Formation mechanism of elongated β–Si3N4 crystals in Fe–Si3N4 composite via flash combustion

Elongated β–Si₃N₄ crystals have a significant influence on the mechanical property of Fe–Si₃N₄ composite. In this paper, the formation mechanism of elongated β–Si₃N₄ crystals in Fe–Si₃N₄ composite was investigated. During the preparation process, β–Si₃N₄ crystals developed in a spiral and layer growth mechanism in the dense areas. They kept growing from the dense areas and formed radially distributed elongated crystals with hexagonal prismatic morphology as time went on. As for the formation mechanism, the (100) crystal plane of β–Si₃N₄ from Si-N-O melt is mainly the vicinal crystal planes growth with different angles from the (100) crystal plane. At the later stage, the crystallization and the diffusion forces in Si-N-O molten phase decreased. However, the short range diffusion remained active and resulted in the gradient distribution of N content near the boundary. With the temperature decreasing, the disappearance of the short range diffusion implied the end of the crystallization process of the elongated β–Si₃N₄ crystals.     

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.     

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.     

Preparation of Si3N4 porous ceramics via foam-gelcasting and microwave-nitridation method

Si₃N₄ porous ceramics with improved mechanical strength were fabricated for the first time by a combined foam-gelcasting and microwave-nitridation method at 1273–1373?K. The Si₃N₄ porous samples prepared at 1373?K/20?min with the porosity of 68.9% had respectively flexural and compressive strength as high as 8.1 and 20.8?MPa, which values were comparable or even superior to those of Si₃N₄ porous ceramics prepared previously by the conventional heating technique at a much higher temperature of 1773–1973?K, indicating that present preparation strategy is feasible to prepare high quality Si₃N₄ porous ceramic at a much milder condition. Moreover, the thermal conductivity of as-prepared Si₃N₄ porous ceramics at 1073?K was as low as 1.697?W/(m?K), suggesting it could be a potentially good heat insulating material for aluminum electrolyte cells.     

A novel approach to fabricate Si3N4 by selective laser melting

Difficulties in sintering refractory ceramics limit their potential high-demanding applications. Selective laser sintering/melting of ceramics is extremely challenging due to poor sinterability of refractories caused by a low thermal shock resistance and an insufficient electron conductivity blocking absorption of laser electron beam energy, etc. Here, we propose a new approach to fabricate Si₃N₄-based complex geometry parts by selective laser sintering. This is a two-step approach including (i) selective laser sintering of silicon powder providing a needed shape, and (ii) nitridation of the as-shaped silicon parts aimed at fabrication of the Si₃N₄ component. Parametric study of the process has been performed for optimization of the sintering parameters, such as laser current, point distance and exposure time. The silicon component of full Archimedes density, 12?GPa Vickers hardness and 432?MPa compressive strength has been produced by SLS technique. Effect of different catalysts (Ni-, Cr-, Co-based) on the nitridation of the shaped silicon parts has been thoroughly studied. The conversion degree of nitridation reaches 50% with Ni-based catalyst subjecting growth of Si₃N₄ nanofibers on the surface of the component.     

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.     

Preparation of Ni-coated Si3N4 powders via electroless plating method

The Ni/Si₃N₄ coated powders were successfully prepared via electroless plating method by using hydrazine hydrate (N₂H₄·H₂O) as a reducing agent. The coated powders were characterized with several techniques such as scanning electron microscope, energy dispersive spectrometer, Transmission electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction to determine particle size, composition, phase and morphology. It indicated that the core–shell structure of Ni/Si₃N₄ has been constructed in the present method, the Ni layer on the surface of Si₃N₄ particles was relatively continuous and uniform, but it is inevitable that only in very small area occurred the aggregation of Ni particles. In principle, the coated process was successful and expectable.     

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 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.     

Porous (SiCw-Si3N4w)/(Si3N4-SiC) composite with enhanced mechanical performance fabricated by 3D printing

SiC whisker and Si₃N₄ whisker-reinforced Si₃N₄-SiC ((SiC_w-Si₃N_4w)/(Si₃N₄-SiC)) composite was synthesized by 3D printing for the first time, by the combination of printed-Si-body nitridation and chemical vapor infiltration-SiC methods. The mechanical properties of the composite could be optimized through the adjustment of SiC_w content and load direction. A SiC_w content of 3?wt% was found to be the optimal scheme, and accordingly, the average bulk density increased by 22.4%, the bending strength increased by 63.6%, the compressive strength parallel to the printing layer increased by 404.8%, and the compressive strength perpendicular to the printing layer increased by 157.1%, compared with the bulks without whiskers. The enhanced mechanical performance was mainly attributed to the process of densification by CVI, and the effect of the homogeneous whiskers bridging, pull-out and deflecting crack to expend energy. The achieved indices meet the requirements for 3D-printed porous ceramic matrix composite targeted for commercial and military field applications.     

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.     

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.     

