Mechanical properties,microstructure, and bioactivity of β-Si3N4/HA composite ceramics for bone reconstruction

Pure hydroxyapatite (HA) as bone graft substitute has excellent osteogenic activity, but it has lower fracture toughness and its biological activity is harmed by the addition of toughening phases, which limits its clinical application. To alleviate the contradiction, columnar β-Si₃N₄ as the toughening phase and La³⁺/Y³⁺ sintering aid as active ions are used in this study to prepare β-Si₃N₄/HA composite biomaterials. According to the results, hardness and toughness of 10 wt% β-Si₃N₄/HA composite prepared by cold press sintering at 1300 °C were 6.44 GPa and 1.69 MPa m^1/2, respectively, being 110% and 140% higher than those of pure HA. Moreover, 10 wt% β-Si₃N₄/HA composite exhibited better protein adsorption capacity and obviously stimulated adhesion, proliferation, and osteogenic differentiation of osteoblast. In addition, it was found that sintering aid not only facilitated the improvement of mechanical properties, but also promoted the formation of 100–200 nm nanostripe structure, which was beneficial to cell adhesion. Determination of La³⁺concentration combined with biological experiments also proved that its concentration range from 4×10⁻⁸ M to 6 × 10⁻⁸M was beneficial to cell proliferation and osteogenic differentiation. In summary, β-Si₃N₄ and sintering aids were shown to improve mechanical properties of HA at maintaining the biological activity of the latter.     

Mechanical properties and microstructure of porous BN–SiO2–Si3N4 composite ceramics

Porous silicon nitride (Si₃N₄) ceramics incorporated with hexagonal boron nitride (h-BN) and silica (SiO₂) nanoparticles were fabricated by pressureless-sintering at relatively low temperature, in which stearic acid was used as pore-making agent. Bending strength at room and high temperatures, thermal shock resistance, fracture toughness, elastic modulus, porosity and microstructure were investigated in detail. The mechanical properties and thermal shock resistance behavior of porous Si₃N₄ ceramics were greatly influenced by incorporation of BN and SiO₂ nanoparticles. Porous BN–SiO₂–Si₃N₄ composites were successfully obtained with good critical thermal shock temperature of 800 °C, high bending strength (130 MPa at room temperature and 60 MPa at 1000 °C) and high porosity.     

Fabrication and properties of a dense SiC NWs-toughened α-Si3N4 composite coating on porous Si3N4 ceramics

A dense SiC nanowires-toughened α-Si₃N₄ coating was prepared using a two-step technique for protecting porous Si₃N₄ ceramic against mechanical damage, and effect of SiC nanowires content on microstructures and properties of the coating were investigated. XRD, SEM and TEM analysis results revealed that as-prepared coatings consisted of α-Si₃N₄, O'-Sialon, SiC nanowires and Y–Al–Si–O–N glass phase. Furthermore, Vickers hardness of the coated porous Si₃N₄ ceramics increased gradually with the increasing SiC nanowires content from 0 to 10 wt%, which is attributed to the gradual improvement in intrinsic elastic modulus (E), hardness (H) and H³/E² of the coatings. But, when the SiC nanowires content was 15 wt%, the thickness of the coating became relatively thinner, so that its protective ability was weakened and Vickers hardness started to decrease accordingly. Meanwhile, the assistance of SiC nanowires enhanced fracture toughness of the coatings obviously because SiC nanowires in the coatings can produce various toughening mechanisms during mechanical damage. When the SiC nanowires content was 10 wt%, its fracture toughness reached the maximum value, which was 6.27 ± 0.05 MPa·m^1/2.     

Dielectric properties of Si3N4–SiCN composite ceramics in X-band

Si₃N₄–SiCN composite ceramics were successfully fabricated through precursor infiltration pyrolysis (PIP) method using polysilazane as precursor and porous Si₃N₄ as preform. After annealed at temperatures varying from 900 °C to 1400 °C, the phase composition of SiCN ceramics, electrical conductivity and dielectric properties of Si₃N₄–SiCN composite ceramics over the frequency range of 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, the content of amorphous SiCN decreases and that of N-doped SiC nano-crystals increases, which leads to the increase of electrical conductivity. After annealed at 1400 °C, the average real and imaginary permittivities of Si₃N₄–SiCN composite ceramics are increased from 3.7 and 4.68 × 10^?3 to 8.9 and 1.8, respectively. The permittivities of Si₃N₄–SiCN composite ceramics show a typical ternary polarization relaxation, which are ascribed to the electric dipole and grain boundary relaxation of N-doped SiC nano-crystals, and dielectric polarization relaxation of the in situ formed graphite. The Si₃N₄–SiCN composite ceramics exhibit a promising prospect as microwave absorbing materials.     

