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.     

Fabrication and wear behaviors of graded Si3N4 ceramics by the combination of two-step sintering and β-Si3N4 seeds

Graded Si₃N₄ ceramics with sandwich-like microstructure were fabricated by the combination of hot-pressing, spark plasma sintering and β-Si₃N₄ seeds. Phase compositions, microstructures, mechanical properties, and wear behaviors were investigated. Main α-Si₃N₄ phase were detected in the outer layers, and only β-Si₃N₄ phase were observed in the inner layers. The outer layer with ultra-fine equiaxed grains were well bonded to the inner layer with a distinct bimodal grain size distribution. Vickers hardness of outer layer (～21.2?GPa) was much higher than that of inner layer (～16.1?GPa), whereas fracture toughness of outer layer (～3.5?MPa?m^1/2) was much lower than that of inner layer (～5.9?MPa?m^1/2), indicative of the hard surface and tough core. Due to the ultra-fine microstructure and high hardness of outer layer, the graded Si₃N₄ ceramics exhibited superior wear resistance with low wear rate.     

Synthesis and properties of AlN/MAS/Si3N4 ternary glass-ceramic composites with in-situ grown rod-like β-Si3N4 crystals

The AlN/MAS/Si₃N₄ ternary composites with in-situ grown rod-like β-Si₃N₄ were obtained by a two-step sintering process. The microstructure analysis, compositional investigation as well as properties characterization have been systematically performed. The AlN/MAS/Si₃N₄ ternary composites can be densified at 1650 °C in nitrogen atmosphere. The in-situ grown rod-like β-Si₃N₄ grains are beneficial to the improvement of thermal, mechanical, and dielectric properties. The thermal conductivity of the composites was increased from 14.85 to 28.45 W/(m K) by incorporating 25 wt% α-Si₃N₄. The microstructural characterization shows that the in-situ growth of rod-like β-Si₃N₄ crystals leads to high thermal conductivity. The AlN/MAS/Si₃N₄ ternary composite with the highest thermal conductivity shows a low relative dielectric constant of 6.2, a low dielectric loss of 0.0017, a high bending strength of 325 MPa, a high fracture toughness of 4.1 MPa m^1/2, and a low thermal expansion coefficient (α_{25–300 °C}) of 5.11 × 10^?6/K. This ternary composite with excellent comprehensive performance is expected to be used in high-performance electronic packaging materials.     

Properties of SiC,Si3N4 and SiO2 ceramic powders produced by vibration ball milling

SiC, Si₃N₄ and SiO₂ powders were ground by a vibration ball mill, the pot and balls of which were made of Si₃N₄ ceramics, in a purified methanol medium. The mean particle diameters of SiC and SiO₂ powders were ca. 0.1 μm and thus ultrafine particles were manufactured. The increasing rate of specific surface area by fine grinding was largest on SiO₂ powders, and the rate decreased gradually with an increase in grinding time. The specific surface area of Si₃N₄ powders increased proportionally with an increase in grinding time. The grindability was obtained in the following order: SiO₂ > SiC > Si₃N₄. The ground SiC powders showed a large lattice strain and little fragmentation of crystallites. The strain and the crystallite size of ground Si₃N₄ powders were moderate.     

Quantitative analysis of toughening effect of crack deflection on Si3N4/Si2N2O composites through improved indentation method

In this study, Si₃N₄/Si₂N₂O composite ceramics prepared by hot pressing were used as an example, and the material fracture morphology and fracture mechanism were analyzed. Based on the formula of fracture toughness measured by an indentation method, a quantitative computation method was proposed to determine the toughened effect of ceramic materials resulting from the crack deflection by the second phase. The grain size and sintering density are increased with the increase of sintering temperature. The toughening effects resulting from the crack deflection is increased, and the main mode of fracture is transformed into the transgranular fracture. The Si₂N₂O grains can play a role in the toughening process because these grains can hinder the cracks extending along the radial direction. However, when the cracks extend in the axial direction, the toughening effect of Si₂N₂O grains is not obvious because of the internal stacking faults in the axial direction. The improved indentation method can quantitatively analyze the toughening effect of the second phase of composite ceramics, and the validity of this method are verified by comparing the fracture toughness of Si₃N4/Si₂N₂O and fine grained β- Si₃N₄ ceramics.     

