosheets (BNNSs) reinforced silicon nitride (Si₃N₄) composites were prepared by surface modification assisted flocculation combined with SPS sintering. In order to improve the dispersibility of the BNNSs in the composites, the liquid phase stripped BNNSs are surface functionalized by a two-step covalently modification. The amino-modified BNNSs (NH₂-BNNSs) and Si₃N₄ powders have opposite surface potential, mixed evenly by electrostatic interaction during flocculation. The results showed that mechanical properties of Si₃N₄ composites were obviously enhanced by adding NH₂-BNNSs. The fracture toughness and bending strength of Si₃N₄ composites added 0.75 wt% NH₂-BNNSs were increased by 34% and 28%, respectively, compared with monolithic Si₃N₄. Toughening mechanisms are synergistic action of the torn, pull-out or bridging of BNNSs and crack deflection mechanisms with microstructural analyzes. The dielectric properties of the Si₃N₄ ceramics are also improved after the addition of NH₂-BNNSs.     

Microstructural and mechanical evaluation of a Cu-based active braze alloy to join silicon nitride ceramics

Self-joining of St. Gobain Si₃N₄ (NT-154) using a ductile Cu-Al-Si-Ti active braze (Cu-ABA) was demonstrated. A reaction zone (∼2.5-3.5 μm thick) developed at the interface after 30 min brazing at 1317 K. The interface was enriched in Ti and Si. The room temperature compressive shear strengths of Si₃N₄/Si₃N₄ and Inconel/Inconel joints (the latter created to access baseline data for use with the proposed Si₃N₄/Inconel joints) were 140 ± 49 MPa and 207 ± 12 MPa, respectively. High-temperature shear tests were performed at 1023 K and 1073 K, and the strength of the Si₃N₄/Si₃N₄ and Inconel/Inconel joints were determined. The joints were metallurgically well-bonded for temperatures above 2/3 of the braze solidus. Scanning and transmission electron microscopy studies revealed a fine grain microstructure in the reaction layer, and large grains in the inner part of the joint with interfaces being crack-free. The observed formation of Ti₅Si₃ and AlN at the joint interface during brazing is discussed.     

Microstructure and mechanical properties of hydroxyapatite obtained by gel-casting process

In the present work, well-shaped HAp green bodies were obtained by the gel-casting process with 50 vol.% slurry. After drying, the microstructure and pore distribution of the green body were investigated. The density, compressive strength and flexural strength of the green body were 1.621 g/cm³, 32.6 ± 3.2 MPa and 13.8 ± 1.0 MPa, respectively. After pressureless sintering at the range of 1100–1300 °C for 2 h, the relative density of the final product ranges from 71.8 to 97.1% th. The maximum value of flexural strength, elastic modulus, hardness and fracture toughness were 84.6 ± 12.6 MPa, 138 ± 7 GPa, 4.45 ± 0.18 GPa and 0.95 ± 0.13 MPa m^1/2, respectively. SEM images show a compact and uniform microstructure; the average grain size was found by using the linear intercept method. XRD and FTIR determined the phase and the radical preserved after sintering.     

Microstructure and mechanical properties of cold sintered porous alumina ceramics

New innovative approach to fabricate porous alumina ceramics by cold sintering process (CSP) is presented using NaCl as pore forming agent. The effects of CSP and post-annealing temperature on the microstructure and mechanical strength were investigated. Al₂O₃–NaCl composite with bulk density of 2.92 g/cm³ was compacted firstly using CSP and then a porous structure was formed using post-annealing at 1200°C–1500°C for 30 min. Brazilian test method and Vickers hardness test were used to determine the indirect tensile strength and hardness of the porous alumina, respectively. Meanwhile, the phases and the microstructure were respectively examined using X-ray diffractometer and scanning electron microscope (SEM) complemented by the 3D image analysis with X-ray tomography (XRT). SEM structural and XRT image analysis of cold sintered composite showed a dense structure with NaCl precipitated between Al₂O₃ particles. The NaCl volatization from the composite was observed during the annealing and then complete porous Al₂O₃ structure was formed. The porosity decreased from 48 vol% to 28 vol% with the annealing temperature increased from 1200 °C to 1500 °C, while hardness and mechanical strength increased from 14.3 to 115.4 HV and 18.29–132.82 MPa respectively. The BET analysis also showed a complex pore structure of micropores, mesopores and macropores with broad pore size distribution.     

Fabrication and properties of in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites

The in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites were fabricated via gelcasting followed by pressureless sintering. SNNWs were well distributed in the porous silicon nitride matrix. The tip-body appearance suggested a VLS growth mechanism. The flexural strength and elastic modulus of the prepared composites can achieve 84.3?±?3.9?MPa and 23.3?±?2.0?GPa respectively (25?°C), while the corresponding porosity was 40.7?vol.%. Remarkably, the strength retention rate of the composites at 1400?°C was up to 66.1%. This is due to the excellent thermal stability of SNNWs and silicon nitride matrix. Also, the fracture toughness of the composites was improved to ～42% larger than pure porous silicon nitride ceramics because of the bridging effect of the NWs and the interlocking effect of β-Si₃N₄ crystals. In addition, a good thermal shock resistance and dielectric properties were indicated. The good overall performance made SNNWs/SN composites promising candidate for advanced high-temperature applications.     