Enhancement of thermal shock resistance in β-Si3N4 coating with in situ synthesized β-Si3N4 nanowires/nanobelts on porous Si3N4 ceramics

A dense β-Si₃N₄ coating toughened by β-Si₃N₄ nanowires/nanobelts was prepared by a combined technique involving chemical vapor deposition and reactive melt infiltration to protect porous Si₃N₄ ceramics in this work. A porous β-Si₃N₄ nanowires/nanobelts layer was synthesized in situ on porous Si₃N₄ ceramics by chemical vapor deposition, and then Y–Si–Al–O–N silicate liquid was infiltrated into the porous layer by reactive melt infiltration to form a dense composite coating. The coating consisted of well-dispersion β-Si₃N₄ nanowires/nanobelts, fine β-Si₃N₄ particles and small amount of silicate glass. The testing results revealed that as-prepared coating displayed a relatively high fracture toughness, which was up to 7.9 ± 0.05 MPa m^1/2, and it is of great significance to improve thermal shock resistance of the coating. After thermal cycling for 15 times at ΔT = 1200 °C, the coated porous Si₃N₄ ceramics still had a high residual strength ratio of 82.2%, and its water absorption increased only to 6.21% from 3.47%. The results will be a solid foundation for the application of the coating in long-period extreme high temperature environment.     

Effect of β-Si3N4 starting powder size on elongated grain growth in β-Si3N4 ceramics

The microstructural evolution of pressureless sintered silicon nitride ceramics prepared from different particle sizes of β-Si₃N₄ as starting powders, has been investigated. When the specimen prepared from as-received β-powder of 0.66 μm in average size, was sintered at 1850°C, equiaxed β-Si₃N₄ grains were observed. As the size of the initial β-powder went down to 0.26 μm, however, the growth of elongated grains was enhanced, which resulted in a whisker-like microstructure similar to that made from α-starting powder. When the sintering temperature was increased to 2000°C, the elongated grains were also developed even in the specimen made from 0.66 μm β-powder. The observed results were discussed with relation to the two dimensional nucleation and growth theory for faceted crystals. In addition, fracture toughness of the specimen consisting of elongated grains, which was prepared from finer powders, increased.     

Effect of the residual phases in β-Si3N4 seed on the mechanical properties of self-reinforced Si3N4 ceramics

In this work, the self-reinforced silicon nitride ceramics with crystal seed of β-Si₃N₄ particles were investigated. Firstly, the seeds were prepared by heating of α-Si₃N₄ powder with Yb₂O₃ and MgO, respectively. Then the self-reinforced silicon nitride ceramics were obtained by HP-sintering of α-Si₃N₄ powder, Yb₂O₃ and the as-prepared seeds which were not treated with acid and/or alkali solution. The results indicated that the introduction of seed with Yb₂O₃ could obviously increase the toughness and room temperature strength of the ceramics. Furthermore, its high temperature strength (1200 °C) could nearly keep higher value as the one of room temperature measured from unreinforced ceramic. However, the seed with MgO abruptly decrease the high temperature strength of the ceramics. The SEM and TEM characterization showed that the rod-like seed particle could favor the toughness and the presence of the Mg promote the formation of crystalline secondary phase.     

Microstructure and mechanical properties of α/β-Si3N4 composite ceramics with novel ternary additives prepared via spark plasma sintering

In this study, α/β-Si₃N₄ composite ceramics with high hardness and toughness were fabricated by adopting two different novel ternary additives, ZrN–AlN–Al₂O₃/Y₂O₃, and spark plasma sintering at 1550 °C under 40 MPa. The phase composition, microstructure, grain distribution, crack propagation process and mechanical properties of sintered bulk were investigated. Results demonstrated that the sintered α/β-Si₃N₄ composite ceramics with ZrN–AlN–Al₂O₃ contained the most α phase, which resulted in a maximum Vickers hardness of 18.41 ± 0.31 GPa. In the α/β-Si₃N₄ composite ceramics with ZrN–AlN–Y₂O₃ additives, Zr₃AlN MAX-phase and ZrO phase were found and their formation mechanisms were explained. The fracture appearance presented coarser elongated β-Si₃N₄ grains and denser microstructure when 20 wt% TiC particles were mixed into Si₃N₄ matrix, meanwhile, exhibited maximum mean grain diameter of 0.98 ± 0.24 μm. As a result, the compact α/β-Si₃N₄ composite ceramics containing ZrN–AlN–Y₂O₃ additives and TiC particles displayed the optimal bending strength and fracture toughness of 822.63 ± 28.75 MPa and 8.53 ± 0.21 MPa?m^1/2, respectively. Moreover, the synergistic toughening of rod-like β-Si₃N₄ grains and TiC reinforced particles revealed the beneficial effect on the enhanced fracture toughness of Si₃N₄ ceramic matrix.     