The influence of mechanochemical activation on combustion synthesis of Si3N4

This study addresses itself in the performance of Si₃N₄ combustion synthesis, occurred in the presence of Si₃N₄ and NH₄Cl powders in N₂ atmosphere of 6 MPa. Mechanochemical activation of Si powder, achieved via high-energy attrition milling up to 24 h, increases the intensity and the efficiency of the reactions between Si and N₂ as well as combustion temperature. Benign processing conditions, anticipated with lower mechanochemical activation of Si powder, low N₂ pressures, and low combustion temperatures, favor formation of α-Si₃N₄.     

Joining of Si3N4/Si3N4 with in-situ formed Si-Al-Yb oxynitride glasses interlayer

Si₃N₄ ceramic was successfully joined to itself with in-situ formed Yb-Si-Al oxynitride glass interlayer. The joints were composed of three parts: (I) Si₃N₄ matrix, (II) oxynitride glass interlayer in which hexagonal or fine elongated β-sialon grains and a few ball-like β-Si₃N₄ grains exist, and (III) diffusion zone in Si₃N₄ matrix containing a thin dark layer and a ~ 25?µm thick bright layer. The seam owned similar microstructure to matrix and was inosculated with the matrix as a whole. The strength of the joint tended to increase with the increase of bonding temperature and reached the value of 225?MPa, when the joints were prepared at 1600?°C for 30?min under a pressure of 1.5?MPa. The high-temperature strength remained 94.7% and 75.2% of R.T. strength when the joints were tested at 1000?°C and 1200?°C, respectively. It may be contributed to the high softening temperature of the Yb-Si-Al oxynitride glass phase formed in the seam. Even suffered to the air exposure for 10?h at 1200?°C, the residual strength of the joints was still 143?MPa, attributed to the existence of YbAG phase.     

In situ preparation of SiC/Si3N4-NW composite powders by combustion synthesis

Large-scale composite powders containing silicon carbide (SiC) particles and silicon nitride nanowires (Si₃N₄-NWs) were synthesized in situ by combustion synthesis (CS). In this process, a mixture of silicon, carbon black, polytetrafluoroethylene (PTFE) and a small amount of iron powders was used as the precursor. The products were characterized by XRD, SEM, EDS and TEM. The particles are equiaxed with diameters in the micron range, and the in situ formed nanowires are straight with uniform diameters of 20-350 nm and lengths of tens of microns. The Si₃N₄-NWs are characterized to be α-phase single crystals grown along the [1 0 1] or [1 0 0] direction. VLS and SLGS processes are proposed as the growth mechanisms of the nanowires. The as-synthesized powders have great potential for use in the preparation of high-performance SiC/Si₃N₄-NW composites.     

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.     

Mechanical characterization of highly porous β-Si3N4 ceramics fabricated via partial sintering & starch addition

In this study, porous β-Si₃N₄ ceramics containing limited amount of Sm₂O₃ and CaO as sintering aids were produced by addition of potato starch (10 and 20 vol.%) and partial sintering. Two different Si₃N₄ powders, α- and β-, were used as starting materials. Scanning electron microscopy investigations revealed that development of elongated β-Si₃N₄ grains were much more pronounced when α-Si₃N₄ starting powder was used. Even though porosity values of the compositions prepared by using α-Si₃N₄ (～57.0–58.4%) is significantly higher than the samples produced by β-Si₃N₄ (42.6%), no significant change was observed for the bending strength, fracture toughness and Weibull modulus. This indicates that microstructural features have a significant contribution to the mechanical properties of the porous materials in terms of bending strength and fracture toughness.     

Neural network modeling and analysis of gel casting preparation of porous Si3N4 ceramics

Based on orthogonal experimental results of porous Si₃N₄ ceramics by gel casting preparation, a three-layer back propagation artificial neural network (BP ANN) was developed for predicting the performances of porous Si₃N₄ ceramics. The results indicated that BP ANN was a very useful and accurate tool for the prediction and optimization of porous Si₃N₄ ceramics performances. By using the developed ANN model, the influences of the compositions on performances of porous Si₃N₄ ceramics were investigated, and some important conclusions were drawn as follows: for the flexural strength of Si₃N₄ ceramics, solid loading has an optimum value where can achieve a maximum value, and the optimum solid loading decreases with the increase of monomer content; the porosity of sintering body monotonically decreases with the increase of solid loading, and it increases with monomer content; the porosity of sintering body monotonically increases with the increase of the ratio of crosslinking agent to monomer.     

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.     