Dielectric properties of silicon nitride ceramics produced by free sintering

The silicon nitride ceramics with a beneficial combination of low dielectric losses and improved physical properties was fabricated by cold isostatic pressing and pressureless sintering. The fine grain microstructure, three-phase composition based on the β-SiAlON, the small amount of the glass phase and relatively small porosity promote a unique combination of a low thermal conductivity 14.51 W m⁻¹ K⁻¹ and low dielectric loss 1.4·10⁻³. A novel method is proposed to overcome the main drawbacks of the commercial and high-cost technologies.     

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 evolutions of SiC whiskers at high temperature and its effects on silicon carbide ceramics

SiC whisker with a single-crystal structure is promising in enhancing the strength and toughness of advanced structural ceramics, owing to its excellent properties. However, studies on its microstructure evolution at high temperature (>2000 °C) are scarce. Herein, SiC whiskers were calcined at 2100 °C, and XRD, SEM, and TEM were employed to analyze microstructure evolutions. Compared with raw whiskers, XRD results indicated serious annihilation of stacking faults after calcination. The annihilation led to the fracture of whiskers and the formation of β-SiC grains, and then partial grains underwent the phase transformation to form hexagonal prism and triangular prism α-SiC grains with diameters of about 10 μm, according to SEM and TEM results. Furthermore, SiC ceramics containing different whisker contents were innovatively fabricated by pressureless solid-state sintering. The flexural strength and fracture toughness of SiC ceramic containing 10 vol% whiskers were 540 MPa and 5.1 MPa m^0.5, resulting in 38% and 11% higher values than those without whiskers, respectively.     

Effect of the addition of polyvinylpyrrolidone as a pore-former on microstructure and mechanical strength of porous alumina ceramics

The effect of the solvent on the properties of porous alumina ceramics was studied when polyvinylpyrrolidone (PVP) was used as an organic pore-former. In particular, porous alumina ceramics were produced by dry-pressing of mixed PVP–alumina powder; the mixing of PVP and alumina powder was achieved via ball milling using water or acetone as solvent, or dry ball milling. Due to the different solubility of PVP in water and acetone, porous alumina ceramics with different pore structures and mechanical properties were obtained. Because of its cylindrical pores being aligned to some extent, the sample prepared using acetone as solvent exhibited the highest bending strength (140.2 MPa) and Young's modulus (57.4 GPa), which were 1.6 times and 3.4 times higher compared to that prepared without PVP. Moreover, the addition of PVP via wet ball milling led to more uniform dispersion of PVP in alumina, hence limiting the grain growth during sintering process and increasing the grain bonding.     

烧结助剂对反应烧结氮化硅陶瓷的影响

以Si粉和C粉为主要原料 ,在氮气流量为1.2L·min- 1,氮化温度为 1380℃ ,保温时间为 2 0h的条件下 ,研究了分别以 10wt%的MgO、Al、Al2 O3和Al2 O3+Y2 O3粉为烧结助剂对反应烧结氮化硅陶瓷的影响。结果表明 :以MgO粉作烧结助剂时 ,试样的主要成分是MgSiO3,另外还有Si2 N2 O ,但没有Si3N4 生成 ;以Al粉作烧结助剂时 ,试样的主要成分是SiO2 ,仅有少量Si3N4 存在 ;以Al2 O3作烧结助剂时 ,试样的主要成分是β Si3N4 和α Si3N4 ;以 2wt%Al2 O3+8wt%Y2 O3作烧结助剂时 ,试样的主要成分为 β Si3N4 ,同时含有少量α Si3N4 。     

Microstructure and mechanical properties of alumina ceramics reinforced by boron nitride nanotubes

Boron nitride nanotubes (BNNTs)/alumina composites were fabricated by hot pressing. The mechanical properties of the composites are greatly dependent upon the content of BNNTs. In comparison with monolithic alumina, the incorporation of BNNTs results in the improvement of bending strength and fracture toughness owing to the effective inhibition of grain growth. A routine toughening mechanism, especially the bridging of BNNTs at grain boundaries and the sufficient physical bonding between BNNTs and alumina matrix, is dominantly responsible for the increase in mechanical properties.     

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.     

Microstructure and mechanical properties of boron nitride nanosheets-reinforced fused silica composites

In order to overcome intrinsic brittleness and poor mechanical properties of fused silica (FS), boron nitride nanosheets (BNNSs) as a novel reinforcement were employed for fabrication of BNNSs/fused silica composites. BNNSs with micron lateral size were homogeneously dispersed with FS powder using a surfactant-free flocculation method and then consolidated by hot pressing. The flexural strength and fracture toughness of the composite with the addition of only 0.5 wt.% BNNSs increased by 53% and 32%, respectively, compared with those of pure FS. However, for higher BNNSs contents the improvement in mechanical properties was limited. Microstructural analyzes have shown that the toughening mechanisms are combinations of the pull-out, crack bridging, and crack deflection mechanisms.