Effect of α-Si3N4 initial powder size on the microstructural evolution and phase transformation during sintering of Si3N4 ceramics

By using the Si₃N₄ ceramic specimens prepared with fine and coarse α-Si₃N₄ powders, respectively, the phase transformation from α- to β-Si₃N₄ and concurrent microstructural evolution during sintering were monitored. For the compact prepared with fine powder, the α/β transformation was completed much earlier than the coarse powder. The higher fraction of pre-existing β-grains in fine powder and its higher reactivity compared to those of coarse one are likely to cause a rapid phase transformation. The growth rate of β-Si₃N₄ grains at the expense of α-Si₃N₄ during phase transformation stage was quite significant while that after they impinge each other was very limited. As a result, the specimens prepared with coarse and fine initial α-Si₃N₄ powders resulted in coarse and fine grained β-Si₃N₄ ceramics, respectively. The specimen prepared with mixture of fine and coarse α-Si₃N₄ powders exhibited the microstructure containing a few elongated large grains and showed an increased value of fracture toughness.     

Combustion synthesis of α-Si3N4 powders using in-situ nano-SiO2 coated Si and Si3N4 reactants

Alpha phase silicon nitride (α-Si₃N₄) powders were synthesized by combustion reaction of the in-situ nano-SiO₂ coated Si and Si₃N₄ reactants with pressurized nitrogen. The combustion temperature profile as well as the product phase composition and morphology were investigated. Regardless of the combustion temperature reached as high as 1800 °C, up to 86 wt% α-Si₃N₄ was obtained in the combustion-synthesized product from the reactants with only 6 wt% SiO₂ addition, which is three times higher than that of without SiO₂ coating, meanwhile, the morphology of Si₃N₄ grains changed from rod-like to equiaxed grain,indicating the in-situ coated SiO₂ tailored the nitridation reaction path of Si successfully by enhancing the silicon monoxide (SiO) gas phase formation.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.     

Synthesis of β-Si3N4 particles from α-Si3N4 particles

This report describes an investigation of the synthesis of β-Si₃N₄ particles from α-Si₃N₄ particles. The β fraction of Si₃N₄ particles was found to depend on temperature, heating time, and the type of crucibles in which the Si₃N₄ particles were heated. When Si₃N₄ particles were heated in a crucible made of carbon, most α-Si₃N₄ particles converted to β-Si₃N₄ after heating at 2000°C for 90 min in an atmosphere of N₂ of 9 kgf/cm². The morphology of the resulting β-Si₃N₄ particles appeared as a whisker shape. When Si₃N₄ particles were heated in a crucible made of boron nitride, most α-Si₃N₄ particles converted to β-Si₃N₄ after heating at 2000°C for 480min in an atmosphere of N₂ of 9kgf/cm². The resulting morphology was equiaxed. It is suspected that the transformation occurs via the gas phase and is affected by the partial pressure of oxygen in the atmosphere.     

Synthesis of pure rod-like α-Si3N4 powder with in situ C/SBA-15 composite

Well crystallized pure rod-like α-Si₃N₄ powder (PRSN) without β-Si₃N₄ was successfully synthesized by carbothermal reduction-nitridation (CRN). In situ carbon/mesoporous silica composite (C/SBA-15) was used as a new kind of raw material. Due to in situ composited carbon, the CRN temperature was decreased and the phase transition from α to β-Si₃N₄ was hindered. The sintering temperature was lowered to 1380 °C and the soaking time of the optimal synthesis condition was reduced to 6 h. Moreover, the as-synthesized rod-like α-Si₃N₄, which is induced by SBA-15, was used to enhance the fracture toughness (K_Ic) of α-Si₃N₄ based ceramics, which was sintered by spark plasma sintering (SPS). Compared with the undoped ceramics (2.9 MPa m^1/2), α-Si₃N₄ ceramics doped with 10 vol% PRSN exhibited a higher K_Ic value (4.9 MPa m^1/2), and lower dielectric loss in MHz frequency range. The results demonstrated that the PRSN powder would be promising for toughening α-Si₃N₄ based ceramics.     