Synthesis of carbon-free Si3N4/SiC nanopowders using silica fume

We have investigated a possible method of synthesizing carbon-free, nano-silicon nitride-silicon carbide (Si₃N₄/SiC) powders from the waste silica fume for the first time, using the integrated mechanical and thermal activation (IMTA) process. This novel process results in the formation of nano-Si₃N₄/SiC powders at 1465 °C with crystallite sizes as small as 45 nm. In order to synthesize carbon-free nano-Si₃N₄/SiC powders, two different approaches, one using the H₂ gas and the other using air, have been studied for their effectiveness in removing the free carbon present. It is found that the H₂ treatment is not very effective although both Si₃N₄ and SiC are stable during the H₂ treatment. In contrast, removing the free carbon using air is effective, and the limited oxidation of nano-Si₃N₄ and SiC can be achieved if the air treatment is terminated soon after the free carbon is eliminated. This study has provided a clear pathway and understanding for effectively synthesizing carbon-free, nano-Si₃N₄/SiC powders from the silica fume.     

Effect of starting Si3N4 powder type and sintering conditions on grain morphology of α-SiAlON ceramics

Abstract

Nitrogen rich multication α-SiAlON ceramics doped with Y-Ce have been densified by gas pressure sintering using α-Si₃N₄ or mixed β/α-Si₃N₄ starting powder. The effects of α-SiAlON nucleation and growth mechanisms were investigated by using di fferent sintering cycles. X-ray diffraction studies after sintering revealed that 21R polytype phase was present in addition to α -SiAlON matrix phase. Microstructural characterisation of sintered materials prepared using α-Si₃N₄ powder in the starting composition revealed a typical equiaxed grain morphology, as expected if the α-SiAlON nucleation step was not applied before grain growth. However, needlelike α-SiAlON grains were observed if a nucleation step was carried out before final sintering. In starting powders containing mixed β/α-Si₃N₄, needlelike grain morphology was also observed. The effects of different Si₃N4 starting powders and sintering conditions on the grain morphology and mechanical properties are discussed.     

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.     

Combined effect of Fe-Si alloys and carbon on Si3N4 stability at elevated temperatures

The combined effect of carbon and Fe-Si alloys on Si₃N₄ was explored by heat treating Si₃N₄ materials at 1500?°C and 1600?°C in flowing nitrogen. The phase compositions and microstructures were characterized by XRD and SEM, respectively. The reaction degree was analysed based on the mass variation in the system. Combined with a thermodynamic assessment, the reaction mechanism was studied and proposed. The results show that the coexistence of Fe-Si alloys and carbon accelerates the phase transformation from Si₃N₄ to SiC and worsens the strength of Si₃N₄ materials. Fe-Si alloys accelerate the deposition of CO gas to free carbon and accelerate the decomposition of Si₃N₄ to Si. The in situ-formed Si can react with carbon, thus accelerating the thermodynamic and kinetic formation of SiC. Along with the growth of pores and the deterioration of the wettability of Fe-Si alloys during this process, the microstructure changes from a network constituted by Si₃N₄ columns/whiskers to porous SiC particles with weak linkages, which leads to the failure of Si₃N₄ materials. Therefore, the combined effect of Fe-Si alloys and carbon is harmful for Si₃N₄ materials at 1500–1600?°C.     

Combustion synthesis of high flexural strength,low linear shrinkage and machinable porous β-Si3N4 ceramics

Porous β-Si₃N₄ ceramics were prepared by combustion synthesis using Si, α-Si₃N₄ and Y₂O₃ powders as raw materials. The effects of α-Si₃N₄ diluent content in pellet on nitriding rate, shrinkage, porosity and flexural strength of porous ceramics were investigated. The results show that porous β-Si₃N₄ ceramic with porosity of 49 % and flexural strength of 151 MPa can be obtained by combustion synthesis when the content of α-Si₃N₄ diluent in raw materials is equal to that of Si, and the linear shrinkage of porous ceramic is only 2.8 %. In addition, the porous ceramic can be drilled and turned by WC drill and turning tool respectively.     

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.     

Thermal shock resistance of in situ formed Si2N2O–Si3N4 composites by gelcasting

Thermal shock resistance of Si₂N₂O–Si₃N₄ composites was evaluated by water quenching and subsequent three-point bending tests of strength diminution. Si₂N₂O–Si₃N₄ composites which was prepared with in situ liquid pressureless sintering process using Yb₂O₃ and Al₂O₃ powders as sintering additives by gelcasting showed no macroscopic cracks and the critical temperature difference (ΔT_c) could be up to 1400 °C. A mass of pores existed in the sintered body and the irregular shaped fibers extended from the pores increased the thermal shock property.     

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.