导电 Si3N4 基复相陶瓷研究进展

Si_3N_4陶瓷具有优异的力学性能和导热性能,然而其固有的高硬度和脆性极大地限制了其加工性能。通过添加导电相改善Si3N4陶瓷的导电性能可实现对Si_3N_4陶瓷的电火花加工。添加的导电相主要包括钛基化合物(TiN、TiC、TiC N、TiB_2)、锆基化合物(Zr B_2、Zr N)和MoSi_2等导电陶瓷以及碳纳米管(CNT)、碳纳米纤维(CNF)、石墨烯纳米片(GNP)等导电碳基纳米材料。本论文详细回顾了Si_3N_4基导电陶瓷的研究进展,并对今后Si_3N_4基导电陶瓷的发展趋势进行了展望。     

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.     

High temperature mechanical properties of porous Si3N4 prepared via SRBSN

The high temperature strength and fracture behavior of porous Si₃N₄ ceramics prepared via reaction bonded Si₃N₄ (RBSN) and sintered reaction bonded Si₃N₄ (SRBSN) were investigated at 800–1400?°C. The weight gain after oxidation for 15?min and the microstructure of the edge and center of the fracture surface clearly show that the internal oxidation of porous SRBSN is unavoidable with porosity of ~ 50% and mean pore size of 700?nm. The oxidation of Si₃N₄ and intergranular Y₂Si₃O₃N₄ phase may responsible for the high temperature strength degradation of SRBSN. Porous Si₃N₄ ceramics prepared with addition of 1?wt% C showed low strength degradation at temperature >?1200?°C.     

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.     

Effect of different preparation methods on the microstructure and mechanical properties of Si3N4 ceramic composites

In this study, Si₃N₄ ceramic composites were fabricated by using ball-milling, titration preparation and urea preparation methods, respectively. The effect of different preparation methods on microstructure and mechanical properties of the Si₃N₄ ceramic composites was investigated. Obviously, the Si₃N₄ ceramic composite prepared by the urea preparation method (U-SN sample) showed better sintering behavior and higher mechanical properties than that prepared by the other two methods. Compared with the Si₃N₄ ceramic composite by the titration preparation method (T-SN sample), we could avoid the complex titration process or uncontrollable pH value during the preparation process of the U-SN sample. Meanwhile, the coated Y-Al precursor layer in thickness of nanometers was more homogeneous than that prepared by the traditional titration method. B-SN represented the Si₃N₄ ceramic composite prepared by the ball-milling method. These samples were all sintered from room temperature to 1750 °C via hot-pressing sintering. The U-SN specimen showed the optimal flexural strength and fracture toughness of being 817 MPa and 6.90 MPa/m², respectively, which could be attributed to its smallest grain size (0.46 µm) among these three samples.     

Wetting of AgCu-Ti filler on porous Si3N4 ceramic and brazing of the ceramic to TiAl alloy

The effect of Ti content on the wettability of AgCu-Ti filler on porous Si₃N₄ ceramic was studied by the sessile drop method. AgCu-2 wt% Ti filler alloy showed a minimum contact angle of 14.6° on porous Si₃N₄ ceramic during the isothermal wetting process. The mechanism of AgCu-Ti filler wetting on porous Si₃N₄ ceramic is clarified in this paper. Porous Si₃N₄ ceramic was brazed to TiAl alloy using AgCu-xTi (x = 0, 2 wt%, 4 wt%, 6 wt%, 8 wt%) filler alloy at 880 °C for 10 min. The effect of Ti content on the interfacial microstructure and mechanical properties of porous-Si₃N₄/AgCu-xTi/TiAl joints are studied. The typical interfacial microstructure of p-Si₃N₄/AgCu-Ti/TiAl joint is p-Si₃N₄/penetration layer (Ag(s,s)+Si₃N₄+TiN+Ti₅Si₃)/Ag(s,s)+Cu(s,s)+TiCu/AlCu₂Ti/TiAl. The maximum shearing strength of the brazed joint was 14.17 MPa and fracture that occurred during the shearing test propagated in the porous Si₃N₄ ceramic substrate for the formation of the penetration layer.     

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.     

A novel aerogels/porous Si3N4 ceramics composite with high strength and improved thermal insulation property

A novel ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite with high strength, low density, good dielectric properties and low thermal conductivity was synthesized by filling ZrO₂-SiO₂ aerogels into the porous Si₃N₄ ceramics through vacuum sol-impregnating. The effects of aerogels on the microstructure and properties of composite were discussed. The results show that aerogels could form a mesoporous structure and significantly decrease the thermal conductivity from 9.8 to 7.3 W m^?1 K^?1. Meanwhile, the density, mechanical and dielectric properties of the porous Si₃N₄ ceramics could not be affected after introducing ZrO₂-SiO₂ aerogels. The composite exhibits high porosity (62.6%), high flexural strength (53.86 MPa) and low dielectric constant (2.86). The ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite shows great potential as radome materials applied in the fields of aerospace.