β-Si3N4及添加β-Si3N4的α-Si3N4的气氛加压烧结

介绍了β－Ｓｉ３Ｎ４及添加β－Ｓｉ３Ｎ４的α－Ｓｉ３Ｎ４的气氛加压烧结,β－Ｓｉ３Ｎ４在ＧＰＳ中具有低于α－Ｓｉ３Ｎ４的烧结活性而且陶瓷显微结构更容易调节,由ＧＰＳβ－Ｓｉ３Ｎ４制备的陶瓷材料晶粒比较均匀,具有较高的力学性能,尤其是高的韦泊模数,添加于α－Ｓｉ３Ｎ４中的β－Ｓｉ３Ｎ４对陶瓷材料显微结构具有明显的调控作用。     

Effect of large β-Si3N4 particles on the thermal conductivity of β-Si3N4 ceramics

Mean-field micromechanics model, the rule of mixture is applied to the prediction of the thermal conductivity of sintered β-Si₃N₄, considering that the microstructure of β-Si₃N₄ is composed of a uniform matrix phase (which contains grain boundaries and small grains of Si₃N₄) and the purified large grains (⩾2 μm in diameter) of Si₃N₄. Experimental results and theoretical calculations showed that the thermal conductivity of Si₃N₄ is controlled by the amount of the purified large grains of Si₃N₄. The present study demonstrates that the high thermal conductivity of β-Si₃N₄ can be explained by the precipitation of high purity grains of β-Si₃N₄ from liquid phase.     

Synthesis of α-Si3N4 whiskers or equiaxed particles from amorphous Si3N4 powders

Silicon nitride (Si₃N₄) particles with different morphologies have been used in many fields. In this work, α-Si₃N₄ whiskers and granular particles with high-phase purity were successfully tailored by the controllable crystallisation process of amorphous Si₃N₄ powders under different N₂ pressure. Impressively, α-Si₃N₄ whiskers were prepared by simply heat treating amorphous Si₃N₄ powders at 1550 °C for 2 h under the low N₂ pressure of 0.2 MPa, whereas equiaxed α-Si₃N₄ particles with uniform size of ~280 nm were obtained under an elevated N₂ pressure of 2.0 MPa. With the evaluated N₂ pressures and temperatures, large scale α-Si₃N₄ whiskers or equiaxed α-Si₃N₄ particles could be produced. The growth mechanisms of the α-Si₃N₄ particles with distinct morphologies were rationally proposed, and these consist of two main growth processes. First, amorphous Si₃N₄ powders decomposed into Si(g) and N₂(g) under high-temperature treatment. Subsequently, N₂(g) dominated the recombination of the evaporated chemical with the Si₃N₄ molecule. The initial N₂ concentration, which plays a key role in tailoring the shape and size of products, was controlled by the N₂ pressure.     

Effect of the addition of β-Si3N4 nuclei on the thermal conductivity of β-Si3N4 ceramics

Various microstructures of β-Si₃N₄ were fabricated, with or without the addition of β-Si₃N₄ seed particles to high-purity β-Si₃N₄ powder, using Yb₂O₃ and ZrO₂ as sintering additives, by gas-pressure sintering at 1950 °C for 16 h. The thermal conductivity of the specimen without seeds was 140 W·(m·K)⁻¹, and the specimen exhibited a bimodal microstructure with abnormally grown grains. The thermal conductivity of the specimen with 24 vol.% seed addition was 143 W·(m·K)⁻¹, and this specimen had the bimodal microstructure with finer grain size than that without the seeded material, but maintained the same amount of large grains (⩾2 μm in diameter) as in the specimen without the seeds. This finding indicates that the thermal conductivity of β-Si₃N₄ is controlled by the amount of reprecipitated large grains, rather than by the grain size of the β-Si₃N₄.     

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.     

Preparation,oxidation property and mechanism of Si3N4/O′-SiAlON composite ceramics

The applications of Si₃N₄ ceramics were significantly restricted because of the disastrous failure resulted from the oxidation weight gain. The generation of O′-SiAlON could effectively address this issue. The effect of N₂ gas pressure on the phase evolution of the Si₃N₄/O′-SiAlON was studied. It was found that high N₂ gas pressure (3 MPa) was more favorable for the formation of the O′-SiAlON than low N₂ gas pressure (0.6 MPa). Furthermore, the effects of SiO₂ content on the phase evolution, microstructure, oxidation properties and mechanism of the Si₃N₄/O′-SiAlON ceramics were investigated. The results revealed that the relative content of the O′-SiAlON phase evidently enhanced from 0 wt% to 18.15 wt%, and the bulk density decreased from 3.01 g/cm^?3 to 2.62 g/cm^?3 with an increase in SiO₂ from 0 wt% to 12.5 wt%. Additionally, the weight gain, oxide layer thickness and roughness similarly reduced from 2.02 mg/cm² to 0.85 mg/cm², 133.87 μm to 2.31 μm and 21.91 μm to 6.34 μm, respectively. The addition of SiO₂ could also reduce bubbles and cracks formation and hinder the diffusion of Al and Y elements from the interior to the surface. Finally, the oxidation resistance mechanism was mainly credited to the pinning effect of O′-SiAlON phases at the grain boundaries.     

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.